Temporolmandibular Joint (TMJ) / Temporomandibular Disorder (TMD).  The temporomandibular joint is formed where the lower jaw bone articulates with the head.  TMD refers to a general class of disorders affecting the bones and muscles of the TMJ region.  Symptoms range from tenderness and swelling, to headaches and neck and back aches.  Generally, a clicking of popping sound when the jaw is opened or closed suggests some form of one of these disorders.