Postoperative fluid collection at the proximal neurovascular bundle level is associated with early pain and reduced knee extensor strength after direct anterior total hip arthroplasty

Background Early anterolateral thigh pain after direct anterior total hip arthroplasty (DAA-THA) is common, and more severe symptoms than expected in the usual postoperative course can be difficult to interpret clinically. This study evaluated whether a landmark-based ultrasonographic pattern of postoperative fluid collection (POFC) at a reference level corresponding to a reported course of the proximal neurovascular bundle is associated with early pain and knee extensor strength. Methods In this prospective cohort study, ultrasonography was performed at 1 week postoperatively in 182 patients (182 hips) who underwent DAA-THA. At a prespecified reference cross-section corresponding to a reported level where the proximal neurovascular bundle crosses the femoral midline, POFC extending medially beyond the femoral midline was classified as a midline-crossing POFC. Co-primary outcomes were proximal anterolateral thigh pain while walking, assessed using a numerical rating scale, and knee extension torque at 1 week. P -values for the co-primary outcomes were Holm-adjusted. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) subscales were analyzed longitudinally. Results Of the 182 hips, 37 were classified as midline-crossing POFC and 145 as non–midline-crossing POFC. Compared with the non–midline-crossing group, the midline-crossing group had greater proximal anterolateral thigh pain during walking (adjusted mean difference, 3.35 points; 95% confidence interval [CI], 2.88–3.81; Holm-adjusted p < 0.001) and lower knee extension torque (adjusted mean difference, − 0.31 Nm/kg; 95% CI, − 0.38 to − 0.24; Holm-adjusted p < 0.001). WOMAC pain scores showed a significant group-by-time interaction. Conclusions Midline-crossing POFC at 1 week after DAA-THA was associated with greater early walking pain and lower knee extensor strength. Ultrasound assessment at this reference level may provide useful adjunctive clinical information for selected patients whose early postoperative anterolateral thigh pain and reduced knee extensor performance appear greater than expected.