3.5 Clinical outcomes
In this study, compared with the preoperative JOA score, there was a statistically significant improvement after surgery in cage subsidence <2 mm group and ≥2 mm groups (P < .05) (Table 1). The recovery rate of the 2 groups had no significant difference (62.1 ± 17.3% and 57.9 ± 14.5%, respectively; P = .212). In addition, we analyzed the reasons for recovery effect. There was a significant difference between kyphotic cohort and lordotic or straight cohort (P < .05). The patients with the spinal cord compression beyond 1 disc level would have a significantly adverse outcome after ACDF approach (P < .001). In addition, there was a nonsignificant relation between the JOA recovery rate and T1 slope by Mann–Whitney U test (P = .668) (Table 4).
Surgical treatment of CSM has 3 approaches including anterior, posterior, and combine anteroposterior. The posterior approach generates an indirect decompression resulting from the posterior shift of the spinal cord. The anterior approach is becoming more common due to the direct removal of the lesion, especially for patients with cervical sagittal kyphosis or circumscribed OPLL with a ≥50% canal occupying ratio.[14,15] However, to remove the heterotopic ossification as much as possible, removal of osseous endplate or vertebral body would be demanded. Cage subsidence seemed to be unavoidable in a majority of the cases after the removal of osseous endplate or vertebral body. However, moderate removal of the endplate can promote early fusion. In addition, Gercek et al defined the titanium cage subsidence as the intervertebral change at least 3 mm after ACDF approach. Because of the obtuse contact face of PEEK cage in this study, we divided the patients into the mild group (<2 mm) and the severe group (≥2 mm).
There are many factors contributed to the cage subsidence. Firstly, some investigations have showed that older patients occurred implant subsidence.[8,16] It is commonly thought that the major causes of cage subsidence in older patients are bone density and cortical endplate decrease. However, Bartels et al deemed that advanced age was not an independent risk factor of cage subsidence after the ACDF approach. In our study, age was not included as a potential risk factor, owing to most patients without severe osteoporosis. Secondly, some investigators thought that it was advantageous to prevent cage subsidence if the contact area between bone surface and the cage was increased. Nevertheless, the use of the end caps is controversial because it decreases the ratio of bone fusion while increasing the contact area. In addition, Marino found that the excisional thickness of endplate was significantly related to cage subsidence after posterior lumbar interbody fusion. By the multivariable logistic analysis, we corroborated the association between the excisional thickness of endplate and cage subsidence after anterior cervical surgery. Meanwhile, the cut-off value showed that excisional thickness over 3.7 mm was significantly associated with cage subsidence. On the contrary, previous studies reported that the early intervertebral fusion could prevent the implant subsidence after anterior surgery.[7,21]
Although various risk factors of cage subsidence have been reported in the previous studies, the loss of cervical sagittal malalignment remains a lack of a concern. Park et al thought that graft subsidence was not correlated with cervical sagittal alignments before and after surgery. However, in our study, there was a contrary consequence for this relationship. Although these results showed a significant statistical relation between the preoperative kyphotic alignment and postoperative cage subsidence, cage subsidence does not necessarily mean kyphosis after surgery. In fact, if the anterior portion of the implant is more subsidence than the posterior portion, the local kyphosis would occur. Nevertheless, one research reported that the implant subsidence usually occurred at the posterior upper endplates of the caudal bodies. We thought the cause was the function of head gravity center. Historically, T1 slope angle was related to the cervical sagittal balance. Therefore, the incidence of cage subsidence should be related to the T1 slope angle. Our study revealed that patients with T1 slope over 19.65° had a high incidence of cage subsidence after ACDF approach. We cannot determine who would obtain a sinking cage, but we could adopt appropriate technique during the surgery by avoiding excessive removal of the endplate and correcting abnormal cervical curvature.
Previous investigations showed that implant subsidence does not have a significant association with clinical outcome after anterior operation.[4,16,22] Our findings corroborated the conclusion of previous investigations. Furthermore, we found that the CSA, C2-C7 SVA distance, and grade of ISI were significantly associated with postoperative outcome. Hence, the reconstruction of cervical alignment was 1 of the important preconditions for a good surgical outcome. In addition, Tang et al deemed that patients with C2-C7 SVA value of approximately 40 mm had a significant association with postoperative poor outcome.
As with all studies, there were some weaknesses and limitations in the current study. Firstly, our investigation is a retrospective and single-center study. Secondly, the number of eligible patients was still not large. Only the sample of a large, multicenter, and randomized study can adequately forecast the risk factors of the cage subsidence. Thirdly, the follow-up term of involved patients were over 12 months after surgery, but a relationship could not be clearly established between long-term outcome and risk factors. Finally, JOA scores were calculated based on patients’ subjective feelings, which inevitably lead to the deviation of outcomes. Although the selection of surgical procedures depended on spinal cord decompression needs, we cannot definitively conclude which is the best surgical procedure for patients with multistage spinal cord compression. In the future, some studies need to ascertain the relationship between adjacent-segment degeneration and cage subsidence after anterior surgery.
Risk factors of cage subsidence included higher T1 slope, more excisional thickness, and inverted CSA. We suggest that patients with high T1 slope angle or preoperative kyphotic deformity should avoid excessive removal of endplate and vertebral body so as to reduce the occurrence of cage subsidence. Poor outcome had significant association with cervical kyphotic sagittal and higher ISI grade.
We thank all of patients for their dedication.
Conceptualization: ShaoQing Li, Yong Shen, Zhanyong Wu.
Data curation: ShaoQing Li, Hao Zhang.
Formal analysis: ShaoQing Li, Zhanyong Wu.
Investigation: ShaoQing Li, Hao Zhang.
Methodology: ShaoQing Li.
Software: ShaoQing Li.
Supervision: Yong Shen, Zhanyong Wu.
Validation: Hao Zhang, Zhanyong Wu.
Visualization: ShaoQing Li, Yong Shen, Zhanyong Wu.
Writing – original draft: ShaoQing Li.
Writing – review & editing: ShaoQing Li.
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Keywords:Copyright © 2019 the Author(s). Published by Wolters Kluwer Health, Inc.
anterior cervical discectomy and fusion; cage subsidence; heterotopic ossification; kyphotic alignment; postoperative outcome