The temporomandibular joint (TMJ) connects the condyle of the lower jaw (mandible) to the temporal bone of the skull in front of the ear. Within the joint there is a disc made of cartilage.
Temporomandibular dysfunction (TMD) is a group of disorders caused by damage to the jaw joints, the muscles controlling the lower jaw and several other related structures.
Although no medical cause of TMD has been identified yet, it has been related with teeth grinding and clenching, stress, jaw injury and arthritis.

The typical TMD symptoms are: radiating pain in the face, jaw or neck, popping/clicking sounds during jaw movements, pain or discomfort when chewing yawning or talking, limited movement or jaw locking (inability to open the mouth widely), changes in the bite, headaches, ringing in the ear (tinnitus) and overall fatigue in the face/head area. These symptoms can be temporary or become a significant, long-term problem for the patient.

The most common treatment options involve self-care instructions (i.e. jaw relaxing exercises), medications and stabilization splints. The latter consists of customized intra-oral appliances (occlusal guards) to help your lower jaw function in a more proper relationship with the upper.
They need regular check-ups and adjustments by the prosthodontist.