Natural occlusions come in all arrangements, and they all work – unless they don't. I have seen many patients for whom "maximum intercuspation" means a few teeth interdigitated, and yet the patient is healthy, comfortable, eats whatever he likes and is exceedingly happy. Why would we EVER change that relationship unless dentistry was required? That's when we must face a problem.
Working at the "acquired occlusion" means accepting that enough of what has been acquired is stable enough to last and will be essentially unchanged by the dentistry or unchanged enough that the patient will adapt to the changes without incident. As greater numbers of teeth become involved it becomes less and less likely that the acquired occlusion can be re-acquired following treatment. The risk is magnified when posterior teeth are part of the treatment plan. At some point the dentist must make the decision to OWN the occlusion. For me that decision is made when the patient meets one of two criteria:
- The signs and symptoms present in the occlusion have exceeded the patient's adaptive capacity.
- The dentistry required will impact the occlusion between first point of contact in a seated joint position and maximum intercuspal position.
When one or both of these criteria is met, the restorative dentist must now fulfill a set of requirements for designing a treatment occlusion.
Requirement 1: The condyle and disk are appropriately positioned. This reference position from which everything is planned will depend on the training and experience of the doctor. It will probably be one of two positions, a muscle mediated position or a joint and muscle mediated position. I use the latter whenever possible, centric relation. The muscle mediated position is my second choice and is required when the patient doesn't have a centric relation position.
Requirement 2: Posterior teeth touch simultaneously at the reference joint position.
Requirement 3: Forces are directed down the long axis of the teeth.
Requirement 4: Front teeth have slightly lighter contact in the maximum intercuspal position. This relates to providing freedom for the movement of the condyle in translation as well as rotation in early opening and late closing.
Requirement 5: Underclench neither teeth nor jaw deflect.
Requirement 6: Canines are the guiding teeth when ever possible. This is primarily for prosthetic convenience but it works well for most patients. There are exceptions.
Requirement 7: Anterior guidance moves as far forward as quickly as possible, eventually transferring the contact to the incisors at end-to-end.
Requirement 8: If group function is appropriate premolars are preferred over molars.
Requirement 9: No balancing interferences. Balancing contacts as deemed necessary by trial and error.
Requirement 10: Lateral crossover is supported on anterior teeth.
Requirement 11: Straight protrusive moves on both centrals smoothly and evenly. Movements of the mandible side-to-side during protrusion create a disharmony as the muscles "look for the contact" and move back and forth.
Requirement 12: Make it smooth.