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Torque Resistance of Different Stainless Steel Wires Commonly Used for Fixed Retainers in Orthodontics

The comforting thought that alignment of the lower labial segment could be maintained over the long-term by a bonded lingual retainer was shaken by the paper by Katsaros et al. (2007) who reported two types of unexpected change in the inclination of teeth in patients wearing bonded flexible spiral wire retainers. The incidence of these complications was approximately 3% and thought to be caused by occlusal forces acting on the teeth, which cause distortion and residual stresses in the retainer wire. This does not occur with single strand wire retainers bonded only to the canines. Not surprisingly, this type of retainer is less effective than a retainer bonded to all six anterior teeth although Booth et al (2008) of a 20-year follow up of patients with bonded canine-to-canine retainers shows both impressive stability of the lower labial segment and longevity of the retainers. As an aside, Rogers and Andrews (2004) provide some useful advice on the successful bonding of lower canine-to-canine retainers. The subject of this commentary describes an in vitro investigation into the resistance to torque of different lingual retainer wires – the inference being that wires that have greater resistance to torque are less likely to cause unwanted tooth movements. The resistance to torque of the retainer wire depends on its stiffness, whether it is annealed or not andwhether or not the retainer is cantilevered or attached to both adjacent teeth. These findings accord with common sense and it is reassuring to have them verified by a scientific study. The clear clinical message in this study is that orthodontists should, for retainers bonded to multiple teeth, design the retainers to have high resistance to occlusal forces. Two points that are often missed in deciding which retention method to use are that bonded retainers can only retain the teeth to which they are attached and so leave the buccal segments unretained. The effectiveness of vacuum formed retainers, which cover the whole dentition, has been one of the significant advances in orthodontics in recent years as shown by Rowland et al in (2007). However, small tooth movements can still occur and compliance may be an issue. The second point is that if one assumes that any bond has a failure rate of, say, 5%, then a retainer with only two attachment points has a theoretical chance of one bond failing of 1−(0.95) or 9.8%. A retainer with six attachment points has a theoretical chance of one bond failing of 1−(0.95) or 26.5%. This corresponds with the findings of Årtun et al. (1997) who found that spiral wire retainers had a failure rate of between 27.3% and 30.8% compared with 9.1% for a retainer involving a single strand wire bonded to the canines only. Thus, the more teeth a bonded retainer is attached to, the higher the maintenance overhead. Perhaps we should see bonded retainers as an adjunct to vacuum formed retainers thus retaining the whole dentition, giving additional support to the labial segment and providing a guard against compliance failure. In this scenario, the combination of a canine-to-canine retainer and avacuum formed retainer might provide sufficient resistance to tooth movement to avoid the need for retainers bonded to every tooth and the consequent complications and maintenance overhead. Another study!