Fracture load and failure patterns of zirconia and 3D-printed resin endocrowns: an in vitro study

Endodontically treated teeth are more prone to fracture and often require durable restorative solutions. Endocrowns offer a conservative alternative to conventional post-and-core systems. This in vitro study compared the fracture load and failure behavior of milled zirconia and 3D-printed resin endocrowns.

Eighteen extracted human molars were endodontically treated and standardized, then prepared for endocrown restorations using a uniform design (two mm occlusal reduction with a four mm intracoronal extension). Specimens were randomly assigned to receive either Computer-Aided Design (CAD)/ Computer-Aided Manufacturing (CAM) milled zirconia (ZR) or 3D-printed resin-based endocrowns fabricated from VarseoSmile Crown Plus (VS) (n = 9 per group). Preparations were digitally captured with an intraoral scanner, restorations were designed using CAD software and fabricated by the respective workflows, and all units were cemented with dual cure self-adhesive resin cement. After controlled storage, specimens underwent compressive loading to failure; maximum fracture loads were recorded, and failure modes were categorized according to whether damage was confined to the restoration or involved the tooth structure. Data were assessed for normality and analyzed using an independent samples t-test with Welch correction at a significance level of p < 0.05.

Statistical analysis revealed a highly significant difference in fracture load between the two groups (p < 0.001), with ZR demonstrating higher strength (3,615.36 ± 792.74 N) compared with VS (1,395.09 ± 149.24 N). However, tooth structure involvement occurred in 8/9 ZR specimens (88.9%), of these, five specimens (55.6% of the total group) were classified as complex/destructive failure, whereas VS failures were simple and confined to the restoration in 5/9 specimens (55.6%), indicating a less severe and potentially repairable failure pattern.

The ZR group showed substantially higher fracture load, but failures more frequently involved tooth structure. VS group exhibited lower strength, yet a higher proportion of simple, potentially repairable fractures confined to the restoration, highlighting a tradeoff between maximum load capacity and repairability. Within the limitations of this in vitro study, these findings support the use of 3D-printed resin endocrowns in selected clinical situations where a tooth preserving endocrown design and digital workflow efficiency are prioritized, and where less severe failure behavior is desirable.