Occlusion (dentistry)

Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the (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.

Incisor and Molar Classification

In order to describe the relationship of the maxillary molars to the mandibular molars, the Angle’s classification of malocclusion has commonly been used for many years. This system has also been adapted in an attempt to classify the relationship between the incisors of the two arches.

Incisor Relationship

When describing the relationship between maxillary and mandibular incisors, the following categories may be referred to:

• Class I: Mandibular incisors contact the maxillary incisors in the middle third or on the cingulum of the palatal surface

• Class II: Mandibular incisors contact the maxillary incisors on the palatal surface, in the gingival third or posterior to the cingulum. This class may be further subdivided into division I and division II:
  • Division I  includes maxillary incisors which are proclined (90%) and these individuals have a greater overjet (horizontal overlap)
  • Division II includes those with retroclined (10%) incisors, which leads to an increase in overbite (vertical overlap)

• Class III: Mandibular incisors occlude with the maxillary incisors on the palatal surface, in the incisal third specifically or anterior to the cingulum
  • In some cases the overjet is reversed (<0mm) and the mandibular incisors lie anterior to the maxillary incisors

Molar Relationship

When discussing the occlusion of the posterior teeth, the classification refers to the first molars and may be divided into three categories:

• Class I: The mandibular first molar occludes mesially to the maxillary first molar, with the  mesiobuccal cusp of maxillary first molar occluding in the buccal groove of mandibular first molar

• Class II: The mesiobuccal cusp of the maxillary first molar occludes anterior to the buccal groove of the mandibular first molar

• Class III: If the mesiobuccal cusp of the maxillary first molar occludes posterior to the buccal groove of the mandibular first molar

Classification of occlusion and malocclusion play an important role in diagnosis and treatment planning. Class I relationships are thought to be “ideal”, however this classification does not take into consideration the positions of the TMJ’s . Class II and III molar and incisor relationships are thought to be forms of malocclusion, however not all of these are severe enough to require orthodontic treatment.

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.

Guidance, Natural Teeth and Function

Mandibular movements are guided by two different systems; the ‘posterior’ determinants and the ‘anterior’ determinants.

1. Posterior Guidance

Posterior guidance refers to TMJ articulations and associated structures (ligaments, disc and musculature) determining mandibular movements.

Lateral Excursion

• The maximum lateral movement of the mandible is approximately 10-12mm

• The primary movement in lateral excursions occurs on the non-working side (NWS) condyle (also called the balancing or orbiting condyle). The NWS condylar head moves in a downward, forward and medial direction. This movement is defined against two separate planes:
  1. Bennet angle - the angle of medial movement to the vertical plane
  2. Condylar angle - the angle of downwards movement to the horizontal plane

• The working side (WS) condyle (also called the rotating condyle) undergoes an immediate, non-progressive lateral shift.  This movement is called the Bennet movement or an immediate side shift.  The condyle is seen to rotate with a slight lateral shift in the direction of movement

Protrusive Movements

• The condylar heads predominantly translate forwards and downwards along the distal face of the articular face in the glenoid fossa. Protrusive movements are restricted by the ligamentous structures to a maximum of ~8-11mm (depending on skull morphology and size of subject)

Retrusive Movements

• The maximum restrusive limit is usually ~1mm however 2-3mm is rarely seen in some patients. As for protrusion, this movement is restricted by the ligamentous structures.

2. Anterior Guidance

Anterior guidance refers to the influence of contacting teeth limiting mandibular movements, whether that may be anterior or posterior tooth contacts, these determinants are anterior to the TMJ. This can be further classified into:

Canine Guidance

• Dynamic occlusion that occurs on the canines during lateral excursions of the mandible.  
• These teeth are best suited to accept horizontal forces in eccentric movements due to their long roots and good crown/root ratio.

Group Function

• Multiple contact relations between the maxillary and mandibular teeth in lateral movements on the WS whereby simultaneous contact of several teeth acts as a group to distribute occlusal forces.
• It is preferable for this guidance to be as anterior as possible e.g. premolars rather than molars, as there is increased force applied when the contacts are closer to the TMJ.

Incisal Guidance

• The influence of the contacting surfaces of the mandibular and maxillary anterior teeth on mandibular movements and is characterised by the overbite and overjet of the maxillary incisors.

Mutually Protected Occlusion

The Journal of Prosthetic Dentistry (2017) defines mutually protected occlusion as ‘an occlusal scheme in which the posterior teeth prevent excessive contact of the anterior teeth in maximal intercuspal position, and the anterior teeth disengage the posterior teeth in all mandibular excursive movements’

In eccentric movements, damaging forces are applied to the teeth and the anteriors are best suited to receiving these. Therefore during protrusive movements, the contact or guidance of the anteriors should be adequate to disocclude and protect the posterior teeth.

In contrast, the posterior teeth are more suited to accept forces that are applied during closure of the mandible.  This is because the posteriors are positioned so the forces are applied directly along the long axis of the tooth and are able to dissipate them efficiently whereas the anteriors cannot accept these heavy forces as well due to their labial positioning.  It is therefore accepted that the posterior teeth should have heavier contacts than the anteriors in ICP and act as a stop for vertical closure.

Additionally, in lateral excursions either canine or group function should act to disocclude the posterior teeth on the WS because, as described above, the anterior teeth are best suited to dissipate damaging horizontal forces, as well as the contact being further away from the TMJ, so the forces created are decreased in strength. Group or canine guidance should also provide disocclusion of the NWS contacts as the amount and direction of force applied to the TMJ and teeth can be destructive due to an increase in muscle activity. An absence of NWS contacts also allows smooth movement of the working side condyle as a contact may disengage the guidance of the condyle and therefore cause an unstable mandibular relationship.

Clinical Relevance of Guidance

Tooth contact involved in guidance is particularly important as these occlude a vast number of times per day and so need to be able to resist both heavy and non-axial occlusal loads. When restoring the occlusal surfaces of teeth, it is likely to change the occlusion and therefore guidance systems. It is unlikely the TMJ will adapt to these changes in occlusion, but rather the teeth adapt to the new occlusion through tooth wear, tooth movement or fracture.  For this reason, it is important to consider these guidance concepts when providing restorations. Guidance should also be considered before restorations as it should not be expected for a heavily restored tooth to provide guidance alone as this leaves the tooth vulnerable to fracture during function.

Deflective Contacts and Interferences

A deflective contact is a contact that displaces a tooth, diverts the mandible from its intended movement. An example of this is when the mandible is deflected into ICP by the RCP-ICP slide, which path is determined by the deflective tooth contacts.  This is often involved in function (e.g. chewing), however in some cases these deflective contacts can be damaging and may lead to pain around the tooth (often associated with bruxism).  However, some patients may be totally unaware of similar deflective contacts suggesting that it is the patient's adaptability rather than the contact that may influence the patient's presentation.

An occlusal interference is any tooth contact that interferes with, or hinders harmonious mandibular movement (an undesirable tooth contact).

The occlusal interferences may be classified as follows:

1. Centric Interference : When the mandible closes and the condyle is in the optimum position in the fossae, a premature contact causes deflection of the mandible in an anterior, posterior and/or lateral direction
2. Working Side Interference : When there is a heavy or early tooth between the maxillary and mandibular teeth on the side that the mandible is moving towards, and this contact disoccludes the anteriors.
3. Non-Working Side Interference : An occlusal contact on the side the mandible is moving away from that prevents harmonious movement of the mandible. These have the potential to be more destructive in comparison to WS interferences due to the obliquely directed forces.
4. Protrusive Interference : Premature contacts that occur between the distal aspects of the maxillary posterior teeth and the mesial aspect of the mandibular posterior teeth.  These interferences are potentially very damaging and may even cause an inability to incise properly due to the close proximity of the interference to the muscle.

See also

• 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 trauma – problems that arise from untreated damaging occlusions