Changes in bite force, masticatory muscle thickness, and facial morphology between primary and mixed dentition in preschool children with normal occlusion.

The study of stomatognathic system development can provide useful data for recognition of deviations from normality. Thus, a longitudinal examination of 14 children with normal occlusion from primary (stage I, mean age 59.21+/-8.40 months) to early mixed dentition (stage II, 77.57+/-5.92) was performed. Bite force was measured with a pressurized tube and correlated with ultrasonographic masseter and anterior portion of temporalis muscle thicknesses (at rest and maximal intercuspation), facial dimensions, age, and body mass index (BMI). Facial dimensions were assessed by standardized frontal photographs as follows: anterior facial height (AFH), lower face height (LFH), and bizygomatic width (BFW). Children with malocclusion, oral tissue/temporomandibular abnormalities, caries, or parafunctional habits were excluded. Results were submitted to the Shapiro-Wilk test, t-test/Wilcoxon's test, and backward stepwise multiple regression (alpha=0.05) for analysis. The results showed that muscle thickness did not differ significantly between the right and the left sides in either stage. Bite force, AFH/BFW ratio, and masseter thickness at rest increased significantly from stage I to II, although temporalis thickness, LFH/AFH ratio, and BMI did not change between the stages. The masseter thickness at rest and the stage of dentition were the most important factors contributing to bite force magnitude; the covariates age, BMI, and facial dimensions did not significantly influence bite force magnitude (power of the test: 96%). In the studied sample, the increase in bite force observed from primary to early mixed dentition was explained by the increase in masseter thickness and the stage of dentition.