Anatomy and Function of the Equine Hyoid Apparatus

By Alexa Linton, Equine Sports Therapist

The hyoid bone and its relationship with the fascia, and the temporomandibular (TMJ) joint between the temporal and mandible bone, are areas of fascination for me. In osteopathy, we are taught to see every part of the body as connected – and not only every part of the body, but everything from the structural, to the fluidic, to the energetic and beyond. The hyoid apparatus, a small collection of bones nestled under the mandible and in front of the cervical spine, and its connection with the fascia exemplify this understanding, with far-reaching connections and the capacity to communicate with structures all the way from the tip of the nose to the tip of the tail. This, of course, can be an awesome thing and a not-so-awesome thing, depending on what is occurring for you and your horse. In this article, we’ll learn how the hyoid can be impacted by both bit and saddle, why scars are such a big deal, what on earth a fascial chain is, and how the TMJ relates to all of this. Put your geek hat on and let’s get started. 

Photo (left): Canstock/Dolgachov. Photo (right): Canstock/erstudio 

Anatomy and Function 

Anatomy is one of my favourite things, and hopefully my enthusiasm is contagious. Just to be clear, both horses and humans are happy homes to hyoids, which have some rather interesting differences, given our similar but different structure and biomechanics. To find a human hyoid, we begin in the cranium and then drop down just a little under the mandible (jaw bone) and above the thyroid cartilage to the hyoid apparatus, suspended from the styloid process of the temporal bones by the stylohyoid ligament. I like to find it by swallowing a time or two, as it moves with this action. This fascinating little bundle of bone, which in humans is made up of a main body, two lesser horns, and two greater horns, is quite literally connected, directly and indirectly, to every muscle in the body, which we’ll explore more in this article.

In horses, the hyoid apparatus is made up of several paired bones: the stylohyoid, epohyoid, ceratohyoid, and thyrohyoid, attaching to the central basihyoid bone. The basihyoid connects to the lingual process, where the tongue is attached. Unlike humans, the hyoid in horses articulates with the petrous portion of the temporal bone via the stylohyoid bone. This joint can be prone to temporohyoid osteoarthropathy, sometimes referred to as “middle ear disease,” and is caused by an overgrowth of bone and potentially a fusion at the temporohyoid joint. This condition can affect the facial and vestibulocochlear cranial nerves (nerves 7 and 8), responsible for balance, facial expression, and eye lubrication. If your horse has one eye that consistently leaks or dries out, or a floppy ear on one side, call your vet to investigate. 

The main functions of the hyoid are to support the tongue, to stabilize the pharynx involved in swallowing, and the larynx involved in breathing and vocalizing, and, due to its complex fascial relationship with the cervical spine, to play a role in maintaining the position of the head. It’s tight relationship with the temporal bones means that it also plays a role in hearing, balance, and, in horses with the hyoid directly articulating with the temporal bone, in steering. 

To find the human hyoid, swallow once or twice and you’ll feel it move. Photo: Candice Camille 

To give you an idea of the potential myofascial impact of this boney apparatus, check out all the muscles that utilize it as a direct anchor. In humans, the four suprahyoid muscles – digastric, stylohyoid, mylohyoid and geniohyoid – attach the hyoid to the tongue, mandible, and cranium. The four infrahyoid muscles – the sternohyoid, sternothyroid, omohyoid, and thyrohyoid – connect it to the sternum, scapula, and thyroid. In horses, the omohyoid muscle connects to the fascia medial to the shoulder joint, which means that an issue in the hyoid can negatively impact the function and mobility of the front limbs, and in a cascade, the ability to engage the hindquarters.  

Fascia, in our current understanding, is a connective tissue that weaves around muscles fibres, tendons, bones, and organs, stabilizing and enclosing structures. Imagine the fascia as one massive, communicating and connected living tissue, which is amazing in itself, but means that tension in the fascia at one point will apply tension and stress on all other areas. In the human body there are 12 fascial lines present. Working with the important concept of fascial lines in the horse and knowing that fasciae connects one muscle or structure to another through a constant chain, we can follow a ventral line from the omohyoid connecting all the way from the head, along the lower neck, into the shoulder and completing in the lower hind limb. We can also follow the hyoid through the sternohyoid muscle (connecting to the sternum), through the deep pectorals, to the rectus abdominus, extending from the sternum to the pelvis, then onto the psoas minor, the biceps femoris and semitendinous muscles of the hamstrings. Everything is quite literally connected and can affect the healthy biomechanics of our horses. In horses, we also see the connection of the occipitohyoid muscle from the poll to the hyoid, which extends through fascial ties down the nuchal ligament and myofascial chains along the spine and back –meaning that an issue in the poll, incredibly common in horses, can affect the hyoid and the health and relaxation of the back.

In researching the anatomical connection between the rider and horse as related to the hyoid apparatus, some fascinating interrelationships appeared. First is one that involves the balance and fit of your bit. The tongue and larynx, as well as the cervical spine, are physically bound to the hyoid, with tongue movement and tension affecting breathing. As you can imagine, a horse who is forced to bend directly at the poll, especially when the atlanto-axis complex is not moving well, or who is wearing a tight bridle or noseband that restricts the mobility of the jaw, will be unable to breathe with ease and will very likely show other signs of stress while under saddle. This is yet another reason why any training that creates false and forced flexion at the poll, such as Rollkur, or training that supports restricted movement of the mouth and jaw, is a very bad idea. Ensuring that your bridle fits properly, with a well-fitted bit that allows for relaxed movement of the jaw and tongue, is essential to a healthy hyoid and beyond.

Speaking of equipment, saddle fit issues impacting the movement of the scapula and shoulder joint can have a direct effect on the balance of the hyoid through the connection of the omohyoid muscle, and vice versa. And as always, good balance of the teeth is a necessity, given the direct bony connection from the hyoid to the temporal bones, and the potential impact of a temporomandibular imbalance on this delicate area.

Detecting an Imbalance

To find out if your horse is experiencing an imbalance of the hyoid apparatus, there is an easy check. Follow the inside of the jawbone with a flat hand and very gently push up on the soft tissue towards the centre of the cranium. Do this on both sides while paying attention to your horse and your safety - this area can be sensitive. If there is a reaction from your horse, generally a raising of the head, the hyoid or surrounding structures may need support. You may observe an imbalance in the level of the eyes, swelling in the temporalis muscle(s) above the eyes, a misalignment of the incisors (at the front of the mouth), or chewing on one side, grinding or sideways movement of the jaw. You may also see behavioural indications such as a reaction to your bit, bridle or saddle, head shaking or shyness, and resistance to flexion at the poll. 

Equipment that restricts the mobility of the mouth and jaw, and training that creates forced and false flexion at the poll, can significantly inhibit breathing as well as balance and movement throughout the horse’s body. Photo: iStock/Mishella

Detect an imbalance in the hyoid apparatus by following the inside of the jawbone with a flat hand and pushing up very gently on the soft tissue towards the centre of the cranium. Do this on both sides, and if the horse reacts by raising his head, look closely for other signs of an imbalance as described in the article. Photo: Devon Gillott

As the hyoid is very sensitive and delicate, it is recommended to work with a trained professional to bring it back into balance. This goes for humans, too. Hyoid imbalances in humans can be just as far-reaching - if you suspect you’ve got some hyoid things going on, look up your closest osteopathic manual practitioner. 

It is important to note that in rare cases the hyoid apparatus can fracture, and in horses it is specifically susceptible to any kind of force placed on the tongue. This is possibly unnecessary advice, but never pull on your horse’s tongue for any reason. As you can imagine, a fracture of the hyoid is very painful and impacts many structures. If you suspect a break, an immediate visit from the veterinarian for x-rays is necessary. 

The omohyoid muscle originates in the subscapular fascia and attaches to the hyoid bone. 

Knowing that the fasciae connects one muscle or structure to another in a constant chain, everything is interrelated and a problem in the hyoid can negatively impact the function and mobility of both the front and hind end.

Scars

Where do scars come in? Scars and adhesions, where the body has laid down collagen fibers irregularly to heal after an injury, are a high priority in the osteopathic methodology because of their impact on mobility, position, and vascularity. A scar, even a small one, can have a wide-reaching impact on the fascial system, and create a “false” fulcrum, or an area that the body uses as an axis for movement, which can place chronic stress on the system and the structures in it. 

Scar tissue is not as functional as the original tissue it replaced, and can tether and pull on the surrounding tissue, restricting movement and stressing the fascial system and its structures. An osteopath can help release the tension and mobilize the adhesion. Photo: Flickr/Candice Camille; Inset photo: Flickr/Candace Gray

If you or your horse have a scar, especially one that does not move well, I highly recommend working with a professional to mobilize this tissue. I have had powerful results in sessions from working to mobilize an immobile scar. Most recently, I worked with a horse who had a traumatic injury to the right mandible and hyoid region with large amounts of scar tissue built up. This deep, older scar was impacting her entire spinal cord through the intense pulling on the hyoid, temporal, mandible, and sphenoid bones, causing her to have a lack of reflex in the muscles of her back. After just ten minutes of basic scar work to release the tension, the sensation was coming into her back muscles. Yes, it can happen that quickly. More on scars and simple ways to work with them in an upcoming article. 

For more on TMJ, read the article TMD – Does Your Horse Have It?

Thanks for coming on this adventure with me, and I wish you and your horses happy hyoids! 

Photo: Canstock/Zabava