Restorative and periodontal considerations of short clinical crowns

The purpose of this review was to examine the periodontal and restorative factors related to restoring teeth with short clinical crowns. Modes of therapy are usually combined to meet the biologic, restorative, and esthetic requirements imposed by short clinical crowns. The complications presented by teeth with short clinical crowns demand a comprehensive treatment plan and proper sequencing of therapy to ensure a satisfactory result. Visualization of the desired result is a prerequisite of successful therapy. Short clinical crowns present many problems to the restorative dentist. Restorations should have proper form, function, and esthetics while promoting the maintenance of tissue health in the surrounding areas. However, adequate tooth structure for achieving these goals may not always be available. While it is difficult to precisely define minimum crown length, this study offers guidelines for defining a short clinical crown and suggests procedures for achieving a predictable result without compromising the periodontium. The consequences of placing a restoration on a tooth with insufficient crown length are discussed and various treatment methods are reviewed.

Moduli of rupture of layered dental ceramics

OBJECTIVE

The purpose of this study was to determine the moduli of rupture of layered beams made of strong core materials veneered with weaker conventional feldspathic porcelain.

METHODS

A three-point flexural test, and formulae derived especially for this purpose were used. Two systems were investigated. The first consisted of Vitadur N (Vita Zahnfabrik), a conventional feldspathic porcelain; and Dicor MGC (L.D. Caulk), a machinable glass ceramic. The second consisted of Vitadur N and In-Ceram (Vita Zahnfabrik), a strong reinforced aluminous porcelain.

RESULTS

Two-way ANOVA and Tukey tests were used to evaluate the effect of numerous factors on the modulus of rupture. The results indicated that the material forming the tensile surface, the material forming the compressive surface and their interaction had a highly significant effect on modulus of rupture (p < 0.05). The effect of the material forming the tensile surface on modulus of rupture was of much greater magnitude than that of the compressive surface. Theoretical curves describing effects of the layers' elastic moduli and thickness on the force-bearing capacity of model beams were plotted. These indicated that for a wide range of thickness ratios and for a wide variety of elastic modulus ratios, the tensile material dominates the force-bearing capacity of layered beams except when a very much lower modulus material forms the compressive layer.

SIGNIFICANCE

Layered prostheses made of strong cores veneered with weaker feldspathic porcelain may be prone to failure when the feldspathic surfaces are subjected to tensile force.