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.
Teeth consist of two parts: the crown, which is visible in the mouth and lies above the gingival soft tissue and the roots, which are below the level of the gingiva and in the alveolar bone.
This bundle of connective tissue fibres is vital in dissipating forces that are applied to the underlying bone during the contact of teeth in function.
This forms the palate of the oral cavity and also supports the alveolar ridges that hold the upper teeth in place.
The TMJ is formed from the temporal bone of the cranium, specifically the glenoid fossa and articular tubercle and the condyle of the mandible, with a fibrocartilaginous disc lying in between.
The temporalis, masseter, medial and lateral pterygoids are the muscles of mastication and these contribute to the elevation, depression, protrusion and retraction of the mandible.
The anterior and posterior belly of the digastric are also involved in the depression of the mandible and elevation of the hyoid bone and are therefore relevant to the masticatory system.
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.
The Index of Orthodontic Treatment Need is a system that attempts to rank malocclusions in terms of significance of various occlusal traits and perceived aesthetic impairment.
[1] Centric relation (CR) describes a reproducible jaw relationship (between the mandible and maxilla) and is independent of tooth contact.
Therefore it is the position that dentists use to create new occlusal relationships as for example, while making maxillary and mandibular complete dentures.
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.
[13] 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.
Posselt (1952) determined that only in 10% of natural tooth and jaw relationships does ICP = CO[14] (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 or where multiple fixed unit prosthodontics are provided, where the occlusion is arranged so that when the mandible is in CR, the teeth are interdigitating.
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.
[15] It was believed in the 1930s that this arrangement was ideal for the natural dentition when providing full occlusal reconstruction in order to distribute the stresses.
[16] On the other hand, unilateral balanced occlusion is a widely used tooth arrangement that is used in current dentistry and is commonly known as group function.
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.
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).
As for deflective contacts, interferences may also be associated with parafunction such as bruxism (although evidence is weak) and may adversely affect the distribution of heavy occlusal forces.
Interferences may also cause pain in the masticatory muscles due to altering their activity,[24] however there is large controversy and debate as to whether there is a relationship between occlusion and temporomandibular disorders.
Psychological and emotional stress can also contribute to the patient's ability to adapt as these factors have an impact on the central nervous system.
[22] In individuals with unexplained pain, fracture, drifting, mobility and tooth wear, a full occlusal examination is vital.
Similarly when complex restorative work is planned it is also essential to identify whether any occlusal changes are required prior to the provision of definitive restoration[25] In some people even minor discrepancies in the occlusion can lead to symptoms involving the TMJ or acute orofacial pain so it is important to identify and eradicate this cause.
[29] Temporomandibular dysfunction commonly presents with muscular tenderness,[25] but pain or palpable soreness associated with the muscles can also be linked to parafunctional activity.
It is important to be able to guide the patient into RCP, as a registration may need to be taken in this position particularly if the occlusion is being reorganised, the OVD is being changed or even just for diagnostic and treatment planning purposes.
Occlusion is a fundamental concept in dentistry yet it is commonly overlooked as it's perceived as being not important or too difficult to teach and understand.
Some of the advantages associated with a working knowledge of these include:[32] Involves simply grinding down involved cusps or restorations and may be indicated after careful examination when: May be required in more severe circumstances and some examples of these include: Achieving a satisfactory occlusal reorganisation involves choosing a desired jaw relationship (either conforming to existing ICP or producing a new ICP coincident with CR), deciding on the intercuspal contacts (removing deflective contacts and adjusting shapes/inclines of teeth), adjusting excursive contacts (removing interferences) and aiming for a mutually protected occlusion.
It is common practice to mount mandibular and maxillary casts (impressions are made of the teeth and poured in dental stone) in an articulator in ICP when constructing restorations that conform to the patient's existing occlusion.