CS2-2. General Principles of Tooth Preparation - Assoc. Prof. Dr. Özay Önöral - 21.02.23.pdf
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1 Assoc. Prof. Dr. Özay ÖNÖRAL COURSE ID Code of Committee: CS-2 Name of Committee: Fixed Prosthetic Restorations Lecturer: Assoc. Prof. Dr. Özay Önöral Topic of the Course: General Principles of Tooth Preparation Duration of the Course: 50 minutes After completion of this course, the stud...
1 Assoc. Prof. Dr. Özay ÖNÖRAL COURSE ID Code of Committee: CS-2 Name of Committee: Fixed Prosthetic Restorations Lecturer: Assoc. Prof. Dr. Özay Önöral Topic of the Course: General Principles of Tooth Preparation Duration of the Course: 50 minutes After completion of this course, the student will be able to: List consequences of tooth loss and determine the need for tooth preparation Plan tooth preparation keeping in mind the space requirements, marginal configurations, and margin placements recommended for different restorations and materials Define resistance and retention forms of tooth preparation Identify the most common causes of failure related to tooth preparation techniques. Describe the clinical instruments and equipment armamentarium that are available to aid efficient and predictable tooth preparation. Establish a prosthetic protocol utilizing the six basic tooth preparation principles of Shillingburg to improve successful, aesthetic, and biological-based dental outcomes for the existing patient base. Contemporary Fixed Prosthodontics, 6th Edition. Book by Junhei Fujimoto, Martin F. Land, and Stephen F. Rosenstiel. Published by Elsevier in 2022. Extra-Coronal Restorations: Concepts and Clinical Application, 2nd Edition. Book by Robert Wassell, Francis Nohl, Jimmy Steele, and Angus Walls. Chapter: Fundamentals of Tooth Preparation. Chapter by James Field, Jimmy Steele, and Robert Wassell. Published by Springer in 2019. Tooth Preparations: Science & Art. Book by Clovis Pagani. Published by Quintessence in 2017. Fundamentals of Fixed Prosthodontics, 4th Edition. Book by Herbert T. Shillingburg, David A. Sather, Edwin L. Wilson, Joseph R. Cain, Donald L. Mitchell, Luis J. Blanco, and James C. Kessler. Published by Quintessence in 2012. Learning Objectives of the Course: Suggested References to Review: Near East University, Faculty of Dentistry 2 Assoc. Prof. Dr. Özay ÖNÖRAL Consequences of Tooth Loss Loss of teeth for various reasons is a very common clinical condition. Restoring the edentulous area quickly is important in preventing possible complications that can be caused by the loss of even a single tooth. (1-Caries formation, 2Premature contacts, 3-Periodontal pockets, 4-Soft tissue loss, 5-Proximal contact loss, 6-Periodontal trauma, 7-Gingival recession, 8-Supraeruption of the opposing tooth to the edentulous area, 9-Malocclusion) Terminological Concepts Occlusal Reduction Lingual Reduction Beveling of Functional Cusp Buccal Reduction Axial Reduction Finish Line Taper is defined as the convergence of two opposite-facing external walls of a crown preparation Occlusal Reduction Finish Line Axial Reduction Tooth Preparation Teeth do not possess the regenerative ability found in most other tissues. Therefore, once enamel or dentin is lost as a result of caries, trauma, or wear, restorative materials must be used to reestablish form and function. Near East University, Faculty of Dentistry 3 Assoc. Prof. Dr. Özay ÖNÖRAL With very rare exceptions, teeth require preparation to receive restorations, and these preparations must be based on fundamental principles from which basic criteria can be developed to help predict the success of prosthodontic treatment. In order to achieve success in fixed prosthetic restorations, it is necessary to make an optimum tooth preparation that is most suitable biologically, mechanically, and aesthetically. To achieve this optimum preparation, tooth preparation must be done by considering certain basic principles stated by Shillingburg. These are: 1. Protection of dental tissues 2. Ensuring retention and resistance 3. Ensuring structural strength 4. Ensuring marginal fit 5. Protection of pulp and periodontal tissues 6. Providing aesthetics 1. Protection of Dental Tissues Restoration should not only replace lost tooth tissue; It should also protect the remaining tooth structure. In preparing for a successful restoration, intact tooth tissue should be preserved as much as possible. For convenience and efficiency, the entire tooth surface should not be prepared; Unnecessary preparation should be avoided. Within the framework of this basic view (minimally invasive), partial crown applications should also be considered where necessary. Apart from this, it is a very big mistake to make insufficient tooth preparation in order to protect the tooth structure. For example, inadequate preparation of a functional tubercle in a molar tooth preparation may result in a fracture of the restorative material and subsequent clinical failure. The preservation of the adjacent tooth structure is another issue that should be examined under this heading. Iatrogenic damage to an adjacent tooth is a common error in dentistry. No matter how carefully the damaged proximal tooth area is polished, it will be more susceptible to plaque accumulation and therefore to caries than normal. Therefore, necessary measures should be taken to protect the adjacent tooth. A metal matrix band placed around the adjacent tooth for protection may be helpful; however, the thin band can be perforated and the underlying enamel damaged. The preferred method is to use the proximal enamel of the tooth that is being prepared for the protection of the adjacent structures. Teeth are 1.5 to 2 mm wider at the contact area than at the cementoenamel junction. Therefore, a thin, tapered diamond can be passed through the interproximal contact area while leaving a slight “lip” or “fin” of enamel without resulting in excessive tooth reduction or necessitating undesirable angulation of the rotary instrument. Near East University, Faculty of Dentistry 4 Assoc. Prof. Dr. Özay ÖNÖRAL 2. Ensuring Retention and Resistance For the restoration to achieve its purpose, it must stand firmly in the oral cavity. No single cement can resist the dislocation of the restoration on its own. The most important factor that keeps a restoration in place is to provide retention and resistance. The most important step in providing retention and resistance is tooth preparation Retention Form with the correct geometric configuration. Resistance Form Retention is the factor that prevents the restoration from being dislodged along the long axis of the tooth or entryway of the restoration. Resistance prevents the restoration from being displaced by horizontal forces and prevents the restoration from moving under occlusal forces. Retention and resistance are interrelated and should generally be considered as a whole. Whether for intra-coronal or extra-coronal restoration, the more parallel the axial surfaces of the tooth are, the greater the retention. It is difficult to prepare with perfectly parallel walls and without undercuts. Therefore, the clinically acceptable taper is specified as 6 degrees between opposing walls. It is possible to obtain retention in optimum conditions with a slope of six degrees, and this slope can be easily done clinically. The diamond bur held parallel to the entrance path of the preparation creates an angle of 2-3 degrees in the area. The three-degree slopes of the opposing surfaces create a 6-degree slope in total, which is sufficient for optimum preparation. Another important factor in increasing retention is surface area. The preparation to be made should be at the optimum level. Thus, the tooth tissues are not removed much and more surface is left for cementation. In other words, the preparation form of large teeth is more retentive than the preparation form of small teeth. The surface area can also be increased by adding some retentive structures such as grooves. Therefore, some retentive grooves can be added to the Near East University, Faculty of Dentistry 5 Assoc. Prof. Dr. Özay ÖNÖRAL restoration if necessary. Maximum retention occurs in preparations with a single path of insertion and long parallel walls. Otherwise, short and conical teeth are disadvantageous in terms of retention as they will have an infinite number of entry/exit paths. Ensuring resistance should not be considered separate from retention. Short walls also lack resistance. As the length of the wall gets shorter, the importance of the slope increases. As the preparation walls get shorter, the axial walls should be prepared as parallel as possible to increase the resistance. In short, walls as long as possible should be prepared for resistance, if this cannot be achieved, extra care should be taken for parallelism. If necessary, the retention grooves prepared parallel to each other can also make an extra contribution to increasing the resistance. 3. Ensuring structural strength Restoration should have sufficient thickness to resist occlusal forces. Although these rates vary according to the type of crown to be applied, in practice it is ideal to prepare 1.5 mm from the functional tubercle and 1 mm from the nonfunctional tubercle. In malposed teeth, the occlusal surfaces may not be parallel to the occlusal plate and therefore these values may vary depending on the situation. Inadequate preparation may cause mechanical complications (fracture, etc.) in the restoration. Obtaining the Near East University, Faculty of Dentistry 6 Assoc. Prof. Dr. Özay ÖNÖRAL optimum level of preparation should not only be achieved by increasing the amount of reduction. The preparation to be made in accordance with the anatomical form of the tooth is the most important factor in providing a homogeneous reduction depth. For example, straightly preparing the occlusal surface of an already short tooth to provide sufficient space for the restoration will further shorten the preparation, resulting in failures. However, in a preparation made in accordance with the form of the central fossa and tubercle crests, the lengths of the buccal and lingual walls will increase and the chance of clinical success will increase. One of the most important measures that can be applied to increase the structural strength in the occlusal section is to pay attention to functional tubercles. A wide bevel should be applied to the palatal tubercle slopes of the maxillary teeth and the buccal tubercle slopes of the mandibular teeth, which have intense occlusal contacts. Otherwise, fractures and perforations will be inevitable when the thin restoration in this region is exposed to functional forces. Excessive reduction to the entire occlusal surface to increase the distance in functional areas will also cause unnecessary tissue loss. For this reason, beveling to the functional tubercles gains importance in terms of increasing the structural strength of the whole preparation. 4. Ensuring Marginal Fit One of the most important factors for a successful restoration is the tight adaptation of the margins to the preparation finish line. The configuration of the finish line of the preparation determines the thickness and shape of the restoration at the margin and thus has an impact on the marginal adaptation. Types of margin geometries are as follows: Near East University, Faculty of Dentistry 7 Assoc. Prof. Dr. Özay ÖNÖRAL The margin geometry that is generally preferred today and can be applied in almost every indication is chamfer. It is the most commonly preferred finish line type due to its rounded angles and wide structure. Shoulder is generally used in all-ceramic restorations. The wide step in the form of chamfer and shoulder edge ends provides resistance against occlusal forces and porcelain fractures are prevented. In the beveled shoulder application, leakage and adaptation are increased by creating a narrow angle at the metal margin. The main disadvantage of this finishing type is the problem of fracture of thin edges. Therefore, beveled shoulder has no application in all-ceramic restorations and should be preferred in metal-containing restorations. Knife-edge application is not a wide-ending finishing line type. For this reason, it is inevitable that the restoration ending will be overcontoured according to the curvature of the tooth. In addition, thin-ending edges can cause fractures in the collar, especially in non-resistant restorations such as all-ceramics. Therefore, it does not have many indications other than the lingual surfaces of lingually inclined mandibular posterior teeth and the overturned side surfaces of teeth that are tipped into edentulous spaces. 4. Protection of Pulp & Periodontal Tissues During tooth preparation, maximum attention should be paid to the protection of periodontal tissues. Damaged tongue, gingiva, pulp, and cheek during the preparation may cause various problems. In order not to damage the gingiva, the margin level should be adjusted as supra-gingival as possible, especially in areas that do not require aesthetics. In addition, the gingiva should be safely removed from the operation area with the appropriate gingival retraction method in step preparations at or below the gingival level. The gingiva, which is damaged in the preparations made without attention, shows a reaction in the form of recession in the following periods. It should not be forgotten that the margins exposed due to recession, especially in the aesthetic region, can cause significant problems. Pulpal injuries, on the other hand, bring along some irreversible problems. Particular care should be taken to prevent these injuries that may occur during full crown preparation. These injuries may be in the form of direct pulp perforation; excessive heat, pressure, chemical irritation, and microorganisms can also cause irreversible pulpitis. In such a case, endodontic treatment will be inevitable and the tooth will lose its vitality. Near East University, Faculty of Dentistry 8 Assoc. Prof. Dr. Özay ÖNÖRAL The pulp temperature rises during tooth preparation. Group I, air turbine, water cooled. Group II, air turbine, dry. Group III, low speed, water cooled. Group IV, low speed, dry. 5. Providing Aesthetics Before starting the preparation, the aesthetic need of the area where the procedure will be performed should be well-evaluated. Especially in areas that are not of primary importance in terms of aesthetics, the protection of dental tissues and function should be prioritized. More care should be taken when preparing the anterior regions, which are of primary aesthetic importance. The amount of reduction to be made in these areas should be large enough that the aesthetic material can provide the desired illusions. For this purpose, a shortening of 2 mm should be made from the incisal edge, while a preparation of at least 1.5 mm should be made from the labial surface. Making extra “critical area” preparation in the labio-incisal Critical Area region, which has the highest aesthetic importance, will increase success. The margin level is also of great aesthetic importance. Basically, there are 3 types of margin levels: supra-gingival, equo-gingival, and sub-gingival. The supra-gingival margin is located above the gingiva and is mainly used in areas where there is no aesthetic concern. The equo-gingival margin is located at the gingival level and can be applied in any region. The Near East University, Faculty of Dentistry 9 Assoc. Prof. Dr. Özay ÖNÖRAL sub-gingival step should be preferred in anterior region where aesthetics is of primary importance. Basically, the advantages of the sub-gingival and supra-gingival margin levels are as follows: Sub-gingival margin: ▪ In cases where dental caries extending into the subgingival area, cervical erosion or restoration is present, but the crown lengthening procedure is contraindicated. ▪ In cases where the proximal contact area extends apically to the gingival crest. ▪ Situations where additional retention and/or resistance is needed. ▪ The border of an aesthetic restoration should be hidden behind the labiogingival crest. ▪ Where root sensitivity cannot be controlled by more conservative procedures such as the application of dentin bonding agents. ▪ Where axial contour modification is required (for example, it can be used to provide retention for a removable partial denture clasp). Supra-gingival margin: ▪ It is easily prepared without causing soft tissue trauma. ▪ Easy to clean. ▪ It is easier to transfer the margin to the impression. It does not require additional retraction procedures. ▪ Restoration margins can be easily checked for fit during follow-up sessions. ANTERIOR TOOTH PREPARATION The bur diameter can be used as the most important reference tool for controlling the depth of cut. For this purpose, a 1-mm diameter bur can be used for controlled cutting (Figure 1). Figures 1-2-3 In order for the preparation to be controlled, the length of the tooth should be shorter than the length of the bur. Therefore, the length of the tooth is shortened by 2 mm by making preparations on the incisal edge first. For this purpose, Near East University, Faculty of Dentistry 10 Assoc. Prof. Dr. Özay ÖNÖRAL reference grooves (in the depth of bur-diameter) are made on the incisal edge (Figure 2). The grooves opened on the incisal edge are then combined to provide the insal edge preparation (Figure 3). In order to keep the depth of cut constant, grooves in the bur diameter parallel to the long axis of the tooth are made on the labial surface. The opened grooves are combined without losing depth, thus providing labial aspect preparation. A preparation of at least 1.5 mm should be made from the labial surface (Figures 4, 5). Making extra “critical area” preparation in the labio-incisal region, which has the highest aesthetic importance, will increase success (Figure 6). Critical Area Figures 4-5-6 In the proximal aspect preparation, in order not to damage the adjacent tooth, a thin flame bur is used to remove a thin enamel layer. It is extremely important that the long axis of the bur is parallel to the long axis of the tooth (Figure 7). The preparation on the lingual aspect is performed with a ball bur in accordance with its anatomy. The important issue here is the protection of the cingulum. The cingulum will contribute to the stabilization and retention of the restoration to be completed (Figure 8). Figures 7-8 Margin preparation on the palatal or lingual surface should be carried out in a way that protects the cingulum, with the axis Near East University, Faculty of Dentistry 11 Assoc. Prof. Dr. Özay ÖNÖRAL of the bur parallel to the axis of the tooth. To provide aesthetics in the anterior region, tooth preparation should be done at the gingival level or at the subgingival level (Figures 9 and 10). The undercut areas are corrected and the preparation is completed in accordance with the tooth morphology (Figure 11). Figures 9-10-11 POSTERIOR TOOTH PREPARATION The preparation is started by controlling the depth of reduction from the occlusal surface and with a bur with a diameter of 1 mm. In accordance with the morphological slopes of the tubercles, preparation is made first on the buccal tubercles and then on the lingual/palatal tubercles. It should have sufficient restoration thickness to resist occlusal forces. Although these rates vary according to the type of crown to be applied, in practice it is ideal to prepare 1.5 mm from the functional tubercle and 1 mm from the non-functional tubercle (Figure 12). While preparing the proximal surface, it should be carried out without damaging the tooth in contact, using a flame tip or fine fissure bur. In addition, matrix tape can be used to protect the tooth in contact (Figure 13). Afterwards, the preparations on the buccal and lingual/palatal surfaces are performed in such a way as to determine the reduction depth of the 1 mm diameter bur. The margin edges should be lowered to the gingival level (Figure 14). Figures 12-13-14 Near East University, Faculty of Dentistry 12 Assoc. Prof. Dr. Özay ÖNÖRAL Straightly preparing the occlusal surface of an already short tooth to ensure adequate restoration will further shorten the preparation and result in failures. However, in a preparation made in accordance with the form of the central fossa and tubercle crests, the lengths of the buccal and lingual walls will increase and the chance of success will increase (Figure 15). Figure 15 Near East University, Faculty of Dentistry 13 Assoc. Prof. Dr. Özay ÖNÖRAL EXTENSION MATERIALS Near East University, Faculty of Dentistry 14 Assoc. Prof. Dr. Özay ÖNÖRAL (https://guident.net/articles/prosthodontics/PRINCIPLES-OF-TOOTH-PREPARATION:-PART-2.html) Near East University, Faculty of Dentistry
