ASD or adjacent segment disease (ASDis) is one of the most common postoperative complications after spinal fusion. The former refers to radiological changes at the segment adjacent to the fused spinal levels, and the latter is defined as degeneration of the adjacent segments that leads to clinical symptoms. The incidence of ASD varies from 5.6–100% in the available literature, which is higher than that of ASDis, whose incidence ranges from 5.2–18.5% [3, 4, 14]. Presently, ASDis is considered one of the main causes of recurrent symptoms, and some patients have to undergo secondary surgery . Therefore, we require a new method for detecting ASD in the light of the higher risk of developing ASDis.
Several studies have reported the prevalence and risk factors of ASD after lumbar spinal fusion; however, the specific mechanisms involved in the incidence of ASD remain controversial [5, 14–16]. Some authors believe that ASD is a natural process of degeneration that is not related to lumbar fusion . However, several recent studies confirmed that segmental fusion may be the main cause of ASD [3, 18, 19]. Rao et al.  reported that the intradiscal pressure and motion at the adjacent segments increased significantly after single-segment lumbar fusion. In a prospective study of 111 patients with more than 10 years’ follow-up, Ekman et al.  found that the incidence of ASD in surgical patients was significantly higher than that in patients receiving conservative treatment, suggesting that both fusion and laminectomy accelerate degeneration in the adjacent segments. Presently, most researchers believe that the development of ASD is related to factors, such as postoperative biomechanical changes, increased concentration stress in the intervertebral discs and facet joints, and hypermobility of the adjacent segments. However, the relevant evidence was derived primarily from the autoptic analysis, biomechanical studies, and finite element analysis [3, 19]; these analyses can not reliably reproduce the postoperative changes in the adjacent during normal daily activities. Radiography was often used to evaluate the degenerative changes in the adjacent segments; however, it only provided static two-dimensional projections rather than immediate and dynamic data. Therefore, we implemented the DIERS system to dynamically monitor the rotation angles of different vertebrae and to study the actual motion of the adjacent vertebrae after lumbar fusion. The advantages of the DIERS system include the following: (1) static measurements can be obtained without exposure to radiation; (2) actual vertebral rotation can be reproduced without considering the position of the pelvis; and (3) dynamic changes in the various vertebrae during walking, a major activity during routine life, can be indicated. The DIERS system analyzed the dynamic parameters related to the human body more accurately, and the measurements were reliable and reproducible, thereby, contributing to the study of the underlying mechanisms of ASD [20, 21].
Recently, several studies reported that the postoperative ROM of the adjacent segments was mostly increased—especially for the upper segment—while varying degree of decrease in the DH was noted. Berg et al.  conducted a 2-year follow-up of 72 patients after lumbar fusion and found that the ROM of the adjacent segments L3-4 and L4-5 increased by 4.3° and 6.0° compared with a decrease of 0.8° in the ROM of L5-S1. In that study, the authors also observed that the postoperative DH decreased slightly. In the current study, we found similar changes in the ROM and DH of the adjacent segments at 2 years postoperatively, and the data did not differ significantly compared to the preoperative data. However, the difference between the previous studies and our study was that we firstly, to our knowledge, found the relative rotation angles with the adjacent vertebrae increased significantly after fusion. This suggests that although the overall stability of the lumbar spine does not change significantly after surgery, the range of rotation of the adjacent vertebrae inevitably increases to compensate for the lost function of the fused segments and to maintain the normal posture during normal activity. Notably, such changes occur earlier than the changes in the DH, intervertebral ROM, or lumbar spinal stability. Consequently, we believe that the relative rotation angles between the fused segments and adjacent vertebrae measured by the DIERS system can detect the occurrence and development of ASD earlier than traditional radiography.
Various risk factors have been proposed regarding the occurrence of ASD . To reduce the effect of the confounding factors, we only included patients aged less than 60 years who underwent single-segment fusion without significant degeneration of the adjacent segments. In our study, 13 patients developed ASD at the final follow-up and 9 patients showed decreased intervertebral spaces. By analyzing the preoperative radiographic images, we found that these 9 patients already had mild degeneration in the adjacent segments before the surgery. Therefore, we believe that the preoperative degeneration of the adjacent segments may be the most important factor leading to postoperative ASD. Another important factor affecting the development of ASD is the location of the fused segment. Many studies have reported that the fusion of the cranial segments is more likely to result in ASD development. Cheh et al.  found that the incidence of ASD after L1-3 fusion was higher than that after L4-5 fusion. Disch et al.  also demonstrated that patients who underwent fusion of the L4-5 segments showed significantly higher risks for ASD development than patients with fusion of the L5-S1 segments. In the current study, we found that the postoperative ROM and DH of the adjacent segments did not change significantly. However, the relative rotation angles with the adjacent vertebrae were greater if the index segments were fused at higher levels, and the change in the upper adjacent vertebra was more significant. The occurrence of such changes may be associatedwith the following factors: (1) the L4-S1 levels contribute almost 50% towards overall lumbar lordosis and provide the highest ROM for flexion/extension movements of the lumbar spine. After fusion, the upper adjacent levels compensate for the segmental immobility of L4-S1; (2) the L2-L4 levels are close to the thoracolumbar segments, which serve as transition zones for spinal curvature. If the upper lumbar segments were fused, the forces will be redistributed to the cranial-adjacent segments because of the special biomechanical characteristics of the thoracolumbar region; and (3) placement of the pedicle screw can damage the superior facet, thereby, inevitably affecting the stability of the upper adjacent segments.
The development of ASD is a chronic process, and only a small number of patients experience clinical symptoms in the short term. Although the incidence of ASD is relatively high, few patients ultimately require surgery. Our results suggest that the clinical symptoms and lumbar spinal function significantly improved after 24 months for most patients. Even the 13 patients who developed radiographic ASD in the present study had clinical outcomes similar to those of their counterparts, suggesting that there was no significant correlation between radiographic ASD and the clinical outcomes in the short term. Notably, although the DH and intervertebral ROM for both groups were comparable at 24 months, the relative rotation angle of the upper adjacent vertebra in the ASD group was significantly greater. These results suggest that the overall structure of the lumbar spine remains stable in the resting state, regardless of the occurrence of ASD; nevertheless, excessive mobility of the adjacent segment persists during walking in the patients with ASD. Therefore, we believe that the relative movement between the adjacent and fused segment may be a more sensitive predictor of the development of ASD.
There are several limitations of our study. First, we only included cases with single-segment lumbar fusion, and patients with multilevel fusion were excluded. Second, the follow-up period of this study was only 2 years, and the long-term changes in the relative rotation angle after lumbar fusion must be investigated further. Finally, to reduce the effect of preoperative degeneration in the adjacent segments, the elderly patients were excluded from the study; this does not conform to the demographic characteristics. These issues will be addressed in the future.