Mastication is defined as the process of chewing food in preparation for swallowing and digestion. Four pairs of muscles in the mandible make chewing movements possible.
These muscles along with accessory ones together are termed as ‘MUSCLES OF MASTICATION’.
These muscles can be divided into:
-Digastric muscle (anterior belly)
- The basic muscles of mastication develop from the mesenchyme of the first branchial arch.
- So they receive all their innervations from the mandibular branch of the trigeminal nerve, all from the anterior division except the medial pterygoid which gets its nerve supply from the main trunk.
- Also they originate from the same origin from temporal and infra-temporal fossa of the skull and are inserted in the mandible.
MOVEMENTS OF MANDIBLE
- Movements that the mandible can undergo are:
1. Depression: as in opening the mouth.
2. Elevation: as in closing the mouth.
3. Protraction: horizontal movement of the mandible anteriorly.
4. Retraction: horizontal movement of the mandible posteriorly.
5. Rotation: the anterior tip of the mandible is “slewed” from side to side.
- These movements of mandible are performed by various muscles involved in it. So, functionally, the muscles of mastication are classified as:
Upper head of lateral pterygoid
Lower head of lateral pterygoid
- It is the largest among all the masticatory muscles and is a fan shape muscle.
- Origin; from the inferior temporal line , floor of the temporal fossa and from the overlaying temporal fascia.
- Insertion; anterior and medial tip of the coroniod process.
- It has been divided into 2 heads:
– Deep head (anterior, middle and posterior fibers)
– Superficial head (much smaller)
– Elevation (anterior fibers)
– Retraction (posterior fibers)
- Nerve supply:
– Anterior division of the mandibular nerve
(by 2 deep temporal nerves)
- Its action is done by;
- The anterior fibers during function act vertically and elevate the mandible.
- The posterior fibers diverge and become horizontal and retract the mandible.
- Blood supply; from the maxillary artery (one of 2 termination of external carotid artery).
- It consist of 2 overlapping heads:
- The origin of the whole muscle is mainly from the zygomatic process, in which:
-The superficial head arises from the lower border of the zygomatic arch.
-The deep head arises from the inner surface of the zygomatic arch.
- Insertion of both the heads is into the outer surface of the ramus of the mandible.
- The superficial head passes downwards and backwards to insert into the lower half of the lateral surface of the ramus.
- While in the deep head, the fibers are more vertically oriented and inserted into the upper half of the lateral surface of the ramus.
- Action of masseter is mainly to elevate the mandible (antigravity action) and also helps in protrusive movement.
- It is the main muscle involved in the elevation of the mandible
- Nerve supply: by the mandibular branch of the trigeminal nerve, from the anterior division(massetric nerve).
- Blood supply is from the maxillary artery which is a terminal branch from external carotid artery.
- One of the interesting properties of this muscle is that, internally, the muscle has many tendinous septa that greatly increase the area for muscle attachment and so increase its power.
- Masseteric Hypertrophy was first described by Legg in 1880.
- A hypertrophied muscle will alter facial symmetry, generating discomfort and negative cosmetic impact in many patients.
- It may also produce functional alterations like bruxism, mandibular prognathism and trismus.
- It can be treated by using Botulinum toxin, RF Electrocoagulation and surgical methods.
- Botulinum toxin can reduce upto one third where as surgical methods can reduce upto two third of the muscle mass.
- It is also called as the Pterygoideus internus (Internal pterygoid muscle).
- It consist of 2 heads which differ in origin:
The deep head originates from the medial surface of lateral pterygoid plate of the sphenoid bone.
While the superficial head originates from the maxillary tuberosity.
The muscle inserts into the inner surface of the angle of the mandible.
- Nerve supply of the muscle comes from the main trunk of the mandibular nerve.
- Blood supply is chiefly from the maxillary artery.
- Elevate the mandible .
- Protrusion of the mandible (lateral & medial pterygoid on one side protrude the mandible to the opposite side).
- Side to side movement (these lateral movements are achieved by lateral & medial pterygoid on both sides acting together to produce side to side movements).
- Also called as the Pterygoideus externus (External pterygoid muscle).
- It is a short conical muscle, having 2 heads:
upper and lower.
- Upper head:
– Origin: infra-temporal surface & crest of the greater wing of sphenoid
– Insertion: enters the TMJ & inserted into:
a) Pterygoid fovea of the neck of the mandible
b) Articular disc
c) Capsule of TMJ (anterior aspect)
- Lower head:
– Origin: Lateral surface of the lateral pterygoid plate
– Insertion: its insertion is same as that of the upper head, it enters the TMJ & gets inserted into:
a) Pterygoid fovea of the neck of the mandible
b) Articular disc
c) Capsule of TMJ (anterior aspect)
- The insertion of the lateral pterygoid in the articular disc occurs in the medial aspect of the anterior border of the disc and thus it plays a role in the T.M.J. diseases especially internal derangement.
- Some of the T.M.J. diseases have been due to an attributed variation of the function and attachment of the superior head as an etiological factor in T.M.J. diseases.
- Nerve supply is from the anterior division of the mandibular branch of trigeminal nerve(nerve to lateral pterygoid).
- Blood supply of lateral pterygoid muscle is from maxillary artery .
- Actions of lateral pterygoid:
- If the Pterygoid muscles of one side act, the other side of the mandible is drawn forward while the same condyle remains comparatively fixed.
- Depression of the mandible .
- Side to side movement (lateral movement) .
- Protrusion of the mandible.
ACCESSORY MUSCLES OF MASTICATION
- It is an accessory muscle of mastication, occupying the gap between mandible and maxilla forming important part of the cheek.
- Its origin is from buccal plate of bone of the sockets of the upper and lower three molars and pterygomandibular ligament.
- Course and insertion
Upper fibers gets inserted into upper lip,
Lower fibers gets inserted into lower lip,
Middle fibers decussate at the angle of the mouth, the upper fibers pass to lower lip while the lower fibers pass to the upper lip .
- Nerve supply is from buccal branch of facial nerve.
- Blood supply is from facial artery.
- The main action of buccinator is to prevent the accumulation of food in the vestibule of mouth.
2. ANTERIOR BELLY OF DIGASTRIC:
- Origin; it arises from the digastric fossa on the lower border of mandible on both sides of symphysis menti.
- Insertion; into the intermediate tendon which is connected to the hyoid bone by a fibrous loop.
- Nerve supply; is through anterior division of mandibular branch of trigeminal nerve.
- Action; its main action is to depress the mandible .
3. MYLOHYOID MUSCLE:
- It form the floor of the mouth.
- Origin is from mylohyoid line on the internal aspect of mandible.
- Insertion; The fibers slops downwards and forwards to inter-digitate with the fibers of the other side to form the median raphe.
- This median raphe insert in the chin from above and the hyoid bone from below.
- Action; Elevates hyoid bone, supports and raises floor of mouth which aids in early stage of swallowing, depress the mandible.
- Nerve supply; by nerve to mylohyoid: which is a branch of Inferior alveolar branch of mandibular nerve, which originates before it enters inferior alveolar canal.
- Blood supply; by Facial artery and Lingual artery.
- This muscle provides a separation between the submandibular and sublingual salivary glands.
- Origin; from inferior genial tubercle (in the midline of inner surface of mandible).
- Insertion; is into the hyoid bone.
- Action; depresses the mandible.
- Blood supply; is through lingual artery.
- Nerve supply; is by hypoglossal nerve.
5. ORBICULARIS ORIS:
- It has two parts: intrinsic and extrinsic part.
- Intrinsic part is a very thin sheet and originates from superior and inferior incisivus. It inserts into the angle of mouth.
- The extrinsic part is actually formed by elevator and depressor muscles of the lips and their angles, and inserts into the angle of the mouth.
- The orbicularis oris functions to close and shut the mouth and formes the most versatile types of grimaces.
Physical Examination Of Muscles Of Mastication
1. EXAMINATION OF MASSETER:
- The patient is asked to clench their teeth and, using both hands, the practitioner palpates the masseter muscles on both sides, making sure that the patient continues to clench during the procedure.
- Palpate the origin of the masseter along the zygomatic arch and continue to palpate down the body of the mandible where the masseter is attached.
- The masseter is most often tender along the central fibers of at its insertion.
- Masseter hypertonicity is found in patients who have premature contacts on the nonworking side.
- Parafunctions such as bruxism and clenching also give rise to masseter pain that is frequently associated with pain in the temporalis muscle.
- The temporalis is palpated in much the same manner
to detect lateral interferences.
3. LATERAL PTERYGOID:
- In patients with nonworking side interferences, the lateral pterygoid muscle on the opposite of the interference is sometimes painful.
- In addition, this muscle will be painful whenever there is a centric slide with an anterior component and the patient is bruxing or clenching in this anterior position.
- The lateral pterygoid, despite its commonality in displaying a spasm, cannot be palpated intraorally.
4. MEDIAL PTERYGOID:
- The medial pterygoid muscle is not usually involved in gnathic dysfunctions but when they are hypertonic, the patient is usually conscious of a feeling of fullness in the throat and an occasionally pain on swallowing.
- Muscles and Malocclusion
- Article published by T.M.Graber. in 1963 June in AJODO.
- This study attempts to balance orthodontic therapy and musculature philosophically
- Orthodontist can balance them physically with appliances.
- It deals with the role of muscles in the etiology and correction of malocclusion.
- Muscles function is normal.
- The teeth are in state of balance with environmental force.
- The open bite problem may arise because of thumb and finger sucking, that gives an excellent example of applied muscles physiology.
- With changes in tongue ,cheek, and lip muscle function, the net effect is narrowing of the maxillary arch and over eruption of post teeth.
CLASS II, DIV I MALOCCLUSION
- Abnormal muscle activity.
- A change in muscle function is a requisite expansion is a treatment objective.
- In hereditary type of class II malocclusion the teeth merely reflect the abnormal antero-posterior jaw relationship, and the excessive over jet is consequence.
- If structural mal-relationship exists, the muscle function adapt to this pattern as best it can in line with the requirement of mastication, deglutition and speech.
- The lip may become hypertrophic as a result.
- The lower incisors buckle as the mandibular segment is flattened by continuously abnormal mentalis muscle activity.
- The curve of spee increases, buccinator muscle activity.
- Openbite also occurs in this abnormal muscle activity can cause the pseudo class II div I.
- t/t for this should creation of normal basal bone relationship that permit muscle function properly and expansion with appliance.
CLASS II, DIV 2 MALOCCLUSION
- The role of musculature is more difficult to establish.
- Activity of the cheek and lip muscles is usually normal but curve of spee is excessive that interferes with the eruption of post teeth.
- Because of this TMJ problems arise like clicking, and pain.
CLASS III MALOCCLUSION
- In this we deal with dominant bone dysplasia, with adaptive muscle function and tooth irregularities reflecting a severe basal dysplasia.
- It has got strong hereditary pattern the upper lip is short and lower lip is hypertrophic.
- During deglutition cycle there is greater mobility of the hyoid bone as the suprahyoid and infrahyoid muscles demonstrate activity.
- Two separate acts are recognized in the chewing process.
- First is a combination of prehension and incision in which the food is secured by the lips and bitten by the front teeth.
- The second is mastication, the major activity during which the food is mashed between the back teeth.
- The total chewing cycle occurs through three phases:
- The opening stroke during which the mandible is lowered.
- The beginning closing stroke during which the mandible is rapidly raised until the entrapped food is felt and
- The power stroke in which the food is compressed, punctured, crushed and sheared.
CHEWING MOVEMENTS AND MECHANICS
- The chewing process generally acts as a 2nd order lever system resulting in compression at TMJ.
- The turning moment generated along mandibular body and ramus creates a sheer at TMJ.
- In 2nd order lever system resistance is present between lever and fulcrum.
- Chewing in humans is actually asymmetrical and unilateral.
- At the working side:
- It possesses the greatest adductor force, but articular emminence is less substantially loaded.
- At the balancing side:
- It possesses the less adductor force and the articular emminence is substantially loaded.
- At the initial action, contraction of inferior head of lateral pterygoid muscle occurs to initiate mandibular deviation to working side.
- Masticatory Muscle Disorders
Some of the common masticatory muscle disorders involve:
- Congenital hyperplasia/ hypoplasia
- Hypermobility/ hypomobility of the muscle
- Muscle pains
- Myositis ossificans etc.
CONGENITAL HYPOPLASIA/ HYPERPLASIA
- It occurs very rarely, and is more common in masseter and orbicularis oris.
- Its oral symptoms include enlargement or decreased size of the affected muscle, which may show an asymmetric facial pattern and stiffness in the temporo-mandibular joint.
- It may or may not be associated with hypermobility/ hypomobility of the muscles.
MUSCLE HYPERMOBILITY/ HYPOMOBILITY
- This disorder involves extreme or diminished activity of the masticatory muscles.
- Its etiology includes various factors such as:
– Decreased/ increased threshold potential of neural activity.
– Facial paralysis
– Nerve decompression
– Secondary involvement of systemic diseases.
- It usually occurs as a result of reflex protective mechanism and myofacial triggers.
- It is usually felt as a non-pulsatile variable aching sensation, with a boring quality. It may also present with tightness, weakness, swelling or tenderness.
- It includes 3 types:
1. local muscle soreness:
it is a primary hyperalgesia with lowered pain threshold due to local factors such as stress, injury, infection etc.
- This may be due to:
1. distortion of blood vessels within the muscle or
2. forceful or sustained contraction repeatedly.
2. Muscle splinting pain:
it is defined as rigidity of the muscle occuring as a means of avoiding pain caused by movement of the part.
it is a reflex protective mechanism.
Splinting of masticatory muscle may occur as a protective mechanism in conditions such as toothache, overstressed teeth, effect of local anaesthetics, trauma etc.
3. Non-spastic myofacial pains:
There is no spasm and pain is the only complaint and this is generally referred to structures outside the muscle proper.
it may be due to atrophied muscle mass because of inactivity, illness or nutritional deficiency.
Zones of referred pain
- The masseter muscle pain refers to the ear, TMJ and the mandibular teeth.
- The temporalis refers to the temple, orbit and maxillary teeth.
- The medial pterygoid refers to the infra-auricular and post-mandibular area.
- The lateral pterygoid always refers its pain to the TMJ.
- Myofacial Pain Dysfunction Syndrome (Mpds)
- Muscular Disorders (Myofascial Pain Disorders) are the most common cause of TMJ pain associated with masticatory muscles.
- Common etiologies include:
1. Many patient with “high stress level”
2. Poor habits including gum chewing, bruxism, hard candy chewing
3. Poor dentition
- Its treatment includes 4 phases of therapy which includes muscle exercises and drugs involving NSAIDs and muscle relaxants.
- A bite appliance is also worn by the patient in the further stages to ‘splint’ the muscle movement.
- It is a condition wherein fibrous tissue and heterotropic bone forms within the interstitial tissue of muscle, as well as in associated tendons or ligaments.
- It is of two types: localized and generalized.
Localized myositis ossificans:
It is caused by trauma or heavy muscular strains or by metaplasia of pluripotential intermuscular cnnective tissue.
- The affected site remains swollen and tender, and the overlying skin may be red and inflamed.
- There may present a difficulty in the opening of the mouth.
- management is done by giving sufficient rest to the muscle and excision of the involved muscle after the process has stopped.
Generalized myositis ossificans:
- In this, formation of bone in tendons and fascia occurs along with subsequent replacement of muscle mass by the bony tissue.
- The masseter muscle is the most frequently involved.
- It usually occurs in children less than 6 years of age.
- It shows an evidence of dense osseous structures in the greater part or whole of the muscle.
- There is a gradual increase in stiffness and limitation in the motion of masticatory muscles. Ultimately, the entire muscle may get transformed into bone resulting in no movement.
Management: there is no specific treatment. The muscles involved are to be excised.
– Nakamura, Zerado and Yoshida concluded that the masseter muscle’s activity level was significantly lower in the malocclusion group than in normal mice. It is, therefore, suggested that malocclusion interferes with optimizing the chewing pattern and establishing appropriate masticatory function.
– It is also suggested that masseter muscle activity decreases following a reduction in masticatory stimulation of the periodontal ligament. Persistence of this condition might inhibit the growth and development of masticatory muscles and their function.
Angle Orthod. 2013;83:749–757
– Rowlerson, Raoul & Daniel concluded that there were significant differences in percentage of occupancy of fiber types in masseter muscle in bite groups with different vertical dimensions.
– Type I fiber occupancy increased in open bites, and conversely, type II fiber occupancy increased in deep-bites.
– The association between sagittal jaw relationships and mean fiber area was less strong, but, in the Class III group, the average fiber area was significantly different between the openbite, normal bite, and deepbite subjects.
– In the Class III subjects, type I and I/II hybrid fiber areas were greatly increased in subjects with deepbite.
Am J Orthod Dentofacial Orthop 2005;127:37–46
– Keisuke, Yasuo and Kazuo employed electrophysiologic techniques (electromyogram) and found that masseter muscle activity decreases during the orthodontic treatment and this must be due to discomfort or pain and the alterations in the occlusal condition produced by the tooth movement or the ortho appliance itself.
The Angle Orthodontist – Vol 66 no3 1996, 223-228
– Easton & David found that there was an increase in action of lateral Pterigoid and Masseter muscles along with the slight increase in mandible in rats after treatment with a protrusive appliance
AM J ORTHOD DENTOFAC ORTHOP 1990;97:149-58
– Carene & Steenberghe proposed that during the first phase of functional treatment ,reflexes in jaw muscles are transiently brought into imbalance. This phase of imbalance could act as a trigger for the mandible to attain a new functional position that subsequently leads to morphologic’ changes.
AM J ORTHOD DENTOFAC ORTHOP 90: 41 O-41 9, 1986.
- The masticatory muscles include a vital part of the orofacial structure and are important both functionally and structurally.
- The effect of muscle forces is three-dimensional although most orthodontists have considered it only one vector that is expansion.
- A change in muscle function can initiate morphologic variation in normal configuration of the teeth and supporting bone, or it can enhance already existing malocclusion.
- It is imperative that the orthodontist appraise muscle activity and that he conduct his ortho therapy in such a manner that the finished result reflect balance b/w structural changes obtain and the functional forces acting on the teeth an investing tissues at that time.
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- The Angle Orthodontist – Vol 66 no3 1996, 223-228
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- Am j orthod dentofac orthop 1990;97:149-58
- Am j orthod dentofac orthop 90: 41 o-41 9, 1986.