OR04 - Fiber Posts, a Fracture Mechanics Approach to Their Failure

Abstract: Glass fiber posts are widely used to restore severely damaged teeth. However, clinical failure has been documented to be more frequent than for tough metallic posts. To explicate the cause, we measured their toughness, anisotropy, and variation using a fracture mechanics approach sensitive to crack initiation and propagation. Glass and carbon fiber posts (n=16) were embedded in epoxy resin horizontally to model clinically-relevant flexural loading and prepared to mid-section by sequential polishing to 1 µm alumina (n=16), loaded with a pyramidal microhardness indenter at 100g for 15s. Indentations and cracks were measured using a digital video micrometer. Fracture toughness, the amount of energy needed to drive crack growth, K_IC, was calculated. Independent t-tests were used to identify differences (α = .05). Glass and carbon fiber posts had low fracture toughnesses, with overall mean K_IC K_IC (standard deviations) in the lateral plane of 0.40 (0.09) and 0.69 (0.16) MPa.m^1/2 respectively, significantly different (p <.01). Anisotropy, the effect of grain/uniaxial fiber orientation, was evident with the along-fiber to across-fiber toughness ratios for glass and carbon fiber posts of 0.71 and .93 respectively, significantly different (p <.05). The coefficients of variation for the K_IC of glass and carbon fiber posts were 26% and 21% respectively, substantially different. Fiber posts had surprisingly low fracture toughness when measured by microindentation, approximately 2 orders of magnitude lower than generally reported for metals such as stainless steel or titanium. Glass fiber posts were also inferior to carbon fiber posts in fracture toughness, anisotropy, and coefficients of variation.