# Effect of polishing process on torque loss ratio and microgap of selective laser melting abutment: an in vitro study

## TL;DR

This study shows that polishing improves the fit and performance of 3D-printed dental abutments.

## Contribution

The study demonstrates that minor back-cutting polishing reduces surface roughness, torque loss, and microgap in SLM abutments.

## Key findings

- Polished SLM abutments had significantly lower surface roughness (6.86 μm) compared to unpolished ones (26.52 μm).
- Polishing reduced torque loss ratio from 58.26% to 24.16% and microgap from 8.69 μm to 2.38 μm.
- Surface roughness correlated strongly with torque loss (r = 0.903) and microgap (r = 0.800).

## Abstract

The purpose of this in vitro study was to investigate the effect of polishing post-treatment process on the torque loss ratio and microgap of Selective Laser Melting (SLM) abutments before and after mechanical cycling test through improving the surface roughness of the implant-abutment interface.

Forty SLM abutments were fabricated, with 20 underwent minor back-cutting, designated as polishing, in the implant-abutment interface. The abutments were divided into three groups: SLM abutments (group A), original abutments (group B), and polished SLM abutments (group C), each containing 20 abutments. Surface roughness was evaluated using a laser microscope. Implant-abutment specimens were subjected to mechanical cycling test, and disassembly torque values were measured before and after. Scanning electron microscope (SEM) was used to measure microgap after longitudinal sectioning of specimens. Correlation between surface roughness, torque loss ratio, and microgap were evaluated. LSD’s test and Tamhane’s T2 comparison were used to analyze the data (α = 0.05).

The Sz value of polished SLM abutments (6.86 ± 0.64 μm) demonstrated a significant reduction compared to SLM abutments (26.52 ± 7.12 μm). The torque loss ratio of polished SLM abutments (24.16%) was significantly lower than SLM abutments (58.26%), while no statistically significant difference that original abutments (18.23%). The implant-abutment microgap of polished SLM abutments (2.38 ± 1.39 μm) was significantly lower than SLM abutments (8.69 ± 5.30 μm), and this difference was not statistically significant with original abutments (1.87 ± 0.81 μm). A significant positive correlation was identified between Sz values and the ratio of torque loss after cycling test (r = 0.903, P < 0.01), as well as Sz values and the microgap for all specimens in SLM abutments and polished SLM abutments (r = 0.800, P < 0.01).

The findings of this study indicated that the polishing step of minor back-cutting can lead to a notable improvement in the roughness of SLM abutments interface, which subsequently optimized the implant-abutment fit. It can be seen that the application of minor back-cutting method has advanced the clinical use of SLM abutments.