Effective IH improvement is crucial for good outcomes after cervical spine surgery. Undesirable postoperative IH has been linked to a higher incidence of postsurgery axial symptoms and ASD [12]. Thus, effective intraoperative restoration and postoperative maintenance of IHs are necessary. However, compared with surgical skills such as IH distraction and reconstruction techniques, few studies have assessed the association between IH maintenance and the clinical efficacy after ACDF using a Zero-p device.
The intervertebral height (IH) was effectively improved in all patients. The average IH rose significantly from 6.72 mm preoperatively to 10.46 mm 1 week before being progressively reduced to 9.58 mm at 3 months, 8.73 mm at 6 months, 7.58 mm at 1 year, and 7.50 mm at the last follow-up. Thus, the IH changed subtly 1 year after ACDF (Fig. 2). Our data revealed no clear association between the postoperative disc height change and any clinical outcomes in the 1st year after ACDF surgery.
A study involving 37 1-level procedures, 50 2-level procedures and 13 3-level procedures evaluated the effects of the IH on overall outcomes after ACDF and found that the IH changed from a preoperative mean of 5.49 ± 1.17 mm to 6.62 ± 1.12 mm at 12 months postsurgery (mean change = 1.13 ± 1.33 mm) [13] (Fig. 3). Here, to avoid biomechanical changes due to adjacent surgical segments that may affect postoperative IH, we only included 1-level ACDF surgery with zero-profile implants.
Achieving complete nerve root decompression in cervical spondylotic radiculopathy patients with intervertebral foramen stenosis is challenging. Frequently, no remission for radicular pain or reoccurrence is observed after temporary relief in such patients. Although the pathogenesis of cervical radiculopathy is not completely understood, stenosis of the intervertebral foramina is considered among its main mechanistic underpinnings [14]. Additionally, narrowing of the intervertebral foramen after surgery is a risk factor for postoperative recurrence of neurological symptoms [15]. Therefore, the intervertebral foramen, as the doorway of the nerve root, plays an important role in radiculopathy and surgical treatment of intervertebral foramen diseases. Here, although the IVF diameter in group A patients was significantly smaller than that in groups B and C 1 year after surgery, this condition was not reflected in the patients’ radicular symptoms. Our study sought to verify the connection between IH variation and overall outcomes, reflecting the 1-year follow-up after surgery.
An in vitro biomechanical study used a calibrated distractor and a subminiature load cell on 17 cadaveric cervical specimens to investigate the effects of IH and distractive forces on a cervical spine model. In that study, the longer was the intervertebral space distance, the greater was the compressive load produced between the implant and vertebral body end plate, which may decrease the fusion rate [16]. Here, the fusion rate in group C patients was significantly lower than that in groups A and B in the first 6 months, possibly because the height of the intervertebral space declined too much in a short time. This effect may concentrate the compressive load in the endplate with the narrowest intervertebral space, negatively affecting bone fusion. This possibility should be tested by analysing the influence of stress distribution on bone fusion using ACDF models with different intervertebral heights through finite elements or biomechanical tests.
Prosthesis subsidence affects 7–25% of patients undergoing ACDF surgery using the Zero-P device [17,18,19]. Our data revealed 14.29% subsidence in group C patients, a level that was significantly higher than that in the other 2 groups. Because previous studies have identified various causes and risk factors for implant subsidence after ACDF [19, 20], it is unlikely that implant subsidence was caused by a single factor. However, our data clearly show that the lower fusion rate caused by an IH change > 4 mm is a non-negligible factor for the higher subsidence rate after ACDF.
We found that IH is strongly associated with the occurrence of ASD, consistent with a prior study [4]. In that study, the average IH variation was 1.8 mm in the ASD group vs 2 mm in the non-ASD group over a two-year follow-up. Li et al. [21] reported that excessive disc space distraction is a considerable risk factor for the development of radiographic ASD after patients had undergone ACDF polyetheretherketone (PEEK) cages with an anterior plate. The main reason may be that distraction of the fusion level by cage insertion exerts significant mechanical stress on the adjacent levels. Although prostheses with different design concepts were used, the same results were obtained in our study; ASD was significantly lower in group B than in groups A and C. Appropriate IH provides a better surgical view during decompression and prosthesis insertion, which is a prerequisite to improve the effectiveness and safety of using the Zero-p implant system. Small IHs might result in inadequate decompression or the formation of pseudarthrosis; conversely, large IHs may elevate mechanical stress on adjacent levels [16].
Our study has the following limitations. Because it was a retrospective study, patient selection bias was unavoidable. Thus, randomized controlled studies are needed to validate our findings. Although our measuring method was conducted according to previous studies, we acknowledge that potentially inherent radiographic imaging error may be a major limitation. Additionally, we did not evaluate effective methods for maintaining the postoperative IH. Given that our study mainly investigated the effects of postoperative IH changes on clinical and radiographic outcomes, further studies must focus on effective interventions for maintaining the postoperative IH at 2 to 4 mm.
