RPC Coomer MA VetMB CertES Diplomate ECVS MRCVS

Suspensory ligament (SL) problems are surprisingly common in horses, affecting not only those in the competition world, but any age and type of horse working hard for its living. Proximal suspensory ligament desmitis (PSLD) rightly commands a healthy fear from those involved in competition work since treatment can be prolonged and outcome by no means guaranteed. This article focuses on what is the suspensory ligament, the type of injuries seen to the upper part of the suspensory ligament, treatment options and prognosis.
ANATOMY
A ligament is a tough fibrous structure which attaches two adjacent structures and has no contraction ability, whereas a tendon connects a muscle to bone. There are close similarities in their microscopic structure and physical properties, although ligaments are generally less stretchy. The SL is the evolutionary descendant of the interosseus muscle and there is one in all four legs. Clenching your fist uses contraction of your interosseus muscles. The interosseus connects the upper part of the cannon bone and small knee/hock bones to the digit. In the horse the SL cannot significantly shorten like a muscle, although there are still muscle fibres within the structure making up 2 to 10 % of the structure; these may play a role in damping vibrations during loading. In its upper part it starts on the back surface at the top of the cannon bone with small branches running up into the back of the knee or hock. It runs down the back of the cannon bone between the splint bones where it is held in place by very tight connective tissue. This is why you can't see or easily feel the ligament, nor appreciate any swelling there. It divides in half around two thirds of the way down: one branch runs to the inside and one to the outside sesamoid bones, just on the back of the fetlock joint. These branches along with the lower part of the body can be easily felt in any equine as the taut vertical structures running down the back of the cannon near the fetlock. The SL can be injured at any point along its structure and classically these injuries are classified into (1) proximal (upper), (2) body injuries (middle), and (3) branch (lower). This article does not discuss injuries to the body or SL branches, which are clinically rather different from PSLD.
DESMITIS
Desmitis is to a ligament what tendonitis is to a tendon, namely inflammation. Hence, PSLD is equivalent to tendonitis localised in the top of the SL. Injury may be caused by tearing of the attachment to the bone, build up of micro damage through trauma, or just over-use. Working a horse when it is tired dramatically increases the risk of SL injury. Always doing a good warm up, warm down and gradually increasing fitness is vital to avoid injuries in equine athletes. Front legs and hind legs can be affected by PSLD. Injury can be primary, through over use and exacerbated by poor conformation and/or foot balance, or secondary to injury elsewhere through compensation. An example of compensatory PSLD can occur with a horse working with fetlock arthritis in the left fore: the right hind has to work harder on the diagonal and so may develop desmitis. Horses with upright (high) pastern and straight limb conformation are at higher risk of injuring the SL because of the forces running up the limb.
RECOGNISING THERES A PROBLEM
Not as easy as it sounds, diagnosis can be problematic because lameness is often very subtle, especially to begin with. The quicker PSLD is diagnosed and treatment commenced the higher the chance of a full recovery. You may only notice a small and unaccountable loss of performance to begin with. The author recently tested blood and supplied iron supplements at the owners request for a mildly anaemic horse with this history. When treatment was unsuccessful and the horse was properly examined by the author two months later, low grade lameness attributable to hind leg PSLD was diagnosed and treatment commenced. It may be that your horse prefers one rein or the other, or throws you off balance on one particular diagonal. These are non-specific alarm bells which should ring and alert you to the possibility of 'performance lameness', a.k.a. subtle lameness. It doesn't necessarily mean your horse has PSLD, it may have another lameness condition, but what it does tell you that experienced veterinary orthopaedic examination should be sought without delay.
DIAGNOSIS
Pain in the suspensory ligament can sometimes be felt by squeezing the top of the SL, but interpretation can be difficult because false positives and negatives are common. Lameness is localised to the area of the upper SL using nerve blocks to rule out pain elsewhere, before blocking the actual origin of the SL. Your vet will need to be able to see improvement, so if loss of performance on a certain rein is all you can see then come prepared to ride for the vet. You can ride after the nerve block and report whether or not the horse feels more comfortable. Having localised it, ultrasound examination of the area is usually carried out to assess the appearance of the SL. Desmitis causes swelling of the ligament, visible on ultrasound as an increased cross sectional area, loss of fibre pattern and less clear definition at the edge of the ligament (Fig 1). More severe cases may develop holes in the ligament. X-rays are also frequently taken to check for bony abnormalities, whilst magnetic resonance imaging (MRI) provides the most thorough evaluation of this area if ultrasound is inconclusive.
Pain in the proximal SL area has traditionally been described as PSLD and frequently carries with it the telltale diagnostic clues described above. As is the case with navicular syndrome, the advent of MRI has made it increasingly clear that this one diagnosis may cover a multitude of separate soft tissue injuries, the end result of which is a degenerative process in the upper SL currently lumped together as PSLD. This is currently an area of active research and will likely change soon. It explains how some horses appear to do much better than others with the same condition.
TREATMENT
Initially rest and anti-inflammatory medication is appropriate, followed by a controlled exercise rehabilitation plan. Optimising foot balance, shortening the toe and providing good heel support with a bar shoe are crucial. If the problem has been recognised early then local injections with anti-inflammatory medication can be useful. Intramuscular injections of Adequan or Cartrophen may also improve healing, but these drugs work better not given at the same time as other anti-inflammatory drugs like bute. If there is a hole in the ligament, direct injection of stem cells or platelet-rich plasma (PRP) is helpful. Shockwave therapy of the ligament is frequently used as part of treatment because this has been shown to increase the proportion of cases returning to soundness. Rehabilitation for the front leg generally is successful in 3 months, but in the back leg can take up to 12 months. Remember that the PSLD may not be the primary lameness problem and so diagnosis and treatment of the primary problem elsewhere needs to be carried out as well.
Surgical treatment has a higher rate of success than conservative treatment alone and can be used for those cases which do not regain soundness following a conservative rehabilitation plan. One of the commonly employed surgical treatments for PSLD cuts the nerve supplying the top of the SL and also divides the tight connective tissue covering the ligament to release pressure. This technique is commonly used in the back leg, but is less commonly required in front. It removes the sensation of pain and hence lameness. By cutting the nerve the muscle fibres within the ligament have been shown to rapidly shrink up and disappear. The significance of this for the horse, if any, has yet to be assessed but some have proposed that it may increase cumulative wear and tear damage to the SL and ultimately result in failure and lameness. Another reported surgical treatment with a similar success rate uses linear splitting of the ligament and leaves the nerve intact.
Proximal suspensory ligament desmitis in the front leg carries a good prognosis with around 90 % of horses returning to soundness after a 3 month rehabilitation plan. If the problem has been present for over 5 weeks then return to soundness is less predictable. Shockwave therapy can increase the chances of chronically affected horses recovering, whilst local injection with anti-inflammatory drugs or counter irritants has also been reported to be beneficial. The situation in the back leg is very different, possibly because spotting hind leg lameness is more difficult and problems tend to go unnoticed for much longer. Most cases improve initially, but full recovery is much less common and only 14 % of horses in one study regained full soundness, all of which had been lame for less than 5 weeks. Shockwave therapy can boost this success rate to around 40 %, but this still leaves well over half of affected horses chronically lame. Luckily, surgery of these cases has been reported to be successful in around 80 % of cases.
TAKE HOME MESSAGE Prevention is much better than cure, so follow an exercise plan which aims to increase fitness gradually and never work your horse hard when it is tired. Make sure you optimise foot balance and pay for good farriery. If you notice a loss of performance, what order of investigation do you instigate? If the answer is riding friend(s), instructor, osteopath, chiropractor, physio, back man, saddler, farrier, quack and the yard dog before the last resort, the vet, then beware! Time and therefore prognosis may be squandered potentially at the expense of long term soundness. Subtle hind leg lameness can be difficult to spot even for experienced vets. If you notice an unaccountable and consistent loss of performance then make arrangements for experienced veterinary examination and make sure you get a definitive diagnosis. Many conditions cause loss of performance, some medical and some orthopaedic, but whatever it is it will be more cheaply treated with a better success rate if there has not been a long delay. PSLD generally carries a good prognosis in the front legs, but can be disastrous in the hind legs. Happily, surgical treatment of these cases carries good success rates.


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