Outcome of surgery for degenerative lumbar scoliosis: an observational study using the Swedish Spine register

In this retrospectively designed study on prospectively collected data, we included patients treated for the first time for degenerative lumbar scoliosis with decompression and/or fusion. A flow chart of the study is presented in Fig. 1. The inclusion criteria were fulfilled by 209 patients. Of these, 155 patients answered to the 1 year and 139 patients to the 2-year follow-up questionnaire.

Classification of the radiological curve pattern or any other radiological information is not included in the Swespine register. We therefore validated the diagnosis by obtaining radiographs from a random sample of 41 patients and another 3 patients with radiographs from our own clinic. The correct diagnosis, degenerative lumbar scoliosis with a Cobb angle of ≥10°, was confirmed for all 44 patients. The mean (SD) preoperative Cobb angle was 21 (7) degrees and the mean (SD) postoperative Cobb angle was 16 (7) degrees. Due to the lack of standardized radiographs in the different clinics we did not attempt to classify the curve patterns.

Patients were divided into two groups based on the extensity of surgery. Isolated decompression with or without fusion of one spinal segment, i.e., two vertebras, were considered as the minor group. Fusions of two or more spinal segments, i.e., three vertebras or more with or without decompression were considered as the major group. In secondary analyses, subgroups within the minor and major groups were compared.

Additional surgeries of the lumbar spine were searched in the Swespine database for the 209 patients until Nov 2, 2015. Any deaths were sought for all 209 patients in the Swedish population register until Nov 2, 2015.

The a priori power calculation was based on the assumption that the effect size of surgery was 0.5. Using the program G*power [17], with power set to 80% and alfa 0.05, the required sample size was 35.

The effect size of surgery was calculated using \(r = z/\sqrt {(n {\text{ observations used to calculate }} z)}\) and estimated according to Cohen [18]. z was obtained using the Wilcoxon signed-rank test.

The cumulative incidence function and competing risks proportional hazards regression were analyzed according to Fine and Gray [19] to describe the risk of additional spine surgery taking the competing risk, mortality into account by applying the PHREG procedure in the Statistical analysis system (SAS) version 9.4 (SAS institute, Cary, NC, USA).

This study was conducted in accordance with the Helsinki Declaration. Ethical approval was obtained from the Stockholm Regional Ethical Board (Number 2012/172-31/4 and 2016/1557-32).

When comparing patients within the minor group, patients undergoing decompression only had similar outcomes compared to those undergoing short fusion or decompression and fusion (all p > 0.05). When comparing patients within the major group, those fused at four or more segments (n = 97) had more previous surgeries (p = 0.048) and had more intermittent consumption and less regular consumption of analgesics than those fused at two or three segments (n = 67) (p = 0.001), but the two groups had similar outcomes (all p > 0.05).

The mean observational period for additional lumbar spine surgery was for more than 95% of the patients longer than the 2-year follow-up, with a mean of 5.4 years, consisting of 1130 person-years, Among the 209 individuals, at least one additional spine surgery was observed in 57 individuals. In 56 individuals, these surgeries were within or in immediate proximity to the area of the index surgery; fusion extending cranially or/and caudally (n = 20), pseudarthrosis (n = 9), implant extraction or repositioning (n = 16), infection (n = 4), and others (n = 7). In one patient, the additional surgery consisted of a decompression on a level above the index surgery. Mortality during the follow-up was seen in 22 individuals. The probability of additional spine surgery was continuously increasing during the follow-up (Fig. 3). The hazard ratio (HR) for at least one additional spine surgical procedure for the major spine surgery group was 2.12 (0.98–4.57) compared to the minor surgery group (1.00; reference). When comparing those fused at four or more segments to those fused at two or three segments, the HR was 1.08 (0.62–1.90).

When comparing preoperative VAS back pain, VAS leg pain, ODI and EQ-5D index between the group without (n = 70) and the group with outcome data at 2 years (n = 139), there were no statistically significant differences (all p > 0.05).