The Effect of the Ratio of Waist Circumference to Thigh Circumference in Obese Patients on the Therapeutic Efficacy of Medial Unicompartmental Knee Arthroplasty for Knee Osteoarthritis.

Background: Medial unicompartmental knee arthroplasty (UKA) is a surgical procedure that replaces only the damaged medial compartment of the knee joint, preserving the healthy lateral compartment. Previous studies have investigated the impact of body mass index (BMI) on the efficacy of UKA for knee osteoarthritis, but the effect of the ratio of waist circumference to thigh circumference in obese patients has not been reported. This study aimed to explore the impact of the waist-to-thigh ratio on the efficacy of medial UKA in obese patients with knee osteoarthritis.

Methods: A retrospective analysis was conducted on the clinical data of 99 patients with knee osteoarthritis who underwent medial UKA at our hospital from February 2021 to March 2023. Patients were grouped based on their waist-to-thigh ratio, with a ratio ≤1.7 classified as the normal group and >1.7 as the obese group. Continuous variables such as age, height, weight, surgical indicators, and pain scores were compared between the two groups using the independent samples t test or Mann-Whitney U test, depending on the normality of data distribution. Categorical variables like gender, comorbidities, and patient satisfaction were analyzed using the chi-square test or Fisher's exact test. Repeated measures ANOVA was used to compare changes in outcome measures over time between the two groups. P < .05 was considered statistically significant. Surgical indicators, hematological indicators, pain status, postoperative recovery, daily living abilities, risk of pressure ulcers and falls, nutritional status, and patient satisfaction were compared between the two groups using the appropriate statistical tests.

Results: This study included 51 patients in the normal group and 48 in the obese group, with no significant differences in baseline characteristics except for gender, BMI, thigh circumference, waist circumference, and waist-to-thigh ratio. The normal group had significantly shorter hospitalization time (5.2 ± 1.3 vs 7.1 ± 2.1 days, P < .001) and surgical time (65.3 ± 11.4 vs 78.6 ± 14.2 minutes, P < .001) compared to the obese group. There were no differences in intraoperative blood loss or time to achieve 90° flexion-extension. Postoperatively, the normal group had lower Visual Analog Scale (VAS) pain scores at all timepoints up to 2 months (P < .05). They also ambulated sooner (2.1 ± 0.6 vs 3.5 ± 1.1 days, P < .001) and discontinued crutches earlier (22.4 ± 4.2 vs 29.1 ± 5.3 days, P < .001) compared to the obese group. Within 1 year, a higher proportion of normal group patients could squat (84.3% vs 62.5%, P = .012). The normal group also had a lower incidence of patellofemoral pain (5.9% vs 18.8%, P = .045).

Conclusion: Patients with a high waist-to-thigh ratio (>1.7) experienced poorer outcomes after medial UKA, including higher postoperative pain, slower recovery, and greater incidence of patellofemoral pain compared to those with a normal ratio. These findings suggest that medial UKA may not be the optimal treatment for obese patients with a disproportionately large waist circumference relative to thigh size. Preoperative weight loss or alternative surgical approaches may be considered for these high-risk patients to improve their outcomes. Further research is needed to develop targeted interventions for this patient population.