Adequately Prepared? A Study Using an Innovative Computer Application to Measure Clinical Crown Convergence Angles Achieved by Students at a UK Dental School

European Journal of Prosthodontics and Restorative Dentistry (2019) 27, 32–38

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Adequately Prepared? A Study Using an Innovative Computer Application to Measure Clinical Crown Convergence Angles Achieved by Students at a UK Dental School

Convergence Angle Dental Student Crown

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Kenneth James Strain*

(BDS, DIP H&T, MFDS RCS (Ed.))

Janine Tiu§

(PhD, BDent Tech, PGDipDentTech)

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James Mackie*

(Pg Dip Dent Tech)

Stephen J Bonsor*

(BDS, (hons), MSc FHEA , FDS RCPS (Glasg)) (BDS MSc, FDS RCS (Eng.) FDS RCS (Ed.) FFGDP)

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Address for Correspondence

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Richard J Ibbetson*

Kenneth James Strain*

Email: k.strain.12@aberdeen.ac.uk

University of Otago

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* University of Aberdeen, Westburn Drive, AB25 3BZ Aberdeen, United Kingdom §

This study examined the total occlusal convergence angles created for full coverage crown preparations by students at a UK dental school. Working casts of 82 clinical crown preparations were scanned using a 3D scanner. Stereolithographic files were uploaded to Preppr, a crown preparation analysis application. Mean bucco-lingual convergence angle were 19.6° (+/-11.7) and mesial-distally 17.8° (+/-11.1). Smallest bucco-lingual convergence angles were achieved for canine teeth with the largest on molar teeth. The smallest mesio-distal values were on canine teeth with the largest on molar teeth. Ideal total convergence angles (4-14°) were achieved in 23% of bucco-lingual preparations and 33% of mesio-distal preparation. Results for clinically acceptable angles (10-20°) were 30% and 40% respectively. There were no statistically significant differences between tooth types for mean bucco-lingual values. (p=0.623), mesio-distal mean values were statistically different by tooth type (p=0.003). Mean values for mandibular molars were significantly higher than for maxillary incisors (p=0.001) and mandibular molars had significantly higher values than maxillary canines (p=0.045). Results in this study were comparable to those of other students and qualified clinicians, with a minority of preparation achieving ideal values.

The effects of different total occlusal convergence angles (TOC) or tapers on the clinical success of full coverage crowns have been extensively investigated both in vivo and in vitro. The angle formed by the opposing walls of a crown preparation when viewed in a given plane forms the total convergence angle.1 The majority of research examining TOC has been laboratory based with a paucity of research examining the convergence angles achieved by clinicians intra-orally.2,3 When assessing TOC, two key concepts must be considered, retention and resistance form. Retention is the ability of a crown to resist forces acting to dislodge it along its path of insertion, resistance is the ability of the crown to resist forces acting to dislodge it in a direction other than the path of insertion.1 Jorgensen found an inverse relationship between TOC angles and retention,4 TOC has also been shown to be a key determinant of the resistance form of an abutment.5 The greater the TOC of a crown preparation, the lower the forces needed to dislodge the restoration.6

Received: 12.05.2018 Accepted: 20.11.2018

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