INTRODUCTION This systematic review with exploratory meta-analysis evaluated the association of apical enlargement and taper with strict periapical healing (SPH) after primary endodontic treatment in permanent mandibular molars. METHODS A search to May 18, 2026, was performed using PubMed, Embase, Web of Science, the Cochrane Library, trial registries, and forward citation tracking. Human clinical studies were eligible when mandibular molar outcomes and apical preparation data were extractable. SPH was defined as the absence of clinical symptoms or signs and a periapical index (PAI) score ≤ 2, or an equivalent criterion. Comparative outcomes were pooled as risk ratios (RRs); single-arm SPH rates were summarized descriptively. RESULTS Five studies met the criteria and were included in the quantitative synthesis. In two initial apical binding file-based randomized trials, +3 enlargement showed a higher SPH rate than +2 enlargement (RR = 0.78; 95% CI, 0.67-0.91; P < 0.001). Evidence was insufficient to determine whether taper independently affected strict healing (RR = 0.90; 95% CI, 0.70-1.15; P = 0.39). The five single-arm datasets reported final apical preparation sizes (FAPS) ≤ #30. For SPH, the descriptive pooled estimate was 72.9% (95% CI, 60.8%-83.5%). CONCLUSIONS Limited randomized evidence suggests that +3 enlargement is associated with a higher SPH rate than +2 enlargement in mandibular molars. Taper remains uncertain as an independent factor and should be regarded as an exploratory variable; however, greater taper may partly compensate when apical enlargement is conservative. Single-arm data showed good SPH with FAPS ≤ #30, but not equivalence with larger apical preparations.
