Attending CPD events has some really important functions, one of which is exposing us to questions and ideas we may not think about every day. At an equine symposium recently, my interest was piqued on a matter I really knew very little about – the role of the hyoid bone and tongue in equine locomotion.

In theory, of course, we know that all parts of the body are connected and that the smallest joint, bone, or dysfunction can have a knock-on effect throughout the whole body. I was curious to see exactly how the tongue and hyoid bone could possibly influence the movement and mechanics of the horse.

 

First, the Equine Anatomy:

The hyoid bone serves as the attachment for the tongue and pharynx, and is made up of paired stylohyoid, epihyoid, ceratohyoid and thyrohyoid bones, with a central basihyoid bone.  The central basihyoid bone supports the lingual process, to which the tongue is attached. The stylohyoid articulates with the temporal bone and allows cranial and caudal movement, somewhat like a pendulum. The base of the tongue is attached to the lingual process, the soft palate and pharynx.

Multiple muscles attach to the hyoid bone and can alter its position and shape.

The main attached muscles are the omohyoideus, sternohyoideus and sternothyroideus.

  • The sternothyroideus originates on the sternum and inserts on the thyroid cartilage.
  • The sternohyoideus originates on the sternum and inserts on the basihyoid bone.
  • The omohyoideus originates on the subscapular fascia near the shoulder joint and inserts on the basihyoid bone.

These three, however, are not the only muscles involved:

  • The thyrohyoideus attaches to the lateral surface of the thyroid cartilage and inserts on the thyrohyoid bone, moving the hyoid bone caudally.
  • The geniohyoideus moves the hyoid rostally.
  • The genioglossus protracts the tongue.
  • The styloglossus, which retracts the tongue.
  • The occiptohyoid connects the hyoid bone to the occiput

Then there are:

  • The mylohyoid
  • The hyoglossus and
  • The cricopharyngeal

So what are the connections?

If we consider the sternohyoideus and sternothyroideus muscles; they directly connect the horse’s tongue and mouth to the chest of the horse. This fascial and muscular connection carries on through the pectoral muscles along the ventral aspect of the horse, along the abdominal muscles and into the pelvis.

The omohyoideus muscle connects to the fascia medial to the shoulder joint, linking into the fascial chain that runs from the head, along the ventral aspect of the neck, medial to the shoulder and all the way to the toe on the hindlimb. This gives a new depth of understanding to the term ‘riding from your leg to your hand’, and the connection between your leg aid and the activity of the hindlimb. In addition, the occiputohyoid muscle connects the hyoid to the poll, creating a direct chain from the shoulder through the hyoid to the poll, and from there through the nuchal ligament and the dorsal muscular chain back to the hindquarter, creating a dynamic, connected chain.

The hyoid also plays an essential role in the horse’s ability to breath optimally, as movement of the hyoid has a direct impact on the suspended pharynx.

How is movement impacted?

We’ve established that the hyoid is not only connected directly to the tongue, forelimb and poll, but indirectly to the hindquarter. Therefore a restriction, pain or any dysfunction in the hyoid may have a direct impact on the mobility of the forelimb, leading to a possibly shortened or stilted gait. This can be transferred to the hindquarter as an inability to engage the hindlimbs. So we see that the hyoid and the tongue affects movement of the head, poll, and jaw, and also of the forelimb. Indirectly it connects to the hindlimb as well, through muscular chains and through the fascia. We can expect to see many areas affected by dysfunction in the hyoid bone, including the ability to perform lateral movement, the range of motion of the cervical spine, tension in the ventral muscles of the neck (and a tendency for the horse to go behind the bit to relieve that tension), a potentially hollow back and a shortened, stilted stride.

What can affect the hyoid bone?

Although the hyoid bone is in a protected area within the horse’s mouth, it is a delicate structure and can be injured, misaligned or even fractured. A bit that causes restriction of the tongue will restrict the movement of the hyoid and associated muscles, causing pain and tension in these muscle groups, affecting the whole myofascial chain and resulting in changes in movement. Practices where the tongue is handled (including during a dental procedure) can lead to a potential unbalancing of the hyoid or, in worst-case scenarios, a fracture of the hyoid bone. Luckily the balance of the hyoid can easily be appreciated from the bottom of the jaw, as well as from the symmetry of the supraorbital fossa. Dysfunction or asymmetry of the temporo-mandibular joint can impact the hyoid, and vice versa, and signs may be closely associated with each other.

And so we see that a small, seemingly insignificant bone suspended inside the jaw can have a massive effect on the whole body movement of our equine patients. Isn’t that so often the case? The smallest component of a picture may have a massive impact on the whole.

If I have missed any information here, or you would like to add something, please feel free to comment with your knowledge, thoughts and theories. I would love to get feedback from you.

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