A lot of TMJ's most confusing symptoms — clicking, locking, a jaw that shifts to one side — trace back to one small structure inside the joint slipping out of place.
The disc's normal job
Inside the jaw joint sits a small, cartilage-like disc that acts as a cushion between the mandible (jawbone) and the skull, letting the joint glide and rotate smoothly. When it's positioned correctly, you don't notice it at all.
Two forms: with reduction and without
In early or milder cases, the disc slips forward but pops back into place as you open your mouth — this is called displacement "with reduction," and the disc's snap back into place is exactly what produces a clicking or popping sound. Our jaw locked open vs. closed guide covers what happens when the disc gets stuck instead: displacement "without reduction," where it stays out of position and physically blocks the joint from opening fully.
Why it happens
The most common cause is trauma to the jaw (see our post on jaw injuries and TMD), though degenerative changes from chronic clenching or grinding, or joint laxity from conditions like hypermobility/EDS, can also allow the disc to shift out of position over time.
An important caveat
Studies have found displaced discs in people with no TMJ symptoms at all — meaning a displaced disc alone doesn't automatically mean you need treatment. What matters clinically is whether it's actually causing pain, restricted movement, or locking, which is something imaging (see our X-ray/MRI guide) combined with a clinical exam can help sort out.