The Palatal Finger Spring: The Bauschinger effect - an assessment

OBJECTIVES: The Bauschinger effect (BE) refers to a spring’s higher load deflection rate (LDR) in the direction of bending. The aim of the study was to assess the role played by the BE on the behaviour of finger springs, compared to non-BE factors. METHODS: RMO® 18/8 0.508mm stainless steel orthodontic wire was used to bend 55 finger springs by hand using loop-forming pliers. Sixteen springs were adjusted outside, and 16 inside the coil, and clamped in a device under load for 14 days. Six springs, three of which were over-bent by 45º and re-bent, were heat-treated at 350º C for 25 minutes. For all the above springs CorelDRAW was used to measure before and after angles. The LDR’s of 10 springs at lengths of 14-20mm were established on a gravity scale device before and after activation. The LDR’s of two springs were measured in the conventional (C) and unconventional (U) mode, and two springs with smaller coils were measured similarly. The LDR’s of three springs were measured with different coil placements. A statistical analysis was performed. RESULTS: Coils adjusted inside opened significantly more than those adjusted outside (P0.05), however, the LDR’s were similar (P0.05), but increased significantly with decreasing spring lengths (P0.05). The LDR’s of springs in U mode were significantly higher than those in the C direction (P0.05). Springs with smaller coils had LDR’s significantly higher than springs with “standard” coils (P0.05). The over-bent and re-bent springs showed significantly less plastic rebound than normally formed springs (P0.05). Increasing the coil-support distance progressively led to a significantly higher LDR (P0.05). CONCLUSIONS: The Bauschinger effect played an important role, but the non-Bauschinger factors were more decisive, particularly spring length. Technicians and Clinicians should be aware of all factors. Conventional springs are preferred. Adjustments should be over-bent and re-bent outside coils.