Growth rotations are most obvious and have their greatest impact on the mandible; their effects on the maxilla are small and are almost completely masked by surface remodelling. In the mandible, however, their effect is significant, particularly in the vertical dimension. Mandibular growth rotations result from the interplay of the growth of a number of structures which together determine the ratio of posterior to anterior facial heights (Fig. 4.14). The posterior face height is determined by factors including the direction of the growth at the condyles and vertical growth at the spheno-occipital synchondrosis. The anterior facial height is affected by the eruption of teeth and vertical growth of the soft tissues, including the masticatory musculature and the suprahyoid musculature and fasciae, which are in turn influenced by growth of the spinal column. The overall direction of growth rotation is thus the result of the growth of many structures.
Forward growth rotations are more common than backward rotations, with the average being a mild forward rotation which produces a well-balanced facial appearance. A marked forward growth rotation tends to result in reduced anterior vertical facial proportions and an increased overbite (Fig. 4.15), and the more severe the forward rotation the more difficult it will be to reduce the overbite. Similarly, a more backward rotation will tend to produce increased anterior vertical facial proportions and a reduced overbite or anterior open bite (Fig. 4.16).
Not only is the vertical dimension affected, but there are also important antero-posterior effects. For example, correction of a Class II malocclusion will be helped by a forward growth rotation but made more difficult by a backward rotation. Growth rotations may also have an effect on the position of the lower labial segment. A forward growth rotation tends to cause retroclination of the lower labial segment which is often associated with shortening of the dental arch anteriorly and crowding of the lower incisors. A possible explanation for this is that, as the lower arch is
carried forwards with mandibular growth, forward movement of the lower incisor crowns is limited by contact with the upper incisors, causing them to crowd. This is common in the very late stages of growth when mandibular growth continues after maxillary growth has finished, although facial growth is only one of a number of possible aetiological factors in late lower incisor crowding.
Thus growth rotations play an important part in the aetiology of certain malocclusions and must be taken into account in planning orthodontic treatment. It is necessary to try to assess the direction of mandibular growth rotation clinically. This is not entirely straightforward since the effect of growth rotation upon the mandible is masked to some extent by surface remodelling, particularly along the lower border of the mandible and at the angle. However, it is possible to make a useful assessment of a patient’s facial growth pattern by examining the anterior facial proportions and mandibular plane angle as described in Chapter 5. Increased facial proportions and a steep mandibular plane indicate that the direction of mandibular growth has a substantial downward component, while reduced facial proportions and a horizontal mandibular plane suggest that the direction of growth is more forwards. It is also helpful to examine the shape of the lower border of the mandible. A concave lower border with a marked antegonial notch is associated with a backward rotation, while a convex lower border is associated with a forward growth rotation (see Figs 4.15 and 4.16).
4.9. GROWTH OF THE SOFT TISSUES
The importance of the oral musculature in orthodontic practice is that it influences significantly the form of the dental arches, since the teeth lie in a position of equilibrium between the lingual and bucco-labial musculature. Therefore they are important factors in the aetiology of malocclusion, and greatly affect the stability of the result after orthodontic treatment.
The facial musculature is well developed at birth, considerably in advance of the limbs, because of the need for the baby to suckle and maintain the airway. Other functions soon develop: mastication as teeth erupt, facial expressions, a mature swallowing pattern (as opposed to suckling), and speech.
The lips, tongue, and cheeks guide the erupting teeth towards each other to achieve a functional occlusion. This serves as a compensatory mechanism for a discrepancy in the skeletal pattern; for example, in a Class III subject the lower incisors may become retroclined and the upper incisors proclined to obtain incisor contact. Sometimes this compensatory mechanism fails, either because the skeletal problem is too severe or the soft tissue behaviour is abnormal. An example of this is where lower lip function worsens a Class II division 1 malocclusion by acting behind the upper incisors rather than anteriorly to them. In the late stages of growth the lips lengthen as they mature, tending to become more competent.
Muscle growth must be coordinated with the growth of the associated bones, with the muscles lengthening as their bony attachments separate. Neuromuscular activity regulates the positions of the jaws, and it has been suggested that the whole process of facial skeletal growth is determined by the soft tissues which surround the bones.
4.10. CONTROL OF FACIAL GROWTH
The mechanisms that control facial growth are poorly understood but are the subject of considerable interest and research. As with all growth and development,
there is an interaction between genetic and environmental factors, but if environmental factors can make a significant impact on facial growth then the possibility exists for clinicians to alter facial growth with appliances.
It is often difficult to distinguish the effects of heredity and environment, but it is helpful to consider how tightly the growth and development of a structure or tissue are under genetic control. Two simple examples illustrate this: gender is genetically determined and does not change no matter how extreme the environmental conditions, while obesity is very strongly affected by the nature and amount of food consumed. Most structures, including the facial skeleton and soft tissues, are influenced by both genetic and environmental factors, and the effect that the latter can have depends upon how tightly growth is under genetic control.
Genetic control is undoubtedly significant in facial growth, as is clearly shown by facial similarities in members of a family. The extent to which the facial skeleton itself is under genetic control has been debated at length in recent decades, with the development of two opposing schools of thought. Growth at the primary cartilages is regarded as being under tight genetic control, with the cartilage itself containing the necessary genetic programming. Therefore those who view growth of the whole facial skeleton as being directly and tightly genetically controlled have looked for primary cartilaginous growth centres in the facial bones. The condylar cartilages seemed to fulfil this role in the mandible, while the nasal septal cartilage was thought to serve a similar function in the maxilla. However, the structure and behaviour of these cartilages is different from primary growth cartilages, and at present it is thought that, while their presence is necessary for normal growth to take place, they are probably not primary growth centres in their own right.
The other school of thought proposed that bone growth itself is only under loose genetic control and takes place in response to growth of the surrounding soft tissues — the functional matrix which invests the bone. This idea looks to the example of the neural growth pattern of the calvarium and orbits, which develop intramembranously and enlarge in response to growth of the brain and eyes. However, the functional matrix theory ran into difficulty with regard to facial growth as there are no similarly expanding structures within the middle and lower face. It has attracted a lot of attention as, if taken to its logical conclusion, it implies that orthodontic appliances can be used to alter facial growth.
There is much yet to be understood about how growth of the face is controlled. As to whether appliances influence facial growth, the truth appears to lie some-where between the two extremes of opinion, but research in this field faces considerable problems, some of which are discussed in Chapter 18 in relation to functional appliances. At present, the evidence is that the impact of current orthodontic treatment methods on facial growth is on average quite small, but there is considerable variation in the response of individual patients.
4.11. GROWTH PREDICTION
It would be extremely useful if we could predict the future growth of a child’s face, particularly in cases which are at the limits of what orthodontic treatment can achieve. For growth prediction to be useful clinically it would need to be able to predict the amount, direction, and timing of growth of the various parts of the facial skeleton to a high level of accuracy.
At present there are no known predictors which can be measured, either clinically on the patient or from radiographs, which will enable future growth to be predicted with the necessary precision. Much work has been done to try to find
measurements which can be taken from cephalometric radiographs which will predict future facial growth to a useful level of precision, but so far with limited success. Assessment of stature (height) and secondary sex characteristics help to indicate whether the patient has entered the pubertal growth spurt, an important observation when functional appliances are being considered. Since growth of the jaws follows a somatic growth pattern, the possibility has been investigated that observation of the developmental stage of other parts of the skeleton would give an indication of the stage of facial development. The stage of maturation of the metacarpal bones and the phalanges as seen on a hand–wrist radiograph is used as a measure of skeletal development, but the correlation of this with jaw growth has been found to be too poor to give clinically useful information.
The best which can be done is to add average growth increments to the patient’s existing facial pattern, but this has only limited value. This can be done manually using a grid superimposed on the patient’s lateral cephalometric tracing, and average annual growth increments are read off to predict the change in position of the various cephalometric landmarks. Computer programs can be used for the same purpose, after the points and outlines from the lateral skull radiograph have been digitized. These programs can refine the prediction process further but they still have to make some assumptions about the rate and direction of facial growth. Unfortunately, the assumption that a patient’s future growth pattern will be average is least appropriate in those individuals whose facial growth differs significantly from the average, and who are the very subjects where accurate prediction would be most useful. As growth proceeds, the rate and direction of growth in an individual vary enough that study of the past pattern of a patient’s facial growth does not allow prediction of future growth to the level of precision required for it to be clinically useful. However, many clinicians find it helpful to assess the direction of mandibular growth rotation (see Section 4.8) on the assumption that this pattern is likely to continue.
Clinical experience has shown that for most patients, whose growth patterns are close to the average, it can be assumed for treatment-planning purposes that their growth will continue to be average.
PRINCIPAL SOURCES AND FURTHER READING
Björk, A. and Skieller, V. (1983). Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years. European Journal of Orthodontics, 5, 1–46.
A summary of the implant work on mandibular growth rotations.
Enlow, D. H. and Hans M. G. (1996). Essentials of facial growth. Saunders, Philadelphia.
The Bible of facial growth.
Houston, W. J. B. (1979). The current status of facial growth prediction: a review. British Journal of Orthodontics, 6, 11–17.
An authoritative assessment of the value of growth prediction.
Houston, W. J. B. (1988). Mandibular growth rotations — their mechanism and importance. European Journal of Orthodontics, 10, 369–73.
A concise review of the aetiology and clinical importance of growth rotations.
Mills, J. R. E. (1983). A clinician looks at facial growth. British Journal of Orthodontics, 10, 57–72.
A clear description of the facial growth processes from a clinical orthodontic viewpoint.