Fundamentals of dental occlusion.

Accurate assessment of occlusion is an important clinical skill for every practicing otolaryngologist. It is an essential part of the examination of patients with facial trauma or acquired or congenital malocclusion, as well as those seeking facial aesthetic surgery. For example, patients seeking mentoplasty should be assessed for micrognathia as well as microgenia. Mentoplasty performed on patients with unrecognized micrognathia can result in an unnatural postsurgical appearance marked by prominence of the labiomental sulcus. Patients with micrognathia can often be better served by orthognathic surgery. (1)

Occlusion is described in three dimensions: anteroposterior, vertical, and transverse. Occlusion may be normal in one of these dimensions and abnormal in another.

The terms used to describe tooth surfaces in the dental arches differ from the more familiar anatomic descriptors. The occlusal surface of the tooth is opposite the end where the root is located (apical end). The mesial side of the tooth laces the dental midline, and the distal surface is away from the dental midline (figure 1). Regionally, tooth surfaces are referred to by the structures they face. The anterior teeth, which include the incisors and canines, have labial outer surfaces. The premolars and molars have buccal outer surfaces. The occluding surfaces of the anterior and posterior dentition differ significantly. Cusps (convexities) and grooves (concavities) characterize the occlusal surfaces of the canines, premolars, and molars. Incisors lack these features: instead they have sharp incisal edges. (2)

[FIGURE 1 OMITTED]

The anteroposterior relationships of the posterior teeth are described by the Angle classification. This system defines the relationship of the molar teeth. (2) In class I occlusion, the mesiobuccal cusp of the maxillary first molar intercuspates with the buccal groove of the mandibular first molar (figure 2). Class II malocclusion occurs when the buccal groove of the mandibular first molar is posterior to the mesiobuccal cusp of the maxillary first molar. In class III malocclusion, the mandibular teeth are anterior or mesial to the maxillary teeth. (2,3) These molar relationships can affect the occlusion of the anterior teeth in predictable but not absolute ways. Normally, the maxillary incisors are anterior to the mandibular incisors. This is the normal overjet relationship, which ranges from 1 to 3 mm.(2) Abnormal anterior relationships include an excess overjet, edge-to-edge contact, and an anterior crossbite in which the maxillary incisors are posterior to the mandibular incisors (figure 3). Usually, patients with a class I occlusion also have a normal overjet while those with a class II malocclusion have an excessive overjet. There are two subdivisions of the class II relationship that apply to the positions of the incisor teeth. In class II, division I, there is an increased overjet. In division II, the central incisors are palatally tilted to compensate for the skeletal abnormality, resulting in a normal or near-normal anterior relationship. The class III malocclusion is typically associated with an anterior crossbite.

[FIGURE 2 OMITTED]

In the vertical dimension, two clinical situations may be encountered:

* An open-bite deformity is seen when the occlusal or incisal surfaces of the teeth do not meet in the same plane during jaw movement. If there is unilateral premature contact between the posterior teeth, a posterior open bite will occur contralaterally. This is commonly seen in unilateral subcondylar fractures.

* An overbite or deep bite describes an increased vertical overlap of the incisor teeth. Normally, the maxillary incisors overlap the mandibular incisors by 1 to 3 mm. (2) When this distance increases, the maxillary incisor teeth obscure the mandibular incisors. In the transverse dimension, the molar teeth intercuspate in such a way that the buccal cusps of the maxillary teeth are lateral or buccal to the buccal cusps of their mandibular counterparts. This normal situation is altered when there is a posterior crossbite. (2)

Malocclusion can have a profound effect on both function and appearance. It contributes to several conditions for which patients seek evaluation, including sleep apnea, congenital craniofacial anomalies, and aesthetic concerns. The documentation of occlusion is an essential part of the diagnosis of these dental-skeletal anomalies.

References

(1.) Zide BM, Pfeifer TM, Longaker MT. Chin surgery: I. Augmentation--The allures and the alerts. Plast Reconstr Surg 1999;104: 1843-53.

(2.) Brand RW, Isselhard DE. Anatomy of Orofacial Structures. 4th ed. St. Louis: Mosby; 1990:213-80.

(3.) Tatum SA. Correction of post-traumatic maxillofacial deformities involving occlusion. Facial Plast Surg Clin North Am 1998;6: 535-56.

Sydney C. Butts, MD; Sherard A. Tatum III, MD

From the Division of Facial Plastic Surgery. Department of Otolaryngology and Communication Disorders. SUNY Upstate College of Medicine. Syracuse. N.Y.