Lower limb compensation mechanisms influencing sagittal balance after lumbar fusion

Background: The adoption of whole-body imaging in routine practice has allowed radiographic analysis of the lower limbs, even in degenerative pathologies. This work aims to characterize the occurrence of knee flexion in relation to global and regional sagittal parameters of the spine in lumbar degenerative pathologies. Methods: This work is a single-center retrospective analysis including patients who underwent lumbar spine fusion for degenerative pathology. All included patients had biplanar whole-body imaging before and after surgery with a minimum of 6 months of follow-up. Exclusion criteria were patients with prior spine surgery and total knee replacement before lumbar spine fusion. Patients were grouped based on preoperative knee flexion threshold (10°) and postoperative changes in knee flexion (ΔKF). Results: Postural parameters of 108 patients (65 M; 43 F) with an average age of 63.02 years were studied. Patients with greater preoperative knee flexion (n = 68) had significantly greater PI-LL mismatch (9.88 +/- 10.48° vs. 0.57 +/- 12.19°) and SVA (5.7 +/- 4.2 cm vs. 2.1 +/- 3.9 cm) than the group with less knee flexion (n = 44). Patients with exacerbated knee flexion postoperatively (n = 57) had significantly greater PI-LL mismatch (p = 0.01) as well as loss of lordosis (p = 0.04) after surgery compared to the group with less flexion (n = 51) without any impact on SVA. Conclusion: Knee flexion angle can help quantify lower limb compensation in relation to spinopelvic misalignment in patients operated on for degenerative pathologies. Nevertheless, further work is needed to fully understand the complex interplay of compensation mechanisms in the face of sagittal imbalance and spinopelvic malalignment.