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A retrospective study was undertaken using treated functional appliance cases. The groups involved 27 Andresen, 19 Bionator, and 23 Fränkel cases. The reflex metrograph was used to measure arch width from study models at the start of treatment, end of treatment including retention, and at least 1 year post-treatment. The effects of each appliance on arch width are compared during and after treatment.
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Gibbs et al. (1992) studied this question.
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