Lockjaw is essentially a disorder that prevents the mouth from opening properly, fully, or to otherwise function improperly.
One of the most famous causes of this disorder is having an infection known as tetanus which is potentially lethal. Tetanus has a few stages of infection and lockjaw occurs later, indicative of a long period of infection. While this one cause may be famous it is not the most common by a longshot.
Many people who develop lockjaw actually have oral or mandible (jaw) related illnesses and problems that result in the disorder. Inflammation of the soft tissue in the mouth is one of the leading causes of lockjaw. Many things can cause the soft tissue in the mouth to be inflamed such as over mastication (too much chewing or use of the mouth) or disorders of the joints in the jaw such as Temporomandibular joint disorder (TMJ or TMJD).
Oral diseases, fevers, or other illness that have symptoms in the mouth can also cause lockjaw to develop. One of the reasons for this is because the muscles in the jaw become worn and exhausted from both tissue swelling and weakness; as a result your muscles can’t open your mouth all the way resulting in this disorder.
Having oral surgery performed is another leading cause of this disorder. It is more common in people who have their wisdom teeth removed; however, over a period of 1 to 2 weeks the problem typically and progressively resolves itself.