The impact of sagittal balance on clinical results after posterior interbody fusion for patients with degenerative spondylolisthesis: A Pilot study

We retrospectively reviewed 220 patients who underwent surgery for degenerative spondylolisthesis from July 2003 to June 2008 at our institute. All patients were operated on by four senior surgeons (KES, WE, CSS and LJS). Eighteen of these patients were selected for this study by applying the following criteria: 1) one or two-level degenerative lumbar spondylolisthesis; 2) treatment by posterior interbody fusion; 3) patients showing radiological solid fusion on follow up computed tomography (CT); and 4) a minimum clinical and radiologic follow-up of 24 months. The following exclusion criteria were applied: 1) more than three-level fusion; 2) a history of a previous spinal operation; and 3) the presence of severe systemic disease, a vertebral fracture, or osteoporosis. All patients visited our outpatient department in June 2010, and a trained nurse collected follow-up clinical data. The medical records of patients were reviewed, along radiographic studies that included preoperative and postoperative radiographs, computed tomography (CT) scans, and magnetic resonance images.

Average patient age was 60.8 years (48-72 years), and there were 8 men and 10 women. Mean symptom duration before surgery was 2.3 years (3 months - 10 years), and patients were observed for an average of 43.2 months (24-84 months). 14 patients underwent single-level fusions, of which 12 were at L4-L5 and 2 at L3-4. There were four two-level fusions, one at L4-L5-S1 and three at L3-L4-L5. Demographic data are summarized in table 1. The study was authorized by the institutional review board of the Samsung medical center. Informed consent was obtained from all patients in accordance with the institutional review board at our institution.

All patients were examined clinically and radiographically before and after surgery at the following times: immediately after surgery and at 3, 6, and 12 months and annually thereafter. At clinical evaluations, all patients completed the Visual Analogue Pain Score (VAS) for back pain and the Oswestry disability index (ODI) questionnaire in order to assess HRQOLs [12]. Clinical outcome was evaluated with improvement in VAS and in ODI. Preoperative and final follow up data were assessed using clinical charts and operative reports.

Sagittal alignments were evaluated preoperatively using 36-inch lateral films of the entire spine and both femoral heads. All films were obtained with the subject standing with arms crossed and knees fully extended with adequate lateral view of overlapping femoral heads and visualization from above the C7 vertebral body to the sacral end plate. These full spine films were evaluated for spinopelvic sagittal parameters including sacral slope(SS), PT, pelvic incidence (PI), sacrofemoral distance (SFD), and sacro-C7 plumb line distance (SC7D) (Figure 1). Other spinal parameters included in this analysis were thoracic kyphosis (TK) and lumbar lordosis (LL). The TK was measured between the upper endplate of the most inclined vertebrae into the thoracolumbar junction zone and the superior plate of C7. The LL was measured using the Cobb method between the sacral plate and the upper endplate of the most inclined vertebrae into the thoracolumbar junction zone, corresponding to the inflection point where the spine transitions from lordosis to kyphosis. Each pre, postoperative and follow up measurement for radiologic parameter was performed twice by two observers, one medical student (KMK) and one neurosurgical -spine surgeon (LSH) who are independent from the operators.

Patients were divided into two groups using improvement of PT at last follow up: group A showed a tendency towards increased and unchanged PT; and group B showed a tendency towards decreased PT to normal range. Radiologic factors and clinical outcomes were compared statistically between two groups.

Statistical analysis was performed using PASW statistical software ver. 18.0 (SPSS Inc., Chicago, IL). The Mann-Whitney U-test was used to compare group clinical and radiological outcomes. Wilcoxon's rank sum test was used to compare differences between pre-, postoperative and final follow-up parameters of clinical and radiological outcomes. Correlation studies were performed using Pearson's coefficients to investigate relations between all radiologic parameters and VAS and ODI improvements.

We noted that a change of pelvic tilt after spinal fusion was associated with improvements in VAS and ODI, and a statistically significant association was observed between lumbar lordosis and improvements in VAS and ODI in patients whose postoperative pelvic tilt was not improved. By classifying patients according to an in improvement in pelvic tilt after operation demonstrated that pain and disability were greater in Group A than in Group B. Furthermore, patients in Group A had higher levels of self-reported pain and disability. In other words, patients with a non-improved PT postoperatively also had lower HRQOLs. In Group A, patients showed a tendency toward a poor clinical outcome. However, interestingly patients who presented with lower LL showed a better clinical outcome, and patients without lower LL in Group A experienced the highest levels of self reported pain and disability. On the other hand, in Group B, patients had better clinical outcomes regardless of LL values and the other parameters.

Actually, it would seem that there is the morphological difference between both groups preoperatively although this difference is not significant statistically. In group B with lumbar hypolordosis, operation could improve the lumbar lordosis into normal range by compression following decreasing PT. In group A with well compensated spondylolisthesis showing normal lumbar lordosis, however, operation could not improve any spinopelvic parameter including lumbar lordosis and pelvic tilt. Finally, the surgical result of group with lumbar hypolordosis was better than those of group with normal lumbar lordosis.

Improvements in PT postoperatively played a significant role in the achievement of a good clinical outcome, as determined by VAS and ODI. Although postoperative PT was not improved, LL also contributed to a good clinical outcome. It is becoming increasingly recognized that studies of spinal alignment should include pelvic position. Although PI determines LL, PT is a positional parameter that reflects compensation to spinal deformity. This study confirms that pelvic position, as measured by PT, is correlated with HRQOL in the setting of adult deformity, and that high PT values reflect compensatory pelvic retroversion due to sagittal spinal malalignment. Furthermore, the significant correlation found between postoperative PT improvement and clinical outcome appears to indicate that resetting PT to a normal value is important for restoring ambulatory function by improving the range of hip extension.

However, Increasing PT is not the only way to compensate a local loss of lordosis. The primary compensation is obtained by hyper extension of the segment above or below the hypolordosis area. This study also confirms the presence of relationships between LL and clinical outcomes, which is consistent with published reports [18, 19]. LL is a key consideration when analyzing radiographic alignment, and is thought to cause chronic lower back pain by increasing traction loadings on posterior elements of the spine, including the paravertebral muscles [20].

Therefore, obtaining lordosis at fused segments may be useful for controlling lower back pain. In case of a hypolordotic fusion, there is a painful compensation in hyper extension at the upper level. The global LL does not change but the patient is not improved. PT remains slightly impaired, but the pain is due to the hyper extension compensation (Figure 2). Such residual lower back pain may result from postoperative change of the zygapophysial joint, the paravertebral muscle, the posterior ligamentous complex and the adjacent segment change.

The issue of spinal balance is attracting attention in the literature. However, conflicting results have been reported about the relationship between radiological factors related to sagittal balance and clinical outcomes after lumbar arthrodesis [10, 21]. With regard to early clinical outcomes, Kawakami et al. noted improved clinical recovery rates in patients that underwent fusion for degenerative spondylolisthesis when the L1 axis S1 interval (their measure of the position of the plumb line in front of the sacrum) was less than 35 mm and when lordosis of the fused segments was achieved [10]. However, although the correction of sagittal plane deformity may be achieved by a number of means, there appears to be relatively little information in the literature regarding degenerative spondylolisthesis. In the present study, no significant relationship was found between radiological factors related to sagittal balance and clinical success except PT and LL.