Friction and archwire engagement in contemporary self-ligating appliance systems

Purpose The aim of this study was to compare classical friction (FR) in passive self-ligating brackets (P-SLBs), active self-ligating brackets (A-SLBs) and a traditional twin bracket, in vitro, and to identify the point of initiation of bracket–archwire engagement. Methods Nine bracket systems of 0.022 in slot size were FR tested: 5 P‑SLB systems; 4 A‑SLB systems; and a control group of twin brackets with elastomeric ligatures. Single upper right central incisor brackets were mounted on a custom metal fixture for testing. Straight sections of various round and rectangular nickel–titanium (NiTi) archwires (0.016, 0.018, 0.018 × 0.018, 0.020 × 0.020, 0.016 × 0.022, 0.017 × 0.025, 0.019 × 0.025, and 0.021 × 0.025 in) were ligated to the bracket and peak static FR (cN) was measured with an Instron Universal Testing Machine. Ten unique tests each utilizing a new bracket and new archwire were conducted for each group in the dry state. Results FR was significantly different between control, P‑SLB and A‑SLB systems ( P < 0.001). P‑SLB groups displayed no significant differences in FR between each other, regardless of archwire size. A‑SLB groups did exhibit significant differences in FR between each other depending on both the bracket system and archwire size. Each A‑SLB system tested possessed a distinctly different pattern of initiation of bracket–archwire engagement. Conclusions FR between the archwire and bracket slot differs between P‑SLB and A‑SLB systems, with a distinct pattern of FR and bracket–archwire engagement for each A‑SLB system. Understanding the different bracket–wire interactions of SLB systems should help orthodontic clinicians to plan effective and efficient biomechanics with the bracket system of their choice.