The equine stifle joint is formed by the union of the tibia, the patella and the femur and consists of two articulations – the femoropatellar joint and the femorotibial joint – with the joint compartment of the femorotibial joint being divided into medial and lateral parts.

These joints are supported and held together by a complex series of ligaments and contain synovial fluid which serves as lubrication, shock absorption and nutrient and waste transportation.

The stifle also has two menisci ('cushions') which sit between the surfaces of the tibia and femur, their purpose mainly being to allow the different shaped ends of the opposing bones to fit together. The patella is the equivalent of the human kneecap and slides up and down in a grove along the bottom of the femur, but it also forms a vital part of the passive stay-apparatus.

This is unique to equids and allows the hind-limb to be essentially 'locked', hence permitting the animal to rest or sleep standing up. The stifle’s role in this is the horse’s ability to fix the patella over a ridge in the femur.

This is a developmental disorder of growing joints where the cartilage protecting the joint surface becomes compromised and fragile. In the case of osteochondrosis dissecans (OCD), it can crack and form flaps, or bone fragments (Fig 1 and 2).

Fig 1 Large OCD flap in the stifle joint

Fig 2 Multiple OCD fragments in a stifle joint

The exact cause is unknown but rapid growth, conformation, hereditary factors, trauma or overuse and dietary imbalances are all known to play a role.

Another type of osteochondrosis is subchondral cystic lesions (SCL), or bone cysts. Diagnosis is usually by radiographs as lameness and swelling can vary, although horses with SCL tend to be more painful than those with OCD (Fig 3).

The treatment for OCD is ideally surgical. In the case of OCD, it is arthroscopic (keyhole) removal of the flaps or chips, while with SCL arthroscopic debridement or placement of a bone screw or an implant have all been described.

Fig 3 A typical bone cyst in a stifle joint

The prognosis depends very much on the size and severity of lesions as well as the development of secondary osteoarthritic changes, but SCL generally have a poorer outcome. Prevention by careful management of the growing horse is infinitely better than cure and is focused on avoiding excessive feeding of the young horses.

Upward fixation of the patella

This condition is commonly referred to as a 'locking stifle' and occurs the stay apparatus function of the stifle becomes activated during the swinging phase of the stride.

It is more common in youngsters or horses with upright hind-limb conformation but can also occur in horses which have lost muscle, particularly in the quadriceps (for example, horses on prolonged periods of box rest for other reasons).

When affected, animals will show a complete inability to move the stifle, often with it being held out in extension behind them. Most horses are able to self-correct this but can sometimes be aided by gently backing them up.

Occasionally, manual correction by your vet under sedation may be required. Mildly affected horses may just show a gait alteration if the fixation is intermittent.

In most young horses, the condition will resolve naturally as they mature and even in older horses, strengthening of the quadriceps using physiotherapy, pole and hill work is usually effective. Non-responsive cases can be treated surgically by splitting of the medial patellar ligament.

Chondromalacia (softening and damage to the cartilage) of the patella is complication of the condition in more severely affected horses which carries a more guarded prognosis.

Osteoarthritis can occur in the stifle as it can with other joints.

Diagnosis is usually made via a combination of examination, joint blocking (by placing local anaesthetic within the stifle joint), stifle radiographs and ultrasonographic assessment.

There are many different treatment options including non-steroidal anti-inflammatory drugs (NSAIDs) such as Bute and various intra-articular medications (where medication is placed directly into the joint). Weight management and physiotherapy also play important roles.

Even small wounds in the region of the stifle joint should be checked by your vet and any penetration into the joint capsule will require surgical flushing.

Fractures of the stifle are most common in hunters and eventers due to direct impact with fixed fences. Prognosis and treatment of stifle fracture depend on location, type and severity and radiographic examination is essential to determine this.

Unfortunately, fractures of the shaft of the femur carry a hopeless prognosis but patellar fractures, mild tibial tuberosity fractures and small condylar fractures can have a good outcome with surgical fixation.

Soft tissue injury to the stifle can be challenging due to the complexity of these supporting structures and their assessment is initially performed using ultrasound (Fig 4).

Unfortunately due to the size and shape of the joint, arthroscopy under general anaesthesia may be required for full assessment of the menisci and the cruciate ligaments. Prognosis and treatment depend on the location and severity of the damage but can include prolonged periods of rest and rehabilitation, stifle arthroscopy and intra-articular medication.

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