| Ceramics in Modern Dentistry
Ceramics have developed enormously in recent years and when combined with new processing techniques this has created new opportunities for the dental application of ceramics. Consequently, the all-ceramic fixed partial denture (FPD) is gaining popularity as an alternative to the well established metal-ceramic bridge. One approach involves using a high strength ceramic rather than metal core, which is glazed with an aesthetic veneer and then cemented in place using conventional luting cements. Versions of these are the glass infiltrated alumina or zirconia cores and core systems made of pure alumina or yttria stabilised zirconia. The system explored in the paper by Stappert et al. is different in that it is based around a resin-bonded ceramic core, not a cemented core, and is made of a lithium disilicate glass-ceramic produced by a hot pressing route.
At present there is little clinical information that can guide the practitioner in the decision making process about new all-ceramic FPD systems. The evidence base consists primarily of laboratory data of fundamental materials properties such as strength and toughness or, as is the case with this paper, data derived from a simulation of the clinical situation. The authors chose to compare the performance of two resin-bonded ceramic three unit FPDs and a cemented metal-ceramic bridge. They assessed how such bridges would perform under simulated cyclic loading conditions using the marginal integrity as the assessment parameter. The marginal integrity was assessed by measurement of the marginal gap and it was found that immediately before and after cementation there was no significant difference in the marginal gap, in the region of 50-70 mm for all three systems and considered to be well within clinically acceptable limits. Thus the quality of construction of the FDPs with respect to their marginal fit is very acceptable.
Fatigue failure due to cyclic or thermal loading is now recognised as a potentially significant contributor to the ultimate failure of dental restorations and this paper shows that thermo-mechanical cyclic loading does not affect the marginal accuracy as determined by no change in the marginal gap. This is very reassuring. However, it should not be assumed from this observation that the restorations are unaffected by the simulated loading conditions as marginal accuracy is only one aspect of many factors contributing to the clinical performance.
For the resin-bonded all-ceramic FPDs an important consideration is the integrity of the bond between the resin and the ceramic. If this is lost then its structural strength can be seriously compromised. Examination of the marginal accuracy will not necessarily pick this up and thus we must look forward to more work being done in this area so as to reassure us that all-ceramic FPDs will deliver a clinical performance that will be acceptable. This paper represents an important step in the right direction. |
