Magnetic Resonance-Based Determination of Local Tissue Infection Involvement in Patients with Periprosthetic Joint Infection Following Total Hip Arthroplasty

<b>Background:</b> Surgical treatment of periprosthetic joint infection (PJI) after total hip arthroplasty (THA) remains challenging, with thorough debridement seen as critical for success. While revision THA is well documented as the standard treatment for PJI, data on infection spread beyond the periprosthetic joint into surrounding soft tissue remain limited-this is the focus of our study. <b>Methods:</b> We retrospectively reviewed 558 patients who underwent a revision THA due to PJI at a single academic institution from January 2011 to December 2023. Out of 558 patients, 46 (8.2%) underwent a Magnetic Resonance Imaging (MRI) scan of their hip joint prior to their revision THA due to suspicion of a PJI. MRI reports were manually chart-reviewed to evaluate patients for evidence of infection spread beyond the constraints of the periprosthetic joint space. <b>Results:</b> Out of 46 patients with hip MRI prior to rTHA, 45 (97.8%) had pathological findings, and 34 (73.9%) had reports suggestive of periprosthetic joint fluid collection. The infected joint effusion extended in 30 cases (65.2%) from the capsule into the surrounding soft tissue, including the greater trochanteric region (17.4%), iliopsoas area (15.2%), anterolateral musculature (13.0%), surrounding soft tissue (10.8%), gluteal (8.7%) and obturator muscles (8.7%), and iliotibial band (4.3%). Capsule thickening was observed in 23.9% of the cases. <b>Conclusions:</b> Our study found that the vast majority (97.8%) of the hip MRIs had pathological findings, with periprosthetic joint fluid collection being a predominant feature in 73.9% of the cases. The extraarticular extension of these fluid collections was observed in over two-thirds (30/34) of affected patients, suggesting that PJI is often not confined to the capsule. MRI studies can help surgeons obtain prior knowledge of these cases and develop a more comprehensive surgical approach for infection debridement, potentially helping improve surgical treatment outcomes after PJI.