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Temporomandibular Disorder (TMD) is a broad term that encompasses disorders of the temporomandibular joint and its associated anatomical structures. The disorder may be intra-articular, due to inflammation, internal structural changes (internal derangement) or degeneration, or it may be extra- articular due to imbalance or over-activity of the jaw muscles, commonly the muscles of mastication or the cervical muscles. There is a strong correlation between postural dysfunction of the cervical spine and TMD. There are numerous other conditions that can cause pain in the TMJ region. It is important to make an accurate diagnosis to ensure that the correct treatment is given and that potentially serious problems are not overlooked.

In an early study on postmortem specimens[1], it found that the lateral pterygoid muscle was connected to the medial aspect of the joint capsule, meniscus and the condyle pterygoid fovea in more than half of the specimens. It indicated that the muscle might have a specific contribution to the TMD.

Please see the page on Muscles of Mastication for further relevant anatomy.

1. Inflammatory conditions within the joint are often caused by direct trauma, such as a blow to the chin or jaw, indirect trauma, such as a whiplash injury, heavy chewing, grinding (bruxism), clenching of the jaw or loss of dental height due to worn down or missing teeth.

• Synovitis - The synovium or the capsule may be inflamed. There is often pain at rest and limited range or pain at the end of range.
• Retrodiscitis - The retrodiscal tissue (the posterior attachment of the articular disc to the mandibular fossa) is highly vascular and innervated and if inflamed, can cause severe pain. The jaw may deviate away from the painful side at rest and with opening.

2. Internal derangement describes conditions where there are structural changes within the joint. This can be caused by direct trauma, such as a blow to the jaw or falling on the chin, indirect tauma, such as a whiplash injury, long term clenching or grinding, heavy or hard chewing or prolonged periods of mouth opening, such as a dental procedure or a general anaesthetic.

• Disc displacement with reduction – The articular disc can become displaced in any direction, but will most commonly displace anteriorly. The disc will be pushed forward during opening and will bunch up. At a certain point in range the disc will reposition or reduce itself causing an audible or palpable click. The jaw will often deviate towards the affected side.
• Disc displacement without reduction – In this more severe version the disc will not reduce causing pain and a loss of range. This is called closed lock. The jaw will often deviate towards the affected side. There will be no click but the patient may report that there was a click at the time when their jaw locked.

3. Arthritis

• Degenerative Arthritis can occur in the TMJ. It can often be seen on plain x-ray or OPG as a flattening of the condylar head, often with some osteophytic formation MRI gives more information with views done in open and closed positions. This shows the position of the joint and disc at the start and end of range. Crepitus can often be felt or can be heard with a stethoscope. It can be age related degeneration, usually seen in the over 50s, or secondary to trauma occurring at a younger age.

• Inflammatory Joint Diseases can affect the TMJ, including rheumatoid arthritis, ankylosing spondylitis, infectious arthritis, Reiter syndrome and gout.

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4. Hyper mobility can result in excessive anterior movement of the jaw and the articular disc. This will result in deviation of the jaw away from the affected side. There are usually some clicking sounds in the TMJ and there may or may not be pain. Hyper mobility may be related to connective tissue disorders such as Marfan syndrome or conditions such as Down Syndrome and Cerebral Palsy. Long term hyper mobility can cause the articular disc to elongate and degenerate. The disc can then fail to reduce on closing, causing the TMJ to become stuck in an open position (Open Lock). This can often occur after opening the mouth to an extreme position, such as when singing or yawning or after a prolonged dental procedure.

1. Muscle Spasm can cause significant pain and limitation of movement of the jaw. This is referred to as trismus. It often affects one or more muscles, commonly the muscles of mastication, especially masseter, temporalis and the pterygoid muscles. Causes include prolonged dental procedures or anaesthetics where the mouth has been held open for extended periods of time, stress, bruxism and postural dysfunction.

2. Cervical Postural Disorders can cause jaw pain. The anterior belly of the digastric muscle runs from the point of the chin to hyoid bone. This attachment means that when the head is protracted forward digastric will exert a posterior force on the mandible. With prolonged cervical protraction as occurs with poor posture or stress-related posture the mandibular condyle is pushed back against the retrodiscal tissue, eventually causing swelling, pain and gradual degeneration of the disc.

3. Temporal Tendonopathy is caused by excessive contraction of the temporalis muscle usually as a result of bruxism. There is tenderness and swelling of the anterior portion of the temporalis tendon palpable just above the zygomatic arch. There may also be tenderness of the temporalis tendon where it inserts onto the coronoid process, palpable just below the zygomatic arch when the jaw is slightly open.

4. Fractures of the mandible often occur at the mandibular symphysis or the condylar neck. Commonly there will be a fracture of the mandibular symphysis combined with a fracture/dislocation of one or both condyles. The mechanism of injury can be a blow to the jaw or a fall onto the chin. Treatment can usually begin within a week or two of surgery to begin early mobilisation of the TMJ and to restore function.

There are different clinical protocols used to establish TMD diagnoses but the Research Diagnostic Criteria for TMD (RDC/TMD)[3] could promote the level of consistency between in research studies [4], it may also be practical in clinical usage.