Growth of the face is a gradual and differential maturational process taking many years and requiring a succession of changes in regional proportions and relationships of various parts (Figure 14-1). An understanding of the mechanisms of growth and development of the face is pertinent to dentistry, in general, and is essential for the practice of dentistry. The objective of this chapter is to present an overall view of events occurring during facial growth and development.*
There are two general head types; the dolichocephalic head is relatively narrow and long, and the brachycephalic head wider and rounder. Each type gives rise to corresponding general facial types. These are the long and narrow (leptoprosopic) facial type, and the round and broad (euryprosopic) facial type.
Although many intermediate types of head forms and facial patterns exist in any general population, these two skull configurations (Figure 14-2) tend to be associated with characteristic facial features. The narrow facial type tends to have a convex profile with a prognathic maxilla and a retrognathic mandible. The forehead slopes because the forward growth of the upper part of the face carries the outer table of the frontal bone with it. A larger frontal sinus than is characteristic of the broad facial type because of the greater separation of inner and outer bony tables of the forehead, whereas the inner table remains fixed to the dura of the frontal lobe of the cerebrum. The glabella and supraorbital rims are prominent, and the nasal bridge is high. There is a tendency toward an aquiline or Roman nose because the more prominent upper part of the nasal region induces a bending or curving of the nasal profile. Because the face is relatively narrow, the eyes appear close set, and the nose is correspondingly thin. The nose also is typically prominent and quite long, and its point has a tendency to tip downward. The lower lip and mandible are often set in a somewhat recessive position because the long dimension of the nasal chambers leads to a downward and backward rotational placement of the lower jaw (the dolichocephalic head form also has a more open cranial base flexure, which adds to the downward mandibular rotation). These factors contribute to a downward inclination of the occlusal plane and a marked curve of occlusion.
The round, broad facial type is characterized by a more upright and bulbous forehead, with the upper nasal part of the face less prominent than in the dolichocephalic face. The nasal chambers are horizontally shorter but wider, in contrast to the narrow but more prominent nasal region characterizing the dolichocephalic head form. The net capacity of the airway in both instances is thus equivalent. There is less protrusion by the supraorbital ridges, the glabella is less prominent, and the frontal sinus is smaller. The nose is shorter vertically as well as horizontally and tends to be more puglike. The nasal bridge is lower, the nasal sides are broader, and the end of the nose often tips upward. The eyes appear widely set and the zygomatic bones seem prominent because the nose and forehead are less prominent. The face appears quite flat and broad, in contrast to the more angular, narrow, deep, and topographically bold appearance of the dolichocephalic face. The cranial base angle of the brachycephalic skull tends to be more closed, and there is a greater tendency toward an orthognathic (straight-jawed) profile.
There are three basic types of facial profiles (Figure 14-3): (1) the straight-jawed, or orthognathic, type; (2) the retrognathic profile, which has a retruding chin and is the most common profile among white populations; and (3) the prognathic profile, which is characterized by a bold lower jaw and chin.
To identify a person’s profile type, imagine a line projecting horizontally from the orbit. Drop a perpendicular line from this just brushing the surface of the upper lip. If the chin touches this vertical line, the profile is orthognathic; if it falls behind or ahead, the profile is retrognathic or prognathic. For a female face, the vertical line generally passes through the nose at a point about halfway along its upper slope. In male faces that are long and narrow, however, the more marked extent of the upper nasal prominence is such that more of the nose sometimes lies forward of the vertical line.
People with a dolichocephalic head form (a characteristic feature of some white populations in northernmost and southernmost Europe, North Africa, and the Middle East) tend to have a retrognathic face. Those with a brachycephalic head form (a characteristic feature of Middle Europe and East Asia) have a greater tendency toward prognathism. Also, Asians commonly have a maxillary and mandibular alveolodental protrusion characterized by labial tipping of the maxillary incisors that results from a protrusive mandibular dentition. Thus there are more malocclusions involving protruding maxillary teeth in some Caucasian populations and more malocclusions involving protruding mandibular teeth with alveolodental protrusion in Asian populations.
An important intrinsic developmental process of compensation functions to offset and reduce the anatomic effects of built-in tendencies toward malocclusions. A genetically determined retrognathic mandibular placement caused, for example, by some rotational factor in the cranial base can be compensated by the development of a broader mandibular ramus. Thus the whole mandible becomes longer and reduces the amount of retrognathism. Because latitude exists for compensatory adjustments, only a relatively slight degree of retrognathism, or some other anatomic imbalance, occurs in most persons (Figure 14-4). For narrow-faced individuals, 3 to 4 mm of mandibular retrognathism (a mild malocclusion with some crowding of the incisors) is typical. A perfect occlusion is hardly to be considered normal because relatively minor dental arch or facial skeleton irregularities are almost universal. Only when the compensatory process fails do severe malocclusions occur.