The Diagnosis of Infection in Metal-on-Metal Hip Arthroplasties.

BACKGROUND: Distinction of aseptic from septic hip arthroplasty failure can be challenging. Some studies report an increased incidence of septic failure with metal-on-metal (MoM) hip arthroplasties. The Musculoskeletal Infection Society (MSIS) have formulated criteria to facilitate the diagnosis of periprosthetic joint infection (PJI). In this study, we determined the prevalence and histologic features of septic MoM hip failure.

METHODS: Overall, 104 cases of failed MoM hip arthroplasty, classified as septic or aseptic by MSIS microbiological criteria, were analyzed. The overall prevalence of septic failure was determined and the nature of the causative organisms noted. The extent of the neutrophil polymorph (NP) infiltrate in periprosthetic tissue in all cases was analyzed by hematoxylin-eosin and chloroacetate esterase staining.

RESULTS: The prevalence of septic MoM hip arthroplasty failure was 6.7%. Infective organisms were coagulase-negative Staphylococcus in 4 cases; Staphylococcus aureus, Streptococcus, and Propionibacterium species were isolated in the remaining cases. Chloroacetate esterase staining facilitated identification of NPs. All cases of PJI contained more than 5 NPs per high-power field (HPF) on average. Four cases of aseptic MoM implant failure contained scanty or scattered NPs (less than 5 per HPF on average).

CONCLUSION: The prevalence of PJI as a cause of MoM hip arthroplasty failure was relatively high compared to other hip bearing combinations; however, the organisms responsible were similar. Histologically, a minority of aseptic MoM implant failures contained some NPs, but the MSIS criteria for the histologic diagnosis of PJI (>5 NPs/HPF) correctly identified all microbiologically confirmed cases of septic failure.