Study Design. Combined prospective randomized controlled trial and observational cohort study of degenerative spondylolisthesis (DS) with an as-treated analysis.
Objective. To determine modifiers of the treatment effect (TE) of surgery (the difference between surgical and nonoperative outcomes) for DS using subgroup analysis.
Summary of Background Data. Spine Patient Outcomes Research Trial demonstrated a positive surgical TE for DS at the group level. However, individual characteristics may affect TE.
Methods. Patients with DS were treated with either surgery (n = 395) or nonoperative care (n = 210) and were analyzed according to treatment received. Fifty-five baseline variables were used to define subgroups for calculating the time-weighted average TE for the Oswestry Disability Index during 4 years (TE = [INCREMENT] Oswestry Disability Indexsurgery− [INCREMENT] Oswestry Disability Indexnonoperative). Variables with significant subgroup-by-treatment interactions (P< 0.05) were simultaneously entered into a multivariate model to select independent TE predictors.
Results. All analyzed subgroups that included at least 50 patients improved significantly more with surgery than with nonoperative treatment (P< 0.05). Multivariate analyses demonstrated that age 67 years or less (TE −15.7 vs.−11.8 for age >67, P= 0.014); female sex (TE −15.6 vs.−11.2 for males, P= 0.01); the absence of stomach problems (TE −15.2 vs.−11.3 for those with stomach problems, P= 0.035); neurogenic claudication (TE −15.3 vs.−9.0 for those without claudication, P= 0.004); reflex asymmetry (TE −17.3 vs.−13.0 for those without asymmetry, P= 0.016); opioid use (TE −18.4 vs.−11.7 for those not using opioids, P< 0.001); not taking antidepressants (TE −14.5 vs.−5.4 for those on antidepressants, P= 0.014); dissatisfaction with symptoms (TE −14.5 vs.−8.3 for those satisfied or neutral, P= 0.039); and anticipating a high likelihood of improvement with surgery (TE −14.8 vs.−5.1 for anticipating a low likelihood of improvement with surgery, P= 0.019) were independently associated with greater TE.
Conclusion. Patients who met strict inclusion criteria improved more with surgery than with nonoperative treatment, regardless of other specific characteristics. However, TE varied significantly across certain subgroups.
Level of Evidence: 3