Craniofacial Growth And Development Biology Essay
The definitions of normal occlusion and malocclusion are introduced as reference for further discussion.
The term 'normal occlusion' is arbitrary but is generally accepted to be a dentition in Class I molar relationship with good alignment of all teeth and represents a situation that occurs in ~ 30-40% of the population (Mossey 1999).
Malocclusion is a difficult term to define. Many authors have provided definitions, for example, "malocclusion is a morphological, and a physiologic, deviation from accepted dentofacial norms for the human species." (Salzman 1974). Daskalogiannakis et al. (2000) defined malocclusion as a significant deviation from normal or 'ideal' occlusion.
The precise definitions of normal occlusion and malocclusion are difficult, but generally in a malocclusion, teeth are misplaced (Proffit 1986). In 1907, Angle proposed a classification of malocclusion, based on the relationship of the mandibular first permanent molars to the maxillary first permanent molars.
Table 1. Angle's classification of malocclusion.
Normal relationship of the molars, but the line of occlusion is deviated due to malposed teeth, rotations, or other causes.
Lower molar distally positioned relative to upper molar, line of occlusion not specified.
Lower molar mesially positioned relative to upper molar, line of occlusion not specified.
In the interest of this review, Class II malocclusion would be focussed for further discussion in relation to the establishment of its management strategies with functional appliances.
Class II malocclusions constitute a major proportion of cases that are treated in the orthodontic clinic. Many studies have attempted to quantify the incidence of malocclusions.
// Talk about oversimplification of Angle's classification.
Angle's classification of Class II malocclsion is characterized by the mandibular teeth in a distal relationship to the maxillary teeth of greater than half a cusp displacement. In fact, he recognized two major categories of Class II malocclusion and subsequently distinguished them by the inclination of the upper incisors. In division 1, the upper incisors are proclined while in division 2, they are retroclined.
Angle's classification primarily involved only the sagittal relationship of the mandibular teeth to the maxillary teeth. The simplicity of Angle's classification has been popular because of its ease of use in communicating a broad overview of the malocclusion. However, several studies have shown that the clinical presentations of Class II malocclusions are diverse and incorporate variations that would ultimately influence treatment strategies.
The validity of using the relationship of the first molars as the main criterion for classifying malocclusions has been questioned, since each Class of malocclusion incorporates many variations that in turn significantly influ- ence the treatment plan. Despite these obvious limitations, Angle's classification is still widely used because of its simplicity as a method of description and communication between dental professionals. Angle7 characterized two types of Class II malocclusions based on the inclination of the maxillary central incisors.
// Talk about the diverse clinical presentation of Class II's and appropriate treatment for each sub-type.
Some authors believed that a variety of skeletal types exist in the Class II population. To investigate the possibility of different types of Class II malocclusion, Moyers et al. (Moyers, Riolo et al. 1980) studied a sample of 697 North American white children using both casts and lateral cephalograms over a period of about 10 years. Cephalometric analyses identified six subgroups based on horizontal variables and five subgroups based on vertical variables.
The six subgroups were labelled types A, B, C, D, E and F as illustrated by Figure <todo>.
Figure <todo>. Representations of the six facial types in Class II based on skeletal and dental relationships to the cranium. The maxilla and mandible are depicted by the large rectangles while the first molars are shown by the smaller squares. The incisors are drawn as short vertical lines when normal or angled when in labioversion. The vertical dotted line represents the ideal profile in orthognathism.
Table <todo>. Class II skeletal and dental characteristics of the six facial types based on horizontal variables.
Skeletal and dental characteristics
Normal skeletal profile.
Normal anteroposterior position of the maxilla and mandible.
Normal occlusal plane.
Mandibular dentition is in normal position on its base but the maxillary dentition is protracted, resulting in a Class II molar relationship with an increased incisal overjet and overbite.
Maxillary prognathism associated with a mandible of normal size, leading to an increased ANB0.
Class II molar relationship with an increased overjet.
Anterior cranial fossa tended to be flat.
Facial dimensions appeared smaller than other Class II types.
Marked Class II profile, as both the maxilla and mandible are retrusive with respect to the anterior cranial base.
Class II molar relationship
Mandibular incisors compensate for the large overjet by tipping labially while upper incisors are either upright or tipped labially,
A small mandible together with a normal or slightly diminished midface.
Retrognathic skeletal profile.
Mandibular incisors are either upright or retroclined while maxillary incisors are very proclined.
Severe Class II profile due to a prognathic maxilla and a normal or even prognathic mandible.
Bimaxillary protrusion of Class II malocclusions are more likely found in this type than any other.
Both the maxillary and mandibular dentitions tended to be protracted on their respective bases and the incisors are often proclined.
Heterogeneous subgroup with the mildest Class II characteristics.
Skeletal profile is less severe than types B, C, D and E.
Mandible is small.
Maxilla may be normal or small.
Most frequent horizontal type.
Difficult to categorize but may be thought of as a milder version of type B, C, D or E.
Figure <todo>. Representations of the five Class II vertical types. The solid lines represent the normal position of the cranial base, palatal, occlusal and mandibular planes. The broken lines indicate the average position for the planes of each of the vertical types.
Table <todo>. Class II skeletal and dental characteristics of the six facial types based on vertical variables.
Skeletal and dental characteristics
Steeper than normal mandibular plane.
Even steeper functional occlusal plane.
Palate is slightly tipped downward anteriorly.
Anterior cranial base tends to be upward.
Anterior face height is significantly greater than posterior face height due to disproportionate growth.
Profile is described by some oral surgeons as the "long face syndrome".
Frequently found in horizontal types C, D and F and to a less extent in some type A's.
Dentoalveolar adaptations depend on both the severity of vertical as well as horizontal dimensions.
The mandibular plane, functional occlusal plane and palatal plane are all flatter than average and tend to be 'parallel' to one another.
Typically 'square' face.
Gonial angle approaches orthogonality.
Cranial base is more horizontal than normal.
Found in all horizontal types but mainly in horizontal types B and E, and as a milder form in F types.
Incisors tend to be upright and in deep-bite.
Most characteristic feature is the palatal plane which is tipped upward anteriorly.
Inadequate growth of upper anterior face.
Steep mandibular plane.
Strong tendency to anterior open-bite.
The mandibular plane, functional occlusal plane, and palatal plane are all tipped downward anteriorly.
Lip line is significantly high on the maxillary alveolar process.
The gonial angle is obtuse.
Virtually all vertical type 4 cases are also found in the horizontal type B cases in which the upper incisors are more proclined than normal. All other horizontal type B cases have upper incisors which are upright.
Lower incisors are retroclined.
Rare and severe form.
The mandibular and functional occlusal planes are normal.
The palatal plane is tipped downward anteriorly.
The gonial angle is the smallest of all the types - "squarish" profile.
Found predominantly in association with horizontal types B and E.
Lower incisors are very proclined.
Upper incisors are nearly upright.
Tendency for bimaxillary protrusions.
The results of Moyers and coworkers (1980) found a variety of Class II malocclusions. Horizontal type A displayed few features of the typical Class II skeletal features but was so classified by some orthodontists because of the Class II dental relationship with increased overjet and overbite due to a protracted maxillary arch. Types B, C, D and E were more clearly differentiated as well as being more severe. Type F is a large group that consisted of mild Class II cases with variable symptoms. In addition, a range of vertical anomalies was found to occur in association with the horizontal types. While communicating Class II malocclusion by Angle's classification has the benefit of ease in communication between clinicians, the findings of Moyers and coworkers illustrated a very important fact that not all Class II malocclusions are the same. This has implications in diagnosis and treatment planning in such a variable and diverse group, i.e. not all Class II malocclusions should be treated the same way.
In recognition of the diversity of Class II malocclusions and the range of treatment strategies available, McNamara (1981) evaluated the lateral cephalometric radiographs of 277 children to determine the relative frequency of key components that would classify a malocclusion as 'Class II'. SNA and SNB angles were used as the criteria of anteroposterior relationship of the maxilla and mandible. It was observed that on average, the maxilla was in a neutral position, otherwise it was more often more retrusive (SNA < 790, 31%) than protrusive (SNA > 840, 14%). The mandibular skeletal position was analysed using SNB and was its retrusive position was found to be the most common single characteristic of the Class II sample (SNB < 790, ~ 60%). However, in a later study which examined the pattern of maxillary protrusion and mandibular retrusion, Rosenblum (1995) found contrasting results to McNamara's investigation. A total of 103 patients from two private practices were analysed at three levels of skeletal maturation. It was observed that the dominant pattern was maxillary protrusion with a normal mandible. Despite variability in the results of studies in the past, the common conclusion was the diagnosis of Class II malocclusion could be difficult due to its diverse presentations, the significance of which implies careful individual analysis before finalizing a treatment plan.
// Talk about importance of recognizing Class II sub-types and dignosis leading to treatment
Hereditary traits in Class II malocclusion
The significance of hereditary traits can be studied via experiments on animals, family investigations and twin investigations.
Animal studies allow the researcher to gain some indications on the inheritance of malocclusion. Crossbreeding experiments with dogs were carried out by Stockard (1941) to induce facial deformities and malocclusions. It was suggested that individual characteristics of the craniofacial complex, including malocclusion, could be inherited independently as dominant traits according to Mendelian principles. Based on his results, Stockard proposed that the increase in prevalence of malocclusion in the modern urban population was most likely due to increased outbreeding. Much criticism had been received because the gene for achondoplasia in the experiment dogs may have been responsible for the inheritance of associated malocclusion. Achondoplasia in animals or humans results in deficient growth of cartilage which manifest as an underdeveloped midface. The malocclusions observed in Stockard's experiments did not actually explain the genetic basis of inherited jaw size discrepancy but rather the extent to which the achondoplasia was expressed. These results could not be reliably extrapolated to explain the relation of cross-breeding and malocclusion in humans.
Evidence of hereditary traits from human family studies and twin studies is considered to be more credible. In his twin study, Lundström (Lundström 1948) reported a 68% concordance of having a Class II malocclusion in monozygotic twins in comparison to 24% concordance in dizygotic twins. Such findings illustrate that given identical genotype, the presence of Class II malocclusion is not always expressed. Extensive cephalometric investigations of the heritability of craniofacial characteristics in Class II division 1 malocclusions have shown that the mandible is more retrognathic when compared to Class I cases (Harris 1963; Harris 1975). A higher correlation was noted between Class II division 1 individuals and their immediate family than from random pairings of unrelated siblings, which implied the theory of polygenic inheritance for Class II division 1 malocclusions. Markovic (1992) investigated the heritability of Class II division 2 malocclusions in twins, triplets and in family pedigrees. From his cephalometric analysis of 114 Class II division 2 malocclusion cases, Markovic performed intra- and inter-pair comparisons on 48 twin pairs and six triplet sets, and found 100% concordance for the malocclusion in monozygotic twin pairs whereas almost 90% discordance in dizygotic twin pairs. Evidence from these studies suggested that genetics is one of the aetiological factors in the development of Class II malocclusions.
Growth and Treatment
General Concepts of Craniofacial Growth and Development
Class II Growth Pattern
The cranial base is located at the junction between the cranium, midface and glenoid fossa(Björk 1955; Ford 1958). It is likely that the growth and development of the cranial base have the potential to influence the growth and development of both cranium and face, in particular, the positional relationship between the maxilla and mandible.
Reports show that angular and linear changes occur between the anterior and posterior cranial base. Lewis and Roche6 reported that, in general, the cranial base ngle in Class I individuals becomes more acute during infancy and then changes more slowly until a few years past puberty. They attribute this change to differential remodeling of the cranial base rather than to flexion at sella. As part of this remodeling, there is resorption on the endocranial surface of the clivus. This (together with downward and forward rotation of the basiocciput and the absence of marked changes in the level of the pituitary fossa or of nasion during growth) would lead to a decrease in the cranial base angle. They also noted that mean cranial base angles tended to be larger in Class II male patients than in Class I male patients.
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