Dental composite resins containing silica-fused ceramic single-crystallinewhiskers with various filler levels

Currently available direct-filling composite resins are susceptible to frac ture and hence are not recommended for use in large stress-bearing posterio r restorations involving cusps. The glass fillers in composites provide onl y limited reinforcement because of the brittleness and low strength of glas s. The aim of the present study was to use ceramic single-crystalline whisk ers as fillers to reinforce composites, and to investigate the effect of wh isker filler level on composite properties. Silica particles were fused ont o the whiskers to facilitate silanization and to roughen the whiskers, ther eby improving retention in the matrix. The composite flexural strength, ela stic modulus, hardness, and degree of polymerization conversion were measur ed as a function of whisker filler mass fraction, which ranged from 0% to 7 0%. Selected composites were polished simulating clinical procedures, and t he surface roughness was measured with profilometry. The whisker composite with a filler mass fraction of 55% had a flexural strength (mean +/-SD; n = 6) of 196 +/- 10 MPa, significantly higher than 83 +/- 14 MPa of a microfi ll and 120 +/- 16 MPa of a hybrid composite control (family confidence coef ficient = 0.95; Tukey's multiple comparison). The composite modulus and har dness increased monotonically with filler level. The flexural strength firs t increased, then plateaued with increasing filler level. The degree of con version decreased with, increasing filler level. The whisker composite had a polished surface roughness similar to that of a conventional hybrid compo site (p > 0.1; Student's t). To conclude, ceramic whisker reinforcement can significantly improve the mechanical properties of composite resins;the wh isker filler level plays a key role in determining composite properties; an d the reinforcement mechanisms appear to be crack pinning by whiskers and f riction from whisker pullout resisting crack propagation.