CONCERNS WITH MODULARITY IN TOTAL HIP-ARTHROPLASTY

Modularity is being diversified in total hip prostheses to increase su rgical latitude in optimizing implant fixation and adjusting hip biome chanics. However, several problems have been clearly identified with i mplant modularity. First generation metal-backed acetabular components have shown deficiencies in the locking mechanism, the congruency and extent of polyethylene liner support, and polyethylene thickness, all of which have been implicated in accelerated polyethylene wear and fai lure. Evidence of screw motion against the metal backing, release of p articulate material, and focal osteolysis have also been observed. At the head/neck junction evidence of corrosion and fretting has been doc umented with both similar-metal and mixed-metal taper combinations. Fe moral prostheses with other sites of modularity present additional con cerns with regard to mechanical integrity and generation of particulat e debris by fretting. The modular junctions of three hip prostheses, t he S-ROM, Infinity, and RMHS, were subjected to wet environment high c ycle mechanical testing in a worst-case loading scenario. Preliminary results at relatively low loads up to three times body weight indicate d gross stability of the modular junctions with evidence of minor fret ting damage. Analysis of water solutions surrounding the modular junct ions after ten to 20 million loading cycles yielded counts of one to t hree micron sized particles totalling several hundred thousand to seve ral million. It is unknown what quantity of particulate material is su fficient to cause macrophage-mediated osteolysis or whether the debris from modular junctions can cause third-body wear of the articulating surfaces. Modular hip prostheses should be examined under stringent te st conditions in order to characterize their fretting behavior and est ablish their mechanical limitations.