Printing orientation and heat treatment effects on fit and hardness of selective laser melting-manufactured cobalt-chromium crowns

Abstract

Objective: To investigate the effects of different printing orientations and heat treatment temperatures on the fit and hardness of cobalt-chromium (Co-Cr) crowns fabricated using selective laser melting (SLM) technology.

Methods: SLM Co-Cr crowns were printed at 0°, 45°, and 90° orientations and subjected to heat treatment at 750 °C, 980 °C, 1150 °C, or none (n = 10 per group). The silicone replica technique was used to assess marginal and internal gaps at 20 measurement points across five areas: occlusal, axial wall, axio-occlusal, cervical, and marginal gaps (MGs). Vickers hardness was assessed at the occlusal surface of crowns. Statistical analyses included two-way ANOVA (α=0.05).

Results: Crowns printed at a 90° orientation exhibited significantly larger marginal and internal gaps than those printed at 0° or 45° orientation, regardless of heat treatment temperature (p < 0.001). Occlusal region showed the largest discrepancies, while axial walls exhibited the smallest. Clinically acceptable MGs (<120 µm) were observed in crowns printed at a 0° orientation without heat treatment and at 0° or 45° with heat treatment at 1150 °C. Crowns treated at 1150 °C demonstrated the lowest hardness values, while those treated at 750 °C or at 980 °C showed significantly higher hardness but larger, clinically unacceptable MGs at certain locations (p < 0.05).

Conclusions: Printing orientation and heat treatment significantly influenced both fit and hardness of SLM Co-Cr crowns. A 0° printing orientation without post-printing heat treatment, and 0° or 45° with heat treatment at 1150 °C, are preferable for fabricating SLM Co-Cr crowns to achieve favorable fit and hardness.

Clinical significance: When single cobalt-chromium crowns are fabricated using selective laser melting technology, a 0° printing orientation without heat treatment, and 0° or 45° orientation with heat treatment at 1150 °C, are considered to achieve acceptable marginal fit, internal adaptation, and appropriate hardness. A 90° orientation may lead to increased marginal gaps.

Keywords: Cobalt-chromium alloys; Dental crowns; Heat treatment; Marginal fit; Printing orientation; Selective laser melting.