The three most common issues I have seen in the clinic around the jaw are: -
                   Inability to open the mouth wide enough
                   Pain in the TMJ
                   Clicking in the TMJ.
The joint where the jaw attaches to each side of the head is called the temporomandibular joint Is abbreviated to TMJ. The above 3 conditions can be attributed by TMJ being dysfunctional.

Jaw (Tempromandibular joint TMJ)

TMJ dysfunction

What you feel: Most people should be able to open their mouth as wide as three fingers(about 35mm). If this is not possible then you consider that there is a restriction. If the restriction is not too significant many people just learn to live with it unless there is associated pain. Some people may experience a click or a sideway movement as they open the jaw others may get sharp pain just in front of the ear. Normally, TMJ issues occur in one side only but affect the total jaw movement.

What is happening: Tight muscles can restrict the amount the jaw opens. An imbalance in muscle tensions can cause the left and right TMJs to move differently which may cause a crack or a sideways movement of the jaw. For the mouth to open symmetrically, both the left and right TMJs must be functioning the same. The actual TMJ comprises of a disc within the joint and may pinch causing sharp pain and restricting the movement of the jaw.

How we might treat this. Cranial osteopathic techniques can work to improve the normal movement of the bone structure[i][ii]. Soft tissue techniques can be applied to the muscles that open and close the mouth. If necessary exercises can be prescribed to increase the flexibility of jaw muscles.