In some temporomandibular disorders, the articular disc remains permanently displaced and does not return to its normal position during jaw movements. This typically follows a period of clicking, where the condyle comes off the disc on closure and pops back on the disc when opening again. Once the disc gets stuck in the displaced position, the click disappears, but prevents the condyle from popping back on during opening. This condition often leads to limited jaw opening and can cause significant discomfort.
Initial Opening
As the jaw begins to open, the condyle moves forward and downward. Since the disc is permanently displaced anteriorly, it obstructs the normal pathway of the condyle. The condyle encounters resistance from the displaced disc, preventing it from moving freely.
Obstruction to Full Range of Motion
This obstruction limits the range of motion, leading to a reduced maximum jaw opening (often referred to as a “closed lock”). The condyle attempts to move forward, but the anteriorly displaced disc acts as a physical barrier. The lack of reduction (return of the disc to its normal position) means the condyle cannot bypass this barrier.
Characteristics of TMJ Lock:
The maximum mouth opening is significantly reduced, typically less than 40 mm (often around 25-30 mm or even less in severe cases). Patients may experience a hard stop when attempting to open their mouth widely, accompanied by pain or discomfort.
Some patients seek therapy and force the opening by various exercises which can increase the range of opening but can contribute to the disc being displaced further down, making it more difficult to recapture and treat.
Understanding the mechanics of disc displacement without reduction helps in diagnosing and treating this form of TMD effectively, aiming to restore normal jaw function and alleviate discomfort.