Restoration design
A range of research activity is ongoing to evaluate and develop new materials and processing techniques for preparing restorations.
Development of CAD/CAM techniques for ceramic restorations
New ceramic materials offer superior mechanical properties to traditional materials. In theory, therefore, thinner restorations can be created, requiring less of the patient's tooth structure to be removed.
The objective of the research at Sheffield has been to evaluate new methods and materials for minimal preparation designs.
Right: A cast ready to be scanned under computer control.
In the first stage of this work we have evaluated how precisely CAD/CAM technology can create restorations from a range of ceramics available to the dentist. One of the critical criteria in this is how acute an angle can be milled in the ceramic before it shows excessive chipping, and studies here have show a considerable degree of variation between different ceramics available.
Future work will evaluate manufacturing tolerances and the resulting fracture resistance of very thin restorations, before clinical evaluation.
Above left: Marginal chipping in acute angle restorations viewed by electron microstopy, scale bars are 100 µm long.
C-factor effects on composite restorations
Composite dental materials have been in clinical use since the late 1950s. One of the characteristics of dental composites is shrinkage during free radical polymerization as monomer molecules are converted into a polymer network, reducing intermolecular spaces. Dental composite polymerization shrinkage ranges between 2 and 6% by volume. Several properties affect the performance of this dental material. Configuration factor (C-factor) is a theoretical concept, which attempts to explain the nature of polymerisation shrinkage of composites. Literature is replete with mention of this factor, however evidence of the validity of this factor is lacking.
Current research at Sheffield is focussed on evaluating models for predicting microleakage, which may occur as a consequence of this shrinkage, and has important clinical consequences. This study attempts to determine whether C-factor or volume of the cavity is the more accurate predictor of microleakage.
Dimensional stability of restorative materials and systems
The restored tooth is a complex compound structure made up of materials with very different physical properties. It is the interaction of these materials in a synergistic and complementary manner that will determine the ability of the restored tooth to function as an effective unit. Thus the optimisation and characterisation of interfaces and the dimensional changes that affect polymer-based dental materials is critical in re-establishing the structural integrity of the damaged tooth.
Dr N. Martin has been active in the design and construction of a novel system for the in-vitro measurement of dimensional changes associated with restorative materials and impression materials. The same technology has been applied to characterise the cuspal deflection of teeth that occurs as a result of the dimensional changes of these materials and the dimensional stability of dental impression materials.
Researchers involved in this work are Pallavi Gaitonde (CAD/CAM), Dr Nicolas Martin (dimensional stability), Effi Tsitrou (CAD/CAM), Suresh Nayar (C-factor) and Prof. Richard van Noort.