Outcome score 60 p=0.005 p=0.01 448 50 40 30 20 10 0 Non-instrumented Instrumented p=0.4 p=0.02 p=0.05 p=0.02 p=0.004 Outcome score p=0.02 p=0.006 70 60 50 40 30 20 10 0 Acta Orthopaedica 2009; 80 (4): 445–450 No reoperations One or more reops p=0.07 p=0.006 p=0.1 p=0.8 p=0.03 Daily activity DPQ Work Anxiety/ Social leisure depression interest DPQ DPQ DPQ Pain index LBPRS PCS SF36 MCS SF36 Daily activity DPQ Work Anxiety/ Social leisure depression interest DPQ DPQ DPQ Pain index LBPRS PCS SF36 MCS SF36 Figure 2. Long-term follow-up scores for all outcome parameters according to study group. PCS: Physical component summary scale; MCS: Mental component summary scale. Figure 3. Outcome score at long-term follow-up according to whether the patient had additional spine surgery after the initial fusion proce- dure. 40) vs. 53 (42–65) (p = 0.007), and SF-36 PCS, 36 (33–39) vs. 29 (24–33) (p = 0.03), and almost in the LBPRS, 21 (16– 26) vs. 25 (17–33) (p = 0.08). The non-union group had a higher use of analgesics with a median DDD of 1.0 (0.14–2.0) as compared to 0.3 (0.1–1.0) in the fused group (p = 0.3). 15 patients (6/51 in the non-instrumented group and 9/43 in the instrumented group) had a self-reported or chart his- tory of repeated surgery to their lumbar spine after their initial fusion procedure: additional decompression at either adja- cent or included levels, or re-fusions. 4 of the instrumented patients had the hardware removed due to loosening. Average age at operation in the reoperated group was 69 (60–79) years, which was no different from the group without reoperations. Functional outcome in the reoperated group was poorer than in single-surgery group (Figure 3). 8 of the reoperated patients reported use of pain medication on a daily basis, as compared to 30 in the group that did not have repeat surgery (p = 0.4). Furthermore, the median DDD was 1.4 (0.1–2.0) in the reop- erated group as compared to 0.3 (0.1–1.0) in the single-sur- gery group (p = 0.3). Comparing drop-outs from the the scores of their last follow- up, drop-outs in the non-instrumented group had poorer scores than the drop-outs in the instrumented group, but not statisti- cally significantly so. Drop-out patients in both groups had poorer work/leisure scores when comparing drop-outs with full follow-up patients in the two groups; this was most pro- nounced in the non-instrumented group. Drop-out patients in the instrumented group generally had poorer anxiety/depres- sion and social interest scores than their full follow-up coun- terparts. Inclusion of the scores of the last follow-up of drop- out patients in the group comparison did not change the results with regard to the significance of differences in the DPQ and LBPRS scales; nor did the overall significance of differences in the fusion status and the reoperation comparisons change when drop-out patients were included, except for the DPQ work-leisure score in the latter, which no longer reached sig- nificance (p = 0.1). Discussion We found functional outcome to be better in older patients who were operated with instrumented fusion as compared to non-instrumented fusion. As it was not a randomized study, several possibilities for bias exist. Patients in the instrumented group were younger and had a slightly longer follow-up. However, after controlling for age and sex by adjusting with norm scores, the SF-36 results were still in favor of the instru- mented group; nor did analyzing outcome scores according to length of follow-up change the fact that the results were better in the instrumented group. Furthermore, inclusion of drop-out patients in the comparisons did not change the results with respect to differences seen between the two groups. Thus, the likelihood of bias introduced by differences in drop-out rate appears to be small. Age was the only demographic variable that differed between the two groups, and it could not explain the differences in SF-36 scores observed. Despite this, it might still represent selection of patients for the instrumented pro- cedure and selection bias favoring instrumentation cannot be ruled out entirely. In general, however, the results obtained in both patient groups are similar to what has been published pre- viously (Glassman et al. 2007, Rampersaud et al. 2008). Thus,