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, but in relation to the overall masticatory system.
Anatomy of Masticatory System
One cannot fully understand occlusion without an in depth understanding of the anatomy including that of the teeth, TMJ, musculature surrounding this and the skeletal components.
The Dentition and Surrounding Structures The human dentition consists of 32 permanent teeth and these are distributed between the alveolar bone of the maxillary and mandibular arch. 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.
The periodontal ligament unites the cementum on the outside of the root and 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.
The teeth are highly specialised and different teeth are involved in specific functions. The masticatory system is largely influenced by these intra and inter-arch relationships and a wider understanding of the anatomy can greatly benefit those who want to understand occlusion.
Skeletal Components The maxilla forms a crucial aspect of the upper facial skeleton. Two irregularly shaped bones fuse at the intermaxillary suture during development forming the upper jaw. This forms the palate of the oral cavity and also supports the alveolar ridges that hold the upper teeth in place. The lower facial skeleton on the other hand, is formed of the mandible, a U-shaped bone, which supports the lower teeth and also forms part of the TMJ. The mandibular condyle and the squamous portion of the temporal bone, at the base of the cranium articulate with one another.
TMJ 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. It is classified as a ginglymoarthrodial joint and can perform a range of gliding and hinge type movements. The disc, which lies in between is composed of dense fibrous tissue and is predominantly avascular and lacking nerves.
Muscles There are various muscles that contribute to occlusion of the teeth including the muscles of mastication and other accessory muscles. 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.
Ligaments There are various ligaments associated with the TMJ and these limit and restrict border movements by acting as passive restraining devices. They do not contribute to joint function, rather exert a protective role. The key ligaments relevant to the TMJ are: The temporomandibular ligament The medial and lateral discal ligaments The sphenomandibular ligament The stylomandibular ligament
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 cheeks 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 for 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. The mixed dentition stage is 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 Classification of occlusion and malocclusion plays an important role in diagnosis and treatment planning in orthodontics. 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 make up Angle's incisal relationship classification:
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 horizontal overlap - this is termed overjet Division II includes those with retroclined (10%) incisors, which leads to an increase in vertical overlap - this is termed overbite 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
Any deviation from the normal relation of teeth (Class I) is considered a malocclusion.
Class I relationships are thought to be “ideal”, however this classification does not take into consideration the positions of the two 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. 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. The index identifies those who would benefit most from orthodontic treatment and onward referral to an orthodontist.
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 interdigitation. This position is usually the most easily recorded and is almost always 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 against the posterior slope of the articular eminence. It is said that in CR, the muscles are in their most relaxed and least stressed state. This position is not influenced by muscle memory, but rather by the ligament which suspend the condyles within the fossa. 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. 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, 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, however this may be an interference contact. On the other hand, 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 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.
Posselt's Envelope of Border Movements Posselt’s Envelope of Border Movement (often referred to as the 'border movements of the mandible') is a schematic diagram of the maximum jaw movement in three planes (sagittal, horizontal and frontal). This encompasses all movements away from RCP, and includes: Protrusive movements: When the mandible moves forward from centric relation, this is considered as protrusion. In Class I occlusion, the predominant contacts occur on the incisal and labial surfaces of the mandibular incisors and the incisal edges and palatal fossa areas of the maxillary incisors. Lateral movements: When the mandible moves to the left or right, the mandibular posterior teeth move laterally across the opposing teeth. For example, when the mandible moves to the right, the right mandibular teeth move laterally across their opponents and this is termed the working side of the mandible (the side towards which the mandible is moving). In contrast, the left mandibular teeth move medially, downwards and anteriorly across their opposing posteriors and this is called the non-working side (the side to which the mandible is moving away from).