Influence of stump resilience on stress distribution in all-ceramic FPDs

Objectives: The aim of this study was to investigate the influence of simulated stump resilience and model materials on stress distribution in a four-unit FPD made of zirconia by means of finite element (FE) analysis.

Methods: A four-unit FPD (teeth 24-27) was scanned optically before and after veneering. Afterwards, three-dimensional volume models of veneering layer and core were constructed by reverse engineering. In a CAD program, other parts like model stumps, cement layer and socket were added. Four different FE models were created. In model 1, socket and rigid fixed stumps were made of Ni-Cr-alloy. Model 2 was similar to model 1, but stumps were embedded resiliently. Model 3 was made of a polyurethane (PUR) socket and resiliently embedded PUR stumps. Model 4 simulated the in vivo situation with a socket made of spongiosa, stumps made of dentin and simulated periodontal ligament (PDL). Maximum principal stresses under axial load of 1630 N at the centre of the middle connector were determined by 3-D FE analysis.

Results: Highest maximum tensile stress was always found to be at the base of the middle connector area. Stress increased with decreasing stump resilience. Highest maximum tensile stresses were 289 MPa in model 1, 331 MPa in model 2, 633 MPa in model 3 and 557 MPa in the model with simulated dentin, spongiosa and PDL.

Conclusion: Stump resilience and materials used for model parts in vitro have important influence on tensile stresses in FPDs. To achieve realistic results in vitro, test specimens should be supported by resilient stumps made of a moderately rigid material, e. g. PUR. In clinical practice risk of failure may rise with increasing resilience of stumps.