Occlusion (dentistry)

Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the maxillary (upper) and mandibular (lower) teeth when they approach each other, as occurs during chewing or at rest.

Static occlusion refers to contact between teeth when the jaw is closed and stationary, while dynamic occlusion refers to occlusal contacts made when the jaw is moving.

The masticatory system also involves the periodontium, the TMJ (and other skeletal components) and the neuromusculature, therefore the tooth contacts should not be looked at in isolation.

Development of Occlusion

As the primary (baby) teeth begin to erupt at 6 months of age, the maxillary and mandibular teeth aim to occlude with one another. The erupting teeth are moulded into position by the tongue, the checks and lips during development. Upper and lower primary teeth should be correctly occluding and aligned after 2 years whilst they are continuing to develop, with full root development complete at 3 years of age.

Around a year after development of the teeth is complete, the jaws continue to grow which results in spacing between some of the teeth (diastema).  This effect is greatest in the anterior (front) teeth and can be seen from around age 4 - 5 years. This spacing is important as it allows space the permanent (adult) teeth to erupt into the correct occlusion, and without this spacing there is likely to be crowding of the permanent dentition.

In order to fully understand the development of occlusion and malocclusion, it is important to understand the premolar dynamics in the mixed dentition stage (when both primary and permanent teeth are present). The permanent premolars erupt ~9-12 years of age, replacing the primary molars.  The erupting premolars are smaller than the teeth they are replacing and this difference in space between the primary molars and their successors (1.5mm for maxillary, 2.5mm for mandibular). is termed Leeway Space. This allows the permanent molars to drift mesially into the spaces and develop a Class I occlusion.

Occlusal Terminology

Intercuspal Position (ICP), also known as Habitual Bite, Habitual Position or Bite of Convenience , is defined at the position where the maxillary and mandibular teeth fit together in maximum intercuspation. This position is usually the most easily recorded and is almost always is the occlusion the patient closes into when they are asked to 'bite together'. This is the occlusion that the patient is accustomed to, hence sometimes termed the Habitual Bite.

Centric relation (CR) describes a reproducible jaw relationship (between the mandible and maxilla) and is independent of tooth contact. This is the position in which the mandibular condyles are located in the fossae in an antero-superior position in against the posterior slope of the articular eminence. In CR, the muscles are in their most relaxed and least stressed state.

When the mandible is in this retruded position, it opens and closes on an arc of curvature around an imaginary axis drawn through the centre of the head of both condyles. This imaginary axis is termed the terminal hinge axis. The first tooth contact that occurs when the mandible closes in the terminal hinge axis position, this is termed Retruded Contact Position (RCP).  RCP can be reproduced within 0.08mm of accuracy due to the non-elastic TMJ capsule and restriction by the capsular ligaments, thus it can be considered a ‘border movement’ in Posselt’s envelope.

Centric Occlusion (CO) is a confusing term, and is often incorrectly used synonymously with RCP. Both terms are used to define a position where the condyles are in CR, however RCP describes the initial tooth contact on closure (this may be an interference contact), whereas CO refers to the occlusion where the teeth are in maximum intercuspation in CR. Posselt (1952) determined that only in 10% of natural tooth and jaw relationships does ICP = CO (maximum intercuspation in CR) and so the term RCP is more appropriate when discussing the occlusion that occurs when the condyles are in their retruded position. CO is a term that is more relevant to complete denture application, where the occlusion of denture teeth is arranged so that when the mandible is in CR, the teeth are in ICP.

Posselts Envelope of Border Movements

Posselt’s Envelope of Border Movement is a schematic diagram of the maximum jaw movement in three planes (sagittal, horizontal and frontal).  This encompasses all movements away from ICP, and includes:

• Protrusive movements: When the mandible moves forward from ICP, this is considered as protrusion. The predominant contacts occur on the incisal and labial surfaces of the mandibular incisors and the incisal edges and lingual fossa areas of the maxillary incisors.
• Lateral movements: When the mandible moves to the left or right, the mandibular posterior teeth move laterally across their opposing teeth in different directions.  For example, when the mandible moves to the right, the right mandibular posteriors move laterally across their opposing teeth and this is termed the working side (the side to which the mandible is moving).  In contrast, the left mandibular posteriors move medially across their opposing posteriors and this is called the non-working side (the side to which the mandible is moving away from).
• Retrusive movements: This is when the mandible moves posteriorly from ICP. Compared with protrusive and lateral movements, retrusive movements are generally considerably smaller with a range of movement around 1 or 2 mm due to restriction by the ligamentous structures.

See also

• Dahl effect
• Malocclusion – "bad bite"
• Maximum intercuspation, formerly known as centric occlusion – the bite in which all the teeth are closed together in their natural and physiologic position
• Mutually protected occlusion – the way front and back teeth protect each other
• Occlusal splint – used to treat malocclusions and bruxism
• Occlusal trauma – problems that arise from untreated damaging occlusions
• Overeruption
• Vertical dimension of occlusion – a type of jaw measurement