| Abstract|| |
Background: Full-coverage porcelain fused to metal crowns is commonly recommended for restoration of extensively damaged teeth. Ability of the dentist to adequately prepare teeth is fundamental to success and longevity of these restorations.
Aims: This study was designed to compare recommended convergence angle and taper values of tooth preparation with clinically practiced values and to assess the factors such as tooth position, operator experience, vitality, and restorative status on convergence angle of prepared teeth.
Setting and Design: It was a descriptive, cross-sectional study design and held at The Aga Khan University Hospital, Dental Section, for a period of 1 year.
Materials and Methods: A minimum of 197 crown preparation models of molar and premolar teeth prepared by residents and specialists were collected in order to achieve the objectives of the study on the basis of convenient sampling technique.
Statistical Analysis: One sample t-test used to compare the mean practiced convergence angle values with its recommended values. Independent sample t-test and one-way ANOVA was used to see difference in the convergence angle values of the teeth prepared by different operators, tooth type, vitality status, and restorative status of teeth.
Results: Mean reported convergence angle and axial wall taper values were 23.7°±8.9° and 11.3°±7.8°, respectively, which is significantly greater (P<0.001) than the recommended values. Convergence angle values were greater for molars as compared to premolars.
Conclusion: There was a considerable disparity between the convergent angles values recorded in this study and the recommended guidelines and are affected by tooth type, vitality, and restorative status of tooth.
Keywords: Convergence angle, porcelain fused to metal crown, taper
|How to cite this article:|
Ghafoor R, Siddiqui AA, Rahman M. Assessment of convergence angle of full-coverage porcelain fused to metal crowns in clinical practice. Indian J Dent Res 2012;23:241-6
Historically, retention has been the main focus of interest in crown dislodgment; however, resistance form is perhaps more crucial because occlusal forces are directed in a lateral or an apical direction. Retention of crown is affected by convergence angle of preparation, height of preparation, height to base ratio of preparation, and type of luting cements. 
|How to cite this URL:|
Ghafoor R, Siddiqui AA, Rahman M. Assessment of convergence angle of full-coverage porcelain fused to metal crowns in clinical practice. Indian J Dent Res [serial online] 2012 [cited 2019 Sep 20];23:241-6. Available from: http://www.ijdr.in/text.asp?2012/23/2/241/100434
Textbook guidelines for axial wall inclination during tooth preparation can vary from 4° to 6° often cited as ideal and 6-14°are acceptable. ,, These recommendations are theoretical and difficult to achieve in clinical practice. Effective taper criteria must define as realistic, measurable goal that the student can visualize and achieve. However, a 12° criterion is more realistic than a 6° criterion for full-coverage crown preparations. Studies , showed the actual convergence angle of most preparations to be greater than 12°. Recommended total occlusal convergence is between 10° and 20°.  In fact, tooth preparation with a convergence angle greater than 25° are statistically less retentive than the optimum convergence. 
Moreover, Mack  estimated that a minimum taper of 6° and convergence angle of 12° was necessary to ensure the absence of undercuts during preparation and found an average clinical taper of 16.5°.
Several clinical studies have been performed in an attempt to establish optimal convergence angles in clinical practice and provide long-term retention. The inverse relationship between retention and convergence angles initially was demonstrated experimentally by Jorgensen  and Kaufman et al.;  Shillingburg et al.,  previously recommended a taper of 6° of abutment preparation in order to achieve maximum retention. Clinical investigations reported convergence angle means for tooth preparation that ranged from 12.2° to 20.1° for dentists with no apparent correlation to their level of education and experience. ,,
Wiskott et al. found a direct linear relationship between the height of axial wall and the degree of resistance of crown. Crowns with more axial wall height had enhanced resistance form. Zuckerman  speculated about the influence of luting medium on crown retention. He identified that strength of cement, geometric consideration, tapering of the opposing walls, and the thickness of cement could affect the effectiveness of the cement bond and retention of the crown.
Jorgensen and Esbensen  approached the problem of the effect of cement thickness upon the retention. They summarized the variations in the phosphate cement film thickness had only a moderate influence upon the retention of complete veneer crown.
Several international studies are available where they worked on extracted teeth (ex vivo) but limited work has been done regarding clinically practiced convergence angle values and factors affecting the convergence angle of prepared teeth. Therefore, the aim of this study was to determine the difference in clinically practiced convergence angle values with respect to its theoretical guidelines and to assess the factors such as tooth position, operator experience, vitality and restorative status on convergence angle, and taper of a tooth preparation development. We hypothesized that "there is a difference in the average clinically practiced convergence angle values with respect to its recommended values (6° axial wall taper and 12° convergence angle)."
| Materials and Methods|| |
It was a descriptive, cross-sectional study design, conducted at the Aga Khan University Hospital, Dental Section. A minimum sample of 197 crown preparations models was collected in order to achieve the objectives of the study. Data were collected retrospectively using convenient sampling technique from die trimmed the plaster models after delivering crown to the patients. We included crown preparation models of molars and premolars from both arches (maxillary and mandibular) prepared by residents (postgraduate students) and specialists (more than 5 years of clinical experience) under normal clinical condition. Pin-indexed crown prepared teeth removed from plaster model and mounted on square-shaped hard wax block to stabilize in a fixed vertical position on horizontal table with white background; then photographs of prepared teeth were taken in mesiodistal and buccolinual direction with Nikon Coolpix 4500 digital camera and tripod stand at 20 cm distance in such a manner that it was perpendicular to long axis of tooth. Photographs of prepared teeth were transferred to AutoCAD 2007 software (Sony Corporation, Tokyo, Japan) for the measurement of convergence angle in both dimensions (mesiodistal and buccolingual) and axial wall tapers (mesial, distal, buccal and lingual) of each prepared tooth.
A minimum sample of 197 crown preparations models were used in order to achieve the objectives of the study having at least 80% power, 5% level of significance, standard deviation of 0.5°, and a bound of error of estimation of 0.1°.
One sample t-test was used to compare the difference in mean clinically practiced convergence angle values with its recommended values (6° axial wall taper and 12° convergence angle). Difference in the convergence angle and axial wall taper values between two groups of teeth (molars and premolars), vitality status of teeth (vital and nonvital), and operator experience (postgraduate residents and specialists) were tested for significance by independent sample t-test. One-way ANOVA model was fit to compare the difference in convergence angles and axial wall taper with respect to restorative status of tooth. The P values below 0.05 were considered statistically significant. The statistical analysis was performed using SPSS-14.0.
| Results|| |
Mean reported convergence angle and axial wall taper values were 23.7°±8.9° and 11.3°±7.8°, respectively. One sample t-test results indicate significant difference (P<0.001) between hypothesized mean and clinically practiced mean convergence angle and taper values [Table 1]. Statistically significant (P<0.001) difference in convergence angle and axial wall taper values were found between two groups of teeth (molars and premolars). Higher values were measured for molars as compared to premolars [Table 2]. Statistically significant (P<0.001) difference was measured in convergence angle and axial wall taper values with respect to vitality status of teeth and we found greater values for nonvital teeth as compared to vital teeth [Table 3]. No statistically significant differences were measured in convergence angle and axial wall taper values of prepared teeth treated by postgraduate residents and specialists [Table 4]. One-way ANOVA comparing the difference in the convergence angle and axial wall taper values with respect to restorative status of tooth and indicating statistically significant (P<0.001) greater convergence angle and axial wall taper values for restored teeth as compared to unrestored have been presented in [Table 5].
|Table 1: Difference between hypothesized and clinically practiced convergence angle and taper values|
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|Table 2: Difference in convergence angle and taper values between two groups of teeth|
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|Table 3: Convergence angle and taper values with respect to vitality status of tooth|
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|Table 4: Difference in convergence angle and taper values between two groups of operator|
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|Table 5: Convergence angle and taper value with respect to restorative status of tooth|
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| Discussion|| |
Theoretical guidelines for axial wall taper and convergence angle during tooth preparation are arbitrary standards and are not consistent with the reality. In clinical practice, axial wall taper and convergence angle of prepared teeth vary from tooth to tooth in different dimensions and depends upon operator experience, vitality, and restorative status of tooth.
Convergence angle and taper values differences with respect to tooth type
Accurate positioning of hand piece or diamond rotary cutting instrument during crown preparation is difficult on the distal aspect of tooth, especially on the molar teeth as compared to premolar teeth. 
Convergence angle and axial wall taper values were significantly different between two groups of teeth. Convergence angle values were significantly less for premolars as compared to molars in both maxillary and mandibular arches because accurate positioning of hand piece is difficult during tooth preparation in the posterior segment of mouth because of limited access. We found wide-range difference regarding distal wall taper and buccal wall taper within same group of teeth and between premolar and molar teeth. These findings were in agreement with Leempoel et al. and Annerstedt et al. studies.
Nordlander et al. conducted a study to measure convergence angles of full-coverage crown preparations in a clinical environment and compared with each other and the ideal taper of 4-10°. And the mean convergence angle found in their study was 19.9° that is slightly less than measured in current study. This disparity in the convergence angle values were probably due to sample size difference and random selection of crown preparation models from clinics in this study. Educational emphasis, the practical application of preparation design routinely exceeds the ideal taper.
Convergence angle and taper values differences with respect to operator experience
Effect of operator clinical experience did not show significant difference in convergence angle and taper values within same group and between two groups of teeth in both dimensions (mesiodistal and buccolingual). Within agreement to Petal.  and Nordlander et al. studies, we also found improvement in convergence angle and taper values with the years of clinical experience and ideal preparation taper is seldom achieved. Given the complex inter-relationships 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.
Convergence angle values with respect to different dimensions of tooth
Mean convergence angle values measured in mesiodistal direction were significantly less as compared to buccolingual direction. Distal and buccal wall taper were greater than mesial and lingual wall, probably because of difficulty in positioning hand piece on buccal and distal aspect of tooth. Moreover, buccal wall taper would be affected by the aim of the clinical operator to reduce bigger thickness in there in order to achieve better aesthetics results. The values obtained in this investigation agreed with the findings of Ayad et al. study, where they measured convergence angle values on extracted teeth in both dimensions (mesiodistal and buccolingual) and found statically significant greater convergence angle values in buccolingual dimension as compared to mesiodistal dimension. Contradictory to current study findings, Rafeek et al. and Al-Omari and Al-Wahadni  measured greater convergence angle values in buccolingual direction as compared to mesiodistal direction.
Convergence angle values with respect to recommended values
Convergence angles for complete crown preparations have been recommended at 4-12°. However, practitioners have difficulty in meeting these recommendations in clinical practice because of limited access and visibility.
In agreement with the results of Nordlander et al.,  mean convergence angle values measured in current study for both group of teeth in both directions were significantly greater with respect to its recommended values, thus the hypothesis of the study was accepted.
Noonan et al. and Sato et al. measured convergence angle and taper values of dental students under normal clinical condition and special (proficiency) testing condition and they found greater values under normal clinical condition as compared to special testing condition. Specialized training definitely helps in improving the clinical practices and achieving the recommended values.
Weed  found that dental students could develop tooth preparations for complete crowns with a taper of 12.7° on typodonts, but their clinical preparations had a mean taper of 22.8°; however it is difficult to achieve recommended taper intraorally as compared to typodonts because of limited access and visibility.
Convergence angle values with respect to vitality status of tooth
Changes in dental pulp electric conductivity and hemodynamic depended on the inclination angle formed during preparation. The optimal safe angle of the lateral wall inclination was determined to be 3-12°. Increase in the angle (> 12 degrees) can lead to traumatic pulpitis. 
Convergence angle of prepared tooth was strongly associated with the vitality status of tooth both in mesiodistal and buccolingual direction. Convergence angle and taper values measured for nonvital teeth were significantly greater than vital teeth. Significant difference was measured regarding mesial, distal, and lingual wall taper of vital and nonvital teeth. These results are in agreement to Ohm, Silness,  and Smith et al. studies, in which they compared convergence angles and taper values microscopically between vital and nonvital teeth. Fear of thermal and vibratory insult to dental pulp during tooth preparation limit the operator to practice conservatively on vital teeth.
Convergence angle values with respect to restorative status tooth
Convergence angle values in both mesiodistal and buccolingual dimensions were directly related to the restorative status of tooth. Statistically significant difference was measured in convergence angle and taper values between restored and unrestored teeth because of change in the cutting resistance during tooth preparation. We did not found sufficient data to support the effect of restorative material on convergence angle and axial wall taper of tooth preparation.
Although several studies had documented the fact that it is difficult to achieve recommended convergence angle and taper values intraorally, but specialized training and regular monitoring will improve operator clinical practices and ultimately the longevity of restoration.
| Conclusion|| |
Within the limitations of this study, it can be concluded that there was a considerable disparity between the convergent angles and taper values recorded in this study and the ideal configurations recommended in fixed prosthodontic textbooks and the dental literature, so the study hypothesis was accepted. Convergence angle values were significantly greater in buccolingual dimension than mesiodistal. Distal and buccal wall taper was significantly greater than mesial and lingual wall taper. Convergence angle and axial taper values are greater for molar teeth as compared to premolar teeth (P < 0.001). Convergence angle and taper values are affected by vitality and restorative status of teeth.
Limitations of the current study are its cross-sectional design and lack of randomization during data collection procedure.
Long-term clinical studies are necessary to fully assess the influence of convergence angle on the longevity of individual crown. Complex inter-relationships 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.
| Acknowledgment|| |
The author is grateful to Dr. Farah Saeed, Consultants of Operative Dentistry, and Mr. Iqbal Azam, Assistant Professor of Biostatistics, Department of Community Health Sciences (CHS), Dr Farhan Raza Khan, senior instructor of Operative Dentistry Residency Programme, for their invaluable support and guidance at crucial steps of this study project. Author is also grateful to University Research Grant of The Aga Khan University Hospital, Karachi, for financial support of study and Architect department of The Aga Khan University Hospital, Karachi, for technical support of the study project.
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Department of Surgery, Dental Section, The Aga Khan University Hospital, Karachi
Source of Support: University Research Grant, The Aga Khan University Hospital, Karachi, Pakistan, Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]