Classification of Jaw Deformities

In the United States, the most widely used classification system for jaw deformities is that provided by the Center for Medicare and Medicaid Services and the National Center for Health Statistics. This classification is part of the International Classification of Diseases, Clinical Modification (ICD-CM), a taxonomy scheme based on the World Health Organization’s International Classification of Diseases (ICD), which is the traditional standard diagnostic tool for epidemiology, health management, and clinical care.²

The latest iteration of the ICD-CM, version 10,³ sorts jaw deformities geometrically into only 3 groups: anomalies of jaw size, anomalies of jaw-cranial base relationship, or unspecified (Table 1)⁴. However, these deformities can affect 6 different geometric attributes: size, position, orientation, shape, symmetry, and completeness.

In both clinical practice and teaching, we have found the ICD-CM classification system incomplete and disjointed. Because of these deficiencies, we have developed what we consider to be a better alternative. It is introduced in the following section.

Geometric Classification of Jaw Deformities

Our classification scheme is presented as a mind-map in Figure 2.⁴ The scheme first classifies jaw deformities as either osseous or dental. Osseous deformities affect the jawbones; dental deformities affect the teeth.

The classification recognizes the jawbones as having 6 geometric attributes: size, position, orientation, shape, symmetry, and completeness. Jaw deformities are classified according to the attribute they affect.

Deformities of size occur when a jaw is either too large or too small. The term hyperplasia indicates pathological enlargement, whereas hypoplasia signifies the failure to attain normal size. Micrognathia is a synonym for mandibular hypoplasia, while macrognathia corresponds to mandibular hyperplasia. The terms macrogenia and microgenia also refer to size, with macrogenia indicating a large and microgenia a small chin.

Abnormal jaw positions occur in all cardinal directions. Prognathism and retrognathism are deformities characterized by abnormal anteroposterior position. By convention, anteroposterior position is assessed in relation to the cranial base. Prognathism occurs when a jaw is too far forward, and retrognathism when it is too far backward. In the transverse direction a jaw can be displaced, in either direction, away from the median plane, a deformity called laterognathia. Vertically, a jaw can be too far down—excessive downward displacement—or too far up—deficient downward displacement.

When a jaw is abnormally oriented, malrotations occur. These malrotations are classified according to the axis on which the abnormal rotation occurs. When a jaw is malrotated around the transverse facial axis, it is said to have abnormal pitch. When malrotated around the anteroposterior axis, the jaw has an abnormal roll, a condition also known as cant.  Finally, when a jaw is malrotated around the vertical axis, it has abnormal yaw.    

Shape refers to figure, the geometric characteristic of an object that is not size, position, or orientation ⁵.  A jaw with abnormal shape is said be distorted.

The human face has reflection symmetry around one plane, the median. For facial symmetry to exist, two conditions must be met.⁶ First, each of the units comprising the face must be symmetrical, a condition called object symmetry. Second, each of the units must be symmetrically aligned to the median plane, a condition called symmetric alignment. Jaws can have deformities of symmetry, either because of object asymmetry or because of misalignment. The terms mandibular asymmetry and maxillary asymmetry refer to abnormalities in object symmetry; whereas, the term asymmetric alignment is used to denote abnormal alignment causing asymmetry.

Completeness refers the wholeness of the jaw. A jaw can be incomplete because one of its processes did not fully develop; for example, agenesis of the condylar process of the mandible, which may be seen in hemifacial microsomia. Completeness can also fail to occur because some of the jaw’s embryologic processes failed to fuse (e.g., cleft), or because of an acquired defect.

The different types of jaw deformities (size, position, orientation, shape, symmetry, and completeness) are frequently correlated.^{6, 7} For example, asymmetric alignment cannot occur in the absence of at least one other deformity (i.e., laterognathia, abnormal roll, or abnormal yaw).

As stated above, jaw deformities can also affect the teeth. Like the ICD-CM³, our classification scheme only considers dental deformities that engender malocclusion. Malocclusion can result from the disarrangement of one or more teeth in their dental arch or because the upper and lower dental arches are not coordinated (Figure 2).

Within a dental arch, deformity may affect the alignment, leveling, or spacing of teeth. Alignment refers to the arrangement of teeth in an arch. In ideal alignment, the incisal edges of the incisors and the buccal-cuspal ridges of the canines, premolars, and molars form an arch. Misalignment can occur because of dental displacement, dental tipping, or dental rotations.  In displacement, a tooth is bodily moved outside the arch. In tipping, a tooth is abnormally inclined. In rotations, a tooth is misaligned because of abnormal rotation around its long axis.

Leveling refers to the vertical arrangement of teeth. Abnormal leveling can affect a single tooth or the whole arch. For this assessment, one measures the vertical position of the teeth in relation to their occlusal plane. In other words, one measures the vertical positions of the lower teeth in relation to the mandibular occlusal plane and the vertical positions of the upper teeth in relation to the maxillary occlusal plane.

An individual tooth is in infraocclusion or supraocclusion when it is located below or above its ascribed occlusal plane.  For the entire dental arch, one judges dental leveling by gauging the curve of Spee. From the central incisor backward to the last molar, the cusps of all teeth should inscribe either a flat plane or a curved plane of slightly upward concavity.  Dental deformity can create a deep or a reverse curve of Spee. A curve of Spee is deep when the cusps of the teeth trace a plane with sharp upward curvature. The curve is reversed when the curvature of the plane has downward concavity.

Within a dental arch, the teeth should be normally spaced; that is, adjacent teeth should touch without crowding one another.  Spacing is abnormal when diastemas are present or when the arch cannot accommodate the teeth. The first condition is excessive dental spacing; the second is dental crowding.

In addition, dental deformities can occur when the upper and lower arches are not harmonized. For normal occlusion to occur, it is insufficient for the upper and lower teeth to be normally arranged in an arch. The upper and lower dental arches must also be coordinated: in position, shape, and tooth size.

Discordant dental arch positions cause malocclusion.  This lack of concordance can occur among all cardinal planes:  anteroposterior, vertical, and transverse.

We appraise anteroposterior occlusal relationships at three different sites. They are: first molar, canine, and central incisors. In this appraisal, the frame of reference is the upper dentition; that is, the examiner judges the anteroposterior position of the lower teeth in relation to hypothetical static upper teeth.

Angle’s molar relationship assesses the position of the buccal groove of the lower first molar in relation to the mesiobuccal cusp of the upper.⁸ In an ideal Class I molar relationship, these landmarks coincide. In a Class II relationship, the lower molar groove is behind the upper cusp; with a Class III it is in front.  A similar assessment is done in the canine region. In a Class I canine relationship, the lower-canine-first-premolar embrasure coincides with the cusp of the upper canine. In a Class II, the embrasure is behind the upper canine cusp; with a Class III, it is in front. Finally, in the incisal region, we measure the overjet. Overjet is the horizontal distance between the incisal edges of the upper and lower central incisors. When the lower incisal edge coincides with the upper, the overjet is zero. When it is behind, the measurement has a positive value; in front, it is negative. The ideal overjet is +2mm. 

Based on these assessments, one classifies the occlusion into neutrocclusion, distocclusion, or mesiocclusion. In neutrocclusion, the molar and canine relationships are Class I and the overjet is normal. In distocclusion, the molar and canine relationships are Class II and the overjet is either greater than normal (Division 1) or normal (Division 2). In mesiocclusion, the molar and canine relationships are Class III and the overjet is smaller than normal, usually negative.

Position discordance between the upper and lower dental arches also occurs in the vertical direction.  Absence of vertical overlap between the upper and lower teeth produces an open bite. It can be can anterior or posterior. 

Excessive vertical-overlap of the anterior teeth results in deep bite. Excessive vertical-overlap of the posterior teeth results in posterior bite collapse. The latter condition can only occur when many posterior teeth are missing and the remaining teeth have no opposing occlusion or when there is complete crossbite of the posterior teeth.

Finally, discordance between the maxillary and mandibular dental arches can also occur in the transverse dimension. Normally, the buccal cusps of the maxillary posterior teeth are lateral to the buccal cusps of the mandibular teeth. When the reverse occurs, we encounter a posterior crossbite.   In extreme cases, all the lower teeth can be inside the upper, a condition known as Brodie bite. Conversely, the upper teeth can be inside the lower, a condition known as scissor bite.

As previously mentioned, the upper and lower arches can occlude abnormally because they have different shapes. For example, a “U” shaped lower arch does not fit a “V” shaped upper.  The lack of shape congruency between the upper and lower teeth results in arch shape-discordance.

Ultimately, to obtain good dental interdigitation in Class I occlusion, the width (mesio-distal size) of the lower teeth must be proportional to the width of the upper.⁹ When this proportionality is absent, the dental arches have a tooth size discrepancy.