Relative and absolute reliability of medial knee joint space width measurement using ultrasound imaging during valgus stress testing

This study aimed to evaluate both the relative and absolute reliability of ultrasound-based measurements of changes in medial knee joint space width under valgus stress in healthy individuals. Seventeen healthy adult males (34 knees) underwent ultrasound assessments under unloaded and valgus-loaded (10 Nm) conditions. The medial knee joint space width was measured using a standardized ultrasound protocol, and changes were calculated as the difference between loaded and unloaded conditions. Intra- and inter-rater reliability between two physical therapists with different experience levels were evaluated using intraclass correlation coefficients, and absolute reliability was assessed via Bland-Altman analysis, standard error of measurement, and minimal detectable change at the 95% confidence level. A significant increase in medial knee joint space width (ranging from 1.33 to 1.41 mm) was observed under valgus stress compared to the unloaded condition (p < 0.001). The intraclass correlation coefficient for intra-rater reliability across two separate measurement days was 0.88 (95% confidence interval: 0.77-0.94), whereas that for inter-rater reliability was 0.84 (95% confidence interval: 0.70-0.92). Bland-Altman analysis revealed only random measurement error, with minimal detectable change at the 95% confidence level values below 1.0 mm (0.51 mm intra-rater; 0.65 mm inter-rater). Ultrasound imaging could provide sufficient reliability to quantify changes in medial knee joint space width during valgus stress testing in healthy individuals. These values suggest that changes exceeding these thresholds may reflect changes beyond measurement error. However, further studies with larger sample sizes and involving patients with ligamentous injuries are needed to confirm the clinical applicability of this method.