Effects of a hollow pontic design with various wall thicknesses on the axial internal fit and failure load of 3D printed three-unit resin prostheses.

Abstract

Statement of problem: A 3-dimensionally (3D) printed 3-unit interim fixed dental prosthesis (FDP) undergoes inward polymerization shrinkage to the pontic region, leading to nonuniform internal fit and improper accuracy. Whether axial wall thickness influences accuracy is unclear.

Purpose: The purpose of this in vitro study was to analyze the axial internal fit and failure load associated with a hollow pontic design of varying wall thicknesses in stereolithography (SLA) 3D printed 3-unit interim FDPs.

Material and methods: A master model for a 3-unit interim FDP with 2 implant abutments was designed. Two master models were produced by using milling and direct metal laser sintering (DMLS). Sixty SLA 3D printed specimens were produced with a hollow pontic design of 4 wall thicknesses (solid and 2.5, 2.0, and 1.5 mm). Fifteen milled specimens were produced with a solid pontic design. The axial internal fit in the coronal section was measured in terms of intaglio and cameo mid-axial wall areas relative to the abutments by using microcomputed tomography (μCT). The failure load was measured by using a universal testing machine at a loading rate of 1 mm/minute. For statistical analysis, parametric tests were performed (α=.05). Horizontal μCT sections were compared qualitatively, and failure patterns were categorized among groups.

Results: The hollow pontic designs with 2.0- and 1.5-mm wall thicknesses presented significantly lower mean intaglio mid-axial gaps than the solid pontic design (P<.001) and similar intaglio and cameo mid-axial gaps in the horizontal μCT sections. The hollow pontic design with the 2.0-mm wall thickness had a significantly higher mean failure load than that with the 1.5-mm wall thickness (P<.001) and a statistically similar mean failure load to that of the solid pontic design (P=.549). As the wall thickness of the hollow pontic decreased from 2.5 to 1.5 mm, the ratio of pontic fracture to complete fracture of the prosthesis increased.

Conclusions: For SLA 3D printed 3-unit interim FDPs, axial internal fit was enhanced by the application of an appropriate hollow pontic design. Considering both the structural strength and fit, a hollow pontic design with a 2.0-mm wall thickness should be selected.