Within-session repeatability of femoral nerve shear wave elastography across fixed sequential Thomas test-derived protocol in healthy young adults

Background Hip and knee influences on femoral nerve mechanical behavior and SWE repeatability remain underexplored. Objective To describe scanner-derived SWE estimates from an ROI over the femoral nerve across five sequential Thomas test-derived positions and assess within-session repeatability. Methods In a single-session repeated-measures study, 19 healthy young adults underwent SWE in supine, Thomas test-derived, modified Thomas test (mTT) with 45° knee flexion, mTT with 90° knee flexion, and mTT with 90° knee flexion plus active cervical flexion. Shear-wave velocity (SWV; m/s) was the principal outcome. The manufacturer-derived apparent Young's modulus (kPa) was recorded as a secondary descriptive outcome. Three consecutive acquisitions were obtained at each position. Within-session repeatability of the three acquisitions was quantified using ICC, SEM, and MDC95. Repeated-measures ANOVA and Bonferroni-adjusted pairwise comparisons assessed differences across the fixed sequential protocol. Results Repeatability of the average of three acquisitions was high (ICC[ 3 ], 0.964–0.984; SEM, 0.09–0.10 m/s and 1.59–2.63 kPa; MDC95, 0.25–0.28 m/s and 4.40–7.29 kPa). Estimates differed across sequential positions (SWV: F = 40.20, p < 0.001, ηp 2 = 0.691; kPa: F = 36.68, p < 0.001, ηp 2 =0.671). Mean SWV increased from 2.87 ± 0.66 m/s in supine to 4.80 ± 0.59 m/s in mTT with 90° knee flexion. Cervical flexion produced no significant additional increase. Conclusion In healthy young adults, scanner-derived SWE estimates obtained from an ROI positioned over the femoral nerve showed high immediate within-session repeatability across a fixed sequential five-position protocol. Across this protocol, the highest apparent stiffness and SWV estimates were observed during the mTT position with 90° of knee flexion.