Diagnostico de posible ubicación

The development of the dentition and the facial skeleton is a series of complex and inter-related processes, beginning with the early embryological formation of the branchial arches and continuing through life with the growth of the skull, mandible and maxilla and the eruption of two dentitions.

Between the ages of 6 and 12 years, the dentition is mixed and it is during these years that many of the problems of facial disproportion and malocclusion become apparent.

In addition, the cranium grows more rapidly than the rest of the skeleton and its pattern of growth (as is that of the facial skeleton) is highly complex. As such, it is perhaps surprising that in the vast majority of individuals harmonious growth occurs in three dimensions, resulting in broadly symmetrical faces with functional occlusions. Despite this, however, there is an enormous variability in forms and dimensions between sexes, individuals and cultures.

Since such variation is universal, it is impossible to consider any particular pattern of growth—either the process or the end result—as ‘normal’.

Assessing the need for any surgical intervention must be on the basis of understanding the technical requirements and considering the relevant cultural, racial and ethical issues. Distinguishing between normal and abnormal variations is not always

possi-ble. However, it is possible in general terms to cate-gorize the different types of anomalies as:

● mandibular

● maxillary

● bimaxillary

● craniofacial.

In each of these cases the variations may be symmetric or asymmetric, congenital or acquired.

Treatment planning may be affected accordingly, although the general principles of treatment are not just dependent on aetiology. In all cases, however, the assessment of the need for surgical intervention must be based on a variety of factors, the principal one being the patient’s own desire for some form of treatment. Assuming this is the case, it is important to be able to recognize which cases are treatable with orthodontics alone and which will also require surgical intervention.

In general terms, orthodontics alone is unlikely to be adequate in patients:

● who cannot retrude into an edge-to-edge occlusion (prognathism—Class 3 occlusions)

● who cannot protrude into an edge-to-edge occlusion (retrognathism—Class 2 occlusions)

● with anterior open bites due to skeletal causes (i.e. not due to thumb- or finger-sucking habits)

● with true asymmetry

● with marked discrepancies in the vertical dimension.

This list is not exhaustive and does not preclude some patients from having treatment that may, at least in technical terms, be a compromise. However, most patients with any of the above problems will require surgery, and it then becomes important to determine the diagnosis of their anomalies. These will be considered in the same order as listed at the start of this section. Many technical terms have been devised to describe the jaw relationships which are useful in considering orthognathic surgery; these are italicized.

Mandibular anomalies

In relation to a normal upper jaw, the lower jaw can be too big or too small. When the mandible is too Fig. 12.8 Periodontal problems associated with a high lingual

fraenum, resulting in direct trauma and poor oral hygiene.

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large the condition is referred to as prognathism;

when it is too small it is termed retrognathia—

literally, backwards positioning of the jaw. A genuinely diminutive mandible is probably better referred to as micrognathic although this term is usually reserved for cases where there is a failure of the ramus to develop. Such cases are often associated with various syndromes of the face including Apert’s and Treacher-Collins’ syndromes.

Prognathism is usually due to excessive growth of the mandible between the ages of 8 and 17 years but occasionally becomes apparent after an adolescent growth spurt. Features of prognathism include reversal of the mandibular/maxillary relationship, a reverse overjet and a diminished or absent overbite. The lower incisors tend to be retroclined and the upper incisors proclined, although they are also less visible on smiling. In addition to the increased horizontal length of the mandible there may be an increase in the vertical length. This vertical excess can be very variable and its management may be addressed by either mandibular shortening or maxillary impaction osteotomies. In addition to the simple excessive developmental growth there are other causes of prognathism, including hormonal disturbances such as would be found in cases of acromegaly. In all cases the enlargement of the mandible is basically symmetrical, although some lateral difference is always possible and the variation in the vertical dimension can be considerable.

It is also possible to have a normal skeletal base relationship (i.e. the teeth in a normal occlusal relationship) and yet have a marked protrusive or retrusive appearance in profile. This is caused by the failure of the chin to develop in proportion to the skeletal bases, excess development being termed progenia and underdevelopment retrogenia. An inad-equate depth of bone between the apices of the anterior teeth and the base of the mandible is correctly termed microgenia but the term is frequently confused with retrogenia. The opposite condition, macrogenia, is also seen on occasion and all these conditions can be treated with different types of genioplasty (surgical sectioning and repositioning of the chin).

In some cases the jaw forms an asymmetric excess;

this occurs commonly in condylar hyperplasia. This idiopathic condition is usually seen in patients between the ages of 20 and 40 years although it is not restricted

to any age. Asymmetric loss of bone can also occur, for example, as a result of trauma or as a result of certain pathological conditions such as hemifacial microsomia, Still’s disease (juvenile rheumatoid arthritis) or severe hemifacial atrophy. Fortunately such conditions are relatively rare; they should be treated in a specialist unit. It should also be noted that asymmetry can be the result of soft-tissue changes such as masseteric hypertrophy, fibrous dysplasia or benign tumours such as neurofibromata.

Maxillary anomalies

The mid-face actually comprises some 23 bones, of which the two maxillae are fused to form the major part. The articulation of the mid-facial skeleton is complex and for many years orthognathic surgery was directed towards the mandible when the problems were really maxillary (mid-facial). Modern orthognathic surgery addresses the concept of the whole facial skeleton and the need to achieve a balance of the hard and soft tissues in three dimensions—lateral, vertical and anterior-posterior.

Additionally, in developmental terms, the formation of the maxillary hard and soft tissues is complex and it is of little surprise that congenital defects in the form of cleft lip and/or palate are the world’s second most common congenital defect (see later).

Maxillary abnormalities can be classified thus:

● protrusion or retrusion and intrusion or extrusion

● dental arches that are normal, widened or narrowed (in relation to the bony skeleton)

● dental arches that slope to one side, a condition usually referred to as a cant.

Abnormal vertical development of the posterior maxilla

Increased posterior facial height may result in the anterior teeth failing to meet (i.e. a reverse overbite, which is commonly referred to as an anterior open bite). A similar condition may result from suppressed eruption of the incisors, such as that caused by thumb-sucking, but the two conditions are treated completely differently with only the former requiring surgical intervention. Isolated instances of reduced vertical dimensions of the maxilla are less common but they often accompany maxillary retrusion.

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Craniofacial anomalies

The details of craniofacial anomalies are outside the scope of this book, but it is important for the dental profession to be aware of the rare conditions such as Crouzon’s syndrome and Apert’s syndrome, which can result in severe facial deformity.

Much of the pioneering work on these groups of patients was carried out by Paul Tessier. It is now possible to perform complex craniofacial reconstruc-tions to ameliorate the worst of the anomalies.

Bimaxillary anomalies

Bimaxillary anomalies are the most common type of facial variation because the growth of one jaw is always, to some extent, dependent on the other. This is clearly demonstrated in children who have cleft lips and palates. Inadequate surgical repair of the cleft in infants often inhibits growth of the maxilla in later years. This may then allow excessive growth of the mandible, resulting in a pronounced class 3 occlusion.

However, it is difficult to justify performing bimax-illary osteotomies when only small movements of two jaws are required; it is therefore sometimes necessary to operate on one jaw and compromise on which jaw is likely to give the best result. Conversely, single-jaw operations when movements in excess of 10 mm are required are more prone to significant relapse. Bimaxillary surgery, when indicated, usually gives more aesthetically and functionally pleasing results and it is always better to avoid compromise when at all possible (Figs 12.9, 12.10).

Fig. 12.9 Preoperative view of a prognathic patient who is about to undergo bimaxillary surgery to correct her facial proportions.

Fig. 12.10 Postoperative view of the patient shown in Fig. 12.9.

Assessing the need for surgical intervention

● Patient choice

● Class 3 or severe class 2 occlusion

● Anterior open bite (skeletal)

● Asymmetry

● Abnormal vertical relationship

● Accurate diagnosis

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THE PLANNING OF OSTEOTOMIES, IN