Tooth Preparation - measuring, understand, and reporting tooth preparation and its influence on fracture of all-ceramic crowns.

Abstract

Tooth preparation for a single complete crown is a fundamental principle in fixed prosthodontics. The geometry of the preparation is clinician-controlled and accepted by the dental community to affect the retention, displacement resistance, and survival potential of a crown. Crowns are routinely placed by clinicians, therefore investigation into the geometry of tooth preparations and its influence on prosthodontic success is important and is of interest to clinicians.

With many geometrical, biological, and technical variables contributing to the clinical success of a dental crown, developing a complete understanding into this complex system is a lifelong challenge. This thesis focuses on the total occlusal convergence angle - the parameter most studied and solely under the control of the clinician. The three objectives of this thesis are: 1. To develop a validated objective method for measuring crown preparation geometry; 2. To report on the geometry of tooth preparations by dentists; and 3. To understand the importance of the total occlusal convergence angles by understanding their effects on fracture mechanisms and hoop stresses.

This thesis begins by investigating methods used to measure tooth preparations. By identifying subjective problems in existing methods, a new method was created and further developed into a measuring program. The measuring program was used in a multipart study to report the geometrical parameters of tooth preparations prepared by general dentists. Finally, the convergence angle measurements were taken further in an in vitro study to investigate and understand its influence on hoop stresses by an axial compressive force and understand this effect on the fracture of glass ceramics.

The systematic review on measuring methods found that the majority of studies measured the convergence angle in two cross sections. Meta-analysis could not be performed as the methods were too varied. The study highlighted the need for a universal standardised measuring method. A new method was proposed and a small validated study carried out. The method was found to be reproducible and less subjective than all previously reported methods.

It was found that convergence angles of tooth preparations by general dentists were greater than the recommended values and marginal widths of preparations fell short of recommended values. Measuring the parameters enabled the calculation of theoretical retention and resistance values of the tooth preparations. Based on the study, the majority of preparations with large angles did not show any resistance form.

Large convergence angles demonstrated a greater mechanical advantage when axially loaded in vitro. An extended finite element model was validated by the experimental model to generate a fracture initiating from the inner surface propagating to the outer surface in alignment with clinical failures.

This study showed that the recommended convergence angle may not be as responsible for clinical success as previous reported. A thorough understanding on each isolated parameter and in combination is needed before the clinical survival can be attributed to the clinician-controlled geometry.