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The jaw is a complex system of muscles, joints, and bones that work together to produce various movements, including opening and closing. Jaw muscles are susceptible to disorders and pain, which can cause discomfort and restrict movement. Disorders such as TMD, RA, bruxism, and infections can cause tightness, locking, and pain in the jaw. Understanding the underlying causes and treatments for jaw muscle disorders is essential for maintaining oral health and preventing further complications. In this context, it is important to know which muscles are responsible for unlocking the jaw.
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What You'll Learn
Jaw-closing and jaw-opening muscles
The jaw muscles are a group of muscles that work together in a complex manner to move the jaw in different directions, including opening and closing movements. The jaw muscles move the jaw in a complex three-dimensional manner during jaw movements. There are three jaw-closing muscles: masseter, temporalis, and medial pterygoid. The jaw-closing muscles have architectural features that suit them for force production. They are characterised by shorter sarcomere lengths at the closed jaw, larger masses of contractile and tendinous tissue, larger physiological cross-sectional areas, larger pennation angles, shorter fibre lengths, shorter moment arms, and lower fibre-length-to-muscle-length ratios.
There are two jaw-opening muscles: lateral pterygoid and digastric. The jaw-opening muscles are better designed to produce velocity and displacement. The length of the sarcomeres is a major determinant of both force and velocity, and the maximal work, force, and shortening range each muscle is capable of producing are proportional to the architectural parameters of volume, physiological cross-sectional area, and fibre length. The mechanical role the muscles play is strongly related to their three-dimensional position and orientation in the muscle-bone-joint system.
Many parts of the central nervous system (CNS) participate in the generation of jaw movements. The face motor cortex is the final output pathway from the cerebral cortex for the generation of voluntary movements, such as opening, closing, protrusive, and lateral jaw movements. Mastication or chewing is a rhythmical movement that is controlled by a central pattern generator in the brainstem. The central pattern generator can be modified by sensory information from the food bolus and by voluntary commands from higher centres.
Jaw muscle disorders can cause pain, weakness, stiffness, rigidity, or swelling, and a restriction of jaw opening. Muscle pain is the most common complaint in patients with jaw muscle disorders and is the most prevalent cause of chronic pain in the orofacial region. Masticatory myalgia is characterised as a dull persistent ache overlying the jaw and temple muscles, with occasional references to other structures such as the head, neck, ear, and teeth. Bruxism, or teeth grinding, can cause tightness or feelings of soreness in the face, neck, and upper or lower jaw, as well as headaches or earaches.
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Disorders and causes of jaw pain
Jaw pain is a common issue affecting millions in the US. It may be mild or severe, and can affect one's quality of life. The pain may be constant or occur spontaneously when the jaw is at rest or during chewing, stretching, contraction, or palpation.
Temporomandibular joint (TMJ) disorder or dysfunction (TMD) is the most common reason for jaw pain. TMD causes pain and tenderness in the jaw joints and surrounding muscles and ligaments. It can also cause locking in one or both hinge joints (temporomandibular joints). These joints are located between the lower jaw and the temporal bone. TMD can also cause an aching, throbbing, or tenderness in or near the ear, jaw, and face. It may also result in headaches, facial pain, shoulder or neck pain, stiffness in the jaw, difficulty opening or closing the mouth, jaw popping or clicking, and tinnitus. TMD is twice as common in women, and people between the ages of 20 and 40 are most likely to develop it.
Other common causes of jaw pain include:
- Teeth grinding (bruxism)
- Gum disease
- Toothache or an abscessed tooth
- Arthritis
- Infection, such as sinusitis or tetanus (lockjaw)
- Mumps
- Stress
- Chewing gum or any other substance in excess
- Rheumatoid arthritis (RA)
- Osteoarthritis (OA)
- Injury to the jaw
- Broken or dislocated jaw
- Heart attack
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Bruxism and teeth grinding
Bruxism, or teeth grinding, is a common condition, with a global prevalence of 22.22%. It involves excessive teeth grinding or jaw clenching and is unrelated to normal functions such as eating or talking. Bruxism can occur during sleep (nocturnal bruxism) or while awake (awake bruxism). Nocturnal bruxism tends to have worse symptoms upon waking, improving throughout the day, while symptoms of awake bruxism may not be present upon waking but worsen as the day progresses.
Bruxism is often related to stress or anxiety, and it can cause several symptoms, including aching jaw muscles, headaches, tooth wear, and damage to dental restorations. In some cases, bruxism can lead to tooth sensitivity, particularly to cold stimuli, as tooth wear reduces the insulating width of enamel and dentin, exposing the pulp of the tooth. Additionally, bruxism can cause or be related to temporomandibular joint dysfunction (TMD), resulting in pain and clicking in the jaw joint, typically felt in front of or inside the ear.
The masseter and temporalis muscle groups are primarily responsible for generating the clenching force in most cases of bruxism. However, some individuals may clench and grind only their front teeth, resulting in minimal involvement of these muscles. The excessive forces generated during bruxism can have detrimental effects on the teeth, periodontium, and the articulation of the mandible with the skull. Over time, teeth can wear down completely if left untreated.
While bruxism can lead to various issues, it does not always require treatment. However, dentists may recommend mouthguards or mouth splints to protect the teeth from damage. These devices are worn at night and can be custom-made by a dentist to fit precisely over the upper or lower teeth. Additionally, addressing stress and anxiety through relaxation techniques, such as breathing exercises, listening to music, or regular exercise, can help manage bruxism. Improving sleep habits, such as maintaining a consistent sleep schedule and creating a relaxing bedtime routine, may also be beneficial.
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Jaw movement and chewing
The human face has about 20 main facial muscles that are essential for chewing and making facial expressions. The jaw muscles work together in a complex manner to generate the force needed for various jaw actions. Jaw movement is three-dimensional, and the jaw can be described as rotating around an instantaneous centre of rotation.
The muscles involved in chewing (also known as muscles of mastication) include the lateral pterygoid, masseter, medial pterygoid, and temporalis. The lateral pterygoid is a fan-shaped muscle that helps the jaw open, while the masseter runs from the cheek to the side of the jaw and helps the jaw close. The medial pterygoid is a thick muscle that also helps the jaw close, and the temporalis is a fan-shaped muscle that assists in jaw closure. These muscles are innervated by the motor branches of the mandibular division of the trigeminal nerve (CNV3) and receive their arterial supply from the maxillary artery's branches.
Mastication or chewing is a rhythmic movement controlled by a central pattern generator in the brainstem. This generator can be modified by sensory information from the food bolus and voluntary commands from higher brain centres. Jaw movements can be classified as voluntary, reflex, or rhythmic. The face motor cortex is responsible for generating voluntary movements such as opening and closing the jaw.
Jaw muscle disorders can cause pain and discomfort. Muscle pain is the most common complaint in patients with jaw issues and can be chronic. It may be felt as tiredness or sharper pain and can be constant or occur during specific activities like chewing, stretching, or contraction. Other symptoms of jaw disorders include weakness, stiffness, rigidity, swelling, and restricted jaw opening. Bruxism, or teeth grinding, can cause tightness and soreness in the jaw and face, leading to headaches and earaches. Infections, such as tooth abscesses, can also affect jaw movement and cause lockjaw.
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Jaw muscle anatomy
The jaw muscles are a group of muscles that work together to move the jaw in various directions, including opening and closing movements. The jaw can be described as rotating around an instantaneous centre of rotation, and its movements are classified as voluntary, reflex, and rhythmical. The temporomandibular joint (TMJ) is a complex hinge joint that connects the lower jaw (mandible) to the skull. There are two TMJs, one on each side of the head, and they work together to allow for smooth movements like opening and closing the mouth, chewing, and speaking.
There are three jaw-closing muscles: masseter, temporalis, and medial pterygoid. The masseter muscle is a powerful muscle responsible for the elevation of the mandible, causing the mouth to close. It originates from the zygomatic arch and inserts along the angle and lateral surface of the mandibular ramus. The masseter is also one of the four muscles responsible for mastication (chewing). The temporalis muscle is the other muscle involved in mastication.
There are two jaw-opening muscles: lateral pterygoid and digastric. The pterygoids are examined by having the patient move the jaw from side to side against resistance and protrude the jaw. Contraction of each muscle causes deviation of the jaw to the opposite side.
Jaw muscle disorders can cause pain, weakness, stiffness, rigidity, swelling, and restricted jaw opening. Masticatory myofascial pain is characterised by a dull ache with trigger points that produce referred pain, which increases during function. Masticatory myalgia is characterised by a dull persistent ache overlying the jaw and temple muscles, with occasional references to other structures like the head, neck, ear, and teeth. Bruxism, or teeth grinding, can cause tightness or soreness in the face, neck, and jaw, as well as headaches and earaches.
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Frequently asked questions
The jaw-closing muscles are the masseter, temporalis, and medial pterygoid.
The jaw-opening muscles are the lateral pterygoid and digastric.
The jaw-closing and jaw-opening muscles work together to generate the force needed for various jaw actions, including opening and closing movements.
Some common issues with the jaw muscles include TMD (temporomandibular joint dysfunction), bruxism (teeth grinding), and infections such as tooth abscesses, which can lead to lockjaw.
