Most orthodontic treatmentat the present time is carriedout during the growthperiod, between the ages of10 and 15 years. Theocclusion and position of theteeth is also establishedduring the growth period,and change after growth hasfinished are of relativelyminor degree.
The skull and jaws at birthAt birth, the skull is far from being merely a smallversion of the adult skull. There are differences inshape, in proportion of the face and the craniumand in the degree of development and fusion of theindividual bones.
At birth, the head forms about one quarter of the total height of thebody. In the adult the head forms about one eighth of total body height.Therefore between birth and maturity the body must grow faster, inproportion to size, than the head.
While the total growth of the head from birth tomaturity is proportionately less than that of therest of the body, the head itself does not growat a constant rate, the two main components ofthe head, that is the cranium and the face,differ in their relative proportion at birth and atmaturity, and therefore they must grow atdiffering rates, neither of which is constant. Thetwo components can be considered separately.
The cranium, which hasgrown rapidly before birth,continues to grow rapidlyup to about 1 year of age,Thereafter the growth ratedecreases, and by about 7years of age the craniumhas reached some 90% ofits final volume There isthen a slow increase in sizeto maturity .
The growth rate of the eyes, and consequently of the eyesockets, follows a similar pattern.Thus the infant, by comparison with the adult, appears to have asmall face, with large eyes, large cranium and retrusive nose.
The growth rate of the face, which ishighest at birth, falls sharply andreaches a prepuberal minimum.Growth rate then increases to a peakat puberty, declining again and tailingoff until growth ceases in lateteenage.Facial growth is normally associatedwith eruption of the primary dentitionbetween 1 and 3 years of age and ofthe permanent dentition between 6and 14 years of age.
Studies of dental arch have suggested that increase in size of the dental arches is particularly associated with tooth eruption. studies of growth of the head have shown that during the same period the total length and height of the jaws increases and the face is progressively positioned downward and forward in relation to the cranium
They also found a close relationship between maximum puberal growth and maximum condylar growth and suggested that mandibular growth continues on average about 2 years longer than maxillary growth. This difference in growth between the two jaws may be important in orthodontic treatment planning
While the possibilities for how the skull grows canbe clear , the question of where growth actuallytakes place?- Bone, unlike most other tissues, cannot grow simply by interstitial division of its living cells to give increasing size.- There are three main mechanisms of bone growth, each of which plays its part in the growth of the skull and jaws:
Cartilaginous growth: the growth of cartilage by cell division, with progressive conversion to bone by ossification . The areas of the skull where cartilaginous growth is possible are mainly in nasal septum and at the head of the mandibular condyle. Growth of the nasal septal cartilage would bring the nose forward from its original position under the front of the cranium. Growth of the mandibular condylar cartilage would increase the total length and height of the mandible.
Sutural growth: the apposition of bone in the area of the sutures between adjacent bones. The bony sutures of the head are such that sutural growth would be capable of increasing the size of the head in all dimensions. It has been suggested that sutural growth move the face in a forward and downward direction in relation to the cranium. Also sutural growth is active in bringing the bones into close proximity.
Periosteal and endosteal growth: the apposition of bone under the periosteal membrane and at the surfaces of the cancellous spaces within the bone.- The apposition of bone on the periosteal surfaces would obviously enlarge the head in all dimensions cause the bones to be excessively thickened and therefore concomitant resorption of bone is necessary in order to obtain the appropriate thickness and strength.- This process called bone remodelling ; which often involves resorption of bone from the outer surface and apposition of bone on the inner surface.
Endosteal resorption and additionof bone from within the cancellousspaces is also necessary to maintain the appropriate thickness of the cortical layer of bone. generally thought that: this method of growth is the most important part in the growth of the headbecause its the most active in the skull and jaws after the first few years of life, when cartilaginous and sutural growth slows.
VariationNormal variation RACIAL VARIATION INDIVIDUAL VARIATIONVariation exists The different ethnicbetween normal groups of mankindindividuals both in Variation in skull and have a tendency togrowth and in the jaw size and form exhibit certain broad between individuals isfinal form and size of patterns of form of the so common and so wellthe head, quite apart skull and jaws although known. occur byfrom the effect of any such patterns are often several reasons.pathological overshadowed by individual variation.conditions.