Type IIB Spino-pelvic Imbalance is an Independent Risk Factor for Flexion Contracture After Total Knee Arthroplasty

Background: Sagittal imbalance leads to adjustments in the pelvis, hips, and knees to keep a level gaze with minimal energy use. People who have a stiff spine adjust their posture by extending their hips, bending their knees, and tilting their pelvis back. This study hypothesized that type IIB Spino-pelvic imbalance may influence the occurrence of flexion contracture after total knee arthroplasty (TKA). Methods: We retrospectively evaluated all patients who received a robotic-assisted TKA between 2019 and 2024 at a single center. Inclusion criteria: patients who had a postoperative Range of Motion (ROM) > 90°, a residual postoperative flexion contracture > 0°, and a final surgery report from the robotic platform confirming full extension at the end of the surgery. Exclusion criteria: hip osteoarthritis or arthroplasty, preoperative stiff knees (full ROM < 70°), and Limb Length Discrepancy (LLD) > one centimeter. Patients were divided into the IA-IB-IIA-IIB categories according to the 2021 Journal of the American Academy of Orthopaedic Surgeons (JAAOS) Spino-pelvic classification. There were two different knee surgeons who meticulously measured the residual flexion contracture with a goniometer. Linear regressions were used to assess whether spino-pelvic alignment would be associated with flexion contracture. Logistic regressions assessed whether a type IIB independently predicted residual flexion contracture. Preoperative ROM, Hip-Knee-Ankle angle (HKA), age, and Body Mass Index (BMI) were set as potential confounders. The model's fit was assessed using the Hosmer-Lemeshow (P > 0.43). Results: A total of 112 patients were included. The postoperative median flexion contracture was two (Interquartiles Range [IQR], one to four) in Cohort 1 and 10 (IQR 7.2 to 14) at the final follow-up. There were 31 patients who were Type IA (27.7%), 20 patients (17.9%) Type IB, eight patients (7.1%) IIA, and 53 patients (47.3%) IIB. A higher incidence of residual flexion contracture was found in IIB patients (24.1, P < 0.001). Logistic regression revealed IIB as an independent predictor of postoperative flexion contracture (B = 0.7, SE = 0.3, Wald = 5.6, P = 0.016, 95% CI [confidence interval] = 4.3 to 6.9). Conclusion: The sagittal spino-pelvic alignment is a predictor of residual postoperative flexion contracture following TKA.