How significant are soft tissue injuries within this complex joint? Matt Smith MRCVS explains

THE stifle is a hard-working joint with a large range of motion, which connects the horse’s thigh and crus (lower limb) regions.

It actually comprises two joints. The femoropatellar joint is located at the front of the stifle, between the ridges at the bottom of the femur and the patellar (the equivalent of our kneecap). The femorotibial joint sits between the bottom of the femur and the top of the tibia, and is divided into medial (inside) and lateral (outside) compartments.

Unlike in most joints, where the opposing cartilage surfaces fit into each other to create a smooth, gliding surface, the femorotibial joint features smooth discs of tissue called menisci, which sit between the ball-shaped ends of the femur and the plateau at the top of the tibia.

A complex array of ligaments supports the stifle and provides stability. On the inside and outside are the collateral ligaments. The patellar ligaments connect the patellar to the tibia, at the front, forming a functional extension of the quadricep muscles. In the centre, the criss-crossed cruciate ligaments connect the bottom of the femur and the top of the tibia.

Tissue injuries

STIFLE injuries are quite common and the soft tissue structures are frequently involved. The range of issues is similar to those seen in the human knee, with meniscal injuries occurring more often than damage to the cruciate and collateral ligaments.

Injury usually follows trauma to the joint. Lameness, often severe, is usually the first clue. While this can improve with rest, moderate lameness typically persists.

Fluid swelling of the stifle joints will be apparent when the lame leg is examined. In the most severe injuries, there may be instability of the joint and obvious swelling around the stifle as a result of internal bleeding. Localising the cause of lameness to the stifle may be possible from examination, but nerve blocks may be necessary.

Ultrasound is generally more useful than X-ray for further investigations, but there are pros and cons with each technique. Fragments of bone that have pulled away from the attachment sites of the menisci and stifle ligaments can be seen with X-ray, as can other abnormalities such as reactive changes within the joints.

While ultrasound is very effective for imaging the menisci and collateral ligaments, significant portions of the menisci between the joint surfaces are not visible. Views of the cruciate ligaments are also limited.

More advanced imaging techniques may be needed to establish a diagnosis. Arthroscopy (keyhole surgery) is often used, as this allows direct examination of the interior of the joints and offers the best treatment in many cases.

Experience with computed tomography (CT) and magnetic resonance imaging (MRI) is developing in the investigation of stifle injuries, as both allow thorough evaluation of the bone and soft tissues. It is likely that these techniques will become more commonly used, but they add significant expense.

Treatment options

MUCH of what we know about soft tissue injuries in the stifle has come from observations during diagnostic investigation of lame horses using arthroscopy.

Injuries of the meniscus most frequently occur within the medial femorotibial joint. In people, the entirety of the upper surface of the menisci can be evaluated with arthroscopy, by applying traction to the knee and pulling the joint surfaces apart to give space to look around with the arthroscope. This is not possible in the horse, however, meaning that portions of the meniscus, particularly towards the centre of the joint, cannot be seen. Injuries in this location are also inaccessible for arthroscopic treatment.

In the majority of cases, arthroscopic surgery involves removal, or “debridement”, of torn and disrupted portions of soft tissue. Partial meniscectomy (removal of the meniscus) can be performed or, with cruciate ligament injuries, torn ligament fibres removed.

During arthroscopic surgery, the cartilage surfaces are also evaluated for any additional damage or signs of degenerative (arthritic) change. The information obtained provides the best guide for the horse’s chances of recovery from injury.

Intra-articular medications may be used as an adjunct to arthroscopic treatment, or, with mild injuries, as the primary treatment. Anti-inflammatory injections are commonly used, typically corticosteroids such as triamcinolone (similar to cortisone, often used in people).

Your vet may recommend alternative intra-articular medications; examples include hyaluronic acid, which is both anti-inflammatory and viscoelastic (shock absorbing), or polyacrylamide gel, which is thought to form a cushion-like membrane after injection. Stem cells have also been researched for treatment of meniscal injuries, and may improve healing.

Following treatment, an effective rehabilitation programme will help the horse back into work.

The prognosis

MANY horses can make a return to full competitive soundness following arthroscopic surgical treatment. The prognosis depends upon the extent of the injury. With meniscal injuries, approximately two-thirds of horses with minor tears recover fully. This falls to closer to 50% with moderately severe tears; in the worst injuries, very few can resume work.

Injuries of the cranial cruciate ligament treated by arthroscopic surgical debridement generally have a fair prognosis, with approximately half of horses reported with this injury resuming a competitive career.

Sadly, horses who sustain a complete rupture of either of the cruciate ligaments are unable to work again. In the most severe soft tissue injuries, stability of the joint may never completely return and arthritis is likely to develop, with associated persistent lameness.

The vet

MATT SMITH MRCVS is a surgeon and partner at Newmarket Equine Hospital (NEH). This referral hospital is one of the largest in Europe, treating a wide range of leisure and sport horses from all over the UK. Matt is a Royal College of Veterinary Surgeons (RCVS) specialist in equine surgery and has a particular interest in orthopaedic problems. 01638 782020, newmarketequinehospital.com

This feature is also available to read in this Thursday’s H&H magazine (1 April, 2021)

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Upward fixation of the patella is a hindlimb gait abnormality that occurs quite commonly in young horses and sometimes also in older horses.

The horse’s stifle is designed so that the patella (kneecap) can hook over the medial trochlear ridge (the protuberance at the bottom and inside) of the femur (see diagram). This locks the leg in place, allowing the horse to stand using minimal energy and also to sleep while standing up. While this may have been of evolutionary benefit when horses needed to run away from predators, it is perhaps less useful in today’s sport horse — particularly when the system fails.

Upward fixation occurs when the patella does not release quickly and easily from the distal femur.

The condition can range from severe, when the leg is completely stuck and dragged behind the horse, to more mild, where the patella “catches” but the limb does not truly become stuck. This intermittent catching may make the horse look jerky in his movement, particularly during downwards transitions.

Conservative treatment

The most common reason for developing upward fixation is weakness of the thigh muscles (the quadriceps and biceps femoris) responsible for releasing the patella.

Horses with overly straight/upright hindlimb conformation or poor foot conformation (long toe, low heel and high inside hoof wall), or those that have suffered trauma to the stifle region, may also be predisposed. In older horses, the cause may be loss of muscle mass, perhaps due to rest.

Diagnosis is based on history and observation, or description of the typical gait abnormality.

Radiographs and ultrasonography are not always essential, but may be useful to identify any underlying abnormalities of the stifle joint, particularly in horses which do not respond to conservative treatment.

Improving the muscle mass of the hindlimbs with strengthening exercises, such as hill work and working over poles, is key to conservative management. Working in small circles may exacerbate the problem, so straight lines are best. Long-reining may be useful in young horses not yet broken to saddle.

As much exercise and turnout as possible is a good idea. Where there is an underlying reason for any sudden loss of muscle mass, an unrelated injury or lameness, for example, this must also be treated.

Where the horse is in poor body condition, particularly in a young or growing animal, it should be ensured that the diet is adequate and that he has been wormed appropriately.

Various feed supplements can help build up muscle mass without adding excessive protein. The aim is for the horse to be neither too fat nor too thin, with adequate muscle mass over the quarters.

Remedial measures

The majority of young horses will grow out of this condition as they mature and with appropriate management. Depending on breed, use and level of training, this will usually happen by the time a sport horse is six — if it happens at all.

More problematic cases may have an underlying conformational issue. While upright hindlimbs cannot be made less straight, conformation of the foot can be altered to an extent.

Some affected horses may improve after appropriate trimming, with shortening of the toe and medio-lateral imbalance correction. Remedial farriery with shortening of the toe and application of a lateral wedge has helped certain cases in the short term, but the significant medio-lateral imbalance sometimes created may have long-term side-effects, by predisposing the horse to collateral ligament damage.

A horse who still has issues after these points have been addressed, or who frequently becomes stuck and unable to release the patella, will require further intervention.

Iodine injected as an irritant around the medial patellar ligament has been used. The aim of this is to encourage scarring, which results in shortening of the ligament, thus stopping the patella hooking over the top of the medial trochlear ridge.

The results of this procedure are inconsistent. I will admit to having limited experience of injecting counterirritants, as I discourage owners from this approach due to its unreliable results and the potential disastrous complication of inadvertent injection of the irritant solution into the stifle joint itself.

Surgical solutions

In the small number of horses who do not respond to strengthening work and conservative management, surgical treatment is preferred.

The standard surgical treatment involves cutting the medial patellar ligament close to its insertion on the tibia, which stops any hooking and instantly solves the problem. This remains the treatment of choice in the severely affected, distressed horse requiring instant relief, and where the patella cannot be released by manipulation of the joint.

Until fairly recently this was the only surgical method used. Its popularity in milder cases waned as it became recognised that the procedure can result in long-term complications, most notably fragmentation of the patella and development of osteoarthritis.

A further downside is that the horse will require rest for between three and five months after surgery.

A more recent surgical option is “splitting” the medial patellar ligament. Instead of completely cutting the ligament, small stab incisions are made at the proximal aspect (top) of the ligament.

This leads to localised inflammation and thickening of the ligament (similar to the rationale behind injection of counterirritants, but without the risk of damaging the joint) and stops the ligament hooking over the top of the medial femoral trochlea. The horse starts walking the day after surgery and improvement is usually seen within two weeks.

Advantages of this procedure over the conventional surgery are the rapid return to exercise and an apparent reduction in complications. A recent study from the University of Copenhagen, following up on 85 horses treated in this way over 14 years, reported a resolution rate of 97.6% — with no long- or short-term complications.

Ref: Horse & Hound; 17 March 2016

Why is a split better than a cut for problematic stifle ligaments? Peter Green MRCVS looks at the latest research

Young horses in particular can experience problems with the mechanism in the stifle that locks the kneecap, or patella, on to the bottom of the femur.

Sometimes the patella gets stuck in the locked position or is not released efficiently when the horse wants to move.

The result is a stiff, straight, fixed back leg that can look quite alarming and can upset the horse, or a snatching action of the back leg as the patella snags on the bone and then flips off.

The condition is known either as “upward fixation” or “delayed release” of the patella. Many young horses grow out of it, and in older animals the condition often resolves as the horse gets fitter.

If it is persistent, surgery is an option. The traditional technique is to cut the thick ligament on the inside of the stifle that links the patella to the tibia. This ligament is an important part of the locking system and if it is cut, the problem is solved. But cutting risks making the patella unstable.

In a more recent technique, the ligament is not cut but split lengthways. This seems to induce a change in the way the ligament functions and prevents unwanted locking or snagging.

Vets in Texas have just published the results of this ligament-splitting operation in 64 horses who had delayed release problems. When they were followed up after surgery, 60% were completely cured, 30% were improved and 10% were not helped by the operation.

The results are not as good as those obtained with the old-fashioned ligament-cutting operation, but there is no risk of causing damage. If it doesn’t work, the ligament can always be cut later.

Just like our knees, a horse’s stifles can suffer soft tissue damage during rigorous sporting activities. Phil Cramp MRCVS explains why equine — and human — athletes are at risk

Knee injuries frequently hit our sporting headlines. We’re accustomed to hearing about famous footballers and rugby players sidelined for long periods following injury, or skiers out of action with knee trouble.

The horse’s stifle is the equivalent of the human knee, so the same soft tissue injuries can occur with equine athletes. Those dressage horses, eventers and showjumpers that put their stifles through extreme degrees of flexion and rotation can experience severe injuries similar to ours.

While there are some differences in the anatomy and the arrangement of the equine patella (kneecap) ligaments, the joint is largely comparable to the human version.

The equine stifle consists of the 3 compartments — the medial femorotibial joint, the lateral femorotibial joint and the femoropatellar joint. There is a complex arrangement of soft tissues in the stifle consisting of 12 ligaments, 2 menisci — pieces of cushioning cartilage — and various other supporting structures.

Such a comprehensive support system is necessary, as a huge amount of force is transmitted through the stifle by the large muscles in this region. The joint undergoes an extensive range of motion, which predisposes these soft tissues to tremendous strain.

Vulnerable structures

4 types of soft tissue injury are seen:

• Collateral ligament injury. These ligaments may undergo partial or complete tearing, the latter usually occurring with severe trauma to either the inside or the outside of the stifle. Such significant trauma might occur in an incident such as a road traffic accident, typically causing additional damage to the menisci and cruciate ligaments.
• Patellar ligament injury. The 3 patellar ligaments — the lateral, middle and medial — transmit the pull of the quadriceps muscle to the tibia. Injury to these ligaments is not common, but is most often seen in jumping horses. Other than as a result of direct trauma, it is more usually part of a chronic strain syndrome.
• Cruciate ligament injury. The cruciate ligament is under tension when the stifle joint is in extension. Subsequently, hyperextension and/or rotation — in essence, a twist — will cause this relatively rare injury.
• Meniscal injury. The menisci are paired “C”-shaped structures that lie between the curved femur and the flat tibia and are attached to the tibia by means of stout ligaments. Meniscal tears are the most common soft tissue lesion in the stifle and account for three-quarters of all soft tissue injuries in this joint. These injuries can be associated with trauma or a fall, but also occur as a result of wear and tear — especially in the equine athlete.

Size matters

Severe traumatic injuries of the stifle can be very obvious and relatively easy to diagnose. Lameness is usually acute and sudden in its onset, accompanied by clear and obvious instability of the joint and abnormalities readily identified by imaging methods.

But less obvious injuries can be much harder to diagnose. The degree of lameness can be variable, flexion tests can be unequivocal and the response to joint-blocking is not always reliable.

The equine stifle is a very large joint, which makes X-rays difficult to interpret and the effect of ultrasonography very limited. The cruciate ligaments, for example, cannot be evaluated with ultrasound. Only a very limited part of the menisci can be imaged.

Even advanced imaging methods such as MRI and computed tomography (CT) scanning are of little diagnostic use. To put it simply, horses are just too big.

Keyhole surgery (arthroscopy) remains the most effective way to diagnose stifle lameness attributable to the menisci, meniscal ligaments, cruciate ligaments and articular cartilage.

Treatment and outlook

Collateral and patellar ligament injuries are best treated with a conservative therapy, such as lengthy rest and rehabilitation.

For cruciate and meniscal ligament injuries, arthroscopic “debridement” — the extraction of damaged tissue — is usually recommended. Arthroscopy allows direct visualisation and assessment of the injury and also enables removal of the torn “fibrils”, or fibres (see box, below).

Occasionally, we will see a defect in the cartilage along with the torn fibres of the medial meniscus. Recent research has shown that there may be a link between these two problems. Removing the fibres reduces the inflammation within the joint and creates a better healing environment.

The biggest limitation with arthroscopy is that we can only see a very limited portion of the stifle. Typically, we are only able to readily access the front of the joint and therefore only the front portions of the cruciate and meniscal ligaments.

Debridement is the mainstay of treatment, but alternatives include direct suturing of the lesion via arthroscopy. Various regenerative treatments such as platelet rich plasma (PRP) and stem cell treatment have also been tried, but there is little research to date to recommend their use.

Soft tissue injuries of the stifle can be severely debilitating for an equine athlete. A period of at least six months’ rest is nearly always required, and the prognosis is not always good.

While the outlook can be relatively promising for mild damage to the collateral and patellar ligaments, prospects are poor for severe cases — especially where the ligament has been completely ruptured.

Cruciate ligament injuries can often be associated with other lesions, such as fractures where the ligament attaches and cartilage damage. The worst scenario is if these are present in combination. Mild, isolated cruciate tears have a favourable outlook, but a significant period of rest will be required.

Approximately 60% of horses with minor meniscal tears return to their previous level of performance, but less than 10% of cases do so after suffering a severe tear. Furthermore, the older the horse, the less good the prognosis is.

How to recognise a stifle injury

The 2 most obvious things to look for are effusion (swelling) of the stifle joint and lameness. Effusion can be very obvious or more subtle, but if you can see a bulge or bump then it usually indicates a problem. The next time you see your vet, ask for a quick demo on feeling for joint effusion.

Lameness is nearly always present with soft tissue injuries of the stifle, but can vary in its severity. Typically, the lameness will be very severe after the injury and the horse will be “toe-touching” or unwilling to bear weight on the affected leg. This lameness will then reduce in severity with time, but will be made worse by performing a flexion test.

Call the vet if you suspect lameness associated with the stifle. The problem can be pinpointed if joint-blocking — where local anaesthetic solution is injected into the stifle joint — renders the horse sound.

Vet casebook note

It is worth noting that the outlook is more positive when it comes to minor soft tissue ruptures.

Small tears of the meniscus and minor tears of the meniscotibial ligaments and of the cruciate ligaments do respond well to debridlement via arthroscopy and intra-articular medications with iRap, platelet rich plasma and stem cells.

Cases such as these are very different from the major trauma discussed in the body of this article.

History was made in Chinese veterinary surgery last month when the first stifle arthroscopy (keyhole procedure) was carried out.

A team led by specialist equine surgeon Cedric Chan, who runs his own practice NW Equine Referrals Ltd, travelled to Shanghai on 27 October to perform surgery on one of China’s top showjumpers, Heraldo.

London based Mr Chan was assisted by anaethetist Claire Booth and Michelle Barber as head nurse, both from the Towcester Veterniary Centre Equine Clinic, Northamptonshire.

“[The surgery] was a complete success for the Chinese and Western veterinary teams,” Mr Chan told H&H.

The procedure took place in a modified stable with the addition of hay bales, a gymnastics mat, ropes from the ceiling and local manpower to move, position and recover the anaesthetised horse safely.

Nonetheless, the actual surgical equipment used was state of the art.

Ms Booth said: “It was very exciting to be involved in the first steps of development of equine surgery in China.”

There are now plans for a hospital similar to the Towcester Equine Clinic to be built at the Shanghai Equestrian Centre, where some of China’s best competition horses are in training.

Equestrianism in China is rapidly expanding alongside the overall wealth of the country.

However Mr Chan added: “The level of equine veterinary care overall in China is similar to Western standards around 20 years ago.”

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Learn more about stifle injuries

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Horses, like many prey species, have the ability to sleep while standing up.

They can do this because they possess the necessary physiological mechanisms to lock their stifles in extension.

Sleeping upright saves valuable seconds should the horse suddenly need to take flight. It also means that they can stand without expending valuable muscular energy.

But this fascinating adaptation can cause trouble for some horses.

Upward fixation of the patella (known as locking patella/stifle) is a fairly common condition that occurs when this natural locking mechanism fails to unlock spontaneously.

It is especially common in horses with straight hind limb conformation, young animals and those with weak thigh musculature or those in poor condition. It can also be inherited.

Certain exercises are particularly beneficial for horses suffering from upward fixation of the patella. These will help build up the musculature required to reduce the risk of the patella locking and include:

• Plenty of work in an extended, forward trot
• Riding up and down hills to help strengthen the hind quarters
• Working in deep sandy or loamy soils, or in shallow water to promote correct muscle development in the stifle area

Every exercise regime should be tailored to suit an individual horse’s needs but, generally speaking, lunging in tight circles should be avoided as it puts strain on the particularly vulnerable areas.

For the full veterinary article on treating locking stifles, see the current issue of Horse & Hound (24 March, 2011)

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More articles on stem cell therapy

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Stem cell treatments for tendon injuries have become commonplace in sports horse medicine.

The stem cells are harvested from the bone marrow of the horse, purified and treated with factors that promote them to grow into tendon cells.

They are then injected into the injured parts of the tendon where they grow to produce good quality tendon tissue.

Without stem cells the horse has to rely upon scar tissue to heal the tendon and this is neither as elastic nor as resilient as proper tendon tissue.

Similar problems occur when the cartilage in the stifle is torn or crushed.

The stifle of the horse corresponds to the knee of a human and, like people, horses have a pad of thick cartilage that separates the bottom of the thigh bone from the top of the gaskin bone.

This cartilage is called the meniscus and it can be injured by excessive stress, twisting or wrenching of the joint.

Once it is damaged it will not heal, but becomes more and more frayed and tattered.

Currently, the best treatment is to remove the loose, frayed portion and allow the fresh, straight edge to heal with scar tissue.

But now vets are hopeful that they can also offer stem cell repair for damaged stifle cartilage.

This would provide a useful treatment and could make the difference between permanent lameness and recovery for some sport horses.

For more of the latest veterinary research and developments, see the current issue of Horse & Hound (17 September, ’09)

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