Assessment of Geometry of Tooth Preparations on Dies for Full Crowns and Bridge Retainers in Selected Dental Laboratories in Nairobi, Kenya

Abstract

Background: The restoration and replacement of teeth with crowns and bridges has been on the increase. The quality of tooth preparation for these prostheses is vital as it establishes the foundation for the prostheses. The recommended guidelines for tooth preparation include provision of adequate retention and resistance, adequate tooth reduction to ensure structural durability of the restorative material and good marginal integrity. Ideal preparations result in functionally durable and aesthetic prostheses that are biologically compatible with pulpal and periodontal tissues. Lack of adherence to these guidelines results in treatment failures which present as loss of retention, dental caries on the prepared teeth and loss of vitality of the natural prepared teeth. The factors that have been reported to influence the preparation geometry include the clinicians’ expertise, the position and anatomy of the prepared tooth in the dental arch and the length of the prosthesis. Aim: To assess the geometry of tooth preparations on dies for full crowns and bridge retainers in selected dental laboratories in Nairobi. Study design: This was a descriptive cross-sectional study which was carried out in selected dental laboratories in Nairobi. Material and methods: Dies of tooth preparations for full crowns and bridge retainers were evaluated. The parallelism of the buccal-lingual and mesio-distal walls was assessed by measuring the convergence angles. This was done by taking photographs of the dies and using computer software AutoCAD (Autodesk Inc., U.S.A.) for analysis. An electronic digital caliper (Masel, U.S.A) was used to measure the height of the prepared tooth. The presence and location of auxiliary retention features were noted in a data collection form. The type, position and number of the prepared teeth in the dental arch were also recorded. The type of margin on both the facial and lingual surfaces of the prepared tooth were recorded. xiv Data analysis and presentation Statistical Package for Social Sciences (SPSS) version 20.0 for Windows was used for data analysis. Computation was done to determine mean convergence angles, occluso-cervical height and the frequency of different crown margin designs. Statistical analysis was done using Chi square (X 2). Results The tooth preparations for single crowns constituted 59% of the dies whereas 41% were preparations for bridge retainers. The maxillary premolar was the most frequently prepared tooth for abutments at 29.9% of the overall abutments, as well as for single crowns at 35.0% of the overall single crowns. The mean buccal-lingual convergence angle was 26.7○ +11.93SD and the mean mesio-distal convergence angle was 22.1○ +10.89SD. Ideal convergence angles, in both mesio-distal and buccal-lingual dimensions, were found in 18.1% of the dies. Majority of which were for single crowns 58.9% followed by abutment preparations for three unit bridges at 23.2%. One hundred and four (33.5%) dies had ideal buccal - lingual convergence angles whereas 206 (66.5%) had non-ideal buccal-lingual convergence angles. A chi square analysis showed a significant difference in the degree of conformity to ideal buccal-lingual convergence angle measurements among the different tooth types (X 2= 16.1, d.f. = 5, p = 0.01). One hundred and twenty five (40.3%) dies had ideal mesio-distal convergence angles whereas 185 (59.7%) had non ideal mesio-distal convergence angles. A chi square analysis showed a non- significant difference in the degree of conformity to ideal mesio-distal convergence angle measurements among the different tooth types (X 2 = 10.48, d.f. = 5, p = 0.06). xv The mean occluso-cervical heights of the dies ranged from 3.0mm +1.0SD to 6.3mm +1.3SD. The maxillary anterior teeth had the highest degree of conformity to ideal occluso-cervical heights at 96.4%. Majority of the molar preparations had inadequate occluso-cervical height with 85.7% of maxillary molars and 72.7% of mandibular molars having occluso-cervical height of less than 4mm. The difference in the degree of conformity in occluso-cervical height among the different tooth types was not statistically significant (X2 = 14.20, d.f = 5, p = 0.06) Auxiliary features of retention were observed in 1.9% of the dies. Auxiliary grooves were placed on the buccal and lingual/palatal aspects of molars. No auxiliary boxes were observed. The variation in the frequency of auxiliary grooves in the maxillary and mandibular molars was not statistically significant (X 2 = 4.187, d.f. = 1, p = 0.05) The finish lines were continuous in 93.2% of the dies. Majority (80.6%) had a shoulder margin on the facial aspect while 88.1% of the dies had a chamfer margin on the lingual aspect. Conclusions Adherence to ideal guidelines for convergence angles of tooth preparations is low. The maxillary premolar was found to be the most frequently prepared tooth for single crowns and bridge retainers. Most of the anterior teeth and premolar preparations had ideal occluso-cervical height in contrast to the molar preparations. Auxiliary features of retention were rarely used. Recommendation  Oral health providers should pay more attention to guidelines when preparing teeth for crowns and bridge retainers. xvi  Encourage continuing professional education courses on tooth preparation for crown and bridge.  There is need for further research to ascertain why the guidelines are not used.