The best method of characterizing the strength of 3-dimensionally (3D) printed occlusal devices is unknown.

The purpose of this in vitro study was to compare the flexural strength, flexural modulus, fracture toughness, and impact strength of flexible and stiff 3D printed occlusal device materials with those of milled and conventionally processed materials under wet and dry conditions.

Specimens were fabricated from flexible (KeySplint Soft, NightGuard Flex 2) and stiff (KeySplint Hard, NightGuard Firm) 3D printed resins using a Digital Light Processing (DLP) printer and milled (ProArt CAD) and heat-polymerized (Excel Formula) materials. Specimen dimensions varied based on testing: flexural strength and flexural modulus (65×10×3.3 mm), toughness (4×8×39 mm, 3-mm cut with a 100- to 400-µm notch), and impact strength (63.5×12.7×4 mm, 1.2 mm, 45-degree notch). Specimens were stored wet (37 °C, 48 h) or dry (37 °C in water and then air dried for 24 h). Flexural strength and flexural modulus and fracture toughness were tested on a universal testing machine until fracture or 15-mm deflection, while impact strength was assessed with an Izod impact machine (7.5 J, 150 degrees). Data were analyzed using 2-way ANOVA and Tukey tests (α=.05).

Milled and heat-polymerized materials demonstrated higher flexural strength and flexural modulus than stiff 3D printed materials, which outperformed flexible 3D printed materials. Fracture toughness was better in milled and heat-polymerized materials under dry conditions, but one flexible 3D printed material outperformed all others under wet conditions. Flexible 3D printed materials exhibited significantly higher impact strength under both conditions.

Flexible 3D printed occlusal device materials excel in impact strength and show variable performance in other mechanical properties depending on material type and testing conditions.