Finite Element Analysis of the Principles and Loosening Force of the Conical Telescopic Crown: A Computer-based Study

Objectives: The primary goal of the present finite element (FE) analysis of a special removable prosthesis, the conical telescopic crown (CTC), was to validate established results based on a rigid model of the CTC and to analyze its characteristic features as a function of the essential material and geometric parameters. Furthermore, the effectiveness of a new element, the composite stop (CS), was investigated. Materials and methods: The study used an axisymmetric FE model containing the inner and outer crown including resin or ceramic veneer, the CS, the cement layer between the interior crown and the tooth, and the upper part of the tooth itself. Results: For a convergence angle (half-cone angle) α = 4° and a moderate chewing force F = 150 N the loosening force decreased from ~ 50 N without to ~ 10 N with CS. Increasing α values yielded a decrease of the loosening force. Adherence between the inner (IC) and outer crown (OC) was achieved for all configurations (α = 2°, 4°, and 6°), except for zirconium crowns with α = 6°. In systems without CS, the maximum tensile stress in the veneer increased proportionally to F, but remained limited in those with CS. Conclusions: The angle α and the coefficient of static friction μ0 emerged as the decisive parameters of the CTC. The computed fitting/loosening behavior agreed well with results of a simple rigid-body model and experiments. The incorporation of a CS allows ceramic veneering of the outer crown. Clinical relevance: The optimal angle α of the CTC is ascribed to a number of customary material combinations for IC and OC. The CS limits the loosening forces of the CTC to values which guarantee non-traumatic removal of the prosthesis. Keywords: conical crown, removable prosthesis, double crown-retained denture, composite stop, ceramic veneer, loosening force control