The pain may be felt over the angle of the jaw (masseter) or in the temple (temporalis), and may be described as a headache or an aching jaw. Most (but not all) bruxism includes clenching force provided by masseter and temporalis muscle groups; but some bruxers clench and grind front teeth only, which involves minimal action of the masseter and temporalis muscles. The temporomandibular joints themselves may also become painful, which is usually felt just in front of the ear, or inside the ear itself. Clicking of the jaw joint may also develop. The forces exerted on the teeth are more than the periodontal ligament is biologically designed to handle, and so inflammation may result. A tooth may become sore to bite on, and further, tooth wear may reduce the insulating width of enamel and dentin that protects the pulp of the tooth and result in hypersensitivity, e.g. to cold stimuli. The relationship of bruxism with temporomandibular joint dysfunction (TMD, or temporomandibular pain dysfunction syndrome) is debated. Many suggest that sleep bruxism can be a causative or contributory factor to pain symptoms in TMD. Indeed, the symptoms of TMD overlap with those of bruxism. Others suggest that there is no strong association between TMD and bruxism. A systematic review investigating the possible relationship concluded that when self-reported bruxism is used to diagnose bruxism, there is a positive association with TMD pain, and when stricter diagnostic criteria for bruxism are used, the association with TMD symptoms is much lower.
This typically involves the masseter muscle and the anterior portion of the temporalis (the large outer muscles that clench), and the lateral pterygoids, relatively small bilateral muscles that act together to perform sideways grinding. Multiple causes
The cause of bruxism is largely unknown, but it is generally accepted to have multiple possible causes. Bruxism is a parafunctional activity, but it is debated whether this represents a subconscious habit or is entirely involuntary. The relative importance of the various identified possible causative factors is also debated. Awake bruxism is thought to be usually semivoluntary, and often associated with stress caused by family responsibilities or work pressures. Some suggest that in children, bruxism may occasionally represent a response to earache or teething. Awake bruxism usually involves clenching (sometimes the term "awake clenching" is used instead of awake bruxism), but also possibly grinding, and is often associated with other semivoluntary oral habits such as cheek biting, nail biting, chewing on a pen or pencil absent mindedly, or tongue thrusting (where the tongue is pushed against the front teeth forcefully).There is evidence that sleep bruxism is caused by mechanisms related to the central nervous system, involving sleep arousal and neurotransmitter abnormalities. Underlying these factors may be psychosocial factors including daytime stress which is disrupting peaceful sleep. Sleep bruxism is mainly characterized by "rhythmic masticatory muscle activity" (RMMA) at a frequency of about once per second, and also with occasional tooth grinding. It has been shown that the majority (86%) of sleep bruxism episodes occur during periods of sleep arousal.