Differences in Facial Form and Facial Growth
The great diversity in facial forms is attributed mainly to differences in shape that come about during morphogenesis and to the role which can be recognized as being played in that process and subsequently during growth by the neuromuscular system and the soft tissues. Variations in facial growth are indicated and rotations of the face, especially of the mandible, are described. The concept of the dentoalveolar compensatory mechanism is discussed. The significance of the different growth regions is elucidated. The role that can be attached to the soft tissues and functional components in the growth of the face is discussed. The guidance due to interdigitation of the teeth is dealt with again. This applies also to the influence of genetic and environmental factors. In conclusion, a simplified idea of a scheme of the dentofacial complex is presented.
The human face displays a multitude of appearances. The proportions between height, breadth, and depth differ remarkably between individuals. The contours within the profile can show notable variation. A great diversity exists in the anteroposterior and vertical relationships of forehead, nose, maxilla and mandible, and chin. The multiplicity of these features exists, not only in the outward appearance, but manifested also in the very variable morphology of the craniofacial skeleton, the relationship between the mandibular and maxillary arches, and the position and mutual relationships of the teeth, particularly the maxillary and mandibular incisors.
Clinically, the most common classification of facial types is based on Angle’s classification,4 which deals with the anteroposterior relationship of the dental arches and jaws. The vertical component is not encompassed.1 A number of facial types subdivided accordingly is illustrated in Figure 8-1. The proportions in breadth are disregarded. They receive relatively little clinical consideration, probably due to the fact that in a growing child, the transverse facial dimensions and proportions—as distinct from the sagittal and vertical dimensions—are scarcely susceptible to external influences.
Fig. 8-1 Some variations in facial form related to the Angle classification with the addition of vertical information.
These drawings present diagrammatic indications of the related bony structures of mandible and maxilla and the positions of the first permanent molars and central incisors.
A Class I situation with harmonious face.
B Class II/2 anomaly in which retroclined maxillary and mandibular incisors go together with a collapsed mouth and lips (dished-in).
C Class II/1 malocclusion with the typical facial configuration often seen.
D Class I/1 malocclusion with an excessive facial height in the lower third, associated with a steep mandibular lower border and a chin point that is located far dorsally.
E Normal facial configuration, with an anterior open bite which is not reflected in the outward appearance.
F Anterior open bite associated with a pronounced increase in height of the lower face and a steep mandibular lower border.
G Class III anomaly in which there is no vertical disproportion.
H Class III anomaly which goes together with a large lower face height and an anterior open bite.
The primitive form of the face is established during the genetically determined morphogenesis in the first three months after conception. Disturbances in this phase due to hereditary or environmental influences (teratogenic factors) can lead to specific anomalies, such as clefts in lip, jaw, and palate. In the first three months, the central nervous system is partly formed, the chondrocranium is laid down, and the basis of the peripheral network of nerves and blood vessels is established. Oriented on these systems, the other structures in the head and neck region develop. It is considered that the laying down and the subsequent growth in the membrane bones is directed primarily by the other contiguous tissues and the growth that those undergo. In this way, for example, the size and shape of the cranial vault bones are determined by the size and form of the brain. The soft tissues developing in the face, and the functional spaces, such as the airway, will have an influence on the membrane bones forming in the facial skeleton. According to this concept, the chondrocranium is given genetic determination of size and shape, but the membrane bones are not.
In fetuses of 5 months, different facial types are clearly discernible which display likenesses to variants met within the newborn and in later years.170 It is probable that the growth pattern of the human head described by Brodie49 is established long before birth.
In the growth of the face that follows birth, a great increase occurs in all dimensions. The increase in the craniofacial skeleton, which is based on the fundamental form, presumably takes place primarily in response to the growth of the soft tissues and the functional demands made of the region. Coincidentally, the developing dentition takes a particular place in the growth of the face. The craniofacial skeleton contains—as described in Chapter 4—specific potentials for rapid growth and special mechanisms to enable it to satisfy those demands placed upon it (adaptability). The second aspect will be returned to later in this chapter; it is discussed fully in Chapter 9.
8.4 Variations in facial growth
The growth of the craniofacial skeleton has been studied deeply and often. The changes that occur in it on the average have been measured countless times. The course of average growth from ages 3 to 18 is given in Figures 5-3 and 5-4.48 In fact, hardly anyone grows strictly according to the average values. Significant individual variations are met with. Two extreme forms of facial growth are illustrated in Figure 8-2, which is taken from Björk. The boy has a low facial height with more vertical growth in the posterior than in the anterior part of the face. The reverse is seen in the girl. A further description is presented in the legend to the figure.
Fig. 8-2 Tracings of lateral cephalometric radiographs of two young persons who received no orthodontic treatment over the period indicated. They have markedly differing facial configurations and patterns of growth. Both had metal implants inserted in maxilla and mandible. In the illustrations A and C, which reproduce the natural head position, superimposition is on the anterior cranial base. The relative change in position of the implants which, of course, did not move, is indicated with arrows. In the illustrations B and D, the superimposition is on the implants. Adjacent is shown a reconstruction of the movement of the mandibular teeth in the occlusal plane.
A In the period from 11 to 17 years of age, the anterior border of the mandibular ramus has moved directly downwards in relation to the anterior cranial base; the mandibular symphysis, however, has moved nearly horizontally to ventral. The mandible is rotated forwards. The posterior facial height had increased more than the anterior facial height. The displacement and rotation of the part of the maxilla related to the palate is similar to that of the mandible, but is less in degree.
B The growth of the condyle was clearly forwards. Resorption has taken place on the superior surface of the posterior border of the ramus; apposition occurred dorsally at the lower part of the posterior border. On the lower border of the mandible, resorption has occurred at the posterior part with apposition anteriorly, including a slight amount at the symphysis. The resorption and apposition shown account for about one-half of the apparent movement which the outline of the mandible would otherwise have had to undergo. The teeth have migrated noticeably to ventral in the mandible, as can be clearly deduced from the arrows drawn. Transversely, however, there is no indication from the occlusal view that any significant movement has occurred.
C This girl displays a Class II/1 anomaly with an anterior open bite associated with an excessive lower face height. The soft tissues draping the teeth appear to be inadequate to obtain an unstrained lip closure. In the period between 10 and 15 years of age. the anterior part of the mandible has descended more than the posterior segment. The mandibular symphysis has moved directly downwards in unison with the premolar region. The mandible as a whole has rotated backwards. The movement of the palatal region of the maxilla has been limited and was chiefly to ventral.
D The amount of growth that has taken place at the condyle is about one-half that measured in the first case. The growth direction was dorsally and a little upwards. Considerable bone apposition took place on the dorsal border of the ramus, with resorption at the anterior border, although to a limited extent. On the lower border of the mandible, there has been minor bone apposition, just as at the symphysis. The permanent molars have moved perpendicularly to the occlusal plane. The same applies more or less to the premolars and canines, but the incisors have gone dorsally. (Figures in mirror-image and slightly modified from Björk.24)
8.5 Rotation of the face and mandible
With the help of radiographic cephalometry using metal implants, research has shown that the average growth pattern incorporates a forward rotation of the face which manifests itself more in the mandible than in the maxilla. The accompanying resorption and apposition compensates for the rotation on the lower border of the mandible to the extent of about one-half of the change; on the dorsal border of the ascending ramus, it accounts for about four-fifths. The nasal floor is displaced downwards more or less parallel to its original situation, since the rotation of the maxilla is fully or almost entirely compensated for by resorption and apposition.38 The vertical growth of the maxillary complex is accompanied by a displacement downwards of the tooth-bearing part of the mandible. Growth at the condyles and descent of the position of the articular fossae gives a downwards displacement of the posterior part of the mandible. When during growth the anterior and posterior parts of the mandible both descend to the same extent, no rotation of the mandible will occur. This is usually not the case. Mostly, the vertical growth of the posterior region predominates and a slight forward rotation takes place.
Björk has further described the rotations of the mandible that occur during facial growth, specifying the centers of rotation.31 Three important types of rotation that he distinguishes are reproduced in Figure 8-3. In addition, the variations in size and direction of growth at the mandibular condyle, such as were found in 21 children, are illustrated. Most of these children had severe orthodontic anomalies which were not treated. There is a strong relationship between the rotation of the mandible and the direction and extent of the condylar growth, something hardly to be wondered at.
Björk also recognized the particular significance of the mandibular rotation in changes that might ensue in the dental arches. The overbite increases with a forward rotation of the mandible without sufficient support in the anterior region. Crowding in the mandibular arch can develop. Further, more with a similar growth pattern, the extension dorsally of the tooth-bearing part of the mandible can be too limited to provide accommodation for the third molar. What occurs is that the lengthening of the body of the mandible, which is related also to the vertical growth of the ramus, is less when the angle of the jaw is acute than when it is obtuse. There can, of course, also be favorable changes in the dental arches that are due to a speci/>
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