Motor-Evoked Potential Analysis of Femoral Nerve Status During the Direct Anterior Approach for Total Hip Arthroplasty

During the direct anterior approach for total hip arthroplasty, a retractor is placed on the anterior wall of the acetabulum to retract the iliopsoas muscle. This step with the retractor has been reported to put the patient at risk for femoral nerve damage. The present study aimed to clarify the effects of the anterior acetabular retractor on the status of the femoral nerve during the direct anterior approach on the basis of transcranial motor-evoked potential (MEP) analysis.Between July 2016 and February 2017, 22 patients underwent primary total hip arthroplasty via the direct anterior approach with MEP analysis. The integrity of the femoral nerve was tested at 3 time points: preoperatively, as a control (first period); just after retractor placement on the anterior wall of the acetabulum (second period); and after the procedure (third period). The association between operative time and femoral nerve status was examined. Postoperative femoral nerve damage was determined by the presence of causalgia and the results of a manual muscle test (MMT) for strength of knee extension.The mean amplitude of the femoral nerve was significantly reduced, from 100% in the first period to 54% (range, 5% to 100%) in the second period (p < 0.01), but then significantly improved to 77% (range, 20% to 100%) in the third period (p < 0.01). In 17 (77%) of the 22 patients, the amplitude of the femoral nerve in the second period was reduced, while only 5 patients (23%) showed no reduction. The mean operative time was 83 minutes (range, 63 to 104 minutes), and no significant correlation was observed between operative time and improvement of femoral nerve status between the second and third periods (p = 0.83 and r = -0.05). All 22 patients had a postoperative MMT grade of 5 for knee extension without causalgia of the femoral nerve.On the basis of the MEP analysis, 17 (77%) of the 22 patients showed a significant reduction of the femoral nerve amplitude despite careful placement of the retractor on the anterior wall of the acetabulum. Although this reduction appears reversible, placement of an anterior retractor should be performed with careful attention to the femoral nerve.Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.