Convergence angles of metal ceramic crowns prepared by dental students Thamer Y. Marghalani, BDS, DScD Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia Statement of problem. Convergence angles of crown preparations achieved by dental students vary from the recommended angles and depend on many variables and factors. Sex of operators and examination conditions in which preparations were made have not been reported in the literature. Purpose. The purpose of this study was to determine the mean convergence angles produced by ﬁfth-year dental students at King Abdulaziz University. Material and methods. Maxillary ﬁrst premolar abutment preparations for metal ceramic restoration were made by ﬁfth-year dental students on typodonts under preclinical examination conditions. A total of 68 preparations, made by 38 male and 30 female students, were randomly selected for evaluation of the buccolingual and mesiodistal convergence angles of each abutment tooth with an optical preparation scanner and digitizer to produce 3-dimensional digital objects. Two-sided t tests were performed to compare the buccolingual and mesiodistal angles between the sexes. Results. The mean mesiodistal angle of all preparations was 11.11 4.79 degrees, and the mean buccolingual angle was 10.49 3.95 degrees. The mean buccolingual angle in preparations by male students was 10.16 3.90 degrees, compared with 11.46 5.06 degrees in preparations by female students. The mean mesiodistal angle was 10.92 4.03 degrees in preparations by male students, compared to 10.66 4.45 degrees in preparations by female students. No statistical difference was observed between the sexes for either angle (buccolingual, P¼.435; mesiodistal, P¼.497). The smallest and largest angles observed were 2.59 and 24.64 degrees. Approximately 12% of male students and 10.29% of female students made preparations with less than a 6-degree (ideal convergence angle) buccolingual or mesiodistal angle. Conclusions. The average convergence angles produced were between 10.16 and 11.46 degrees. Several students were able to achieve ideal convergence angles. Male and female students achieved similar buccolingual and mesiodistal convergence angles. (J Prosthet Dent 2014;-:---)
Clinical Implications Ideal convergence angle of crown preparation is achievable by dental students, regardless of their sex, when measured in preclinical examination conditions after the students have gained some clinical experience during their training. Differences in the angles achieved may be due to the variations between the preclinical and clinical conditions.
Fixed dental prostheses have been used to replace missing teeth and coronal tooth structures. They are one of the restorative options preferred by patients to replace a missing tooth.1 Adequate retention properties are important for the success of single-unit or multiple-unit prostheses.1,2 Retention is dependent on many factors, such as the occlusocervical height, surface texture,
type of cement, use of bonding, close adaptation of retainers to the abutment, and convergence angles of opposing surfaces.1,2 Most importantly, retention depends on the tooth preparation being as close to parallel as possible to the axial walls.1,2 However, parallel preparation walls are not always possible to achieve for many reasons,3 including differences in the hand-eye
coordination of the individuals making the preparations, vision, and accessibility. Parallel surfaces prevent the complete seating of the prosthesis; therefore, opposed tooth surfaces that create angles that are as small as possible, yet not completely parallel, are necessary to maximize retention. Minimizing convergence angles during tooth preparations for ﬁxed prostheses
Associate Professor and Consultant in Prosthodontics, Oral and Maxillofacial Prosthodontics Department.
1 Scanner and digitizer (PrepScan; KaVo). is also essential for conserving tooth structures.1,2 Convergence angles are measured in degrees and deﬁned in the dental literature as the taper of a crown preparation or the angle formed between opposing axial walls when teeth are prepared for crowns or ﬁxed dental prostheses.4 This term is best described as the total occlusal convergence.4 In some reports, the taper angle is described as the angle formed between a wall of the preparation and the long axis of the tooth, whereas the convergence angle is the combination of the taper angles formed by opposing walls.5 Other reports use engineering terms such as “rate of taper” and “cone angle.” The International Organization for Standardization (ISO) guidelines deﬁne taper as the ratio of the difference between the diameter of 2 sections to the distance between the sections.6,7 In this study, the term “convergence angle” is used to describe the angle formed by 2 opposing walls of the preparation. Prothero8 ﬁrst reported on convergence angles and recommended a range of 2 to 5 degrees. Theoretically, ideal convergence angles should range from 4 to 6 degrees, but angles of 4 to 14 degrees are considered acceptable.1,2,9-11 Several in vitro studies have evaluated the convergence angles produced by dentists.7,12-18 Others evaluated convergence angles produced in clinical settings.13,19-29 Many studies have found a strong negative relationship between
convergence angle and retention.30 One in vitro study found that the maximum tensile retention value was measured at a convergence angle of 5 degrees when tensile loads were applied to cemented crowns.31 Another study measured maximum tensile retention values at convergence angles of 6 to 12 degrees.20 A minimal taper angle of 12 degrees has been suggested to ensure that there are no undercuts in preparations.32 Other studies recommended convergence angles of 2 to 6 degrees,1,2,10,19 whereas laboratory studies found that ranges of 10 to 16 degrees are acceptable.12-16 Bowley and Kieser33 found that axial inclinations greater than 10 degrees in 3- to 4-mm-long axial walls reduced the maximum surface area for complete crown preparations. Auxiliary means of retention are required when retention is compromised owing to suboptimal heights and inclinations in the preparation.34-36 The convergence angle may not affect the strength of the attachment of luting resin to dentin in crowns with minimal axial height.37 Some new selfadhesive luting resins allow the crown to be bonded to nonretentive tooth preparations, resulting in a bond strength that is greater than the internal cohesive strength of the ceramic restoration.38 This study reports the average total occlusal convergence angle in tooth preparations for crowns fabricated by dental students at King Abdulaziz University and compares the convergence
The Journal of Prosthetic Dentistry
2 Lingual view of 3-dimensional digitization of premolar scan. (M, mesial; D, distal.) angles of tooth preparations produced by male and female students. The null hypothesis was that convergence angles produced by male and female students would not signiﬁcantly differ.
3 Distal view of buccolingual convergence angle (buccolingual angle) measurements with MeshLab software. (B, buccal; L, lingual.)
MATERIAL AND METHODS This is an observational study collecting tooth preparation characteristics and data after annual practical examinations on which tooth preparations were performed on mannequins by ﬁfthyear dental students. Students that performed the tooth preparations had completed their ﬁrst year of ﬁxed prosthodontics preclinical training experience, including training on laboratory mannequin typodonts in the previous year. They had recently started their clinical training. After some clinical experience, they were examined for a tooth preparation on a mannequin. Direct vision or interference from proctors, instructors, or evaluators was not allowed during the examination. No special instructions about the examination were given to the students or proctors. Students and proctors were not informed that convergence angle measurements
would be performed on the prepared teeth. The time allowed for the preparation and fabrication of an interim restoration was 1 hour. Of 126 preparations, a total of 68 from 38 male and 30 female dental students were randomly selected for evaluation. Randomization was done with the randomization function in a spreadsheet (Excel 2011; Microsoft Corp) to generate the serial number of selected students. The preparations for a metal ceramic crown for the maxillary ﬁrst premolar were scanned and digitized with an optical preparation scanner and digitizer (PrepScan; KaVo) to produce a 3-dimensional (3D) rendering (Fig. 1). The 3D digitization of each preparation was evaluated for buccolingual (BL) and mesiodistal (MD) convergence angles (Fig. 2). Measurements were made with software (MeshLab v1.3.0; open source software available at meshlab.sourceforge.net) (Fig. 3).
Statistical summary of convergence angles from preparations made by students in examination setting
Convergence Angle Mean SD Median Minimum Maximum
Males and females combined
BL, buccolingual; MD, mesiodistal. All measurements are in degrees.
Data were analyzed with software (SPSS Statistics v18.0; SPSS Inc). Mean, median, and SD values of the convergence angles were obtained. Two-sided t tests were performed separately to identify differences in BL and MD convergence angles from preparations made by male and female students (a¼.05).
RESULTS Preparations from 68 students (30 females and 38 males) were analyzed. Table I summarizes the statistics of convergence angles (BL and MD) for preparations made by the students. The mean BL angle of all preparations was 10.49 3.95 degrees, and the mean MD angle was 11.11 4.79 degrees. The mean BL angle was 10.16 3.90 degrees in preparations by male students and 11.46 5.06 degrees in preparations by female students. The mean MD angle was 10.92 4.03 degrees in preparations by male students and 10.66 4.45 degrees in preparations by female students. The smallest and largest measured angles were 2.59 and 24.64 degrees. Approximately 12% of students made preparations with BL angles