An in vitro investigation of the fracture strength of root-filled - posterior teeth restored with polymer full-coverage crowns

Objectives: To investigate, by an in vitro study, the fracture strength of root-filled teeth restored with polyetheretherketone (PEEK) full-coverage crowns.

Materials and Methods: Two single-rooted maxillary second premolar Typodont teeth were prepared according to the standardized preparation guidelines for two different full-coverage crown restorations: computer-aided design/computer-aided manufacturing monolithic lithium disilicate crowns (E.max) for the control samples and machine pressed monolithic PEEK crowns for the test samples. Teeth were duplicated in a polyurethane-based resin, with properties analogous to human dentin; n = 5 for each category. Replicas were embedded in polyurethane-based resin material using a retaining copper ring, with a simulated 200-μ periodontal ligament. Subsequently, all samples received orthograde root canal treatment using standardized preparation and obturation techniques. Crowns were cemented with resin-modified glass ionomer cement using a standardized cementation force. All teeth were subjected to a monotonic, axial static load of 2500 N to the point of fracture.

Results: All control group samples displayed failure as a combined crown/tooth fracture. The crowns in the test group samples (PEEK) did not fracture under the experimental loads. Under the maximum loads, the crowns exhibited plastic deformation visible as an indentation created on the occlusal surface. Subsurface cohesive damage of the interface cement was noted. T-test showed that the resistance to fracture of the test group compared to the control group was highly significant (P < 0.001).

Conclusions: Within the limitations of this study, PEEK crowns help preserve the remaining tooth structure and provide more predictable protection when compared to E.max crowns. They demonstrate failure by surface deformation and subsurface cohesive damage to the interface cement layer. It is unknown if this failure mode would occur under normal masticatory loads.