clinical Fixed II 2024 333.pdf

Bridge Design Bridge design is an art rather than a science and based on: The clinical experiences of the dentist The clinical condition of the patient NO FIRM RULES are given for selecting any particular design, but personal opinion mastering the deci...

Bridge Design Bridge design is an art rather than a science and based on: The clinical experiences of the dentist The clinical condition of the patient NO FIRM RULES are given for selecting any particular design, but personal opinion mastering the decision in selecting one design than the other. Treatment Modalities for Replacing Missing Teeth Treatment modalities for replacing missing teeth: 1. Implant-supported fixed dental prosthesis (1st option). 2. Resin-bonded fixed dental prosthesis (2nd option). 3. Conventional tooth-supported fixed dental prosthesis (3rd option). 4. RPD (last option). Implant-supported fixed dental prosthesis Advantages of implants: ▪ Preservation of tooth structure. Missed tooth or teeth can be replaced with implant and saving defect-free adjacent teeth from the destructive tooth reduction. ▪ Preservation of bone. ▪ Provision of additional support. ▪ Retrivability. ▪ Resistance to disease. ▪ Improved esthetics, function and speech. Span length: ▪ Single tooth: single tooth can be replaced by a single implant, saving defect-free adjacent teeth from the destructive of tooth reduction. ▪ Multiple. Span configuration: ▪ No distal abutment. ▪ No favourable distribution of abutments. * 4 Resin-bonded fixed dental prosthesis Posterior resin-bonded restoration Anterior resin-bonded restoration Span length: ▪ Single tooth ▪ Possible for 2 incisors Anterior resin-bonded restoring two mandibular incisors Span configuration: abutment mesial and distal to pontic. Abutment alignment: ▪ less than 15°inclinations mesio-distally. ▪ should be in the same facio-lingual plane. ▪ preparations are not easily modified because of minimal reduction. Abutment condition: Defect-free abutments. Occlusion: can not be used for replacement in presence of deep vertical overlap as well as abnormal occlusal habits. Periodontal condition: No mobility. Ridge form: ▪ Moderate resorption. ▪ No gross soft tissue defects. 5 General features: Will suit for young patients. Conventional tooth-supported fixed dental prosthesis If the abutment teeth are periodontally sound, FPD can be expected to provide a long life of the patient. Disadvantages and complications of replacing teeth with conventional fixed dental prosthesis (Why conventional FPD is not the 1st treatment modality in restoring a missing tooth?) 1) Possible damage to tooth and pulp: it is necessary to remove a considerable amount of healthy tooth structure. 2) 2ry caries: there is a risk of micro-leakage and caries. 3) Effects on periodontium: especially with sub-gingival margins. Also, poorly maintained supra-gingival margins can produce a destructive effect on periodontium. 4) High cost and discomfort associated with dental procedures Span length: ▪ Posterior span: 2 or fewer. ▪ Incisors: 4 or fewer. Span configuration: usually mesial and distal abutments, but can be used with short cantilever pontic. Abutment alignment: less than 25° inclinations can be accommodated by preparation modification. 6 Abutment condition: ▪ Good if abutments need crowns. ▪ Non-vital teeth can be used if there is sufficient coronal tooth structure. Occlusion: favorable loading. Periodontal condition: ▪ Good alveolar support. ▪ Favorable crown-root ratio. ▪ No mobility. ▪ Favorable root morphology. Ridge form: ▪ moderate resorption. ▪ no gross soft tissue defects. Basic Design of Conventional FPD Fixed-fixed design It has a rigid connector at both ends of the pontic. i.e. the abutment teeth are rigidly splinted together. The abutment teeth must be prepared to have a common path of insertion and the FPD can be cemented in one piece. 7  The retainers should have approximately the same retention to reduce the risk that forces applied to the FPD will dislodged one retainer from its abutment leaving the FPD suspended from the other abutment (if the dislodged retainer remain in its place but loose, the oral fluid will enter the space between the retainer and the abutment and caries rapidly develop leading to failure). Fixed-supported (movable)design It has a rigid connector and a semi-rigid connector that allows some vertical movement of the mesial abutment. The semi-rigid connector should: i. resist separation of the pontic from the retainer. ii. resist also lateral movement of the pontic. The abutment teeth do not have to be prepared parallel to each other. 8  The semi-rigid connector can be separated before cementation and so the two parts of FPD can be cemented separately. Cantilever design It provides support for the pontic at one end only (the pontic may be attached to a single retainer to be two or more splinted together). The abutment tooth or teeth may be mesial or distal to the pontic but in small FPD they usually distal. The cantilever is a destructive design. o.Cause: the pontic act as lever when the occlusal forces are applied to the pontic and result in: i. Rotation and tipping on the abutment. ii. Unfavorable magnitude of forces concentrated on abutment tooth leading to destructive effect on the periodontium. 9 Three cases only this design is used: o.Missed upper lateral incisor: a full coverage retainer is constructed on the canine as it has the longest root in the oral cavity o Missed upper premolar either first or second: the abutment is in good vertical alignment with good periodontal support. o.Missed upper second molar: the first molar is used as an abutment and the pontic of upper second molar is suspended from it with reduced bucco-lingual dimensions. In this case, it acts as occlusal stopper to prevent over-eruption of the opposing dentition 10 Spring cantilever design This design is restricted to the replacement of upper incisor tooth. One pontic can be supported by a spring cantilever bridge. The only two cases in which this design is constructed: o.Multiple diastema between anterior teeth. o o.To preserve intact anterior teeth while posterior teeth need to be crowned. It is rarely used these days and have been replaced by implant or resin-bonded bridge. Criteria for Selecting a Bridge Design Conservation of tooth structure * All conventional bridges are destructive. As a tooth-supported restoration, the most conservative design is resin-bonded bridge. Therefore, it is now considered as first choice whenever possible. If not, we must choose a conservative design as possible and avoid adding additional abutments unless it is urgently needed. Support 11 * Ante suggested that the root surface area of abutment teeth must equal or surpass the root surface area of teeth to be replaced. i.e., a given area of periodontal membrane could support up to twice its normal occlusal load. * From the evidence accumulating based on clinical observations, this concept is wrong. When periodontium is healthy, bridges can be successful with a little as one-quarter of the support suggested by Ante. Such bridges can be successful for many years. o Explanation: the occlusal load on a bridge is determined by muscles of mastication. These are under the control of the neuromuscular mechanism, which is influenced by proprioceptive receptors in the periodontium of the supporting teeth. As a result, occlusal loading is modified by the presence or absence of natural teeth. Cleansability * Any chosen design must offer an access for cleaning around the pontics and retainers. Appearance * Complete coverage (PFM or all-ceramic) is more favorable than partial coverage with incisal display of metal. Some Proposed Bridge Designs Missed maxillary central incisor 12  1 st choice: Single implant-supported crown replacing the missed central incisor (considering the advantages of implant-supported restoration). If there is any contra-indication for implant placement, consider another treatment option. Resin-bonded bridge: Abutment: central and lateral incisors. Retainers: resin-bonded retainers on central and lateral incisors. Pontic: metal-ceramic - modified ridge-lap pontic design. If there is any contra-indication for resin-bonded (e.g., abutment discoloration, rotated abutment, improper width of edentulous space or proximal caries), consider another treatment option. Fixed-fixed bridge: Abutment: lateral and central incisors. Retainer: metal-ceramic or all-ceramic. Pontic: metal-ceramic or all-ceramic (material) – ovate or modified ridge-lap pontic (design). Connector: rigid connectors. 13  Fixed-fixed with palatal loops: in case of multiple diastema Fixed-supported bridge Cantilever bridge on lateral and canine, if there is central diastema Spring-cantilever, if premolar(s) require a full- coverage restoration Missed mandibular central incisor Implant-supported crown. Resin-bonded bridge using lateral and central incisors as abutments. Rotate, mal-posed or mobile abutment will contra-indicate the use of resin bonded. Fixed-fixed bridge using lateral and central incisors as abutment teeth. Teeth reduction could easily reach the pulp and the patient should be advised. Endodontic treatment and radicular restoration would be necessary. 14 Missed maxillary first premolar Implant-supported crown. Resin-bonded bridge if the teeth are in normal alignment and muscles of mastication not well developed. Fixed-fixed bridge: using canine and second premolar as abutments. Cantilever bridge using second premolar and first molar as abutments, if second premolar and first molar are restored will need restoration. N.B., a canine-guided occlusal scheme would be necessary to prevent excessive forces on the cantilever pontic (it that is not possible, do not use cantilever). Missed maxillary first molar Implant-supported crown. Fixed-fixed bridge using second premolar and second molar as abutments. Fixed-supported bridge using second premolar and second molar as abutments (the minor retainer on second premolar). Missed maxillary canine Implant-supported crown. 15  Fixed-fixed bridge using lateral incisor, central incisor and first premolar as abutments. N.B., restore the occlusion to group function. Missed all maxillary incisors Implant-supported restoration Fixed-fixed bridge: depending on the antero- posterior distance; using canines as abutments if the arch curvature is slight and ̸ or canines are large. or canines and first premolars as abutments to counteract the lever arm created by the curve of the anterior segment of the arch. Double abutments are often used to assure maximum retention. or canines and premolars as abutments if the curve of the anterior segments needs longer resistance arm. Pier (intermediate) abutment * Pier abutment is a common situation e.g., a missing first premolar and first molar 16 * During function, the terminal abutments tend to intrude and the pier abutment act as a fulcrum. This movement results de- bonding of the less retentive terminal retainer with subsequent failure of the restoration * The de-bonding is related to the development of extrusive forces at a terminal retainer as the other retainer is loaded. * The use of non-rigid connector is advised to overcome this risk. The female part of the non-rigid connector is placed at the distal aspect of the pier abutment while the male part is placed in the mesial of the pontic. This will nullify the fulcrum effect. * The use of a non-rigid connector in a posterior 5-units FPD with a pier abutment is contra-indicated in: If the abutment teeth present significant mobility. If the span between the abutments is longer than one tooth. 17  If the distal retainer and pontic are opposed by RPD or an edentulous ridge while the anterior retainers are opposed by natural dentition, allowing the distal terminal abutment to super-erupt. Mesially inclined second molar abutments * With this mesial tilting, it is impossible to prepare the abutment teeth for a FPD along the long axes of the respective teeth and achieve a common path of insertion. * The treatment of choice is orthodontic uprighting of molar. o Advantages: ▪ Place the abutment tooth in a better position for preparation. ▪ Place the abutment tooth in a better position for distribution of forces under occlusal loading. ▪ It helps to eliminate bony defects along the mesial surface of the root. 18 * A proximal half-crown on the distal abutment. This retainer can be used only if the distal surface itself is untouched by caries or decalcification. If there is a sever marginal ridge height discrepancy between the distal of the second molar and the mesial of the third molar as a result of tipping, the proximal half-crown is contraindicated. * The non-rigid connector is another solution. * A telescopic crown and coping can be used as a retainer on the distal abutment. A full crown preparation with heavy reduction is made to follow the long axis of the tilted molar. An inner coping is made to fit the tooth preparation and a crown that will serve as the retainer for the FPD is fitted over the coping. 19 This allows for total coverage of the clinical crown while compensating for the discrepancy between the paths of insertion of the abutments. 20  Gingival Displacement for Impression Making Gingival displacement (Tissue dilatation): it is the process of pushing the gingival tissue away from the tooth surface temporarily for accurate impression making. Gingival tissue displacement is indicated during: * Examination: for better examination of the cervical area covered by the free gingiva. * Preparation: for accurate preparation and positioning of the finish line. * Impression making: help to displace the gingival tissue to expose the preparation margins. * Finishing and cementation: to be sure that there is no overhanging or open margins for better marginal fitness. Aim of Gingival displacement: 1. To create a proper space horizontally, so that: ⇒ The impression material can record the tooth structure at and beyond the preparation margins. ⇒ Provides for sufficient strength of impression material to prevent distortion or tearing when the impression is removed or poured with stone. 2. To create clean and dry field from serum seepage or blood. 3. To protects and maintained the health of the gingiva. 21  Healthy gingiva following adequate management of gingival tissues Factors governing the preservation of gingival position and form a. Pretreatment periodontal health: Before gingival displacement, it is important to assess the periodontal health of the teeth, and healthy, firm, non- bleeding tissues should be present. b. Periodontal biotype: Two biotypes describe the tissue thickness and the amount of scalloping of the interdental papilla: ⇒ Thick, flat biotype * The gingiva is thicker and more fibrotic. * Greater probing depth and the gingival margin is usually located on enamel. * The distance between CEJ and the bony crest is about 2 mm. Appearance of gingival Appearance of gingival tissue tissue in a thick, flat biotype in a thin, scalloped biotype 22  ⇒ Thin, scalloped biotype * The gingiva is Thin, delicate with a limited amount of attached gingiva. * The probing depth is shallow and the gingival margin is frequently not on enamel. * There is an increased distance between the CEJ and the bone. c. Biologic width It is of integral importance to the maintenance of the healthy periodontium that no restorative effort violates the junctional epithelium or connective tissue fiber apparatus. The biologic width is about 2.04 mm (epithelial attachment: 0.97 mm, connective tissue attachment: 1.07 mm). it is widely accepted that restorations should not extend beyond 0.5 to 1.0 mm into the gingival sulcus. Maintaining a 3.0-mm safety zone between the crest of the alveolar bone and the margin of a crown is recommended. d. Displacement cord positioning force Minimally traumatic tissue displacement is necessary to maintain a healthy periodontium. Heavy forces can cause: ▪ injury to the gingival tissue and disturb the blood. ▪ gingival recession. 23  Gingival recession caused by heavy force exerted during tissue displacement e. Displacement time It is important to reduce gingival displacement as much as possible. The cords need to stay in the sulcus for about 4 minutes before impression-making to produce sufficient crevicular width expansion. Materials for Tissue Displacement Displacement Cords It is the most popular method for tissue displacement in fixed prosthodontics. Types: Braided cords * It has a consistent and tight weave. * The braiding makes them resistant to separation during placement and considered to be easy to manipulate and pack into place. * It absorbs the medicaments efficiently and do not split or tear during placement. 24  * Both serrated or smooth packing instruments can be used with braided cords without any concern to the clinician. Knitted (woven) cords * It becomes more popular years due to their ease of placement and their ability to expand when wet, making them easy to insert in the sulcus. * Their design requires the use of thin, smooth, and non-serrated instruments. Serrated instruments can lift loosely woven cords out of the sulcus. It is suggested to start the cord placement in the interproximal area where tissue is more easily displaced. A periodontal probe can be effectively used when the gingiva is thin and delicate and there is minimal sulcus depth. Use of a periodontal probe for cord placement in the case of thin gingiva with minimal sulcus depth 25  There is a lack of standardization in cord size and clinical efficacy, making the choice of displacement cord based on the personal preference of the clinician. The cords can be dispensed from container (which could lead to cross-contamination), a clicker, or precut and individually wrapped (which eliminates guessing the length of the cord needed for each tooth). Clicker cord dispenser. Hemostatic Medicaments Several hemostatic medicaments are available to be used in conjunction with displacement cords: Epinephrine It was common in the paste. It is known to cause adverse cardiovascular problems (hypertension, increased heart rate) and/or other symptoms such as anxiety, increased respiratory rate, tachycardia, and, in rare instances, death. There is a general consensus that epinephrine should be avoided for tissue displacement given the significant number of complications that can occur specifically in patients with cardiovascular problems. 26  It did not produce superior displacement compared to other medicaments. Astringents They are metal salts that cause contraction of the gingival tissue by contracting small blood vessels. They precipitate tissue and blood proteins that physically inhibit bleeding by decreasing exudation and making the surface of the gingival tissue tougher. Extended duration of contact with the gingival tissues can cause delayed healing or tissue damage. The contact with the prepared tooth surfaces must be minimized because of its acidic PH which may removes the smear layer that can cause postoperative sensitivity and an adverse effect on the bonding mechanism of adhesive cements. They are available in liquids or gels and the most commonly used are: ❖ Aluminum chloride ❖ Ferric sulfate ❖ Aluminum sulfate ❖ Aluminum potassium sulfate (Alum) ❖ Nasal and ophthalmic decongestants (e.g. Afrin) Cordless displacement materials They have been recently introduced as an alternative to the liquid hemostatic medicaments. 27  They are less traumatic to the gingival tissue than conventional displacement cords, less painful to the patient, and quicker to deliver. They are available in different forms (paste, foam, or gel) and meant to be injected or packed into the sulcus. Types: ⇒ Viscus paste contain 15% aluminum chloride an 85% fillers (mostly a Kaolin matrix). ⇒ PVS paste that works by generating hydrogen and expanding in the sulcus. It can also be used in conjunction with a compression cap to enhance the displacement effects of the material. Techniques for Tissue Displacement Mechanochemical Tissue Displacement Single cord technique A single cord presoaked with hemostatic medicament packed in the sulcus is the easiest technique. It is useful when the prepared margins are supragingival or when the sulcus depth is not deep enough for a second cord to be located apical to the finish line. Excellent results are obtained when the soft tissue maintains its lateral displacement and the finish line is visible after tissue displacement. The cord can be removed before making the impression or left in the sulcus to control bleeding and tissue fluids if the 28  finish line is completely visible and the cord is positioned below the finish line with unprepared tooth structure present occlusal to the cord. Double cord technique It takes additional time to place a second cord and the gingival displacement associated with 2 cords has the potential to induce more gingival trauma. It is used when the finish line is located sufficiently below the gingival margin that 2 cords can be placed into the sulcus. A small diameter cord (presoaked with hemostatic medicament) is positioned at the base of the gingival sulcus to prevent hemorrhage and seepage. Then, a second cord larger in diameter and also impregnated with hemostatic medicament, is placed into the sulcus on top of the first cord. The small diameter cord remains in place at the time the impression is made to reduce the collapse of the gingival tissue, control hemorrhage, and reduce the tearing of the impression material. Before making the final impression, the second cord is removed while the smaller diameter cord is left in place. Surgical Tissue Displacement Electrosurgery It is used to reduce hyperplastic tissues, expose the gingival margins, and prevent bleeding. 29  It is also used to widen the gingival sulcus without reducing the height of the gingival margin. Purpose: to remove several layers of cells from the inner lining of the gingival sulcus, creating tissue displacement Careful patient selection and a thorough review of the medical history are required because its use is strongly contraindicated in patients with pacemakers and implanted cardioverter defibrillators. Also, it should be used with caution in esthetically sensitive areas because healing is unpredictable following the removal of the gingival tissue. Electrosurgery used on the lingual surface of a mandibular first molar to obtain tissue displacement Rotary gingival curettage it is important to have an attached gingiva free of inflammation with an acceptable width of marginal gingiva and a high degree of keratinization. It removes the inner epithelium of the gingival sulcus along with some of the underlying connective tissue. The results after healing are not predictable. It is a not recommend technique as the changes in gingival position would produce a negative esthetic result. 30 Laser tissue sculpting for tissue displacement It is an alternative surgical technique for tissue displacement. It generates an intense (high energy) beam of coherent monochromatic light converted into thermal energy (heat) when it enters the soft tissue. The result is the vaporization or ablation of the targeted tissue, hence a controlled coagulation that increases hemostasis. Diode lasers are most commonly used for tissue displacement because of their low wavelength (near infrared spectrum). Other types of lasers can also be used for tissue displacement: Neodynium-doped yttrium aluminum garnet (Nd:YAG) and Erbium-doped yttrium aluminum garnet. The use of a laser results in minimal or no postoperative pain and can sometimes be used selectively without anesthesia. They displace gingival tissues and the result is less hemorrhage and gingival recession. 31  Impression Materials and Techniques Impression Materials Irreversible hydrocolloids (Alginate) Advantages: * Low cost and good wetting properties. * It can be used in the presence of saliva and blood with a moderate ability to reproduce details because of its hydrophilic nature. * Easy to use and easy to mix with sufficient setting time. * Hydrophilic. * It does not require any special equipment. * It produces impressions with reasonable surface detail. 32  Disadvantages: * Poor dimensional stability caused by loss of water creates distortion and shrinkage if it is not poured within 10 minutes. * It can be poured only once. * Low tear resistance which can be a real problem when attempting to record the gingival sulcus. Indication: * For diagnostic casts in fixed prosthodontics. Reversible hydrocolloid (Agar) It is not commonly used in dental practice today because of the need for expensive conditioning baths and water cooled trays. Polysulphides The first introduced elastomers and unpopular materials now. Setting time: * Long (excess of 10 minutes) Viscosities: 1. Light bodied (low viscosity) 2. Medium or regular bodied 3. Heavy bodied (high viscosity) advantages: * Long working time. * Excellent tear resistance. 33  Disadvantages: * Messy to handle. * Objectionable odour. * Dimensional instability because: 1. Continued setting reaction after the apparent setting time. 2. Evaporation of water produced as a by-product of the setting reaction. * The impression should be removed with a single, swift pull. Example: COE-FLEX (GC Amirica). Polyethers A popular polyether impression material is Impregum (Espe GmbH). Advantages: * Hydrophilic and can be used in a moist environment. * no by-product formation (very good dimensional stability). * Newer polyether impression materials are slightly more flexible than the older products, making them easier to remove from the mouth. * Impressions should be poured within 48 hours of them being recorded. * Adequate tear resistance. * very good elastic properties. 34  Disadvantages: * Because of the nature of the material absorbing water, the impression should not be submerged in water for a period of time because it could lead to distortion. * It has a high elastic modulus and consequently are relatively rigid when set, hence considerable force may be required to remove the impression from both the mouth and the stone cast. This may preclude their use in cases where severe undercuts are present. Viscosities: 1. Low viscosity. 2. Medium viscosity. 3. high viscosity. Example: Imprigum (3M Espe). Silicones Types: * Condensation curing (Type I) silicones. * Addition curing (Type II) silicones. * Light * Medium * Heavy * Very high viscosity or putty material (The high filler loading was initially devised to reduce the effects of polymerization shrinkage). The putty is commonly 35  combined with a low viscosity silicone (a procedure known as the putty-wash technique). Very hydrophobic. Condensation curing silicones The setting reaction produces a volatile by-product (ethyl alcohol). Loss of the by-product leads to measurable weight loss accompanied by shrinkage of the impression material on storage. The dimensional changes of condensation silicones are slightly greater than those of polysulphides, but the changes in both types of material are small in comparison to the changes which occur with alginate. To produce the most accurate models, impressions should be cast within 6 hours of being recorded. Example: Speedx (Coltene). Addition curing silicones (Polyvinyl siloxanes or PVS) PVS impression material is one of the most favored impression materials in dentistry. They are considered the most dimensionally stable impression materials. No by-product is produced Although there is no by-product formed, there is often a secondary reaction that can release hydrogen It is recommended to wait at least 60 minutes before pouring a PVS impression (some manufacturers claim that they can be poured immediately). 36  Great detail reproduction and can be poured multiple times because of their high tear strength and high elastic recovery. Caution should be taken to avoid contact of the material with latex rubber dams or latex gloves, which may leave a sulfur or sulfur compound that inhibits polymerization of the material. Moreover, gingival retraction soaked cords containing sulfur may also contribute to the inhibition. Hydrophilic polyvinyl siloxane: o.Newer PVS impression materials have been advertised as hydrophilic, suggesting that they can perform adequately under moist or wet conditions. o.They contain intrinsic surfactants that improve their wettability and facilitate the pouring process with gypsum materials. o However, hydrophilic PVS seems to remain hydrophobic when it is still in the liquid, un- polymerized state and its wetting abilities are compromised in the presence of moisture. As a result, their surface detail reproduction is inconsistent when moisture control is not maintained. Example: Extrude (Kerr), Express (3M), Virtual (Ivoclar). Vinyl siloxanether (Vinyl polyether siloxane) A new impression material that combines the properties of polyether and PVS. 37  Introduced in the dental market in 2009 (Identium, Kettenbach Co) It combines the ease of removal of PVS with the hydrophilicity (wetting properties) of polyether. Impression Trays Requirements of an impression tray: * It must be rigid and does not flex to avoid impression distortion. * It should be dimensionally stable. * It should provide an even space for the impression material. * It should extend to include all prepared teeth and adjacent teeth without impinging on other areas. * It should provide a positive mean for retention of the impression materials e.g., perforations and or adhesives. Types of trays: Stock trays - metallic or plastic. - Use of non-rigid plastic trays may result in flexure of the side walls of the tray during the impression procedure; subsequent tray rebound on removal from the mouth produces an inaccurate cast and, ultimately, poor restoration fit. - Supplied either for sectional or for full arch. 38  - Advantages: o Time conserving eliminating the need for fabrication of a special tray. - Disadvantages: o.Consume more impression material. o Require sterilization. Personalized special (custom) trays - A custom tray improves the accuracy of elastomeric impression by limiting the volume of the material. - It allows uniform impression material thickness. - It minimizing distortion and material waste, and are also more comfortable for patients. - They can be from: 1. Auto-polymerizing acrylic resin. 2. Thermoplastic material (It can be softened in a water bath and adapted manually or vacuum former with a heating element). 3. Photopolymerizing resin. Impression Techniques Mixing methods: The impression pastes can be mixed together with: A. Manually mixing on mixing pads. B. Auto mixing. 39  * With pre-packed cartridge with a disposable mixing tip attached. * The cartridge is inserted in a gun-like device and the base and catalyst are extruded into mixing tip where mixing occurs as they progress to the end of the tube. The material can be directly placed on the prepared tooth and impression tray. * Advantages: o.Proper proportioning. o Uniform mixing. o Less air is incorporated into the mix. o Mixing time is reduced. C. Machine mixing * Machine mixer (Electrically driven pumps for proportioning of materials from larger tubes through a mixing tip). * It is convenient and produces void-free impression. Impression techniques: Traditional impression techniques: ➔ Copper band impression technique. ➔ Monophase impression technique (one mix, step). ➔ Double mix one step technique. ➔ Double mix two step technique (putty-wash). Other impression techniques: ➔ Optical impression. 40  ➔ Impression techniques for implant. Single (individual) copper band impression technique: It is indicated for the impression of a single full metal crown preparation, used either with impression compound or with rubber base impression materials. Single mix, one step (monophase) technique: It is used with a single viscosity (e.g., medium or regular body) Always in a custom tray. Steps: * Check the accuracy of the special tray in the mouth. * Paint the fitting surface and the rim of the tray with an even thickness of the appropriate adhesive and allow it to dry. * Displace the gingival tissues (e.g. with an impregnated retraction cord). * Isolate and dry the field (if the material is hydrophobic). * On a mixing bad, dispense two equal lengths of the base and catalyst of the impression material. * Mix with a stiff spatula, first using a circular motion and then flattening the spatula against the pad in a figure 8 motion limiting the number of times you lift the spatula from the pad to avoid incorporation of air voids. this is done till you obtain a homogenous mix or use the auto mixing and omit the previous step. 41  * Load part of the material in the syringe and the remainder in the tray. * Remove the retraction cord. * Immediately inject the material into the crevice of the prepared teeth starting from the most inaccessible area and going systematically around the preparation and the interdental areas of the adjacent teeth then seat the loaded tray firmly. * Check the setting of the material. After the complete setting remove the tray with a downward straight pull. Double mix, one step technique: It is used with two different viscosities (light and heavy body). Both stock and custom trays may be used, however using a custom tray is more recommended. Steps: * Select a suitable stock or special tray and paint with adhesive. * Displace the gingival tissues. * Isolate and dry the field. * On two separate mixing pads dispense two equal volumes of the base and catalyst of both the light and heavy body of the same material. * Mix the light body as with the single step then load the syringe or use auto-mixing. * Mix the heavy body with a spatula until you achieve a homogenous mixture then load the tray. 42  * Remove the retraction cord. * Immediately inject the light body around the preparation. * The tray loaded with the heavy body is then seated in the mouth before the light body has set thus forcing the light body into intimate contact with the preparation and the gingival crevice. * After setting, remove the impression with a downward straight pull. Double mix, two step (Putty wash) technique: It is used with the putty and light bodies’ viscosities. Because of the putty viscosity, a stock tray is always used. It represents the most common technique. Steps: * Select a suitable stock tray and paint it's fitting surface with adhesive and allow it to dry. * Mix the putty by hands for 30 seconds. * Form the mixture into a road shape and load the tray. * Cove the loaded tray with polyethylene sheet spacer then seat the tray into the mouth to take a preliminary impression. * After complete setting, remove the tray from the patient’s mouth and peel off the spacer and remove any excess with a sharp knife. 43  * On a mixing pad, dispense two equal lengths of the base and catalyst of the light body and mix till you obtain a homogenous mixture. * Load one third of the amount in the back of the syringe and the reminder in the tray. * Remove the retraction cord and immediately inject the light body around the preparation until the tooth is covered. * Then, seat the loaded tray into the patient’s mouth exerting no pressure until the material completely sets. * Remove the tray after setting with a downward straight pull. A space for light body can be created by one of the following: 1. Polyethylene sheet spacer covering the putty before seating in the patient’s mouth. 2. Record the putty before teeth preparation. 3. Remove a layer of set putty to provide a space for wash step. Problems with putty wash technique: 1. Recoil Although the putty wash is the most common technique used, it suffers from a unique problem which is the recoil. This recoil results in a poorly fitting restoration. Explanation: A considerable force is needed to seat the set putty into the mouth with light body over it. This force 44  causes outward flexing of the tray walls and residual stresses within the material. With removal, the tray walls rebound resulting in dies which are undersized bucco-lingually. Spacing reduces this problem but can not eliminate it completely. 2. seating of the set putty is difficult which results in a stepped occlusal surface of the restoration (require excess occlusal adjustment). Impression Defects (Flows) I. Visible impression flows 1. Finish line not visible. Causes: * Gingival inflammation and bleeding * Subgingival finish line * Localized gingival overgrowth * Retraction cord displaced from sulcus 2. Air bubbles in critical places Causes: * Air trapped during mixing, tray loading or tray seating * Hand mixing * Inadequate covering of the margin area 45 3. Voids or drags. Causes: * Poorly adapted tray * Inadequate gingival displacement and control of crevicular fluids * Premature syringing of impression material intra- orally prior to seating the tray * Tray movement during setting of the impression material. * Exceeding the working time of the impression material. 4. Unset impression material on surface of impression and cast. Causes: * Contamination of the impression by ingredients of latex rubber gloves. * Residual from temporary cement. * Use of expired impression materials. * Inadequate mixing of impression material. 5. Lack of impression details. * Thick blood or saliva. * Inadequate retraction. * Incorrect storage of impression material. * Use of expired impression materials. * Exceeding the working time. 46 6. Tearing at the margins. Causes: * Residue from provisional. * Insufficient retraction. 7. Tray-tooth contact. Causes: * Improper tray selection. * Improper tray seating in the mouth. 1. Delamination. Easy separation between impression layers (e.g., between light and heavy bodies). Causes: * Exceeding the working time. * Improper storage condition. * Contamination of putty with provisional. * Use of light and heavy bodies from different brands. II. Invisible impression flows They resulting in an apparently good fit of the restoration on the die but a poor fit on the tooth. It occurs because of: * Tray and impression recoil: ▪ Faulty putty-wash technique. ▪ Use of non-rigid tray. 47  ▪ Attempting to reline a defective localized area with light bodied material (i.e. try to repair a visible flow as voids can turn it invisible flow). * Detachment of impression from tray * Permanent deformation Disinfection of Conventional Impressions Disinfection should take place immediately on removal from the mouth Disinfection protocols consist of 2 steps: * Rinsing the impression with tap water immediately after removal from the patient’s mouth to reduce the number of blood-borne pathogens that can be transferred to the stone casts. * Spraying the impression with an appropriate disinfecting agent or immersing it in a chemical solution for a specified amount of time. Digital Impressions Dr Francois Duret from France pioneered optical impressions in 1971. In the early 1980s, Professor Mormann from Switzerland was the first to patent and design a handheld intraoral scanner, which was the first-generation Chairside Economical Restoration of Esthetic Ceramics (CEREC). 48 Examples: - CEREC (Sirona, Dentsply) * This system has been around for the longest time. * The Omnicam unit is the latest version, and is available as a cart system that includes the camera and computer, all of which can be connected or simply transmit files to a milling system. * The device works on the principle of triangulation (a method to determine the 3D geometry of real- world objects. The light or laser source is positioned at a fixed distance from a sensor or camera. As light/laser is reflected from the scanned object, it falls on the camera. The position of points on the object can be calculated using the angle of the reflected light). * This device is a closed system in that the software is only compatible with the company’s milling unit and the image files cannot be exported and used with other milling systems. The work. s- Planscan (Planmeca, USA). - True Definition Scanner (3M ESPE, United States). - Trios 3 (3 Shape, Denmark). - iTero Element (Align Technology, United States). - CS 3600 (Carestream, United States). Advantages of intraoral scanning: 1. Real-time visualization and evaluation. 49  2. Easy to correct, manipulate, or recapture images. 3. Segmental image capture. 4. Archival digitally, therefore no need to store physical casts. 5. Do not need to disinfect before sending information to the laboratory. 6. No damage or wear and tear of the stone casts. 7. Swift communication with the laboratory via the Internet. 8. Self-assessment for tooth preparations. 9. Increased patient satisfaction. 13. Some systems have color scanning, shade selection. Disadvantages of intraoral scanning: 1. Initial cost of equipment and software maintenance fees. 2. Learning curve can be difficult for some individuals 3. Difficult to capture occlusion information for complex prosthodontics treatments. 4. Closed systems restrict options for transferring STL (standard tessellation language) files. 5. Cannot capture subgingival margins if obscured with blood, saliva, or tissue. 6. Scanning patterns need to be followed as per manufacturer’s recommendations. 50  Pontic Design Pontic: An artificial tooth in a fixed partial denture that replaces a missing natural tooth, restores its function and appearance and usually fills the space previously occupied by the clinical crown. Requirements of a pontic: 1. It should provide good esthetics. 2. It should restore function. 3. It should preserve residual ridge. 4. It should be biocompatible. 5. It should be comfortable for the patient. 6. It should be able to maintain good oral hygiene. 7. It should stabilize adjacent and opposing teeth. Pontic space The space for the pontic may be reduced by tilting or drifting of the adjacent teeth into the edentulous space. The situation can be managed by orthodontic repositioning of the abutment teeth. Residual ridge contour Ideal ridge contour: * It has a smooth, regular surface of attached gingiva, which facilitates maintenance of a plaque-free environment. 51 * Its height and width should allow placement of a pontic that appears to emerge from the ridge and mimics the appearance of the neighboring teeth. * It must be free of frenum attachment. * It has an adequate facial height to sustain the appearance of interdental papillae. Loss of residual ridge contour may lead to: * Un-esthetic open gingival embrasures (black triangles). * Food impaction. * Percolation of saliva during speech. Loss of residual ridge Loss of residual ridge contour contour leading to un-esthetic open leading to food entrapment gingival embrasures (arrow) Classification of ridge deformity (Siebert classification): * Class I defects: facio-lingual loss of tissue width with normal ridge height. * Class II defects: loss of ridge height with normal ridge width. * Class III defects: a combination of loss in both dimensions. 52  Pontic Classification Pontic designs are classified according to mucosal-contacting or non-mucosal contacting into: I. Mucosal contact: - Ridge lap (Saddle) - Modified ridge lap - Ovate - Conical II. No mucosal contact - Sanitary (Hygienic) - Modified sanitary Sanitary or hygienic pontic Design: * Occluso-gingival thickness should be a minimum of 3 mm and with space between the pontic and ridge for easy cleaning. * It is an all convex design mesio-distally and facio-lingually (Fish belly design). Sanitary pontic 53 Advantages: * It allows easy cleaning, because its tissue surface remains clear of the residual ridge. * Permits easier. * Plaque control by allowing gauze strips and other cleaning devices to be passed under the pontic and seesawed in shoe-shine fashion. Disadvantages: * Entrapment of food particles which may lead to tongue habits that may annoy the patient. * It is the least tooth-like design. Recommended location: * Teeth seldom displayed during function (i.e., the mandibular molars). Contra-indication: * Where esthetic is important. * Minimal vertical dimension. Modified sanitary pontic Design: * Concave mesio-distally and convex facio-lingually. 54  Modified sanitary pontic Advantages: * Increased connector size. * Less susceptible to tissue proliferation that can occur when a pontic is too close to the residual ridge. Saddle or ridge lap pontic Design: * It has a concave fitting surface that overlaps the residual ridge bucco-lingually. Advantages: * Esthetics. Disadvantages: * Difficult to maintain oral hygiene. * Can cause tissue inflammation. 55 Recommended location: * It should be avoided because the concave gingival surface of the pontic is not accessible to cleaning with dental floss, which will lead to plaque accumulation and tissue inflammation. Modified ridge lap pontic Design: * Lingual surface: Slight deflective contour. * Ridge contact: Only on crest of ridge facially (Contacting area of pontic is convex). * Tissue contact should resemble a letter T. Advantages: * Good esthetics. * Easy cleaning. Disadvantages: * Moderately easy to clean. 56 Recommended location: * Areas of the mouth that are visible during function (maxillary and mandibular anterior teeth and maxillary premolars and first molars). Conical pontic Design: * Rounded and cleansable with a small tip. Advantages: * Good access for oral hygiene. Disadvantages: * Poor esthetics. Recommended location: * Replacement of mandibular posterior teeth where esthetics is a lesser concern. 57 Contra-indication: * Unsuitable for broad residual ridges, because the emergence profile associated with the small tissue contact point may create areas of food entrapment. Ovate pontic Design: * Its convex tissue surface resides in a soft tissue depression or hollow in the residual ridge, which makes it appear that a tooth is literally emerging from the gingiva. * Socket-preservation techniques should be performed at the time of extraction to create the tissue recess from which the ovate pontic form will emerge. For a preexisting residual ridge, soft tissue surgical augmentation is typically required. When an adequate volume of ridge tissue is established, a socket depression is sculpted into the ridge with surgical diamonds or electrosurgery. Advantages: * The most esthetically appealing pontic design. * its emergence from the ridge appears identical to that of a natural tooth. * its recessed form is not susceptible to food impaction. 58  * The broad convex geometry is stronger than that of the modified ridge lap pontic, because the unsupported, thin porcelain that often exists at the gingiva-facial extent of the pontic is eliminated. Disadvantages: * meticulous oral hygiene is necessary to prevent tissue inflammation resulting from the large area of tissue contact. * the need for surgical tissue management and the associated cost. Recommended location: * Maxillary incisors, canines, and premolars with desire for optimal esthetics. 59  Provisional Restorations Provisional Restorations = Temporary Restoration = Interim Restoration Definition: Provisional restoration is a temporary fixed restoration that provides service to the patient and protects the prepared tooth till the permanent restoration being fabricated at the lab. Functions of a Provisional Restoration: 1. Provides esthetics, phonetics and masticatory function. 2. Protects the prepared tooth from fracture. 3. Protects the pulp from thermal and chemicals. 4. Maintains the health of the periodontium, should not be overhang or improper margins. 5. Maintains proximal contact and occlusal relations. 60 6. Previews of final restoration. Requirements of Ideal Provisional Restorations: 1. Reasonably esthetic appearance. 2. Biocompatibility (nonirritant to pulp and soft tissue). 3. Easily fabricated in short time. 4. Easily modified and repair. 5. Capable to receive high polish. 6. Dimensional stability. 7. Adequate strength to withstand force of mastication. Types of Provisional Materials * Poly methyl methacrylates * Poly R’ methacrylates ( R’ is Ethyl or Butyl group etc.) * Bis-acryl composites * Bis-GMA composite resin (light cured composites) Poly methyl methacrylates (PMMA ) 1. Long time used material for provisional restoration. 2. Consistent and good performer. 3. Easily repaired and modified. 4. Easily polish but tend to gain stain over time. 61 5. Exothermic setting (irritation to the pulp). 6. Significant polymerization shrinkage. 7. Strong odor. Poly ethyl methacrylate (PEMA) 1. Developed to compensate the limitations of PMMA 2. Decreased heat of polymerization 3. Reduced polymerization shrinkage 4. Reduced strength vs PMMA Bis-acryl composites 1. Very low heat generation at polymerization 2. Very low polymerization shrinkage 3. Auto mix 4. Provides good marginal fit and abrasion resistance. 5. Brittle, facture upon removal 6. Less stain-resistant 7. Expensive 7. Limited shades 8. Because of dissimilar chemistry, flowable composite do not easily bond to the Bis-acryl material, so repair or modification is difficult, may be it is easier to remake than repair. 62 Bis-GMA composite resins (light cured composites) 1. Eliminates the problems associated with both methacrylates and Bis-acryl materials. 2. Provides good marginal fit, as Bis-acryl, and lower shrinkage and lower exothermic setting. 3. Good polishability. 4. Less brittle than Bis-acryl and show good fracture resistance. 5. Can be repaired with flowable composites. 6. Cured on demand. 7. Available in a different of shades with good long-term color stability. 8. Expensive than the methacrylates. 9. Requires transparent carrier. **** Requirements of any Provisional Technique * Reproduce External Surface Form A. Prefabricated B. Custom * Capture Internal Surface Form - Direct 63  - Indirect - Indirect/direct * External Surface Form A. Prefabricated Forms - Usually needs modifications. - Relined with resin to establish marginal fit. 1. Cellulose acetate crown 2. Polycarbonate crown 64  3. Aluminum, Nickel-chrome crown 4. Prefabricated light cured composite crown More recent introduced (Protemp™ crown, 3M ESPE). Tooth-colored prefabricated crowns. Available in a variety of shapes and sizes. They are a simple and esthetic solution for provisional. Quickly and easy adjusted prior to light curing. No impression or matrix. Trimmed the margin before adapting to the preparation. After adaption to the preparation, light-cured, then removed and finished. 65  Flowable composite may be added to the Crown and light-cured. * External Surface Form B. Custom Forms Preferred in most cases 1. Clear vacuum formed matrix from diagnostic cast 2. PVS putty 3. Dual arch tray and impression 66 Capturing Internal Form (details preparations) A. Direct * Formed directly in the patient’s mouth. * Adequate time. * Patient exposed to chemicals and heat of polymerization. * Technique sensitive to control; - Shrinkage - Marginal adaptation - Occlusion * Difficult for FPDs. 67 B. Indirect * Formed outside the mouth on the stone cast at the laboratory. * Time consuming. * No tooth exposure to chemicals and heat of polymerization. * Recommended for FPDs. 68 C. Indirect-direct technique * Thin shell obtained from minimally prepared abutment on the cast. 69  * Shell adjusted in lab for contour and occlusion. * Left over size in length and width at the margin. * Reline at chair side with fluid resin mix. * Recommended for FPDs and large multi-unit cases. 1 mm prepara(on on abutments on the cast at the lab lab. Check Provisional fit Prepara(on intraoral Refine gingival Excess reline 70  Final provisional restora(on Excess cement Advantages of indirect- direct technique Decrease time at chair side. Patient will expose to less chemicals and heat. Minimal adjustment is required if done properly. Cementation Cementation with weak cement like; 1. Zinc oxide eugenol 2. Zinc oxide without eugenol (Eugenol interfere with polymerization of resin) Cementation Procedures Clean up tooth and dry. Dry the restoration. Isolate the tooth. Petroleum jelly on the external surfaces to facilitate cleans up. Dispense equal parts of base/catalyst. 71  Remove excess to prevent irritation to periodontium. Instruction to the patient. 72 References 1. Rosenstiel S, Land M, Fujimoto J. Contemporary Fixed Prosthodontics. 5th Ed, 2016. Mosby, Inc. 2. Ahmad I. Prosthodontics at a Glance. 1st Ed 2012. Wiley-Blackwell. 3. Herbert T. Shillingburg. Fundamentals of fixed prosthodontics. 3rd ed, 1997. Quintessence Publishing. 4. Ting-Shu S, Jian S. Intraoral Digital Impression Technique: A Review. J Prosthodont. 2015;24(4):313-21. 5. Punj A, Bompolaki D, Garaicoa J. Dental Impression Materials and Techniques. Dent Clin North Am. 2017;61(4):779-796. 6. Baba NZ, Goodacre CJ, Jekki R, Won J. Gingival displacement for impression making in fixed prosthodontics: contemporary principles, materials, and techniques. Dent Clin North Am. 2014;58(1):45-68. 73