Bone-implant Interface Mechanics of in vivo Bioinert Ceramics.

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We have previously demonstrated that there was no significant difference between the affinity of bone to bionert ceramics and stainless steel in a histological study. In this study, the bone-implant interface shear strength of alumina ceramics (Al2O3), zirconia ceramics (ZrO2), stainless steel (SUS316L) and sintered hydroxyapatite (HA) were compared in 13 dogs using a transcortical push-out model of the femur 4 and 12 weeks after implantation. The interface shear strength of HA was significantly greater than that of alumina ceramics, zirconia ceramics and stainless steel (P<0.001). There was no significant difference between bioinert ceramics and stainless steel. Young's modulus of stainless steel is higher than that of cortical bone, but Young's modulus of bioinert ceramics is much higher than both cortical bone and stainless steel. With the stimulus of weight-bearing, the shearing stress along the interface becomes greater as the difference between Young's modulus of the implant and that of bone increases, and functional compatibility is believed to decline. Consequently, bioinert ceramics offer little advantage in terms of biomechanical compatibility to bone over metal alloys. We recommend a review of the use of bioinert ceramics at the bone-implant interface in orthopaedic prostheses.