Dawson has 5 requirements for occlusal stability.
The first is that there are stable contacts on all teeth of equal intensity in centric relation.
So what is centric relation. It is relationship of the mandible to the maxilla when the properly aligned condyle-disc assemblies are in the most anterior-superior-medial position in the glenoid fossa. This cartoon shows what that looks like. Additionally it shows that in an opening motion, the condyle initially rotates about its axis and as opening increases, it begins to translate down the articular eminence while continuing to rotate. The lateral pterygoid muscle stabilizes the joint throughout this movement. And the inclination of the articular eminence is the posterior determinant of the movement of the mandible.
The second detail embedded within Dawson’s first criteria is that all teeth have equal intensity contact in centric relation – in an occlusal relationship known as centric occlusion. But where should these contacts be located? For posterior teeth, they should be between functional cusp tips and flat surfaces of the opposing central pits or marginal ridges. For anterior teeth, there should be light contact between mandibular incisal edges and just above the cingulum of maxillary teeth.
Dawson’s second criteria is that anterior guidance is in harmony with the envelope of function. What this means is that the maxillary anterior teeth need to be inclined or contoured in such a way that when you perform functional movements, like biting or chewing, the path of your mandible does not lead to the incisal and facial surfaces of your mandibular incisors bumping into the incisal or lingual surfaces of your maxillary incisors. This is sometimes called outside-in movement because your mandible is travelling from outside your mouth back inside when you close down during chewing.
The third through fifth criteria are that all posterior teeth disclude during mandibular protrusive, lateral working, and lateral non-working movements. Basically this means that the anterior teeth keep the posterior teeth separated everywhere aside from when all the teeth are touching in centric occlusion. This is sometimes called inside-out movement.
These concepts imply that lingual surfaces of the maxillary anterior teeth need to BOTH separate posterior teeth during protrusive, lateral working, and lateral non-working movements AND avoid anterior interferences when in function.
Sometimes the position where all the teeth fit together, maximum intercuspal position (MIP), does not allow the condyle-disc assemblies to be in the most anterior-superior-medial position in the glenoid fossa. This means that MIP is not the same as CO. This can happen when there is a posterior interference on a molar that gets in the way of the mandible rotating into MIP while the condyle-disc is in CR. Therefore, the lateral pterygoid needs to pull the condyle to slide down the articular eminence and allow the mandible to rotate into MIP (some people call this distraction). And that means that the patients must now function with the condyle-disc assembly in this position. Since the lateral pterygoid is holding it there, it puts strain on that muscle which can cause pain for our patients. Additionally, since the condyle is functioning on the slope of the articular eminence (see original post for review of anatomy), the condyle has the ability slide upwards and backwards. This movement of the condyle (upwards and backwards) would allow the posterior teeth to rub – which can cause tooth wear, fracture, loosening or movement.
What do you do if the patient is showing signs of occlusal instability (pain, tooth wear, tooth loosening)? One option is to determine if a discrepancy between MIP and CO is causing muscle pain. A method to evaluate this etiology is to place a Lucia jig or leaf gauge between the patients teeth and determine if the interocclusal relationship is altered once the tooth contacts are removed. It may take a period of time with the teeth separated for the muscles to “deprogram. If a MIP/CO discrepancy is noted after “deprogramming”, an interocclusal record may be taken with the teeth separated in CO. Then you can mount the patients casts in this CO position in an articulator with a facebow record. In this mounting, not all teeth will be touching and any interferences will be the first contacts between teeth. The occlusion can be examined on the casts in the articulator to see if adjusting teeth will allow the removal of these interferences for MIP to be achieved with the centric contacts and anterior guidance described in our earlier posts. This is known as occlusal equilibration. If these adjustments would cause too much enamel removal or require increase of interocclusal space, then other options, such as restorative treatment or orthodontics may be considered.
About the Authors
Dentinaltube is a website, youtube channel and instagram account run by faculty at the UAB School of Dentistry. Dr Nate Lawson is the director of the Division of Biomaterials, Dr. Augusto Robles is an Operative Dentist and an Associate Professor in the Division of General Dental Sciences, and Dr. Celin Arce is a board-certified prosthodontist and Assistant Professor in the Division of Prosthodontics.