The test takes five seconds and needs neither a doctor nor a machine: open your mouth as wide as you can and try to fit four fingers vertically between your front teeth. If four fit, the jaw is working as it should. If only two or three fit, the opening is restricted and that is a sign worth checking with a specialist. That simple, and almost nobody does it - because the jaw is the one joint in the body we use all day and never think about.

Physiotherapist Carlos Trigo says the thing that changes the whole picture: it does not have to hurt badly for there to be a problem. "The discomfort develops gradually and gets attributed to stress or neck pain, while the source is the jaw itself," he says. That is also why people spend years seeing the wrong specialist - the neck gets treated, the headache gets treated, and the joint producing both goes unexamined.

The causes are banally everyday. Bruxism - clenching and grinding the teeth - along with stress, disrupted sleep and habits like chewing gum for long stretches all load the joint. Much of the clenching happens at night, when nobody can control it, and during the day, when nobody notices they are doing it.

The list of signs Trigo gives is worth reading slowly: stiffness or fatigue in the jaw in the morning, tiredness while chewing, difficulty opening the mouth fully, pain spreading towards the temples, ears or neck, frequent headaches and wear on the teeth. Clicking on its own is not an alarm - it becomes one if it comes with pain or restricted movement.

When is it worth seeing a specialist? When the trouble lasts days or weeks, when it keeps coming back, when it interferes with eating or speaking, when it limits how far the mouth opens, or when it arrives together with headaches and neck pain. Here that usually means a person buys a painkiller first and waits for it to pass - and then waits again the following month.

The good news is that the solution is rarely dramatic. "A personalised conservative treatment achieves significant improvement without invasive procedures," Trigo says, provided you find what is actually causing the clenching. That is the part most often skipped - the joint gets treated while the cause, usually stress and poor sleep, stays untouched. The question is not whether the jaw hurts, but why the body has spent months clenching something nobody asked it to.