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:
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