When a structural deformation or nerve compression is so severe that simple decompression alone does not produce satisfactory outcomes for degenerative lumbar disease, extensive decompression and lumbar vertebral fusion to maintain stability are widely-conducted surgical treatments.

Although some fusion methods and fixation systems have been developed to achieve successful fusion in spinal surgery, a long-term follow-up after a strong fusion has revealed degenerative changes at the adjacent segments due to the loss of mobility of the fusion site and the mechanical load caused thereby [1,2]. Degenerative changes at the adjacent segments include segmental instability, spinal stenosis, intervertebral disc lesion, retro-spondylolisthesis and fracture [3-5]. These poor outcomes are known to follow the accelerated degenerative changes at the adjacent segments after fusion [2,6,7]. As these complications are observed during a long-term follow-up, a cautious application of the fusion itself as well as new alternatives have been suggested. Therefore, measures to reduce and treat degenerative changes after fusion are discussed, along with the increased interest in causative factors related to the prevention and treatment reported by many studies. Nevertheless, these are still controversial.

On the basis of previous studies, in order to determine its causative factors, we statistically analyzed the correlation between possible factors and the degenerative change in adjacent segments among patients with radiographic changes during middle- or long-term follow-up of over 5 years after fusion with pedicle screw. In addition, we investigated the correlation between a radiological degenerative change at the adjacent segments and the actual clinical symptoms in order to show whether the radiological change was an index of the actual abnormality. We define that the degenerative change in the adjacent segment with a radiographic change is 'adjacent segment degeneration', and the adjacent segment degeneration with clinical symptom is 'adjacent segment disease'.

Decompression and lumbar vertebral fusion have been widely conducted as surgical treatment for lumbar degenerative change. Spinal fusion provoked a conflict between benefits secured just after the surgery and future problems. The complications of lumbar vertebral fusion, such as intervertebral disc degeneration at adjacent segments, instability, fatigue and fracture, were observed during middle- and long-term follow-up [3-5]. Although many researchers have pointed out the degenerative lesions at the adjacent segments occurring frequently after fusion as major causes of these late complications [2,6,7], their causes, frequencies and risk factors are still controversial.

It is the predominant view that the degenerative lesion at the adjacent segments can be part of normal aging, and that the reduced mobility and the mechanical load following lumbar fusion accelerates the degeneration [1-3,10,11]. This biomechanical change at the adjacent segments is affected by the range of fused segments and the sagittal angle, and stronger fixation of fused segments is known to be associated with a larger effect as more stress is put on the adjacent segments [12,13]. Cunningham et al. [14] reported that the pressure of the adjacent intervertebral disc became larger by 45% in their cadaveric study, and Lee and Langrana [15] found that the load at the adjacent segments was raised in their biomechanical study on lumbosacral fusion. For the influence of gender, Kumar et al. [16] and Ha et al. [17] showed no significant difference in the rate of the degenerative change by gender, while Etebar and Cahill [18] insisted that the rate of the change at the adjacent segments was higher in females after menopause. This study did not reveal any significant difference by gender.

Many researchers believe that older age leads to more change at the adjacent segments [10,18-21]. As reasons for that, Aota et al. [20] pointed out that it was more difficult for the spine to adapt to postoperative biomechanical change in the elderly, whereas Etebar and Cahill [18] reported that osteoporosis negatively affected the existing degenerative procedure. However, we found no significant correlation between age and the degenerative change at the adjacent segments. As for the effect of lifestyle, Gillet [22] and Cho et al. [23] reported that differences in lifestyle could influence the adjacent segments, and Ahn et al. [24] reported that manual workers and residents in rural areas had around 47 times higher risk compared with those in urban areas. However, no significant correlation with residential area was observed in this study.

When the cases with preoperative instability or intervertebral disc degeneration at the adjacent segments were reviewed, Aota et al. [20] reported that instability deteriorated after the surgery in all cases with preoperative anterior displacement of ≥3 mm. Ha et al. [21] showed that preoperatively, more severe degenerative change in the adjacent joints was associated with more radiological change in the joints, and no degenerative change was observed for more than 5 years of follow-up in cases without preoperative degenerative change in the joints. However, this study did not show any direct correlation between preoperative adjacent disc degeneration on MRI and postoperative degenerative change at the adjacent segments.

Schlegel et al. [13] revealed that the change at the adjacent segments occurred early in the fusion when a device was used for fixation, and Rahm and Hall [2] insisted that posterior intervertebral body fusion increased the load over adjacent segments because the remaining mobility after bone fusion was excluded along with stronger initial fixation. However, Kim et al. [25] and Ha et al. [21] found no significant difference in the frequency of the degenerative change by the fixation system or fusion method. This study also showed that the correlation between the fusion method and degenerative change was not significant. Moreover, there was no significant difference in the degenerative change by the number of fusion segments. Although Aota et al. [20] and Etebar and Cahill [18] insisted that the changes at the adjacent segments became larger due to more stress on the segments for multi-level fusion, Kettler et al. [26] and Ha et al. [17] reported that there was no correlation between the number of fusion segments and the change. This study also indicated no significant correlation between them.

For the change at the adjacent segments by the sagittal angle, reduced lordotic angle has been reported to promote the degenerative change early by leading to a concentrated load of segmental motion at the adjacent segments [27,28]. Herkowitz and Kurz [29], Cho et al. [23] and Ahn et al. [24] stated that it was critical to maintain the lumbar lordotic angle during follow-up after fusion, and decreased lordotic angle eventually stimulated the degenerative change at the adjacent segments. For the segmental sagittal angle, Ahn et al. [19] found that the decrease in the fusion segment lordotic angle by 10° was associated with 3.2 times the increase in the degenerative change. In this study, the degenerative change was observed more frequently in the cases where the fusion segment lordotic angle per level was <15° after the surgery.

The incidence rate of the degenerative change at the adjacent segments after lumbar vertebral fusion has been reported variously as between 19.4% and 40% [3,20,21,25]. When it was investigated whether the degenerative change provoked clinical symptoms, Booth et al. [6] reported that a radiological degenerative change was found in many cases in >5 years after lumbar vertebral fusion; however, there were almost no cases with this symptom; other previous studies also revealed no correlation between radiological change and clinical symptoms [2,18,23,30]. This study found that unsatisfactory outcomes were observed in only 11 among 21 cases with the degenerative change at the adjacent segments, and that the correlation between radiological change and surgical outcomes was not statistically significant. However, it was reported that if clinical symptoms were observed again along with the degenerative change at the adjacent segments, surgical treatments were necessary in 8% to 16.8% of cases [14,20,25]. This study also revealed that three (14.3%) out of the 21 cases needed surgical treatment.

During the long-term follow-up after pedicle screw fixation and fusion, gender, age, residential area, fusion method, the number of fusion segments, and the degree of preoperative adjacent disc degeneration on MRI showed no significant relationship with the postoperative degenerative change at the adjacent segments; however, the correlation between the fusion segment lordotic angle per level and the postoperative degenerative change was significant. Therefore, efforts to restore the fusion segment lordotic angle per level to >15° are most important and are considered to be able to reduce the degenerative change at the adjacent segments.