The Hype About The Hyoid

Hyoid Apparatus: Who would have thought that a small and fragile bony structure located at the ‘floor’ of the horse’s mouth between the two halves of the horse’s mandible (lower jaw) would be so important to a horse’s athletic performance?

Much has been written about the hyoid apparatus’ affect on the horse’s airway during exercise, but not enough attention is being paid to its influence on biomechanics – especially by riders.

The hyoid apparatus is a Y-shaped group of bones that joins onto the skull and provides attachment for several muscles, including the tongue.

The muscles that control the tongue and airways, including those that attach to the hyoid apparatus are small and sit deep between the horse’s ‘jawbones’ (the mandibular rami). These muscles control the position and function of the tongue, and also contribute to the opening of the pharyngeal airway during breathing.

What perhaps isn’t known, is that some of the muscles that attach to the hyoid apparatus also extend upwards to the horse’s skull (occipitohyoid) and downward to the sternum (sternohyoid and sternothyroid), and the scapula or shoulder blade (omohyoid).

These muscles can be seen and felt in the ventral area of the horse’s neck.

Making connections

Looking at the image on the left as well as the anatomical illustrations in the following pages, you can see there is a direct muscular connection between the hyoid apparatus that sits between the bones of the lower jaw (the mandibular rami), upwards to the skull (the poll) and downwards to the trunk and the forelimbs.

Although the hyoid is attached to muscles that contribute to opening the airway as well as those involved in chewing and swallowing, there is an important relationship between the hyoid apparatus and the horse’s locomotion, that is, the horse’s performance.

Let’s look at the anatomy in more detail.

Consider, for example, the sternohyoid and the sternothyroid muscles. They connect the horse’s tongue directly with the trunk via the sternum.

Arising from the sternum we also find the pectoral and abdominal muscles, and their fascia – this whole muscle/fascia chain continues ventrally connecting the pectoral muscle region with the abdominal muscles and the pelvis.

These pectoral and abdominal muscles are the ones that contribute to ‘lifting and rounding’ of the horse’s trunk enabling the hindlimbs to come underneath and become ‘engaged’.

Now consider the omohyoid muscle that runs from the fascia around the shoulder and shoulder blade (the scapula) to the hyoid apparatus.

The scapula is attached to the trunk and neck of the horse by muscles, of which omohyoideus is one. This muscular attachment is called a synsarcosis and is unlike other joints in the body because there is no bony connection of the forelimb to the trunk of the horse.

This arrangement allows the forelimb to have a greater amount of movement in a forward and backward direction, one of the important factors for locomotion in the horse.

Any alteration of tension in the omohyoideus can potentially affect the shoulder blade’s motion, thus affecting the stride length and gait.

Alteration in function or tension in the sternohyoid and sternothyroid can ultimately influence some the muscles and fascia responsible for engagement of the hindlimb.

Omohyoideus, sternohyoid and sternothyroid although important in locomotion, are also considered accessory respiratory muscles, as they attach to the hyoid apparatus.

So, we’ve established that that hyoid is connected directly to the tongue, forelimb and poll region, and also via muscle and fascia chains to the hindlimb.

Any dysfunction, pain, or limitation of motion of the hyoid can potentially affect the mobility of the forelimb during locomotion, affecting the performance of the equine athlete.

What about the tongue muscles?

Mastication (chewing of food) is clearly important for the horse’s nutrition and behaviour. Two muscles that are responsible for tongue action in chewing are called the mylohyoid and geniohyoid muscles and, as their name suggests, they are attached to the hyoid apparatus.

Other muscles which run from the hyoid apparatus to the tongue are named the genioglossus, hyoglossus and styloglossus – these not only depress and retract the tongue but synchronise with respiration.

Depression of the tongue plays a critical role in dilating and stabilising the nasopharynx during breathing.

So, what can adversely affect activity of the tongue muscles? Pain.

Tongue pain can result from a dental issue and needs to be dealt with, as it directly affects the horses’ ability to chew and digest food. Another potential cause of pain and/or dysfunction of the tongue can be the bit and how it’s used, a restrictive or overtight noseband and, in racehorses, tongue ties.

The tongue is pain sensitive, so anything that compresses and affects the position of the tongue can potentially cause a dysfunction. Equipment that restricts the function of the tongue will limit the movement of the hyoid and associated muscles, causing pain and tension in these muscle groups. This, in turn will affect the whole myofascial chain and result in changes to the horse’s movement and biomechanics.

These are all important issues we should consider in terms of the horse’s welfare and performance.

In addition, there are other very small muscles attaching parts of the bony hyoid apparatus to each other, to the tongue, and to the cartilages that form the nasopharynx and larynx of the horse.

Styloglossus retracts the tongue (pulls it back in the mouth). Genioglossus moves the tongue forward and moves the basilohyoid bone.  Hyoglossus works with genioglossus to depress and retract the tongue, and they are synchronous with respiration, meaning that the activity of these muscles correlates well with airway aperture during breathing.

Of course, having good airway function during exercise and being able to chew appropriately for good nutrition are both important for performance.

Thus, if there is dysfunction of this seemingly small and insignificant bone suspended inside the jaw area of the horse, there can be wide ranging effects on gait and performance, including breathing, in the exercising horse.

Some of the specific dysfunctions in movement may involve a tendency for the horse to go behind the bit to relieve tongue pressure and to relieve tension in the ventral muscles of the neck.

The horse may be reluctant to go forward or engage the hindquarters, or move in a lateral direction, due to the connections of the hyoid indirectly by muscles and fascia to the pelvic end of the horse.

As a physiotherapist assessing horses with neck pain or tension, or horses that have problems going forward or accepting the bit I find it is always useful to assess the hyoid apparatus and its related muscles.

How can I tell if my horse has a hyoid dysfunction?

If your horse seems to have pain in the mouth, or is not accepting the bit, dysfunction of the hyoid bone could be a component of this. However, if you suspect mouth pain, a dental assessment is the first important step.

You, as an owner, can very gently feel the the area between the horse’s lower jaw bones (the rami of the mandible). Tenderness or tension there is not necessarily a diagnosis of a hyoid problem  as there could be dentition issues which require professional attention.

Asymmetry (unevenness) of the muscles of mastication that you can easily see  – the muscles of the cheek (masseter) and the area above the eye and below the ear (temporalis) – may indicate a temporo-mandibular joint (TMJ) problem.

Dysfunction or asymmetry of the temporo-mandibular joint can impact the hyoid and vice versa, and the signs may be closely associated with each other. Asymmetry of teeth can affect the function of the TMJ, which is another reason to have a dental assessment.

If your horse holds a lot of tension in the ventral area of the neck, where the long strap muscles run from the sternum and shoulder blade to the hyoid, there could be a hyoid dysfunction.

Ex-racehorses who have been subject to the practice of tongue-tying (as an attempt to improve airway function) could be prone to problems of the hyoid apparatus. Further, horses who have been windsuckers may have placed tension on the hyoid apparatus and the TMJ.

What do I do?

If you suspect that hyoid apparatus dysfunction may be the cause of pain or poor performance in your horse, the first step is to have the teeth checked by your dental veterinarian. Most equine dentists are considerate in moving the tongue around so that the hyoid bone is not put under undue stress.

Step two is to have a qualified equine practitioner such as a veterinary/animal physiotherapist to assess your horse. Of course, you should always check the qualifications of your equine practitioner.

As an animal physiotherapist, in my teaching or treatment, if I suspect hyoid apparatus is contributing to a horse’s performance, I will first make a comprehensive assessment including history, equipment used and evaluation of the gait and movement.

It is then important to examine the movement and function of the jaw, head and poll, including the muscles involved in mastication, the muscles of the neck, and the muscles and fascia that run on from the sternal attachment of the hyoid apparatus.

This examination is made through observation and detailed palpation, feeling the joints and muscles and their movement. Very gentle assessment of tongue motion, as well as feeling around the area of the cricoid cartilage from which small muscles run to the hyoid and tongue, can be useful.

Nevertheless, as the hyoid and associated structures are so delicate, I strongly advise all horse owners not to undertake any manipulation, however gentle it may seem, of the tongue or TMJ.

Examination of the movement and function of the TMJ and the most cranial neck joints is also important, as they are anatomically and functionally related to the hyoid apparatus.

In addition, questions about diet and previous dentistry, the bit and bridle that are used, and the horse’s previous occupation are important in formulating an assessment of the hyoid apparatus.

Take home message

So now you know, as the old song goes, that the neck bone is connected to the jawbone and the jawbone is connected to the chest bone… which is indirectly connected to other areas of the horse!

The take home message of this article should be a better understanding of the anatomy of the hyoid and acknowledgement that a dental, tongue or hyoid dysfunction will have a cascading effect along the entire body.

Tongue ties are best avoided. Bits and bridles should be chosen carefully and be adjusted in ways that allow the hyoid apparatus to function freely.

As well as choosing the right equipment, a crucial factor of your training will be the pursuit of ‘lightness’ and self-carriage – that state of flow when the horse maintains his gait, line and posture on the lightest of contacts.

And, of course, all horses benefit from a support team that is made up of qualified experts in different fields (coaches, nutritionists, physios, veterinarians, etc) all working together to protect their welfare as well as improve their training and performance.

References:

Holcombe & Ducharme, (2008) Hyoid Apparatus, in Chapter 3.1 Upper airway function of normal horses during exercise, in Equine Exercise Physiology, Hinchcliff, Geor and Kaneps (Eds), Saunders, Elsevier, pp 170-192

Kuryszko, J & Lyczewska-Mazurkiewicz, S (2004) Equine Masticatory Organ Part III, Acta of Bioengineering and Biomechanics, 6 (1), p 25-31

Manfred, J & Clayton, H (2005) Radiographic study of bit position within the horse’s oral cavity. Equine and Comparative Exercise Physiology. 2(3) p 195-201

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This article, “The Hype About the Hyoid” by Dr Lesley Goff was printed in the July-August 2020 issue of Horses and People Magazine.