In a healthy temporomandibular joint (TMJ), opening and closing the mouth involves smooth, coordinated movements without pain, discomfort, or abnormal sounds.
Opening the Mouth: Initial Hinge Movement
The first part of opening the mouth involves a hinge-like motion of the TMJ. The condyle (the rounded end of the lower jaw) rotates within the mandibular fossa (the socket in the temporal bone of the skull). This initial movement allows for the first 20–25 mm of mouth opening.
Translational (Sliding) Movement
After the initial hinge movement, the condyle and the articular disc (a small, fibrous cushion within the joint) slide forward along the articular eminence (a bony ridge in the temporal bone). This sliding action allows for further opening of the mouth, typically up to about 40–50 mm in a healthy adult. The movement is smooth and controlled, with the disc remaining properly positioned between the condyle and the temporal bone.
Closing the Mouth: Translational (Sliding) Movement
When closing the mouth, the process reverses, starting with the condyle and disc sliding back along the articular eminence into the mandibular fossa. The movement is smooth, ensuring that the disc remains properly positioned to cushion the joint.
Hinge Movement
The final part of closing involves the hinge motion, where the condyle rotates back to its resting position within the mandibular fossa. The upper and lower teeth come together in a precise and controlled manner, without any deviation or misalignment. The teeth coming together into occlusion support the condyle in its seated position. This is an important key to a healthy TMJ and often missed when assessing the TMJ and the possibility of a disorder. If the posterior teeth do not support the condyle in seated position in a currently healthy TMJ, it is highly possible that disc displacement will eventually occur, as discussed in the next part of this video series.
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Characteristics of Healthy TMJ Movement:
- Smooth and Painless: Movements occur smoothly without pain, discomfort, or resistance
- Symmetrical: The jaw opens and closes symmetrically without deviation to one side
- No Abnormal Sounds: There are no clicking, popping, or grinding sounds during movement
- Full Range of Motion: The mouth opens fully (approximately 40-50 mm) without restriction or difficulty.