Adaptation, Trueness and Precision of Inlay and Onlay Restorations Fabricated By Additive and Subtractive Manufacturing: A Systematic Review

Objectives: The accuracy of indirect restorations plays a critical role in their longevity and clinical performance. Inlays and onlays as conservative alternatives to full crowns, require optimal marginal and internal adaptation to prevent microleakage, recurrent caries and loss of bond integrity. With the increasing integration of Three-Dimensional (3D) printing in dentistry, a systematic evaluation of additive versus subtractive fabrication for these restorations is warranted. Objective: This systematic review aimed to synthesize evidence comparing the adaptation, trueness and precision of inlay and onlay restorations fabricated by Additive Manufacturing (AM) with those produced by Subtractive Milling (SM) or conventional methods. Methods: This review followed the PRISMA 2020 guidelines. A comprehensive search was conducted in PubMed, Embase, Web of Science and Scopus from inception to October 30, 2025. Eligible studies included in vitro experiments on natural teeth or standardized dies prepared for inlay or onlay restorations, with AM as intervention and SM or conventional fabrication as comparators. Outcomes of interest were Marginal Gap (MG), Internal Gap (IG), Absolute Marginal Discrepancy (AMD), Overall Discrepancy (OD), trueness and precision. Risk of bias was assessed with the QUIN tool. Results: A total of 866 records were identified through database searches. After removal of duplicates and screening against eligibility criteria, seven in vitro studies involving 402 specimens were included in the review. Across studies, AM restorations demonstrated MG values between 54-290 µm, generally comparable to SM (58-509 µm) and conventional methods (50-108 µm). IG values for AM ranged from 35-280 µm versus 62-509 µm for SM. Trueness outcomes often favored SM, particularly with zirconia, while limited evidence suggested favorable precision or reproducibility with AM, although the metrics used differed across studies. Many reported gap measurements were within the historically cited 120-µm benchmark, although substantial method- and site-specific exceedances were observed. Risk of bias was graded as low in 2 studies and medium in 5 studies. Conclusion: Under laboratory conditions, additive manufacturing yielded adaptation comparable to subtractive milling and conventional methods, although outcomes varied among studies and measurement approaches. Trueness often favored certain milled materials, while the limited evidence on precision and reproducibility sometimes favored AM. These findings support AM as a potentially viable restorative approach. However, clinical equivalence or superiority cannot be established from the currently available in vitro evidence.