Radiographic changes in the cervical spine have been observed in patients with AS, although the prevalence rates differ among reports [
17–
19]. Cervical involvement occurs less frequently than lumbosacral AS involvement and at a later time in the course of the illness, because ankylosis usually progresses caudally to cephalad in the cervical spine [
17]. Ankylosis can lead to a loss of lumbar spinal lordosis and progressive kyphosis of the cervical and thoracic spine. Cervical involvement increases with age and disease duration in AS and is more frequent in symptomatic and structurally severe forms of the disease [
18]. Moreover, cervical spine involvement may progress until the patients are unable to look forward and subsequently show the chin-on-chest deformity [
18]. However, the extent and degree of cervical involvement could vary by disease duration because AS progresses over the whole life of the patients and progresses caudally to cephalad [
17–
19]. One study identified the risk of cervical involvement concerning disease duration and showed that 19.6%, 29.9%, 45.1%, and 70.0% of the patients had radiological involvement after 5, 10, 15, and 20 years, respectively [
18]. The same group also reported that 21.3% of the cervical vertebral body eroded, which was similar to the anterior resorption of the vertebral bodies [
18]. Furthermore, Lee et al. [
2] reported that 48.6% showed radiological changes to the cervical spine. The average disease duration in the current study was 16.9±3.9 years (21.9±4.7 years and 11.9±2.9 years in groups 1 and 2, respectively) and 33.6% of the patients had complete ankylosis of the cervical spine. Of the 80 patients with complete ankylosis, 28 patients (35%) showed anterior resorption of the cervical vertebral bodies. However, the current study showed relatively less cervical involvement than any other previous reports [
18,
19]. We believe that this may be because the patients enrolled in this cohort study were selected. After all, they had complete ankylosis of the cervical spine without subaxial motion.
Martel et al. [
19,
20] reported that the cervicothoracic segments tend to be horizontal with the patient in the upright position, which is associated with complete ankylosis and extreme kyphosis of the thoracolumbar spine. Continual neck hyperextension becomes necessary to allow the patient to look forward rather than look down. As a result of this continual stress, physiological bone resorption occurs at the anterior, convex surface of the cervical arch, by Wolff’s law [
21]. Additionally, the anterior resorption of the cervical vertebral body in AS, a growth disturbance at ring epiphyses, has been reported [
2]. Moreover, Abe et al. [
22] reported that atrophic changes, similar to anterior resorption, in the cervical vertebral body after cervical fusion in adolescent patients were caused by endplate destruction during anterior interbody fusion (AIF) and instrumentation-induced stress shielding effect. Abe et al. [
22] also stressed that the effects of altering load transmission may be greater after posterior spinal fusion with instrumentation than in anterior fusion with instrumentation because the normal alignment of the cervical spine is lordotic. However, this morphological change in AS in the current study occurred without endplate destruction and instrumentation-induced stress shielding effect. In addition, this morphological change can develop in adult patients who undergo AIF of the cervical spine [
23]. These radiological findings were similar to the in-waisting sign (a phenomenon of becoming narrow in the waist of the fused vertebral body) of the vertebral body developing after the AIF of the cervical spine [
23,
24]. Moon et al. [
24] also reported the morphological adaptation of bone grafts and fused vertebral bodies after noninstrumented AIF and suggested that the vertebral in-waisting of the surgically fused anterior cervical column indicated maintenance of normal function at the adjacent motion segments. This process is explained by Wolff’s law and Frost’s flexure drift laws [
25–
27]. Drift is the basis of bone remodeling. Two kinds of drift, osteoclastic and osteoblastic, remodel bony architecture by moving its periosteal and cortical endosteal surfaces through tissue space, usually traversing to its longitudinal axis [
23]. Ha et al. [
28] also reported spontaneous osteophyte resorption and sclerosis disappearance at the fused segments by Wolff’s law after lumbar fusion instrumentation. Like the in-waisting sign after AIF, the gradual loss of motion at each segment of the spinal column in AS leads to the squaring of the vertebral bodies, which is explained by the flexure drift law [
24]. However, Moon et al. [
24] postulated that vertebral bodies are only squared without in-waisting in AS, unlike AIF of the cervical spine. In the current study, 28 of the 80 patients with complete ankylosis of the cervical spine showed anterior bony resorption that was similar to the in-waisting sign after cervical AIF. Interestingly, the anterior resorption of the cervical vertebral bodies developed at C4, C5, and C6 without the involvement of C3 and C7 in all patients. The most severe anterior resorption developed at C5. The average lateral diameter measured in patients who had anterior resorption of the C5 vertebra was 84.2%±7.8%. This could be closely related to the motion of the cervical spine because these segments have larger motion than any other segments in the cervical spine. Moreover, segments that need more motion for horizontal gaze are more likely to develop resorption of the anterior vertebral bodies by Wolff’s law. Therefore, more malalignment due to severe kyphosis of the thoracolumbar spine could assume that more anterior resorption may develop because the cervical spine shifts to be more lordotic to allow the patient to obtain a horizontal gaze. Two parameters in the current study were identified as significant (C2–C7 lordosis [
R2=−0.428,
p=0.021] and T1–T4 kyphosis [
R2=−0.375,
p=0.045]) in the correlation analyses between the extent of anterior resorption and radiographic parameters. This study hypothesizes that more motion happens to allow for the horizontal gaze and maintain cervical lordosis and more stress concentration at the convex curvature, leading to more resorption by Wolff’s law. However, in the current cohort study, no statistical difference was observed between the two groups in terms of spinopelvic alignments including T1 tilt, C2–C7 SVA, thoracic kyphosis, C7 SVA, and T1PA. Therefore, the particular segments involved in developing anterior resorption by Wolff’s law varied, possibly because of their dependence on the preceding ankylosis pattern. Slobodin et al. [
8] reported that the C0–C2 junction involvement is frequently involved in patients with advanced AS. In the current study, the C0–C2 junction involvement was noted in 7 (8.8%) of the 80 patients with cervical involvement. However, no statistical difference was observed between the two groups. In addition, anterior resorption of the vertebral bodies was not affected by ankylosis of the C0–C2 junction.