Jaw Relation Registration
The jaw relation is any position that the movable mandible can adopt relative to the fixed maxilla. In definition terms, jaw relation in a healthy occlusal system means the constantly reproducible, centric position of the mandible where the condyles are located deep in the mandibular fossae without pressure or tension and where maximal intercuspation and centric loading of the teeth exist. Jaw relation registration is intended to rediscover this centric occlusion or habitual intercus-pation position.
In a fully dentate, normal dentition, the position of the mandible relative to the maxilla is clearly fixed by the occlusal pattern of the teeth, the TMJs, and the musculature. In a reduced residual dentition, the situation of opposing teeth may be so impaired that a definite centric position cannot be established with the models in the way it can with fully dentate models. In edentulous jaws, the centric occlusion position cannot be established with models alone. It has to be reconstructed by suitable methods.
Relation of the mandible to the maxilla can be determined in three dimensions.The central position analogous to centric occlusion is established by the occlusal height (vertical relation) and by the centric position (horizontal relation), which corresponds to the physiologic hinge-axis position (Fig 8-24). To reconstruct this occlusal position, baseplates are fabricated and then used in different types of jaw relation registration. The recording takes place in two phases: establishing the vertical relation and establishing the horizontal relation.
The vertical relation (occlusal height) can be determined on the basis of the interrelations among four variables:
- Adoption of the physiologic rest position as the neuromuscular position of the mandible. In the relaxed state, there is a very small distance between the rows of teeth, which is adopted by the resting muscular tension.
- Smallest speaking distance resulting from pronunciation of consonants or series of numbers.
- Statistical mean in edentulous jaws: 38 to 42 mm distance from the mandibular to the maxillary vestibular fornix.
- Establishment of harmonious facial proportions according to the golden ratio; the distance from the tip of the nose to the upper lip and from the upper lip to the tip of the chin is set at a length ratio of 3:5. The facial proportions can also be determined by average-value measurements of cranial dimensions (craniometry), whereby the distances between the forehead point, root of the nose, subnasal point, and tip of the chin are equal.
The horizontal relation or physiologic hinge-axis position is determined by the following methods:
Adopting the most retruded mandibular position by:
- Guiding it back by hand.
- Getting the patient to swallow.
- Having the patient touch the superior dorsal edge of the plate with the tip of the tongue. A wax ball can be placed exactly in the middle at the edge of the vibrating line on the baseplate, which the patient then has to touch with the tongue. As a result, the mandible is forcibly pulled back. In most cases, this is a forced position in which the condyles are in contact with the dorsal limit of the joint, hence about 1 mm behind the normal position. These methods are used for edentulous jaws.
- Gothic arch tracing (intraoral support pin registration) in which a centrally attached tracing stylus (or pin) records the horizontal mandibular movement on a tracing plate. Only this method yields the physiologic hinge-axis position.
- Electronic registration methods.
Centric records are used to transfer the physiologic hinge-axis position or centric position of the mandible relative to the maxilla onto the model situation; they are used to code the models for mounting in an articulator. A centric record can be prepared as a wax squash bite, with occlusal plates made of thermoplastic acrylic, silicone, or composite impression materials.
Hand-guided occlusal registration involves recording the jaw relation with wax occlusion rims, usually for edentulous jaws in which the mandible is guided into the centric position by hand. Occlusal height and horizontal occlusal position can be located by the aforementioned methods. In addition, the occlusal plane and fullness of the lips are established, and other orientation marks for the artificial teeth are traced.
Figures 8-25 to 8-31 illustrate various considerations for jaw relation registration.