THE stifle joint is a large hinge joint found in the horse’s upper limb and is the equivalent to the human knee. It is formed with the articulation of the lower part of the femur, the upper tibia and the patella.

Although we loosely refer to the stifle as one joint, it is actually split into three smaller joints – the medial femero-tibial joint, the lateral femero-tibial joint and the femero-patella joint. Recognising these individual joints is important when localising specific lesions which affect the specific joints.

As with all joints, the stifle joint is held together by a collection of ligaments.

There are two cruciate ligaments within the femoro-tibial joint and two strong collateral ligaments on either side of the leg. Their function is to stabilise the joint. In addition there are two crescent-shaped menisci which sit between the femur and tibia; these menisci are cartilage discs that help to make up the joint surface of the stifle and reduce concussion in the joint. Besides their separate functions, these structures also have separate pathologies and treatment needs.

A unique function of the stifle is its role in the stay apparatus. The stay apparatus allows the stifle joint to lock, and in doing so the horse can maintain a weight-bearing, straight hind limb for long periods of time while relaxing the other hind limb.

Injury of the stifle joint can be divided into two categories, acquired disorders and developmental disorders. Acquired disorders are similar to those of any other joint and include arthritis, degenerative joint disease, fractures and trauma.

Developmental disorders include osteochondritis dissecans (OCD) and sub-chondral bone cysts. These disorders will generally present in young horses with the clinical signs initially quite subtle but can then worsen when the horse begins training.

Diagnosis of a stifle joint problem will often be reached by the clinical appearance of an inflamed sore stifle. If a clinical examination fails to localise the cause of lameness, then local analgesia or nuclear scintigraphy can be useful tools in pinpointing the problem.

The most common stifle lameness seen in flat racehorses are the developmental disorders mentioned above, OCD and sub-chondral bone cysts. They are caused by an abnormal development of cartilage and bone.

OCD lesions are usually seen on the medial ridge of the lower femur. These lesions can cause lameness in young racehorses but in our experience, the lameness will most often resolve with rest and anti-inflammatory medication. In severe cases of OCD, these bony fragments can be removed with arthroscopy, as in later life the OCD lesions can lead to degenerative joint disease.

Sub-chondral bone cysts are most commonly seen in the medial condyle of the lower femur and can be a significant cause of lameness. In some horses these lesions are found as incidental radiographic findings which are of no clinical significance. In our experience when they are the cause of lameness they can be challenging to treat.

Our initial approach to treatment is the same as those horses with OCD lesions - rest and anti-inflammatory medication.

Horses which do not respond to conservative management will require more invasive procedures to alleviate the lameness. These procedures include debridement of the cyst and ultrasound guided injection of the cyst with corticosteroids under general anaesthesia. This procedure carries a good success rate for a return to training, although a long convalescence time is required in addition to the surgery, meaning the horse can be out of full training for up to six months.

Another relatively common but less serious condition affecting the stifle is upward fixation of the patella, also known as stifle locking.

This condition is caused by a malfunction in the horse’s stay apparatus. As mentioned before, the stay apparatus allows the horse to maintain a weight-bearing straight leg for a long time, and one part of this anatomy is the locking of the patella over a notch on the lower femur.

There are a group of muscles responsible for raising the patella on and off the ridge and failure to release the patella may result in the stifle being locked in extension. The condition is most commonly seen in ‘unfit’ horses and most resolve with strengthening and conditioning of the hind musculature.

FRACTURES

Fractures of the bones forming the stifle joint are uncommon and are usually as a result of a kick, fall or direct trauma.

Injuries to the soft tissue structures which help support the joint are also uncommon and prognosis will depend on the structure involved and the extent of the lesion within that structure.

The collateral ligaments are poorly protected by overlying tissue and are vulnerable to direct trauma.

The menisci and cruciate ligaments can tear from rotation or over-extension of the stifle joint.

Initial treatment will involve anti-inflammatory therapy and rest until the lameness resolves. In those horses which respond poorly to conservative treatment, further investigation and treatment with arthroscopy is indicated.

Thankfully the conditions outlined above are relatively uncommon but should any horse in training at Mark Johnston Racing be found to have a stifle joint issue then all diagnostic work and treatment, including referral to Newmarket Equine Hospital if necessary, will be covered in the daily training rate.

A graduate of University College Dublin, Laois native John Martin is in practice at Mark Johnston Racing. This article first appeared in the Kingsley Klarion.