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.
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), termed Leeway Space. This allows the permanent molars to drift mesially into the spaces and develop a Class I occlusion.