The temporomandibular joint. These are the two joints that connect the jawbone to the skull under the zygomatic arch. TMJ syndrome involves pain in the joint, clicking in the joint from degradation of the sinovial fluids, and sharp, shooting pain when chewing. The two main causes are malocclusion (improper tooth alignment) and tension. Some people grind their teeth, others clench their jaws, perhaps from the inability to say what is felt. Chiropractors and osteopaths love helping these folks, some even specializing in TMJ work.
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