MANDIBULAR AXES & MANDIBULAR MOVEMENTS

Anatomy and Physiology of Temporomandibular Joint

Temporomandibular joint (TMJ): It is the articulation of the condyle of the mandible, and the inter-articular disc; with the mandibular fossa (glenoid fossa) of the temporal bone. The joint has a capsule and an articulating disc. It is considered as a compound joint (a compound joint is one with more than two bones articulating); in TMJ, the articular disc acts like the third bone.
TMJ CONSISTS OF THE FOLLOWING PARTS
1. The mandibular fossa (glenoid fossa) of temporal bone.
2. The condyle or head of the mandible.
3. Synovial cavity.
4. The articular disc or (meniscus).

Meniscus is found between the condyle and the glenoid fossa. It divides the synovial joint or TMJ into upper and lower (superior and inferior) compartments. Each compartment acts as a separate joint during function. The presence of the meniscus also distinguishes the TMJ from most other joints in the body, making it a bone-to-tissue (mandible to disc) and tissue-to-bone (disc to skull) articulation.

The muscles that control the movement of the mandible
There are three groups of muscles:
1. Closing muscles.
2. Gliding muscles.
3. Opening muscles.


CLOSING MUSCLES
The temporalis, masseter and medial pterygoid muscles supply the power for pulling the mandible against the maxilla (elevating and closing the mandible).

GLIDING MUSCLES
The lateral pterygoid muscle connects the mandible to the lateral pterygoid plate in such a way as to act as the steering mechanism for the mandible and act to protrude the jaw or to move it laterally.

OPENING MUSCLE
The muscles that depress (open) mandible consist of three groups, suprahyoid muscles, infrahyoid muscles, and platysma.


1. Temporomandibular and capsular ligaments.
2. Sphenomandibular ligament.
3. Stylomandibulalar ligament.

MANDIBULAR AXES & MANDIBULAR MOVEMENTS
Good prosthodontic treatment bears a direct relation to the structures of the temporomandibular articulation, since occlusion is one of the most important parts of treatment of the patients with complete dentures. The temporomandibular joints affect the dentures and likewise the dentures affect health and function of the joints.
The mandibular bone has specific relationships to the bones of the cranium. The mandible is connected to the cranium at the two temporomandibular joint by the temporomandibular and capsular ligaments. The sphenomandibular and stylomandibular ligaments also connect the bones in such a way as to limit some motions of the mandible.

MANDIBULAR MOVEMENTS
The mandibular movements can be classified as following

Based on the dimension involved in the movement
1- Rotational
a- Rotation around the transverse or hinge axis.
b- Rotation around the anteroposterior or sagittal axis.
c- Rotation around the vertical axis.

2- Translational or gliding
They are considered as basic movements of the mandible.

O The upper compartment shows anteroposterior gliding movement, when this movement takes place, the condyle and the disc move as a single unit against the glenoid fossa.
O The lower compartment shows hinge movement, during hinge movement the condyle moves against the articular disc and the glenoid fossa, which together act as a single unit. True condylar rotation is 12° with the maximum incisal separation of 22 mm. See figure (5-14)

Based on the type of movement
1- Hinge movement.
2- Protrusive movement.
3- Retrusive movement.
4- Lateral movement.
a- Lateral rotation or (laterotrusion).
? Right.
? Left.
b- Lateral translation or (Bennett movement).
? Immediate side shift.
? Progressive side shift.
? Precurrent side shift.

BASED ON THE EXTENT OF MOVEMENT
1- Border movement
a- Extreme movement in the sagittal plane.
b- Extreme movement in the horizontal plane.
c- Extreme movement in the frontal plane.
d- Envelope of motion.
2- Intra-border movement
a- Functional movement.
? Chewing cycle.
? Swallowing.
? Yawing.
? Speech.
b- Para-functional movement.
? Clenching.
? Bruxism.
? Other habitual movements.