This application proposes studies which address the problem of suboptimal fixation of porous coated implants due to poor bone ingrowth.
Two specific aims are described. The first is to determine if different initial interface strains produce different amounts of bone ingrowth. The second is to determine if interface tissue adapts to limit strains within a range of 200 to 2500 microstrain as has been proposed for cortical bone. The recent development of an homogenization sampling procedure by the applicant provides the capability for estimating tissue strains realistically for the first time. The technique, combined with a unique, well-established canine model system which allows controlled implant loads to be applied to exposed trabecular bone of the canine distal femur, will be used to test the hypothesis that interface tissue adapts to limit strain within a homeostatic range. Three different groups of animals will be subjected to three different magnitudes of loading, ranging from 0 to 8 pounds. The animals will be sacrificed at 6 months post surgery and bone cores will be obtained. Type I pro- collagen and osteoclast resorption will be measured in the experimental limb and compared to the contralateral limb to determine how close the bone is to being at equilibrium. Backscattered scanning electron microscopy (SEM) will be used to construct a 3-D digitized image of the ingrowth in each platen region. Micro-computed tomography (micro-CT) will be used to quantify bone architectural changes. Images from both SEM and micro-CT will be used to construct 3-D microstructural finite element models of the implant tissue interface and the surrounding bone. Tissue level strains will be estimated using the homogenization sampling procedure and used to test the hypotheses concerning tissue adaptation. If trabecular bone tissue does adapt to exist within a homeostatic strain range, this would provide criteria for designing porous coated implant interfaces that may achieve better bone ingrowth fixation.
