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.