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Objectives and questions: Stable standing posture results from the interplay of the spine, hip, knee, and ankle joints. Given the influence of gravity, deformities at one level such as a stiff, straight lumbar spine must be compensated for at another (e.g., increased knee flexion) to maintain a balanced standing posture. However, whether such compensations remain adaptive or contribute to further permanent deformities such as knee flexion deformity (KFD) remains unclear. The aim of this study was to evaluate the impact of reduced spinopelvic mobility (SM) on KFD in patients undergoing total knee arthroplasty (TKA). The hypothesis was that reduced SM correlates with increased KFD.

Material and methods: A retrospective analysis was conducted on patients undergoing TKA. The inclusion criteria were: primary image-based robotic-assisted TKA, preoperative sitting/standing sagittal spinopelvic radiographs, and intraoperative KFD data. The exclusion criteria included: revision TKA, history of spine surgery/pathologies, and history of knee surgery/pathologies other than osteoarthritis.

Spinopelvic parameters—sacral slope (SS), pelvic incidence (PI), and pelvic tilt (PT) were measured by two observers. Patients were stratified according to classifications of SM: standing/sitting differences in PT ≥ 20° and < 20°, SS ≥ 10° and < 10°, and Dorr’s classification (DC).

KFD was recorded twice using the robotic system intraoperatively; once in the native state following placement of arrays and limb registration (KFDb), and secondly after osteophyte removal and bone cuts (KFDa). Correlations between KFDb/a and patient groupings according to SM were assessed using Spearman correlation coefficient (CC). Additionally, linear regression models were employed to evaluate the validity of SM classifications as predictors of KFDb/a.

Results: A total of 221 patients (271 knees) were included. The mean differences between standing and sitting were SS 11.3° (SD 9.1) and PT 6.9° (SD 9.8). Overall, 30 knees (11%) had a difference in PT ≥ 20°, 129 knees (48%) in SS < 10°, and DC categorized 131 (48%) as normal, 64 (24%) as stuck standing, 74 (27%) as stuck sitting, and 2 (1%) as kyphotic. The mean KFDb was 5° (SD 6) and the mean KFDa 2° (SD 2).

No significant correlation was found between SM classifications and KFDb/a (CC < 0.1 for all). SM classifications failed to predict KFDb/a (p > 0.5 for all).

Discussion and conclusions: Reduced SM does not correlate with KFDb/a in patients undergoing TKA, suggesting that increased knee flexion is as a reversible rather than a permanent compensatory mechanism in global sagittal malalignment.