Abstract:[Objective] To investigate the relationship between the ultrasonic morphology of the medial stable structures of knee osteoarthritis (KOA) and the clinical presentations of the knee. [Methods] A total of 202 patients with unilateral knee KOA hospitalized in our hospital from March 2023 to March 2024 were included in this study. The ultrasound measurements and clinical data of the patients were collected. The correlation between ultrasonic measurements and the clinical presentations of KOA was analyzed. [Results] According to the self-reported knee instability score, the patients were divided into two groups. Of them, 140 patients were fall into the unstable group, accounting for 69.3%; while other 62 patients (30.7%) were in the stable group. The unstable group proved significantly greater than the stable group regarding to WOMAC [(45.2±4.8) vs (29.8±3.8), P<0.001], VAS [(6.3±1.7) vs (4.4±1.5), P<0.001], JLCA [(5.4±1.3)° vs (2.9±1.1)°, P< 0.001], the thickness of the medial collateral ligament [(3.5±1.4) mm vs (2.9±1.3) mm, P<0.001], extrusion of medial meniscus [(3.8±1.4) mm vs (2.2±0.9) mm, P<0.001], whereas the former was significantly less than the latter in terms of HSS score [(65.3±5.1) vs (88.6±3.2), P< 0.001], knee flexion-extension ROM [(118.4±5.3)° vs (130.5±5.13)°, P<0.001], HKA angle [(170.9±3.7)° vs (176.2±3.1)°, P<0.001], the muscle thickness [(1.8±0.6) cm vs (2.4±0.5) cm, P<0.001] and the cross-sectional area of the medial vastus muscle [(9.1±2.2) cm2 vs (11.3± 2.4) cm2 , P<0.001]. There was no significant difference in K-L rating between the two groups (P>0.05). Paired correlation analysis showed that the thickness and cross-sectional area of the vastus medialis muscle were significantly positively correlated with self-reported knee instability score, HSS score, HKA angle and knee flexion-extension ROM (P<0.05), while significantly negatively correlated with WOMAC,VAS and JLCA angle (P<0.05). On the contrary, medial collateral ligament thickness and medial meniscus extusion were significantly negatively correlated with self-reported knee instability score, HSS score, HKA angle and knee ROM score (P<0.05), but were significantly positively correlated with WOMAC, VAS and JLCA angle (P<0.05). There was no correlation between the above ultrasound indicators and K-L rating. As results of multiple linear stepwise regression, the cross-sectional area of vastus medialis muscle (B=-0.101, P<0.05), thickness of medial collateral ligament (B=0.112, P<0.05) and medial meniscus extrusion (B=0.132, P<0.05) were the factors affecting knee joint function. [Conclusion] For patients with self-reported knee instability, ultrasound detection of medial knee structure has clinical guiding significance.