807 Peak Force of the Posterior Ligamentous Complex is Lower in Patients with Adult Spinal Deformity Requiring Revision Surgery

Abstract

INTRODUCTION: Proximal Junctional Kyphosis (PJK) is observed in up to 40% of those undergoing fusion surgery for adult spinal deformity (ASD). Despite common use of surgical techniques reinforcing ligamentous integrity (e.g. tethering, augmentation), it is unknown whether ligamentous biomechanics are impaired in this population. METHODS: Sample: 23 consented individuals with a diagnosis of ASD undergoing primary (N=14) or revision (N=9) multilevel spinal fusions (>4 levels). Intraoperative biopsies of the PLC were harvested 2 levels below the upper instrumented level (primary) or at the failed junctional segment (revision). Peak force (strength) and elastic modulus (stiffness) were compared between primary and revision groups using independent student t-tests, and relationships between biomechanical parameters and patient characteristics evaluated. RESULTS: Participants in the revision group were older (73.3(3.6) years) than the primary group (54.7(19.2)years), p=0.02, but did not differ in number of fused vertebrae (12.0(4.3); primary and (11.3(3.4); revision), p=0.71). Mean pre-operative Vitamin D levels were 40.6(11.5), spino-vertebral angle (SVA) was 54.4(56.8) degrees, Pelvic Tilt (PT) was 24.1(11.0) degrees, Pelvic Incidence-Lumbar Lordosis mismatch (PI-LL) was 15.3(22.4) degrees, and Proximal Junctional Kyphosis (PJK) was 9.4(8.1) degrees. Mean post-operative SVA was 8.3(28.2) degrees, PT was 20.6(6.9) degrees, PI-LL was 4.4(10.0) degrees, and PJK was 12.0 (10.5) degrees. Peak force was significantly lower in the revision group (143.1(72.5) N) compared to the primary group (238.5(109.8) N), p=0.03. There were no differences in ligament stiffness between the primary (5.9(2.0) MPa) and revision groups (5.3(3.1) MPa), p=0.92). There was a significant association between greater peak force and lower age (r=-0.56, p=0.003), larger ligament area (r=0.50, p=0.015), lower vitamin D levels (r=-0.63, p=0.015), and lower pre-operative PI-LL (r=-0.53, p=0.014). There was also a significant association between greater ligament stiffness and greater post-operative PJK (r=0.60, p=0.013). CONCLUSIONS: Peak force of the posterior ligamentous complex in individuals undergoing revision surgery was <60% of those undergoing primary surgery, suggesting an important role of ligament in stabilization of the proximal junctional interface in patients with ASD, particularly in older individuals.