A4 Bruxism

Topic

Bruxism is a parafunctional activity of the masticatory muscles that results in the clenching and grinding of teeth without any functional purpose. Unlike normal chewing, this parafunctional activity serves no digestive or communicative function; instead, it exerts force on the teeth in a repetitive and involuntary manner.

Regarding the masticatory muscles, the electromyographic activity recorded during bruxism episodes exceeds 60% of the muscles' maximum voluntary contraction—far surpassing the 10% to 20% typically recorded during normal chewing. This heightened electromyographic activity in the masticatory muscles translates into the abnormally high forces that characterize bruxism.

The forces generated by bruxism can be three to ten times greater than those produced during normal chewing, reaching peaks of up to 1,500 newtons. These forces act on the teeth in two distinct ways, depending on whether clenching or grinding is occurring: clenching applies force statically, whereas grinding applies force accompanied by the sliding movement of one tooth surface against another.

Grinding—which involves sliding one tooth surface against another—exerts a sliding load on the enamel, causing abrasive wear. The volume of enamel lost to this wear increases with greater sliding loads and longer sliding distances, while it decreases when the enamel is harder and more resistant to scratching.

Both clenching and grinding apply forces repeatedly and cyclically to the tooth-supporting structures; this repetitive cyclic loading causes bone fatigue in the alveolar process and the temporomandibular joint. The greater the number of loading cycles experienced by the alveolar process and the temporomandibular joint, the more bone fatigue accumulates in these structures—even when no single force would be sufficient on its own to cause damage.

The combination of abrasive enamel wear and accumulated bone fatigue manifests clinically in several consequences of bruxism: microfractures appear in enamel weakened by wear; flexural stress concentrates at the cervical neck of the tooth, eventually causing abfraction in that area; and the temporomandibular joint, already fatigued by cyclic loading, becomes overloaded.

Taken together, these consequences demonstrate how the very parafunctional activity of the masticatory muscles that defines bruxism ultimately compromises the dental enamel, the bone supporting the tooth, and the temporomandibular joint. While muscle activity during normal chewing remains within moderate ranges of force and contraction, in bruxism it is sustained well above those levels; consequently, dental and periodontal structures suffer damage that does not occur during normal chewing.

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