Masseter muscle thickness and treatment outcomes in children with Class II division 1 malocclusion

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ABSTRACT

Aim

To investigate if treatment outcome in patients with Class II division 1 malocclusion treated by a variety of approaches is affected by masticatory muscle capacity.

Material and methods

Seventy-four children with Class II division 1 malocclusion were included in the present study. These were divided into 54 children (10.4 ± 1.6 years of age) treated with fixed appliances including headgear and/or Class II intermaxillary elastics, 12 children (9.9 ± 1.9 years of age) treated with functional appliances and 8 children treated with fixed appliances and other means of treatment. Ultrasonographic masseter muscle thickness measurements were taken before treatment, while lateral cephalograms were taken before and after treatment. Statistics: Multivariate linear regression analysis was used to assess the association between masseter muscle thickness and treatment outcomes, as well as the gonial angle and treatment outcomes.

Results

All children showed dentoalveolar sagittal improvement during treatment. In children treated with fixed appliances and headgear and/or intermaxillary elastics, no association was found between pre-treatment masseter muscle thickness or the gonial angle and treatment outcomes. Children treated with functional appliances however showed a negative association between pre-treatment masseter muscle thickness and mandibular first molar mesialisation and maxillary molar distalisation during the functional phase of treatment. A positive association was also found between the pre-treatment gonial angle and maxillary incisor retroclination and mandibular incisor proclination during the functional phase of treatment. These associations were however not maintained at the end of the two-phase treatment even in this group.

Conclusion

In children with Class II division 1 malocclusion treated by means other than functional appliances, treatment outcome is not associated with masseter thickness. This may be the case since the application of intermaxillary forces and torque application may offset the functional component in tooth movement. Outcomes following functional appliance treatment may be partly determined by the masticatory muscles. During functional treatment, children with a weaker masticatory system show greater dentoalveolar compensation. This is however not maintained during the second phase of fixed appliance treatment.

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