Recruitment of Compensatory Mechanisms in Sagittal Spinal Malalignment Is Age and Regional Deformity Dependent A Full-Standing Axis Analysis of Key Radiographical Parameters

Study Design. Retrospective review, full-body radiographical analysis of adult patients with sagittal spinal malalignment (SSM). Objective. To investigate the compensatory mechanisms involved in the sagittal plane of the body after progressive spinal sagittal malalignment and to study the impact of age on compensatory mechanism recruitment. Summary of Background Data. Patients with SSM recruit compensatory mechanisms to maintain erect posture and horizontal gaze. Mechanisms such as pelvic retroversion, knee flexion, and pelvic shift have been proposed, but how they contribute and how age affects their recruitment are poorly understood. Methods. Retrospective review of adult patients with SSM who underwent full-standing axis stereoradiography (EOS imaging). Radiographical measurements were performed with Surgimap. Patients were categorized on the basis of the mismatch between pelvic incidence (PI) and lumbar lordosis (PI-LL). Compensatory mechanisms were normalized to each patient's PI-LL and compared by mismatch groups. In addition, patients were subcategorized into 2 age groups (>= 65 and < 65 yr) and compared within the same groups of mismatch. Results. A total of 161 patients with a mean age of 62.93 +/- 12.8 years. Mean sagittal vertical axis = 62.3 +/- 61.5 mm; pelvic tilt (PT) = 29.2 degrees +/- 8.4 degrees; and PI-LL = 21.0 degrees +/- 14.9 degrees. Mismatch groups were as follows: group 1: PI-LL 0 degrees -10 degrees; group 2: 10 degrees - 20 degrees; group 3: 20 degrees 30 degrees; and group 4: > 30 degrees. There were signifi cant differences between all groups with regard to thoracic kyphosis (TK), PT, knee fl exion angle, and pelvic shift by analysis of variance (P < 0.001). As PI-LL increased, TK and PT contribution to the compensation cascade decreased and knee fl exion angle and pelvic shift contribution increased. Patients with PI-LL of more than 30 degrees who were older had signifi cantly less PT and more TK than patients with similar PI-LL who were younger. Conclusion. Spinopelvic mismatch is an important driver in SSM. Pelvic retroversion and fl attening of TK (reduction) become exhausted with increasing mismatch, at which point there seems to be a steady transfer of compensation toward signifi cant participation of the lower limbs. Further analysis suggests differential recruitment of these compensatory mechanisms based upon age.