Equine Massage: A Practical Guide (46 page)

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of the horse’s mandible, known as the “ramus,” is twice as long as in the human. When chewing, this extra leverage will affect the equine TMJ with greater force, causing more severe damage to the structures.

Anatomy of the Equine TMJ

Skeletal Structure

The bony structures of the equine TMJ are the temporal bone of the skull and the mandible.The mandible articulates on either side of the skull, at the temporomandibular joints.

When the horse chews, the mandible moves in a rotating fashion (medio-lateral), with minimal cranio-caudal displacement.

This movement forms a figure 8 during mastication.

Temporomandibular Articulation

The temporomandibular joint is a unique joint that simultaneously combines a synovial and a condylar joint; it links the condylar process of the mandible to the articular surface of the temporal bone.

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12.1 Bones of the TMJ

(1) skull

(2) mandible

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❖ A
synovial joint
is a special form of articulation, which permits the union of the bony elements by surrounding them with an articular capsule enclosing a cavity lined by a synovial membrane that produces the transparent, viscid fluid called synovia. The
synovial fluid
provides lubrication to the joint.

❖ A
condylar joint
is a joint in which an oval head of one bone moves in an elliptical cavity of another, permitting all movements except an axial rotation.This combination allows the TMJ free motion to open by hinge action, then to glide sideways in order to perform many different movements, principally for good mastication.

Inside the TMJ there is a thick, fibro, cartilaginous, inter-articular disk located between the mandible and the temporal bones, dividing the synovial joint capsule into 2 cavities.The disc function is to improve the performing functions of the TMJ against mechanical constraints.The hinge movement occurs in the lower cavity, and the lateral gliding and slight protrusive movements (anterior and posterior movements of the mandible) occur in the upper, more capacious cavity, where the oval head of the mandible articulates in the elliptical cavity of the temporal bone of the skull. On each of its sides, the temporomandibular joint gets extra 1

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12.2 Lateral View of the TMJ

(1) cronoid apophysis

(2) disco-temporal articulation

(3) disco-mandibular articulation

(4) zygomatic arch

(5) articular disc

(6) articular capsule

(7) head of mandible

(8) neck of mandible

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support from a collateral and a caudal ligament. Palpation of the TMJ will reveal the placement of this joint.

Nerve Supply

The facial (adjust) nerve and the mandibular nerve provide the nerve supply to the TMJ. These two nerves divide into many branches as seen in figure 12.3:

❖ The caudal auricular nerve

❖ The rostral auricular nerve

❖ The mesenteric nerve (not seen in figure 12.3)

❖ The mandibular alveolar nerve

❖ The mylohyoid nerve (not seen in figure 12.3)

❖ The masticatory nerve

❖ The pterygoid nerve (not seen in figure 12.3)

❖ The infraorbital nerve

❖ The cervical branch of the facial nerve

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12.3 Nerve Supply of the TMJ

(1) rostral auricular nerve

(5) caudal auricular nerve

(2) infraorbital nerve

(6) facial nerve

(3) masticatory nerve

(7) cervical branch of facial nerve

(4) mandibular alveolar nerve

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Vascular Supply

The common carotid artery and the external jugular vein provide the blood circulation to and from the upper neck and head area.

These vessels divide themselves into many branches. The most adjacent vessels to the TMJ are:

❖ The occipital artery and vein located posterior to the TMJ

❖ The transverse facial artery and vein, located laterally to the TMJ

❖ The masseteric artery and vein, located latero-distally to the TMJ

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12.4 Arteries of the TMJ

(1) supra orbital artery

(10) linguofacial artery

(2) angular artery [eye]

(11) external carotid artery

(3) intraorbital artery

(12) thyroid artery

(4) maxillary artery

(13) common carotid artery

(5) mandibular alveolar artery

(14) superficial temporal artery

(6) dorsal nasal artery

(15) caudal auricular artery

(7) lateral nasal artery

(16) occipital artery

(8) facial artery

(17) internal carotid artery

(9) masseteric artery

(18) vertebral artery

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12.5 Veins of the TMJ

(1) deep facial vein

(8) external jugular vein

(2) angular vein of the eye

(9) transverse facial vein

(3) dorsal nasal vein

(10) rostral auricular vein

(4) lateral nasal vein

(11) dorsal auricular vein

(5) facial vein

(12) maximillary vein

(6) buccal vein

(13) masseteric vein

(7) external linguofaciabular vein

Fascia of the Head

A continuation of the cervical fascia, both the deep and superficial layers, the fascia of the head may be divided into 2 groups:

❖ The
superficial fascia,
which forms an almost continuous layer except over the natural orifices—the eyes, nostrils, mouth, and ears—and provides attachment for the various cutaneous muscles

❖ The
deep fascia,
which can be further subdivided into the temporal fascia, the buccal fascia, and the pharyngeal fascia Muscles of Mastication

A good knowledge of the anatomy of the muscles and bones involved in the proper functioning of the TMJ is crucial for your understanding of their interrelation. It will contribute greatly to your expertise in assessing this condition and in treating it with massage.

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The muscles responsible for closing the jaw are:

❖ The masseter muscle

❖ The temporalis muscle

❖ The pterygoideus medialis and lateralis

❖ The buccinator muscle

❖ The mylohyoideus muscle

The muscles responsible for opening the jaw are:

❖ The digastricus muscle

❖ The occipitomandibularis (deep) muscle

❖ The sternothyroideus muscle

The lateral swinging of the jaw from side to side during mastication is achieved by the alternate, unilateral contraction of the pterygoideus muscle—both the medialis and lateralis bundles on each side. Take note that the mandibular nerve lies between the two muscle bundles, so when applying massage over that area, start with a gentle pressure and only increase your pressure while observing the feedback signs given by your horse.

When there is a TMJ problem, upon palpation there is usually a marked variation within the temporal and masseter muscles.

These muscles will be more developed on the side where most of 2

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12.6 Muscles of the Jaw, Deep

(1) rostral portion of digastricus muscle

(2) stylohyoid

(3) digastricus (caudal portion)

(4) occipitomandibular portion of digastricus
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12.7 Muscles of the Jaw, Superficial

(1) occipitomandibularis muscle

(2) masseter muscle

(3) sternothyroideus muscle

the mastication occurs and underdeveloped on the side of little use. The extent of this variation is proportional to the degree of severity of the TMJ dysfunction.

Causes of TMJ Dysfunction

Syndrome

The TMJ dysfunction syndrome is caused by inappropriate alignment of the joint and/or laxity of the supporting ligaments and muscles. Many of the following factors can trigger the TMJ dysfunction syndrome to develop:

❖ Dental problem and/or faulty teeth

❖ Upper cervical vertebrae misalignment

❖ Arthritis of the TMJ

❖ Trauma to the jaw, head, or upper neck

❖ Violent traction of the reins and bit

❖ Equine gastric ulcer syndrome

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Signs and Symptoms of TMJ

Dysfunction Syndrome

Here is a list of some of the most common signs and symptoms associated with TMJ dysfunction syndrome that you may observe in your horse’s behavior:

❖ The horse starts shaking his head regularly, especially during exercise

❖ The horse avoids contact with the bit

❖ The horse plays with his tongue (tongue lolling)

❖ Some swelling over the joints

❖ Spasms, hyper tonicity, or trigger points in the mastication muscles

❖ When jaw is closed there might be some bruxism (grinding of the teeth); not to be confused with Equine Gastric Ulcer Syndrome

❖ Difficulty opening the mouth when bridling

Upon visual and palpation evaluation, you may encounter the following symptoms, even if minimal, in the early development stage:

❖ Muscle, fascia, and ligament tightness

❖ Jaw moving unevenly to one side

❖ Mastication action is tender to perform

❖ Mastication action only happens on one side

❖ There might be some restriction to open the jaw properly and fully

❖ There begins to be an offset alignment between the upper and lower rows of teeth

❖ Noticeable wear of the molars

❖ There might be some trigger points in the mastication muscles TMJ Problems

Two types of lesions can develop in the TMJ due to the articular disc slipping forward or backward from its normal position as seen in figure 12.8.

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12.8 Healthy TMJ

(1) disc

(2) temporomandibular articulation

(3) third molar [maxillary]

(4) third molar [mandible]

Caudal Lesion

A
caudal lesion
of the TMJ is a situation where the articular disc has slipped forward, and the head of the mandible has slipped backward, touching the temporal bone at the zygomatic arch, by the retro-articular process landmark as seen in figure 12.9. This will result in a reduced mouth opening for the horse and a greater difficulty in feeding himself.

Retro-articular process

12.9 Caudal Lesion of the TMJ

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Retro-articular process

12.10 Rostral Lesion of the TMJ

Rostral Lesion

A
rostral lesion
of the TMJ is a situation where the articular disc has slipped backward, behind the head of the mandible, and between the retro-articular process of the zygomatic arch of the temporal bone as seen in figure 12.10, causing the mandible to protract.

If only one side is affected, the mandible will move towards the side opposite the luxation. The horse will be able to open his mouth, but will have problems closing it. This might cause him some difficulty in feeding.

Palpation

As you proceed with palpation, constantly assess the horse’s feedback signs (especially his eyes) to evaluate the presence of pain from inflamed muscles, ligaments, or the joint itself. Palpate the masseter, the temporalis, the pterygoideus medialis and lateralis, the occipitomandibularis, the digastricus, and the buccinator muscles for tone, tenderness, inflammation, and the eventual presence of trigger points and stress points. Referring to figures 12.4 and 12.5, note where the veins, arteries, and nerves are located. It is important to know what lies under your fingers.

Palpate the TMJ joint by placing your index finger on the horse’s face in front of and slightly below the auditory canal of the ear (do not place your fingers in the auditory canal). With your other hand, force the horse to open and close his jaw.You should be able to tell if the jaw is opening evenly. During your palpation
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you may feel and/or hear a small “click.” This is a sign of joint restriction and muscle tension.

Checking the Protraction and the

Retraction of the Mandible

Place yourself in front of the horse’s mouth with one hand on the bridge of the horse’s nose and the other on his mandible. Gently push the maxilla to the rear (caudally) while pulling the mandible to the front (rostrally).You should feel some play. If the horse’s jaw protrudes, that is a good sign. Lack of protrusion could indicate a TMJ condition. Then reverse your pressure over the mandible to evaluate the movement to the rear (caudally). Lack of retraction could also indicate a TMJ condition.