In vitro simulation of clinical fabrication of direct dental restorations

摘要

The clinical placement of direct composite restorations involves many complex variables. The investigations described in this thesis had the aim of simulating some key aspects of behaviour under controlled in vitro conditions. The first experimental section was concerned with two aspects of the rheology of the unset pastes, namely packing stress dynamics and stickiness. A self-standing rheological instrument for the rapid in vitro characterisation of packing behaviour of resin-composites was developed and applied to unset composites at 23°C and 37°C. It was found that the relative viscosity of all materials rapidly decreased with increasing temperature. A new instrument for measuring stickiness was developed to have a basis for the selection of materials showing good adaptation to the cavity floor and walls, and optimized approximal contact areas. The effect of temperature on the stickiness property was evaluated. The result shows that the composites known to have a higher filler loading usually exhibited low stickiness and that the increase in temperature led to an increase in stickiness. The second experimental section studied several factors involved in the clinical setting process, mainly, depth of cure, exotherm and shrinkage. These were assessed in connection with light cure units of conventional and high intensity. One of the hypotheses to be tested was whether any useful network-polymerisation was attained by application of the 1, 2, 3, or 5 s cure times from the high intensity light unit. Most measurements were preformed at 23°C. but some specimen groups were also preconditioned at 37°C. Depth-of-cure values obtained by (Apollo 95E) were acceptable at (5 s) cure, but these values were reduced significantly with lower exposure times (1 and 2 s). The exotherm of the specimens cured by (Apollo 95E) also showed inadequate cure with 1-3 s irradiation. Shrinkage-strain kinetics were studied by using the "bonded-disc" technique, in which the axial shrinkage of a thin disc was recorded. The effect of Apollo 95E curing was investigated. It was concluded that the shrinkage strain increased significantly with temperature, because of greater network conversion. The several studies with the high intensity light source showed that it only gave acceptable results of clinical significance when irradiation was for a period not less than 5 sec.