The movements that the mandible can make and the names of the important positions within this range of movements are following videos
Terminal hinge axis (THA) and the retruded arc of movement (or closure)
This is an axis which passes through both condyles and about which the mandible rotates in its most retruded (comfortable) position of the condyles – the retruded arc of closure. This is a clinically reproducible movement and recording it is often useful in making crowns and bridges and essential in making complete dentures. The THA can be measured in individual patients but it is sufficient for most applications to use an average THA such as that recorded by an ear-bow.
Intercuspal position (ICP)
This is the position of maximum contact and maximum intercuspation between the teeth. It is therefore the most cranial position that the mandible can reach. The term ‘centric occlusion’ has been used to describe this position, but this is confused with ‘centric relation’ (see below) and may also imply centricity of the condyles in their fossae, centricity of the midline of the mandible with the midline of the face, or centricity of the cusps within the fossae of the opposing teeth, none of which may be the case. The term ‘centric occlusion’ is therefore better not used.
Retruded contact position (RCP)
This is the most retruded position of the mandible with the teeth together. It is a clinically reproducible position in the normal conscious patient. Patients with conditioned patterns of muscle activity may not be able to manipulate the jaw into it, even with assistance by the dentist. In less than 10% of the dentate population the RCP coincides with the ICP. In the remainder the RCP is up to 2 mm or more posterior to the ICP. The term ‘centric relation’ has been used to describe this position, but it has the same disadvantages as the term ‘centric occlusion’ and will not be used. ‘Centric occlusion’ (CO) and ‘centric relation’ (CR) are terms sometimes used in complete denture construction where they mean different things to ICP and RCP.
Those patients who have a discrepancy between the RCP and ICP usually close straight into the ICP from the postural or rest position when the movement is made subconsciously. Patients sitting in dental chairs making voluntary, conscious movements when asked to do so by the dentist often make bizarre movements rather than closing into the ICP directly. These aberrant movements and contacts are the result of patients trying too hard to help and not understanding what is required. Students have been heard to ask non-dental or medical patients to bite on their ‘posterior’ teeth!
However, contact does occur in the range between the ICP and RCP during empty swallowing (particularly nocturnal swallowing), during the mastication of a tough bolus and during parafunctional activity. Thus the mandible can slide from the ICP in four main directions with the teeth in contact, or in an infinite number of directions at angles between these main pathways. The four excursions are:
• left lateral
• right lateral.
Movements between the ICP and RCP are usually guided by a limited number of opposing pairs of cusps of posterior teeth. The angle of the slide between RCP and ICP, its length and the individual pairs of teeth that produce it are important and should be examined. Of even greater importance is any unevenness of the movement producing bulges or lumps in the path of movement. These disturbances to the smooth movement of the mandible are one form of occlusal interference.
In forward movement of the mandible with the teeth together. It is usually the incisor teeth that guide the movement. This will not be the case in anterior open bites or in Class III incisor relationships. The angle and length of movements will be determined by the incisor relationship so that, for example, in a Class II Division II incisor relationship with an increased overbite and reduced overjet, the movement of the mandible has to be almost vertically downwards before it can move forwards. Anterior guidance is important when making anterior crowns or bridges. Sometimes, when the teeth are a normal shape, it is helpful to reproduce the patient’s existing guidance as accurately as possible; on other occasions, for example, with worn teeth, it is unnecessary or undesirable to do so, and in fact the purpose of the treatment may be partly to alter the incisor guidance.
Left and right lateral excursions
In lateral excursions the side that the mandible is moving to is known as the working side and the opposite side the non-working side. The term ‘balancing side’ has been used to refer to the non-working side, but since it implies a balanced occlusion, balancing or stabilizing a complete denture base, it should not be used in reference to natural teeth.
The contacts on the working side are either between the canine teeth only (canine-guided occlusion) or between groups of teeth on the working side (group function). Occasionally, individual pairs of posterior teeth will guide the occlusion in lateral excursion, but this is not regarded as ideal. The canine-guided occlusion is considered to be protective of the posterior teeth which disclude in lateral guidance.
Contact on the non-working side in lateral excursions should not normally occur. It does sometimes occur after extractions and over-eruption and occasionally following orthodontic treatment, particularly when this treatment has been carried out with removable appliances that have allowed the posterior teeth to tilt. Contact may also occur in cases of posterior crossbite where the lower teeth are placed buccally to the upper teeth.