Quantitative ultrasound evaluation of medial compartment knee osteoarthritis under standing weight-bearing condition

To evaluate the diagnostic value of ultrasound for joint space width in knee osteoarthritis under standing weight-bearing conditions and identify reliable sonographic parameters for osteoarthritis severity grading, especially end-stage knee osteoarthritis.

This cross-sectional study enrolled 256 patients with knee pain and suspected knee osteoarthritis from June to December 2025. All participants underwent standing weight-bearing radiography and ultrasound of the affected knee at 20° and 45° flexion. Radiographic assessments (Kellgren-Lawrence grading, joint space width, minimal joint space width) were performed by two senior orthopedists, while ultrasound evaluations (joint space width, total femoral and tibial osteophyte height, medial meniscal extrusion) were conducted by another two experienced orthopedists blinded to radiographic findings. Interrater reliability was analyzed using the weighted kappa coefficient and intraclass correlation coefficient. The Pearson product-moment correlation coefficient quantified the correlation between sonographic and radiographic measurements, and receiver operating characteristic curve analysis determined the diagnostic efficacy of ultrasound parameters for Kellgren-Lawrence grading.

Participants had a mean age of 55.9 ± 11.7 years, mean body mass index of 25.22 ± 4.32 kg/m², and 81.6% were female. Interrater reliability for all measurements was moderate to excellent (intraclass correlation coefficient/weighted kappa: 0.54-0.92). With increasing Kellgren-Lawrence grades, radiographic and ultrasound joint space width decreased, while total femoral and tibial osteophyte height and medial meniscal extrusion increased. A total of 29 patients (11.3%) were reclassified to higher Kellgren-Lawrence grades at 45° versus 20° flexion. High correlations were observed between radiographic and sonographic measurements. For differentiating Kellgren-Lawrence Grade 3 from 4, ultrasound joint space width at 45° flexion had a higher area under the curve (0.839, 95% confidence interval: 0.766-0.912; cutoff: 1.75 mm) than at 20° (0.825, 95% confidence interval: 0.745-0.905; cutoff: 1.95 mm).

Sonographic joint space width measured at 20° and 45° knee flexion under standing weight-bearing conditions yielded AUC values similar to those obtained from weight-bearing radiographs for diagnosing medial compartment KOA. Ultrasound may act as a potential supplementary imaging alternative to conventional standing radiograph for grading the severity of medial compartment KOA.