Horses rely heavily on the proper function of the temporo-mandibular joint (TMJ). The ability to move the jaw correctly — side to side, forward and back, up and down — affects not only their ability to masticate, which is the very first step in proper digestion, but also body balance and biomechanics.
Yet proper TMJ function often gets overlooked by veterinarians. I practice and teach a dental paradigm based on the principle that the TMJ is the “master link” in equine biomechanics. In balanced equine dentistry, we work toward maintaining equilibrium or a balanced state of repose between two or more antagonistic forces that exactly counteract each other. In doing so, we adjust the angles and planes of the cheek teeth and incisors. This maximizes the surface-to-surface contact between these biophysical planes and achieves optimal jaw biomechanics. Ultimately, we are looking to achieve three-point harmony between the incisors, molar tables and TMJ.
Anatomy
The TMJ is a complex array of bones, muscles, ligaments, fascia, nerves and cartilage. It is a synovial joint between the condyles of the mandible and the temporal bone. It is a very strong, tight encapsulated joint with an articular disc that allows for the gliding movement of the condyle. The horse’s TMJ is considerably more complex than a human’s because of three factors:
1. It is connected to the ears and horses have movable ears with 20 or more muscles. 2. The hyoid bone articulates with the temporal bone. 3. Horses have teeth erupting continuously throughout their lives.
Bones
1. The temporal bone is the most cranial bone in the TMJ mechanism. It is part of the maxilla. It is also home to the auditory or Eustachian tube. This is where balance and equilibrium are recorded through the vestibulocochlear nerve. Tightness in the muscles or temporo-mandibular joint disorder (TMD) can adversely affect the horse’s balance and proprioception. Also, the trigeminal nerve rests on the inside of the temporal bone. One of the three branches of the trigeminal nerve is the motor nerve to the muscles of mastication.
2. The mandible at birth consists of left and right halves that have a cartilaginous joining at the center, much like the pubic bone of the pelvis. These two halves fuse together and become a complete jawbone when the horse is two to three months old. The mandible is considered one of the most important “pattern setters” in the body. A pattern setter is a body part or condition that sets up patterns of habitual compensatory movement in other parts of the body. The “right handedness” of a horse, for instance (see sidebar), can often be dissected back to TMJ dysfunction. I believe this is also why limited range of motion in the TMJ can cause limited or asymmetric pelvic movement.
3. The hyoid apparatus is group of seven bones that cradles the larynx and pharynx, connects to the tongue and articulates with the temporal bone. Excessive tongue pulling, cribbing, throat surgery, and even chronic cough can create restriction in the hyoid apparatus, which can adversely affect the TMJ.
Muscles
Though several muscles are responsible for jaw movement, I will review the three that are most influential and palpable.
• The masseter muscle is the largest. It can be easily observed while a horse is chewing. It brings the upper and lower jaws together. • The temporalis muscle is the next major muscle and also easily observable. It works in conjunction with the masseter muscle. Its primary function is to close the mandible. • The pterygoid muscles attach from the central bone of the cranium, or sphenoid bone, and insert onto the medial aspect of the mandible. The function of the pterygoid muscles is to pull the mandible from side to side. Horses with TMD often exhibit great discomfort when palpating the medial pterygiod muscle trigger point.
The horse by nature is a grazing animal, and will graze up to 16 hours a day if allowed. When the head is dropped down in the grazing position, the mandible comes down and forward in the joint capsule and the atlantoaxial joint opens. This relaxes the muscles and connective tissue around the TMJ, and the upper and lower molars achieve proper contact. Many performance horses are kept in stalls and fed from hay bags or elevated feeders. This does not promote healthy TMJs.
Symptoms of TMD
These are many and varied and include low performance levels, imbalanced gaits or lameness, uneven tooth wear, head shaking, cribbing, teeth grinding, signs of headache, behaviors such as spookiness, ADD, ear sensitivity, head shyness and bitting difficulty, being a poor keeper, muscle wasting (especially on the top line), difficulty flexing the poll and engaging the hindquarters, and an unwillingness to change leads.
How it affects riding
Most riders use reins to communicate with their horses. They expect the horse to interpret their signals. If there is pain or imbalance in the TMJ, the signals are polluted. Any deviation from normal will alter the rider’s communication. If she asks the horse to move to the left by picking up the rein and adjusting her body, and it causes him pain, it is actually telling him not to go forward. The horse starts to break down millisecond by millisecond. First he will perceive a physical inability which soon turns into an emotional situation. He will not feel good because he cannot do what the rider is asking.
The balance of the teeth determines the readiness of flexion which must be obtained and maintained to facilitate correct maneuvers. If a horse’s jaw is restricted in anterior-posterior (A/P) motion, he cannot drop his jaw into neutral, raise his withers to allow the head to go forward and down, lift the lower cervicals and engage his hindquarters. Pain in the jaw will likely kill the desire for specific movements and sometimes for any movement at all. [insert diagram – to come]
The most common causes of restricted A/P motion are: • Incisor abnormalities, especially overjet or overbite • ATRs (accentuated transverse ridges) • Hooks on lower 11s or upper 6s • Missing molars • Wave complexes
By paying closer attention to the balance and function of the TMJ in your equine patients, you can help preserve their overall health and performance levels.
The right-handed horse
• Incisor wedge sending maxilla to right, mandible to left. [insert image – to come] • Atlas superior on left. • Horse camps on left front leg, has larger left foot, often more prominent shoulder muscles, often hollow behind left shoulder. • Pelvis is most often in reciprocal relation with the jaw, creating a right posterior ilium. In other words, the pelvis is rotated forward and upward on the right. A full 75% of horses I see have a reciprocal pelvis to the incisor wedge; 25% mirror the wedge. Dr. Judith Shoemaker taught me almost two decades ago that horses have different levels of proproiceptive awareness. Some try hard to correct their imbalances, while others just adapt to leaning. • Ribcage expanded on left, contracted on right. Already bending to right and very difficult to bend left. • Tail often held to left.
References
Gronberg, Pauli. (2002) ABC Of The Horse, A Handbook of Equine Anatomy, Biomechanics, and Conditioning. www.pg-team.fi.
May, Kevin J., DVM. (2008) Interrelationships Between Equine Acupuncture, Chiropractic And Dentistry. Procc. of 34th IVAS 2008 international congress, Keystone, CO, USA.
Rooney, James R. Clinical Neurology of the Horse. KNA Press Inc. 1971 June, First Edition.
Upledger, John E., DO, FAAP. Craniosacral Therapy II, Beyond The Dura. Eastland Press Inc. 1987
Willoughby, Sharon L., DVM, DC. (1992) Sacropelvic Module
Dr. Heather Mack graduated from the University of Pennsylvania School of Veterinary Medicine in 1991. In the early 1990s, she received certification from both the International Veterinary Acupuncture Society (IVAS) and the American Veterinary Chiropractic Association (AVCA). She began her studies of Traditional Chinese Medicine and acupuncture in 1985 while attending Columbia College. She runs an Equine Sports Medicine practice and has been breeding, raising, training, observing herd dynamics and healing horses at her Mystic Canyon Ranch in Idaho for over 14 years. balancedequinewellness.com
