Non-invasive measurement of lumbar mobility using a flexible curve ruler in patients with total hip arthroplasty

This study aimed to evaluate the reliability and validity of lumbar mobility measurements obtained using a flexible curve ruler (FCR) in comparison with radiographic assessment.

46 postoperative patients who had undergone total hip arthroplasty (THA) were included. Lumbar lordosis (LL) measured using the FCR was calculated using a trigonometric equation based on tracing the spinal curvature at the Th12, L4, and S2 levels in standing and flexed-seated positions. Lumbar mobility was defined as the change in LL between standing and flexed-seated positions and was classified as stiff (ΔLL ≤ 20°), flexible (20° < ΔLL ≤ 40°), or hypermobile (ΔLL > 40°). Radiographic lumbar mobility was estimated from FCR-derived measurements using linear regression analysis.

FCR-derived standing and flexed-seated LL showed moderate to strong correlations with radiographic LL (r = 0.65 and 0.77, respectively; p < 0.01), but were underestimated by 25° and 17°. Lumbar mobility measured by FCR correlated moderately with radiographic values (r = 0.63, p < 0.01) and was underestimated by 6°. Regression analysis indicated that radiographic lumbar mobility can be estimated from FCR measurements (R² = 0.39, p < 0.01). Using the above calibrated model, the agreement with the radiographic classification was 74%.

FCR-based measurements of lumbar alignment and mobility in postoperative THA patients demonstrated moderate to strong correlations with radiographic assessments. Although this method cannot completely replace radiographic evaluation, it may provide a non-invasive approach for repeated assessment of lumbar mobility when radiographic assessment is not feasible.