Guidance Factors in Mandibular Movement
Mandibular movements are determined by three guidance factors (Fig 7-8):
- Condylar (TMJ or ligament) guidance is the limitation of movement by the TMJ. The condyle is guided both on the condylar path and by the articular disc. The joint capsule and articular ligaments limit these movements. It is mainly the free mandibular movements that are limited by the capsule and ligaments.
- Tooth guidance arises from the spatial arrangement of antagonizing occlusal patterns; in this case, movement is performed along the occlusal tooth surfaces as effective chewing movements.
- Neuromuscular guidance takes place via the active neuromuscular system of the masticatory muscles. This guidance produces a coordinated movement habit in fixed patterns.
Function of condylar guidance
In a technical articulator device for imitating mandibular movement, neither ligament guidance nor neuromuscular guidance can be mimicked. The parts of the articulator can only be moved against
each other via rigid condylar guidance; this is why precise analysis of condylar guidance is necessary. The most common form of movement in the TMJ is rotary sliding: rotation with simultaneous translation. Real mandibular movements take place around three spatial axes, in which the condyles of the TMJ form the centers of rotation (Figs 7-9 and 7-10).
In a pure opening movement, there is one rotation point in each condyle, through which a hinge axis can be placed that can be projected onto the external skin at each condyle. Other rotation axes are necessary in lateral movements. When the nonworking condyle slides forward, downward, and inward on its condylar path during lateral movement (Fig 7-11), the working condyle on the opposite side must rotate around several axes that are perpendicular to each other (see Fig 7-10):
- Hinge axis: Because a lateral movement is only possible when a slight opening movement takes place, the rotating (working) condyle turns around the hinge axis.
- Vertical axis: Because the nonworking condyle describes a section of a circular arc forward and inward on the condylar path, the rotating condyle has to turn around a vertical axis.
- Sagittal axis: Because the nonworking condyle slides downward and forward on the condylar path, the rotating condyle has to turn around a horizontal-sagittal axis.
When the condyles slide forward during forward (protrusive) movement, they move in line with the sagittal condylar path inclination downward as far as the articular tubercle, which means
the mandible has to drop down in the dorsal area (Fig 7-12). If the mouth remains closed, the incisors come into contact but the posterior teeth do not touch.