Fracture strength of additively manufactured one-piece endodontic crowns in resins for definitive use: Effect of material type, margin configuration, and pulp chamber depth

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STATEMENT OF PROBLEM: Additive manufacturing enables the fabrication of definitive resin 1-piece endodontic crowns, but information on how resin type and preparation design affect their fracture strength is limited. PURPOSE: This in vitro study aimed to evaluate the effects of material type, margin configuration, and pulp chamber depth on the fracture strength of additively manufactured (AM) 1-piece endodontic crowns compared with those subtractively manufactured (SM). MATERIAL AND METHODS: One-piece endodontic crowns were designed by using the scans of 4 mandibular first molar typodonts prepared with either butt joint or 1-mm shoulder margins and pulp chamber depths of 2 mm or 4 mm. These designs were used to fabricate specimens either additively (Tera Harz TC80DP [GR], FREEPRINT Crown [FP], Crowntec [CT]) or subtractively (Tetric CAD [TC]) (n=7). Scans of the prepared typodonts were used to additively manufacture resin dies, and each specimen was cemented to its corresponding die using a dual-polymerizing resin cement. After cementation, all specimens were subjected to cyclic loading (1.2 million cycles, 49 N) before the load-to-fracture test. Fracture load data were analyzed with a generalized linear model and Bonferroni-corrected post hoc tests (ɑ=.05). RESULTS: All specimens survived cyclic loading and were subjected to the load-to-fracture test. The interaction among all main factors affected measured values (P=.028). CT led to the highest fracture load among the specimens with butt joint margins and 4-mm depth, while TC led to the highest values among those with shoulder margins, regardless of the pulp chamber depth (P≤.002). Butt joint margins resulted in higher values for GR and CT specimens with 4-mm depth, while shoulder margins led to higher values for FP and CT specimens with 2-mm depth, as well as for TC specimens (P≤.020). A depth of 4 mm resulted in higher values for all specimens with butt joint margins and for TC specimens with shoulder margins, whereas 2-mm depth led to higher values for FP specimens with shoulder margins (P≤.016). CONCLUSIONS: The fracture strength of the tested 1-piece endodontic crowns was affected by the material type, margin configuration, and pulp chamber depth. However, all specimens had acceptable mean fracture load values according to the previously reported masticatory forces of the molar region.