Understanding how our teeth meet and function can help us appreciate the inter-relationship of the masticatory system: the teeth, bone, gingivae, temporo-mandibular joint (TMJ) and the muscles of mastication.
This, in turn, will open our eyes to the cause and effect scenario, and help us identify why parts of the system may be breaking down. This knowledge will assist us in communicating effectively with our patients, thus helping them to understand the benefits of treatment for the longevity of their dentition.
An outline for patient screening
Understanding the principles of occlusion can help us see why teeth sometimes break or why patients grind. We can learn to ask the right questions and ascertain what corrective treatments are suitable.
There are four determinants of occlusion:
1. The two temporo-mandibular joints
2. The occlusion itself
3. The way the teeth meet
4. The neuromuscular system.
We must fully understand how these parts function together and individually in harmony and stability.
The temporo-mandibular joints relate our mandibular arch to the maxilliary arch. The ligaments attach the mandible to the skull and are supported by the muscles of mastication. This is the only point of reference between the mandible and the maxilla.
Looking at the way teeth meet, we need to think about how the occlusal surface of one tooth meets its antagonist (another occlusal surface) in a stable relationship. We also need to understand how any movement away from this should be harmonious, with no interference and completely stress-free. We also need to understand the range of mandibular movement and how this relates to the morphology.
The elements of occlusion
The elements of occlusion are as follows:
• Centric relation/centric occlusion
• Canine guidance
• Centric stops
• Neutral zone
• Axial inclinations versus lateral forces.
Centric relation (CR)
This is when the heads of the condyles are in their most superior position within their sockets, with the disc properly aligned and full neuromuscular release, irrespective of tooth position or vertical dimension. The lateral pterygoid muscle is relaxed and the elevator muscles are contracted.
Centric occlusion (CO)
Also known as habit bite is the relationship of the occlusal surfaces of one arch to those of the opposing arch at physical rest position.
Ideally we would like CR to equal CO to create harmony between teeth and joints. If CR does not equal CO there is a disturbance of the timing of the contraction and release of antagonistic muscle groups. This CR interference causes constant contraction of the lateral pterygoid muscles, causing problems in some patients and could be the weak link in the system, which differs from patient to patient and can affect:
• Joints – tenderness, noises and pain on loading
• Muscles – tenderness, headaches, neck and shoulder pain
• Bone – angular bony defects, accelerated bone loss
• Teeth – sensitivity, wear and fracture.
The normal range of motion is 35-45mm opening with no deviation, with protrusion at 8-12mm and lateral excursion of 8-12mm (both being equal).
This looks at the relationship between the upper and lower teeth when the mandible moves laterally. Ideally, as the mandible glides to one side the canines guide and separate all anterior and posterior teeth (Figures 1 and 2), protecting teeth from fractures, worn cusps, angular boney defects (Figure 3), recession or anterior wear (Figures 4 to 9).
The side to which the mandible is moved is called the working side and the opposite side is the balancing side. When assessing lateral excursion we would then note any working or balancing interferences or anterior guidance or group function.
A stable centric occlusion will have:
• Teeth in the neutral zone
• Posterior teeth with a cusp-fossa relationship
• Anterior teeth with a light stable stop or an acceptable substitute.
The soft tissues, lips, cheeks and tongue create opposing forces. If one force is greater than another, teeth can move out of alignment. For example, a tongue thrust may create an anterior open bite or the drifting of an incisor outside of the neutral zone will carry on drifting as the opposing forces are not equal.
Cusp to fossa relationship This is how the teeth interdigitate. Ideally the teeth should meet cusp tip into opposing fossa (Figure 10).
This is the point at which the upper and lower anterior teeth meet lingually and stop, usually at the cingulum area. When assessing anterior stops we consider the angles of occlusion: over bite, open bite, edge to edge, etc.
This is where the cusp tips contact their opposing fossa or marginal ridge.
The long axis of each tooth should bare the forces of occlusion to allow for the best possible support. Teeth can withstand tremendous forces in an up and down motion. Forces parallel to the long axis create resisting forces from the supporting tissues, i.e. the ligament.
We may see tooth migration, bone loss, widened periodontal ligament, wear and fractures.
In my previous articles I have spoke of ‘comprehensive care’. A sound understanding of occlusion is paramount in offering our patients freedom from disease. Understanding the inter-relationship of the teeth, periodontium, TMJ, occlusion, function and aesthetics will help a clinician identify signs of early breakdown before symptoms are apparent and can assist in minimising the need for dentistry.
Dawson PE (2006) Functional Occlusion: From TMJ to Smile Design. 1st edition, Mosby
Photography courtesy of Dr D Bloom and Dr J Padayachy of Senova Dental Studios, Watford, Herts and directors of CoopR8 Seminars Ltd.