A Gradient of Potential for Modifying Craniofacial Growth
Section snippets
Relative Craniofacial Growth
Differences in the relative growth of the various craniofacial components have long been established.8, 9 Among the first to quantify relative facial growth differences was Hellman,10 who described a lengthening and deepening of the face as a whole over time; regionally, he noted relative lengthening posteriorly and relative widening and deepening inferiorly. Hellman’s cross-sectional comparisons indicated that, from early ages on, facial width reaches maturity earlier than facial depth, which
The GH-Deficient Child
Growth hormone deficiency (GHD) has profound effects on overall body size and maturation. Although growth deficits occur earlier,21 children are normally 2 to 5 years of age before growth failure is detected. Even at those young ages, skeletal maturation is already delayed by approximately 2 years.17, 19, 22, 23, 24, 25, 26 Stature is affected even more than skeletal maturation.18, 25, 27 Simply to be diagnosed as GHD, the child’s growth rate must fall below the 3rd percentile.
Craniofacial Growth Status of Untreated GHD Children
GHD produces characteristic differences in the craniofacial complex. Children with GHD have an immature facial appearance,22, 24, 28, 29 with nose shape characterized as “saddled.”28, 30 The maxilla is generally small and recessed.30, 31, 32 The mandible is also small, retrognathic and hyperdivergent,30, 32, 33 with open-bite tendencies.29 The gonial angle, mandibular plane, and palatal plane angles tend to be larger than normal.32 Posterior facial height is proportionately smaller than
Growth Changes of Children Treated for GHD
Treatment of GHD produces “catch-up” growth, with substantial increases in skeletal maturation and somatic growth, especially during the first year of replacement therapy.18, 19, 25, 38, 39, 40 Studies evaluating craniofacial growth of treated GHD children are conflicting due to small sample sizes and relatively weak designs. In addition, most studies evaluating craniofacial growth of treated children with GHD have been cross-sectional, which limits their ability to describe actual growth
Animal Models for Studying Relative Growth
For both practical and ethical reasons, it is not possible to perform prospective clinical trials evaluating GH-induced catch-up growth in humans. Animal studies are needed to control extraneous sources of variation, compare treated with untreated subjects, and to evaluate the effects of stopping treatment.
VandeBerg and colleagues recently established that wild-type Lewis rats between 4 and 9 weeks of age provide excellent models for studying relative craniofacial growth.51 They showed that
Animal Studies Evaluating GH Supplementation
A number of animal studies have shown that GH supplementation has different effects on the various craniofacial components. Collins and colleagues, who used hypophysectomized rats, reported greater effects of GH on the growth of the lower than upper jaw, with growth of the posterior mandible contributing greatly to the effects observed.55 Miura and colleagues also showed a differential increase in dimensions of the craniofacial complex following injections of GH in 21 day-old GH-normal rats.56
Possible Mechanisms of Postnatal Growth Gradients
Variability in response potential across craniofacial components is important because it provides information about morphological integration. Cheverud’s quantitative genetic analyses have demonstrated that covariances across structures reflect developmental and functional relationships.57, 58, 59 If, as he suggested, the phenotypic covariance structure provides an indirect measure of the genotypic covariance structure, the CFGMG reflects genetic relationships between component parts, which
Conclusions
- 1
The craniofacial growth maturity gradient provides a basis for better understanding how environmental and epigenetic factors produce variability across structures.
- 2
Unlike Scammon’s typological model of differential growth, which focuses on differences between structures, the craniofacial growth maturity gradient emphasizes the morphological integration of the component parts.
- 3
The various components of the craniofacial complex exhibit substantially different response potentials to environmental
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