Quantifying the Contribution of Lower Limb Compensation to Upright Posture: What Happens if Adult Spinal Deformity Patients Do Not Compensate?

Abstract

Study designThis is a multicenter, prospective cohort study.ObjectiveThis study tests the hypothesis that elimination of lower limb compensation in patients with adult spinal deformity (ASD) will significantly increase the magnitude of sagittal malalignment.Summary of background dataASD affects a significant proportion of the elderly population, impairing functional sagittal alignment and inhibiting overall quality of life. To counteract these effects, patients with ASD use their spine, pelvis, and lower limbs to create a compensatory posture that allows for standing and mobility. However, the degree to which each of the hips, knees, and ankles contributes to these compensatory mechanisms has yet to be determined.MethodsPatients undergoing corrective surgery for ASD were included if they met at least one of the following criteria: complex surgical procedure, geriatric deformity surgery, or severe radiographic deformity. Preoperative full-body x-rays were evaluated, and age and PI-adjusted normative values were used to model spine alignment based upon three positions: compensated (all lower extremity compensatory mechanisms maintained), partially compensated (removal of ankle dorsiflexion and knee flexion, with maintained hip extension), and uncompensated (ankle, knee, and hip compensation set to the age and PI norms).Results288 patients were included (mean age 60 y, 70.5% females). As the model transitioned from the compensated to uncompensated position, initial posterior translation of the pelvis decreased significantly to an anterior translation versus the ankle (P.Shift: 30 to -7.6 mm). This was associated with a decrease in pelvic retroversion (PT: 24.1 to 16.1), hip extension (SFA: 203 to 200), knee flexion (KA: 5.5 to-0.4), and ankle dorsiflexion (AA: 5.3 to 3.7). As a result, the anterior malalignment of the trunk significantly increased: SVA (65 to 120 mm) and G-SVA (C7-Ankle from 36 to 127 mm).ConclusionRemoval of lower limbs compensation revealed an unsustainable truncal malalignment with two-fold greater SVA.