Spine Journal - 2026-04-13 - Journal Article
Increasing Levels of Craniocervical Junction Fusion Increase Cervical Annulus Stress and Facet Joint Forces While Decreasing Cervical Motion.
Anderson JM, Karren E, Dailey AT, Brockmeyer DL, Ellis BJ
Topics
Key Takeaway
Each incremental extension of craniocervical junction fusion exponentially increases subaxial facet joint forces—up to 24,722% above baseline in occiput-C5 constructs—and annulus fibrosus stress up to 1,833% above baseline.
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Summary
This finite element study quantified the biomechanical consequences of four CCJ fusion constructs (unfused, C1-C2, occiput-C2, occiput-C5) on subaxial annulus stress, facet joint forces, and cervical ROM across three patient-specific models. Facet joint forces in adjacent unfused segments increased 606%, 1,772%, and 24,722% for AAF, OCF, and eOCF constructs respectively, while annulus stress increased 161%, 408%, and 1,833%; all pairwise differences were significant (p<0.0005). Total cervical ROM decreased from 46.9° (AAF) to 10.2° (eOCF), with each additional fused level producing a nonlinear, exponential increase in mechanical stress transfer.
Key Limitation
Only three female patient models were analyzed, making it impossible to assess the influence of sex, bone mineral density, cervical alignment (lordosis/kyphosis), or construct rigidity on the magnitude of stress redistribution.
Original Abstract
BACKGROUND CONTEXT
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.
PURPOSE
To elucidate the biomechanical effects of facet joint forces and annulus fibrosus stresses on the subaxial cervical spine caused by fusion in the CCJ.
STUDY DESIGN/SETTING
A finite element (FE) analysis of the adult cervical spine.
PATIENT SAMPLE
Three patient-specific FE models developed from a 26-year-old woman, a 59-year-old woman, and a 64-year-old woman.
OUTCOME MEASURES
Subaxial annulus fibrosus stresses, facet joint forces, and total cervical range of motion were analyzed.
METHODS
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.
RESULTS
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.
CONCLUSIONS
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.