The effects of cortical bone viscoelasticity on the short-term fixation of press-fit cylindrical intramedullary rods

摘要

Cementless femoral implants have demonstrated widespread clinical success, particularly in the patient populations for which various cemented techniques have been troublesome [1-4]. Long-term fixation and consequent clinical stability occurs primarily via bony ingrowth into a porous-coated implant surface. The adequacy of this biologic fixation depends in part upon the initial or short-term fixation of the implant with respect to the adjacent bone [5,6]. Short-term fixation refers to the post-operative limitation of relative motion between the porous-coated implant surface and the adjacent bone structure. This relative motion, or micromotion, may be limited by utilizing a porous implant coating in concert with a stem press-fit both increase the frictional resistance to motion [7]. The holding power of the press-fit over time is dependent upon the viscoelastic nature of cortical bone. Data has long been available in the literature for the viscoelastic behavior of cortical bone in the longitudinal direction [8], however, a transverse viscoelasticity model is required to evaluate a press-fit since it generates considerable radial and circumferential stress but very little axial stress. Only recently has such a model become available [9]. The objective of this study was to parametrically evaluate the effect of transverse cortical bone viscoelasticity on initial diaphyseal fixation of an implant for various degrees of press-fit and coefficients of friction between the implant and cortical bone. It is well known that as the amount of mechanical interference between the bone and implant increases, so does the radial stress at the bone-implant interface; however, the tendency of these stresses to relax with time, as represented by the viscoelastic material model for the cortical bone, increases nonlinearly with the stress magnitude. Consequently, it was hypothesized that there is an amount of stem-bone interference beyond which no additional gains in initial fixation are attained due to the relaxation of the radial stresses within the cortical bone.