The truth about the jaw joint

The horse’s head is a complex area for vets to examine, diagnose and treat. Modern diagnostic imaging techniques, such as CT scans have deepened our understanding of the complexities of the many different structures involved.

A horse has seven pairs of air-filled sinuses within the skull, and between 36 and 44 teeth. The majority of these teeth have very long roots. This means that interpreting an X-ray image can be tricky, as these structures may be superimposed on top of each other.

X-ray pictures may be sufficient for diagnosis of an obvious skull fracture, however they do not always provide your vet with a clear answer when looking for subtle changes, such as when trying to localise an infected tooth root or sinusitis. The vet will need to know precisely which tooth to remove or which sinus needs to be drained, so accurate imaging is important.

It can be even harder when dealing with a vague problem, such as headshaking, when one needs to rule out any abnormality in the head that could be contributing to this debilitating condition.

It is possible to see inside some of the different compartments within the skull. Vets can make a small hole or surgically open a bone flap in the skull and then pass a flexible endoscope inside the sinuses. There are also sophisticated rigid endoscopes with tiny cameras that are used to look at the teeth and examine the back of the long, narrow oral cavity inside a horse’s mouth.

Unlike a person, we cannot just ask the horse to open wide and then look inside.

An open and shut case?

Examination of the moving parts of the skull can be difficult. This includes the jaw joint, properly called the temporomandibular joint, or TMJ.

There is one of these joints on either side of the horse’s head, connecting the skull and the lower jawbone. The TMJ is crucial to enable the mouth to open and shut and it works hard, as a horse in the wild will spend 75% of his day grazing. Despite this, surprisingly little is known about these structures.

In recent years there have been huge efforts to learn more about and manage equine teeth. There are many dental treatments performed, yet the hinge that keeps the jaw moving has been largely ignored.

This may be because TMJ disease is rare in the horse, but equally it may be because we are unaware when a horse’s jaw hurts. Horses are prey animals and tend not to show pain, because their instinct is to conceal any vulnerability on the basis that if they look weak they are likely to be attacked.

Whenever a horse grazes, the two TMJ joints will be in almost constant motion and are crucial for proper chewing. The jaw joint is unusual in that it has a meniscus — a small pad of fibrocartilage — which acts as a cushion within the joint. Most joints do not have these in-built shock absorbers, although they are found in the human knee and the equivalent equine stifle joints.

The TMJ has a special sort of fibrocartilage made up of fibres, like normal connective tissue, rather than the “hyaline” (transparent) cartilage found elsewhere.

Canadian research has shown that as a horse chews the jaw moves in three dimensions — not only up and down, but also horizontally across and backwards and forwards. This makes sense when one thinks of a horse’s high-fibre diet. The jaws need to work quite hard to chew hay, grass and feed.

Despite performing the same repetitive action many millions of times, the TMJ rarely seems to go wrong. There are almost no published reports describing disease of this joint, apart from the occasional severe infection or fracture. This has led to speculation that perhaps jaw joint disease is more common, yet under-recognised. Alternatively, it may be truly rare and the joint is a particularly healthy structure.

There is also the question whether it is affected by a horse’s eating position. Is it better if his head is down at ground level, in the natural position, or is it affected when the head is up and eating from a haynet or manger, which some suspect may harm a horse’s back and have an impact on teeth wear?

Looking for links

In people, there may be links between dental disease and TMJ pain. In horses, the joint is very different to man and there is no firm proof that dental disease and TMJ pathology is linked.

Having said that, one might think horses with very irregular teeth might be more likely to develop TMJ problems, but this has never been proven. Even so, it can be imagined that an awkward head carriage and a tight noseband with overgrowths of the check teeth may well make the jaw ache.

It is possible to do a nerve block and anaesthetise an individual TMJ joint to test what difference that makes. This is similar to putting local anaesthetic in a limb joint, such as when investigating lameness.

This sort of clinical test only works when there is a clear difference, such as a change in lameness or way of going, that can be seen after the nerve block.

Logic would suggest that horses with painful jaw joints would be reluctant to eat or would show problems when ridden, such as headshaking, head shyness, or fighting the bit. Nerve blocks can be tried, but such tests are not going to be easy to interpret for more subtle signs. Equally, there can be many other reasons to explain problems such as headshaking or a reluctance to accept the bit.

Until more is known, be cautious if TMJ problems are suggested. Have the problem investigated properly and watch carefully for signs of discomfort. If your horse does have proven TMJ pain, then hopefully your vet will record it to expand the knowledge of this mysterious equine joint.

Ref Horse & Hound; 17 November 2016