Mandibular Border Movements
In centric occlusion, the teeth are in uniform contact on all sides with the antagonists; in this position, the periodontal tissues
are centrically loaded (Fig 7-3a). The mandible is at its closest to the maxilla in this position. The condyle of the mandible lies without pressure deep in the articular fossa. Centric occlusion is normally adopted as a reflex to a wide opening position, so that this position can also be regarded as the habitual occlusion or intercuspation. An opening movement of approximately 10 mm can be made out of centric occlusion as a pure hinge movement (Fig 7-3b). However, the mandible has to be forcibly held back to do this. For wider jaw opening, the mandibular condyle slides forward and downward on the oblique condylar path; thus, a gliding movement is added to the pure hinge movement.
For maximum opening movement, the condyle now slides as far downward and forward as necessary until it comes to rest on the horizontal surface of the articular tuberosity (Fig 7-3c). The condyle is thus slid out of the articular fossa. So that the joint is not dislocated, any further movement is prevented by the articular ligaments. This special movement is enforced by the lateral pterygoid muscle, which, with a few muscle fibers, attaches to the articular disc and draws it forward. The joint capsule itself can be stretched so that it is also involved in this movement without being abnormally stretched.
A particular problem when analyzing mandibular movements is that functional movements cannot be exactly reproduced. It is assumed that all functional movements lie within a range of border movements. The border movements are therefore mapped in all three planes so that the whole range of functional movements can be outlined. The inferior incisive point and the midpoints of the condyles are chosen as measuring points for tracing this pattern of border movements. The maximum movement paths of the points are thus transferred to the median or sagittal plane, frontal plane, and horizontal plane. When these are combined, the result is an envelope of border movements in space, within which all mandibular movements take place (Fig 7-4).The border movement of the mandible in each of these planes is described below.
Sagittal plane: Movements out of centric occlusion are possible, both as movements with tooth contact and as free mandibular movements. This yields reproducible limit positions, including the following (Fig 7-5):
- Centric occlusion
- Maximum protrusion of the mandible
- Endpoint of pure hinge movement
- Maximum opening point
- Rest position
The lines connecting the limit points correspond to the possible border movement paths.
Frontal plane: Plotting of mandibular movement shows a border movement that is reflected on the line of the sagittal limit points. From centric occlusion, the mandible drops and moves over the occlusal contact out to the right or left limit position and from there in an even curve to the maximum opening point.
Horizontal plane: Movement of the mandible creates a rhomboid, symmetric movement path of the mandibular incisive point, which in principle can be reproduced at each centric stop in the contact area of the antagonists (Fig 7-6). The movement path arises from mapping of the in-terocclusal record and here corresponds to the Gothic arch or needlepoint tracing (Fig 7-7).
The needlepoint or Gothic arch is based on the tracing of protrusive and lateral movements during interocclusal record registration. For this purpose, a tracer should be attached centrally to a baseplate, then used to track the horizontal border movements on a registration plate in the opposing jaw.
Posselt's diagram can be used for twodimensional plotting of the pattern of border movements of the mandibular incisive point in the sagittal, horizontal, and frontal planes. The record of the area covered by the border movements created by joining the limit points of free mandibular movements in all planes of space can be referred to as Posselt's envelope. All mandibular movements take place within that space.