This classification, which effectively divides TMD into 2 syndromes, is followed by the American Academy of Orofacial Pain. However, since most people with TMD could be placed into both of these groups, which makes a single diagnosis difficult when this classification is used. The Research Diagnostic Criteria (RDC/TMD) allows for multiple diagnoses in an attempt to overcome the problems with other classifications. RDC/TMD considers temporomandibular disorders in 2 axes; axis I is the physical aspects, and axis II involves assessment of psychological status, mandibular function and TMD-related psychosocial disability. Axis I is further divided into 3 general groups. Group I are muscle disorders, group II are disc displacements and group III are joint disorders, although it is common for people with TMD to fit into more than one of these groups. By duration
Sometimes distinction is made between acute TMD, where symptoms last for less than 3 months, and chronic TMD, where symptoms last for more than 3 months. Not much is known about acute TMD since these individuals do not typically attend in secondary care (hospital). Signs and symptoms
Signs and symptoms of temporomandibular joint disorder vary in their presentation. The symptoms will usually involve more than one of the various components of the masticatory system, muscles, nerves, tendons, ligaments, bones, connective tissue, or the teeth.The three classically described, cardinal signs and symptoms of TMD are:
Pain and tenderness on palpation in the muscles of mastication, or of the joint itself (preauricular pain – pain felt just in front of the ear).
In addition to those already mentioned, examples include temporomandibular joint pain dysfunction syndrome, temporomandibular pain dysfunction syndrome, temporomandibular joint syndrome, temporomandibular dysfunction syndrome, temporomandibular dysfunction, temporomandibular disorder, temporomandibular syndrome, facial arthromyalgia, myofacial pain dysfunction syndrome, craniomandibular dysfunction (CMD), myofacial pain dysfunction, masticatory myalgia, mandibular dysfunction, and Costens syndrome. The lack of standardization in terms is not restricted to medical papers. Notable internationally recognized sources vary in both their preferred term, and their offered definition. For example:

"Temporomandibular Pain and Dysfunction Syndrome – Aching in the muscles of mastication, sometimes with an occasional brief severe pain on chewing, often associated with restricted jaw movement and clicking or popping sounds." (Classification of Chronic Pain, International Association for the Study of Pain). "Headache or facial pain attributed to temporomandibular joint disorder." (International Classification of Headache Disorders 2nd edition (ICHD-2), International Headache Society). "Temporomandibular joint-pain-dysfunction syndrome" listed in turn under "Temporomandibular joint disorders" (International Classification of Diseases 10th revision, World Health Organization). By cause and symptoms
It has been suggested that TMD may develop following physical trauma, particularly whiplash injury, although the evidence for this is not conclusive. This type of TMD is sometimes termed "posttraumatic TMD" (pTMD) to distinguish it from TMD of unknown cause, sometimes termed "idiopathic TMD" (iTMD). Sometimes muscle-related (myogenous) TMD (also termed myogenous TMD, or TMD secondary to myofascial pain and dysfunction) is distinguished from joint-related TMD (also termed arthogenous TMD, or TMD secondary to true articular disease), based upon whether the muscles of mastication or the TMJs themselves are predominantly involved.