The lateral pterygoid muscle is a craniomandibular muscle that plays a crucial role in the craniofacial system. It is the key muscle of the inferior temporal region. The Lateral pterygoid muscle is active during mastication and during mandibular movements such as protrusion (forward movement of the mandible), abduction (depression of the mandible), mediotrusion (movement of the mandibular condyle towards the midline), and particularly during speaking, singing, and clenching. Among all the muscles of mastication, it is the only muscle with fibers arranged horizontally. Functionally, the muscle is analogous to the temporalis muscle. The fan-shaped temporalis muscle exhibits a continuous range of movements anteroposteriorly, while the lateral pterygoid muscle exhibits a continuous range of functional movements mediolaterally as well as superoinferiorly. The lateral pterygoid muscle has two heads or bellies. The inferior belly is three times larger than the superior belly.
The lateral pterygoid muscle has a triple function. Simultaneous contraction of the bilateral inferior belly of the lateral pterygoid muscle with the medial pterygoid muscle results in protrusion of the mandible. This action results from the anterior and medial alignment of the fibers of the muscle to the bilateral mandibular condyle. This bilateral contraction, when assisted by the suprahyoid and infrahyoid muscles, results in depression of the mandible (opening of the jaw). Among all the four muscles of mastication (medial pterygoid, lateral pterygoid, masseter, and temporalis), the lateral pterygoid is the only muscle that depresses the mandible. The action of all the other three muscles of mastication results in elevation of the mandible. The unilateral contraction of the lateral pterygoid muscle with the ipsilateral medial pterygoid muscle results in lateral mandibular movement to the contralateral side. This movement is observable during both functional and parafunctional lateral excursive movement of the mandible, i.e., during chewing stroke as in mastication and clenching, respectively.
Unlike the jaw-closing muscles, this jaw depressor does not contain muscle spindles. Due to the absence of muscle spindles, the lateral pterygoid muscle plays a secondary role during mandibular depression. Muscle spindles (stretch receptors) are required to detect any change in the working length and velocity of the muscle. During functional movements, this prevents excessive stretching of the muscle.
The lateral pterygoid muscle plays a crucial role in controlling the function of the jaw and temporomandibular joint. It also appears to function during the generation of horizontal forces required during mastication and parafunctional activities. There have been suggestions that during clenching, some fibers of the superior head of the lateral pterygoid muscle become active; this prevents the condyle of mandible from being displaced posteriorly, thus, avoiding the exertion of pressure on the sensitive postcondylar structures.
The entire masticatory musculature originates from the mesoderm of the first branchial arch. Hence, the lateral pterygoid muscle also develops from the mandibular arch. During the third month of intrauterine life, the muscle gets inserted into the mesenchyme, which condenses around the developing condyle of the mandible. However, a part of the tendon of the muscle sweeps above the condyle and inserts into the portion of the Meckel's cartilage (cartilaginous bar of the mandibular arch) that later on forms the head of the malleus. A part of the tendon also gets inserted into the articular disc of the temporomandibular joint.
The lateral pterygoid muscle receives nerve supply from the pterygoid branch of the second part of the maxillary artery.
The lateral pterygoid muscle receives innervation from the mandibular branch of cranial nerve V (trigeminal nerve). The main trunk of the mandibular nerve divides into the anterior and the posterior division. The anterior division gives off three motor branches. One of the motor branches is the nerve to the lateral pterygoid, which is a paired nerve (one for each belly) innervating the lateral pterygoid muscle. According to some authors, this muscle may also receive innervation from the anterior, middle, and posterior deep temporal nerves, masseteric nerve, and sometimes from the buccal nerve. They innervate different parts of the muscle resulting in selective activation, which ensures coordination of movements and reduces stress on the temporomandibular joint.
The lateral pterygoid muscle has been described classically as a biceps muscle as its fibers have their origin from two muscle fascicules. Hence, the muscle normally has two bellies or heads, namely superior and inferior. The origin of the superior belly is from the infratemporal surface, and the crest of the greater wing of the sphenoid bone, whereas the origin of the inferior belly is from the lateral surface of the lateral pterygoid plate. The insertion of the superior belly is into the anteromedial aspect of the temporomandibular capsule and its disc. In contrast, the inferior belly inserts at the neck of the condyle of the mandible and pterygoid fovea (anterior fovea). The two bellies become separated via a horizontal septum of connective tissue that is approximately 10mm thick proximally. Each belly is surrounded by its own fascia, which joins about 10mm in front of the temporomandibular joint. The inferior belly is three times larger than the superior belly.
The inferior belly functions to depress the mandible (open the mouth), protrude the mandible, and helps in sideward movement. It is inactive during the closing of the mouth. On the other hand, the superior belly of the lateral pterygoid muscle contracts when the mouth closes, and the teeth get clenched. During dynamic testing, the timing of the contraction of fibers of the superior belly has been seen to coincide with the activity of the temporalis and masseter muscles. However, it is involved mostly during the terminal closing movement of the mandible, thereby stabilizing the joint by directing the forces exerted during mastication on to the articular eminence.
On the lateral side (superficial surface), this muscle is related to the ramus of mandible, maxillary artery, tendon of the temporalis, and the masseter muscle. On the medial side (deep surface), the muscle is related to the mandibular nerve, the middle meningeal artery, the sphenomandibular ligament, and the upper part of the medial pterygoid muscle. Deep temporal and masseteric nerves and vessels emerge from the upper border of the superior belly. The lingual and inferior alveolar nerve and vessels pass beneath the lower border of the inferior belly, and the second part of the maxillary artery runs between the two bellies to enter the pterygomaxillary fissure.
The lateral pterygoid muscle is generally two-headed (95.5%). However, the literature also describes variations in the form of a single-headed muscle(2.5%) and a three-headed muscle (2.5%). The three-headed variant of the muscle consists of the superior, medial, and inferior head.
The lateral pterygoid muscle is located deeply in the inferior temporal fossa. The inaccessibility of this region and its surrounding tissue makes anatomical dissection of the muscle very difficult.
The spasm of the lateral pterygoid muscle can be very painful and can result in trismus (locked jaw). The patient may require analgesics or muscle relaxants for treating the same. Some authors believe that during temporomandibular disorders, this muscle plays an important role. This involvement may be due to the absence of coordination between the superior and the inferior bellies of the muscle. This lack of coordination leads to disturbance in the horizontal positioning of the intra-articular disc relative to the condyle. Clinical examination of patients with such disorders reveals pain in the region of this muscle during jaw movements and with palpation behind the tuberosity region. In the internal derangement of the temporomandibular joint, there are implications that the superior belly of lateral pterygoid muscle plays a role in causing anterior dislocation of the disk. The prolonged contraction of the muscle places forward traction on the disk, resulting in anterior displacement of the disk.
Clinical significance in Prosthodontics: Long term completely edentulous patients wearing complete dentures with worn-out occlusal surfaces of the artificial teeth tend to position their mandible in the forward and lateral position. The dentist may notice different anteroposterior/lateral teeth contact positions during subsequent appointments, which poses difficulty in fabricating a new complete denture, particularly during the recording of the maxillomandibular relations. There have been suggestions that this variation in the position of the mandible during resting jaw posture is a result of an attempt to attain a reasonable bite with worn-out dentures. Some authors believe that both the lower as well as upper belly of lateral pterygoid muscle play a significant role in maintaining the mandible at this new anterior position. The variations in the levels of activity between the two bellies on each side determine the lateral and anteroposterior positioning of the jaw.
The origin of both the bellies of the lateral pterygoid muscle is medial to their insertions. Thus, the wide opening of the mandible may temporarily result in the mandible undergoing distortion in the transverse plane. Therefore, during impression making of the lower arch, if the mouth is widely open, it may result in an impression that is not accurate in the transverse dimension due to mandibular flexion.
Some parafunctional movements of the jaw are characterized by protrusion and/or side-to-side movements of the mandible. Such movements are associated with large jaw-closing muscle activity, necessitating sizeable horizontal force vectors to overcome the frictional resistance between the teeth. The inferior belly of the lateral pterygoid muscle and the superior belly play a vital role in the generation of these horizontal forces.
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