Anatomy

The hyoid apparatus consists of a selection of small bones that articulate together. Its name means “shaped like the letter upsilon (Y)”, and it is situated at the base of the skull; between the cheeks of the horse. The hyoid apparatus connects to the skull via the temporohyoid joint. At this joint, the hyoid apparatus articulates with the skull. The hyoid apparatus gives biomechanical form and function to the larynx, pharynx and the tongue. 

Every muscle in the horses body eventually connects to the hyoid. 

The bones that make up the hyoid apparatus are the only bones in the body not to connect to another bone – they are held in place solely by ligaments.

Components of the Hyoid Apparatus:

• Stylohyoid (paired)
  • Articulates with the petrous part of the temporal bone, allowing the stylohyoid bones to move cranially and then caudally like a pendulum. 
  • Largest bones in the hyoid apparatus
• Epihyoid
• Ceratohyoid
  • Attaches to distal end of the stylohyoid via the epihyoid bone.
  • Articulation with the ceratohyoid to stylohyoid bone lengthens the stylohyoid-ceratohyoid unit.
• Thyrohyoid
• Central basihyoid
  • The base of the tongue attaches to the lingual process of the basihyoid.
  • Can be argued to be the most important component. 

Muscles are attached to the hyoid apparatus; their contraction determines its position and shape which in turn determines the shape of the larynx and nasopharynx.

Examples of muscles attached to the hyoid apparatus:

• geniohyoideus – movement of the hyoid bone rostrally.
• genioglossus – contraction = protracts the tongue and pulls the basihyoid bone rostrally (towards the nose).
• sternohyoid
• sternothyroideus
• omohyoideus
• thyrohyoideus
• hyoglossus
• styloglossus – contraction = retraction of the tongue

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Muscular Connections

The tongue connects to the hyoid apparatus. 

The hyoid apparatus has muscular connections from the throat to the forelimbs (omothyroid), shoulder (omohyoid) and sternum (sternohyoid).

Sternohyoid = hyoid to sternum; Omohyoid = basihyoid to medial scapula/subscapular fascia; these muscles provide a direct connection from the hyoid apparatus to the shoulder of the horse via the ventral neck. Tension within these muscles, along with sternothyroid, results in restricted shoulder movement and the development of hypertonicity. Contraction of these muscles can put strain on the temporomandibular joint (TMJ). Additionally, hypertrophy and hypertonicity of the sternocephalic muscle can occur when the horse strains against the bit as a result of negative pressure upsetting the hyoid apparatus.

The muscle chain continues ventrally, connecting the pectoral area to the abdominal muscles.

The abdominal muscles are connected to the pelvic muscles.

JUST A MUSCULAR CONNECTION?

“We connect all three main junctions of the horse through the bones of the hyoid in the horse’s jaw when we connect the muscles from the scapula and the sternum up to the hyoid; then from the hyoid to the occiput and finally from the poll to the nuchal ligament which then connects with the supraspinous ligament.”

“Dr. Ridgway presented it to us, muscle pathology of the long hyoid muscles ‘goes beyond just TMJ pain, it affects the entire balance of the body’.  Specifically, he clarified for us that a contracted omohyoid muscle results in the following:  it retracts the tongue back into the throat; interferes with the bit; locks the horse’s jaw; limits lateral flexion; interferes with shoulder freedom and range of motion; and interferes with balance and proprioception. When these long muscles are contracted, they mimic the body’s response to fear—they are a part of the Fright and Flight Muscle Groups.  Dr. Ridgway reminded us that when humans react to emotional stress, we tighten our neck muscles, clench our teeth, and hunch our shoulders.   It’s the same with horses.”

The Masterton Method

Connection to the TMJ

Small muscles of the hyoid apparatus connect to the TMJ and the poll. The TMJ has a dual purpose of mastication and registering of postural information. Therefore, it is an important anatomical location for nerves that control proprioception and balance; the highest concentration of mechanoreceptors (sensory nerves that report shape change) are found in the periphery of the disc and at the attachment site of local ligaments.The TMJ articulates with the hyoid apparatus.

The hyoid and cranial nerves are intricately connected with the jaw.

Function of the Hyoid Apparatus

The series of bones in the hyoid apparatus are responsible for the suspension of the larynx and tongue from the skull. The main function the hyoid is to support the tongue. It also places a role in balance.

BEAR IN MIND: Movement of the tongue by the equine dentist during a treatment can upset the balance of the hyoid.

Dysfunction

Mouth health – due to the muscular connection from the tongue to the core, pectoral and pelvic muscles of the body… discomfort in the mouth from a poorly fitting/used/type of bit, harsh hands of the rider, poor dentition can influence whole body posture and locomotion. Also, a negative memory associated with the bit can cause the onset of “bridle lameness”. When the horse extends the cervical vertebrae longitudinally, stretches down and chews/licks softly this can indicate correct engagement and suppleness of the ventral chain. Engagement of both ventral and dorsal muscle chains is important as they work in synergy to produce correct dynamic posture and thus promote correct and healthy overall muscular development. This whole-body engagement is commonly referred to as the circle of muscles. 

“A stiff poll and jaw, holding behind the vertical or a rider that hangs on to the reins can inhibit the engagement of the ventral muscle chain and therefore the hindlimb. This is why over-bending to the inside or pulling on the reins can inhibit hindlimb engagement.”

Susan E. Harris (2017) Horses Gaits, Balance and Movement: The natural mechanics of movement common to all breeds

Misalignment of the Hyoid.

Tension of the tonguecan result in tension within the muscles surrounding the sternum and the ventral neck (due to the muscular connections of the sternohyoid and omohyoid). Tension in the sternum means that the horse is unable to engage the pectoral muscles and lift through the back… an essential posture for healthy dynamic movement and collection.

Windsucking can also put stress on the hyoid apparatus and TMJ. Hypertonicity of the hyoid muscles can develop as a consequence.

Temporohyoid osteoarthopathy (THO) is also known as middle ear disease. This is a condition where the bone surrounding the temporohyoid joint is proliferated, leading to joint fusion and decreasing articulation. Aetiology is not clearly understood, but may be resultant from an inner ear/guttural pouch infection or degenerative. Fusion of the temporohyoid joint results in decreased range of motion and flexibility. This makes actions of swallowing and head/jaw/neck movement vulnerable to causing a fracture at the location where the hyoid bones attach the skull (Walker et al. 2002; Divers et al. 2006; Palus et al. 2012). The fracture commonly occurs in the stylohyoid bone. Clinical signs of a fracture include: rapid worsening of clinical signs associated with temporohyoid joint fusion, compression of the cranial nerves 7 & 8 (vestibulocochlear and facial) which are responsible for facial expression, eye lubrication and balance. Early clinical signs include: pain when pressure is applied at the base of the ear/throatlash area, tossing of the head, reluctance to accept the bit, reluctance to perform in specific head position. Once there is significant thickening of the bone there may be marked ataxia, lip and ear droop. Diganostics involve radiographs (to detect a thickening of the bone), upper airway endoscopy (to confirm diagnostics). The upper airway endoscopy involves placing the endoscope into the guttural pouch as this enables a view of the stylohyoid bone articulating with the skull. The endoscopy of a horse with THO will show enlargement of the stylohyoid bone. Checking both guttural pouches is essential as this disease can be bilateral. CT scans can provide a significantly more detailed image by detecting both bony and soft tissue changes to the hyoid apparatus, skull and inner ear. Treatment involves antibiotics, tarsorrhaphy (temporary surgical closure of the eyelids to protect the cornea), ceratohyoidectomy (removal of the ceratohyoid bone to decrease the pressure of the hyoid apparatus to the skull and thus decrease risk of fracture and incidence of pain). Prognosis is better for animals who have not suffered from pre-existing nerve damage prior to treatment; this can result in nerve deficits such as a head tilt or ear droop). Prognosis is also lower in cases where a fracture has already occurred. Further research into THO.

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Tying down the Tongue

Before research into the hyoid apparatus, I was unaware of the morbid practice of tying a horses tongue to the mandible (jaw) or outside of the mouth to stop the horse from placing the tongue over the bit. Such measures are a poor attempt to improve airway function and therefore performance, along with decreasing airway noise.

Bottom line:research has shown these practices do not influence the position of the hyoid apparatus. The passive action of pulling the tongue from the mouth does differs dramatically from active muscle contraction. This is due to tying down of the tongue causing protrusion rather than depression of the tongue, and it is depression that causes the action of the extrinsic tongue muscles to dilate and stabilise the airways.

A horse MUST be able to confidently stretch over his back and down into a rider’s hands with his nose in front of the vertical, finding his full range of motion, while keeping his own free-flowing elastic balance. This is the starting point of relaxation, balance, and impulsion that builds to collection.

Classically Naturally

Assessment

Assessment of Pain and Sensitivity:Application of digital pressure to 1cm below and cranial to the TMJ. There should be no reaction, yet sensitivity within this region is indicative of pain within the joint.

Assessment of Hyoid Imbalance:Hypertonicity of the hyoid musculature = often have lateral imbalances of the hyoid bones. When viewing the horses head from underneath, the tongue can be located in the depression between the two ramus of the mandibles. Applying gentle pressure underneath can identify a bony structure, which is the basihyoid bone. Lumps around this area are likely saliva glands. Feeling the location of this bone can determine deviation of the hyoid. Sliding the tips of the fingers along the inside curvature of the mandible upwards can move the hyoid gently to determine tension.

Sternocephalic muscle: assess this muscle for hypertonicity, hypertrophy. When moving, this muscle should be relaxed and swing gently from side to side.

Reaction to pain in the region of the hyoid can be adverse and quick; be cautious and slow.

Treatments

Under-Scapula Release – Masterton Method (when applying this technique, the hyoid, TMJ and poll are all being assessed and treated too)

Light therapy on acupuncture points – further information linked here.

Tongue Release – this practice is controversial, as too much force on the tongue can easily place too much pressure on the hyoid bones. An article with more depth on this topic, accompanied by videos, is linked here. An example of the Tongue Release by the Masterton Method is linked here.

Hyoid Mobilisation:This practice involves placing the fingertips on the medial surface of the mandible, just to the side of the tongue. The fingers are slid upwards along the surface of the inner mandible. Starting on the looser side first, and then moving on to the stiffer side. Look out for signs of release. Hold the pressure; melt the tissue.

References

Holcombe, S. and Ducharme, N. (2008) Equine Exercise Physiology

Temporohyoid Osteoarthropathy by the ACVS

Susan E. Harris, Horse Gaits, Balance and Movement featured in Equine Wellness Magazine (2015)

Susan E. Harris (2017) Book.

Hinchcliff, K., Kaneps, A., Geor, R. (2008) Equine Exercise Physiology: The Science of Exercise in the Athletic Horse 

Rohwerder, B., Buss, A., Boening, K. (2016) Computed tomographic diagnosis: Traumatic lingual process fracture and surgical resection, Equine Veterinary Journal: Case Report

Bar Valley Equine Therapy – TMJ and Hyoid Release: The Biomechanics

The Rubber Curry Comb Blog Post