Sore jaws

True pathology of the temporomandibular joint is an uncommon cause of a number of relatively common signs. Robyn Graham MRCVS explains how the problem is identified and treated

- Robyn Graham

THE term temporomandibular joint (TMJ) pain has been used broadly for a number of years for horses with poor performance and head shaking. However, true TMJ pathology is very rare.

The temporomandibular joints connect the jaw with the rest of the skull on each side of the head, and provide articulation between the mandible (lower jaw) and the maxilla (upper jaw). They are two independent joint compartments separated by a biconcave disc (meniscus).

This disc and the articular surfaces of the joint are unusual in that they consist of a fibrocartilaginous material that is able to adapt to the loads of mastication chewing over time.

This differs from other commonly investigated joints in the musculoskeletal system, which are protected by hyaline cartilage (which is more flexible but can take less mechanical stress).

This difference may have implications when it comes to the treatment of problems within the joint.

Pain or discomfort, or both, of the TMJ is commonly reported in human medicine and can be associated with a wide variety of clinical signs. Similarly, horses with pathology of the region can present with a varying array of signs, including:

• Intermittent head shaking 

• Difficulty eating, poor appetite, or both 

• Changes in mood (the horse is dull or depressed) 

• Sticking the tongue out during exercise 

• Reluctance or refusal to collect which can present as sticking the nose in the air, reluctance to go forward, or bucking or rearing.

Given that stress is applied to the joint when pressure is exerted to the bit via tension through the reins, especially when this occurs in a downwards direction, clinical signs may be exacerbated during collected work.