The Difference in Thigh Circumferences Between 5 and 10 cm Proximal to the Suprapatellar Border Serving as an Indicator of Joint Effusion in Patients With Knee Osteoarthritis: A Cross-Sectional Study.

Objective Knee effusion is a clinically significant symptom associated with pain and functional impairment in patients with knee osteoarthritis (KOA). This study aimed to investigate whether the difference in thigh circumferences measured at two distinct proximal sites could serve as a useful indicator of "knee swelling," particularly "joint effusion," in KOA patients. In this study, knee swelling was assessed using thigh circumference, and joint effusion was quantitatively measured using ultrasonography. Methods A total of 176 KOA patients who were admitted for total knee arthroplasty between June 2023 and June 2024 were included. Preoperative assessments involved measuring the thigh circumference 5 and 10 cm proximally from the superior border of the patella using a tape measure, and quantifying suprapatellar pouch area as an indicator of joint effusion using sagittal ultrasonographic imaging. The association between the suprapatellar bursa area and femoral circumference at 5 and 10 cm proximal to the suprapatellar border, and the difference between these circumferences was determined using Spearman's correlation. Additionally, A simple linear regression analysis was performed, with thigh swelling as the independent variable and joint effusion as the dependent variable. Statistical significance was set at p < 0.05. Results Data from 96 patients (mean age 73.8 ± 8.0 years; Kellgren-Lawrence grade II: 7, III: 50, IV: 39) were analyzed. The mean thigh circumference at 5 and 10 cm proximal to the patella was 40.2 ± 4.4 and 43.8 ± 4.8 cm, respectively, with a mean difference of 3.6 ± 1.2 cm. The average sagittal area of the suprapatellar bursa was 132.4 ± 98.8 mm². The correlation between suprapatellar bursa area and thigh circumference at 5 cm proximal to the superior border of the patella was not significant (r = 0.028, p = 0.789). Similarly, there was no significant correlation between the suprapatellar bursa area and thigh circumference at 10 cm proximal to the superior border of the patella (r = -0.047, p = 0.648). However, a significant correlation was found between suprapatellar bursa area and the difference in thigh circumference measurements at 5 and 10 cm proximal to the superior border of the patella (r = -0.248, p = 0.015). The regression analysis revealed a significant association only between the area of the suprapatellar bursa and the difference in thigh circumference between the two sites (p < 0.05, r = 0.258). Discussion While individual thigh circumference measurements showed no significant relationship with joint effusion, the difference between the two measurement sites was significantly associated with effusion. These findings suggest that calculating the difference between thigh circumferences may reduce the influence of muscle mass, fat, and edema, thereby enhancing the estimation of knee effusion. Conclusion Assessment of knee swelling using the difference in thigh circumference 5 and 10 cm proximal to the suprapatellar border is a useful and simple method of assessing joint effusion in patients with KOA.