TEMPOROMANDIBULAR JOINT (TMJ): Muscles and Movements

MUSCLES

Muscles that provide action to the TMJ are the masseter, temporalis, and pterygoid along with a group which are classed under suprahyoid and infra-hyoid muscles. This latter group contribute to deglutition as well as mandibular movements.

MASSETER

This is a rectangular muscle with three layers, superficial, middle, and deep, all acting to elevate (close) the mandible. It also contributes to the act of protrusion . This is a fan-shaped muscle occupying the whole temporal fossa with converging fibers to a tendon along a broad attachment. In its three parts (anterior, middle, and posterior), the fibers act strongly on elevation as well as retrusion. The middle is also active on protrusion. Lateral deviation of the jaw is provided by the ipsilateral posterior fibers and the contralateral middle fibers.

LATERAL PTERYGOID

This is comprised of upper and lower parts of heads which act on mouth opening and also on closing (eccentric contraction or giving up its concentric action slowly). Lower head is strongest on protrusion and on lateral deviation to the opposite side. The upper head is attached to the anterior portion of the meniscus and is responsible for the meniscus being pulled forward of the condyle (gliding motion) to cushion rotation of the condyle on jaw opening. Resistance to initial jaw opening activates the upper head and then the lower head.

MEDIAL PTERYGOID

This is a thick quadrilateral muscle that strongly assists in the elevation of mandible and also protrusion. It also acts to laterally deviate the mandible to the opposite side .

SUPRAHYOID MUSCLES

DIAGASTRIC

This has two bellies that loop through an attachment from the hyoid bone. With a fixed hyoid, it will depress the mandible; with a fixed mandible, elevation of hyoid will occur. Muscles bilaterally, always act together and are especially active on maximal depression of mandible.

 

STYLOHYOID

This functions to raise and retract the hyoid elevation of the hyoid bone  which elongates the floor of the mouth.

 

MYLOHYOID

This muscle is active on mandible depression, some activity on retrusion and on the ipsilateral side of lateral deviation. It will elevate the floor of the mouth on initial deglutition with accompanied elevation of the hyoid bone.

 

GENlOHYOlD

This muscle acts to depress mandible when the hyoid is fixed. With a fixed mandible, it will elevate the hyoid and move it forward (antagonist of stylohyoid.) Also, it is active on protrusion and the ipsilateral side of lateral deviation.

 

INFRAHYOID MUSCLES

This includes sternohyoid, sternothyroid, thyrohyoid, and omohyoid.These muscles act as antagonists to the suprahyoid group by depressing the hyoid bone and in acting as fixators of the hyoid during suprahyoid activities.

Mandibular actions by muscle function are summarized below.

DEPRESSION (JAW OPENING)

The lateral pterygoid muscles are prime movers. Digastric, geniohyoid and mylohyoid contribute in descending order when the jaw is already opened and against resistance.

ELEVATION (JAW CLOSING)

Temporalis, masseter, and medial pterygoids are prime movers, each working simultaneously. Lateral pterygoids engage on resistance to closing with the posterior temporalis retracting the condylar head.

PROTRUSION

Lateral pterygoids and medial pterygoids show the strongest activity; however, there is evidence of activity also in the middle temporalis, masseter, digastric, and geniohyoid muscles. All parts of temporalis active with some activity in the digastric and mylohyoid.

LATERAL DEVIATION

On ipsilateral side, posterior fibers of temporalis, digastric, mylohyoid, and geniohyoid act in unison. On contralateral side, lateral and medial pterygoids are prime movers, with some evidence of activity in the middle fibers of temporalis and to a lesser degree in the digastric, mylo, and geniohyoid muscles.

INNERVATION AND BLOOD SUPPLY

The extrinsic innervation of TM joint, as most joints of our body, has both specific articular nerves and nonspecific articular branches of related muscle nerves. Of these joint receptors, as classified by Wyke”, all four groups (groups I, II, III and IV) are found in the TMJ.

GROUP l

Signal static joint positions, intra-articular pressure changes, direction, amplitude and velocity of joint movement.

GROUP II

Signal onset and cessation of joint movement.

GROUP Ill

(Confined to lateral ligament) signal when the extreme of available range on opening is reached.

GROUP lV

(Pain receptor system) is found in the capsule, articular fat pad, and articular blood vessels (posterior compartment).

The auriculotemporal and masseteric branches of the third division of the trigeminal nerve innervate the muscles of the TMJ. Specifically, the muscles of mastication (masseter, temporal, medial, and lateral pterygoids) as well as the anterior belly of the digastric. The mylohyoid has motor innervation from the trigeminal nerve (cranial nerve V). Whereas, the facial nerve (cranial nerve VII) innervates the posterior belly of the digastric and stylohyoid muscle. The blood supply is from the superficial temporal, middle meningeal and associated branches of the maxillary artery.

MOVEMENTS OF THE TMJ

The basic anatomical function of the mandible is to: 1) open and close; 2) protrusion and retrusion; and 3) lateral deviation which are provided through two types of basic movements. These movements are termed rotation and translation. Rotation occurs in the lower portion of the joint. The rotational axis passes through the head of the condyle so that there is a hinged type movement occurring . Translation occurs in the upper portion of the joint, and this motion of the condyle and the meniscus is relative to the articular eminence . the axes of mandibular movements do not shift significantly during either motion. The rotary motion causes successive regions of the condylar articulating surfaces to come into relation on a fixed point; this is arthrokinematically termed a glide. This gliding will only permit a depression of the mandible with no forward movement occurring. Because a continuance of this rotary glide will cause an impingement of structures posteriorly, the condyle must assume a translation or roll occurring almost simultaneously with rotation. This allows a resulting motion about the axis of the articular eminence as well as the condylar head. On jaw opening, the condyles are found to rotate at the very beginning of opening to about mid opening; the translatory movement is then obvious, permitting the condyle to slide forward just under the eminentia. Limited condylar rotation will present limited jaw opening.  It is apparent that the function of the upper lateral pterygoids in drawing the meniscus anteriorly is necessary and critical in preparation for condylar rotation. The meniscus, with its irregular shape, acting to stay ahead of the condyle, provides for congruent contour and lubrication especially at the beginning and end of motion. With these two conditions in effect, the lower lateral pterygoids will function to provide jaw protrusion (translatory condylar movement) and if required lateral deviation. Lateral deviation occurs to the opposite side of the contracting pterygoid. There has been little investigation on the exact mechanism of this deviation, nor is there agreement on the site of axis about which motion occurs.