The use of the Epoch (Zimmer, Warsaw, IN) composite femoral stem for total hip arthroplasty (THA) has gained considerable popularity in recent years since being redesigned in the image of the Versys (Zimmer, Warsaw, IN) fully coated stem system. Proposed advantages of this composite stem include an extensive area of titanium fiber metal coating for porous ingrowth, multiple neck and metaphyseal options, gradated lengths based on component diameter, and its low modulus of elasticity. The material components include a solid cobalt-chromium-molybdenum alloy core and a polyetheretherketone (PEEK) polymer matrix - which are designed to closely match the stiffness of a normal femur to potentially reduce femoral stress shielding. The device has performed well in clinical trials at short- and long-term follow-ups, demonstrating excellent implant stability, histologic osseous ingrowth, and good proximal bone density preservation [1, 4, 6, 13]. A prospective study on 31 cases demonstrated no stem component revision at 10.1 years mean follow-up, but acetabular component revision in 8 cases (3 for liner dissociation, 5 for polyethylene wear) . Complications with this stem have been minimal, and the perceived benefits are attractive [4, 6, 10]; consequently, the Epoch composite femoral stem is being used with increased frequency.
We describe two cases of a previously unreported mechanism of failure for the Epoch composite femoral stem. In both cases, the patients developed late, chronic prosthetic infections with subsequent loosening of the femoral component and delamination of the PEEK from the cobalt-chromium-molybdenumalloy core. The fibermetal coating remained well-fixed to the host bone and the PEEK composite, yet was dissociated from the central core. Both patients required two-stage exchange procedures to eradicate the infection and manage the septically loose components.
HSS Journal, an academic peer-reviewed journal published three times a year, February, July and October. The Journal accepts and publishes peer reviewed articles from around the world that contribute to the advancement of the knowledge of musculoskeletal diseases and disorders.