Determining Joint Values
The condylar paths are set in fully adjustable articulators after the tracks of the condylar paths have been traced and measured or by using eccentric positional records (interocclusal records).
The interocclusal record method exploits the Christensen phenomenon, such that the rows of teeth open on the mediotrusive side and hence space is left for the material of the positional record. The following positions of the jaws in relation to each other are established:
- Centric occlusion using the centric record, where the rows of teeth are about 3 mm apart
- Protrusive position using a protrusive record; in protrusion, the posterior teeth open and provide space for the record
- Lateral positions using laterotrusive records to the right and left on the mediotrusive side
The positional records are made of a tough elastic material, either silicone or wax reinforced with metal chips or textile inserts. With the facebow, the models are first inserted into the articulator in centric occlusion. The condylar paths are brought into an average inclination or into the neutral position, and the locks are loosened. The models are then pushed into the protrusive position and fixed with the interocclusal record. The joints can then be set to the sagittal condylar path inclination and locked.
To set the Bennett angle, the models are fixed one after another in the lateral positions with the interocclusal records; the condylar path of the nonworking condyle, which follows the movement of the upper arm of the articulator, is then adjusted to the Bennett angle. This setting of the condylar inclinations must be repeated and checked several times. The protrusive record is used as the interocclusal record, which must be correct after settings have been made with the lateral occlusions.
The advantage of the interocclusal record method is that it compensates for system errors in the arbitrary facebow technique. The interocclusal record method does not produce individual joint values but rather articulator settings for mandibular movements within a range of border movements that is defined by the interocclusal records. It is assumed that tooth guidance and condylar guidance exactly correspond mechanically.
Individual registration of the joint values is done via extraoral, spatial tracing of the condylar movement paths with a tracing arch that is rigidly fixed to the mandibular dentition (Fig 8-49). The tracing arch corresponds to the axis locator (Almore facebow), whose tracers are oriented to the hinge-axis position. Vertical and horizontal tracing flags are arranged over the hinge-axis position. The vertical flags are used to trace the sagittal condylar path inclinations and are precisely aligned with a reference plane.
The horizontal flags are used for tracing the Bennett angle and are fixed roughly in the sagittal condylar path inclination. The flags can be fixed in the correct position with a headband. During mandibular movements, the tracing needles, which can be pressed on by elastic force, trace the condylar movement paths onto the tracing flags. In protrusive movements, the protrusion paths are produced on the vertical tracing flags as S-shaped curved lines (shape of sagittal condylar path) (Fig 8-50); on the horizontal flags, the protrusion paths are produced as straight lines running forward.
During lateral movements, the mediotrusion paths are produced on the horizontal tracing flags; with the forward-running protrusion paths, these mediotrusion paths enclose the Bennett angle. Mediotrusion paths are also traced on the vertical flags. However, these patterns of sagittal condylar paths traced during lateral movements are steeper and longer than those resulting from pure protrusive movement (Fig 8-51).
The Fischer angle denotes the angle between the tracings of the sagittal condylar path during protrusive movements (protrusion path) and during lateral movement (mediotrusion path) of up to 10 degrees. The track of movement of the nonworking condyle is longer during lateral movements than during protrusive movements, and it deflects caudally.This difference in the movement paths arises from the tracing error of the registration kit, which tilts when tracing the lateral movement in the horizontal plane so that the tracing points on the flags deflect further caudally and forward.