Effects of Central Retainer Depth on Stress Distribution and Fracture Resistance in Endocrown-restored Maxillary Premolars: 3D Finite Element and in vitro Analysis.

OBJECTIVE: To explore how central retainer depth affects stress distribution and fracture resistance in endocrown-restored premolars. METHODS: A 3D finite element (FE) model of an intact maxillary premolar was established as a negative control. Five endocrown-restored FE models were established, differing in the extension of the central retainer inside the pulp chamber: 0, 2, 3, 4 and 5 mm. The remaining pulp cavity was filled with composite resin. Stress distributions were analysed under oblique loads. Thirty freshly extracted human maxillary premolars were randomly divided into three groups after endodontic treatment. The extension of the central retainer was set to 0 (G0), 2 (G2) or 4 mm (G4). After endocrown manufacturing and cementation, all specimens were subjected to oblique loads in a universal testing machine until fracture occurred. Fracture resistance was analysed by one-way analysis of variance (ANOVA). Fracture patterns were analysed using a Fisher exact test (α = 0.05). RESULTS: The FE analysis showed that within a range of 0 to 4 mm, increasing retainer depth led to decreased peak von Mises Stress (VMS) values in the cement layer and tooth cervix. At a retainer depth of 5 mm (without base material), peak VMS values were higher than those parameters at a depth of 4 mm. The mean fracture resistance values increased with the increasing retainer depth. G4 exhibited significantly higher fracture resistance than G0. The proportion of irreparable fractures increased with retainer depth. CONCLUSION: A central retainer with composite resin used as base material and extending approximately 1 mm apical to the cementoenamel junction is recommended for endocrown-restored premolars.