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Jaws work by moving in opposition to each other and are applied for biting, chewing, and handling food.
The mandible or lower jaw consists of a horizontal arch, which contains the teeth and includes blood vessels, and nerves. There are two vertical parts (rami) that produce movable hinge joints on either side of the head. This links with the glenoid cavity of the temporal bone of the skull.
The rami also produces attachment for muscles necessary in chewing. The center front of the arch is reinforced and buttressed to form a chin, a development unique to humans.
The upper jaw or maxilla is firmly and securely fastened to the nasal bones at the bridge of the nose, eye socket, to the roof of the mouth, and to the zygomatic bone (cheekbone). The arched lower portion of the maxilla holds the upper teeth. The internal part of the bone is called the large maxillary sinus.
In the human fetus and infant, both the upper and lower jaws are composed of two halves. These fuse at the midline a few months after birth.
The Muscles in the Jaw
The jaw muscles affect the jaw in a combined three-dimensional manner during movements. There are two jaw-opening muscles (digastric and lateral pterygoid) and three jaw-closing muscles (temporalis, masseter, and medial pterygoid).
The primary functional system of muscle is the motor unit. The internal structure of the jaw muscles is complex, with many complicated pennate (feather-like) internal architecture. Within the jaw muscles are compartments with specific directions of muscle fibers.
This indicates that all jaw muscle is capable of generating a variety of force vectors (magnitude and direction) needed for a particular jaw movement.
In the formation of any desired movement, the central nervous system (CNS) activates motor units in different muscles. Movements are categorized as voluntary, reflex, and rhythmical. Various parts of the CNS participate in the production of jaw movements.
The face motor cortex is the last producing pathway from the cerebral cortex. It is designed to produce voluntary movements, such as opening, closing, protrusive, and lateral jaw movements.
Reflexes show pathways that assist in the refinement of a movement and can be used by the higher motor centers for complex movements.
Mastication or chewing is a rhythmical action that is managed by a central pattern generator in the brainstem. The central pattern generator can be altered by sensory information from the food bolus and by voluntary CNS commands.
The muscles that close the jaw are much more robust than the ones that open it.
Closing is generated by three large muscles on each side: The temporalis, medial pterygoid, and the masseter.
The opening is generated by the digastric and lateral pterygoid muscle and by some smaller muscles below the mandible.
The digastric is a small joined muscle placed in the anterior compartment of the neck. It belongs to a set of muscles called the suprahyoid muscles.
Besides the digastric, this set also comprised the mylohyoid, geniohyoid, and stylohyoid muscles. As the name suprahyoid indicates, these muscles are perceived as superior to the hyoid bone, and together with the adjoining tissue, they form the floor of the mouth.
Added hint regarding the digastric muscle is found in its own name. It originated from the Greek word “dis” indicating double or twofold and Latin “gaster” meaning belly. This means that the structure of this muscle has two muscle bellies.
Digastric has two main roles:
- Depression of the mandible when the hyoid bone is set.
- Elevation of the hyoid bone and larynx when the mandible is set.
By depressing the mandible, the digastric muscle helps the act of opening the mouth, which is needed when acting against resistance. This process of the digastric muscle, together with the rest of the suprahyoid muscles, improves the action of chewing, swallowing, and speech.
This explains why the suprahyoid muscles are sometimes attributed to as accessory muscles of mastication.
Conversely, when the mandible is fixed, the digastric muscle raises the hyoid bone and the larynx. The superior motion of the hyoid and larynx results in lessening or closing of the epiglottis, blocking the entrance to the trachea. Thus, it limits the inhalation of food or liquid during swallowing.
The digastric muscle is composed of two parts: Anterior and posterior bellies.
It is united by an intermediate tendon. Moreover, each of the digastric muscle bellies has a unique point of source.
- The posterior belly starts at the medial surface of the mastoid notch of the temporal bone. From here it moves anteroinferior towards the hyoid bone, entering the stylohyoid muscle before joining the intermediate tendon of the digastric muscle.
- The anterior belly of the digastric muscle starts from the digastric fossa of the lower border of the mandible. It will then approach the midline near the mandibular symphysis (symphysis menti). This piece of the muscle stretches posteroinferiorly from the mandible, connecting with the intermediate tendon
Above the hyoid bone, these two muscle bellies consolidate as the intermediate tendon.
The intermediate tendon of the digastric muscle is surrounded by a U-shaped fibrous tissue sling, created by a thickening of the investing layer of the deep cervical fascia. This sling is fastened on the superior side of the body of the hyoid bone.
It works as a pulley, enabling the intermediate tendon to slide anteriorly and posteriorly.
Lateral pterygoid muscle
The lateral pterygoid is a two-headed, fan-shaped muscle found in the infratemporal fossa of the skull. It is one of the four masticatory muscles, along with the temporalis, masseter muscles, and medial pterygoid.
All these muscles work upon the temporomandibular joint (TMJ) to allow chewing (mastication) and biting. The lateral pterygoid provides this purpose by protruding and depressing the mandible when engaging bilaterally. The same thing goes when turning the mandible when contracting unilaterally.
The lateral pterygoid is established deep to the temporalis and masseter muscles, crossing between the sphenoid bone and temporomandibular joint. Its muscle belly is divided by a small horizontal gap into two heads: Superior (upper) and inferior (lower).
The superior head is made by the most superomedial fibers of the muscle. It consists of muscular slips that are located from the infratemporal crest of the greater wing of the sphenoid bone.
The inferior head is much broader than the superior one. It starts from the lateral surface of the lateral pterygoid plate of the sphenoid bone.
Fibers from both heads meet to course posterolaterally, in a predominately straight plane.
The superior fibers enter into the anteromedial portion of the articular capsule and articular disc of the temporomandibular joint. Meanwhile, the inferior fibers enter into the pterygoid fovea on the neck of the condyloid process of the mandible.
The above attachment onto the temporomandibular joint (TMJ) allows the muscle head to work on the superior compartment of the joint and to create the gliding motions of the disc and mandibular condyle. The inferior head works on the interior compartment of the TMJ, producing a hinge-like turn that happens between the mandibular condyle and the inferior surface of the articular disc.
Being a masticatory muscle, the lateral pterygoid helps in chewing and biting movements by controlling the actions of the mandible. The sphenoid attachment of the muscle is always secured, indicating that the direction of pull is oriented towards it.
Respective compression of the left and right lateral pterygoid muscles results in rendering and rotation within both temporomandibular joints. The inferior heads stretch the mandibular condyle anteriorly, which occurs in rotation of the condyle against the inferior surface of the articular disc. At the same time, the superior heads stretch the articular capsule and disc in the same anteriorly to cushion the movements of the condyle.
The effect is the anterior translation of the disc and condyle, occurring concurrently with the turn of the condyle. This is the protrusion and depression of the mandible.
Alternate compression of the digastric and geniohyoid muscles settles the movement of opening the jaw. The retrodiscal fat pad within the TMJ expands and restricts anterior joint translation during protrusion and depression. Its natural elasticity creates the force that initiates the closing of the mouth.
In jaw closing, the identical translation and rotation movements happen within the TMJ, they just occur in the opposite direction.
Here, the inferior heads of the muscle eccentrically compress to smoothen the posterior translation of the articular disc and mandibular condyle. Thus, it offsets the net pull of temporal and masseter muscles which bring the mandible posteriorly.
Side-to-side actions of the jaw occur when the inferior head contracts unilaterally, rotating the mandibular condyle anteromedially.
The movement happens in synergy with the compression of the ipsilateral medial pterygoid muscle. This combined muscle activity turns the jaw to the opposite side and is seen, for example, when crushing food between the teeth on one side of the mouth.
The masseter muscle is one of four muscles that works during mastication. It’s primary function is to close the jaw in connection with two other jaw-closing muscles, the temporalis and medial pterygoid muscles.
The incidence and timing of movement in the jaw-closing muscles are apparent when speaking. Various degrees of activity between muscles can be observed when repeating the same syllables.
These data are considered concerning the group of muscle coordination.
The temporalis muscle is a small, fan-shaped muscle located within the temporal fossa of the skull. Along with the lateral pterygoid, medial pterygoid, and masseter muscles, it belongs to the set of masticatory muscles.
The temporalis muscle moves partially, from the temporal bone to the coronoid process of the mandible. The main role of this muscle is to create the movements of the mandible at the temporomandibular joint. Thus, promoting mastication.
Its anterior part moves the mandible dorso-cranially (elevation), while its posterior fibers pull the mandible posteriorly (retrusion).
The temporalis muscle is a large muscle that fills most of the temporal fossa. Its entry point crosses the entire surface of the fossa below the temporal line. Additionally, some fibers start from the temporal fascia.
The temporalis muscle is classified into the anterior and posterior sections. The anterior fibers work inferiorly, in an almost upright direction, while its posterior fibers are steered almost horizontally.
Both anterior and posterior fibers meet onto a tight tendon that runs medial to the zygomatic arch. The tendon enters onto the apex and medial surface of the coronoid process of the mandible.
The temporalis muscle is the most solid muscle of the temporomandibular joint. So much so that it’s the main retractor of the mandible.
The compression of the posterior fibers of the temporalis muscle ends in the backward motion of the mandible (retrusion). The contraction of its anterior fibers affects the mandible dorso-cranially (elevation). In unison, these activities help the closing of the mouth and the approximation of the teeth.
Additionally, the unilateral compression of the temporalis muscle plays an essential role in the side-to-side movements of the jaw.
The masseter muscle is a paired, firm, thick, and rectangular muscle that is starting from the zygomatic arch and continues down to the mandibular angle. It is composed of a superficial and a deep part.
It is one of the masticatory muscles, a set of muscles that also involves the temporal muscle, lateral pterygoid muscle, and medial pterygoid muscle. Its distinct roles are elevation and protrusion of the mandible, as well as supporting the articular capsule of the temporomandibular joint.
The superficial part of the muscle that extends over its deep part starts from the maxillary process of the zygomatic bone. It then runs anteroposteriorly to the masseteric tuberosity located on the lower portion of the superficial side of the ramus of the mandible.
The deep part consists of vertically organized muscle fibers. This part arises from the entire length of the inferior border of the zygomatic arch and enters onto the superficial side of the ramus of the mandible. As such, the muscle can be quickly palpated from the oral cavity along the cheek.
Furthermore, some of the deep fibers expand into the anterior capsule and articular disc of the temporomandibular joint.
It raises the mandible causing a strong jaw closure. The compression of the superior part, which goes diagonally to the front, moves the mandible forward (protrusion). Thus, the muscle maintains the tension of the articular capsule of the temporomandibular joint.
Medial Pterygoid Muscle
The medial pterygoid is a thick quadrilateral muscle that attaches the mandible with the maxilla, sphenoid and palatine bones. It belongs to the set of masticatory muscles, along with the lateral pterygoid, masseter, and temporal muscles.
The medial pterygoid muscle consists of two heads: Superficial and deep.
Although having distinct origins, each head inserts on the inner surface of the mandible, producing an axis for a strong pull of the bone. One-sided compression of the medial pterygoid causes the turn of the mandible, while bilateral compression elevates and protrudes it. In synergy with other masticatory muscles, these movements help in chewing.
The superficial and deep medial pterygoid muscles are divided by the inferior head of lateral pterygoid muscle at their source.
- The deep portion produces the bulk of the muscle and starts from the medial surface of the lateral pterygoid plate of the sphenoid bone.
- The smaller, superficial portion starts from the maxillary tuberosity and the grooved surface of the pyramidal process of the palatine bone.
As these two sets of fibers slide posterolaterally, they enter the triangular impression found on the medial surface of the ramus and the angle of the mandible. The fibers fasten via a secure tendinous lamina that extends from the mandibular foramen to the mylohyoid groove anteroinferior.
Due to its posteroinferior course, and the fact that the mandibular attachment is always mobile, the medial pterygoid muscle can display several actions:
- Unilateral contraction slightly medially turns the mandible. When this movement happens simultaneously with the contraction of the ipsilateral lateral pterygoid, it ends in a noticeable movement. And that is the mandible pivots anteriorly and medially. Also, alternating compression of the medial pterygoid and lateral pterygoids creates side-to-side movements of the mandible.
- Bilateral contraction raises the mandible. This movement, when coupled with the bilateral contraction of the lateral pterygoids, protrudes the mandible.
The medial pterygoid normally works together with other masticatory muscles. Therefore, its movements combined with those of other masticators helps with chewing and grinding food grinding between the maxillary and mandibular teeth.
Jaw Muscle Disorders
Jaw muscle disorders are distinguished by pain that is normally aggravated by function. They are seen in 45% of patients, mostly women, with temporomandibular disorders (TMDs).
Various contributing factors can cause jaw muscle disorders. Unfortunately, research and studies cannot fully determine potential risk factors.
The masseter and medial pterygoid muscles serve mainly as sources of bite force, whereas the temporalis and lateral pterygoid muscles are necessary for jaw movements and durability. Overuse, in terms of sustained movement and high-level compressions without rest periods, is connected with raised intramuscular pressure. This can cause local ischemia, developed cell membrane permeability, edema, and cellular damage.
Muscle pain is commonly described as a constant deep dull ache, tightness, or pressure. The onset is usually gradual and may vary from a feeling of tiredness to severe sharp pain.
The pain could emerge from trauma, sustained or vigorous contractions, and stretching of ischemia. But, it may also be due to other structures like the temporomandibular joints.
Local situations, such as inflammation, activated pain receptors. Hence, you feel the pain.
A thorough evaluation of the jaw muscles involves a systematic history and clinical examination. The clinical research has two main purposes: To assess jaw function and to possibly make controlled and regulated prompting of the patient’s pain.
The treatment of jaw muscle disorders is aimed toward reducing pain and improving function. It should generally be reversible, evidence-based, or based on well-established clinical practice.
Jaw Joint and Muscle Strain/Sprain: A Common Complication From Dental Care
Jaw joint and muscle strain or sprain can usually happen after lengthy dental procedures. This includes needing to open the mouth wide and other factors brought by dental care.
Procedures like crown preparation, mandibular blocks, periodontal, endodontic, dental extractions, etc., can end up in jaw joint and muscle strain. Additionally, a finger that rests continuously in the mouth puts forces on the jaw and creates hyperextension of the joint and strain of the muscles.
Severe jaw joint and muscle strain after dental procedures are more prevalent than formerly thought. So much so that more than 50% of patients with temporomandibular disorders (TMD) describe their pain onset as a direct result of dental care.
In one research of 164 patients with TMD, trauma was the initiating agent in 50.6% of cases, with 61% of these cases coming from trauma and strain during a dental procedure. Among young adults, 23% of all jaw pain problems had their onset after third-molar removals.
Other reasons for jaw trauma incorporate motor vehicle accidents, intubation during general anesthesia, yawning, blows to the jaw, or hard and continued chewing.
Jaw Joint and Muscle Strain/Sprain Treatment Technique
One of the usual non-infectious difficulties resulting from dental care is jaw joint and muscle strain/sprain.
Jaw joint and muscle sprain are identified by acute joint pain, limited range of movement, muscle tenderness, and muscle and joint dysfunction. If left untreated, these can proceed into a chronic condition.
Luckily, we’ll share with you treatment procedures that effectively handle 1st and 2nd-degree jaw strain.
The four procedures of the MEAT method for 1st and 2nd-degree jaw injuries include:
This involves self-care practice in the proper use of the jaw and avoiding risk factors that can hinder recovery. With 1st or 2nd degree injuries, a MEAT procedure and precise use of the jaw improves blood flow, promotes healing, and restores jaw function.
Risk factors such as jaw overuse, muscle bracing, and parafunction, should be withdrawn.
Recovery of normal ability can be accomplished with exercises that concentrate on restoring normal range of motion. Also, posture and relaxation activities lessen the strain on the muscles and support healing.
For pain cure, acetaminophen, anti-inflammatories, topical, or hydrotherapy (alternating heat and cold) lessen pain and aid movement, exercise, and healing.
For more severe pain, the Center for Disease Control has formed opioid prescribing guidelines and gives a comprehensive user guide. Failure to implement sufficient pain control can restrict the patients’ willingness to move and can have an adverse effect.
The QuickSplint is a quick anterior bite splint that can be installed on the same day at the chairside. This is to protect the jaw and is similar to a protective elastic bandage for ankle and wrist injuries.
The splint is worn at night and it can lessen jaw-closing muscle movement (e.g., jaw clenching or teeth grinding). It also hinders the maximum bite force, and helps to promote healing of acute temporomandibular pain and related limited jaw opening.
The flat occlusal surface enables the mandible to move loosely instead of closing into a particular dental relationship. It also promotes healing and normal function.
The benefits over regular lab-manufactured splints include the following:
- Cast placement (no impression)
- Custom fit
- Large enough to evade aspiration or swallowing
- Temporary (up to four weeks) to reduce occlusal changes
- Provide time to place a long-term splint.
Early Treatment Can Prevent Progression to a Chronic Condition
In case of jaw joint and muscle strain, it is necessary to take appropriate measures immediately. This is to encourage fast healing to resolve pain and decrease any risk factors that can delay recovery.
These actions must carry enough treatment of the injury and decrease complicating risk factors. Such factors include parafunction, muscle bracing, and jaw overuse.
Failure to give immediate treatment can result in a rise to a chronic pain condition.
If pain is kept untreated, peripheral and central sensitization happens due to the neuroplasticity of the nervous system. Local tissues become more fragile and susceptible to mechanical stimulation.
As the pain becomes chronic, added signs of sensitization including allodynia (sensitivity to normal touch) and hyperalgesia (more further pain with normal stimulation) may emerge.
First symptoms include headaches, neck pain, earaches, and jaw dysfunction (i.e.,clicking and locking).
Since oral and facial constructions are necessary to eating, communication, and hearing, and strongly influence appearance, self-esteem, and personal expression, chronic orofacial pain can influence the quality of life, functioning, emotional state, and dental care.
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