- Email: [email protected]
The taper of clinical preparations for fixed prosthodontics
The taper of clinical prosthodontics
preparations
Jeffrey Nordlander, D.D.S.,” Shigeru O&i, Ph.D.****
Weir, D.D.S., M.A.,**
Veterans
Administration
Medical
Dennis Center,
San Francisco,
M
AND
METHODS
Teeth were prepared by the participants to attempt a 4- to 1O-degree convergence angle. The dies examined in this study were collected from clinically successful crowns and fixed partial dentures made for either all-metal or ceramometal crowns. A total of 88 working dies including full-coverage preparations by eight general practice residents (GPRs) and 120 dies by two prosthodontists (specializing in fixed prosthodontics) were included. The dies were sorted into maxillary and mandibular groups and further divided into anterior,
*Former Prosthcdontic Resident; currently private practice, Sacramento, Calif. **Director, Prosthodontic Residency Program; Associate Clinical Professor, University of California, San Francisco, School of Dentistry. ***Staff Prosthodontist; Clinical Associate Professor, University of Pittsburgh, School of Dental Medicine, Pittsburgh, Pa. l ***Biostatistician, Veterans Administration Medical Center, Palo Alto, Calif. 148
Warren
Stoffer, D.M.D.,**+
and
Calif.
any dental schools and textbooks advocate preparation designs that include an ideal wall taper of 2 to 5 degrees per side for fixed prosthodontics.’ In addition to serving as a clinical objective, wall taper in this ideal range has been used in numerous laboratory studies where standardized dies or preparations were used to evaluate fixed prosthodontic materials or techniques. The ideal wall taper is based on clinical observation and laboratory research begun by Jorgenson2 and expanded by Kaufman et al.3 A strong relationship exists between the retentive force of cemented castings and the convergence angle of machined test dies. The convergence angle equals the sum of the taper of two opposing preparation walls, thus if opposing walls each taper 2 to 5 degrees, the convergence angle would equal 4 to 10 degrees. Ohm and Silness4 and Weed et al.5 also studied the convergence angle of crown preparations done by dental students under supervision. This study measures the convergence angles of fullcoverage preparations performed in a clinical environment.
MATERIAL
for fixed
premolar, and molar categories. It was noted whether the preparations were for single crowns or fixed partial denture retainers. The convergence angles of all of the preparations were determined by projecting the faciolingual and mesiodistal silhouettes of the dies with an overhead projector (Fig. 1). All of the dies were positioned and stabilized with clay over the lens by one investigator to record the outline of the appropriate side. The sharp image focused on paper was traced. Lines were drawn parallel to the traced axial walls in the gingival one third and extended until they met to form an angle above the tracing. This angle was measured with a protractor to give the convergence angle (Fig. 2). Some tracings presented slightly curved axial walls that required interpolation in placing lines. Examiner reliability was evaluated by having two investigators draw the reference lines and measuring the resulting convergence angles. Variation was + 2 degrees. The data were analyzed for differences in means among examiners and among tooth positions by using analysis of variance techniques.
RESULTS The mean convergence angles of the two groups of investigators’ preparations show no statistically significant difference (19.6 degrees and 20.1 degrees; p > .37) (Table I). The data related to the GPR and prosthodontists were pooled to increase the power of determining differences in mean convergence angle readings for the different views. The sample size for each group was based on the desire to detect a 2-degree difference for an expected variance of 100 when the type I error is 5% (alpha = .005). These data are presented graphically in Fig. 3. Anterior to posterior positions for maxillary and mandibular arches combined are shown in the first three columns. The comparison among positions is significantly different for both the faciolingual and mesiodistal aspects 0, < .OOl). The comparison of the maxillary versus mandibular arches is shown in the next two columns and is significantly different (p < .Ol). In addition, the difference between fixed partial denture retainer preparations and crown preparations is presented (p > .23). AUGUST
1988
VOLUME
60
NUMBER
2
TAPER
OF CLINICAL
PREPARATIONS
Fig. 1. Projection
of preparation
taper.
Patient: No. 38 Tooth: No. 8
Table I. Mean faciolingual
(F-L) and mesiodistal (M-D) convergence angles of dies Anterior
Maxillary Residents (N = 51) Prosthodontists (N = 64) Total (N = 115) Mandibular Residents (N = 37) Prosthodontists (N = 57) Total (N = 94)
Premolar
Restoration:crown
Molar
F-L
M-D
F-L
M-D
F-L
18.4
17.2
13.3
17.0
21.6
18.5
19.3
12.6
16.8
15.9
24.6
24.8
19.0
14.3
14.6
16.6
23.4
22.4
26.7
18.0
18.2
16.6
27.7
30.4
20.3
17.6
17.4
17.2
26.1
26.9
23.1
17.8
17.7
17.0
26.6
28.0
An@: F-L: 14’
M-D
Prep.
DISCUSSION Mean convergenceanglesin the ideal range of 4 to 10 degreeswere not obtained. An analysisof the tracings for the percentage of preparations achieving ideal convergenceangle in a given view (Table II) showshow rarely a single preparation achieved the ideal in both faciolingual and mesiodistalviews. Further, certain dimensions were more difficult to prepare to ideal taper than others. Anterior teeth were rarely prepared to an ideal taper in the faciolingual dimension.The difficulty in that part of the mouth is probably related to the short cingula normally found on anterior teeth. The data on convergence angles show other trends as well. First, mean mandibular convergenceangleswere greater in general THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 2. Example of convergence angle worksheet. FM-D = mesiodistal.
L = faciolingual;
than were maxillary angles. Second,premolar preparations tended to have smaller anglesthan anterior preparations, which had smaller anglesthan molar preparations. These differences were especially evident in the mandibular arch. The combined means scoresfor convergence of the 149
NORDLANDER
q n
Fig.
of preparations with convergence angles within the ideal range (4 to 10 degrees)
Maxillary Anterior Premolar Molar Mandibular Anterior Premolar Molar F-L
= Faciolingual;
M-D
M-D
9.4 35.5 0.0
30.2 25.8 3.2
8.0 15.6 2.7
16.0 15.6 0.0
Both
1.9 16.1 0.0 4.0 6.25 0.0
= mesiodistal.
participants in this study (19.6 degrees for residents and 20.1 degrees for prosthodontists) are comparable to those reported in previous studies. Ohm and Silnes$ (mean convergence angle of 21.1 degrees) and Weed et aL5 (mean of 21.6 degrees) reported scores from preparations done by dental students. The close comparison between those studies and the current study indicates that the skill of the operator is not a governing factor in preparation taper. Other factors may account for the differences in convergence angles noted among categories in this study. Access, particularly in the molar region is an obvious factor. In the anterior region where access is optimal, mesiodistal convergence angles still exceeded the ideal range, suggesting that visual assessment of taper may be a limitation. The range of convergence angles produced clinically in a segment of an arch is fairly small, and the visual sense of taper is affected by length of the preparation. This illusion can also be demonstrated by 150
Mesiodistal Faciolingual
Taper of clinical preparations.
3.
Table II. Percentage
F-L
ET AL
looking at the end of a sharpened pencil, which most dentists would consider an over-tapered preparation. Yet the convergence angle of the pencil is approximately 20 degrees, close to the mean angle values found in this and other studies. The method of judging preparation taper might be another visual limitation. Typically, a dentist holds a mirror at a fixed point above the prepared tooth and looks at all surfaces to ensure a compatible draw. In anterior regions where interarch distance is maximal, that point can be far enough from the tooth to permit visualization of less-tapered walls. However, teeth in the molar region are more difficult to visualize and evaluate because their greater circumference separates the prepared walls and requires the sight lines to extend further above the tooth to meet and form the visual cone for evaluation. The reduced interarch space in this region may require increasing the wall taper to bring the convergence point to a location that can be seen in mirror evaluation. The anatomy of the tooth is another factor. Short lingual surfaces of anterior teeth and lingual inclination of lower posterior teeth may explain larger mean convergence angles in preparations. Reduction of axial surfaces in two planes also produces a short wall surface in which to judge desired taper. Strong differences in preparation taper between individual restorations and fixed partial dentures were not shown. Increasing the convergence angle to achieve a common path of draw for multiple preparations is not a factor in preparation taper.
SUMMARY Convergence angles of full-coverage preparations were measured in a clinical environment and compared AUGUST
1988
VOLUME
60
NUMBER
2
TAPER
OF CLINICAL
PREPARATIONS
with each other and the ideal taper of 4 to 10 degrees. Despite educational emphasis, the practical application of preparation design routinely exceeds the ideal taper and casts a different light on retention and resistance characteristics described in both laboratory and theoretical work. Comparison of preparations done by residents and by prosthodontists in this study showed that ideal preparation taper is seldom achieved. Given the complex interrelationships of clinical, theoretical, and mechanical factors that determine the retention and resistance characteristics of a preparation in vivo, it is advisable to design preparations that blend retentive characteristics with functional demands. Because it is difficult to assess preparation taper intraorally, efforts should be directed to using other retentive devices, especially on posterior preparations where ideal taper is difficult to achieve.
CONCLUSIONS 1. The ideal convergence angle of 4 to 10 degrees is seldom achieved in clinical practice. 2. Mean convergence angles for mandibular preparations were greater than mean maxillary convergence angles. 3. Premolar convergence angles tended to be smaller than anterior convergence angles. Both were smaller than molar convergence angles. 4. No significant differences were found between the
Stain potential Michael
S. Lute, D.D.S.,*
The University
REFERENCES 1.
2. 3.
Tylman SD. Theory and practice of fixed prosthodontics. 7th Ed. St Louis: The CV Mosby Company, 1978;108. Jorgensen KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Stand 1955;13:35-40. Kaufman EG, Coelho DH, Colin L. Factors influencing retention of cemented gold castings. J PROSTHET DENT 1961;11:487-
502. 4. Ohm E, Silness J. The convergence 5.
angle in teeth prepared for artificial crowns. J Oral Rehabil 1978;5:371-5. Weed RM, Suddick RP, Kleffner JH. Taper of clinical and typodont crowns prepared by dental students [Abstract]. J Dent Res 1984;63:286.
Reprint requests to: DR. DENNIS J. WEIR DIRECTOR, PROSTHODONTIC DENTAL 160
RESIDENCY
VETERANS
MEDICAL
ADMINISTRATION
CENTER
4150 CLEMENT ST. SAN FRANCISCO, CA 94121
composites
and Charles E. Campbell**
of Texas Health Science Center, Dental Branch, Houston, Tex.
tain on microfilled compositesis a clinically manifest problem. Maintenance of the esthetic properties of microfilled compositesmay determine the clinical life span of these restorative materials. Staining of m:icrofilled compositerestorations may be related to the type of matrix resin, particle size and percent of filler, degree of light curing, length of time between finishing and polishing, degree of a glossy smooth-polishedsurface, water sorption, typ of staining agent, and duration of contact with the staining agent. *Lieutenant, U.S. Navy Tex. **Senior dental student. JOURNAL
We thank Ms. Hilary Pritchard, Department of Restorative DenSan Francisco, for her editorial tistry, University of California, assistance.
of four microfilled
S
THE
mean convergence angles of crowns and fixed partial denture retainers. 5. Auxiliary retention should be used in the molar region because these preparations were found to have large convergence angles.
Reserve,
OF PROSTHETIC
Dental
DENTISTRY
Corps,
Corpus
Christi,
This study compares the staining potential of four substancescommonly thought of as “stainers” that are repeatedly usedthroughout the day and on a daily basis and compares and correlates the stainability of four commonly usedmicrofilled composites.
MATERIAL
AND METHODS
Four microfilled compositeshaving similar qualities were selected:Durafill (universal shade,batch No. 141, Kulzer Inc., Irvine, Calif.), Heliosit (shade22, Vivadent Inc., Buffalo, N.Y.), Prisma Microfine (shadeLY, batch No. B-66, The L. D. Caulk Co., Milford, Del.), and Silux (shade universal, batch No. 5502u, 3M Co., St. Paul, Minn.). 151
preparations
Jeffrey Nordlander, D.D.S.,” Shigeru O&i, Ph.D.****
Weir, D.D.S., M.A.,**
Veterans
Administration
Medical
Dennis Center,
San Francisco,
M
AND
METHODS
Teeth were prepared by the participants to attempt a 4- to 1O-degree convergence angle. The dies examined in this study were collected from clinically successful crowns and fixed partial dentures made for either all-metal or ceramometal crowns. A total of 88 working dies including full-coverage preparations by eight general practice residents (GPRs) and 120 dies by two prosthodontists (specializing in fixed prosthodontics) were included. The dies were sorted into maxillary and mandibular groups and further divided into anterior,
*Former Prosthcdontic Resident; currently private practice, Sacramento, Calif. **Director, Prosthodontic Residency Program; Associate Clinical Professor, University of California, San Francisco, School of Dentistry. ***Staff Prosthodontist; Clinical Associate Professor, University of Pittsburgh, School of Dental Medicine, Pittsburgh, Pa. l ***Biostatistician, Veterans Administration Medical Center, Palo Alto, Calif. 148
Warren
Stoffer, D.M.D.,**+
and
Calif.
any dental schools and textbooks advocate preparation designs that include an ideal wall taper of 2 to 5 degrees per side for fixed prosthodontics.’ In addition to serving as a clinical objective, wall taper in this ideal range has been used in numerous laboratory studies where standardized dies or preparations were used to evaluate fixed prosthodontic materials or techniques. The ideal wall taper is based on clinical observation and laboratory research begun by Jorgenson2 and expanded by Kaufman et al.3 A strong relationship exists between the retentive force of cemented castings and the convergence angle of machined test dies. The convergence angle equals the sum of the taper of two opposing preparation walls, thus if opposing walls each taper 2 to 5 degrees, the convergence angle would equal 4 to 10 degrees. Ohm and Silness4 and Weed et al.5 also studied the convergence angle of crown preparations done by dental students under supervision. This study measures the convergence angles of fullcoverage preparations performed in a clinical environment.
MATERIAL
for fixed
premolar, and molar categories. It was noted whether the preparations were for single crowns or fixed partial denture retainers. The convergence angles of all of the preparations were determined by projecting the faciolingual and mesiodistal silhouettes of the dies with an overhead projector (Fig. 1). All of the dies were positioned and stabilized with clay over the lens by one investigator to record the outline of the appropriate side. The sharp image focused on paper was traced. Lines were drawn parallel to the traced axial walls in the gingival one third and extended until they met to form an angle above the tracing. This angle was measured with a protractor to give the convergence angle (Fig. 2). Some tracings presented slightly curved axial walls that required interpolation in placing lines. Examiner reliability was evaluated by having two investigators draw the reference lines and measuring the resulting convergence angles. Variation was + 2 degrees. The data were analyzed for differences in means among examiners and among tooth positions by using analysis of variance techniques.
RESULTS The mean convergence angles of the two groups of investigators’ preparations show no statistically significant difference (19.6 degrees and 20.1 degrees; p > .37) (Table I). The data related to the GPR and prosthodontists were pooled to increase the power of determining differences in mean convergence angle readings for the different views. The sample size for each group was based on the desire to detect a 2-degree difference for an expected variance of 100 when the type I error is 5% (alpha = .005). These data are presented graphically in Fig. 3. Anterior to posterior positions for maxillary and mandibular arches combined are shown in the first three columns. The comparison among positions is significantly different for both the faciolingual and mesiodistal aspects 0, < .OOl). The comparison of the maxillary versus mandibular arches is shown in the next two columns and is significantly different (p < .Ol). In addition, the difference between fixed partial denture retainer preparations and crown preparations is presented (p > .23). AUGUST
1988
VOLUME
60
NUMBER
2
TAPER
OF CLINICAL
PREPARATIONS
Fig. 1. Projection
of preparation
taper.
Patient: No. 38 Tooth: No. 8
Table I. Mean faciolingual
(F-L) and mesiodistal (M-D) convergence angles of dies Anterior
Maxillary Residents (N = 51) Prosthodontists (N = 64) Total (N = 115) Mandibular Residents (N = 37) Prosthodontists (N = 57) Total (N = 94)
Premolar
Restoration:crown
Molar
F-L
M-D
F-L
M-D
F-L
18.4
17.2
13.3
17.0
21.6
18.5
19.3
12.6
16.8
15.9
24.6
24.8
19.0
14.3
14.6
16.6
23.4
22.4
26.7
18.0
18.2
16.6
27.7
30.4
20.3
17.6
17.4
17.2
26.1
26.9
23.1
17.8
17.7
17.0
26.6
28.0
An@: F-L: 14’
M-D
Prep.
DISCUSSION Mean convergenceanglesin the ideal range of 4 to 10 degreeswere not obtained. An analysisof the tracings for the percentage of preparations achieving ideal convergenceangle in a given view (Table II) showshow rarely a single preparation achieved the ideal in both faciolingual and mesiodistalviews. Further, certain dimensions were more difficult to prepare to ideal taper than others. Anterior teeth were rarely prepared to an ideal taper in the faciolingual dimension.The difficulty in that part of the mouth is probably related to the short cingula normally found on anterior teeth. The data on convergence angles show other trends as well. First, mean mandibular convergenceangleswere greater in general THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 2. Example of convergence angle worksheet. FM-D = mesiodistal.
L = faciolingual;
than were maxillary angles. Second,premolar preparations tended to have smaller anglesthan anterior preparations, which had smaller anglesthan molar preparations. These differences were especially evident in the mandibular arch. The combined means scoresfor convergence of the 149
NORDLANDER
q n
Fig.
of preparations with convergence angles within the ideal range (4 to 10 degrees)
Maxillary Anterior Premolar Molar Mandibular Anterior Premolar Molar F-L
= Faciolingual;
M-D
M-D
9.4 35.5 0.0
30.2 25.8 3.2
8.0 15.6 2.7
16.0 15.6 0.0
Both
1.9 16.1 0.0 4.0 6.25 0.0
= mesiodistal.
participants in this study (19.6 degrees for residents and 20.1 degrees for prosthodontists) are comparable to those reported in previous studies. Ohm and Silnes$ (mean convergence angle of 21.1 degrees) and Weed et aL5 (mean of 21.6 degrees) reported scores from preparations done by dental students. The close comparison between those studies and the current study indicates that the skill of the operator is not a governing factor in preparation taper. Other factors may account for the differences in convergence angles noted among categories in this study. Access, particularly in the molar region is an obvious factor. In the anterior region where access is optimal, mesiodistal convergence angles still exceeded the ideal range, suggesting that visual assessment of taper may be a limitation. The range of convergence angles produced clinically in a segment of an arch is fairly small, and the visual sense of taper is affected by length of the preparation. This illusion can also be demonstrated by 150
Mesiodistal Faciolingual
Taper of clinical preparations.
3.
Table II. Percentage
F-L
ET AL
looking at the end of a sharpened pencil, which most dentists would consider an over-tapered preparation. Yet the convergence angle of the pencil is approximately 20 degrees, close to the mean angle values found in this and other studies. The method of judging preparation taper might be another visual limitation. Typically, a dentist holds a mirror at a fixed point above the prepared tooth and looks at all surfaces to ensure a compatible draw. In anterior regions where interarch distance is maximal, that point can be far enough from the tooth to permit visualization of less-tapered walls. However, teeth in the molar region are more difficult to visualize and evaluate because their greater circumference separates the prepared walls and requires the sight lines to extend further above the tooth to meet and form the visual cone for evaluation. The reduced interarch space in this region may require increasing the wall taper to bring the convergence point to a location that can be seen in mirror evaluation. The anatomy of the tooth is another factor. Short lingual surfaces of anterior teeth and lingual inclination of lower posterior teeth may explain larger mean convergence angles in preparations. Reduction of axial surfaces in two planes also produces a short wall surface in which to judge desired taper. Strong differences in preparation taper between individual restorations and fixed partial dentures were not shown. Increasing the convergence angle to achieve a common path of draw for multiple preparations is not a factor in preparation taper.
SUMMARY Convergence angles of full-coverage preparations were measured in a clinical environment and compared AUGUST
1988
VOLUME
60
NUMBER
2
TAPER
OF CLINICAL
PREPARATIONS
with each other and the ideal taper of 4 to 10 degrees. Despite educational emphasis, the practical application of preparation design routinely exceeds the ideal taper and casts a different light on retention and resistance characteristics described in both laboratory and theoretical work. Comparison of preparations done by residents and by prosthodontists in this study showed that ideal preparation taper is seldom achieved. Given the complex interrelationships of clinical, theoretical, and mechanical factors that determine the retention and resistance characteristics of a preparation in vivo, it is advisable to design preparations that blend retentive characteristics with functional demands. Because it is difficult to assess preparation taper intraorally, efforts should be directed to using other retentive devices, especially on posterior preparations where ideal taper is difficult to achieve.
CONCLUSIONS 1. The ideal convergence angle of 4 to 10 degrees is seldom achieved in clinical practice. 2. Mean convergence angles for mandibular preparations were greater than mean maxillary convergence angles. 3. Premolar convergence angles tended to be smaller than anterior convergence angles. Both were smaller than molar convergence angles. 4. No significant differences were found between the
Stain potential Michael
S. Lute, D.D.S.,*
The University
REFERENCES 1.
2. 3.
Tylman SD. Theory and practice of fixed prosthodontics. 7th Ed. St Louis: The CV Mosby Company, 1978;108. Jorgensen KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Stand 1955;13:35-40. Kaufman EG, Coelho DH, Colin L. Factors influencing retention of cemented gold castings. J PROSTHET DENT 1961;11:487-
502. 4. Ohm E, Silness J. The convergence 5.
angle in teeth prepared for artificial crowns. J Oral Rehabil 1978;5:371-5. Weed RM, Suddick RP, Kleffner JH. Taper of clinical and typodont crowns prepared by dental students [Abstract]. J Dent Res 1984;63:286.
Reprint requests to: DR. DENNIS J. WEIR DIRECTOR, PROSTHODONTIC DENTAL 160
RESIDENCY
VETERANS
MEDICAL
ADMINISTRATION
CENTER
4150 CLEMENT ST. SAN FRANCISCO, CA 94121
composites
and Charles E. Campbell**
of Texas Health Science Center, Dental Branch, Houston, Tex.
tain on microfilled compositesis a clinically manifest problem. Maintenance of the esthetic properties of microfilled compositesmay determine the clinical life span of these restorative materials. Staining of m:icrofilled compositerestorations may be related to the type of matrix resin, particle size and percent of filler, degree of light curing, length of time between finishing and polishing, degree of a glossy smooth-polishedsurface, water sorption, typ of staining agent, and duration of contact with the staining agent. *Lieutenant, U.S. Navy Tex. **Senior dental student. JOURNAL
We thank Ms. Hilary Pritchard, Department of Restorative DenSan Francisco, for her editorial tistry, University of California, assistance.
of four microfilled
S
THE
mean convergence angles of crowns and fixed partial denture retainers. 5. Auxiliary retention should be used in the molar region because these preparations were found to have large convergence angles.
Reserve,
OF PROSTHETIC
Dental
DENTISTRY
Corps,
Corpus
Christi,
This study compares the staining potential of four substancescommonly thought of as “stainers” that are repeatedly usedthroughout the day and on a daily basis and compares and correlates the stainability of four commonly usedmicrofilled composites.
MATERIAL
AND METHODS
Four microfilled compositeshaving similar qualities were selected:Durafill (universal shade,batch No. 141, Kulzer Inc., Irvine, Calif.), Heliosit (shade22, Vivadent Inc., Buffalo, N.Y.), Prisma Microfine (shadeLY, batch No. B-66, The L. D. Caulk Co., Milford, Del.), and Silux (shade universal, batch No. 5502u, 3M Co., St. Paul, Minn.). 151