# Slot fidelity and ligation-dependent tribology in personalized brackets made by casting versus selective laser melting: an in vitro study

## TL;DR

This study compares two manufacturing methods for orthodontic brackets, finding differences in slot accuracy and friction behavior that could affect treatment outcomes.

## Contribution

The study introduces a novel in vitro comparison of casting and selective laser melting for orthodontic brackets, revealing fabrication-ligation interactions affecting friction.

## Key findings

- Casting and SLM produced similar slot height accuracy but SLM had greater angle errors.
- Friction behavior varied by ligation type and fabrication method, reversing rankings between steel and elastomeric ligatures.
- Support-facing walls showed reduced accuracy in both methods, suggesting design adjustments for SLM brackets.

## Abstract

Reliable chairside adoption of digital orthodontics depends on micrometre-scale slot fidelity and stable bracket–wire tribology. A single computer-aided design for a personalized second-premolar bracket was manufactured by lost-wax casting and by selective laser melting (SLM) (n = 36 per method). Slot height and inter-wall angle were measured on both the support-facing and non-support surfaces. Static and dynamic friction were evaluated using stainless-steel rectangular wires ligated either with tightly twisted stainless-steel ties or with elastomeric modules. Mean slot height was 480.88 ± 73.90 µm for casting and 421.47 ± 32.03 µm for SLM, against a nominal 480 µm. Overall height error did not differ between methods (P = 0.673), whereas angle er-ror was greater for SLM (17.76 ± 11.29°) than for casting (9.56 ± 8.88°, P < 0.001). The support-facing wall consistently showed reduced accuracy: in casting, height error in-creased on the support side (P = 0.001); in SLM, both height and angle errors increased on the support side (both P < 0.001). Across conditions, static friction exceeded dynamic friction (all P < 0.001). With steel ligation, friction was higher for casting than SLM (median static 6.00 N vs 4.35 N, P = 0.007; median dynamic 5.03 N vs 3.83 N, P = 0.011). With elastomeric ligation, the ranking reversed, with higher friction for SLM (median static 2.95 N vs 2.05 N, P < 0.001; median dynamic 2.54 N vs 2.03 N, P = 0.003). In this standardized local in vitro model, findings suggest a surface-dependent reduction in accuracy at the support-facing wall and a fabrication-by-ligation interaction that may alter the frictional ranking between steel and elastomeric ligations. Cast brackets showed more consistent full-wire seating, whereas SLM brackets may benefit from calibrated design offsets and targeted finishing to limit undersized or tapered slots; validation beyond this setup is needed.

The online version contains supplementary material available at 10.1038/s41598-025-34810-w.