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Injection of a drug into a joint is often used as treatment where pain is caused by synovitis — an inflammation of the soft tissues — generalised trauma or osteoarthritis.

If there is obvious swelling and pain on manipulation of the joint, with lameness that is accentuated after flexion for approximately one minute, it is highly likely that the painful joint is contributing to lameness. In the absence of these clinical signs it is usually necessary to inject local anaesthetic solution into the joint to see if the joint itself is the source of pain. If it is, there should be significant improvement in the lameness or performance after this local anaesthetic “block”.

The most common of the injected medications are hyaluronan — often termed hyaluronic acid or “acid” — and corticosteroids.

Hyaluronan is a normal constituent of joint fluid and is produced by the synovial membrane — the cells that line the inside of the joint capsule. If a joint is inflamed, the quantity and quality of hyaluronan produced is reduced. This has potentially deleterious consequences, because hyaluronan is responsible for lubricating the soft tissues of the joint and has a role in the nutrition of the joint cartilage.

It may also have some anti-inflammatory role. Injecting a synthetic version of the substance is thought to encourage the joint to normalise its own production of hyaluronan.

A vicious circle

Inflammation within a joint has a number of potentially serious consequences.

The resulting enzyme production can lead to a loss of important constituents of the joint cartilage. If the architecture of the joint cartilage is damaged, its shock absorbing capabilities will be reduced. Abnormal loads will then be transmitted through to the underlying bone, which may respond by becoming stiffer. This can result in a vicious circle of compromised function of a number of joint structures, resulting in progressive osteoarthritis.

Joint inflammation can also stimulate the production of an abnormal amount of synovial (joint) fluid. Although the fluid is important for joint lubrication and nutrition of the cartilage, an excessive amount may increase pressure within the joint and cause pain. The smooth running of the structure may then be affected, resulting in low-grade loss of stability that, again, may compromise function.

Corticosteroids are potent anti-inflammatory drugs that, if used judiciously, can stop this vicious circle. They act rapidly and have a potentially valuable role in the management of joint inflammation. There is a downside, however, as in the short term, corticosteroids can increase the loss of important structural components known as proteoglycans from the joint cartilage. This effect can be reduced by injecting hyaluronan at the same time.

Because of their powerful anti-inflammatory effect, corticosteroids also have the potential to reduce the joint’s ability to respond appropriately to challenge from bacteria and may mask early clinical signs associated with joint infection. When infection is then recognised, it may be quite advanced and extremely difficult to treat.

An additional and fortunately uncommon problem occasionally associated with the use of corticosteroids is the development of laminitis. This can be rapidly progressive, resistant to treatment and possibly life threatening.

Joint infections may benefit horses with osteoarthritis in the lower hock joints

Infection risk

Appropriate medication can be effective for damaged joints that are painful and cause lameness or reduced performance. There is an increasing trend, however, to inject the joints of upper-level competition horses on a regular, so-called prophylactic (or preventative) basis. Coffin, front fetlock joints, lower hock joints and stifles may be treated several times a year in an effort to maximise the horse’s performance.

It makes sense to perform regular assessments of these high-level athletes to determine whether there is any change in the way their joints look, feel and react to flexion. If a joint that was previously “tight” has mild increased filling, and the horse slightly resents flexion when he normally does not, this indicates something abnormal — even if the horse is sound. Therefore, it seems logical to treat the joint before a clinical problem develops.

If a rider complains of a subtle change in a horse’s way of going, it is reasonable to presume that this may be due to low-grade pain. Identifying the source of this pain and dealing with it before it becomes a major problem may involve treatment of a joint or physiotherapy, or both.

Treating a busy competition horse presents some challenges. It is essential to be aware of his competition schedule and the rules under which he will be competing.

As some of these treatments involve prohibited substances, there must be sufficient time between injection and any subsequent drug tests.

It is standard practice to treat a lame horse in the relatively clean environment of a veterinary clinic, where a surgical scrub can be performed before an intra articular (joint) injection is administered using a sterile technique. However, many competition riders prefer preventative injections to be carried out at their own yard and are reluctant to have a horse clipped in preparation.

It is crucial that the skin is cleaned thoroughly. The risk of introducing infection is real and the consequences potentially devastating. I am aware of several high-level competition horses globally that have developed career-threatening infections after “routine treatment” — that is, where several joints have been injected as part of a so-called “maintenance programme”, despite the absence of either prior lameness or performance problems.

Can routine injections ever be justified? No evidence suggests that regular joint medication prevents the development of osteoarthritis, nor that it enhances the performance of a clinically normal horse — especially if he is already going well.

Careful adjustment of the horse’s training schedule and the way in which his feet are trimmed and shod, along with consideration of the surfaces on which he trains, could well be more important. Might it not be better to persuade the rider to reduce his or her weight to reduce the risk of overload of the horse’s joints?

I am a great advocate of routine monitoring of competition horses, with the veterinary surgeon, physiotherapist, farrier, rider, groom, nutritionist and trainer all working together. Detailed clinical records must be kept so that changes in the appearance, response to flexion or range of motion of a joint can be recognised, and any abnormal muscle tension can be noted.

If a joint shows signs of injury despite the absence of detectable lameness, then treatment is, in my opinion, both justified and indicated. However, I question the value of the repeated treatment of symptomless joints when considering the cost, efficacy and the inherent risk to the horse, albeit small.

Joint medication may have become fashionable, and some riders may become psychologically dependent upon it, but the question must always be asked: is it of any real benefit to the horse?

Ref Horse & Hound; 23 August 2018