Malocclusions of dentofacial deformities are either hereditary or environmentally determined. There are several studies available focusing on the ratio determining environmental and genetic factors in the manifestation of malocclusions. Varying results of these investigations, however, have shown that there are rather genetic effects than dental components that assert predominance on skeletal components of the facial appearance. Thus, a strong influence of heredity on facial features is obvious at a glance, it is easy to recognise familiar tendency in relation of the maxilla as well as the shape of the jaw, and the nasal arch. Two of the most likely deformity types that run in families are prognathic mandible and long face pattern. Environmental influences during the growth and development of the face, jaws, and teeth largely affect genetically determined deformities. Identification of the environmental effects is almost impossible as usually they do not affect the developing body in an isolated way. According to the results of a large scale study published recently, there are only less than 10% of the occlusion disorders in which any origin can be identified.
HEREDITARY DENTOFACIAL DISORDERS
Manifestation of genetic effects can be identified easily through their family heredity. Genetic factors play a role in the development of malocclusions in two ways. The first one would be an inherited disproportion between the size of the teeth and the size of the jaws, which would produce crowding or spacing. The second one would be an inherited disproportion between the size or shape of the upper and lower jaws, which would cause improper occlusal relationships. It is hard to determine the contribution of the genetic factors to the development of individual malocclusions.
ACQUIRED INHERITED DENTOFACIAL DISORDERS
A part of the acquired malfunctions develops during the embryonic stage. Physical impacts on the embryo including birth traumas, as well as bone fractions and their consequences experienced in the developing stage, all result in skeletal disorders. Indeed, muscle dysfunctions can also serve as basis for developmental disorders.
Developmental disorders of dentition lead to congenital absence of teeth, abnormal or supernumerary teeth, and eruption disorders.
Embryonic developmental disorders
Adverse environmental influences can manifest as early as in the embryonic stage leading to developmental disorders of various severity. Defects in embryonic development usually result in death of the embryo. Only a relatively small number of recognisable conditions producing dentofacial problems are compatible with long-term survival. The time of the environmental effect delong-termines the type of the developmental disorder, which usually belongs to a certain syndrome. From the third week after fertilisation, maternal high alcohol consumption results in embryonic alcohol syndrome with characteristic midface deficiency. Effects on the embryo on the third and fourth weeks are manifested in disorders such as hemifacial microsomia or mandibulofacial dysostosis. Hemifacial microsomia is caused by defective migration of ganglion cells resulting in that lateral areas being farther from the midline of the face remain undeveloped. It is primarily a unilateral and always an asymmetrical disorder, in which the external ear is deformed typically, and both ramus of the mandible and associated soft tissues (muscle, fascia) are deficient or missing. The best evidence suggests that mandibulofacial dysostosis or Treacher Collins syndrome arises because of excessive cell death (cause unknown) in the trigeminal ganglia. People with this disorder have typically both the maxilla and mandible underdeveloped, the abnormality is symmetrical and affects both sides of the face. Both syndromes develop as a result of certain drugs taken in the early stage of the pregnancy.
Various facial clefts represent the majority of inherited developmental disorders affecting the maxilla and the mandible. Their appearance strongly correlates with impacts being present at the time of the fusion. Cleft lips originate in false fusion of the medial and lateral nasal prominences, which is due in the sixth week of gestation in normal cases. Cleft lip may develop only either side (unilateral cleft lip) or they can affect both sides (bilateral cleft lip). Since the fusion of these processes during primary palate formation creates not only the lip but the area of the alveolar ridge containing the incisors as well, it is likely that a notch in the alveolar process
PATHOLOGY
will accompany the cleft lip. Closure of the primary palate is followed by closure of the secondary palate by the elevation of the palatal shelves in nearly 2 weeks‘ time, which means that an interference with lip closure can also directly affect the palate. About 60% of individuals with a cleft lip also have a palatal cleft. An isolated cleft of the secondary palate is the result of a problem that arises after lip closure has been completed. It can be full or partial, and the exact place of emergence has also close correlation here with the time of the effect causing the defect. It is not rare that the defect occurs at the very last moment of the bilateral formulation, and it is manifested in the form of uvula bifida. The width of the mouth is determined by fusion of the maxillary and mandibular processes at their lateral extent. Failure of fusion in this area could produce an exceptionally wide mouth, or macrostomia, and in a more severe case, an obliquely directed cleft of the face. Responsibility of several external factors in which clefts of the lip and palate develop has been clarified, so it is well-accepted by now that maternal cigarette smoking is one of the aetiological factors in the development of cleft lip and palate in the form of fetal hypoxia.
Another major group of craniofacial malformations is the various ossification disorders, which are mainly represented by the individual synostoses. Synostosis syndromes typically develop during the last stage of fetal life, and their common feature is early closure of the sutures between the cranial and facial bones. Early closure of a suture leads to characteristic distortions depending on the location of the early fusion. Crouzon‘s syndrome is the most frequently occurring synostosis. It is characterised by underdevelopment of the midface and eyes, which seem to bulge from their socket. Crouzon‘s syndrome arises because of prenatal fusion of the superior and posterior sutures of the maxilla along the wall of the orbit. Premature fusion in the orbital area prevents the maxilla from translating downward and forward thus contributing to the severe underdevelopment of the middle face. The characteristic protrusion of the eyes is largely an illusion – the eyes appear to bulge outward because the area beneath them is underdeveloped; however, there may be a component of true extrusion of the eyes, because intracranial pressure increases when cranial sutures become synostosed.
The association between developmental disorders established as the result of teratogenic effects in fetal life and the time of the impact is described below:
The links between development disorders established as the result of teratogenic effects proceeding in fetal life and the time of the impact
Drugs, viruses, harmful chemicals, and radiation are identified as teratogenic agents being the cause of dentofacial developmental disorders. Here is an incomplete list of the most frequent ones:
PATHOLOGY
Teratogenic agents identified being cause of dentofacial developmental disorders Skeletal growth disturbances
Facial injuries apparent at the moment of birth are the result of a physical impact on the developing face during fetal life or it may be acquired during delivery. On rare occasions, the arm may be pressed into the face in the uterus causing severe growth disturbance of the maxilla. Sometimes the head of the fetus is flexed tightly against the chest in the uterus preventing the mandible from growing forward normally. The result is an extremely small mandible at birth, usually accompanied by a cleft palate. In this case, the restriction on displacement of the mandible forces the tongue upward and prevents normal closure of the palatal shelves. This extreme mandibular deficiency at birth is called Pierre Robin syndrome, which leads to respiratory difficulty due to the disproportion between the reduced volume of the oral cavity and the size of the tongue; it has an adverse effect on the whole neonatal development. Because the pressure against the face causing the growth disturbance is not present any more after birth, there is a possibility of normal growth thereafter. In many cases, however, the underdeveloped mandible requires long conservative therapy and frequently surgical treatment as well. In case of children with an underdeveloped mandible, auxiliary devices used in surgery were blamed for the adverse effects on temporomandibular joints during difficult births. It is, however, refuted by contemporary developmental theories, which declare that the condylar cartilage is not critical for the proper growth of the mandible. The disproof is confirmed by the prevalence of mandibular underdevelopment, since it has not decreased over the past decades in spite of the fact that the use of forceps in deliveries has actually been ceased in clinical practice.
Among childhood injuries, the fracture of condylar neck of the mandible can cause further growth disturbances.
Fortunately, the condylar process tends to regenerate well after an early fracture, and the appearance of consecutive mandibular underdevelopment is much less. Overall analysis results suggest that about 75% of the children had normal further growth. In the remaining cases, scarring developing during the therapy or the healing process may cause growth disturbance in the affected side frequently leading to severe asymmetry.
Muscle dysfunction
Facial muscles can affect jaw growth in two ways. First, skeletal development at the point of the muscle arisen or connected to bone largely depends on the activity of the muscle. Second, according to functional matrix theory, soft tissue matrix is decisive in the growth of the jaws, which mainly consists of facial muscles. Loss or underdevelopment of the individual facial muscles can occur in the uterus, but it is most likely to result from damage to the motor nerve. The result would be underdevelopment of that part of the face. Muscle weakness syndromes and muscular dystrophy have similar consequences, these pathologies, however, emerge symmetrically, and are also manifested in an increased growth of the total face length beside full open bite.
Otherwise, excessive muscle contraction can restrict growth in the same way as scarring after an injury. This effect is seen most clearly in torticollis, a twisting of the head caused by excessive tonic contraction of the neck muscles on one side. The result is severe facial asymmetry because of growth restriction on the affected side.
Disturbances of dental development
PATHOLOGY
Disturbances of dental development may accompany major congenital defects but they are mostly independent from other malocclusions.
Congenitally missing teeth
Congenital absence of teeth results from disturbances during the initial stages of tooth formation. The term hypodontia implies the absence of only a few teeth, the term oligodontia refers to congenital absence of many but not all teeth, while anodontia is describing the total absence of teeth. The lack of primary tooth buds results in the lack of the relevant permanent tooth buds, while the lack of the permanent tooth buds does not refer automatically to the lack of primary tooth buds. Anodontia and oligodontia are usually associated with a systemic abnormality, ectodermal dysplasia, which is also characterised by thin, sparse hair and an absence of the sweat glands. Whereas oligodontia may occur in a patient without congenital syndromes. Anodontia and oligodontia are rare, but hypodontia is relatively common. It is a general rule that individual missing tooth buds always belong to the most distal teeth of the given group. If a molar tooth is congenitally missing, it is almost always the third molar; if an incisor is missing, it is nearly always the lateral one; if a premolar is missing, it is almost always the second rather than the first one. Rarely is a canine the only missing tooth.
Malformed and supernumerary teeth
Abnormalities in tooth size and shape result from disturbances during the stage of morphodifferentiation. The most common abnormality is a variation in size, particularly of the maxillary lateral incisors and the second premolar. About 5% of the total population have a significant tooth size discrepancy because of disproportionate sizes of the upper and lower teeth. Occasionally, tooth buds may fuse or geminate during their development.
Fusion results in teeth with separate pulp chambers joined at the dentin, whereas gemination results in teeth with a common pulp chamber by bud fissure. Normal occlusion is all but impossible in the presence of geminated, fused or otherwise malformed teeth. Supernumerary or extra teeth always result from disturbances during the initiation and proliferation stages of dental development. The most common supernumerary tooth appears in the maxillary midline and is called a mesiodens. Supernumerary lateral incisors, premolars and molars develop very rarely. The presence of an extra tooth obviously has great potential to disrupt normal occlusal development.
Multiple supernumerary teeth are most often seen in the congenital syndrome of cleidocranial dysplasia, which is characterised by missing clavicles beside tooth eruption difficulties.
Interference with eruption
For a permanent tooth to erupt, the overlying bone as well as the primary tooth roots must resorb. Similarly, gingiva should also promote tooth eruption. Supernumerary teeth, sclerotic bone, and heavy fibrous gingiva can obstruct eruption. In patients with less severe interferences with eruption, delayed eruption of some permanent teeth contributes to malocclusion when other teeth drift to improper positions in the arch. Eruption of permanent teeth can be delayed by radicular resorption of the affected primary tooth, which can also be caused by the ankylosis (ossification) of the roots of the primary tooth. In such cases, delayed permanent tooth eruption usually does not result in malocclusion.
Ectopic eruption
Occasionally, malposition of a permanent tooth bud can lead to eruption in the wrong place. Ectopic eruption is most likely to occur in the eruption of maxillary first molars. If the eruption path of the maxillary first molar carries it too far mesially at an early stage, the permanent molar is unable to erupt, and the root of the second primary molar may be damaged and resorbed. The mesial position of the permanent molar results in the shortening of the arch, which will lead to lack of space at incisor level and consecutively to crowding.
Lack of space is frequently the reason for the altered eruption path and ectopic eruption. For example, the position of ectopic canines in the maxilla is to be blamed for the narrowed space due to adjacent teeth, which erupted earlier in most cases.
Early loss of primary teeth
Permanent molars are likely to drift mesially and forward during their eruption in the absence of occlusal contacts. Mesial drift of the permanent first molar after a primary second molar is lost prematurely, if this loss occurs more than one year before the eruption of the homologous second permanent molar, consecutively it leads to a shortened dental arch in most cases. Loss of primary first molars or canines provides favourable conditions for the distal drift of the homologous first permanent premolars and canines, which is significantly promoted by the pressure from the lips.
PATHOLOGY
Traumatic displacement of teeth
Dental trauma can lead to the development of malocclusion in three ways:
1. Damage to permanent tooth buds from an injury to primary teeth.
2. Drift of permanent teeth after premature loss of primary teeth.
3. Direct injury to permanent teeth.
Trauma to a primary tooth can displace the permanent tooth bud underlying it. If the trauma occurs while the crown of the permanent tooth is forming, enamel formation will be disturbed. If the trauma occurs after the crown is complete, the crown may be displaced relative to the root. Root formation may stop, leaving a permanently shortened or bent root. The usual cause of dilacerations is mechanical trauma to the primary incisors.
DENTOFACIAL DEFECTS ACQUIRED DURING GROWTH
Environmental impacts may also trigger or aggravate malocclusion in the postnatal developmental period. Some of these effects are associated with physiological functions, and others are the result of bad habits.
Physiological functions are, for example, feeding habits of the baby, holding of the head during sleep, type of the developed swallowing and respiration. Bad habits developed and maintained during life may contribute to the development of the individual deviations by upsetting muscle balance between lips and tongue. Such bad habits are, for example, chewing different objects; sucking and chewing of lips and cheek; bruxism, etc.
Corrective interventions based on a comprehensive and modern approach should correct malocclusion by the elimination of the causes. The stability of adulthood treatments confirms that causes of deviations are derived in the developmental period.