Effect of adding a hard-reline material on the flexural strength of conventional, 3D-printed, and milled denture base materials

Abstract

Statement of problem: Novel 3-dimensionally printed resin and milled polymethyl methacrylate materials have been marketed for computer-aided design and computer-aided manufacturing (CAD-CAM) denture base fabrication. However, information on the flexural strength of digitally fabricated denture base material is limited, and little is known about how they are affected by a hard-reline procedure.

Purpose: The purpose of this in vitro study was to assess the flexural strength of 6 digitally manufactured denture base materials and to assess the effect of a hard-reline procedure on their flexural strength.

Material and methods: A total of 140 strips of denture base material were fabricated from a conventional heat-polymerized polymethyl methacrylate (L199), 3 brands of milled polymethyl methacrylate (IBC, DSL, and ADH), and 3 brands of 3D-printed resin (DFD, ADB, and DrFD) (n=20). Ten specimens in each group did not receive any treatment, and 10 were relined with a hard-reline material (ProBase Cold Trial Kit). Specimens were then subjected to a 3-point flexural strength test using a universal testing machine at a crosshead speed of 5.0 mm/min. A 1-way ANOVA test followed by the Tukey multiple comparison test was used to detect the difference in flexural strength and the strain at fracture of the different types of denture base materials (α=.05). The comparison of flexural strength between with and without hard-reline was analyzed using an unpaired t test (α=.05).

Results: All materials, with or without the hard-reline, met the International Organization for Standardization (ISO) 20 795-1:2013 standard for flexural strength (65 MPa). The milled materials (DSL>IBC≈ADH) showed higher flexural strength than the 3D-printed or conventional materials (DrFD>DFD≈ADB≈L199) without a hard-reline. No statistical difference in flexural strength was found among the hard-relined denture base materials (P=.164). All 3 milled materials showed reduced flexural strength after relining, while the relined conventional (L199) and 3D-printed materials (DFD and ADB) showed notably higher flexural strength; printed DrFD showed no significant difference (P=.066). In terms of strain at fracture, the milled materials displayed higher values than those of the conventional or 3D-printed materials (P<.05).

Conclusions: All digitally fabricated denture base materials were within acceptable limits for clinical use, even after hard relining. Flexural strength was highly dependent on the type of material. Hard relining affected the flexural strength of most of the digitally fabricated denture base materials.