Principles of Tooth Preparation PDF

Principles of tooth preparation 8. The finish line used for bucco occlusal margin of maxillary partial veneer crown: Requirements:  It provides an acute angle with a proper bulk of metal.  Enamel must be protected by a finishing bevel to prevent chipping.  Types: a. Narrow (0.3-0.5 mm...

Principles of tooth preparation 8. The finish line used for bucco occlusal margin of maxillary partial veneer crown: Requirements:  It provides an acute angle with a proper bulk of metal.  Enamel must be protected by a finishing bevel to prevent chipping.  Types: a. Narrow (0.3-0.5 mm) → finishing bevel perpendicular to the path of insertion. b. Contra bevel→ when esthetic requirements are minimal and the presence of deep bite. II- MECHANICAL CONSIDERATIONS:  The design of tooth preparations for fixed prosthodontics must adhere to certain mechanical principles; otherwise, the restoration may become dislodged or may distort or fracture during service. Mechanical considerations can be divided into four categories: A. Providing retention form. B. Providing resistance form. C. Preventing deformation of the restoration (structural durability). D. Achieving a common path of insertion. A. Retention form:  It is the quality of a preparation that prevents the removal of the restoration along its path of insertion or long axis. Certain forces (e.g., when the jaws are moved apart after biting on very sticky food) act on a cemented restoration in the same direction as the path of placement.  Path of insertion is an imaginary line along which the restoration will be placed onto or removed from the preparation. Dr. Mazen A. Attia Page 59  Principles of tooth preparation  Factors affecting the retention form: 1. Magnitude and direction of the dislodging forces. 2. Geometry of the tooth preparation. 3. Roughness of the fitting surface of the restoration. 4. Materials being cemented. 5. Type of the luting agent. 1. Magnitude of the dislodging forces:  The magnitude of dislodging forces is dependent on the stickiness of food, generally the greatest removal forces are from the exceptionally sticky food (e.g., caramel). 2. Geometry of the tooth preparation:  Most fixed dental prostheses depend on the geometric form of the preparation rather than on cement for retention because most of the traditional types of cement (e.g., zinc phosphate) are non-adhesive, i.e., they act by increasing the frictional resistance between tooth and restoration.  Cement is effective only if the restoration has a single path of placement, i.e., the tooth is shaped to restrain the free movement of the restoration (Fig. 16).  Theoretically, maximum retention is obtained if a tooth preparation has parallel walls. However, it is neither desirable nor practical to prepare a tooth this way because slight undercuts that prevent the restoration from seating are then created (Fig. 17).  Undercut: It is defined as the divergence between opposing axial walls in a cervico-occlusal direction, i.e., the cervical circumference of the prepared tooth at the margin narrower than at the occluso-axial junction (reverse taper). Dr. Mazen A. Attia Page 60  Principles of tooth preparation Fig. 16: For effective retention, a tooth preparation must constrain the movement of a restoration. A B Fig. 17: A, for a crown to seat and have the optimal retention, all axial walls should have a 6- degree taper from cervical to occlusal. B, an undercut is formed if opposing walls diverge. A tooth prepared with an undercut does not permit the crown to seat as it cannot pass over the divergent walls. A. Taper:  It is defined as the convergence of two opposing external walls of a tooth preparation. The extension of those surfaces forms an angle described as the angle of convergence. The recommended convergence angle between opposing walls is 6 degrees, which has been shown to optimize retention.  It is not necessary to deliberately tilt a rotary cutting instrument to create a taper, because this invariably leads to over convergence. Rather, teeth are readily prepared with a rotary instrument of the desired taper held parallel to the long axis of the tooth and the taper of the instrument will produce the desired axial wall taper on the completed preparation (Fig. 18). Dr. Mazen A. Attia Page 61  Principles of tooth preparation Fig. 18: A, Tilting away from the tooth creates an undercut, opposing axial preparation walls diverge in an occlusal direction. B, Tilting toward the tooth results in an excessive convergence angle of the preparation. B. Length:  A slight convergence, or taper, is clinically desirable in complete crown preparations. If this taper is small, the movement of the cemented restoration will be effectively restrained by the preparation and will have a limited path of withdrawal; thus, retention will be increased. As taper increases, there is an unlimited path of withdrawal (freedom of displacement) thus, retention will be reduced. C. Preparation features:  Includes axial grooves (V-shaped, U-shaped, Box-shaped grooves, pinholes, ledges (Fig. 19). Fig. 19: Retention from an excessively tapered preparation could be increased by the addition of proximal grooves or pinholes, because these will limit the path of withdrawal. Dr. Mazen A. Attia Page 62  Principles of tooth preparation D. Surface area:  Crowns with long axial walls are more retentive than those with short axial walls and molar crowns are more retentive than premolar crowns of similar taper because of increasing of friction related to the overall surface area. E. Stress concentration:  Cohesive failure occurs through the cement layer because the strength of the cement is less than the induced stresses.  These stresses are not uniform throughout the cement but are concentrated around the junction of the axial and occlusal surfaces as they are usually sharp. Rounding the internal sharp line angles reduces stress concentration on the cement and thus increases the retention of the restoration. F. Type of preparation:  Different types of preparation have different retentive values that correspond to the surface area of the axial walls if other factors (e.g., taper) are kept constant. Thus, the retention of a complete crown is about double that of partial coverage restorations. 3. Roughness of the fitting surface of the restoration:  When the internal surface of restoration is very smooth, retentive failure occurs not through the cement but at the cement-restoration interface.  Therefore, retention will be increased if the fitting surface of the restoration is roughened i.e., sandblasting the fitting surface with 50 µm of alumina. This should be done carefully to avoid abrading the polished surfaces or margins. Dr. Mazen A. Attia Page 63  Principles of tooth preparation 4. Materials being cemented:  The more reactive the alloy, the more adhesion with certain luting agents. Therefore, base- metal alloys are better retained than high-gold content metals. 5. Type of luting agent:  The type of luting agent chosen affects the retention of a cemented restoration. Adhesive resin cements are more retentive than conventional cements. B. Resistance form:  It is the quality of a preparation that prevents dislodgement of the restoration by forces directed in an oblique or horizontal direction (loading restoration during eccentric contact between posterior teeth).  Mastication and para-functional activity may subject the prosthesis to substantial horizontal or oblique forces. These forces are too heavy, especially between the posterior teeth which would tend to displace the restoration around its gingival margin. Therefore, the displacement is in a rotation path and the axis of rotation is at the gingival margin (Fig. 20). Fig. 20: The resistance area (RA) of a complete crown is placed under compression when a lateral force (F) is applied. Dr. Mazen A. Attia Page 64  Principles of tooth preparation  The tipping path is the path along which the restoration could be displaced under the displacing lateral occlusal forces.  Rotation is prevented by any areas of the tooth preparation that are placed in compression, called resistance areas. These areas of the tooth structure lie outside the tipping path and will resist the displacing forces (Fig. 21). Fig. 21: The tipping path is the path along which the restoration could be displaced under the displacing occlusal forces.  Factors affecting the resistance form: 1. Magnitude and direction of the dislodging forces. 2. Geometry of the tooth preparation. 3. Physical properties of the luting agent. 1. Magnitude and direction of the dislodging forces:  In a normal occlusion, biting force is distributed over all the teeth; most of it is axially directed.  However, if a patient has a biting habit such as pipe smoking or bruxism, it may be difficult to prevent large oblique forces from being applied to a restoration. Therefore, the complete tooth preparation and restoration must be able to withstand considerable oblique forces as well as the normal axial ones. Dr. Mazen A. Attia Page 65  Principles of tooth preparation 2. Geometry of the tooth preparation:  As with retention, preparation geometry plays a key role in attaining desirable resistance form. The tooth preparation must be shaped so that particular areas of the axial wall prevent rotation of the crown. A. Degree of occlusal convergence:  Slight occlusal convergence between the prepared axial surfaces will leave a reasonable amount of tooth structure outside the tipping path that will give good resistance to the displacing forces. B. Occluso-gingival height of the preparation:  Excessive occlusal reduction may cause severe shortening of the preparation, so in case of two preparations having the same occlusal convergence, normal occluso-gingival height will preserve sufficient tooth structure outside the tipping path to resist the displacing forces, while severe occlusal reduction will lead to very short occluso-gingival height; therefore, no sufficient tooth structure will be left outside the tipping path to resist the displacing forces (Fig. 22).  Accordingly, occlusal reduction should not exceed one third of the occluso-gingival height. C. Types of the preparation and features:  A partial coverage restoration (e.g., 3/4 crown) will have less resistance than a complete coverage crown because the buccal surface is left unprepared, so it has no buccal resistance areas.  Resistance must be provided by grooves and will be greatest if they have walls that are perpendicular to the direction of the applied force (Fig. 23). Dr. Mazen A. Attia Page 66  Principles of tooth preparation Fig. 22: Excessively increasing the convergence of axial walls or excessively shortening the preparation will decrease greatly the tooth structure outside the tipping path. Fig. 23: The resistance areas on the buccal surface of a complete coverage preparation are compensated in a partial coverage preparation by mesial and distal grooves. Dr. Mazen A. Attia Page 67  Principles of tooth preparation There are three forms of the axial grooves:  V-shaped grooves: the restoration tends to be displaced under the horizontal component of the occlusal force. They are used in anterior teeth with limited dimensions (Fig. 24). Fig. 24: V-shaped grooves.  U-shaped grooves: The lingual wall of the groove is perpendicular to the proximal surface; thus, a thick rib of metal will resist the displacing force (Fig. 25). Fig. 25: U-shaped grooves.  Box-shaped grooves: provide high resistance to displacing force. Used in case of proximal caries or beside the connector when used as a retainer (Fig. 26). Fig. 26: Box-shaped grooves. Dr. Mazen A. Attia Page 68  Principles of tooth preparation  The relation between the axial grooves and the tipping path will depend on: a. The length of the groove: The longer the groove the more the resistance to displacing forces. b. The position of the groove: Placing the groove more buccal will give enough tooth structure lingual to the groove that will resist the displacing force. c. The depth of the groove: The U-shaped and box-shaped grooves will resist the displacing forces better than the V-shaped grooves. N.B. The grooves interfere with the rotational movement of the restoration along its tipping path by reducing the rotational radius thus that portion of the walls of the grooves near the occlusal surface of the preparation will interfere with the displacement. 3. Physical properties of the luting agent:  Resistance to displacement is affected by physical properties of the luting agent such as compressive strength and modulus of elasticity.  The compressive strength of zinc phosphate cement must exceed 70 MPa at 24 hours. Glass ionomer cement and resin cement have higher compressive strength when compared with zinc polycarboxylate cement.  Increasing temperature has a dramatic effect on the compressive strength of luting agents. An increase from room temperature (23 oC) to body temperature (37 oC) halves the compressive strength of reinforced zinc oxide eugenol cement and a rise in temperature to (50 oC) (equivalent to hot food) reduces the compressive strength by over 80%. C. Preventing deformation of the restoration:  A restoration must have enough strength to withstand masticatory forces without permanent deformation during function; otherwise, it will fail. Dr. Mazen A. Attia Page 69  Principles of tooth preparation 1. Alloy Selection:  Type I and Type II gold alloys are satisfactory for intra-coronal cast restorations, but they are too soft for crowns and fixed partial dentures, for which Type III, or Type IV gold alloys are chosen.  Nickel-chromium alloys have a high modulus of elasticity and thus they are indicated when large forces are anticipated, such as in cases of long-span FPDs. 2. Adequate tooth reduction: I. Occlusal reduction:  There should be a minimum alloy thickness of about 1.5 mm over functional cusps (buccal in the mandible, palatal in the maxilla). The less stressed non-functional cusps can be protected with less metal (1 mm is adequate in most circumstances) for a strong and long-lasting restoration.  Occlusal reduction should be as uniform as possible, following the cuspal planes of the teeth; this will ensure that sufficient occlusal clearance is combined with preservation of as much tooth structure as possible (Fig. 27). Fig. 27: Anatomic occlusal reduction is conservative of tooth structure and gives rigidity to the restoration. Dr. Mazen A. Attia Page 70  Principles of tooth preparation II. Functional cusp bevel:  The functional cusp bevel is performed on the lingual inclines of the maxillary lingual cusps and the buccal inclines of mandibular buccal cusps to provide sufficient space for an adequate bulk of metal in that area of heavy occlusal contact (Fig. 28).  Lack of occlusal bevel results in, a- thin metal area or its perforation and b- over contoured restoration and occlusal prematurity. Fig. 28: The functional cusp bevel provides sufficient space on the functional cusp to produce a crown with enough thickness to resist deformation. III. Margin design and axial reduction:  Tooth reduction should provide sufficient room for the bulk of metal at the margin to prevent distortion. For example, a featheredge finish line produces a thin weak margin that is liable to deformation compared with the thicker restoration of a chamfer finish line.  Over reduction→ pulp exposure.  Under reduction→ thin walls thus liable to distortion, bulky restorations, bad esthetics, and periodontal problems (Fig. 29).  In case of partial coverage restorations, distortion of the restoration margin on the occlusal surface is prevented by keeping preparation margins approximately 1 to 1.5 mm away from occlusal contact areas. Dr. Mazen A. Attia Page 71  Principles of tooth preparation Fig. 29: A very thin finish line will lead either to a restoration with very thin margin liable to distortion or an over contoured restoration. D. Path of insertion:  A path of insertion must be selected that will allow the margins of the retainers to fit against the respective preparation finish lines with the removal of minimum sound tooth structure. This path should not encroach upon the pulp or adjacent teeth.  The path of insertion could be classified into: A. For a single restoration: a) Line of insertion It is a single direction through which the restoration could be precisely seated on the prepared tooth. b) Range of insertion It denotes the converging angle of two opposing surfaces, within its limits; the restoration could be precisely seated on the prepared tooth. B. For a bridge: a) Common line of insertion It is the single direction through which all the retainers of a fixed-fixed bridge could be precisely seated on the corresponding abutment teeth. Dr. Mazen A. Attia Page 72  Principles of tooth preparation b) Common range of insertion It denoted the sum of the smallest converging angles- of the mesial plus of the distal surfaces of the abutment teeth, within its limits, all retainers of a fixed-fixed bridge could be precisely seated on the corresponding abutment teeth.  The path of insertion must be considered in two dimensions: facio-lingually and mesio- distally.  The facio-lingual orientation of the path can affect the esthetics of metal-ceramic or partial veneer crowns. For metal-ceramic crowns, the path is roughly parallel with the long axis of the teeth. A facially inclined path of insertion on a preparation for a metal-ceramic crown will leave the facio-occlusal angle too prominent, resulting in over contouring of the restoration (Fig. 30).  The mesio-distal inclination of the path must be parallel to the contact areas of adjacent teeth. If the path is inclined mesially or distally, the restoration will be held up at the proximal contact areas (i.e., locked out). This is a problem when restoring a tilted tooth. In this situation, making the path of insertion parallel with the long axis of the tooth will cause the contacts of the adjacent teeth to encroach on the path of insertion (Fig. 31). Dr. Mazen A. Attia Page 73  Principles of tooth preparation Fig. 30: a-The path of insertion of a preparation for a metal-ceramic crown should be parallel to the long axis of the tooth. b- If the path is directed facially; the prominent facio-incisal angle may create esthetic problems of over contouring or opaque show-through. c- If the path is directed lingually; the facial surface will intersect the lingual surface, creating a shorter preparation. It also may encroach on the pulp. Fig. 31: The path of insertion should be parallel to the adjacent proximal contact.  All undercuts must be eliminated, or it will prevent the restoration from seating. To evaluate the preparation taper, view it with one eye from a distance of 30 cm. The entire finish line should be visible to one eye from one fixed position with no obstruction by any part of the prepared tooth. If both eyes are open when the preparation is viewed, an undercut may remain undetected (Fig. 32). Dr. Mazen A. Attia Page 74  Principles of tooth preparation Fig. 32: Checking undercuts using one closed eye.  Where it is difficult to survey the preparation with direct vision use a mouth mirror. To verify parallel paths of insertion of bridge abutments, the image of one preparation is centered in the mirror. Then using firm finger rests the mirror is moved bodily without changing its angulations until the image of the second preparation is also centered. If the angulations of the mirror must be changed to see all the finish lines; therefore, there is a discrepancy between the paths of insertion of the preparations (Fig. 33). Fig. 33: Checking the path of insertion using a mouth mirror. Dr. Mazen A. Attia Page 75  Principles of tooth preparation  If direct vision is possible, the mesial surface of the anterior abutment should be convergent with the distal surface of the posterior abutment. In contrast, the distal surface of the anterior abutment should be divergent from the mesial surface of the posterior abutment (Fig. 34). Fig. 34: Checking the path of insertion by direct vision. Factors affecting the path of insertion: 1. Reduction of axial surfaces  The reduced surfaces should be parallel to each other or with slight occlusal convergence. 2. Roundation of axial line angles  Remove cervical triangular undercuts and obtain a continuous smooth finish line. 3. Alignment of the proximal grooves  Buccal and lingual walls of the groove in relation to each other should be parallel or with slight occlusal divergence (convergence will make an undercut) (Fig. 35).  The lingual wall of the groove in relation to the lingual surface of the prepared tooth should be parallel or with slight occlusal convergence (divergence will make an undercut).  The buccal wall of the groove in relation to the lingual surface of the prepared tooth should have slight occlusal convergence or divergence (no problem in the path of insertion). Dr. Mazen A. Attia Page 76  Principles of tooth preparation  The axial wall of the groove should be parallel to the opposing proximal surface of the preparation or with slight occlusal convergence. a b Fig. 35: a-Buccal and lingual walls of the groove, b-Buccal wall in relation to the lingual surface. III- ESTHETIC CONSIDERATIONS:  Most Patients prefer their dental restorations to look as natural as possible. However, esthetic considerations should not be pursued at the expense of the prognosis of the patient's long- term oral health or function.  At the initial examination, a full assessment is made of the appearance of each patient, noting which areas of teeth show during speech, smiling, and laughing. The patient's esthetic expectations must be discussed and related to oral hygiene needs and the potential for the development of future disease. The final decision regarding an appropriate restoration can then be made with the full cooperation and informed consent of the patient. 1. All-ceramic restorations:  Some of the most pleasing esthetic restorations are all-ceramic crowns, inlays, onlays, and veneers. They can mimic the original tooth color better than the other restorative options.  The newest materials have improved physical properties and can be strengthened by using adhesive resin luting agents. Dr. Mazen A. Attia Page 77  Principles of tooth preparation  Not all-ceramic crown preparations are conservative of tooth structure, as a wide heavy chamfer margin must be prepared around the entire tooth to ensure increased material thickness and material strength. For the same reason, additional reduction on the lingual surface is needed for these restorations.  A minimal material thickness of approximately 1 to 1.2 mm is necessary to ensure optimal esthetics. This limits the use of these restorations on facio-lingually thin teeth and teeth with large pulps, as in young individuals. 2. Metal-ceramic restorations: A. Facial tooth reduction:  A minimum reduction of 1.5 mm typically is required for optimal appearance. Adequate porcelain thickness is essential for preventing direct light reflection from the highly pigmented opaque porcelain. The most critical areas are the gingival and incisal third, therefore, opaque modifying stains are often used in these areas (Fig. 36). Fig. 36: Adequate porcelain thickness is essential for preventing direct light reflection from the highly pigmented opaque porcelain. The most critical areas are the gingival and incisal thirds. Dr. Mazen A. Attia Page 78  Principles of tooth preparation  The labial surfaces of anterior teeth should be prepared for metal-ceramic restorations in two distinct planes. If they are prepared in a single plane, insufficient reduction in either the cervical or the incisal area of the preparation will result (Fig. 37). Fig. 37: A-Reduction of the facial surface for a tooth to receive a metal-ceramic crown should be done in two planes, one parallel with the path of insertion, and one parallel with the incisal two-thirds of the facial surface of the tooth, B-If only one plane is produced, opaque porcelain may show through, C-or the labial surface may be over contoured, D-or the pulp may be encroached upon. B. Incisal reduction:  The incisal edge of a metal-ceramic restoration has no metal backing and can be made with a translucency like that of a natural tooth structure.  An incisal reduction of 2 mm is recommended for good esthetics. Excessive incisal reduction must be avoided because it reduces the resistance and retention form of the preparation. Dr. Mazen A. Attia Page 79  Principles of tooth preparation C. Proximal reduction:  The proximal surfaces of anterior teeth will look most natural if they are restored as the incisal edges, without metal backing. This will allow some light to pass through the restoration in a manner like what occurs in a natural tooth.  If there is a fixed partial denture, the need for connectors will make this impossible so to prevent excessive metal display, connectors in anterior bridges should be placed towards the lingual (Fig. 38). Fig. 38: Connectors in anterior bridges should be placed towards the lingual. D. Labial margin placement:  Supragingival margin placement has many biological advantages. The restorations are easier to prepare properly and easier to keep clean.  Subgingival margins may be indicated for esthetic reasons to hide the dark metal-ceramic junction at the margin referred to as "the black line", particularly when the patient has a high lip line (Fig. 39). Dr. Mazen A. Attia Page 80  Principles of tooth preparation Fig. 39: The dark metal-ceramic junction at the margin is called "the black line".  If the patient has a low lip line, a metal supragingival collar may be placed because the metal is not seen during normal function and metal margins generally have a more accurate fit than porcelain margins. However, the use of a labial supragingival metal collar in anterior teeth is often rejected by the patients (Fig. 40). Fig. 40: Supragingival metal collar.  Metal collars can be hidden below the gingival crest, although there will be some discoloration if the gingival tissue is thin.  Margins should not be placed so far apically that they encroach on the attachment. Extension of the margin to within 1.5 mm of the alveolar crest will lead to bone resorption. The margin should be placed subgingivally midway between the crest of the free gingiva and the base of the sulcus (Fig. 41). Dr. Mazen A. Attia Page 81  Principles of tooth preparation Fig. 41: The margin should be placed subgingivally midway between the crest of the free gingiva and the base of the sulcus.  Good appearance can be restored with a metal-ceramic restoration having a supragingival porcelain labial margin sometimes called a "collarless" design. The metal is trimmed at the margin area of about 1 mm and the margin is constructed entirely with a special type of porcelain called "shoulder porcelain"(Fig. 42). Fig. 42: Shoulder porcelain is used to produce porcelain labial margin. Dr. Mazen A. Attia Page 82  Principles of tooth preparation 3. Partial coverage restorations:  Esthetic partial coverage restorations depend on the accurate placement of the potentially visible facial and proximal margins.  If a partial coverage restoration is poorly prepared, the patient may demand its replacement with a metal-ceramic crown, and the result is an unnecessary loss of tooth structure. A. Proximal margin:  Placement of the proximal margins (particularly the mesial which is the more visible margin) is critical to the esthetic result of a partial-coverage restoration.  The margin should be placed just buccal to the proximal contact area, where metal will be hidden by the distal line angle of the neighboring tooth (Fig. 43). Fig. 43: The margin should be placed just buccal to the proximal contact area.  The distal margin of posterior partial-coverage restorations is less visible than the mesial margin. Therefore, it is advantageous to extend the preparation further beyond the contact point for easier preparation and finishing of the restoration and better access to oral hygiene. Dr. Mazen A. Attia Page 83  Principles of tooth preparation B. Facial margin:  The facial margin of a maxillary partial-coverage restoration should be extended just beyond the occluso-facial line angle. A short bevel is needed to prevent enamel chipping.  If the buccal margin of metal is correctly shaped, it will not reflect light to an observer. As a result, the tooth appears to be a little shorter than normal and not as though its buccal cusp is outlined in metal (Fig. 44). Fig. 44: The facial margin of a partial crown should be shaped so that light is not reflected directly to the observer.  When mandibular partial cast crowns are made, metal display is unavoidable because the occlusal surface of mandibular teeth can be seen during speech. A chamfer is recommended for the buccal margin because it provides a greater bulk of metal around the highly stressed functional cusp (Fig. 45). If the appearance of metal is unacceptable to the patient, a metal- ceramic restoration with porcelain coverage on the occlusal surface can be made. Dr. Mazen A. Attia Page 84  Principles of tooth preparation Fig. 45: A substantial chamfer is recommended for the functional buccal cusp of a mandibular partial cast crown. It provides a greater bulk of metal in a stressed area.  Anterior partial coverage restorations can be fabricated to show no metal. The facial margin is extended just beyond the highest contour of the incisal edge but not quite to the inciso- labial line angle. Here the metal will protect the tooth from chipping but will not be visible (Fig. 46). Fig. 46: The incisal edge is not completely covered. The restoration margin is located between the highest point of the incisal contour and the inciso-labial angle. Dr. Mazen A. Attia Page 85