Amoxicillin did not Reduce Modic Change Oedema in Patients with Chronic Low Back pain - subgroup Analyses of a Randomised Trial (the AIM study)

Study Design. Exploratory subgroup analyses of a randomised trial (Antibiotics In Modic changes (AIM) study). Objective. To assess the effect of amoxicillin versus placebo in reducing Modic change (MC) oedema in patients with chronic low back pain (LBP). Summary of Background Data. The AIM study showed a small, clinically insignificant effect of amoxicillin on pain-related disability in patients with chronic LBP and MC type 1 (oedema type) on magnetic resonance imaging (MRI). Methods. A total of 180 patients were randomised to receive 100 days of amoxicillin or placebo. MC oedema was assessed on MRI at baseline and one-year follow-up. Per-protocol analyses were conducted in subgroups with MC oedema on short tau inversion recovery (STIR) or T1/T2-weighted MRI at baseline. MC oedema reductions (yes/no) in STIR and T1/T2-series were analysed separately. The effect of amoxicillin in reducing MC oedema was analysed using logistic regression adjusted for prior disc surgery. To assess the effect of amoxicillin versus placebo within the group with the most abundant MC oedema on STIR at baseline (‘STIR3’ group), we added age, STIR3 (yes/no), and STIR3×treatment group (interaction term) as independent variables and compared the marginal means (probabilities of oedema reduction). Results. Compared to placebo, amoxicillin did not reduce MC oedema on STIR (volume/intensity) in the total sample with oedema on STIR at baseline (odds ratio 1.0, 95% confidence interval (95%CI) [0.5, 2.0]; n=141) or within the STIR3 group (probability of oedema reduction 0.69, 95%CI [0.47, 0.92] with amoxicillin and 0.61, 95%CI [0.43, 0.80] with placebo; n=41). Compared with placebo, amoxicillin did not reduce MC oedema in T1/T2-series (volume of the type 1 part of MCs) (odds ratio 1.0, 95%CI [0.5, 2.3], n=104). Oedema declined in >50% of patients in both treatment groups. Conclusions. From baseline to one-year follow-up, amoxicillin did not reduce MC oedema compared with placebo. Level of Evidence. Level 2.