Failure Load and Failure Modes of Indirect Posterior Restorations Combining Occlusal Coverage with Axial Extension: A Systematic Review and Exploratory Meta-Analysis of In Vitro Studies

Indirect posterior restorations combining complete occlusal coverage with extension onto an axial surface are described inconsistently, and their biomechanical advantage remains uncertain. This review compared failure load and a review-defined severe-failure category of morphology-defined axial-extension restorations with occlusal-only restorations and full-coverage crowns. PubMed, Scopus, Embase, and Web of Science Core Collection were searched from inception and last checked on 22 July 2026; citation searching supplemented the database searches. Eligible studies tested restorations meeting the morphological definition in extracted human posterior teeth. Two reviewers independently selected studies, extracted data, and appraised risk of bias using RoBDEMAT with QUIN as a supplementary appraisal. Multiple eligible intervention arms were combined, and shared comparators were counted once. Exploratory random-effects models estimated average effects across heterogeneous laboratory systems using ratios of means (RoMs) and mean differences (MDs) for failure load and risk ratios (RRs) for review-defined severe failure. Eight studies were included. Five independent studies contributed to the main comparison, restricted to single-surface axial-extension versus occlusal-only restorations. Failure-load estimates were inconclusive (RoM 0.98, 95% CI 0.88 to 1.09; MD -23 N, 95% CI -195 to 149). Severe-failure events were sparse and inconsistently defined across the source studies; exact counts were therefore primary, and the pooled RR was exploratory (RR 1.10, 95% CI 0.65 to 1.86). Three studies compared axial-extension restorations with crowns, but these heterogeneous comparisons were not pooled and remained inconclusive. Current in vitro evidence is insufficient to determine whether the evaluated axial-extension designs affect failure load or the occurrence of review-defined severe failure. Imprecision, methodological heterogeneity, and indirectness for clinical performance preclude claims of clinical superiority. A routine strength benefit of the evaluated axial-extension designs cannot be inferred. Standardized preparation definitions, testing protocols, and failure reporting are required. Registration: OSF, DOI 10.17605/OSF.IO/9PXE5 (retrospective).