We shall give our principal attention to the skeleton, which has been the object of the bulk of craniofacial research and is relatively easy to measure. There is no doubt that the enveloping soft tissues exert important influences on the skeleton, but these are difficult to measure directly. The methods available for studying growth utilize animals (dyed slides of tissue; evaluation of hormonal, hereditary, and dietary factors; and surgery) and humans (embryologic, genetic, and, especially, cephalometric studies).
Two types of growth
Some bones of the cranium and the face are of cartilaginous, or endochondral, origin. Others begin as membranes and are thus formed by different types of calcification, a distinction that persists only until the end of the growth period. It is important to differentiate between these two types of growth because endochondral growth is regulated, in large part, by hereditary factors, while membranous growth responds readily to forces emanating from the surrounding environment, although its prefunctional form is guided by genetic determinants. The cranial base, which develops from the primary activity of its oriented sutures, is a good example of endochondral bone growth. The bones of the cranial cap, in contrast, are membranous in origin. They are also separated by sutures, but these sutures function only secondarily to fill in the spaces that arise as growth proceeds and the developing cranial bones are propelled outward by the developing cerebrum.
Growth is controlled by general and local factors. General factors include genetic, hormonal, neural, nutritional, health, and socioeconomic influences. Local factors include cartilaginous, osseous, muscular, and aponeurotic structures and functional forces.
Here discussion will be limited to the growth of the cranial base, which is the point of support for the whole face. The cranial base consists of the horizontal portion of the frontal bone, the crista galli apophysis of the cribriform plate of the ethmoid bone, the sphenoid bone, the petrous segments of the temporal bone, and the body and the lateral masses of the occipital bone. These osseous segments are separated by synchondroses, which are active growth centers. Their orientation can be transverse or longitudinal so that they can grow in both length and width. Because it is placed obliquely, the spheno-occipital suture also allows growth in height. Surface bone remodeling can occur through apposition and resorption of osseous tissue as well as through the action of the synchondroses.
The angulation of the cranial base can influence the position of the maxilla and the mandible. This is what Björk (1963) has called “anterior rotation” or “posterior rotation” of the face. No therapeutic efforts can affect the growth of the cranial base because its outcome depends, essentially, on hereditary factors.
Middle third of the face
The bones of the face develop in two ways: by sutural growth and by remodeling.
The sutural system that unites the various osseous units of the face with each other and with the cranial base is fairly complex. These sutures are syndesmoses, which unite primarily bones that are membranous in origin. They have no inherent growth potential but, as is the case at the cranial cap, they behave like “automatic joints of expansion and shrinkage that operate through adaptive connective tissue proliferation and marginal calcification” (Delaire 1971, 1978). These are the “functional units” that Moss (1982) asserts have the primary responsibility for the movement and development of the osseous segments. Making these phenomena complex are the great number of syndesmoses, the variation in their orientations, the variation in the timing and extent of their activity, and the rapid decrease in the intensity of their action as growth proceeds.
Remodeling, which becomes more important as sutural activity declines, is expressed as surface apposition in some regions and resorption in others. It leads to morphologic changes, which include the development of the sinuses.
The mandible is primarily a membranous bone that forms around Meckel’s cartilage, which, after serving as a guide for development, disappears. The growth of the mandible takes place partly in response to the activity of condylar cartilage and partly through recontouring.
The alveoli have traditionally been considered to be osseous tissue that is created when the teeth/>