Unilateral posterior crossbite (UPCB), often from dysfunctional and para-functional causes, leads to positional mandibular asymmetries that can develop at a very young age into skeletal mandibular deviation; hence the interest of early maxillary expansion. The purpose of this retrospective study was to compare the impact of maxillary expansion by Quad Helix (QH) on mandibular skeletal asymmetry correction before and after 7 years of age.

All children with UPCB and skeletal mandibular asymmetry, who were treated by QH in the orthodontic department of Montpellier between February 2017 and August 2018 and had a radiography file at baseline (T0) and 12 months later (T1) were retrospectively included. The patients were divided into two groups according to age: under 7 years of age with QH adjusted on second primary molars (early group G1) and over up to 13 years old with QH adjusted on first permanent molars (late group G2). The differences between the right and left sides of the mandible in corpus length ΔL (main eligibility criterion) and ramus height ΔH (secondary criterion) were compared between groups. X² test, Fisher's exact test and Wilcoxon rank-sum tests were used for baseline comparisons. A multifactorial analysis allowing adjustment on possible confounding factors was used with R software.

Out of 67 patients files only 40 were completed and analysed: 13 in G1 and 27 in G2. These groups were comparable at baseline except for the age parameter. Taking into account the initial severity of asymmetry, the analysis of covariance showed a significant intergroup difference with higher correction in the early group of the corpus asymmetry (+1.0; P=0.008). On the contrary, no significant differences were observed between the groups in the ramus asymmetry correction.

Within the limits of this retrospective study, the early unilateral posterior occlusion correction by QH can better reduce both positional and contour mandibular asymmetries in patients under 7 years of age.