Vertebral Ankylosis Is Associated with Reduced Cervical Extensor Muscle Bulk and Increased Fatty Degeneration

<b>Background/Objectives:</b> Ankylosing spondylitis and diffuse idiopathic skeletal hyperostosis produce long-segment spinal ankylosis, altered biomechanics, and high fracture risk in the cervical spine. Paraspinal muscle degeneration ("spine-specific sarcopenia") has been linked to pain, disability, and worse outcomes after cervical spine surgery, but the relationship between vertebral ankylosis and cervical paraspinal muscle health is unknown. We aimed to evaluate the association between vertebral ankylosis and cervical paraspinal muscle health using MRI-based measures of muscle quantity and quality. <b>Methods:</b> Adult patients with cervical vertebral ankylosis and available cervical MRI were identified at a single academic center and propensity score-matched 1:1 to patients without ankylosing conditions based on age, sex, body mass index, American Society of Anesthesiologists class, and comorbidity index. Axial T2-weighted images at C2-3 through C7-T1 were used to manually trace bilateral deep extensor and deep flexor muscles to obtain bilateral cross-sectional areas (CSAs) at each level. Extensor fatty infiltration was graded using the Goutallier classification. CSAs and Goutallier grades were compared between the matched groups. <b>Results:</b> Compared with matched controls, patients with vertebral ankylosis demonstrated significantly smaller deep extensor CSA at multiple cervical levels and higher Goutallier grades in the lower cervical spine and at the cervicothoracic junction. Deep flexor CSA tended to be smaller in the ankylosis group, but differences did not reach statistical significance. <b>Conclusions:</b> Vertebral ankylosis is associated with poorer cervical paraspinal muscle health, characterized by reduced extensor muscle bulk and increased fatty degeneration. These findings support conceptualizing ankylosing spinal conditions as disorders of both bone and muscle and highlight the cervicothoracic extensors as a potential target for risk stratification and rehabilitation strategies.