A retrospective study.
We investigated the risk factors in adjacent segment degeneration (ASD) after more than 5 years of follow-up of lumbar spinal fusion.
There are many concerns regarding ASD followed by lumbar spinal fusion. However, there is a great deal of dispute about the risk factors.
A total of 55 patients who were followed up for more than 5 years after lumbar fusion were observed. Gender, age, residence, fusion method, number of fusion segments and radiological measurements were analyzed. In the radiological measurement, disc height, lumbar lordotic angle (LLA), fusion segment lordotic angle and fusion segment lordotic angle per level (FSLA per level) were estimated. In preoperative MRI, Pfirrmann's classification was used. The clinical result was evaluated by the criteria of Kim and Kim. Statistical univariate analysis was performed with the chi-square test by using SPSS ver. 12.0. Multivariate logistic regression analysis was conducted with SAS ver. 9.
There were 21 patients with adjacent segment degeneration. Further, there was little relationship between ASD and gender, age, residence, fusion method, number of fusion segments, degree of preoperative adjacent disc degeneration in MRI, or preoperative and postoperative LLA. However, the frequency of ASD was significantly low in cases where FSLA per level was >15° (
In patients followed up for more than 5 years after lumbar spinal fusion, the most important factor in the prevention of ASD was the restoration of FSLA per level to >15°.
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 [
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'.
The subjects of this study were 55 patients who had undergone pedicle screw fixation and spine fusion of three or fewer segments due to degenerative lumbar disease. The patients had been followed up for over 5 years. Their mean age at operation was 50.2 years (range, 34-67 years) and they consisted of 18 males and 37 females. Their mean follow-up period was 8 years and 6 months (range, 60-190 months). All of the surgery was performed by one orthopedic surgeon. The fusion methods were posterolateral fusion and posterior lumbar interbody fusion in 24 and 31 cases, respectively (
The 55 subjects were retrospectively investigated with their medical records and radiological findings.
Criteria of degenerative change at adjacent segments: radiological degenerative change at the adjacent segments was considered to exist when anterior or posterior displacement of >3 mm was found on the X-ray of the sagittal plane of the closest upper segment and the closest lower segment at the last follow-up, when the height of the intervertebral disc relative to that of the upper interbody had declined by 20% and when a segmental motion instability of more than 15° was observed on the X-ray of the sagittal plane with flexion and extension.
Gender, age and lifestyle by residential area were analyzed as patient-related factors, which could have some influence. Age was examined by dividing the patients into two groups: ≥50 years and <50 years of age (mean, 50 years). The effect of differences in lifestyle was examined by classifying residential areas into urban and rural categories.
Magnetic resonance imaging (MRI) was used to investigate whether there had been a degree of preoperative adjacent disc degeneration. Patients recording grade ≥III in the five-grade classification of Pfirrmann et al. [
As factors related to surgical treatment, fusion method (posterolateral fusion or posterior lumbar interbody fusion) and the number of fusion segments (one, two and three segments) were analyzed.
Measures from the radiological images taken just after surgery were evaluated as surgical outcomes. First, the postoperative lumbar lordotic angle was classified, with its mean value of 40° as a standard, into ≥40° and <40°, and the meaning of each group was analyzed. Next, the fusion segment lordotic angle per level was calculated by dividing the lordotic angle of the fusion site or a Cobb's angle between the upper endplate of the fusion segment and the lower endplate of the fusion segment by the number of fused segments. It was also divided, with its mean of 15°, into ≥15° and <15°, and its correlation with the degenerative change of adjacent segments was assessed.
The relationship between degenerative change in the adjacent segments and clinical outcome was assessed. The clinical outcomes were divided into satisfactory (excellent, good) and unsatisfactory (fair, poor) on the basis of the assessment base of the criteria of Kim and Kim [
To verify the significance of each factor, a univariate analysis was performed with the chi-square test by using SPSS ver. 12.0 (SPSS Inc., Chicago, IL, USA). Multivariate logistic regression analysis including all factors was also performed with SAS ver. 9 (SAS Institute, Cary, NC, USA), and the odds ratio of the significant factors was calculated. The significance level was
Degenerative change at the adjacent segments was observed in a total of 21 cases. The change occurred at the upper segments (14 of retrolisthesis, seven of decreased height of the intervertebral disc, seven of segmental motion instability, and one of spondylolisthesis) in 18 cases, at the lower segments (one of retrolisthesis, two of decreased height of the intervertebral disc and one of spondylolisthesis) in four cases, and there was one case that showed the change at both the upper and lower segments.
According to the analysis of patient-related factors, the subjects included 18 males and 37 females, and the postoperative degenerative change was found in seven males and 14 females; hence, there was no significant difference by gender (
When the influence of the degree of preoperative adjacent disc degeneration was investigated by MRI, the degenerative change at the adjacent segments was found at the last follow-up in 11 out of 29 cases with and ten out of 26 without preoperative degenerative change; the difference was not statistically significant (
When the difference by the fusion method was investigated, the degenerative change was found in 11 and 10 out of 24 and 31 cases treated with posterolateral fusion and posterior lumbar interbody fusion, respectively, and the difference was not statistically significant (
When the influence of the postoperative lumbar lordotic angle on the degenerative change at the adjacent segments was investigated, the change was found in ten and 11 cases out of 29 and 26 with the angle ≥40° and <40°, and the difference was not significant (
For the correlation between the degenerative change of adjacent segments and clinical outcomes, the change was shown in 18 out of 44 cases with satisfactory clinical outcomes and three out of 11 with unsatisfactory outcomes. As there was no significant difference, the radiological change did not imply unsatisfactory clinical outcomes (
Among the 21 cases with change at the adjacent segments, three needed a revision surgery (14.3%, 5.5% out of the total subjects); two of these were surgically treated for spinal stenosis at the upper adjacent segments, and the other one was treated for segmental instability.
When the multivariate logistic regression analysis on risk factors was conducted-with independent variables as gender, age, residential area, fusion method, degree of preoperative adjacent disc degeneration on MRI, the number of fusion segments, and the lumbar lordotic angle and the fusion segment lordotic angle per level at the fusion site after the surgery-the fusion segment lordotic angle per level of <15° increased the risk of degenerative change at the adjacent segments 4.666 times (range, 1.015-21.439 times) (
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 [
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 [
Many researchers believe that older age leads to more change at the adjacent segments [
When the cases with preoperative instability or intervertebral disc degeneration at the adjacent segments were reviewed, Aota et al. [
Schlegel et al. [
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 [
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% [
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.
No potential conflict of interest relevant to this article was reported.
This work was supported by the Soonchunhyang University Research Fund.
Summerized data of 55 patients followed up for over 5 years after spinal fusion
ASD, adjacent segment degeneration; preop., preoperative; MRI, magnetic resonance imaging; Postop., postoperative; N, no; PLF, posterolateral fusion; U, unsatisfactory; Y, yes; PLIF, posterior lumbar interbody fusion; S, satisfactory.
Univariate analysis of risk factors for adjacent segment degeration
ASD, adjacent segment degeneration; ADD, adjacent disc degeneration; MRI, magnetic resonance imaging; LLA, lumbar lordotic angle; PLF, posterolateral fusion; PLIF, posterior lumbar interbody fusion; FSLA, fusion segment lordotic angle.
Multivariate logistic regression analysis including all risk factors
CI, confidence interval; ADD, adjacent disc degeneration; MRI, magnetic resonance imaging; LLA, lumbar lordotic angle; FSLA, fusion segment lordotic angle.