LAUSR

With great interest, we read the article entitled “Complications of cervical total disc replacement and their associations with heterotopic ossification: a systematic review and metaanalysis” written by Nicholas Hui et al. in European Spine Journal [1]. The authors conducted a systematic literature review and meta-analysis to investigate the pooled rate of complications after cervical total disc replacement (CTDR) and analyze the association between heterotopic ossification (HO) and other complications. We highly appreciate their contribution to this important topic; however, we noted some issues in the article may mislead the readers, and a few clarifications will benefit the readers of the journal. In this article, the results suggested that dysphagia was inversely associated with HO after adjustment for age and length of follow-up. The authors attributed the inverse correlation to the relatively intact and undamaged peripheral nerve containing progenitor cells in the endoneurium, which may play a role in HO formation. However, HO may share risk factors with postoperative dysphagia such as smoking, long operation time, lack of meticulous hemostasis, inappropriate intraoperative retraction, postoperative cervical curvature and its change. Thus, we believe that the findings of the study should be interpreted with caution. Our other concern was about the severity of HO in this study. The associative evaluation between HO rate and other complications was carried out; however, the severity of HO was not considered. The authors pointed out that the fusion across intervertebral disc was probably one of the main contributors in the aspect of cervical biomechanics to the association between HO and adjacent segment degeneration (ASDegeneration), which primarily happened in advanced HO (McAfee grade III and IV). Hence, if possible, the correlation analysis should be conducted between the high-grade HO and ASDegeneration to further corroborate the considerations in biomechanics because low-grade HO (McAfee grade I and II) may not lead to secondary biomechanical changes in the index or adjacent segments. Moreover, it is noteworthy that patients without HO may have a low range of motion (ROM) at the index level while normal ROM could be observed in some patients with high-grade HO [2]. Therefore, ROM at surgical segment should be considered to obtain a comprehensive understanding if available. We agree with the theory for the natural spinal degeneration contributing to HO and ASDegeneration. Li et al. [3] found that the severity of preoperative cervical spondylosis and degeneration at the disc replacement level were significantly correlated with the occurrence of postoperative HO. Furthermore, Zhou et al. [4] revealed that the degree of spondylosis at the index level before surgery has a positive correlation with the incidence of postoperative ossification through a quantitative analysis. As the authors discussed in the article, the concordance between ASDegeneration and HO in this meta-analysis is largely due to the degeneration of cervical spine preoperatively, and their presences during follow-up were probably the spontaneous continuation of preoperative condition. Once again, we congratulate the authors for their hard work and hope that the readers may benefit from it.