- Email: [email protected]
Systematized procedure of crown preparation
HOFFMANANDRUBIN
4. Pipko DJ, Fine D, Sadeek M. Vacuum formed temporary splints. Dent Dig 1970;76:270-4. 5. Sotera AJ. A direct technique for fabricating acrylic temporary crowns using the Omnivac. J PROSTHETDENT 1973;29:577-80. 6. Halterman CA. Creating and developing a temporary acrylic crown technique. Quintessence Dent Tech 1981;5:3-8. 7. Breeding LC. Indirect temporary acrylic restorations for fixed prosthodontics. J Am Dent Assoc 1982;105:1026-7. 8. Cannistraci AJ. A new approach to impression taking for crown and bridge. Dent Clin North Am 1965; Mar:33-42. 9. Amsterdam M, Fox L. Provisional splinting-principles and technics. Dent Clin North Am 1959;Mar:73-99. 10. Miller IF, Belsky MW. The full shoulder preparation for periodontal health. Dent Clin North Am 1965;Mar:83-102.
Systematized
procedure
11. Hoffman JM. Replication by silicone materials in crown and bridge construction. NY State Dent J 1987;53:40-3. 12. Kile WL. An improved periodontal dressing technique. the acrylic splint. J Mich State Dent Assoc 1968;50:20-3. 13. Warnick ME. Cast crown restoration of a badly involved abutment to fit an existing removable partial denture. Dent Clin North Am 1970;14:631-44. Reprint
requests
to:
DR. JULE~HOFFMAN SCHOOLOFDENTALANDORALSURGERY COLUMBMUNIVERSITY NEW YoRK,NY 10032
of crown preparation
E. V. Bass, M.D.Sc.,* and M. C. Kafalias, B.D.S., M.D.S., University of Sydney, Faculty of Dentistry, Sydney, Australia
M.S.D.**
The theoretical considerations of the marginal fit of crowns are discussed and the conventional approach in tooth-cutting procedures is addressed. Gingival marginal surface irregularities described as “peak and valley” and “stepping” effects can leave a cement line in excess of 80 pm after cementation. A regimented method of tooth reduction for metal ceramic crowns, using specially designed rotary cutting instruments, is described. The suggested technique accomplishes the crown preparation more rapidly with a consistent standard of excellence. (J PROSTHET DENT 1989;62:400-6.)
F
or decades, various techniques for tooth preparation of intracoronal and extracoronal restorations have been proposed. In addition, thousands of shapes of commercial diamond and tungsten carbide rotary burs are available. The current equipment and materials are unquestionably superior to what was available a few years ago. However, the dentist is disconcerted by the infinite array of instruments and how they relate to uncomplicated, efficient tooth preparation that is comfortable for the patient. A technique is suggested using the instruments and methods advocated for the postdoctoral students at the University of Sydney School of Dentistry that have produced predictable, reliable results.
TOOTH PREPARATION The optimal tooth preparation is elusive and rarely in the complete control of the dentist. The length, size, and angulation of the tooth, the remaining tooth structure, and surface area for retention are commonly compromised. Parallelism of the preparation has received the greatest attention because the taper of the preparation is more *Senior Registrar. **Associate Professor, Department of Operative Dentistry. 10/l/13381
400
commonly determined by the dentist than by other fact0rs.l The degree of taper and its effect on retention has been widely researched, from in vitro studies with precisely controlled conditions2-4 to measuring the taper of dies used for clinical fixed restorations.5y 6 Most authors recommend a taper range of 2 to 7 degrees,5>7 but it is difficult to visually determine whether a preparation is free of undercuts at less than a taper of approximately 17 degrees. One studys found that the convergence angles of clinical tooth preparation for single crowns varied from 12 to 37 degrees. A uniformity of axial tooth reduction should be achieved for sufficient space of the restoring material, for example metal and porcelain. Experience has suggested that an axial reduction of preparation from 1.2 to 1.5 mm is suitable if a metal ceramic is placed, a 1.5 to 2 mm reduction of the occlusal surface, and a minimum of 0.5 mm for metal surfaces. The facial reduction should be uniform from mesial to distal proximal contacts of the preparation. The occlusal reduction of the tooth should establish a 1.5 to 2 mm clearance from the opposing dentition in the excursive movements of the mandible. The most critical part of tooth preparation is the development of the gingival margins. The casting should fit the tooth preparation satisfactorily while allowing adequate
OCTOBER1989
VOLUME62
NUMBER4
SYSTEMATIZED
Fig.
PROCEDURE
OF CROWN
PREPARATION
1. Facial surface with gingival orienting groove.
space for a luting agent on the axial and occlusal surfaces of the preparation. The minimum cement film thickness is approximately 20 brn. The casting should be accomplished in the laboratory with a controlled “passive fit” of approximately 40 pm or 20 pm space on the opposite sides of the preparation. If the casting “slide-fits,” or worse, “pressure-fits,” the cement commonly prevents a precise seating of the restoration, resulting in a disappointing marginal fit and elevated occlusion. A well-defined loose fit of the restoration is obtained by treating the stone die with a die spacer that is applied to the axial and occlusal/incisal surfaces. This ensures the precise contact of the casting to the tooth at the crucial gingival margin. Jorgensen and Finger9 have described in detail their disciplined method of producing more consistent, acceptable castings. Theoretically, a casting with a cement film of approximately 20 pm is ideal, but a film of 50 pm does not present a noticeable biological risk. lo A cement line of 50 grn is the limit that can be observed visually under favorable conditions, and a gingival gap of this size can just be detected with a sharp, pointed explorer.
Surface
roughness
The tooth preparation should not be finished with burs that produce a smooth surface, which compromises retention, or excessive roughness, which results in marginal inaccuracy. A surface prepared with a coarse-grained diamond point leaves a roughness index (Rt) of 35 km, which is clinically unacceptable. (Rt is the distance from the highest point to the lowest point on the rough surface.) After completion of a surface with a medium-grained diamond point, a roughness of approximately 20 wrn is evident. Normally this roughness on the axial surfaces provides sufficient retention of the cemented restoration. The cement particles are lodged between the grooves without interfering with marginal adaptation and retain the restoration.”
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig.
2. Linguai surface with lingual chamfer groove.
Fig.
Gingival
3. Proximal surface reduction.
marginal
surface
The critical features in finishing the gingival part of the preparation are as follows: 1. Enamel rods are angled at approximately 100 to 110 degrees to the long axis of the tooth but 90 degrees at a cavosurface margin. The important angulation is the cavqsurface angle (to the external surface of the tooth) and not the angle to the prepared axial surface. This is crucial but is not usually clarified in descriptions of tooth preparations. The unsupported enamel rods are dislodged at this cavosurface margin of the preparation during cementation of the restoration, resulting in a marginal gap. 2. The gingival surface of the preparation should be finished with a substantially smaller Rt than the axial wall. If the Rt at the gingival part of preparation is greater than 20 Km it is recorded by the impression materials and reproduced on type II die stone material.‘” This roughness is transferred and remains on the gingival surface of the casting. Because it is impossible to interdigitate the peaks of a casting surface into the valleys of a tooth preparation, peak-to-peak interfaces of casting to preparation surfaces are common. 401
BASS
Fig. 4. Facial surface with depth-orienting
AND
KAFALIAS
grooves.
Fig. 6. A, Lingual cingulum axiai surface reduction. B, Concave lingual surface reduction. Fig. 5. Completion of labial reduction. If the Rt at this gingival surface is 30 pm with a peak-topeak microsurface interface, there is a gap between the valleys equaling 60 km. A luting agent has a minimal thickness of 20 pm and if this is added to the 60 pm gap between the valleys, a cement film thickness of 80 pm results. Tooth preparation with a flat-ended tip of a mediumcoarse diamond bur creates a “stepping”on the shoulder of the preparation. This is especially evident on the concave labial shoulder and results in a greater discrepancy between the casting and the tooth. Therefore it is advantageous to produce a smooth, even surface at the gingival surface of the preparation. A tungsten carbide finishing bur (20,000 rpm) produces a surface smoothness of approximately 2 to 5 prn.l”p l3 If this tungsten carbide bur is directed to the shoulder of the preparation in a horizontal plane (shank secured at right angles to the long axis of the tooth), the stepping effect can be controlled, creating a smooth surface without a peakand-valley effect.
Tooth preparation for metal ceramic
on upper central crown
incisor
Step 1. Facial surface (gin&al orienting groove). A high speed, medium-grit diamond wheel (Fig. 1) is used 402
to make the initial square groove level with the crest of the gingiva, following the labial contour from proximal-toproximal surface. The bur is angled toward the tooth so that the shank of the bur contacts the tooth surface and a self-limiting depth of 0.5 mm is produced in the groove. This groove creates a uniform ledge but is not the final shoulder preparation that forms the gingival limitation to facial reduction. The shoulder is completed at a later stage. Step 2. Lingual surface (lingual chamfer groove). A fluted tungsten carbide finishing bur (Fig. 2) is used to develop a quarter-circular groove level with the crest of the lingual gingiva, following the gingival tissue contour from proximal-to-proximal surface similar to step 1. In this instance the shape of the cutting surface of the bur produces a smooth chamfer-finishing margin to a depth of 0.5 mm and with a surface h of 3 to 5 pm. The bur speed is 15,000 to 20,000 rpm without a coolant. This sequence of the preparation becomes the final lingual chamfer. Step 3. Proximal surface reduction. A high-speed, round-ended, medium-grit diamond bur of 3 degrees taper on each side and 0.5 mm diameter at the tip is used to prepare the proximal surfaces of the tooth (Fig. 3). The cut is made level with the interproximal papilla from labial gingival orienting groove to lingual chamfer groove. The long axis of the bur is held parallel to the path of insertion, al-
OCTOBER
1989
VOLUME
62
NUMBER
4
sYsrEmAnzeD PROCEDURE 0F cRowN PREPARATION
Fig. 7. Incisai reduction.
lowing the 3-degree taper of the cutting surface to produce an optimal path of insertion. A rounded ledge is produced on the gingival proximal surface of the tooth. The gingival finishing margin is completed later. Step 4. Facial surface and depth-orienting grooves. A medium-grit, high-speed, self-limiting diamond tip is used to establish the depth-orienting grooves on the labial surface. The diamond cutting tip is 1 mm in diameter and 1 mm long. The bur is held at right angles to the labial surface and 3 or 4 grooves are placed (Fig. 4). Each groove originates at the facial gingival orienting groove and extends to the incisal edge. The self-limiting design of the cutting point produces a controlled contoured depth. Step 5. Completion of facial reduction. A high-speed, flat-ended, medium-grit diamond point with a 3-degree taper on each side and 1 mm tip diameter (Fig. 5) is used to remove the remaining facial tooth structure by joining the contoured depth-orienting grooves. This is accomplished in two stages. The gingival half is completed first and the long axis of the bur is held parallel to the path of insertion for the restoration. It is essential that this is completed before the incisal half is prepared and before the lingual axial surface is prepared so that the maximum available vertical tooth structure faciolingually can be preserved for optimal taper. This bur creates a 1 mm wide gingival shoulder that is at a go-degree angle to the faciogingival half of the preparation. The incisal half of the labial surface is completed by joining the depth orienting grooves. Step 6. Lingual surface reduction. Lingual cingulum axial surface reduction. A high-speed, smooth-ended, medium-grit diamond point of 3-degree taper on each side and 1 mm diameter at the tip is used to reduce the lingual cingulum axial surface (Fig. 6, A). This bur is held parallel to the same path of insertion as for the gingival half of the facial surface preparation. The smooth end is applied to the finished chamfer established in step 2 and is moved from
THE
JOURNAI.
OF PROSTHETIC
DENTISTRY
Fig. 8. Proximal axial line angles preparation.
Fig. 9. Finishing the labial shoulder. proximal surface to proximal surface, following the curvature of the tooth. Concave surface. A high-speed, oval-shaped, round-tip, medium-grit diamond point is used to reduce the concave surface of the lingual surface from cingulum to incisal edge (Fig. 6, B). Generally 1 mm of tooth structure is removed. The amount of tooth structure removed in depth depends on the design of the metal framework of the restoration and should also be checked during excursive movements of the mandible. Step 7. Incisal reduction. The oval-shaped diamond point seen in Fig. 6, B is used to reduce the Incisal edge so that it will be 1.5 mm below the incisal edge of the adjacent central incisor or, if the adjacent central tooth is absent, to 1.5 mm below the estimated incisal level of the final crown (Fig. 7). Step 8. Proximal axial line angles. A high-speed, round-ended, medium-grit diamond bur I mm in diameter at the tip with a 3-degree taper on each side is then used to blend the proximal axial line angles into the buccal and lingual surfaces (Fig. 8). Step 9. Finishing the labial shoulder. A slow-speed (10,000 to 20,000 rpm) tungsten carbide finishing bur is used to reshape the gingival ledge of the facial surface in 103
BASS
Fig. 10. Finishing the interproximal
shoulder.
Fig. 11. Teeth before preparation.
step 5. This bur is held at right angles to the labial surface. The angulation of the cutting blades produces a slope of 110 to 120 degrees to the faciogingival half of the axial wall of the tooth preparation that is the angulation of the enamel rods. This labial slope is finished 0.5 mm below the crest of the gingiva (Fig. 9). This bur has a rounded polished end that prevents cutting of the axial wall of the preparation during the necessary changes of the angulation of the bur in the horizontal plane while the shoulder is refined. The bur is also sloped and polished from the shank to the cutting blades, allowing displacement of the free gingiva without trauma and controlled placement of the margin 0.5 mm below the crest of the gingiva. The horizontal application of this bur smoothes the surface roughness produced by the medium-grit diamond bur and removes unsupported enamel rods on the cavosurface margin. The surface roughness of this facial slope has an Rt of 3 to 5 pm and the stepping effect produced’ by an endcutting rotary instrument is minimized. Step 10. Interproximal shoulder. A tungsten carbide end-cutting finishing bur, parallel-sided, with a lo-degree bevel at the cutting end is used at a speed of 15,000 to 30,000 rpm without a coolant to finish the interproximal shoulder. 404
AND
KAFALIAS
Fig. 12. Teeth prepared for metal ceramic crowns.
The bur is moved faciolingually from the proximal surface of the completed facial shoulder to the finished lingual chamfer. It is maintained at an angle to the long axis of the preparation so that the cutting end lies along the direction of the enamel rods. The lo-degree bevel prevents the corners of the end-cutting surface from producing undercuts in the gingivoaxial line angle (Fig. 10). Fig. 11 demonstrates endodontically treated maxillary central incisor teeth with palatally displaced lateral incisors. The maxillary central and lateral incisors were prepared by using the method and cutting burs described, with a total preparation time of 1 hour and 22 minutes. Fig. 12 emphasizes the smoothness of the facial shoulders compared with the horizontal grooves on the facial axial walls of the prepared teeth.
Posterior
teeth
The preparation of posterior teeth for a complete gold crown or a metal ceramic crown is similar to the maxillary central incisor. Unless the esthetic requirements of a particular patient are demanding, a metal collar 1 to 1.5 mm wide is placed on the facial surface for metal ceramic crowns on posterior teeth.
CONCLUSIONS Standard tooth preparation for a crown requires undivided concentration simultaneously in the three planes of preparation: length, depth, and width. The medium-grit diamond burs used to prepare the gingival surface produce irregularities described as peak-andvalley and stepping effects that can leave a cement line in excess of 80 pm after cementation. The optimal fit of crowns at the gingival margin require tooth preparation finished with tungsten carbide finishing bum producing an Rt of approximately 5 pm, eliminating the peak-and-valley effect. An innovative application of a cutting rotary bur to finish the facial shoulder and eliminate the stepping effect was described. Programmed tooth preparation using specially designed cutting burs reduces the diversity of concentration to one
OCTOBER
1989
VOLUME
62
NUMBER
4
SYSTEM;
TIZED
PROCEDURE
OF CROWN
PREPARATION
plane of preparation at a time. This technique accomplishes the tooth preparation more rapidly, with a reliable, predictable standard of excellence. REFERENCES 1. Thayer KE. Fixed prosthodontics. Chicago: Year Book Medical Publishers, 1984;31-6. 2. Rosenteil E. The taper of inlay and crown preparations. Br Dent J 1975;139:436-8. 3. Abdullah SI, Mohammed H, Thayer KE. Factors in the failure of cemented full crowns. J Can Dent Assoc 1974;40:721-4. 4. Woolsey GD, Matich IA. The effect of axial grooves on the resistance form of cast restorations. J Am Dent Assoc 1978;97:978-80. 5. Mack PI. A theoretical and clinical investigation into the taper achieved on crown and inlay preparations. J Oral Rehabil 1980,7:255-65. 6. Hyde PF. The mechanical principles of cavity preparation for the cast gold restoration [M.D.S. thesis]. Sydney: University of Sydney, 1984. 7. Shillingburg HT Jr, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981;7983.
Considerations
in measurement
8. Ohm E, Silness J. The convergence angle in teeth prepared for artificial crowns. J Oral Rehabil 1978;5:371-5. 9. Jorgensen KD, Finger W. Die-spacing technique by &fusion precipita tion. Stand J Dent Res 1979;87:73-8. 10. Jorge” D. Efficient and high-quality cavity preparation ergonomy preparation sets. Lemgo. West Germany:Gehr Rrasseier Gmhh & Co KG, 1981;4. 11. Felton BA, Kanoy BE, White JT. Surface roughness of crown preparation: effect on cemented casting retention [Ahs~ract]. J Dent Ree 1986;65:312. 12. Craig RG. Restorative dental materials. St T,ctws: CV Mosbv Co, 1980;203. 13. G&art E. The Rio-set will make things a lot easier r’or vou. Dusseldorf, West Germany: Hager & Meisinger Gmbh. 1985. Reprint requests toDR. M. C. KAFALIAS UNITED DENTAL HOSPITAL UNIVERSITY OF SYDNEY 2 CHALMERS ST. SYDNEY 2010 AUSTRALIA
of marginal
fit
J. Robert Holmes, D.D.S., M.S., M.Ed.,* Stephen C. Bayne, M.S., Ph.D.,** Gene A. Holland, D.D.S., M.S.,*** and William D. Sulik, D.D.S., M.S.**** Medical University of South Carolina, College of Dental Medicine, Charleston, SC., and University of North Carolina, School of Dentistry, Chapel Hill, N.C. The terminology describing “fit” and the techniques used for measuring fit vary considerably in the literature. Although fit can be most easily defined in terms of “misfit,” there are many different locations between a tooth and a restoration where the measurements can be made. In this work, the measurements of misfit at different locations are geometrically related to each other and defined as internal gap, marginal gap, vertical marginal discrepancy, horizontal marginal discrepancy, overextended margin, underextended margin, absolute marginal discrepancy, and seating discrepancy. The signiicance and difference in magnitude of different locations are presented. The best alternative is perhaps the absolute marginal discrepancy, which would always be the largest measurement of error at the margin and would reflect the total mislit at that point. (J PROSTHET DENT 1989;62:406-8.)
T
he marginal “fit” of any dental restoration is vital to its long-term success. Lack of adequate fit is potentially detrimental to both the tooth and the supporting periodontal tissues. Unlike physical and mechanical properties of materials, the fit of a restoration has never been strictly defined. The reference points for measurements *Assistant Professor, Department of Crown and Bridge, Medical University of South Carolina, College of Dental Medicine.
**Associate Professor, Department of Operative Dentistry, University of North Carolina, School of Dentistry. ***Professor, Department of Fixed Prosthodontics, University of North
Carolina,
School of,Dentistry.
****Part-time Associate Professor, Department of Fixed Prosthodontics, University of North Carolina, School of Dentistry. 10/l/12238
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
and the discriptive terminology defining fit vary considerably among investigators. Often the same term is used to refer to different measurements, or different terms are used to refer to the same measurement. This is a constant source of confusion in reporting and comparing studies. Studies have reported measurement of fit relative to marginal adaptation, lW3internal adaptation,*? 6 vertical seating,6-E radiographic appearance,g and clinical adaptability as judged by experienced practitioners.eT lo Two common techniques are measurement of embedded and sectioned specimens,ly 11,l2 and measurement of specimens (or their replicas) by direct visuahzation?*13 Mechanical devices, such as the tracing jig14 to measure relative distortion at the margin during porcelain firing cycles, have also been used frequently.
Several studiessp l5 have evaluated fit
405
4. Pipko DJ, Fine D, Sadeek M. Vacuum formed temporary splints. Dent Dig 1970;76:270-4. 5. Sotera AJ. A direct technique for fabricating acrylic temporary crowns using the Omnivac. J PROSTHETDENT 1973;29:577-80. 6. Halterman CA. Creating and developing a temporary acrylic crown technique. Quintessence Dent Tech 1981;5:3-8. 7. Breeding LC. Indirect temporary acrylic restorations for fixed prosthodontics. J Am Dent Assoc 1982;105:1026-7. 8. Cannistraci AJ. A new approach to impression taking for crown and bridge. Dent Clin North Am 1965; Mar:33-42. 9. Amsterdam M, Fox L. Provisional splinting-principles and technics. Dent Clin North Am 1959;Mar:73-99. 10. Miller IF, Belsky MW. The full shoulder preparation for periodontal health. Dent Clin North Am 1965;Mar:83-102.
Systematized
procedure
11. Hoffman JM. Replication by silicone materials in crown and bridge construction. NY State Dent J 1987;53:40-3. 12. Kile WL. An improved periodontal dressing technique. the acrylic splint. J Mich State Dent Assoc 1968;50:20-3. 13. Warnick ME. Cast crown restoration of a badly involved abutment to fit an existing removable partial denture. Dent Clin North Am 1970;14:631-44. Reprint
requests
to:
DR. JULE~HOFFMAN SCHOOLOFDENTALANDORALSURGERY COLUMBMUNIVERSITY NEW YoRK,NY 10032
of crown preparation
E. V. Bass, M.D.Sc.,* and M. C. Kafalias, B.D.S., M.D.S., University of Sydney, Faculty of Dentistry, Sydney, Australia
M.S.D.**
The theoretical considerations of the marginal fit of crowns are discussed and the conventional approach in tooth-cutting procedures is addressed. Gingival marginal surface irregularities described as “peak and valley” and “stepping” effects can leave a cement line in excess of 80 pm after cementation. A regimented method of tooth reduction for metal ceramic crowns, using specially designed rotary cutting instruments, is described. The suggested technique accomplishes the crown preparation more rapidly with a consistent standard of excellence. (J PROSTHET DENT 1989;62:400-6.)
F
or decades, various techniques for tooth preparation of intracoronal and extracoronal restorations have been proposed. In addition, thousands of shapes of commercial diamond and tungsten carbide rotary burs are available. The current equipment and materials are unquestionably superior to what was available a few years ago. However, the dentist is disconcerted by the infinite array of instruments and how they relate to uncomplicated, efficient tooth preparation that is comfortable for the patient. A technique is suggested using the instruments and methods advocated for the postdoctoral students at the University of Sydney School of Dentistry that have produced predictable, reliable results.
TOOTH PREPARATION The optimal tooth preparation is elusive and rarely in the complete control of the dentist. The length, size, and angulation of the tooth, the remaining tooth structure, and surface area for retention are commonly compromised. Parallelism of the preparation has received the greatest attention because the taper of the preparation is more *Senior Registrar. **Associate Professor, Department of Operative Dentistry. 10/l/13381
400
commonly determined by the dentist than by other fact0rs.l The degree of taper and its effect on retention has been widely researched, from in vitro studies with precisely controlled conditions2-4 to measuring the taper of dies used for clinical fixed restorations.5y 6 Most authors recommend a taper range of 2 to 7 degrees,5>7 but it is difficult to visually determine whether a preparation is free of undercuts at less than a taper of approximately 17 degrees. One studys found that the convergence angles of clinical tooth preparation for single crowns varied from 12 to 37 degrees. A uniformity of axial tooth reduction should be achieved for sufficient space of the restoring material, for example metal and porcelain. Experience has suggested that an axial reduction of preparation from 1.2 to 1.5 mm is suitable if a metal ceramic is placed, a 1.5 to 2 mm reduction of the occlusal surface, and a minimum of 0.5 mm for metal surfaces. The facial reduction should be uniform from mesial to distal proximal contacts of the preparation. The occlusal reduction of the tooth should establish a 1.5 to 2 mm clearance from the opposing dentition in the excursive movements of the mandible. The most critical part of tooth preparation is the development of the gingival margins. The casting should fit the tooth preparation satisfactorily while allowing adequate
OCTOBER1989
VOLUME62
NUMBER4
SYSTEMATIZED
Fig.
PROCEDURE
OF CROWN
PREPARATION
1. Facial surface with gingival orienting groove.
space for a luting agent on the axial and occlusal surfaces of the preparation. The minimum cement film thickness is approximately 20 brn. The casting should be accomplished in the laboratory with a controlled “passive fit” of approximately 40 pm or 20 pm space on the opposite sides of the preparation. If the casting “slide-fits,” or worse, “pressure-fits,” the cement commonly prevents a precise seating of the restoration, resulting in a disappointing marginal fit and elevated occlusion. A well-defined loose fit of the restoration is obtained by treating the stone die with a die spacer that is applied to the axial and occlusal/incisal surfaces. This ensures the precise contact of the casting to the tooth at the crucial gingival margin. Jorgensen and Finger9 have described in detail their disciplined method of producing more consistent, acceptable castings. Theoretically, a casting with a cement film of approximately 20 pm is ideal, but a film of 50 pm does not present a noticeable biological risk. lo A cement line of 50 grn is the limit that can be observed visually under favorable conditions, and a gingival gap of this size can just be detected with a sharp, pointed explorer.
Surface
roughness
The tooth preparation should not be finished with burs that produce a smooth surface, which compromises retention, or excessive roughness, which results in marginal inaccuracy. A surface prepared with a coarse-grained diamond point leaves a roughness index (Rt) of 35 km, which is clinically unacceptable. (Rt is the distance from the highest point to the lowest point on the rough surface.) After completion of a surface with a medium-grained diamond point, a roughness of approximately 20 wrn is evident. Normally this roughness on the axial surfaces provides sufficient retention of the cemented restoration. The cement particles are lodged between the grooves without interfering with marginal adaptation and retain the restoration.”
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig.
2. Linguai surface with lingual chamfer groove.
Fig.
Gingival
3. Proximal surface reduction.
marginal
surface
The critical features in finishing the gingival part of the preparation are as follows: 1. Enamel rods are angled at approximately 100 to 110 degrees to the long axis of the tooth but 90 degrees at a cavosurface margin. The important angulation is the cavqsurface angle (to the external surface of the tooth) and not the angle to the prepared axial surface. This is crucial but is not usually clarified in descriptions of tooth preparations. The unsupported enamel rods are dislodged at this cavosurface margin of the preparation during cementation of the restoration, resulting in a marginal gap. 2. The gingival surface of the preparation should be finished with a substantially smaller Rt than the axial wall. If the Rt at the gingival part of preparation is greater than 20 Km it is recorded by the impression materials and reproduced on type II die stone material.‘” This roughness is transferred and remains on the gingival surface of the casting. Because it is impossible to interdigitate the peaks of a casting surface into the valleys of a tooth preparation, peak-to-peak interfaces of casting to preparation surfaces are common. 401
BASS
Fig. 4. Facial surface with depth-orienting
AND
KAFALIAS
grooves.
Fig. 6. A, Lingual cingulum axiai surface reduction. B, Concave lingual surface reduction. Fig. 5. Completion of labial reduction. If the Rt at this gingival surface is 30 pm with a peak-topeak microsurface interface, there is a gap between the valleys equaling 60 km. A luting agent has a minimal thickness of 20 pm and if this is added to the 60 pm gap between the valleys, a cement film thickness of 80 pm results. Tooth preparation with a flat-ended tip of a mediumcoarse diamond bur creates a “stepping”on the shoulder of the preparation. This is especially evident on the concave labial shoulder and results in a greater discrepancy between the casting and the tooth. Therefore it is advantageous to produce a smooth, even surface at the gingival surface of the preparation. A tungsten carbide finishing bur (20,000 rpm) produces a surface smoothness of approximately 2 to 5 prn.l”p l3 If this tungsten carbide bur is directed to the shoulder of the preparation in a horizontal plane (shank secured at right angles to the long axis of the tooth), the stepping effect can be controlled, creating a smooth surface without a peakand-valley effect.
Tooth preparation for metal ceramic
on upper central crown
incisor
Step 1. Facial surface (gin&al orienting groove). A high speed, medium-grit diamond wheel (Fig. 1) is used 402
to make the initial square groove level with the crest of the gingiva, following the labial contour from proximal-toproximal surface. The bur is angled toward the tooth so that the shank of the bur contacts the tooth surface and a self-limiting depth of 0.5 mm is produced in the groove. This groove creates a uniform ledge but is not the final shoulder preparation that forms the gingival limitation to facial reduction. The shoulder is completed at a later stage. Step 2. Lingual surface (lingual chamfer groove). A fluted tungsten carbide finishing bur (Fig. 2) is used to develop a quarter-circular groove level with the crest of the lingual gingiva, following the gingival tissue contour from proximal-to-proximal surface similar to step 1. In this instance the shape of the cutting surface of the bur produces a smooth chamfer-finishing margin to a depth of 0.5 mm and with a surface h of 3 to 5 pm. The bur speed is 15,000 to 20,000 rpm without a coolant. This sequence of the preparation becomes the final lingual chamfer. Step 3. Proximal surface reduction. A high-speed, round-ended, medium-grit diamond bur of 3 degrees taper on each side and 0.5 mm diameter at the tip is used to prepare the proximal surfaces of the tooth (Fig. 3). The cut is made level with the interproximal papilla from labial gingival orienting groove to lingual chamfer groove. The long axis of the bur is held parallel to the path of insertion, al-
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1989
VOLUME
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NUMBER
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sYsrEmAnzeD PROCEDURE 0F cRowN PREPARATION
Fig. 7. Incisai reduction.
lowing the 3-degree taper of the cutting surface to produce an optimal path of insertion. A rounded ledge is produced on the gingival proximal surface of the tooth. The gingival finishing margin is completed later. Step 4. Facial surface and depth-orienting grooves. A medium-grit, high-speed, self-limiting diamond tip is used to establish the depth-orienting grooves on the labial surface. The diamond cutting tip is 1 mm in diameter and 1 mm long. The bur is held at right angles to the labial surface and 3 or 4 grooves are placed (Fig. 4). Each groove originates at the facial gingival orienting groove and extends to the incisal edge. The self-limiting design of the cutting point produces a controlled contoured depth. Step 5. Completion of facial reduction. A high-speed, flat-ended, medium-grit diamond point with a 3-degree taper on each side and 1 mm tip diameter (Fig. 5) is used to remove the remaining facial tooth structure by joining the contoured depth-orienting grooves. This is accomplished in two stages. The gingival half is completed first and the long axis of the bur is held parallel to the path of insertion for the restoration. It is essential that this is completed before the incisal half is prepared and before the lingual axial surface is prepared so that the maximum available vertical tooth structure faciolingually can be preserved for optimal taper. This bur creates a 1 mm wide gingival shoulder that is at a go-degree angle to the faciogingival half of the preparation. The incisal half of the labial surface is completed by joining the depth orienting grooves. Step 6. Lingual surface reduction. Lingual cingulum axial surface reduction. A high-speed, smooth-ended, medium-grit diamond point of 3-degree taper on each side and 1 mm diameter at the tip is used to reduce the lingual cingulum axial surface (Fig. 6, A). This bur is held parallel to the same path of insertion as for the gingival half of the facial surface preparation. The smooth end is applied to the finished chamfer established in step 2 and is moved from
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Fig. 8. Proximal axial line angles preparation.
Fig. 9. Finishing the labial shoulder. proximal surface to proximal surface, following the curvature of the tooth. Concave surface. A high-speed, oval-shaped, round-tip, medium-grit diamond point is used to reduce the concave surface of the lingual surface from cingulum to incisal edge (Fig. 6, B). Generally 1 mm of tooth structure is removed. The amount of tooth structure removed in depth depends on the design of the metal framework of the restoration and should also be checked during excursive movements of the mandible. Step 7. Incisal reduction. The oval-shaped diamond point seen in Fig. 6, B is used to reduce the Incisal edge so that it will be 1.5 mm below the incisal edge of the adjacent central incisor or, if the adjacent central tooth is absent, to 1.5 mm below the estimated incisal level of the final crown (Fig. 7). Step 8. Proximal axial line angles. A high-speed, round-ended, medium-grit diamond bur I mm in diameter at the tip with a 3-degree taper on each side is then used to blend the proximal axial line angles into the buccal and lingual surfaces (Fig. 8). Step 9. Finishing the labial shoulder. A slow-speed (10,000 to 20,000 rpm) tungsten carbide finishing bur is used to reshape the gingival ledge of the facial surface in 103
BASS
Fig. 10. Finishing the interproximal
shoulder.
Fig. 11. Teeth before preparation.
step 5. This bur is held at right angles to the labial surface. The angulation of the cutting blades produces a slope of 110 to 120 degrees to the faciogingival half of the axial wall of the tooth preparation that is the angulation of the enamel rods. This labial slope is finished 0.5 mm below the crest of the gingiva (Fig. 9). This bur has a rounded polished end that prevents cutting of the axial wall of the preparation during the necessary changes of the angulation of the bur in the horizontal plane while the shoulder is refined. The bur is also sloped and polished from the shank to the cutting blades, allowing displacement of the free gingiva without trauma and controlled placement of the margin 0.5 mm below the crest of the gingiva. The horizontal application of this bur smoothes the surface roughness produced by the medium-grit diamond bur and removes unsupported enamel rods on the cavosurface margin. The surface roughness of this facial slope has an Rt of 3 to 5 pm and the stepping effect produced’ by an endcutting rotary instrument is minimized. Step 10. Interproximal shoulder. A tungsten carbide end-cutting finishing bur, parallel-sided, with a lo-degree bevel at the cutting end is used at a speed of 15,000 to 30,000 rpm without a coolant to finish the interproximal shoulder. 404
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Fig. 12. Teeth prepared for metal ceramic crowns.
The bur is moved faciolingually from the proximal surface of the completed facial shoulder to the finished lingual chamfer. It is maintained at an angle to the long axis of the preparation so that the cutting end lies along the direction of the enamel rods. The lo-degree bevel prevents the corners of the end-cutting surface from producing undercuts in the gingivoaxial line angle (Fig. 10). Fig. 11 demonstrates endodontically treated maxillary central incisor teeth with palatally displaced lateral incisors. The maxillary central and lateral incisors were prepared by using the method and cutting burs described, with a total preparation time of 1 hour and 22 minutes. Fig. 12 emphasizes the smoothness of the facial shoulders compared with the horizontal grooves on the facial axial walls of the prepared teeth.
Posterior
teeth
The preparation of posterior teeth for a complete gold crown or a metal ceramic crown is similar to the maxillary central incisor. Unless the esthetic requirements of a particular patient are demanding, a metal collar 1 to 1.5 mm wide is placed on the facial surface for metal ceramic crowns on posterior teeth.
CONCLUSIONS Standard tooth preparation for a crown requires undivided concentration simultaneously in the three planes of preparation: length, depth, and width. The medium-grit diamond burs used to prepare the gingival surface produce irregularities described as peak-andvalley and stepping effects that can leave a cement line in excess of 80 pm after cementation. The optimal fit of crowns at the gingival margin require tooth preparation finished with tungsten carbide finishing bum producing an Rt of approximately 5 pm, eliminating the peak-and-valley effect. An innovative application of a cutting rotary bur to finish the facial shoulder and eliminate the stepping effect was described. Programmed tooth preparation using specially designed cutting burs reduces the diversity of concentration to one
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VOLUME
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plane of preparation at a time. This technique accomplishes the tooth preparation more rapidly, with a reliable, predictable standard of excellence. REFERENCES 1. Thayer KE. Fixed prosthodontics. Chicago: Year Book Medical Publishers, 1984;31-6. 2. Rosenteil E. The taper of inlay and crown preparations. Br Dent J 1975;139:436-8. 3. Abdullah SI, Mohammed H, Thayer KE. Factors in the failure of cemented full crowns. J Can Dent Assoc 1974;40:721-4. 4. Woolsey GD, Matich IA. The effect of axial grooves on the resistance form of cast restorations. J Am Dent Assoc 1978;97:978-80. 5. Mack PI. A theoretical and clinical investigation into the taper achieved on crown and inlay preparations. J Oral Rehabil 1980,7:255-65. 6. Hyde PF. The mechanical principles of cavity preparation for the cast gold restoration [M.D.S. thesis]. Sydney: University of Sydney, 1984. 7. Shillingburg HT Jr, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981;7983.
Considerations
in measurement
8. Ohm E, Silness J. The convergence angle in teeth prepared for artificial crowns. J Oral Rehabil 1978;5:371-5. 9. Jorgensen KD, Finger W. Die-spacing technique by &fusion precipita tion. Stand J Dent Res 1979;87:73-8. 10. Jorge” D. Efficient and high-quality cavity preparation ergonomy preparation sets. Lemgo. West Germany:Gehr Rrasseier Gmhh & Co KG, 1981;4. 11. Felton BA, Kanoy BE, White JT. Surface roughness of crown preparation: effect on cemented casting retention [Ahs~ract]. J Dent Ree 1986;65:312. 12. Craig RG. Restorative dental materials. St T,ctws: CV Mosbv Co, 1980;203. 13. G&art E. The Rio-set will make things a lot easier r’or vou. Dusseldorf, West Germany: Hager & Meisinger Gmbh. 1985. Reprint requests toDR. M. C. KAFALIAS UNITED DENTAL HOSPITAL UNIVERSITY OF SYDNEY 2 CHALMERS ST. SYDNEY 2010 AUSTRALIA
of marginal
fit
J. Robert Holmes, D.D.S., M.S., M.Ed.,* Stephen C. Bayne, M.S., Ph.D.,** Gene A. Holland, D.D.S., M.S.,*** and William D. Sulik, D.D.S., M.S.**** Medical University of South Carolina, College of Dental Medicine, Charleston, SC., and University of North Carolina, School of Dentistry, Chapel Hill, N.C. The terminology describing “fit” and the techniques used for measuring fit vary considerably in the literature. Although fit can be most easily defined in terms of “misfit,” there are many different locations between a tooth and a restoration where the measurements can be made. In this work, the measurements of misfit at different locations are geometrically related to each other and defined as internal gap, marginal gap, vertical marginal discrepancy, horizontal marginal discrepancy, overextended margin, underextended margin, absolute marginal discrepancy, and seating discrepancy. The signiicance and difference in magnitude of different locations are presented. The best alternative is perhaps the absolute marginal discrepancy, which would always be the largest measurement of error at the margin and would reflect the total mislit at that point. (J PROSTHET DENT 1989;62:406-8.)
T
he marginal “fit” of any dental restoration is vital to its long-term success. Lack of adequate fit is potentially detrimental to both the tooth and the supporting periodontal tissues. Unlike physical and mechanical properties of materials, the fit of a restoration has never been strictly defined. The reference points for measurements *Assistant Professor, Department of Crown and Bridge, Medical University of South Carolina, College of Dental Medicine.
**Associate Professor, Department of Operative Dentistry, University of North Carolina, School of Dentistry. ***Professor, Department of Fixed Prosthodontics, University of North
Carolina,
School of,Dentistry.
****Part-time Associate Professor, Department of Fixed Prosthodontics, University of North Carolina, School of Dentistry. 10/l/12238
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and the discriptive terminology defining fit vary considerably among investigators. Often the same term is used to refer to different measurements, or different terms are used to refer to the same measurement. This is a constant source of confusion in reporting and comparing studies. Studies have reported measurement of fit relative to marginal adaptation, lW3internal adaptation,*? 6 vertical seating,6-E radiographic appearance,g and clinical adaptability as judged by experienced practitioners.eT lo Two common techniques are measurement of embedded and sectioned specimens,ly 11,l2 and measurement of specimens (or their replicas) by direct visuahzation?*13 Mechanical devices, such as the tracing jig14 to measure relative distortion at the margin during porcelain firing cycles, have also been used frequently.
Several studiessp l5 have evaluated fit
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