Mechanical characteristics of implant/tissue interphases.

Abstract

In this study, tissue integration, material properties and micromechanical behavior of implant/hard tissue interfaces were determined analytically and experimentally. A global-local modeling scheme was developed to account for the coupling effect of the global shape of implants and the microstructure of implant coatings and ingrown tissues on the implant mechanical fields. A new mechanical model based upon homogenization theory was developed quantifying local stresses of a bimaterial interphase composite. Both experimental tests and numerical analyses were used to validate the model. The model predicted the same orthogonal elastic properties as those measured experimentally, and estimated interfacial stresses with 80% accuracy. As determined by the model, the implant-tissue interphase composite was macroscopically orthotropic. Local interfacial strains and stresses were inhomogeneous. This dissertation demonstrates that the surface architectures of the osseointegrated implant determine interfacial strains and stresses, and control osseointegration. Interphase characteristics such as amount of bone ingrowth, potential for material failure, and degrees of tissue damage were analyzed by using the resulting stresses and strains predicted from the homogenization method. The results of an ingrowth study, which statistically correlates tissue strains to actual ingrowth data from a canine tibia implant, support the hypothesis that tissue strains regulate remodeling of osseointegrated tissue. In addition, a strain magnitude of 1500 $\mu\varepsilon$ serves as the threshold determining whether bone formation or resorption occurs at the interphase. Results also demonstrated that proximally coated total hip replacement (THR) provides better bone fixation and less risk of failure than fully coated THR, due to more efficient load transfer of the former through the interphase. As a result of less bone ingrowth on fully coated THR, tissue microdamage was largely concentrated on the proximal femur. The architecture and location of both tissue and implant coating had significant effects on local stresses and strains and in determining the bone ingrowth pattern and predicted success of prostheses. The research enhances the information that is significant in the selection of porous coated prostheses for clinical usage.