STUDY DESIGN:
Combined prospective randomized controlled trial and observational cohort study of intervertebral disc herniation (IDH), an as-treated analysis.

OBJECTIVE:
To determine modifiers of the treatment effect (TE) of surgery (the difference between surgical and nonoperative outcomes) for intervertebral disc herniation (IDH) using subgroup analysis.

SUMMARY OF BACKGROUND DATA:
The Spine Patient Outcomes Research Trial demonstrated a positive surgical TE for IDH at the group level. However, individual characteristics may affect TE. No prior studies have evaluated TE modifiers in IDH.

METHODS:
IDH patients underwent either discectomy (n = 788) or nonoperative care (n = 404) and were analyzed according to treatment received. Thirty-seven baseline variables were used to define subgroups for calculating the time-weighted average TE for the Oswestry Disability Index (ODI) across 4 years (TE = ΔODI(surgery) -ΔODI(nonoperative)). Variables with significant subgroup-by-treatment interactions (P < 0.1) were simultaneously entered into a multivariate model to select independent TE predictors.

RESULTS:
All analyzed subgroups improved significantly more with surgery than with nonoperative treatment (P < 0.05). In minimally adjusted univariate analyses, being married, absence of joint problems, worsening symptom trend at baseline, high school education or less, older age, no worker's compensation, longer duration of symptoms, and an SF-36 mental component score (MCS) less than 35 were associated with greater TEs. Multivariate analysis demonstrated that being married (TE, -15.8 vs. -7.7 single, P < 0.001), absence of joint problems (TE, -14.6 vs. -10.3 joint problems, P = 0.012), and worsening symptoms (TE, -15.9 vs. -11.8 stable symptoms, P = 0.032) were independent TE modifiers. TEs were greatest in married patients with worsening symptoms (-18.3) vs. single patients with stable symptoms (-7.8).

CONCLUSION:
IDH patients who met strict inclusion criteria improved more with surgery than with nonoperative treatment, regardless of specific characteristics. However, being married, without joint problems, and worsening symptom trend at baseline were associated with a greater TE.