Cervical osteotomy performed for ankylosing spondylitis remains challenging due to the high risk of neurovascular compromise. In 1958, Urist first reported on a case of cervical kyphosis treated using an opening wedge osteotomy similar to the Smith-Petersen osteotomy [15]. This technique was able to achieve significant correction of deformity but involved lengthening of the anterior column, which potentially threatened anterior soft tissue structures, such as trachea and esophagus). In 2007, Tokala et al. first described the cervical PSO procedure [6]. Similar to PSO on the thoracolumbar spine, PSO on the cervical spine has the advantages of structural stability and a wider cancellous contact surface for bony union, over extension-type osteotomy [19]. However, in some cases, a single-stage posterior PSO may not be suitable to correct extremely severe kyphotic deformity. VCD is a relatively new technique for spinal osteotomy, which was first described in a series of nine patients with severe thoracolumbar kyphosis [9]. Nevertheless, the use of this technique on the cervical spine has not been described. In this study, we modified the cervical PSO by shifting the hinge of correction backwards to the anterior 1/3 of the C7 vertebral body. Measurements of vertebrae dimensions provided by CT scanning determined the operating space for the osteotomy. We defined the anatomic morphometry of the C7 vertebra based on gender. The VBD and VBH determined the antero-posterior and supero-inferior diameter, respectively, for each vertebral body. The results of our study show that the average C7 VBD and VBH values are similar to those obtained by Keskin [20], Koller [21] and Chen [22]. We also found significant differences between genders for these parameters.
Instead of a “V” shaped osteotomy, the mPSO was a “Y” shaped osteotomy. In theory, the mPSO could achieve greater correction using the same resection in height of the posterior wall. Based on the results of the simulated osteotomy, mPSO was determined to be able to obtain a mean of 59.1° of correction at the osteotomized level, while PSO provided a correction of 41.9°. However, in our series, the results show that we achieved a mean correction of 31.6° by performing mPSO. This finding is inconsistent with the average of 40–60° correction achievable using classic PSO [3, 6]. Possible explanations are that both Tokala and Deviren manually extended the head and closed the wedge osteotomy using a Mayfield clamp or halo ring. In our practice, we performed in situ rod bending to restore cervical lordosis, which yielded a limited result. Second, the cases reported on in our research (average age 36.8 years old) were much younger than those reported on by Deviren (average age 70 years old). In general, younger patients have less cervical lordosis and Deviren et al. needed a larger degree of osteotomy to maintain normal cervical alignment. Third, most of the patients had a loss of horizontal gaze as the chief complaint. Thus, restoration of the normal visual field has become the main purpose of cervical osteotomy. We used CBVA measurements as the basis for kyphosis correction and evaluation of treatment outcome. Therefore, we individualized the kyphosis correction plan based on the CBVA of each patient. For the illustrative case, the simulated correction angle was larger than the actual correction angle required when we set the apex of wedge osteotomy at the anterior 1/3 point of the mid-vertebral line. As a result, we finally set the apex of wedge osteotomy at approximately the anterior 1/5 point during the surgery and achieved a satisfactory outcome (CBVA decreased from 48° to − 3°).
Mehdian et al. implied of the importance of assessing the contribution of the hips, thoracolumbar spine, and the cervicothoracic spine to overall flexion deformity. They also suggested that a severely compromised frontal visual field from cervical kyphosis was best addressed by performing a cervical osteotomy [23]. The Illustrative case shown in Fig. 2 presented with sagittal imbalance, restricted frontal visual field and severe flexion deformities of the hips. Total hip replacement was performed based on his complaint. Next, a lumbar osteotomy was performed and improved forward vision to some extent but was not sufficient to restore horizontal gaze. Finally, a cervical osteotomy was performed, and the patient was able to maintain his frontal visual field. Simmons et al. recommended that slight under correction of kyphosis deformity was more beneficial to patients than overcorrection, since there needs to be a compromise between being able to look straight ahead when standing and walking, and still being able to work at a desk or drive a car [24]. In their practice, they usually corrected flexion deformity to approximately 10° of the flexion. We achieved dramatic CBVA correction with a mean angle of 44°.
The size of C6 and T1 facets to be excised should be carefully assessed before surgery and intraoperatively to avoid any compression of the nerve roots. Increased resection area provides ample space for osteotomy and instrumentation but may result in inadequate contact of the posterior column on closure of the osteotomy site, causing a gap, which may have an impact on the stability of the posterior column. In our morphometric measurements, the resected lengths of the C6 inferior facet and the T1 superior facet were 4.7 ± 2.1 mm and 6.5 ± 2.6 mm, respectively, for males. Intraoperatively, the C6 inferior facet or the T1 superior facet should be further removed, if the C7 and C8 nerve roots are not free. Reduction of the deformity was found to be the most critical step for a successful osteotomy, while intraoperative manual reduction was found to be the most basic and the most commonly reported technique [6, 23,24,25]. This technique required that the surgeon hold a Mayfield clamp or halo ring, while an assistant monitored the spinal dura and nerve roots. In our study, a plaster bed was placed on the Jackson table and the patient was placed in the prone position on the plaster bed with several rectangular paddings filled between his ventral side and the plaster bed (Fig. 3). After osteotomy, with the head fixed, the assistant closed the osteotomy site by manually holding the shoulders of the patient and extending the neck, while another assistant removed the uppermost padding. This procedure was repeated multiple times to obtain the desired correction angle.
This study was limited by a small sample size of cases, since severe cervicothoracic kyphosis following ankylosing spondylitis is a rare condition. However, previous studies have reported on only a few cases undergoing cervical PSO [3, 6], and our study reported on the largest number of cases with cervicothoracic kyphosis following ankylosing spondylitis.