There is still some confusion regarding the use of the acronyms TMJ and TMD. TMJ is an acronym for Temporo-Mandibular Joint. Everyone has a TMJ; this is an anatomical structure and does not denote pathology. On the other hand, TMD is an acronym for Temporo-Mandibular Disorder, meaning pathology. Therefore, the correct term for pathology of the TMJ, its supporting and surrounding structures, and muscles is TMD. TMD is a collection of diseases/disorders (rather than one disorder with a single cause) with a number of causative factors. Treatment for TMD is usually symptomatic and palliative because it is often not possible to identify the cause for the individual patient. A cure for TMD is not usually possible. Rather, TMD patients deal with their symptoms long term, i.e., typically, no one treatment can resolve symptoms completely. Further, the symptoms of TMD are "fluctuatant" and cyclic, where symptoms appear and reappear.

The cause of TMD is "multi-factorial" (a number of causes). Some of the causes are: clenching and bruxing of the teeth due to stress or habits, trauma, developmental and degenerative conditions and changes, dental/facial structural abnormalities, sometimes the occlusion ("bite"), and so forth.

Some practitioners have included joint sounds as a diagnosis of TMD. However, the American Dental Association (ADA), and the American Association of Orthodontists (AAO) do not consider joint sounds, "popping", clicking, grinding, and crepitus per se as TMD. TMD is limited to pain, and/or dysfunction. Pain may be related to various causes such as trauma, degeneration of the joint, tears of the TMJ meniscus, and so on. Pain is somewhat easy to identify, but dysfunction is not, and it includes, but not limited to, decreased range of opening the mouth. This maximum opening should be 30 mm or greater. Or, dysfunction may be a deviated path of opening, limitations of lateral excursions or movements of the mandible, or a subluxation of the TMJ.

QUESTIONS REGARDING TMD? 

Is orthodontic treatment associated with TMD?
Can orthodontic treatment prevent or minimize future development of TMD?
Generally, the literature demonstrates that orthodontic treatment plays a neutral role in regards to TMD. It does not help nor does it hurt the TMJ.

Does orthodontic treatment harm the TMJ?
No. Orthodontic treatment does not harm the TMJ.
(Gianelly AA. Orthodontics, condylar position, and TMJ status. Am J Orthod Dentofacial Orthop1989; June )

No, using an exhaustive search of 960 articles, 31 articles met the inclusion criteria. The data in this meta-analysis do not indicate that traditional orthodontic treatment increases the prevalence of TMD. (Kim MR, Graber TM, Viana MA.Orthodontics and temporomandibular disorder. Am J Orthod Dentofacial Orthop 2002;121(5):438-46.)

Does orthodontic treatment help the TMJ?
At this time, there does not appear to be any scientific basis for believing that malocclusion ("bad bite") per se is a predisposing factor for the development of TMD. Therefore, it is difficult to defend the widely believed myth that orthodontic treatment will prevent or minimize later development of TMD.
(American Academy of Pediatric Dentistry. Treatment of temporo- mandibular disorders in children: Summary statements and recommendations. J Amer Dent Assoc 1990; 120:265-269.)

Presently there are no studies that suggest that orthodontic or restorative treatment either prevents or promotes the development of symptoms of TMD.
(Tallents R, Catania J, Sommers E. Temporomandibular joint findings in pediatric populations and young adults: A critical review. Angle Orthodontist 1991; 61(1)7-15.)