Douglas E. Padgett, Timothy M. Wright

     The introduction of total hip arthroplasty, in its modern form, by Sir John Charnley has been one of the most impactful medical advances of the last century, providing patients with pain relief, improved function, and better quality of life. Since then, surgeons and engineers have sought to improve surgical techniques and prosthetic designs and materials in an attempt to make a great intervention even better. The use of modularity, particularly at the femoral stem-neck junction, was an attempt to reproduce femoral offset and version and to restore limb length apart from stem fixation and appeared attractive in concept. Initial bench testing by Viceconti et al.1 demonstrated minimal fretting and no evidence of corrosion at the taper junctions. These findings, coupled with the long-term success of a predicate modular stem, the S-ROM (DePuy, Warsaw, Indiana)2, seemed to assuage the fears of failure related to these junctions. Despite little data to support any improvement in clinical outcomes, modular hip stems were introduced into the marketplace and their use was embraced by many.

      This recent disturbing report by Cooper and associates represents a collaborative multicenter effort3 detailing a series of eleven patients (a total of twelve hips) who underwent primary cementless total hip arthroplasty employing a modular femoral stem design (Rejuvenate; Stryker, Mahwah, New Jersey) consisting of a titanium-molybdenum-zirconium-iron alloy body (TMZF; Stryker) mated with a modular cobalt-chromium alloy neck (Rejuvenate; Stryker). The articular surfaces included both ceramic (eight hips) and cobalt-chromium (four hips) bearings against highly cross-linked polyethylene. All eleven patients presented with new onset pain at a mean of 7.9 months postoperatively. The authors describe a careful workup, including serologic studies (complete blood count, C-reactive protein, and erythrocyte sedimentation rate), the results of which varied from slightly elevated to normal. Ten of the twelve hips underwent aspiration as part of a protocol to rule out infection, and all ten had negative results. Serum metal ion analysis demonstrated a disproportionate elevation of cobalt and, to a lesser extent, chromium, with normal levels of titanium. Cross-sectional imaging on ten of the twelve hips with use of the metal artifact reduction sequence (MARS) magnetic resonance imaging (MRI) technique demonstrated either large fluid collections and/or pseudotumor formation in all but one.

     On the average, 8.6 months transpired from the onset of pain until a diagnosis was established and revision surgery was performed. Due to presumptive adverse local tissue reaction related to the implant system, all patients underwent revision at a mean time of 15.2 months after the index procedure. Revision surgery was difficult in every case. Seven of the twelve hips required an extended trochanteric osteotomy to remove the implant; of the remaining five hips revised without osteotomy, three sustained a fracture during or after surgery. Histology of submitted tissue samples demonstrated marked necrosis and dense lymphocytic infiltrates in the majority of cases. Polymorphonuclear leukocytes, which one might observe in acute inflammatory conditions such as infection, were rare. Pale-green chromium phosphate particles, byproducts of corrosion, were observed in many of the samples. This finding was consistent with perhaps the most compelling piece of evidence for the cause of failure, the presence of severe corrosion at the femoral neck-body junction and, to a lesser degree, at the articular femoral head-neck junction in all of the retrieved femoral components.