ACDF is the treatment of choice for symptomatic cervical spondylosis in patients when conservative treatments, such as medication or physiotherapy, have failed . Patients with arm pain with neural foramen stenosis due to osteophytes or hypertrophy of the uncovertebral joint should be treated with ACDF, as well as UPR. ACDF with complete UPR is known to improve pain in the arm better and faster . However, inadequate removal of the uncinate process has been reported to contribute to poor outcomes in cervical spondylosis cases . In our study, the ACDF with UPR group had better arm pain in the immediate post-operation period than the ACDF without UPR group. As the uncinate process is an important structure to maintaining the stability of adjacent vertebral bodies in the spinal axis, we investigated whether sagittal alignment or subsidence is affected by removing the uncinate process.
Subsidence occurs as a natural process during the course of an interbody fusion procedure and is described as settlement of a body with a higher elasticity modulus (e.g., graft, cage, spacer) into a body with lower elasticity modulus (e.g., vertebral body), leading to a change in spine structure . However, upon excessive subsidence, interbody spaces are narrowed and kyphosis of the spine occurs. This introduces instability of the screw-plate and screw-bone (e.g., pull-out, change of angulation, breakage of the instrumentation) . To the best of our knowledge, end-plate preparation, type of cage and size, multilevel fusion, recombinant human bone morphogenetic protein-2 (rhBMP-2), process of instrumentation, and bone quality are significant factors of subsidence . In our study, when the ACDF with complete UPR and ACDF without UPR were compared under the same conditions, subsidence was significantly higher when complete UPR was performed after 3 years on average. Considering these reasons, it would seem that end-plate preparations would be performed more in the process of UPR in the ACDF with UPR group. However, between the ACDF with UPR and ACDF without UPR groups, clinical outcomes were not significantly different. This is because the foramen is widened due to the UPR, such that, even if subsidence occurs, radiculopathy due to pressing of the root does not occur. Overall, in the case of one-level ACDF, it is difficult to find a significant adverse effect of subsidence. However, caution against subsidence is needed, and a large-scale and long-term follow-up study of multiple-level ACDF with UPR is necessary.
Sagittal balance has been suggested for cervical spine treatment. T1 slope determines the sagittal balance of the cervical spine, and this parameter is related with C2–C7 angle . Previous studies have reported that C2-C7 lordosis is closely related to the other cervical and thoracic parameters (cervical lordosis, thoracic kyphosis) . Cervical sagittal imbalance influences health-related quality of life patient (HRQOL) . St-SVA and C2–C7 SVA are closely associated with the clinical results of neck pain and HRQOL . The
A study by Tang et al. suggested that increasing cervical SVA is a cause for
clinical concern of cervical malalignment as reflected by poor HRQOL scores . In our study, C2-C7 lordosis, segmental angle, disc height, C2-C7 SVA, St-SVA, T1 slope, and SCA were not different between ACDF with UPR and ACDF without UPR group, although the differences significant in segmental angle, disc height, C2-C7 SVA, St-SVA, and SVA at last follow-up and preoperatively were statistically between the two surgery groups (p<0.05). Accordingly, there were no differences in clinical outcomes between the two groups.
Global cervical spine lordosis was not influenced by single-level ACDF . This is the natural mechanism of the human body, which keeps the head on a neutral axis in the optimal horizontal plane for the visiovestibular system and re-establishes sagittal balance . In our study, single-level ACDF with UPR did not affect sagittal balance, although parameters of segmental angle, disc height, C2-C7 SVA, St-SVA, and SVA were worse. Thus, long-term follow-up and a large scale study of multiple-level ACDF with UPR or ACDF in kyphotic cervical spine are necessary.
Limitations of this study
Our study had a few limitations. The number of patients who underwent removal of the uncinate process was small. Also, cases with a bilaterally UPR were rare. Also, because our study did not have a randomized controlled design, we could not completely control the possibility of selection bias. Additionally, because our study size was small, we were limited in our ability to make comparisons between the groups for several factors known to affect prognosis. Failure to indicate the extent to which the uncinate process was removed as an objective indicator was also a limitation. However, the results of this study suggest that when performing ACDF with complete UPR, the risk of subsidence should be considered. Prospective studies will be conducted using well-guided evidence-based protocols with adequate controls.