Is the cause of my tmj mostly due to cartilage/bone wear (from chewing gum, etc) or from tired muscles?

Many causes: A thorough history and exam are needed to hopefully determine the triggers for your problem. Chewing gum, grinding, clenching, etc. Can be contributing factors.

TMJ: Your muscles may be in spasm from grinding or clenching your teeth. Or the cartilage that should be located between your upper jaw ( temporal bone) and the condyle of the lower jaw is out of position, blocking normal motion, causing the condyle to push on the nerves at the back of the tm joint . See a dentist that treats TMJ for a definitive diagnosis which will determine proper treatment.

TMJ: The cause is not well understood and disputed among dentists. For a great resource on the subject see: http://www.Nidcr.Nih.Gov/nidcr2.Nih.Gov/templates/commonpage.Aspx?Nrmode=published;nrnodeguid=%7b53fd7a9d-b819-4ce4-8afb-65bbd352bd26%7d;nroriginalurl=%2foralhealth%2ftopics%2ftmj%2ftmjdisorders%2ehtm;nrcachehint=guest#causes.

TMJ pain: The most common cause of TMJ disfunction/pain is bruxism (chronic teeth grinding and/or clenching). Other common causes are a bad bite and injury to the joint. See your dentist, or a dentist who specializes in tmd, for an examination, diagnosis and treatment recommendations.

TMJ-TMD: TMJ disorder is the second most frequent cause of orofacial pain after dental pain. The common symptom are: headache, jaw pain, neck pain, eye pain, earache. Proper diagnosis is the key for the successful treatment. This disorder will not resolve on its own with home care. Rec.: orofacial pain specialist for evaluation and treatment.

List: Causes of TMJ syndrome are not completely understood.. Causes may include, misalignment (malocclusion) of or trauma to the teeth or jaw, teeth grinding (bruxism),, poor posture, stress or anxiety, arthritis or other inflammatory musculoskeletal disorders, excessive gum chewing. Treatment can be complex and beyond the scope of this answer. See a TMJ expert for guidance.