Pattern of craniofacial growth

At birth, the face and jaws are underdeveloped compared with those in the adult. More growth, therefore, occurs of the facial skeleton than of the cranial structure postnatally. Growth patterns have been established for four major body tissue systems – lymphoid, neural, general or somatic, and genital – and it is important to have an appreciation of these as some patterns are followed by tissues involved in craniofacial growth (Fig. 9.1). Lymphoid growth is rapid up to about 10 years but undergoes involution as the genital growth is accelerating at puberty. Neural growth, however, is virtually complete by 6–7 years, while somatic growth increases in early childhood, then slows, before increasing dramatically at puberty.

The pattern of neural growth affects skeletal growth of the calvarium and orbit, whereas the somatic growth pattern is followed approximately by the mandible and maxilla. More precisely, the jaw growth pattern falls between that followed by the neural and general body tissues, with the mandible aligning itself more to the latter than the maxilla. The spurt in jaw growth at puberty almost coincides with the spurt in height, on average at 12 years in girls and 2 years later in boys, although considerable individual variation exists.

For both the maxilla and mandible, on average, growth in width is completed in advance of that in length, which ceases before growth in the vertical dimension. The transverse dimensions of the jaws and dental arches do not tend to alter during puberty, as growth in width is largely completed before the growth spurt. Growth in length usually continues until 14–17 years in girls and 17–19 years in boys, while vertical growth may extend into the late teens in girls and into the twenties in boys. For males and females, growth of the maxilla in all dimensions is completed before that of the mandible.

Growth continues into adult life, with vertical changes in the facial skeleton predominating over anteroposterior changes, and least change taking place laterally. In the twenties, growth tends to resume slightly in females who overall exhibit a backward mandibular rotation. Although a late resumption of growth is not witnessed in males, a forward rotation of the mandible is usual in adulthood. Irrespective of the direction of mandibular growth rotation, compensatory adjustment occurs in the occlusal relationships. Facial growth should, therefore, be seen as a process that continues well into adult life and not as one that is complete in the late teens.

Control of facial growth

Both genetic and environmental factors impact on the regulation of craniofacial growth but the exact mechanisms are unclear. In theory, genetic control may be expressed primarily via bone, cartilage or the adjacent soft tissues (‘functional matrix theory’); the current concept is a merging of these second and third theories, bone largely having being discounted. Sutures and periosteal tissues do not determine craniofacial growth primarily, as sutural growth is reactive rather than inherently programmed. The maxillary sutures are not growth centres, but rather growth sites. The synchondroses of the cranial base and the nasal septum, to a lesser degree, probably act as independent growth centres but the cartilage of the mandibular condyle appears to react to, rather than initiate, growth.

Growth of the cranium and of the orbit in direct response to growth of the brain and of the eyes respectively lend support to the ‘functional matrix theory’. Growth of the cranial base, although mainly by endochondral growth coupled with bony replacement at the synchondroses, may be influenced by growth of the brain. Cartilaginous growth of the nasal septum and growth of the surrounding soft tissues probably contributes to moving the maxillary position forward but the amount apportioned to the former is unknown.

The soft tissues, including the masticatory muscles, are also contributors to spatial translation of the mandible and to addition of new bone at the condyle.

Growth prediction

At present, no method is available to predict accurately the amount, direction and timing of facial growth. Instead, the assumption is usually made that for most patients whose direction and amount of facial growth are about average, the likelihood is that their growth pattern will follow the same pattern through orthodontic treatment.

Methods attempting to predict the pattern of facial growth to assist orthodontic treatment planning include:

As the amount and direction of growth may not be ‘average’ in an individual, there is a considerable amount of imprecision with regard to ascertaining clinically useful growth prediction. The assumption is often made, however, that the direction of mandibular growth rotation is likely to continue.

Growth of the craniofacial skeleton

Craniofacial growth can be considered conveniently in relation to the calvarium, the cranial base, the maxillary complex and the mandible.

Growth rotations

The trend is for the facial skeleton to grow downwards and forwards away from the cranial base, although implant studies have indicated that rotations of both the maxilla and mandible occur during growth. These have more marked effects on the mandible than on the maxilla, where remodelling disguises their true impact. Mandibular growth rotations represent a growth imbalance in anterior and posterior facial heights. The direction of condylar growth and the vertical magnitude of growth at the spheno-occipital synchondrosis influence posterior facial height. Growth of the masticatory and suprahyoid musculature, including associated fascia and influenced partly by the vertical growth changes in the spinal column, affects the anterior facial height together with the eruption of teeth.

While mandibular growth rotations occur in all individuals, these are particularly different where the vertical facial proportions are markedly reduced or increased. A forward rotation, characterised by greater growth in posterior than in anterior facial height, is more common than a posterior growth rotation, where the change in facial height ratio is opposite to that observed in forward rotation (Fig. 9.2). Where forward rotation of the mandible is extreme:

Conversely, a backward rotational pattern of mandibular growth results in:

The pattern of rotation has an impact on treatment. Whereas a forward rather than a backward rotation aids correction of a class II skeletal discrepancy, it also tends to increase overbite. Where the rotation is marked, overbite reduction is more difficult. Growth rotations also influence the inclination and anteroposterior position of the lower incisors. A forward growth rotation leads to retroclination of the lower incisors and an increase in lower labial segment crowding. With a posterior mandibular growth rotation, the incisors become upright, shortening the dental arch and producing crowding of the lower incisor area. Lower labial segment crowding is common in late teens particularly where mandibular growth continues after maxillary growth has ceased but other factors are likely contributors (see below).

Soft tissue growth

At birth, the orofacial musculature is well developed to allow suckling and breathing, and it soon responds to other functional demands of mastication, speech, facial expression and changes to the swallowing pattern. The light pressures from the lingual and buccolabial musculature affect tooth position, guiding the teeth towards a functional relationship and compensating, where possible, for any skeletal discrepancy. However, where a severe skeletal discrepancy or abnormal soft tissue behaviour exists (e.g. a lip trap with a class II division 1 malocclusion), the dento-alveolar compensatory mechanism will be insufficient.

Facial musculature lengthens with facial growth which increases the likelihood of lip competence. In girls, the greatest overall changes in soft tissues occur between 10–15 years but nasal growth is complete generally by age 16. In boys, however, the greatest soft tissue changes usually occur later (from 15 years) and most will be complete by late teens. In adulthood, soft tissue profile changes take place in both sexes in concert with, but are much larger than, those of the facial skeleton. Notably, the nose tends to lengthen and the lips move down relative to the teeth so, over time, less of the maxillary and more of the mandibular incisors become exposed on smiling; the fullness of the lips also decreases. An increase in chin prominence is common in males, whereas in females, the chin may become more retrusive or show minor change.

Development of the primary dentition

The gum pads, containing the primary teeth, enlarge and widen following birth, with the lower lying slightly behind the upper by the time the first primary teeth (lower incisors) start to erupt at about 6 months of age. These are followed closely by the other incisors. The first primary molars erupt 3–4 months later, followed by the primary canines and the eruption of the second primary molars at about 3 years. The incisors tend to be upright, and anterior spacing is normal. Spacing is most common mesial to the upper canine and distal to the lower canine – the anthropoid or primate spaces. With a 1–2 mm increase in the intercanine distance, spacing between the incisors often increases as the child grows. In the absence of generalised spacing of the primary teeth, crowding of the permanent teeth is likely. The overbite, which is often initially ‘deep’ in terms of lower incisor crown coverage, reduces, and by 5 years of age an edge-to-edge occlusion with incisor attrition is common.

Development of the permanent dentition

At about 6 years, the eruption of the first permanent molars, followed by the permanent incisors, signifies the transition from the primary to the permanent dentition, commonly referred to as the ‘mixed dentition phase’. The permanent successors are slightly larger than the primary teeth and the first permanent molars need to be accommodated. Existing space is present between the primary teeth. Additional space is provided by minor modifications in arch length, arch width and intercanine distance (Table 9.1). Once the primary dentition is fully erupted, however, the dental arch size remains more or less constant anteriorly apart from a modest change in shape with some growth in the intercanine distance. In addition, growth at the back of the arches is necessary to accommodate the permanent molars and to maintain the arch relationship while the face grows vertically.

The permanent lower incisors develop lingual to their predecessors and are frequently misaligned on eruption, but this usually resolves with intercanine growth. The upper anterior teeth develop palatal to their primary predecessors and are accommodated:

The upper permanent lateral incisors usually move distally and labially with eruption of the central incisors, but they may be trapped palatally in crowded arches. The upper central incisors are often distally inclined when they first appear. An associated diastema tends to reduce as the lateral incisors erupt. At this time, the upper central and, to a greater extent, the lateral incisors are divergent, the latter because of pressure on their roots from the unerupted canines; this has been referred to as the ‘ugly duckling’ phase of dental development, terminology that is best avoided in the company of concerned parents.

The maxillary canines migrate from their palatal developmental position to lie labially and distally above the roots of the lateral incisors, leading to approximation of the incisor crowns as they erupt.

The combined mesiodistal widths of the primary canines and molars in each quadrant are slightly greater than those of the permanent canines and premolars. This difference in dimension is known as the ‘leeway space’ and is about 1 mm in the upper arch and 2.5 mm in the lower arch. The larger leeway space in the lower arch, probably in combination with mandibular growth, allows greater forward movement of the lower first permanent molar, converting a ‘flush terminal plane’ relationship of the primary molars to a class I occlusion (see below).

Six features (keys) of a good (‘ideal’) permanent occlusion have been described by Andrews:

In addition, the following functional occlusal relations should exist:

Maturational changes in the occlusion

The occlusion of any child must be seen as dynamic and responding to changes in the facial skeleton. As the face continues to grow throughout the late teens and into adulthood, changes in the dentition and occlusion follow:

Classification for diagnosis

Classification to assess treatment need and treatment outcome

Skeletal problems

The majority of anteroposterior skeletal problems are caused by inherited jaw proportions, which are strongly genetically determined. Inherited characteristics (e.g. mandibular deficiency) account for almost all of moderate class II malocclusion, while the added insult of environmental soft tissue influences is likely in more severe cases. For class III malocclusion, mandibular prognathism has a strong racial and familial tendency although mandibular posturing, possibly caused by tongue or pharyngeal size, may stimulate growth and influence jaw size secondarily. Maxillary deficiency is also most likely due to inherited jaw dimensions and a simple environmental factor seems unlikely, but its exact aetiology is almost completely unidentified.

Vertical jaw proportions are also inherited, but soft tissue postural effects (e.g. anterior tongue position or mandibular postural changes induced by partial nasal obstruction) may contribute in particular to anterior open bite. Other environmental influences, such as a high lower lip line, may contribute to deep overbite.

A unilateral crossbite with displacement is often caused by a functional alteration, but it is usual for a skeletal crossbite to have an additional genetic input.

Crowding

Crowding is the most common orthodontic problem and is caused in part by a reduction in jaw and tooth size over the centuries. Interpopulation breeding has also been implied as arch width is influenced by jaw size, which is under tight genetic control.

Environmental influences, for example early loss of primary teeth due to caries or trauma or a digit-sucking habit, may also be instrumental in the aetiology of crowding. Also implicated is a softer, less abrasive modern diet which results in less interproximal tooth wear and less demands on jaw function; the latter may have added to a general tendency for smaller jaw size which was already happening. Soft tissue pressure of sufficient duration (more than 6 hours per day) in combination with the developmental tooth position may be responsible for a localised crossbite or malalignment.

History

Initially the dentist must identify:

It is important to document whether the patient is unconcerned with the appearance of their teeth, particularly in the presence of obvious malocclusion, as any attempt to persuade the patient to undertake treatment is likely to be met with indifference. Attitude to treatment is best assessed from response to enquiries about their perception of orthodontic treatment for their peers, and by observing carefully their reaction when shown photographs or examples of appliances. Adults usually have a high level of dental awareness, are able to specify their concerns and motivation is high for treatment.

Medical history

A health questionnaire should be completed by each patient or a parent, and the findings verified by a clinical interview. Several medical conditions may impact on orthodontic treatment. Some are given below; where any uncertainty exists, liaison with the relevant medical specialist(s) is required.

Dental history

The nature, extent and frequency of previous dental treatment together with the level of patient co-operation should be recorded, along with details of daily oral hygiene practices. A history of early loss of primary teeth or of incisor trauma should be noted. If orthodontic treatment has been carried out previously, details relating to extractions and appliance type should be recorded. If treatment was abandoned, the patient must be questioned carefully for the reasons.

Social history

The ease with which regular appointments can be attended and any forthcoming events that may influence attendance should be noted.

Extraoral examination

The skeletal pattern, soft tissues of the lips, tongue position during swallowing, speech, temporomandibular joints and mandibular path of closure should be assessed and the presence of any habits noted.

The relationship of the mandible to the maxilla should be assessed in all three planes of space: anteroposteriorly, vertically and laterally. Before proceeding, it is important to ensure that: