Experimental determination of frictional loads in tooth levelling using edgewise multibracket appliances

The significance of experimentally observed friction between orthodontic brackets and the connecting archwire for multibracket therapy is still debated. This publication is intended to provide detailed and validated information for the discussion of this issue during the initial alignment and levelling phase.

Frictional forces were determined by using in vitro experiments of the simulated levelling of a malpositioned maxillary central incisor. For each experiment a straight archwire was placed in the slots of three aligned brackets. Levelling was simulated by bending the wire using a multistep displacement of the middle bracket. All force and moment components corresponding to each step were recorded at the central and lateral brackets. In vitro frictional forces and the corresponding friction coefficients were determined through direct comparison of experimental results and, additionally, using numerical methods. Different types of conventional orthodontic wires, made of stainless steel [SS] and titanium molybdenum alloy [TMA] were tested.

Results indicated that calculated friction coefficients (SS ~0.1; TMA ~0.2) agreed with published reference data. The mesiodistal forces were proportionally more affected (~100% increase) by friction than the levelling forces (10%-20% reduction). Comparison with numerical models demonstrated that carefully dislodging and reinserting the wire is a viable strategy to remove frictional forces.

The findings indicate that vertical tooth movement was only moderately influenced by friction when conventional SS or TMA wires were used. Clinically, the results suggest that an effective way to reduce friction may be re-opening wire ligatures or using passive self-ligating brackets.