Effects of levelling of the curve of Spee on the proclination of mandibular incisors and expansion of dental arches: a prospective clinical trial.

Abstract

OBJECTIVES: To investigate the effects of levelling the curve of Spee (COS) on the inclination of the mandibular incisors and the width of the mandibular arch. METHODS: Fifty patients, 10-18 years of age, were selected using the following inclusion criteria: nonextraction treatment in the mandibular arch; eruption of all mandibular teeth; no spaces in the mandibular arch; no crowding in the posterior mandibular segments; a mandibular irregularity index greater than 2.5. The depth of the COS, the amount of crowding of the mandibular anterior dentition and the intercanine and intermolar widths were measured on standardised photographs of the casts. The inclinations of the mandibular incisors were measured on cephalometric radiographs. The paired t-test was used to analyse changes in the intercanine and intermolar widths and incisor inclinations before and after treatment, whilst the Wilcoxon signed ranks test was used to examine changes in the COS with treatment. The data were further analysed with a regression analysis to determine the measurements that predicted a reduction of the curve of Spee at the 5 per cent level of significance. RESULTS: The COS showed a median decrease of 0.9 mm, with 50 per cent of the cases ranging between 0.4 mm and 1.4 mm. The sole predictor of curve flattening was the lower incisor to mandibular plane angle. CONCLUSIONS: The COS is mainly 'flattened' by proclining the mandibular incisors. For 1 mm of levelling the mandibular incisors were proclined 4 degrees, without increasing arch width.