Abstract
All-ceramic dental restorations are now regularly being used in clinical situations. Forces applied to ceramic dental restorations can result in failure due to cracking. Dental restorations may be subjected to significant loads in the mouth and small contact points between opposing teeth may result in markedly high stress when a high bite force is encountered. Today’s clinician faces a complex decision when choosing a ceramic for a particular indication. The market offers an extensive palette of materials, and their selection is often based on material strength measured in vitro, which may not always predict the true performance characteristics a material will have when used in vivo. The proposed outcomes of this research comprised the development of a protocol for measuring and describing intra-oral stress. The objective was to provide a reference base for the design of restorations and the selection of the appropriate restorative material for patients. The investigation followed a four–phase approach:
• Develop a bite force transducer suitable for measuring tooth to tooth bite forces in the posterior region of the mouth.
• Record the maximum voluntary posterior bite force and occlusal contact area in healthy human volunteers.
• Develop a device and method to record tooth movement.
• Establish the tooth movement of a sample of healthy human volunteers.
The bite force transducer was developed and utilised to record the maximum voluntary bite forces on the posterior teeth of 40 participants. Their inter-occlusal contact points were identified by digitally scanning the teeth and calculating the surface area by accompanying software. Bite forces from as low as 83.9 Newton (N) to the highest at 1642.8 N, with a mean of 430.4 N (Standard Deviation (SD) = 279.4) were recorded. Inter-occlusal contact points as small as 0.065 mm2 were measured. The resultant stress that 21.8% of the teeth tested may experience has been shown to potentially exceed the flexural strength of even the strongest dental ceramic available on the market.
Teeth are supported by the periodontal ligament which allows controlled movement of the tooth when a force is applied. A novel device capable of recording the vertical movement of a tooth was developed and utilised to record vertical tooth movement of the second maxillary premolars of 20 participants. A mean tooth displacement of 73.8 µm (sd = 22.5 µm) was recorded. The mean recovery to zero was 24.3 µm (sd = 17.1 µm). This mechanism of displacement and recovery may play a role in the protection of teeth during high stress, as during such events, teeth are initially moving rapidly to dissipate high stress concentrations.
To ensure a positive long-term outcome for all-ceramic restorations, the potential stress a restoration may experience should be an important consideration when material choices and design parameters are decided. If all-ceramic restorations are considered for implant-supported cases, the absence of the periodontal ligament and the resultant rigidity of the whole structure should be a consideration in material choice as well as inter-occlusal contact design.
Intra-oral stress is an important factor in the long-term survival of an all-ceramic restoration. The lower average bite forces of females and their higher rate of tooth displacement may provide a lower intra-oral stress environment. This widens the choice of all-ceramic materials to include the use of more translucent materials, with better aesthetics but lower strength for a wider range of teeth for female patients, if the contact point sizes are effectively controlled.