Maturational and functional related differences in rat craniofacial growth.

Background: Whilst decreases in masticatory muscle function have been linked with increased prevalences of craniofacial dysmorphology and malocclusion in humans, the relative susceptibility of the different craniofacial components remains poorly understood.

Methods: Thirty-two wild-type male Sprague-Dawley rats were randomly assigned to two groups (n=16 each), one raised on a soft diet (SD) and the other on a hard diet (HD). Body weights and three radiographs (lateral, dorso-ventral, and tibial X-rays) were taken at baseline (T1=23 days old) and every 2 weeks thereafter for 8 weeks (T5=79 days old). The X-ray images were scanned, standardised points were digitised, and linear measurements were calculated. Multilevel statistical models were used to describe longitudinal absolute growth changes and statistically evaluate group differences. Relative maturity curves were generated for each measurement based on the animals' T5 status. The experimental effect was calculated as the absolute and relative growth differences between the HD and SD groups.

Results: The HD group was significantly heavier than the SD group at T5, but no differences in tibial length were observed. Eight of the 20 craniofacial measurements (40%) showed significant size differences at the end of the experiment, with the SD group showing deficiencies in each instance. All of the vertical measurements, as well as most of the mandibular (67%) and transverse (67%) measures, showed absolute growth deficits in the SD group. Relative maturity curves demonstrated considerable variation among craniofacial structures (ranging from 42 to 98%). The neurocranial measures were the most mature; the mandibular measures were the least mature; the viscerocranial measures, which were most variable, tended to be intermediate. Whilst unrelated to the absolute experimental effect, the structures' relative maturity explained almost 70% (r=-0.82) of the relative experimental effect.

Conclusion: The results of this study support the notion that masticatory function is a key determinant of the craniofacial growth pattern and that its effects are modulated by the relative growth potential of the different craniofacial components.