Many trials have substantiated the theoretical advantages of CDA over fusion in recent decades, such as preservation of motion through the operated segments; however, the documented ROM of CDA showed wide variability among patients. Several investigations suggest that the preoperative ROM scale attributes to the variability of ROM after CDA , but there is a shortage of data on whether limited or excessive segmental mobility should be considered as a suitable indicator for CDA. In the current study, patients were divided into the small-ROM (ROM≤5.5°) and large-ROM (> 12.5°) groups according to their preoperative index-level mobility. There was a significant difference in the distribution of operated levels between the groups, where C4/5 was more prone to hypermobility before surgery. This observation could be a reflection of the relatively spared disc disease at C4/5 compared with other segments [20, 21]. Patient symptoms relieved after surgery regardless of preoperative segmental mobility; this may be attributed to the complete decompression of the spinal cord or nerve roots, as well as disc height restoring and reconstructing stability of the cervical spine.
Radiographic features were different between the two groups. Patients with limited segmental ROM showed significantly less global and segmental lordosis, and ROM and shorter DH. This may be due to the degenerative cascade concept; that loss of proteoglycans and water in the nucleus pulposus causes disc height loss, leading to excessive motion and instability at the early stage of disc degeneration and losing segmental ROM at the late stage. These patients also suffered from relatively severe cervical spine degeneration. △ROM significantly increased by 3.05° in discs with preoperatively limited ROM, which parallels the observations of a previous study ; however, unlike the current study, there were no clinical studies that had reported on the outcomes of CDA in discs with excessive motion. By contrast, the changes in ROM paradoxically decreased by 4.77° in discs with preoperative hypermobility. Many factors, such as overstretch of the surrounding soft tissue, prostheses design , the inconsistent axis of rotation , and development of HO , could lead to decreased ROM after CDA. These findings indicate that segmental ROM could be physiologically restored by CDA using Prestige-LP discs in some cases with loss of mobility and that the technique could partly reduce mobility in some degenerative segments with excessive motion, to achieve “dynamic” re-stability.
The key concern for patients who had excessive ROM preoperatively was that the associated hypermobility would cause increased stress loading on the facet joints and accelerate their degeneration leading to additional neck pain. However, based on the clinical and radiographic outcomes, we suggested that segmental mobility preservation at the index level and the maintenance of motion through the posterior elements did not place patients at risk of increased neck pain. Thus, we supposed that selected patients with preoperatively limited or excessive segmental ROM were good candidates for CDA.
Although there is no consensus on the mechanism of HO, its development has been associated with variables such as age, sex, disc height, residual exposed endplate, and mismatch of the prosthesis [17, 23, 24]. We found that segments with preoperatively limited ROM has significantly less HO than those with excessive ROM at the last follow-up. This was in contrast to previous studies by Tu et al., who reported that HO was similar for patients in less-mobile and more-mobile groups . For further analysis, some segments in the negative △ROM subgroup were found to be more prone to severe HO, especially those with preoperatively limited ROM. One possible explanation could be that limit-ROM discs inherently degenerated more before surgery. Zhou et al.  reported that patients with more severe preoperative cervical spondylosis had higher rates of ossification formation after CDA with Bryan discs. Wu et al.  demonstrated that patients diagnosed with soft-disc herniation had significantly less HO (6.25%) than those diagnosed with spondylosis (58.33%). In the current study, 11 segments with preoperatively limited ROM developed HO in the negative △ROM subgroup; however, 8 of them (72.7%) had been diagnosed with cervical spondylosis before surgery. This observation perhaps indicated that patients with preoperative cervical spondylosis are not optimal candidates for CDA if the index-level ROM is limited.
There were several limitations in the study. First, it was a retrospective study carried out at a single institution presenting inherent weaknesses and limited generalizability of the findings. Second, we evaluated the levels as long as they met the inclusion criteria before surgery. However, different surgery types or levels in subaxial cervical spine may affect outcomes. The small sample size did not present adequate-subgroup data to cover all potential factors. In the current study, factors such as age, sex, and primary cause did not have any significant effect on the results, other than a tendency between the two groups, which may also attribute to the small sample size. Third, the study was limited to the quantity of motion of Prestige-LP discs, whose FDA trials defined inclusion criterion of segmental ROM is in the range 2°-20°; thus, the results may not represent any other type of prostheses. Forth, HO formation was a time-dependent complication after CDA. Due to the extensive time span of the study and the relatively small sample of long-term follow-up cases, the results may not have been precisely evaluated.