Efficiency of Orthodontic Adhesives: Influence of Saliva and Shear Direction—In Vitro Study

TL;DR

This study found that saliva contamination reduces the bond strength of orthodontic adhesives, while applying force at a 45-degree angle improves it under dry conditions.

Contribution

The study introduces shear force direction as a new variable in orthodontic bond-strength testing and evaluates adhesives under clinically relevant saliva contamination.

Findings

Saliva contamination significantly reduces shear bond strength across all adhesive types.

Applying shear force at 45° increases bond strength in dry conditions.

Adhesive remnant index scores vary under contamination, indicating complex failure modes.

Abstract

This in vitro study evaluated the shear bond strength (SBS) and adhesive remnant index (ARI) of orthodontic molar tubes bonded using conventional, hydrophilic, and self-etch adhesives under dry and saliva-contaminated conditions, while also assessing the impact of shear force direction. Extracted molars were bonded with Transbond XT™ (T), Transbond MIP™ (M), or Scotchbond Universal™ (S) under dry or saliva-contaminated conditions. Debonding was performed at 90° or 45°, introducing a clinically relevant but underexplored variable in orthodontic bond-strength testing. ARI scores were assessed via stereomicroscopy and visual inspection. Statistical tests (Kruskal–Wallis and Mann–Whitney) showed no significant SBS differences among adhesives under identical conditions (p > 0.05). However, all adhesives exhibited significantly reduced SBS under saliva contamination (p < 0.001; T: 5.4 vs. 4.1 MPa; M: 5.7 vs. 3.6 MPa; S: 5.5 vs. 4.5 MPa). In dry conditions, SBS was significantly higher with 45° debonding (p < 0.05). Under contamination, SBS varied by ARI score (p = 0.05), with ARI 0 specimens showing higher SBS than ARI 3. These findings confirm that moisture reduces bond strength across adhesive types, while 45° force application enhances SBS under dry conditions. ARI score variability under contamination may reflect complex failure modes.

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