Proximal component modularity in THA - At what cost? An implant retrieval study

Background While modular femoral heads have been used in THA for decades, a recent innovation is a second neck-stem taper junction. Clinical advantages include intraoperative adjustment of leg length, femoral anteversion, and easier revision, all providing flexibility to the surgeon; however, there have been reports of catastrophic fracture, cold welding, and corrosion and fretting of the modular junction. Questions/purposes We asked whether (1) the neck-stem junction showed the same degradation mechanisms, if any, as the head-neck junction, (2) the junction contributed to THA revision, (3) the alloy affected the degree of degradation, and (4) the trunion machine finish affected the degradation mechanisms. Methods We compared 57 retrievals from seven total hip modular designs, three cobalt-chromium-molybdenum and four titanium based: Bionik® (four), GMRS® (four), Margron® (22), Apex® (five), M-series® (five), ZMR® (two), and S-ROM® (15). Macroscopic inspection, microscopy, and micro-CT were conducted to determine the effects of materials and design. Results The cobalt-chromium-molybdenum components showed crevice corrosion and fretting of the neck-stem taper, whereas the titanium components had less corrosion; however, there were several cases of cold welding where disassembly could not be achieved in theater. Conclusions Even with modern taper designs and corrosion- resistant materials, corrosion, fretting, and particulate debris were observed to a greater extent in the second neckstem junction. Titanium-based modular arthroplasty may lessen the degree of degradation, but cold welding of the components may occur. Clinical Relevance Degradation of the second junction contributed to 8 cases of metallosis and two cases of aseptic lymphocyte-laminated vascular-associated lesions contributing to revision.