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When treating a horse, you might notice me placing my fingers beneath its jawline and feeling around. I'm checking for tension around the hyoid apparatus that could be pulling it out of alignment and affecting surrounding structures. Why am I doing this?
by Andria Watson-Massie Equine Sports Therapist, The Equine Healing Center
The hyoid bone plays a pivotal yet often overlooked role in horses. While it is recognized for its impact on the horse's airway during exercise, its influence on biomechanics and horse locomotion has received more attention in equestrian circles over the last 10 years.
Located in the throat area beneath the jaw, this small bone structure supports the tongue and larynx and is essential for respiration and swallowing. But that's not all. The hyoid's unique anatomical design and intricate connections to structures extending from the tip of the nose to the tail make it vital for horse locomotion and performance.
Despite its small size, dysfunction of the hyoid bone can have extensive effects on a horse’s gait and performance, especially during exercise. This often manifests as the horse's tendency to go behind the bit to alleviate tongue pressure and reduce tension in the neck’s ventral muscles.
Reluctance to move forward, engage hindquarters, or perform lateral movements can all stem from indirect connections of the hyoid bone to the pelvic region. When assessing horses with neck pain or tension or those having difficulty going forward or accepting the bit, I find it is always useful to assess the hyoid apparatus and its related muscles.
In this article, I’ll address why the hyoid bone has begun to capture significant attention in veterinary and equine rehabilitation circles and the profound importance this small bone has on a horse’s performance.
Anatomy of the Equine Hyoid Bone
The equine hyoid bone has been largely underrepresented in anatomical diagrams until relatively recently. The hyoid bone is a complex and delicate structure linked to the skull and provides attachment for important muscles that radiate down through the chest and forelimbs. It consists of several interconnected bones that form a Y-shaped apparatus, each with its specific function and connection to surrounding structures like the tongue, the poll, and the temporomandibular joint (TMJ)
Components of the Hyoid Bone:
Stylohyoid: (Longer pair) These bones connect with the temporal bone at the base of the skull, allowing for cranial and caudal movement. They’re the largest bones in the hyoid apparatus.
Epihyoid: A smaller bone that connects and articulates the stylohyoid and ceratohyoid.
Ceratohyoid: Positioned between the epihyoid and the thyrohyoid.
Thyrohyoid: (Shorter pair) Connects the hyoid apparatus to the thyroid cartilage of the larynx.
Basihyoid: The central bone of the apparatus, supporting the lingual process where the tongue attaches. Arguably one of the most important components.
Each component is connected to surrounding structures via small muscles, ligaments, and fascia (connective tissue). Overall, they collectively contribute to the hyoid's functional significance in supporting breathing, swallowing, and communication.
The Significance of the Hyoid Bone in Equine Biomechanics
Every muscle in the horse's body eventually connects to the Hyoid. The hyoid bone forms an intricate network of muscular and structural connections that extend upwards to the skull and downwards to the trunk and forelimbs, thereby influencing the horse’s respiratory, vocal, and locomotive functions. The contraction of the muscles alters the position and shape of the hyoid bone, which in turn alters the shape of the larynx and nasopharynx. Here are the three major connections:
Sternohyoideus and Sternothyroideus: These muscles link the horse’s tongue and mouth to the sternum in the chest. From this connection, the fascia and muscles extend to the pectoral and abdominal muscles, which continue into the pelvis. This pathway is pivotal for the ‘lifting and rounding’ of the horse’s trunk, allowing the hindlimbs to engage more effectively beneath the body. Any functional changes or tension in the sternohyoid and sternothyroid muscles can influence the engagement of the hindlimbs and, ultimately, movement.
Omohyoideus: Originates near the shoulder joint and inserts on the basihyoid bone, linking the hyoid into the fascial chain that runs from the head, down the inside of the neck, across the middle of the shoulder, and extends all the way to the toes on the hindlimb. Any change in tension in the omohyoideus results in restricted shoulder movement and limits the contraction of muscles along this chain, which affects the horse's stride and balance.
Occipitohyoid: This muscle creates a direct chain from the shoulder through the hyoid to the poll, and from there through the nuchal ligament and the dorsal fascial line back to the hindquarter. It's a dynamic and continuous chain that extends from the front to the rear of the horse.
The Hyoid's connection to the Tongue (Swallowing and Breathing)
I've explained how the basihyoid bone supports the lingual process to which the base of the tongue is attached. The hyoid bone's position and structure directly affect the horse's respiratory system. It anchors the tongue, stabilizes the larynx, and facilitates the opening and closing of the airways during breathing.
Pain or dysfunction in the tongue restricts the tongue's ability to move, which in turn restricts the movement of the hyoid and related muscles, resulting in tension and pain in these muscle groups. This restriction ultimately affects the entire myofascial chain and alters the horse’s movement and biomechanics.
Common Disorders Associated with the Hyoid Bone
The hyoid bone is a delicate structure susceptible to injury, misalignment, and even fracture, even though it is located in a protected area within the horse's mouth. Although these conditions are rare in general, they are the most common that affect the hyoid bone. They include:
Temporohyoid Osteoarthropathy (THO): A degenerative condition that causes the stylohyoid and temporal bones to fuse at the temporohyoid joint. Fusion of the joint results in a decreased range of motion and flexibility, which makes the stylohyoid bones more vulnerable to fracture from head or neck movement.
Fractures can occur due to trauma, such as sudden jerks or kicks.
Restrictive bits and bridles can impede tongue movement, causing pain and tension in the hyoid and surrounding muscle groups.
Mishandling of the tongue such as pulling it around during a dental or practices like tongue-tying (intended to improve airway function) can lead to misalignment or, in severe cases, fractures of the hyoid bone.
How would I suspect a hyoid injury in my horse?
You should be attentive to some of these signs, as they may indicate Hyoid restrictions in your horse:
Mouth pain or difficulty accepting the bit
Changes in gait, Reluctance to move forward or engage the hindquarters
Holding their head to one side when ridden
Difficulty swallowing
Incorrect canter lead
One-sided chewing
Head shaking or tossing
Struggling to soften onto the bit
Preference for one rein over the other
Teeth grinding
Opening their mouth when ridden
You can gently feel the area between your horse's jawbones to check for hyoid imbalances. Tenderness or tension in this area doesn't necessarily mean a hyoid problem but could indicate issues needing professional care. If your horse exhibits significant tension in the ventral neck area, it could be a sign of hyoid dysfunction.
Diagnosing Hyoid Imbalance
Consult your veterinarian if you suspect your horse's pain or poor performance is due to hyoid apparatus dysfunction. Since the hyoid and its associated structures are sensitive and delicate, you should work with a trained professional for proper assessment and treatment.
First, I observe the horse's gait and movement, as they could provide clues to potential issues. Then, I conduct a comprehensive assessment of the horse's history, including diet, previous dentistry, exercise routine, and equipment used.
A hands-on examination is necessary to assess the movement and function of the jaw, head, and poll. In the examination, I’m looking out for pain-indicative behaviors, pain/sensitivity to touch, and imbalance of the Hyoid. Through careful palpitation, I assess muscles involved in chewing, neck muscles, and fascia that extend from the sternum to the hyoid apparatus. I feel for the joints and muscle movement and gently assess tongue motion. I also check the movement and function of the TMJ, cranial neck joints, and sternocephalic muscles, as they are anatomically and functionally linked to the hyoid.
In rare cases where a fracture is suspected, x-rays are initially used to evaluate the bone, and if still inconclusive, advanced imaging techniques such as CT scans may be employed to obtain a definitive diagnosis.
Treatment and management
This depends on the severity and specific condition. Extreme conditions may involve surgery. For general Hyoid imbalances, manual therapies like hyoid mobilization can work wonders. This technique involves working your fingers along the surface of the inner mandible and observing for signs of release. This would be demonstrated if required. Laser therapy on acupuncture points, myofascial release, and under-scapula release are other effective non-invasive therapies.
How the Hyoid Affects the Horse's Movement- Key Takeaways
Here are the important details to note:
We've established that the hyoid bone is directly connected to the tongue, forelimb, and poll region and indirectly linked through muscle and fascial chains to the hindlimb.
Tension around the hyoid apparatus can cause tightness in the poll, TMJ, jaw, neck, shoulders, and forelegs.
Because these structures are interconnected, any dysfunction, pain, or restricted movement of the hyoid bone can impair forelimb mobility or hinder the horse's ability to engage its hind limbs.
Hyoid imbalances may present as an inability to perform lateral movements, a shortened stride, a limited range of motion in the cervical spine, and tension in the neck's ventral muscles, resulting in a horse going behind the bit.
Case Study
When Midnight, a 9-year-old Thoroughbred gelding and competitive dressage performer, arrived at our clinic, he exhibited symptoms of head tossing, reluctance to move forward, and resistance to the bit, which severely impacted his performance.
Upon examination, I noticed that Midnight showed significant discomfort when touched at the poll, and his movements were stiff and lacked the graceful fluidity one should expect. A physical examination revealed tension in the muscles associated with the hyoid apparatus and a reduced range of motion in the TMJ.
Based on these findings, I began a series of physical therapy sessions targeting the neck and poll area. We also adjusted Midnight's bridle and bit to relieve pressure on the hyoid region. Within two weeks of treatment, Midnight's movement improved significantly, and he could return to training with greater freedom and flexibility.
Conclusion
It is often said that the smallest cog in a machine can be critical to its entire operation. This is certainly true of the hyoid bone in horses. It's easy to assume the hyoid is only useful in respiration and swallowing, but there is a significant relationship between this apparatus and the horse's locomotion. In this article, I have explained how a problem that starts at the top of your horse in the hyoid apparatus can cause tightness, tension, and restrictions all the way down to the foot.
If you have any questions or believe your horse could benefit from treatment in the hyoid region, along with a comprehensive assessment, reach out to me. I’d love to hear from you.
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