Background and Objective: Literature suggests the use of thinner indirect partial coverage restorations. The purpose of this in vitro study was to evaluate the influence of cavity design optimisation and biosubstitution with various cuspal and fissure designs, for indirect lithium disilicate partial coverage restorations. The influence on the initial crack formation and subsequently the catastrophic fracture force were evaluated.

Method: Forty molars were prepared and randomly divided into four test groups (A, B, C and D). Indirect partial coverage restorations were milled with different cuspal and fissure thicknesses of lithium disilicate ceramic. Group A (cusp 1; fissure 0.7 mm thickness) and group B (cusp 0.8; fissure 0.5 mm) received a composite restoration in the class I preparation additionally. Group C (cusp 1; fissure 1.7 mm) and group D (cusp 0.8; fissure 1.5 mm) had ceramic included in the class I cavity. The restored specimens were thermo-cycled and force application was completed with a Universal testing machine in two phases. Phase one was the determination of initial first crack formation with various pre-set forces. Phase two was the maximum catastrophic fracture force. Statistical analysis was completed with the Kruskal-Wallis test and confirmed by the Dunn-Sidak multiple comparison test (p< 0.05).

Results: The catastrophic fracture force in group A (2460N) was significantly greater than group B (1523N), with no difference between group C (3142N) and group D (2591N).

Conclusion: The cuspal thicknesses of 1 mm for indirect partial coverage restorations were advisable, with or without composite in the class I cavity.