2009-21-01-37-44-full - Saudi Dental Journal

ISSN (Print) 1013-9052
EISSN 1658-3558

The Saudi Dental Journal,
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Riyadh 11563,
Kingdom of Saudi Arabia

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2009-21-01-37-44-full

Assessment of convergence angle of full veneer preparations
carried out by practitioners with different levels of experience

Khalil Al Ali,* BDS, MSc Khalid A. Al Wazzan, ** BDS, MS Mohammad D. Al Amri, * BDS, MS, FRCDC
Abdul-Majeed Al-Shahrani, *** BDS Mohammed Al-Shahrani, *** BDS Hussain Al-Qahtani, *** BDS

Abstract

OBJECTIVES: To investigate the convergence angle (CA) of dies achieved by clinicians with different levels of experience and to study the effect of the number and location of abutments on the CA. MATERIALS AND METHODS: Two hundred and ninety one dies were obtained and mounted with the occlusal plane of the prepared teeth parallel to the floor. Photographs of facial and proximal aspects were taken for each die. The photographs were transferred into a personal computer and an AutoCAD software program was used to measure the bucco-lingual and mesio-distal CA of each preparation. Analysis of Variance (ANOVA) was used to test the significance at 95% confidence level. RESULTS: The greatest mean CA was in the teeth prepared by general practitioners (25.7°) while the smallest was in teeth prepared by prosthodontists (16.9°). Statistically significant difference was found among CA values of the three groups (P<.0001). Statistical significant difference was also found between the CA of the preparation for single crown and for fixed partial denture abutments. No statistically significant differences were found between the CA of the anterior and posterior tooth preparations. CONCLUSIONS: The prosthodontists produced a lower convergence angle of prepared teeth compared to students and general practitioners. There is a need for all groups to exert extra effort to reduce the CA during tooth preparation. The recommended 6° taper is difficult to be achieved clinically.

INTRODUCTION

Fixed dental prostheses are commonly recommended for the restoration of extensively damaged or missing teeth. The success of such restorations requires optimum tooth preparation.^1-3One of the fundamental principles of tooth preparation is the retention and resistance form.^1-3 Retention features prevent the dislodgement of the prosthesis along its path of insertion, whereas resistance features prevent prosthesis dislodgement when oblique, non-axial, forces act on the tooth.³
The convergence angle (CA) of a preparation is the calculated taper of two opposite axial walls.3 Textbooks recommended guidelines for an acceptable CA after tooth preparation varied from 4 to 14° with 6° frequently cited as ideal.^1-3 Jorgensen4 tested the retention of crowns at various CAs by applying a tensile force to a cemented crown and found that the maximum tensile retentive values were recorded at CA of 5°. Wilson and Chan5 found the highest retention of cemented cast crowns occurred with CAs between 6° and 12°.

Several clinical and laboratory studies showed the actual CA of most preparations to be greater than 12° (Table 1).^6-18 Eames et al.⁶ found a mean CA of 20° on 50 dies selected from a commercial laboratory. Leempoel et al.7 measured 132 dies selected from models in the laboratory of two dentists. They reported a mean CA
of 21°, with a range of 15.5° and 30.2°.⁷Nordlander et al.⁸ surveyed 208 tooth preparations by general practitioners and prosthodontists and reported an overall mean of 19.9°. Furthermore, Kent et al.⁹ evaluated the CA of 418 dies of preparations made over a span of 12 years by Shillingburg and reported a CA ranged from 8.6° to 26.6°, with a mean of 14.3°. Annerstedt et al.¹⁰ assessed the CA in 478 full crown preparations performed by general practitioners and dental students. They reported a mean CA of 21°.

Nevertheless, other studies of laboratory and clinical preparations made by dental students have shown greater CA than recommended in textbooks.^11-15 Ohm and Silness¹¹ measured the CA of teeth for artificial crowns prepared by final year dental students. They reported a mean CA of 19.2° mesio-distal and 23° bucco-
lingual for vital teeth, and 12.8° mesio-distal and 22.5° bucco-lingual for non vital teeth. Al-Omari and Al-Wahadni¹² measured the CA of full metal-ceramic crown preparations made by final-year dental students and found the mean CAs, facio-lingually and mesio-distally, were 22.4° and 25.3°, respectively. Ayad et al.¹³ compared the CA for complete crowns prepared by dental students at three dental schools. Their results showed the mean bucco-lingual CA ranged from 15.6° to 19.8°, whereas the mean mesio-distal CA ranged from 14.1° to 19.4°.¹³ Furthermore, Noonan and Goldfogel¹⁴ assessed 909 prepared teeth by dental students and reported a mean CAs of 19° for normal clinic conditions and 15.7° for examination conditions. Moreover, Poon and Smales¹⁵ found a CA of teeth prepared for cast crowns ranged from 21.4° to 27.8°.

Studies^16-18 found that dental students could produce a smaller CA on typodont teeth than on their clinical cases. Weed ¹⁶ found that dental students could develop tooth preparation with a CA of 12.7° on typodonts. However, their clinical preparation had a mean CA of 22.8°. Another study¹⁷ reported the mean CA of bench top preparations to be 13.5°, compared to 24.7° for the clinical examination cases. Smith et al.¹⁸ evaluated the CA of 321 plastic teeth prepared for full veneer crowns by preclinical dental students working under examination conditions. They reported a CA range from 12.2° to 19.3°.¹⁸
A prepared tooth with more parallel opposing walls has greater retention and more conservative tooth preparation.^1-5 However, a preparation with parallel walls is difficult to accomplish in the mouth without the risk of creating undercuts into the tooth preparation.^1-3,11 Therefore, Mack19 recommended a minimum CA of 12° to ensure the absence of undercuts.

There are different methods that have been used to measure the CA of a prepared tooth.^10-14,18,20 These methods include, for example, tool marker microscope, goniometric microscope and overhead projector. Also, the CA can be measured using a series of photocopied images of the same dies which could be magnified to allow precise measurement.¹⁴ A new technological method is using the AutoCAD software to measure the CA with a high degree of accuracy.^{21, 22} AutoCAD is an architecture engineering computer aided design software application for 2D and 3D design and drafting. AutoCAD includes a full set of basic solid modeling and 3D tools which have been used in several dental investigations.^23-36
Despite the several studies that investigated the CA, there is no information in the literature regarding the effect of the level of experience, number and location of abutments for fixed prosthesis on the CA especially in Saudi Arabia. Therefore, the present study was undertaken to investigate (1) the CA that can be achieved by clinicians with different levels of experience, (2) the effect of tooth location on the CA (anterior versus posterior teeth), and (3) the effect of single crown preparation versus fixed partial denture (FPD) abutments preparation on the CA.

Materials and Methods

Segmented master working casts of fixed dental prostheses with removable dies were randomly collected from private laboratories as well as from the production laboratory of College of Dentistry, King Saud University, Riyadh, Saudi Arabia. The casts were divided into three groups according to the educational qualifications of the clinicians. The first group was the final year undergraduate students in the College of Dentistry, King Saud University. The second group represented the general dentists working in private dental clinics in Riyadh while the third group comprised prosthodontists from the private sectors.

The dies of prepared tooth/teeth under investigation were fixed on the cast base before photographing with all the other teeth removed. A digital camera (Cyber-shot® S750 Digital Camera DSC-S750, Sony, Japan) with 7.2 mega pixel was mounted on a tripod stand (VCT-1500L tripod, Sony, Japan) to take the photographs according to manufacturer's instructions. Each die was placed at 10 cm distance away from the camera and the surveyor was used to position the occlusal surface parallel to the floor. For each die, two different aspects, facial and proximal, were photographed. All the photographing procedure was performed by a single investigator.

Photographs were then transferred into a personal computer and an AutoCAD program (AutoCAD 2005 version N.63.0, Autodesk Inc., CA 94903, USA) was used to measure the CA of each preparation. For each photograph, two lines were drawn over the right and left contours of the axial walls of the die, mid-buccal and mid-lingual of the proximal view or mid-mesial and mid-distal for the facial view. The lines were drawn from the finish line extended coronally. Another two lines were drawn parallel to the long axis of the tooth contingent with the internal finish line (Fig. 1). The angle created by the first and second lines (the line parallel to the long axis of the tooth and the line parallel to the axial walls) was measured to determine the CA of one side. The total CA of the preparation in each view was computed by adding the two sides' angles (Fig. 1).

All the CA measurements were performed by a single investigator who was trained on AutoCAD for calculating the CA with a calibration intraexaminer's
agreement above 95%. The collected data were analyzed and the descriptive findings were presented. Analysis of Variance (ANOVA) was used to test the significance at 95% confidence level.

Results

The study was on 291 master casts. One hundred and two (35%) dies were prepared by students, 110 (38%) dies were prepared by general practitioners, and 79 (27%) dies were prepared by prosthodontists. Furthermore, 175 (60%) dies were for single full veneer crown and 116 (40%) were abutments for FPD. In addition, 137 (47%) dies were for anterior teeth and 154 (53%) were for posterior teeth.

The means and standard deviations of the CA of the facial and proximal views for each group are listed in Table 2. The mean CA of the tooth preparations for the students was 19.2°, for the general practitioners, was 25.7° and for the prosthodontists was 16.9°. The differences in the mean CA between the general practitioners and the prosthodontists was statistically significant (P<.0001). The overall mean CA for all the groups were 21°. Furthermore, there was significant difference between the mean CA of the proximal view (23.4°) and the facial view (18.6°) (P<.001).

No statistically significant difference was found between the mean CA for both the anterior teeth preparations (21.4°) and for posterior teeth (20.7°) (P= .641). Moreover, there was a statistical significant difference in the mean CA between the teeth prepared for single complete crown (22°) and those prepared as FPD abutments (19.6°) (P<.001).

Discussions

The CA is one of other several factors that may affect the adequacy of tooth preparation. Many dental schools and textbooks advocate an ideal CA of 4° to 10° for crowns and FPD abutments.^1-3 The results of the present study showed that all of the three groups produced higher values of CA than the recommended angles. These results are in concurrence with previous laboratory and clinical studies.^7-18 It appears that the textbook recommendations are theoretical and difficult to achieve in clinical practice. However, more concern, time and effort should be assumed by all the clinicians during tooth preparation. Goodacre et al.³¹ proposed that the ideal convergence angle for preparing teeth for complete crown should range between 10° and 20°.

The results of this study showed that the mean CA achieved by students was 19.2°. The results are in concurrence with other studies.^12-17,20 Al-Omari and Al-Wahadni¹²reported a mean CA of 23.9° for teeth prepared by final-year dental students. Similarly, Weed et al.¹⁷ reported the mean CA of 21.6°. In addition, Weed¹⁶found that dental students could develop tooth preparation with a CA of 12.7° on typodont teeth, but their clinical preparation had the mean CA of 22.8°.

Although the general practitioners have more dental clinical experience than undergraduate students, yet they produced the highest CA. These results were consistent with the results of Annerstedt et al.¹⁰ who reported CA for general practitioners and students of 22.1° and 19.4°, respectively.¹⁰ This could be explained by the fact that undergraduate students were under close and direct supervision of the faculty during the clinical procedures and the students' concern for having better grades. On the other hand, the observation of a larger CA produced by general practitioners in private clinics could be due to the limited time set for each patient or because of different clinical experiences. In the present study, although the prosthodontists achieved the smallest CA among the three groups, the produced angle values were higher than the recommended CA.

The findings of this investigation showed the mean bucco-lingual CA was greater than the mesio-distal CA in the three groups. This finding is in agreement with previous studies. ^{8,10-12,14,18,38} Ohm and Silness¹¹ found a mean CA of 19.2° mesio-distally and 23° bucco-lingually in vital teeth, and 12.8° mesio-distally and 22.5° bucco-lingually in non vital teeth. Al-Omari and Al-Wahadni¹² reported the mean mesio-distal and bucco-lingual CA were 22.4° and 25.3°, respectively. The same results were found by Djemal and Setchell³⁸ who reported a mean CA of 24.5° bucco-lingually and 17.18° mesio-distally. This could be due to the difficulty to inspect the buccal and lingual aspects of the crown preparation for undercuts especially the mandibular molars and therefore, it is likely to have greater CA.⁸Also, the anatomical curvature of the buccal surface and presence of the cheek and the tongue are other factors that might mislead the bur direction during preparation. Short lingual or palatal surfaces of anterior teeth and lingual inclination of lower posterior teeth may explain larger bucco-lingual CA. Finally, the proximal surfaces of the anterior and premolar teeth have a better accessibility and visibility leading to a smaller proximal CA. However, other studies found the mesio-distal CA was greater than the bucco-lingual CA.^7,9,15,20
The findings of the present study showed no difference in the mean CA between the anterior and posterior teeth. It seems that the difficulty in accessing posterior teeth was opposed by damaging the short lingual wall of an anterior tooth. However, this finding is in disagreement with previous studies which reported a higher mean CA in posterior teeth when compared with anterior teeth.^8,10,12
Poon and Smales15 found preparations with no preservation of the cingulum had significantly higher bucco-lingual CA (25°) than the preparations preserved the cingulum height (17.1°). This difference in results among the studies could be due to differences in the measuring method and sample size and human factors.

With regard to the AutoCAD measuring method, it seems that AutoCAD software is useful and accurate tool to assess the CA of prepared teeth. AutoCAD can be used by the dental clinicians to improve the psychomotor skills.

In the present study, the CA prepared for single crown was more than that of FPD abutment. This could be due to the fact the teeth in need for crowning usually are grossly destroyed and/or heavily restored. Also, the concern of not harming the neighboring teeth could lead to more tapering and over preparation. In addition, for the FPD preparation, clinicians are usually more concerned about tapering to guarantee one path of insertion for both abutments. Nordlander et al.⁸ found no significant difference in the mean CA prepared for single crowns and FPD abutments.

For the three groups studied, there is a need to exert extra effort to reduce the CA during tooth preparations. Several studies have shown the relationship between increased CA and decreased retention and resistance forms.^4,5,39 Increasing CA from 5° to 10° reduces the retention by 40-50%.4 A uniform tooth preparation with minimum CA should be strictly applied when possible. However, a balance should exist between the presence of CA and the requirement for retention and resistance. In other words, the CA should be as small as possible to increase the retention and resistance of the prosthesis, but at the same time some degree of convergence is necessary to reduce the axial discrepancy which consequently improves the crown seating.

In the present study, no efforts were made to assess the difference, if any, between vital teeth and those with post and core. In previous studies, this was found to have an impact on CA.^11,18 More long term clinical studies with larger sample size are needed to fully assess the influence of CA on the longevity of single crown and FPD. AutoCAD software is reliable and accurate instrument that can be used as educational tool for clinical assessment.

Conclusions

Within the limitations of this study it can be concluded that:

1.    The recommended CA of 6° is difficult to achieve clinically.
2.    The least mean CA was achieved by the specialists while the general dentists produced the highest CA.
3.    The mean CA for complete crowns was greater than the CA for FPD abutments.
4.    The mean CA of anterior teeth was greater than that for posterior teeth.
5.    The mean buccolingual CA of the preparation was less than the mean mesiodistal CA.
6.    There is an indication for exerting extra effort to reduce the CA during tooth preparation.

ACKNOWLEDGMENT

The authors give special thanks to Engr. Wyle Abdul-Rahim for his assistance in the AutoCAD training and Dr. Naseer Khan for his help in the statistical analysis. This study was funded by the College of Dentistry Research Center (CDRC) of King Saud University, Riyadh under Registration No. F-1157.

References

Malone WF, Koth DL. Tylman's theory and practice of fixed prosthodontics. 8th ed. St Louis: CV Mosby, 1989:2-45.
Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 3rd ed. St Louis: Mosby Inc., 2001: 166-201.
Shillingburg HT, Hobo S, Whitsett LD, Jacobi R, Brackett SE. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence Publishing, 1997: 119-138.
Jorgensen KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Scand 1955;13:35-40.
Wilson AH, Chan DC. The relationship between preparation convergence and retention of extracoronal retainers. J Prosthodont 1994;3:74-78.
Eames WB, O'Neal SJ, Monteiro J, Miller C, Roan JD, Cohen KS. Techniques to improve the seating of castings. J Am Dent Assoc 1978;96:432-437.
Leempoel PJ, Lemmens PL, Snoek PA, van't Hof MA. The convergence angle of tooth preparations for complete crowns. J Prosthet Dent 1987;58:414-416.
Nordlander J, Weir D, Stoffer W, Ochi S. The taper of clinical preparations for fixed prosthodontics. J Prosthet Dent 1988; 60:148-151.
Kent WA, Shillingburg HT, Duncanson MG. Taper of clinical preparations for cast restorations. Quintessence Int 1988; 19:339-345.
Annerstedt A, Engstrom U, Hansson A, Jansson T, Karlsson S, Liljhagen H, Lindquist E, Rydhammar E, Tyreman- Bandhede M, Svensson P, Wandel U. Axial wall convergence of full veneer crown preparations. Documented for dental students and general practitioners. ActaOdontol Scand 1996;54:109-112.
Ohm E, Silness J. The convergence angle in teeth prepared for artificial crowns. J Oral Rehabil 1978; 5: 371-375.
Al-Omari WM, Al-Wahadni AM.Convergence angle, occlusal reduction, and finish line depth of full crown preparations made by dental students. Quintessence Int 2004;35:287-293.
Ayad MF, Maghrabi AA, Rosenstiel SF. Assessment of convergence angles of tooth preparations for complete crowns among dental students. J Dent 2005;33:633-638.
Noonan JE, Goldfogel MH. Convergence of the axial walls of full veneer crown preparations in a dental school environment. J Prosthet Dent 1991; 66:706-708.
Poon B, Smales R. Assessment of clinical preparations for single gold and ceramometal crowns. Quintessence Int 2001;32:603-610.
Weed RM. Determining adequate crown convergence. Tex Dent J 1980;98:14-16.
Weed RM, Suddick RP, Kleffner JH. Taper of clinical and typodont crowns prepared by dental students [Abstract]. J Dent Res 1984; 63:286.
Smith CT, Gary JJ, Conkin JE, Franks HL. Effective taper criterion for the full veneer crown preparation in preclinical prosthodontics. J Prosthodont 1999;8:196-200.
Mack PJ. A theoretical and clinical investigation into the taper achieved on crown and inlay preparations. J Oral Rehabil 1980;7:255-265.
Patel PB, Wildgoose DG, Winstanley RB. Comparison of convergence angles achieved in posterior teeth prepared for full veneer crowns. Eur J Prosthodont Resto Dent 2005;13:100-104.
Assungao WG, Gomes EA, Tabata LF, Gennari-Filho H. A comparison of profilometer and AutoCAD software techniques in evaluation of implant angulation in vitro. Int J Oral Maxillofac Implants 2008;23:618-622.
Assungao WG, Tabata LF, Cardoso A, Rocha EP, Gomes EA. Prosthetic transfer impression accuracy evaluation for osseointegrated implants. Implant Dent 2008;17:248-256.
Assungao WG, Tabata LF, Barao VA, Rocha EP. Comparison of stress distribution between complete denture and implant-retained overdenture-2D FEA. J Oral Rehabil 2008;35:766-774.
Coimbra ME, Penedo ND, de Gouvea JP, Elias CN, de Souza Araujo MT, Coelho PG. Mechanical testing and finite element analysis of orthodontic teardrop loop. Am J Orthod Dentofacial Orthop 2008;133:188 e9-13.
Palaisa J, Ngan P, Martin C, Razmus T. Use of conventional tomography to evaluate changes in the nasal cavity with rapid palatal expansion. Am J Orthod Dentofacial Orthop 2007;132:458-466.
Verri FR, Pellizzer EP, Rocha EP, Pereira JA. Influence of length and diameter of implants associated with distal extension removable partial dentures. Implant Dent 2007;16:270-280.
Gautam P, Valiathan A, Adhikari R Stress and displacement patterns in the craniofacial skeleton with rapid maxillary expansion: A finite element method study. Am J Orthod Dentofacial Orthop 2007;132:5 e1-11.
Constante IG, Davidowicz H, Barletta FB, de Moura AA. Study of the areas and thicknesses of mesiobucal root canals prepared by three endodontic techniques. Braz Oral Res 2007;21:118-126.
Gergi R, Sabbagh C. Effectiveness of two nickel-titanium rotary instruments and a hand file for removing gutta-percha in severely curved root canals during retreatment: An ex vivo study. Int Endod J 2007;40:532-537.
Iqbal MK, Banfield B, Lavorini A, Bachstein B. A comparison of LightSpeed LS1 and LightSpeed LSX NiTi rotary instruments in apical transportation and length control in simulated root canals. J Endod 2007;33:268-271.
Basting RT, da Trindade Rde C, Florio FM. Comparative study of smile analysis by subjective and computerized methods. Oper Dent 2006;31:652-659.
Iqbal MK, Rafailov H, Kratchman SI, Karabucak B. A comparison of three methods for preparing centered platforms around separated instruments in curved canals. J Endod 2006;32:48-51.
Masiero AV, Barletta FB. Effectiveness of different techniques for removing gutta - percha during retreatment. Int Endod J 2005;38:2-7.
Iqbal MK, Karabucak B, Brown M, Menegazzo E. Effect of modified Hedstrom files on instrumentation area produced by ProFile instruments in oval canals. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2004;98:493-498.
Iqbal MK, Firic S, Tulcan J, Karabucak B, Kim S. Comparison of apical transportation between ProFile and ProTaper NiTi rotary instruments. Int Endod J 2004;37:359-364.
Biz MT, Figueiredo JA. Morphometric analysis of shank-to-flute ratio in rotary nickel-titanium files. Int Endod J 2004;37:353-358.
Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: An art form based on scientific principles. J Prosthet Dent 2001; 85:363-376.
Djemal S, Setchell D. An investigation of upper anterior crown preparations. [Abstract #616]. J Dent Res 1995;74:899.
Kaufman E, Coelho D, Colin L. Factors influencing the retention of cemented gold castings. J Prosthet Dent 1961;11:487-502.

Tables and Figures

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