Fixed Prosthodontics Introduction PDF

Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics Introduction Fixed prosthodontics is the specialized area of dentistry concerned with replacement and restoration of teeth by artificial substitutes that are not readily removable from the mouth. Its focus is to restore function, esthetics, and comfort”. Indications for Fixed Prosthodontics 1) One or two adjacent teeth are missing in the same arch. 2) The supportive tissues are healthy. 3) Suitable abutment teeth are present. 4) The patient is in good health and wants to have the prosthesis placed. 5) The patient has the skills and motivation to maintain good oral hygiene. Contraindications for Fixed Prosthodontics 1) Necessary supportive tissues are diseased or missing. 2) Suitable abutment teeth are not present. 3) The patient is in poor health. 4) The patient is not motivated to have the prosthesis placed. 5) The patient has poor oral hygiene habits. 6) The patient cannot afford the treatment. Terminology Crown: is an artificial replacement that restores missing tooth structure by surrounding most or all of the remaining structure with a material such as cast metal, porcelain or a combination of materials such as metal and porcelain. Full Metal Crown: is a full coverage all metallic restoration rebuilding the prepared clinical crown of the natural teeth.N.B. Mainly used in posterior teeth. Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics Jacket Crown(Ceramic crown): is a non metallic restoration rebuilding the prepared teeth mainly for esthetic. Veneered Metal Crown: It is a full cast metal crown having an acrylic or porcelain facing on the labial or buccal surface. Full Veneered Metal Crown: is a full cast metal crown having all surfaces faced with porcelain. Post Crown: is a full coverage crown that gains its retention by a post inserted in the prepared root canal. It has a radicular part inserted in the prepared root canal and a top part outside the root canal. Three-Quarter 3\4 Crowns (partial Veneered Crown): Restore the occlusal or incisal surface and three of the four axial surfaces the proximals, palatal/lingual, and retained in position by grooves prepared on the prepared proximal surfaces. (not including the facial surface). Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics Reverse Three-Quarter Crowns: Restore all surface except the lingual/palatal surface, usually used with lower premolars. Seven-Eighths Crowns: Are extensions of the three-quarter crown to include a major portion of the facial surface, except the mesio-buccal cusp of maxillary molars Pinledge Retainer: Refers to the modification of an anterior three-quarter crown preparation to obtain primary retention and resistance from long parallel pins in the lingual/palatal surfaces of the clinical crown Inlay Retainer: It is a classic intracoronal cast restoration that restores the proximal and occlusal surfaces but does not protect or cover the cusps entirely. Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics Onlay Retainer: Is a modification of the inlay with cusp overlays on the occlusal surface to protect the integrity of the remaining tooth structure. Temporary Crown: Is a crown made and used temporarily for the protection of the prepared tooth till the crown or bridge is completed and ready for cementation Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics One-half crown veneers: Restore the occlusal and mesial surfaces, as well as portions of the facial and lingual surfaces. It is a modified form of three-quarter crown A) Normal three-quarter crown with an intact facial surface. B) Proximal half crown. Here, the distal surface is left intact. Laminates: Are veneer restoration that restore the facial surface of a tooth for esthetic purposes. They are fabricated from resin or dental porcelain. They bond to etched enamel with a composite resin luting agent. Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics Bridge Bridge: (Fixed Partial Denture), (Fixed Prosthesis), fixed Replacement (Fixed Appliance): Is a masticating or incisive surface of metallic and/or nonmetallic material spanning an edentulous space in the dental arch and firmly anchored at one or both ends to the adjoining teeth. Abutment Tooth: Is the prepared natural tooth which supports and retains the bridge at one or both terminals. Retainer (Attachment), (Abutment Piece): Is a restoration rebuilding the prepared tooth and connecting the pontic to the abutment. Pontic: (Dummy) is that part of the bridge which acts as the actual substitute for the lost tooth and is suspended between the retainers replacing the lost natural tooth functionally and esthetically Connector Joint: Is that part of the bridge uniting the pontic (s) with retainer (s) joining the component parts of the bridge Pier Abutment: Is an isolated tooth where the adjacent anterior and posterior teeth are missing Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics Fixed Fixed Bridge: (Fixed Bridge), (Rigid Bridge), (Fixed Rigid Bridge), Stationary-Fixed Bridge.Is a bridge where the abutment pieces and Pontics are all soldered together and the bridge is cemented at both ends to the abutment teeth Fixed Supported Bridge: (Fixed-Movable), (Broken Stress Bridge), (Semi-Rigid), (Fixed- Semirigid Bridge). Is a bridge which is not actually joined to one of the terminal abutment teeth but is connected to it by means of a non-rigid connector allowing some individual movement of the abutment tooth. Removable Bridge: (Precision Attachment Bridge), ( Removable partial Denture with internal attachment). Is a bridge where each retainer consists of two parts, one fixed to the abutment tooth, and one soldered to the pontic and the bridge can be removed at the will of and by the patient for cleansing purposes Cantilever Bridge: (Swing on Bridge) , (Free end Bridge) is a bridge where the pontic is fixed to and derives its support from double retainers at one end only while the other end is unsupported. Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics Spring Cantilever Bridge: Is a cantilever bridge where the pontic is at the end of slightly resilient curved arm derving its support from an abutment remote from the edentulous space Immediate Bridge: (Immediate replacement Bridge), (Immediate Fixed Partial denture). Is a bridge with root extension pontic and is constructed before extraction of the tooth and seated in position immediately after the extraction at the same visit Temporary Bridge: Is a bridge made and used temporarily for the protection of the prepared teeth and maintenance of the space till completion and cementation of the bridge. A minimum-preparation bridge (resin bonded bridge, adhesive bridge, Maryland bridge): Is attached to the surface of minimally prepared (or unprepared) natural teeth and therefore occupies more space than the original dentition Implant supported bridge: A single implant may support a single tooth prosthesis or a series of implants may support a prost hesis replacing a number of teeth. Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics CLASSIFICATION OF BRIDGES 1. Depending upon location - Anterior bridges (It is the anterior bridge that doesn’t cross the canine line)  Unilateral  Bilateral - Posterior bridges (It is the posterior bridge that doesn’t cross the canine line) -Complex: It is a bridge that involve anterior and posterior segments crossing the canine line 2. Depending on number of teeth  2 Unit bridges  3 Unit bridges  etc……….. 3. Depending upon the tooth reduction  Conventional preparation bridge  Minimal preparation bridge  Hybrid bridge 4. Depending on support - Tooth supported FPD :  Conventional or rigid FPD  Cantilever FPD  Resin bonded FPD -Implant supported FPD :  Screw retained FPD.  Cement retained FPD. 5-Depending on material used:  Cast metal FPD  All ceramic FPD  Metal ceramic FPD  Resin veneered FPD. Dr.Mohamed Adel Anwar Introduction of fixed prosthodontics 6- Depending upon construction :  Cast metal FPD  CAD CAM assisted FPD  Direct fibre reinforced 7- Depending upon retention : A) Simple bridge  Fixed – fixed bridges  Fixed supported bridges  Cantilever bridges (fixed-free bridge)  Spring Cantilever bridges  Removable bridge B) Compound bridges It is a combination of two or more simple bridges Dr.Mohamed Adel Anwar Treatment planning Treatment Planning It is formulating logical sequence of treatment designed to restore patient’s dentition in good health and with optimal function and appearance. It must be presented in written form to the patient and discussed with him in details. It must informs the patient about the present condition, the extent, time and cost of the proposed treatment as well as the level of home care and professional follow up that will be required to achieve success. It is based on data collected from the patient’s history, clinical examination, diagnostic cast analysis and radiographic evaluation. On planning treatment, one must pay attention to patient’s needs and correlate them to the range of the treatment available. For long term success of fixed partial denture, proper evaluation of abutment must be done. Treatment plane is concerned mainly with the following: 1. Identification of patient’s need. 2. Correction of existing disease paying attention to its initiating factors and its resistive factors. 3. Prevention of future disease. 4. Restoration of function. 5. Improvement of appearance. The sequence of the treatment plane must be as follows: 1. Treatment of symptoms. 2. Stabilization of deteriorating conditions. Dr.Mohamed Adel Anwar Treatment planning  Replacement of defective restoration.  Removal of caries.  Crown recontouring.  Proper oral hygiene instructions.  Fluoride treatment.  Dietary advice. 3. Definitive therapy a. Oral surgery. b. Periodontic. c. Endodontic. d. Orthodontic. e. Fixed prosthodontics then removable partial denture. f. Occlusal adjustment. On designing a treatment plane for prosthesis, we must evaluate certain factors: 1. Abutment. 2. Biomechanical forces 3. Retainer selection. 4. Pontic selection. 5. Connector selection. Abutment Is a tooth that carries or supports a prosthetic restoration either crown or bridge. It may be a natural tooth either vital or non-vital, or an implant abutment. Many factors must be put into consideration to judge a tooth as good abutment. Dr.Mohamed Adel Anwar Treatment planning 1. Factors related to the crown.  Crown length.  Crown form.  Degree of mutilation. 2. Factors related to the root.  Root length and form.  Root number.  Crown root ratio. 3. Factors related to the Periodontium.  Ante’s law.  Periodontal health.  Tooth mobility.  Tooth axial alignment. 4. Others:  Occlusion - Opposing occlusion. - Type of bite. - Occlusal plane. - Presence of parafunctional habits. Crown length is a determining factor of whether the tooth is considered a good abutment or not. Short teeth are not ideal abutment for partial coverage restorations as they do not provide sufficient retention. They can be used as abutments for complete coverage retainers but with the supplement of subgingival finish line or even addition lengthening through periodontal surgery. Dr.Mohamed Adel Anwar Treatment planning Crown form interferes with the resultant preparation; tapered crown interferes with the preparation parallelism, necessitates the use of full coverage retainer to improve their retention and esthetic qualities. Anterior teeth with poorly developed cingulum and short proximal walls, mandibular premolars with poorly developed lingual cusp and short proximal surfaces and incisors with thin highly translucent incisal edge that contraindicates the use of partial coverage retainers as it will be esthetically unacceptable. Degree of mutilation; size, number and location of carious lesions or restorations plays an important role in abutment selection and so the type of retainer used. Some fractured or carious teeth are not restorable and must be removed. Root length, form and number, abutment must retain stability for the retainer to function normally and preserve healthy oral conditions. Abutment tooth must have good anchorage in bone enough to resist occlusal forces. Long rooted teeth are good abutments than short rooted ones. Parallel sided roots with developmental depressions are better than conical smooth roots as they resist additional occlusal forces. Multi-rooted teeth, provides better stability than single rooted teeth. Also multi-rooted teeth with curved roots give better support than those with fused roots. Crown/ root ratio is a traditional guideline in which the percentage of tooth length projecting out of the alveolar bone to that embedded in bone. It is liable to change through out life due to apical migration of the alveolar bone level and aggressive periodontal disease. Ratio 1: 1.5 is considered satisfactory whereas 1: 1 is minimal and may require the use of additional abutment in relation to the Dr.Mohamed Adel Anwar Treatment planning number of replaced teeth, tooth mobility and periodontal health, also crown lengthening may be performed. However, it can be used in certain conditions: - Artificial opposing occlusion. - Opposing is mobile and periodontally involved. - Normal occlusion. - Divergent root form with elliptical cross section. - Motivated patient with excellent oral hygiene and plaque control. Ante’s law stated that “combined periodontal ligament area of the abutment must equal or exceed that of the tooth or teeth to be replaced. It may be affected by - length and cross section of the root. - Root form. - Root number. Dr.Mohamed Adel Anwar Treatment planning - Alveolar bone support as in old age there is gingival recession that cause bone loss. - Axial inclination and drifting. - Endodontic surgery or resorption. The width of periodontal ligament space normally is 0.18- 0.25mm indicating normal function whereas, narrowed width is related to non functional side and widened space denotes prematurity and looseness. The location of epithelial attachment varies it may be on enamel, cement enamel junction or cementum. It affects the length of the clinical crown which in turn affects the type of preparation and finish line location. Bone loss is a sign of periodontal health inadequacy that alters the convenience of the tooth as an abutment or necessitates the use of additional abutment or even changing treatment plan to removable prosthesis. Patient must be motivated to maintain his oral health in good conditions and instructed to use different oral hygiene measures before proceeding in restorative treatment. The magnitude of mobility and cause must be properly evaluated before deciding whether the abutment is suitable or not. According to miller’s, mobility value of one is considered acceptable, value of two necessitates the assessment of the cause and consideration of replaced teeth number. If it is due deflective occlusal contact, correct it and plan for short span prosthesis to determine suitable abutment. However, if tooth mobility is due to bone loss and a long span is planned, the abutment must be splinted to adjacent sound tooth. A tooth with miller’s mobility value of three in not suitable as abutment and is indicated for extraction. Dr.Mohamed Adel Anwar Treatment planning Properly aligned tooth is a good abutment as a retentive preparation can be developed. Malaligned teeth necessitate excessive reduction or even devitalization to maintain mechanical principles of reduction, unless orthodontic correction is done. Minor malalignment (tipping or rotation) of teeth direct the clinician to full coverage restoration to achieve adequate retention and acceptable esthetic results. Vital sound properly aligned teeth with healthy periodontium where bone crest is 1.5mm to CEJ are ideal abutments. However a vital tooth may be carious, fractured, mutilated or periodontally affected. Carious teeth must be properly excavated and remaining tooth structure evaluated for the proper restorative technique. Caries must never be left in abutment teeth. Mutilated teeth can be used as abutments after restoring them with proper restoration either pin retained or pin ledge restorations. Traumatized teeth must be properly inspected for their vitality, cracks or fracture. Ellis classified fractured teeth into four classes.  CL I chipping of the incisal edge, fracture will be included in the reduction.  CL II fracture of incisal angle without pulp involvement that needs composite filling then reduction.  CL III horizontal fracture involving both incisal angles without pulp exposure. Composite restoration will be done before reduction.  CL IV fracture of incisal angle with pulp exposure. Endodontic treatment must be carried out followed by post and core buildup then reduced for fixed prosthetic restoration. Periodontally affected teeth are classified into four classes according to the amount of bone loss and its direction. Dr.Mohamed Adel Anwar Treatment planning  CL I vertical bone loss less than 3mm apical to CEJ and can not be detected by x-ray.  CL II vertical bone loss more than 3mm with or without 1mm horizontal bone loss and detected in x-ray.  CL III horizontal bone loss more than 1mm, through and through but occluded with gingival tissues and allows passage of probe from facial or lingual or palatal surfaces. Bone loss is grossly evident in x-ray.  CL IV horizontal bone loss through and through and not occluded by gingival and obviously detected on x-ray. Dr.Mohamed Adel Anwar Treatment planning CL I needs scaling, pocket elimination before reduction, whereas CLII necessitates alveoloplasty and gingivoplasty before reduction. CL III is treated by hemi-sectioning in case of mandibular molars, removing half tooth in the side of the affected root (half crown and root) and changing the molar into premolar or root amputation by removing the affected root only. Amputated teeth can be used as abutments but supplemented with additional abutments. Implant abutments are components of the implant system that screw directly into the implant, fixed restoration then will be retained to them. According to retention they are either screw retained or cement retained. Several types of abutments are present. Standard abutment which is screw retained and allow margin placement subgingival or supra-gingival, fixed abutment that has finish line preparation and it is a cement retained implant abutment, angled abutment that is used where correction of implant angle is needed to improve esthetics or biomechanical qualities and it is either screw or cement retained, non segmented abutment that is used in case of soft tissue thickness is less than 2mm and it is a screw retained abutment and tapered abutment which is also a screw retained implant abutment. Biomechanical forces: It is affected by the 1. Span dimension: either span length or width affects the amount of forces. The more the number of pontics the more will be the bending of the FPD that results in porcelain fracture, connector breakage, retainer loosening and unfavorable soft tissue response. This necessitates the use of additional abutments, the use of Ni Cr alloy because of its high yield Dr.Mohamed Adel Anwar Treatment planning strength and the presence of well developed abutment that gives maximum retention. 2. Inter abutment axis: abutments on straight line resist movement in one direction i.e. mesiodistal but those on curved path resist both mesiodistal and buccolingual movements e.g. canine case. Also pontics must be close to the interabutment axis to reduce the torque on them and alter stresses to be favorable. 3. Span position: forces acting on posterior bridge are more than those acting on anterior bridge that necessitates proper selection of bridge design, retainer, solder and alloy used for construction. 4. Arch form and curvature: tipping force are created on incisal pontics in case of v- shaped arch that recommend the use of additional abutments at a distance from the inter abutment axis equal to the length of lever arm (premolars). There are several situations where we need to restore or replace missing teeth: 1. Replacing missing single tooth. 2. Replacing missing more teeth. 3. Special situations. Replacing missing single tooth: The first choice is implant as it is a conservative, preserving the adjacent tooth structure. Resin bonded bridge is the second conservative choice that is based on resin bonding of pontic with metal treated wings to the enamel of the palatal surface of the abutments. Retention is dependent on mechanical and chemical adhesion. Fixed fixed bridge, where mesial and distal abutments around the Dr.Mohamed Adel Anwar Treatment planning pontic will be reduced to allow construction of three unit bridge. Fixed free bridge or cantilever bridge, where the pontic is retained and supported at only one side e.g. missing lateral, a retainer on the canine will be attached to the pontic. Fixed supported bridge, where a retainer is attached to the pontic on one side while the other side gains support from a rest or precision attachment on another abutment, usually the anterior abutment. E.g. missing lateral, retainer will be attached to canine and a palatal rest is supported on the central. Replacing more than one tooth: a) Missing anterior teeth, in case of missing mandibular anterior teeth a fixed fixed bridge is designed using canines as abutments. In case of missing maxillary incisors, the line of treatment is dependent on the arch curvature. Pointed v-shaped arch requires the use of additional abutment posteriorly to counteract forces acting on the pontics that tend to tip abutments as they are not in straight line. b) Missing two posterior teeth or less, FPD is the line of treatment where favorable conditions are available. c) Long edentulous span, where there is 3 missing posterior teeth. Implant supported prosthesis or RPD. FPD is used in case of favorable conditions in case of replacing teeth in the maxillary arch although it is risky. d) Free end span, in case of bilateral free span, RPD is chosen. In case of unilateral, implant supported FPD, RPD or a short cantilever is indicated in case of missing lower 6 using premolars as abutments. Pontic in this case will act as stopper. e) Multiple edentulous areas with intermediate abutments, a combination of fixed and removable PD may be used. Dr.Mohamed Adel Anwar Treatment planning f) Long span and free end span case is treated by RPD, precision attachments or implants. Special situations: a) Pier abutment = isolated = intermediate abutment: It is an abutment with edentulous spans on both sides. It acts as a fulcrum transmitting unfavorable forces to the terminal abutments, tension between retainer and abutment causing intrusion or unseating which in turn causes loosened retainer, leakage and caries. To neutralize these forces a non rigid attachment must be used. N.B. A rigid attachment distributes even stresses than non rigid one. However, it is more desirable to use non rigid attachment in case of a tooth with decreased periodontal attachment. Non rigid connector provides stress breaking action transfers shear stress to bone rather than to the connector and minimizes mesiodistal torque of the abutment by allow it to move independently. It is formed of a key and a keyway. The key is located in the mesial side of the distal pontic as mesial movement will seat the key in the key way that is located in the distal contour of the pier abutment. Dr.Mohamed Adel Anwar Treatment planning b) Tilted molar: it has the following problems; difficulty to establish common path of insertion with the mesial abutment, overeruption of the opposing leading to discrepancy of the occlusal plane, decreased mesiodistal pontic space, if the third molar is present it is tilted or drifted with the second molar and so it will encroach the path of insertion of the FPD and food accumulation interproximally leading to caries and periodontal disease, lastly destructive lateral forces to the periodontium will cause resorption. The solutions are: orthodontic uprighting the tilted molar and removing the third molar to facilitate the uprighting. It takes three months. Selective grinding of second molar to upright it as much as possible if it is drifted within 25˚. This may lead to overtapering of it that necessitate the use of additional means of retention as buccal and lingual grooves. Half crown retainer on the distal abutment especially in case of tight contact between 7 and 8. Telescopic crown on the distal abutment. Reduction of the tilted molar is following its long axis and a coping is fitted to it. A crown is fitted over the coping, not involving the mesial undercut. It acts as a retainer following the path of insertion of anterior abutment. The use of non rigid connector, where a full coverage preparation is done for the tilted molar parallel to its path of insertion and reduction of the premolar will include a box preparation on its distal surface to accommodate the keyway in it. Last solution is correction of opposing occlusal plane discrepancy to establish balanced occlusion. Dr.Mohamed Adel Anwar Treatment planning Dr.Mohamed Adel Anwar Treatment planning c) Maxillary canine replacement: - On dealing with this tooth we are faced with a complex bridge as we are facing arch curvature where teeth in different arch segments move in different directions (both facio-palatal movement of anterior teeth and buccolingual movement of posterior teeth). – Maxillary canine lies outside the interabutment axis extending between maxillary 1st premolar which is the smallest posterior tooth and lateral incisor the weakest tooth in the arch. Forces on the FPD will be directed labially so we have to use additional abutment and so abutments will be 1, 2 and 4. – In this complex bridge no other missing tooth can be replaced but a RPD must be used. d) Missing upper lateral: either a fixed free PD or a fixed supported PD with a rest on the central can be the line of treatment. A cantilever is used on favorable abutments i.e. those having long clinical crown, adequate crown root ratio and healthy periodontium. However, no occlusal contact should exist on the lateral pontic in both centric and eccentric excursions. Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics Biological considerations of fixed prosthodontics Success of fixed prosthetic work is dependent on preserving the famous triad :  BIOLOGIC CONSIDERATIONS (Health of oral tissues)  MECHANICAL CONSIDERATIONS (Integrity & Durability of restoration)  ESTHETIC CONSIDERATIONS (Appearance) Biological consideration deals with the health of the oral tissues, mechanical consideration affects the restoration’s integrity and durability whereas esthetic consideration affects the appearance of the patient. Biological consideration is an important issue that must be attained and maintained during each phase of the fixed prosthesis work ( pre-operative, operative and post operative). The oral cavity has a number of damage prone structures, the pulp and dentine of the tooth, the Periodontium in the form of the gingiva and the periodontal ligament, soft tissues in the form of lip, cheeks and tongue and finally the TMJ. As a rule avoid unnecessary damage during prosthetic procedures. This must be the rule during all the steps either before, during and after prosthetic treatment. Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics 1. Preoperative phase: Before starting prosthetic treatment, biologic consideration can be achieved by:  Comprehensive diagnosis and treatment plan that objects to correct an existing disease, to prevent future disease, to restore function and improve esthetics. This necessitates the following:  Proper evaluation of patient’s medical history as this may give an idea about certain precautions that you have to put into consideration, special medication that you have to prescribe to fit the patient’s condition or may be the need to postpone, delay or even cancel the proposed treatment due to the patient’s physical and psychological conditions.  Proper evaluation of occlusal forces and type of bite.  Proper motivation to maintain good oral hygiene to assure success of the prosthesis.  Having knowledge of the available materials and techniques to choose between extra-coronal and intra-coronal restorations or between metallic or non metallic restorations as well as the indications and contraindications for use o f different types of prosthesis.  Proper decision taking to remove or restore teeth and proper selection of abutment teeth and evaluation of the span length and the appropriate bridge design.  Following a sequential mouth preparation that adheres to: 1. Relief of symptoms. 2. Removal of etiologic factors. 3. Repair of damage. 4. Maintenance of dental health. Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics 2. Operative phase: This is divided into during preparation and after preparation. a. During the prosthetic procedure every step you make can cause damage to the oral structures unless that it is done meticulously. It is related to: 1- Prevention of damage to - Adjacent teeth - Soft tissues - Pulp 2-Conservation of tooth structure. - Preventing damage of adjacent teeth can be achieved through either the use of metal matrix band around the tooth need to be protected. However, the band may perforate and damage occurs. The production of enamel lip during proximal reduction by cutting on expense of the tooth to be prepared as the width of the proximal contact over the cement enamel junction is 1.5-2 mm and then by fine tapered stone this lip is removed, opening the contact without injuring the adjacent tooth. - Preventing damage to soft tissues (cheek, lip & tongue) is done by proper retraction by mirror or saliva ejector tip and maintaining support to avoid slipping while working. - Preventing pulp damage through preventing thermal, chemical and bacterial irritations. Thermal irritation arises during cutting due to the friction between the stone and tooth surface. To avoid thermal irritation a coolant must be used in the form of air- water jet and not air only to avoid aspiration of the odontoblasts that will lead to post operative pain and sensitivity as well as clogging of the rotary instrument that will reduce its cutting efficiency, high Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics speed hand piece use with intermittent light pressure and the use of sharp rotary instruments. Chemical irritation comes from putting some of the restorative materials on freshly cut dentine and also from the use of some chemicals to clean the tooth after reduction. Bacterial irritation can be avoided by removing all the caries and sealing the cavities. - Conservation of tooth structure is important and is achieved by optimal reduction of the tooth structure as the remaining dentine is inversely proportional to pulpal response. Also the use of partial coverage preparation is more conservative than complete coverage. Avoid over tapered reduction. Occlusal reduction following the occlusal anatomy. The use of conservative marginal design. Avoid unnecessary apical extension of the preparation that necessitates more reduction. It is also important to differentiate between dentinal and pulpal pain on dealing with deep preparations to choose the ideal line of treatment. Dentinal pain pulpal pain -Sharp, lancinating. -Dull throbbing -Easily localized -Poorly localized -Stimulated by touch, cold, -Responds slowly to heat and acid, dehydration. increased venous pressure. -Origin: pain fibers around - origin: pain fibers around pericytes. odontoblasts -cold elicits pain. - cold relieves pain. Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics 3. After the preparation: o Cleansing the tooth structure: avoid the use of caustic chemicals to remove debris from the cut tooth surface. o Tissue retraction procedure: it is indicated during examination, preparation, impression taking and cementation procedures. Methods: 1. Mechanical, where the cord is applied using a blunt instrument to avoid any trauma to the gingiva. It should not be left more than 15- 20 minutes otherwise permanent gum recession will occur. 2. Mechanical chemical method; by using cord impregnated in different chemicals as : - Epinephrine 8% but it is contraindicated in case of cardiac patients, otherwise epinephrine syndrome may occur in the form of tachycardia, hypertension and post operative depression. – Tannic acid 20- 100% or aluminum chloride 5- 25% but it was found to cause gingival inflammation or tooth discoloration. – ferric sulphate 13.3%, it can stain gum yellow brown for few days. – Alum solution 100% which is the safest method. 3. Surgical method, indicated in case of gingival hyperplasia. It is done either by (1)lancet or (2) electrosurgury that is contraindicated in patients with pacemaker, delayed healing or thin attached gingiva. 4. Rotary curettage, it should be done to healthy gingiva to avoid shrinkage during healing. 5. Laser, which is faster, more efficient, painless, bloodless and more sterile. Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics o Impression taking procedures: 1. Proper selection of tray size to avoid trauma of the tissues. 2. In case of impression compound material, avoid introducing it into the oral cavity too hot, otherwise irritation of the soft tissues and the prepared tooth will occur. 3. Polyether impression materials produce allergic manifestations. 4. Assure complete removal of excess material from the sulcus. o Temporary restoration procedures: 1. The indirect technique is more preferable as it protects the freshly cut dentine from both thermal and chemical irritation due to the exothermic reaction and the free monomer resulting from polymerization. 2. The custom made are more preferable than the ready made due to their proper contact, contour and fit. 3. Newly introduced materials as micro-filled composite and urethane dimethacrylate show better biocompatibility than polymethyle methacrylate. 4. Provisionals must be smooth and highly polished, have proper axial contour and contact, well adapted smooth margins and provides harmonious occlusion. o Cementation procedures: dentist must be aware of the effects of the various luting cements. (1) zinc oxide & eugenol luting agents are palliative, obtundant and sedative on the pulp, (2) zinc phosphate causes pulpal irritation due to its low PH. The use of frozen slab technique accelerates pH rise and decreases the irritation. (3) Zn Polycarboxylate, shows relative pulp biocompatibility due to its rapid rise of PH to 6and 7, its large molecular size and low toxicity of the polyacrylic acid and its ability to complex with proteins limits diffusion through the dentinal tubules. (4) Glass ionomer Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics cement, although it has an anticariogenic property, it may cause post cementation hypersensitivity that can be reduced by: - slight hydration of the tooth before cementation by placing a drop of water on the tooth during mixing, that is gently blown off just before placing the restoration on the tooth. - allowing the cement to set hard to the touch before removing the excess.- placing a varnish on the margins of the restoration after removing the excess. (5) Resin modified glass ionomer showed less solubility and reduced sensitivity. (6) Composite resin cement, irritant to the pulp, however this irritation can be decreased by using dentine bonding agents that seals the dentinal tubules. o Restorative materials: the effect of the different restorative materials must be evaluated; (1) porcelain, highly glazed and polished porcelain is biocompatible with the gingiva, however the solubility of the non-adhesive cements may leave marginal gap leading to gingival irritation. (2) Gold casting, ionically neutral that doesn’t irritate the pulp. The highly polished shows gingival biocompatibility. (3) Base metal castings, nickel containing alloys may cause allergy in some patients. Beryllium containing alloys are carcinogenic. (4) Amalgam, corrosive, darkens tooth structure, galvanism and marginal leakage that decreases by time. To solve these problems use high copper alloys, varnish application and cement base under the restoration. (5) Composite resin, pulp irritation, polymerization shrinkage and gap formation. The use of suitable liner will protect the pulp. (6) gold foil restorations, non corrosive and shows superior adaptation to the walls however, their application pounding force causes pulp reaction. Dr.Mohamed Adel Anwar Biological considerations of fixed prosthodontics 5. Post operative phase: After cementation of FPD a sequence of continued post operative appointments is designed to: o Monitor pt’s dental health. o Stimulate meticulous plaque control habits. o Identify any incipient disease. o Introduce any needed corrective treatment before irreversible damage occurs. Considerations affecting future dental health -Overcontoured restorations accumulate more plaque leading to periodontal problems and caries. -Reduced Occlusal preparation will cause occlusion problem, ceramic chipping & fracture. -Preparation should be even, uniform, sufficient, smooth without ledges. -Better margin placement from biological aspect is the supragingival finish line. -Better margin adaptation will prevent future leakage and recurrent caries. -Sufficient space for Occlusal Scheme (for better functioning). -Supraeruption / tilted teeth may requires Endodontic treatment to get sufficient reduction. -If cusps are weak and may fracture, intracoronal restorations may increase stresses so CROWNS are better solution. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation Mechanical Considerations Mechanical considerations can be divided into, providing retention form, resistance form and preventing deformation of the restoration. Retention: is the ability of the preparation to resist the crown restoration from removal along its path of insertion. Resistance: is the ability of the preparation to resist the dislodgment of the restoration by forces directed obliquely or horizontally to the restoration. Path of insertion: An imaginary line along which the restoration can be inserted and removed without causing lateral force on the abutment. Forces develop on teeth from a myriad of angles. A force placed on a retainer can result from mastication, bruxism, dietary intake and also a log of unpredictable stresses. So this element of the Fixed Partial denture must not be compromised otherwise it can lead to failure and the restoration. The following factors must be considered in deciding whether retention and resistance are adequate for a given fixed restoration. These include: 1. Magnitude of dislodging force 2. Geometry of the tooth preparation 3. Taper 4. Surface area 5. Stress concentrations. 6. Type of preparation 7. Roughness of the fitting surfaces of the restorations 8. Materials being cemented. 1. Magnitude of Dislodging Forces Forces that tend to remove a cemented restoration along path of withdrawal are small as compared to those that tend to unseat it or tilt it e.g. pulling with floss under the connectors. Generally the greatest removal forces arise when exceptionally sticky food, e.g. bubble gum is eaten or chewed. The magnitude of the dislodging forces depends on the stickiness of the food and the surface area and texture of the restoration. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation 7. Roughness of the Surface Being Cemented When the internal surface of a restoration is very smooth, retentive failure occurs not through the cement but rather at the cement restoration interface. Air abrading has been shown to increase the retention of the castings by 64%. 8. Materials Being Cemented Retention will be affected by both the casting alloy and the core or buildup material. It is said that more retentive the alloy, the more adhesion there will be with the luting agents. Therefore the base metal alloys i.e. nickel, cobalt and chromium are more retentive and better retained than less reactive high gold content metals. 2. Geometry of the Tooth Preparation Fixed prosthesis depend on the geometric form of the preparation rather than on adhesion for retention. The cement is effective only if the restoration has a single path of withdrawal i.e. the tooth is shaped in a manner to restrain the free movement of the restoration. A preparation is cylindrical only if the two horizontal cross sections of the prepared axial tooth surfaces are coincident. A partial denture will be retentive if the sections are coincident and perpendicular movement is prevented by grooves. However, if one wall of the complete crown preparation is over tapered, it will no longer be cylindrical and the cemented restoration will not be constrained by the preparation because the restoration then has multiple paths of withdrawal. Under these circumstances, the particles of the cement will tend lift away from rather than slide along the preparation and the only retention will be a result of the limited adhesion of the cement. 3. Taper Selection of the appropriate degree of taper for tooth preparation is very important. Too small taper may lead to unwanted undercuts and too large will no longer be retentive. The recommended convergence between opposing wall is 6 degrees. The tooth should be prepared with instrument of the desired taper that is held at a constant angulation. 4. Surface Area Provided the restoration has a limited path of withdrawal, its retention is dependent on the length of this path or more precisely on the surface area in sliding contact. Therefore crowns with long axial walls are more retentive than those with short axial Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation walls and molar crown of same taper are more retentive than premolar crown of the same taper. 5. Stress Concentration When a retentive failure occurs, cement is often found adhering to both the tooth preparation and the fitting surface of the restoration. In these cases, cohesive failure has occurred through the cement layer because the strength of the cement was less than the induced stress. It has been proved that changes in the geometry of the preparation (e.g. rounding of the internal line angles) reduces stress concentrations and hence increases the retention of the restoration. 6. Type of Preparation Different types of the preparations have different retentive values and these correspond to the surface area of the axial walls, provided other factors (e.g. taper are kept constant). Thus the retention of a complete crown is almost double of partial coverage restoration. Additional methods of gaining retention:- One method of increasing retention without lengthening axial surfaces is with grooves or boxes. Pins are also used to increase retention. Four ways to resist displacing forces and increase retention are: 1. Preparing a Suitable Gingival Finish Line Whenever possible, the finish line should be placed in an area where the margins of the restorations can be finished by the dentist and kept clean by the patient. Placement of the finishing lines creates a barrier by preventing the cement to come in contact with the oral fluids and thus these finishing lines help in preventing microleakage and ultimately the retention and longevity of the restoration is increased. They also provide support to the metal and porcelain or acrylic used in restoration. There are four basic types of finishing lines shoulder, bevel shoulder, chamfer and knife-edge. 2. Contouring and Placing Suitable Contact Areas 3. Incorporating Occlusal Locks i.e. Dovetail, Boxes and Grooves 4. Adding Tapered or Parallel Pins Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation Factors affecting retention&resistance in fixed partial dentures 1. Length of Span In addition to the increased load placed on the periodontal ligament by long span bridge, the longer spans are less rigid and so less retentive. 2. Curvature of Arch Arch curvature has its effect on stresses occurring in a fixed bridge. When pontics lie outside the inter abutment axis line, the pontics act as a lever arm which will produce a torquing movement which leads to loss of retention of bridge. 3. Type of Bridge There are two types of bridges made according to the prevalent condition and position of abutments in the arch. a. Rigid connector b. Non-rigid connector. A completely rigid restoration is not indicated for all situations requiring a fixed prosthesis. In many instances, an edentulous span will occur on both sides of a tooth creating a free standing pier abutment. The use of a form of non-rigid connector can lessen these hazards. The non-rigid connector is a broken stress mechanical union of the retainer and pontic instead of usual rigid solder joint. 4. Occlusion Interference with undesirable occlusal contacts produce deviation during closure of maximum intercuspation, hinder smooth passage to and from the intercuspation position and lead to deflective occlusal force on the bridges which may lead to damaging effects on abutment and also on the retention of the casting. There are four types of occlusal interferences, centric, working, non-working and protrusive. All these interferences should be removed on suitable articulator and a harmonious occlusion should be achieved in the final casting. 5. Periodontal Condition The abutment tooth must be able to provide good support for the bridge. This support is related to both the amount of root and the amount of bond present. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation 6. Tooth or Teeth Being Replaced A bridge replacing a maxillary canine is subjected to more stresses than the mandibular since forces are transmitted outward (labially) on the maxillary arch against the inside of the curve (its weakest point). When a cantilever pontic is employed to replace a missing tooth, the forces applied to the pontic have an entirely different effect on the abutment tooth. The pontic acts as a lever which tend to be depressed under forces with a strong occlusal vector. 7. Type of Retainer Used There are two types of retainers which are generally used 1-Intra coronal 2-Extra coronal In the intra coronal retainers, the retention is obtained between the inner wall of the tooth preparation i.e. the internal wall of the prepared cavity and the casting. On the other hand, in extra coronal retainers, the retention is obtained between the outer wall of the tooth preparation and the inner wall of the retainer. 7. Materials Employed in the Construction of Retainers The material used in the construction of the fixed partial dentures calls for certain requirements which help to increase the longevity of the restoration. Cobalt chromium or nickel chromium alloys generally used for making fixed bridges fulfill majority of these ideal requirements. On the other hand acrylic is generally weak, is not rigid and cannot provide strong connectors. It also has lower compressive and tensile strength compared to other alloys and is thus easily subjected to fracture. Hence acrylic is used for interim on temporary restorations in the mouth. 8. Arch Position of the Abutment Teeth and Retention When the abutment teeth are more or less parallel to each other, complete or partial crown retainers can be made. If the abutment teeth are not parallel, complete crown retainers with a common path of insertion are not feasible. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation 9. Spring Cantilever Bridges and Retention This bridge provides a method of supporting a pontic at some distance from the retainers. This type of bridge is both tooth and tissue supported. A gold bar which fits in contact with the palatal mucosa connects the pontic to the retainers. Different tooth preparation and ways of acheiving retention &resistance in each Complete Cast Crown Preparation After the occlusal reduction is completed, the guiding grooves are placed on the axial walls. When these guiding grooves are placed, the dentist should be sure that the shank of the diamond is parallel to the proposed path of withdrawal of restoration. A diamond taper bur with a taper of 3-6° should be used and thus an identical taper on the preparation wall will result. Place the cervical chamfer concurrently with axial reduction. Width of the chamfer should be approximately 0.5mm which will allow adequate bulk of metal at the margin. The Metal Ceramic Crown Preparation Factors affecting retention that should be taken into consideration while preparation 1. The completed reduction of the incisal edge on an anterior tooth should allow 2mm of adequate material thickness to permit translucency in the completed restoration. Caution must be used here to prevent over reduction because excessive occlusal reduction shortens the axial wall and thus is a common cause of inadequate retention and resistance form of completed restoration. 2. Labial reduction of 1.5mm should be done for the adequate retention of metal and porcelain and the shoulder preparation should have a 90° butt joint. 3. Reduction of the proximal and linguo-axial surfaces should be done with a diamond held parallel to the path of withdrawal of the restoration giving an approximate taper of 6o. If this is not followed, a slightly more taper or discrepancy in taper of two walls will result thus affecting retention. 4. In a completed restoration, all the line angles and point angles should be rounded. This will help in reducing the stress con- centration and thus will enhance retention. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation THE PARTIAL VENEER CROWN PREPARATION Posterior Teeth Three Quarter Crown 1. During axial reduction place grooves for axial alignment in the centre of the lingual surface and in the mesiolingual and distolingual transitional line angles. These grooves should be made parallel to the long axis of tooth. 2. During proximal reduction the proximal grooves are placed parallel to the path of withdrawal. The groove should not be deeper than 1mm and is best done with a tapered carbide bur. The grooves prepared should resist lingual displacement of the periodontal probe. 3. If additional bulk is needed to ensure rigidity of the restoration it can be provided with an occlusal offset. This V-shaped groove extends from the proximal grooves along the buccal cusp. Anterior Partial Veneer Three Quarter Crown Preparation With the advent of metal ceramic restorations the use of partial veneers on anterior teeth has lessened somewhat during recent years. However two types of partial veneer anterior crown prepar- ations are still done. 1. Maxillary canine three quarter crown. 2. Pin ledge preparations. To enhance the retention and resistance form of the preparation a slightly exaggerated chamfer on the lingual aspect of the tooth should be placed and a guiding groove in the middle of the cingulum wall. The mesial and the distal proximal grooves provide most of the retention form for the anterior partial veneer crowns. They are made with a 170L carbide bur and converage at an angle of 3-5o degree. Pin Ledge Preparation and Retention A pin ledge is occasionally used as a single restoration generally to reestablish anterior guidance, in that case only the lingual surface is prepared. More commonly, however, it is used as a retainer for a fixed partial denture or to splint periodontally compromised teeth. Retentive features for all ceramic restoration An all ceramic restoration remains the most aesthetic restoration for duplicating individual anterior teeth. Adequate tooth reduction is created to achieve space for the porcelain bulk required for the strength of the restoration. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation Retentive features to be taken into consideration during each step of the preparation Incisal Reduction There should be an adequate incisal reduction of 2mm otherwise brittle failure of the material occurs. Facial Reduction The facial reduction is performed with a coarse flat end diamond to remove the labial surface while establishing a preliminary shoulder. The incisal 2/3 rd of the facial surface should be inclined lingually to provide uniform porcelain and ensure suitable aesthetics. Insufficient tooth reduction on the facial surface can lead to either a tooth thin coverage contoured restoration. This can also lead to the failure of the restoration. Proximal Reduction Excessive taper of the proximal surface should be avoided which can also lead to loss of retention by decreasing the surface area and also the parallelism of walls. Lingual Reduction Proper lingual reduction is very important for the strength and retention of the restoration. The lingual surface of the tooth is generally reduced in two planes. First cingulum shoulder is placed with a flat ended tapered diamond to crest a 0.75mm shoulder in the cingulum with a 20-50 taper. The cingulum reduction is now completed. A flame shaped or wheel shaped diamond is used to form the lingual concavity of the anterior teeth. Inadequate tooth reduction of the lingual surface can lead to loss of clearance and also diminished strength for the porcelain which can overall lead to loss of retention of the restoration. Proper Finish Line A proper marginal finish line is very important for the retention. Inadequate finish line in some areas of the preparation can lead to microleakage thus leading to the loss of retention. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation Sharp Points and Undercuts All the sharp points and undercuts should be removed or rounded off to prevent the accumulation of the stresses and thus prevent the subsequent failure of the restoration. Retention in endodontically treated teeth It has been demonstrated experimentally that endodontically treated teeth are weaker and more brittle than vital teeth. So for this reason attempts have been made to strengthen the teeth by removing part of the root canal filling and replacing it with a metal post. Also when the teeth will be serving as an FPD abutment, a complete crown becomes mandatory. Under these circumstances, the retention and support most be derived from within the root canal. Canal Retention It is recommended that the root canal should be enlarged only to amount necessary to enable the post to fit snugly for strength and retention. Retention in porcelain laminate veneers To ensure a uniform thickness and the retention of the laminate veneer, the following criteria must be met: a. There should be a uniform reduction on the labial surface of the tooth and the preparation should remain within the enamel whenever possible. b. The margin of the porcelain laminate veneer should generally be hidden within the embrasure area. A modified chamfer finish line ensures correct enamel preparation exposing correctly aligned enamel rods for increased bond strength at the cervical margin thus increased retention. It also ensure an adequate bulk at the margins and hence it increases the strength. Etching the porcelain is also said to be a predominated factor in producing the retention. RETENTION FOR CERAMIC INLAYS AND ONLAYS Ceramic inlays and onlays provide a durable alternative to posterior composite resins for patients demanding aesthetic restoration. For maximum retention following points should be taken cared of: Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation The outline and the reduction of the tooth is governed by the existing restorations and caries. Now here is resin bonding, the axial wall undercuts can be blocked out with GI cement preserving additional enamel for adhesion and thus the increased retention of the restoration. However undermined and weakened enamel should always be removed. The outline should avoid occlusal contacts. Areas to be onlayed need 1.5mm of clearance in all excursions to prevent ceramic fracture and thus increase the longevity of the restoration. In this preparation, it is preferred that the margin is kept supragingival, if this is not possible, crown lengthening is advisable. All the internal line angles should be rounded to prevent stress concentration and to thus enhance retention. A 90o butt joint should be given for ceramic inlay margin. Bevels are contraindicated because bulk is needed to prevent fracture and thus increase the longevity. Final retention is achieved during the bonding of the inlay as it is done with a resin luting cement. In this procedure acid etching is done which creates micro tags and help in mechanical retention. RETENTION IN RESIN BONDED BRIDGES The retention of this prosthesis depends on the adhesive bonding between the etched enamel and the metal casting. To enhance retention in these restorations, significant clinical crown length should be present. If there is insufficient moisture control, retention is minimized. Short clinical crown and narrow embrasures are also a contra indication for resin retained FPD because in these type of teeth, surface area is reduced and thus the retention. If a patient has parafunctional habits, this restoration should not be given because they lead to early failure of the restoration. ROLE OF LUTING CEMENTS IN RETENTION The type of luting agent chosen affects the retention of cemented restoration. Five kinds of luting agents are most commonly used: 1. Zinc Phosphate 2. Zinc Polycarboxylate 3. Glass ionomer 4. Zinc oxide eugenol 5. Resin bonded cement. Dr.Mohamed Adel Anwar Mechanical principles of tooth preparation The Retention of restorations has been achieved primarily by mechanical interlocking of the cement into irregularities on the internal surface of the fabricated restoration and the tooth preparation. Polycarboxylate and glass ionomer cements adhere directly to calcified tissues by chemical attraction to calcium ions in addition to mechanical interlocking. True adhesion between cement and tooth is desirable because of potential to reduce microleakage between the tooth and the restoration. Retention failure has been shown if the internal surface of the surface of the restoration is very smooth. So it is recommended to air abrade the internal surface of the casting with 50μm alumina. Retention has been seen to be more with more reactive alloys i.e. nickel, cobalt and chromium are more retentive and better retained than less reactive high gold content metals. Film Thickness of the Luting Cement There is a conflicting evidence on the effect of increased thickness of cement film on retention of the restorations. But its proved there, uniform thickness of cement between restoration and tooth provides more retention than a non-uniform thickness. A film thickness of 2.5μm or less has been preferred for successful restoration. CONCLUSION Retention in fixed partial denture, is one of the important factor in the success of fixed partial dentures. There is no single factor on which retention is totally dependent. In fact retention comprises of a list of factors, all of which have to be taken into consideration during all the stages starting from tooth preparation to the final cementation. Even if a single factor is neglected it can affect the retention of the casting which further has a direct influence on the longevity of the restoration. Dr.Mohamed Adel Anwar Marginal Integrity Marginal Integrity Introduction: The longevity of the restoration in the biological environment of the oral cavity depends a lot on the adaptation of the margins to the finish lines of the preparation. The quality of this adaptation is dependent on the professional skills of the dentist, lab procedures (professional skills of the technician and the laboratory itself), materials used by the dentist and the technician, and the type of the finish line of the preparation. Definition: -It is the border of the preparation where the prepared tooth structure meets the unprepared surface of the tooth. -It may be also well defined as I. Line of demarcation. II. Peripheral extension of tooth preparation. III. The terminal portion of the prepared tooth. Features of finish lines: Finish line must be distinct, uniform, and smooth and should follow alveolar bone crest and free gingival margin. Requirements of finish lines: It should be easy to prepare, easy to duplicate in impression, conservative and provide sufficient strength to restoring material. Dr.Mohamed Adel Anwar Marginal Integrity Functions of finish lines: i) During visual evaluation of the tooth preparation, it is a measure of the amount of the tooth structure already removed. It also delineates the extent of the cut in an apical direction. ii) The finish line is one of the features that can be used to evaluate the accuracy of the impression made for induct procedures. iii) On the die, a distinct finish line helps in the evaluation of the quality of the die and aid in trimming it accurately. iv) The correct marginal adaptation of the pattern depends on an obvious finish line. v) The evaluation of the restoration is also aided by a proper finish line. Criteria for successful finish line design: I. Acceptable marginal adaptation ii. Tissue tolerant surface iii. Adequate contour: conservative finish lines like feather edge and knife edge produce overcontoured restoration leading to periodontal problems gingival recession, unaesthetic black triangular spaces, alveolar bone loss. iv. Adequate strength: finish line design should provide adequate strength to restoring material. Classification of finish line design configuration: I. Based on configuration of finish line a. Feather edge b. Knife edge c. bevel d. shoulder e. chamfer Dr.Mohamed Adel Anwar Marginal Integrity ii. Based on location of finish line a. Supra gingival b. Equigingival c. sub gingival iii. Pardo's classification: Ÿ Inclined vertical Feather edge, shoulder with bevel Ÿ Horizontal margins Shoulder, chamfer Feather edge I. ADVANTAGE: Most conservative II. DISADVANTAGE: Over contoured restorations Not recommended now Knife edge I. It is most conservative type of finish line. ii. It gives >135 cavosurface angle. iii. Pointed end tapered fissure bur is used The easiest finish line preparation is the knife-edge (Fig. 4-8). Fig. 4-8. Knife edge preparation Dr.Mohamed Adel Anwar Marginal Integrity Unfortunately, its use can create a number of problems. Unless it is carefully cut, the finish line may fade out and the technician sometimes cannot determine it especially if periodontitis exists. The second problem is that the thin margin of the restoration is difficult to wax and cast accurately. To avoid the distortion of the margins during clinical and laboratory procedures, the third problem, the overcontouring is created. This can lead to plaque accumulation and subsequent gingivitis and periodontitis. Despite these disadvantages, the knife- edge can be used on very short teeth with small diameter, tilted teeth, on teeth with very convex axial surfaces, in furcation areas. INDICATIONS I. Large pulp chambered tooth ii. Finish line on cementum iii. MOD onlay ADVANTAGES I. Easy to prepare ii. Most conservative iii. Burnishable type of finish line iv. Ideal for marginal adaptation DISADVANTAGES I. Indistinct margin ii. Over contoured restoration iii. Marginal distortion iv. Difficult to wax and cast Dr.Mohamed Adel Anwar Marginal Integrity Bevel: It may be well defined as SLANTING EDGE. Functions of bevel: it improves marginal seal, produces strongest enamel margin, improves retention and resistance form of the preparation, creates sliding joint effect and produces burnishable margins. INDICATION of bevel: Facial margin of maxillary partial coverage restoration Inlay margin Onlay margin Shoulder finish line : is the finish line design for tooth preparation in which the gingival floor meets the external axial surfaces at approximately a right angle. Flat-end tapered diamond end cutting diamonds are used to prepare shoulder finish line.The shoulder is the finish line of choice for the all-ceramic crowns (Fig. 4-5). Fig. 4-5. Shoulder finish line It is used also facially for metal-ceramic crowns for anterior teeth. The wide edge provides resistance to occlusal forces and minimizes the stresses that might lead to the porcelain fracture. Also it provides enough space for esthetics. Dr.Mohamed Adel Anwar Marginal Integrity However, although this is more esthetic finish line, it is also the most destructive one. If a strict 90 degree angle is created, this concentrates stress in the tooth and can lead to coronal fracture. Types of the shoulder are 1- shoulder 2-radial shoulder 3-shoulder with bevel. 4-sloped shoulder Dr.Mohamed Adel Anwar Marginal Integrity Sloped shoulder: finish line design for tooth preparation in which the gingival floor meets the external axial surfaces at approximately 120. It is indicated in facial margin of metal ceramic crown. Dr.Mohamed Adel Anwar Marginal Integrity Radial shoulder: Shoulder finish line with rounded gingivoaxial 0 line angle and90 cavosurface angle. Radial shoulder on all ceramic preparation combines the support of ceramic with stress reducing radial shoulder. Dr.Mohamed Adel Anwar Marginal Integrity Shoulder with bevel: it is used in facial margin of metal-ceramic crowns, proximal box of inlays and onlays and occlusal shoulder of onlays and mandibular three fourth crowns. Chamfer finish line A chamfer is an obtuse angled gingival termination. It is a concave extra coronal finish line that possesses greater angulations than a knife-edge with less width than a shoulder The chamfer finish line ( Fig. 4-3) is the preferred one for full veneer metal restorations. It is broadly used also lingually for metal- ceramic restorations. Dr.Mohamed Adel Anwar Marginal Integrity Fig. 4-3. Chamfer finish line This finish line has been shown experimentally to exhibit the least stress, so that the cement, underlying it, will have less likelihood of failure. It can be cut with a round- end tapered diamond. A heavy chamfer, or a wide chamfer (Fig. 4-4), can serve also for a ceramic crown, but it is not as good as the shoulder. Fig. 4-4. A heavy chamfer finish line Dr.Mohamed Adel Anwar Marginal Integrity Factors deciding placement of finish lines: Aesthetics: The subgingival finish line suits for the high lip line and equigingival and supragingival suits for low lip line patients. Biological width: it is the combined dimension of epithelial attachment (0.97mm) and connective tissue attachment (1.07mm) coronal to alveolar bone crest. It is measured by bone sounding. Minimizing transgingival probing depth by sulcus depth measures the biological width. Biologic width violation causes gingivitis, periodontal pocket formation, recession and tooth-restoration interface display. Depending on margin placement, types of finish lines A) Supragingival finish line B) Equigingival finish line C) Subgingival finish line Supragingival finish line is used in low lip line cases. Advantages are as under: I. Easy preparation ii. Easy to finish iii. Easy to duplicate iv. Easy to varify fit of restoration v. Easy mentainance Dr.Mohamed Adel Anwar Marginal Integrity Subgingival finish line: it is best avoided unless indicated. Indications: I. Aesthetics II. Subgingival caries III. Erosion IV. Abfraction V. Dentinal hypersensitivity Rationale of subgingival finish lines: 1- Tooth-restoration interface latency 2- To maximize resistance and retention form of tooth preparation 3- To make significant contour alteration Dr.Mohamed Adel Anwar Esthetic consideration in dental restoration Esthetic Considerations in Dental Restorations Esthetics is the sense of beauty and it means merely pleasant. It is the discipline that deals with retaining and maintaining the ultimate in appearance. General factors influencing esthetics:  Cultural factor that develops from the surroundings. We recognize that older teeth are darker in color, longer than normal due to gingival recession and have short clinical crowns due to attrition. Also we recognize that female teeth are white, small in size, have rounded corners and smooth.  Artistic factor that develops unconsciously and affects our perception of form and size. It is affected by illumination and lines. Illumination is related to the appearance of certain light areas and the recession of dark areas. It has relation to two factors; (1) proportion, golden proportion produces harmony. An object is considered beautiful if it is properly proportioned. A ratio of approximately 1:1.619 between succeeding terms is considered pleasant. In the oral cavity this is clear in the prominent position of the maxillary centrals at the midline with their widest crowns followed by the laterals that have the narrowest crowns and the canines that are the next widest. However, from frontal view, the apparent teeth size should be progressively smaller from the midline distally by ratio of 0.619 (2) Symmetry that is related to perception of order and balance depending on the production of parallel lines and proper ratios. Certain lines act as guidelines for developing harmonious orientation and arrangement. Dr.Mohamed Adel Anwar Esthetic consideration in dental restoration - Vertical reference line which is the facial midline that helps to locate the dental midline, the medio-lateral tooth position. The central midline should coincide with the facial midline. - Horizontal reference lines that include; the inter-pupillary line, lip line, smile line or incisal plane and the gingival form. The inter- pupillary line must be parallel to the incisal plane of maxillary teeth and gum margin. Appearance zone: it is also known as oral frame. It includes lips, teeth and soft tissues occupying the one third of the face. To achieve attractiveness in appearance there must be harmony between facial structures, balance and symmetry. Factors affecting to oral esthetics: - Lip line, the length and curvature of lips influence the amount of incisal display in function and at rest. The upper lip serves to evaluate the exposed length of maxillary incisors at rest and the position of gingival margin during smile. Lower lip affects the buccolingual position of the incisal edge and the curvature of the incisal plane. - Incisal plane or smile line, it should be perpendicular to the midline and parallel to the inter-pupillary line. It may be (1) straight that is related to age and attrition, (2) convex that is normal looking, where the incisal plane of central incisors and canines cusp tips are on the same level, and (3) concave that is called reverse smile line and it is formed when the canines are longer than the central incisors. - Gingival form; health, color, texture and the amount of exposed gingiva during smile affect esthetics. Moderate gingival exposure between no gingiva is seen and 3mm display. Beyond that is unpleasant as in case of gummy smile where more than 3mm of gingiva is displayed. Dr.Mohamed Adel Anwar Esthetic consideration in dental restoration - Phonetics; to assure the production of proper length of maxillary incisors ask the patient to sound "f" or "v", the incisal edges must be opposite to the inner edge of the vermillion border of the lower lip. The incisal edge of the lower incisors must be 1mm below and behind the edges of maxillary teeth and it can be detected by asking the patient to sound "s". This also assures the presence of proper anterior guidance. - Age: Young age Old age Light colored teeth Darker in color Lower chroma Higher chroma More surface texture Less surface texture and smooth High epithelial attachment Low epithelial attachment due to recession Significant incisal Less significant embrasure Small gingival embrasure Large Convex incisal plane Straight Light characterization Marked and small facets Maxillary anteriors display Mandibular anteriors display - Sex: Feminine Masculine More rounded line and point angle Angular Pronounced incisal embrasures Less More translucent incisal edges Stronger characterization Dr.Mohamed Adel Anwar Esthetic consideration in dental restoration Guidelines to preserve esthetics during clinical procedures: 1. Soft tissue management: during each step attention must be paid to avoid soft tissue damage and to accomplish work with least trauma to preserve adjacent teeth intact, lip and check and gingiva in good health. The production of finish line whether supra gingival or subgingival must be weighed putting into consideration both the advantages and disadvantages of each of them. The use of sharp cutting instrument to avoid pulpal trauma supplemented with intermittent cutting and coolant. The use of retraction cord of proper size using a blunt instrument for its placement and gentle pressure to avoid gingival trauma. Also proper timing of insertion and period ranging from 15- 20 min to avoid permanent injury of the gingiva. Impression taking using suitable trays and proper technique to avoid gingival and soft tissue trauma as well as to allow recording all details. The use of properly finished temporization that will only last 2- 3 weeks and no more. The removal of residuals of cement to prevent gingival inflammation. 2. Tooth reduction: Cutting in right direction and on two planes to avoid over or under reduction or production of undercut. Sufficient reduction is essential for proper color production as minimal required thickness of porcelain is 1.5- 2 mm according to the type of retainer used. Insufficient reduction most properly occurs in two areas; labio- incisal by cutting on one plane and at the cervical third due to improper production of a shoulder finish line. Both cases will cause esthetic failure of the restoration as the technician will solve the problem either by over- contouring to develop proper color or properly contoured life less restoration due to the display of the opaque color. Dr.Mohamed Adel Anwar Esthetic consideration in dental restoration 3. Shade matching: failure during this step is related either to incorrect shade selection, insufficient reduction, improper lab work and miss following color selection guidelines. There are other factors that the dentist must have knowledge about to be able to send them to the technician to achieve successful final restoration. 4. Tooth color and variation: there are variations in color among teeth and in the same tooth. In the tooth, cervical third is darker than the rest of the tooth due to the reduced thickness of enamel (0.3mm) so dentine color is displayed. Incisal shows translucency and bluish due to the predominance of enamel (1.5mm). Translucency is also present at the proximal line angles and cusp tips where there is only enamel and no dentine. Among teeth, teeth get darker as we go posteriorly. Canine is the most intensely pigmented tooth while laterals are the least pigmented. Posterior teeth are darker than anterior teeth. 5. Tooth form, size and position: restoration must be an exact duplication of the original tooth in size, form and position. Preserving the size by same positioning of line angles and same size and location of incisal embrasure. 6. Degree of gloss: it enhances the natural appearance of the restoration as it affects the reflection of light. 7. Surface characterization; it is represented in the production of white lines simulating hypo-calcification, brown lines denoting old teeth. 8. Illusion and perceived size: this is produced in restoration during porcelain buildup. To develop an illusion of smaller size, place the mesio-facial and disto-facial line angles close to each. Large size is developby putting them apart. Increasing the length is developed by placing vertical developmental grooves, but to shorten it simulate a root Dr.Mohamed Adel Anwar Esthetic consideration in dental restoration portion by placing cemento- enamel junction incisally and pink porcelain to simulate gingiva. To make the canine more obvious move the canine tip more distally whereas if you want to make less obvious move it mesially. Dr.Mohamed Adel Anwar Full veneer metal crown Full Veneer Metal Crown There are numerous situations, when full veneer crowns have to be used. A number of studies have shown that full veneer crowns exhibit much more retention and resistance. The selection of a full veneer crown becomes mandatory when the tooth is small or when the occlusal forces are strong, for example if the tooth serves as an abutment for a long- span fixed or removable partial denture. The full veneer crowns also have to be used, if the partial veneer ones have been considered to a lack of esthetics or of proper restoration the tooth. Nowadays, beside the full metal crowns, metal- ceramic and all- ceramic crowns are very often used. Indications for full metal crown a) Teeth with extensive caries or large amalgam in order to protect the remaining tooth structure from fracture. b) As retainer for FPD in case of long span for maximum retention. c) To protect endodontically treated teeth. d) Recontouring of posterior over erupted teeth for better occlusal relation. e) For patients with high caries index. Contraindications a) Areas where esthetic is important. b) Less than maximum retention is required. Advantages: 1. Strong. 2. More conservative and easy to be prepared. 3. Provide more retention and resistance compared to partial veneer crowns. Disadvantages a) Removal of large amount of tooth structure. b) Not esthetic. c) Vitality test could not be applied. Dr.Mohamed Adel Anwar Full veneer metal crown Full Metal Crown Preparation When all of the axial surfaces of a posterior tooth have been attacked by decalcification or caries, or when those surfaces have been previously restored, the tooth is a candidate for a full metal crown. It can also be used, if we need to change the shape of the tooth for a removable partial denture. The preparation for a full veneer crown begins with occlusal reduction, we need about 1.5 mm of clearance on functional cusps and about 1.0 mm on nonfunctional ones. Depth orientation grooves can be placed to easily assess if the reduction is sufficient on every stage of the preparation (Fig.7-1). Fig. 7-1. The round-end tapered diamond is used to make depth orientation grooves on the triangular ridges and in the primary developmental grooves. Fig. 7-2. The depth-orientation grooves should be 1.5 mm deep on the functional cusps and 1.0 deep on the nonfunctional cusps for precious metal alloys. Dr.Mohamed Adel Anwar Full veneer metal crown If reduction begins without orientation marks, time will be wasted in repeated checks for adequate clearance. A round- end tapered diamond is used to place the grooves on the functional cusp and on the rest of the occlusal surface. If there is already some clearance with the opposing tooth because of malpositioning or fracture of the tooth being prepared, the reduction has to be made less and only after estimation of the existing clearance. The tooth structure remaining between the orientation grooves is removed, keeping the occlusal surface in the configuration of the geometric inclines of any posterior tooth (Fig. 7-3). Fig. 7-3. Planar occlusal reduction: round-end tapered diamond and no. 171 bur. The functional cusp bevel, placed on the buccal inclines of mandibular buccal cusps and the lingual inclines of maxillary lingual cusps, is an integral part of the occlusal reduction. Failure to place this bevel can produce a thin casting, resulting in perforation, or an overcontouring, resulting either in poor occlusion, or reduction of the opposing tooth. Occlusal clearance can be checked by giving the patient a 2.0 mm thick strip of red utility wax to bite. If somewhere the reduction is insufficient, the wax will be detectable as a transparent spot on the light. Additional tooth structure then should be removed from indicated areas and rechecked. The buccal and lingual walls are reduced with tapered with round end diamond, simultaneously forming a chamfer finish line. The interproximal surfaces are initially cut with a short needle diamond (Fig.7-5 ). This can be made either in occlusogingival, or in buccolingual”sawing” motion, being careful not to damage the adjacent tooth. Dr.Mohamed Adel Anwar Full veneer metal crown Fig. 7-5. Complete axial reduction: short needle diamond and tapered with round end diamond stone. Fig. 7-6. Contact opening with a short needle diamond. Once sufficient maneuvering space has been obtained, the tapered with round end diamond stone is used to plane the walls and simultaneously form a chamfer finish line, connecting the vestibular and lingual surfaces and trying to avoid forming undesirable sharp edges. After finishing of the occlusal plane and functional cusp bevel a retentive groove is made. All of the axial surfaces and the chamfer finish line are smoothed with finishing bur. Special care should be taken in rounding the corners from the buccal or lingual surfaces to the proximal surfaces to insure that the finish line will be smooth and continuous. The final step in the full veneer preparation is the placement of a seating groove with a no. 171 L bur. This usually will be on the buccal surfaces of mandibular teeth or lingual surfaces of maxillary teeth. The seating groove will prevent any rotational movements during cementation, creating only one path of insertion, and will help guiding the casting to place. Dr.Mohamed Adel Anwar Full veneer metal crown Fig. 7-10. Features of a mandibular full metal crown and the function served by each. Full- metal crowns. Clinical-laboratory procedures There are numerous situations, when full veneer crowns have to be used. A number of studies have shown that full veneer crowns exhibit much more retention and resistance. The selection of a full veneer crown becomes mandatory when the tooth is small or when the occlusal forces are strong, for example if the tooth serves as an abutment for a long- span fixed or removable partial denture. The full veneer crowns also have to be used, if the partial veneer ones have been considered to a lack of esthetics or of proper restoration the tooth. Nowadays, beside the full metal crowns, metal- ceramic and all- ceramic crowns are very often used. All the way, the full veneer crowns are overused now and it would be better to choose less destructive techniques whenever possible. Construction of the full metal crown. Clinical and Laboratory steps 1.Anesthesia 2.Tooth preparation 3.Gingival retraction 4.Impression taking 5.Temporary crown making Dr.Mohamed Adel Anwar Full veneer metal crown 6.Gypsum cast fabrication (Dies and working casts) 7.Wax pattern fabrication 8.Casting 9.Correction of ready crown on the cast model 10.Try in oral cavity and cementation Gypsum cast fabrication A die is a positive replica of the individual prepared tooth on which the margins of the wax patterns are finished. These are individual tooth replicas prepared for easier handling during wax pattern fabrication and finishing of inaccessible areas of the cast. Ideal requirements of a die system The die should be easy to remove and replace in its original position. The die must be stable when placed in the cast. Working cast with a removable die system In this system a special type of working cast is prepared and the dies are carefully sectioned so that the individual dies can be removed and replaced in their original position in the cast. Wax pattern fabrication Apply die spacer to die for: Compensation the alloys shrinkage during metal framework casting Provision space for cement. To prevent the wax from sticking to the die stone, coat the die thoroughly with die lubricant and allow it to soak in for several minutes. Dr.Mohamed Adel Anwar Full veneer metal crown Casting. The various steps in a casting procedure are: Spruing the wax pattern Attaching the sprue to the crucible former Investing the pattern in a casting ring Burnout of the wax pattern Casting Recovery Finishing And Polishing Correction of ready crown on the cast model by the technician. Try in oral cavity and cementation. During try in the following features are checked in the cast restoration 1. Proximal contact 2. Marginal integrity 3. Stability 4. Occlusion 1) Checking for proximal contacts the proximal contact between the crown and natural tooth should allow the passage of floss. Ideally the contacts should be stable and easy to maintain. 2) Checking for marginal integrity Margin adaptation with a gap around 30 μm is clinically acceptable. Testing whether the casting binds to the tooth surface, is helpful to determine the marginal integrity. This can be done using the following material: pressure Dr.Mohamed Adel Anwar Full veneer metal crown indicating paste, powdered sprays, elastomeric detection paste. Marginal integrity can be assessed by moving a sharp explorer from the restoration to the tooth and from the tooth the restoration. 3) Checking for stability The restoration should not rock or rotate when a force is applied Instability produced by a small positive nodule on the fitting surface can be corrected by trimming. 4) Checking for occlusion Occlusal discrepancies are one of the most common errors that occur during the fabrication of a fixed partial denture. Occlusal adjustment during eccentric movements (clinical correction) is necessary. Cementation is the process by which the restoration is cemented to the tooth using a suitable luting agent. Preparing the casting: the casting should be cleaned by sandblasting with 50μm alumina or by steam, followed by ultrasonic or organic cleaning. Next the operatory side is isolated with cotton rolls. The cement should be mixed to a luting consistency. A thin coat of cement should be applied on the internal surface of the casting. The tooth surface is dried and the prosthesis is inserted with a firm, rocking dynamic seating force. A static load will lead to fracture. Next the margins of the retainers are examined to verify the fit of the prosthesis. Excess cement should be removed with an explorer. Floss can be used to remove the excess cement in the inter-proximal surface. Occlusion should be checked with articulating paper. The patient should be advised to avoid loading for the first 24 hours. Dr.Mohamed Adel Anwar Porcelain Fused to Metal Crown Porcelain Fused to Metal Crown Porcelain fused to metal (PFM) crown is the most widely used fixed restoration. It is a full metal crown having a facial surface (or all surfaces) covered by ceramic material. It consists of a ceramic layer bonded to a thin cast metal coping. It combines the strength and accurate fit of cast metal coping with the cosmetic of ceramic. So, this type combines the advantages of the strength of full metal crown and esthetic of all ceramic crown. Disadvantages of PFM crown 1. Removal of substantial amount of tooth structure. Dr.Mohamed Adel Anwar Porcelain Fused to Metal Crown 2. Subject to fracture because of the brittle nature of porcelain. 3. Shade selection can be difficult. 4. Inferior esthetic compared to porcelain jacket crown. 5. Discoloration of the gingival margin may occur with time. 6. More expensive. Indications of PFM crown 1. Teeth need to be completely covered for esthetic demand. 2. As a retainer for fixed partial denture. 3. Similar to those of full metal crown. Contra-indications of PFM crown 1. Teeth with large pulp (because of the possibility of pulp exposure during preparation). 2. Intact buccal wall where a more conservative retainer can be used. 3. Teeth with short crowns. 4. Patient with bad oral hygiene. Dr.Mohamed Adel Anwar Porcelain Fused to Metal Crown Preparation Requirements:  Deep facial reduction to provide enough space for the metal coping and porcelain and shallower reduction on the other surfaces covered with metal only.   Shoulder, radial shoulder, or heavy chamfer can be used as a gingivo-facial finishing line, whereas chamfer or knife edge finishing line is used for the remaining surfaces covered with metal only. Since this restoration is a combination of metal & porcelain, tooth preparation likewise is a combination. Tooth preparation of PFM crown (for anterior teeth) Fabrication of silicone index The silicone index acts as a guide to check the amount of tooth structure removal. Dr.Mohamed Adel Anwar Porcelain Fused to Metal Crown Incisal reduction 2 mm should be removed from the incisal edge to allow for adequate translucency of the restoration. Flat-end tapered diamond bur is used, placed parallel to the incisal inclination (with a slight palatal inclination in the upper incisors and labial inclination in the lower incisors). Labial reduction PFM crown preparation requires deep facial reduction to give enough space for metal and porcelain, and thus avoiding over contouring and poor esthetic which would inevitably occur when no enough tooth structure is removed. The amount of labial reduction is 1.5-2 mm. Advantages of adequate reduction (deep facial reduction) 1. The restoration will properly contour (effect on esthetic & gingival health). 2. The shade & translucency of the restoration will match that of the adjacent natural tooth.  0.5 mm for the metal coping.   1 mm for porcelain (0.2 mm opaque layer, 0.5 mm body “dentin” layer, and 0.3 mm incisal “enamel” layer). Dr.Mohamed Adel Anwar Porcelain Fused to Metal Crown Because of the anatomy of the tooth labially, it should be reduced in two planes corresponding to the two geometric planes of the labial surface: a gingival plane and an incisal plane. Advantages of two plane reduction 1. To follow the anatomy of the surface. 2. To avoid hitting the pulp. 3. To give enough space for the metal and porcelain layers, so that avoiding poor esthetic or over contour. Dr.Mohamed Adel Anwar Porcelain Fused to Metal Crown a.Gingival plane Three D.O.G (1.5 mm in depth) are placed in the gingival third of the labial surface parallel to the long axis of the tooth. b.Incisal plan

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