Overstrain on the longitudinal band of the cruciform ligament during flexion in the setting of sandwich deformity at the craniovertebral junction: a finite element analysis

Abstract

BACKGROUND CONTEXTIn the setting of “sandwich deformity” (concomitant C1 occipitalization and C2–3 nonsegmentation), the C1–2 joint becomes the only mobile joint in the craniovertebral junction. Atlantoaxial dislocation develops earlier with severer symptoms in sandwich deformity, which has been hypothesized to be due to the repetitive excessive tension in the ligaments between C1 and C2. PURPOSETo elucidate whether and how the major ligaments of the C1–2 joint are affected in sandwich deformity, and to find out the ligament most responsible for the earlier development and severer symptoms of atlantoaxial dislocation in sandwich deformity. STUDY DESIGNA finite element (FE) analysis study. METHODSA three-dimensional FE model from occiput to C5 was established using anatomical data from a thin-slice CT scan of a healthy volunteer. Sandwich deformity was simulated by eliminating any C0–1 and C2–3 segmental motion respectively. Flexion torque was applied, and the range of motion of each segment and the tension sustained by the major ligaments of C1–2 (including the transverse and longitudinal bands of the cruciform ligament, the alar ligaments, and the apical ligament) were analyzed. RESULTSTension sustained by the longitudinal band of the cruciform ligament and the apical ligament during flexion is significantly larger in the FE model of sandwich deformity. In contrast, tension in the other ligaments is not significantly changed in the sandwich deformity model compared with the normal model. CONCLUSIONSConsidering the importance of the longitudinal band of the cruciform ligament to the stability of the C1–2 joint, our findings implicate that the early onset, severe dislocation, and unique clinical manifestations of atlantoaxial dislocation in patients with sandwich deformity are mainly due to the enlarged force loaded on the longitudinal band of the cruciform ligament. CLINICAL SIGNIFICANCEThe enlarged force loaded on the longitudinal band of the cruciform ligament can add to its laxity and thus reducing its ability to restrict the cranial migration of the odontoid process. This is in accordance with our clinical experience that dislocation of the atlantoaxial joint in patients with sandwich deformity is mainly craniocaudal, which means severer cranial neuropathy, Chiari deformity, and syringomyelia, and more difficult surgical treatment.