Increasing Levels of Craniocervical Junction Fusion Increase Cervical Annulus Stress and Facet Joint Forces While Decreasing Cervical Motion.

Abstract

Preventing cervical adjacent segment disease (ASD) is a priority in spine surgery, because its onset can result in recurrent symptoms and undermine the durability of surgical intervention. However, the biomechanical effects of craniocervical junction (CCJ) fusion on the adjacent annulus and facet joint forces that contribute to the risk of ASD after surgery are poorly understood. To elucidate the biomechanical effects of facet joint forces and annulus fibrosus stresses on the subaxial cervical spine caused by fusion in the CCJ. A finite element (FE) analysis of the adult cervical spine. Three patient-specific FE models developed from a 26-year-old woman, a 59-year-old woman, and a 64-year-old woman. Subaxial annulus fibrosus stresses, facet joint forces, and total cervical range of motion were analyzed. The previously validated FE models from occiput to C7 were modified to simulate four fusion constructs: unfused, atlantoaxial fusion (AAF, C1-C2), occipitocervical fusion (OCF, occiput-C2), and extended OCF (eOCF, occiput-C5). Each model was loaded with physiological torques and applied motions in flexion/extension and right/left axial rotation. Results were collated via data clustering, and multilevel linear regression was used for analysis. FE analysis demonstrated that forces measured in adjacent and unfused facet joints significantly increased with fusion length by 606±429%, 1772±1449%, and 24,722±21,729% in the AAF, OCF, and eOCF constructs, respectively. Annulus fibrosus stress also increased significantly by 161±93%, 408±211%, and 1833±1398% in the AAF, OCF, and eOCF constructs, respectively. Differences in annulus fibrosus stress and facet joint force were significant among all fusion levels (p<.0005) and were seen regardless of load direction or distance from fusion. Range of motion significantly decreased (p<.0005) as the fusion length increased, with mean ranges of motion of 46.9±5.0, 32.1±4.2, 24.0±3.4, and 10.2±1.0 degrees in the AAF, OCF, and eOCF models, respectively. Our findings indicate a clear connection between the length of CCJ fusion and factors that have been linked to ASD development. Furthermore, our evidence shows that each additional joint fused significantly limits cervical range of motion. We recommend caution to avoid extending CCJ fusions beyond necessity.