Investigation of bone response to implant materials by electron microscopy and computer simulation

Abstract

(cont.) implementation of this scintigraphic method for quantitative studies of osteoblast-mediated mineralization in vitro. A 2-D truss finite element model is used to study the remodeling of trabecular bone. Using strain energy density (SED) as the optimization object and the trabecular width as the optimization variable, an optimal structure with minimum SED was achieved. This structure is similar to real bone in the dense outside, porous inside, and orientation of the trabeculae. The bone density distribution pattern also matched with previous result by other people. Different implants were introduced to simulate the replacement for the femoral head. It has been proved that the difference in Young's modulus between bone and implant materials is the main reason for the long-term bone loss (stress screening). This problem can be alleviated by proper implant design and resurfacing instead of replacing the whole femoral head.

Initial fixation with bone and the long term bone loss are two main problems associated with total hip replacement (THR), which are studied by electron microscope and computer simulation in this thesis. Bare Titanium-6 wt% Aluminum-4 wt% Vanadium (Ti64) implants, Ti64 implants with plasma-sprayed hydroxyapatite (PSHA), and Ti64 implants with electrochemically-deposited hydroxyapatite (EDHA) were implanted into canine trabecular bone for 6 hours, 7 days, and 14 days to study the initial bone formation on these implant materials. Scanning electron microscope (SEM) results showed that at 7 days PSHA had a higher bone apposition ratio than Ti64 and EDHA samples; however, at 14 days, the bone apposition on EDHA increased to be similar to PSHA, much higher than that on Ti64. By transmission electron microscope (TEM), a layer of new bone tissue was observed on PSHA coating surface; in contrast, no much bone was found on EDHA surface. At 14 days, substantial bone was found on both EDHA and EDHA coating surface. Technetium-99m-methylene-diphosphonate (Tc-99m-MDP) labeling was used to quantify mineralization of cultures of MC3T3 osteoblast-like cells in vitro on tissue culture polystyrene (TCPS). The gamma signal from labeled samples was imaged with a gamma camera and compared with the calcium content in the same samples determined by inductively coupled plasma. The high correlation (0.88) between these two values validated that radiotracer uptake method as a quantitative analytical tool for certain mineralization studies in vitro. There was an association between mineralization and radionuclide uptake in the MC3T3 cultures on titanium alloy, but the attenuation of the gamma photons by the metal resulted in a less robust correlation. The results warrant