For a sharp-eyed individual, it doesn’t take much to spot a stifle problem: The telltale swelling in the stifle area is a big hint. But the stifle is a complicated joint. Knowing the particular part of the stifle that’s affected and the precise cause of the disorder is critical for implementing the correct therapy and maximizing recovery.

The stifle is a complex hinge joint in the upper hind limb of the horse, and the largest joint in the horse’s body. Its role is to provide unfettered flexion and extension of the hind limb, which allows the horse to move forward with smooth impulsion. The stifle also participates in the passive stay apparatus, which locks the joint to allow the horse to maintain a weight-bearing, straight hind limb for long periods of time while relaxing the other hind limb. Although comparable to the knee joint in humans, the equine stifle is angled when the horse stands still, whereas the human knee is upright.

The stifle consists of several different articulations between the patella (knee cap), the femur (the long upper bone of the hind leg), and the tibia (the inner of the two bones that extends down from the knee to the hock). Explains Melinda H. MacDonald, DVM, PhD, Dipl. ACVS, Assistant Professor of Equine Surgery at the University of California, Davis, "The femorotibial joint cavity of the stifle is further subdivided into medial and lateral (inside and outside) compartments. Two crescent-shaped menisci sit between the femur and tibia; these menisci are cartilage discs that provide congruency between the two bones and help reduce concussion in the joint. Two cruciate ligaments within the femorotibial joint function to stabilize the joint.

"There are also two strong collateral ligaments supporting the joint on either side of the leg," she continues. "The patella is supported from the top (or base) by the quadriceps femoris muscle, on either side by the femoropatellar ligament, and below by three strong patellar ligaments. With normal hind limb movement, the femoropatellar joint moves in unison with the femorotibial joint."

Besides their separate functions, these elements also have separate pathologies and treatment needs. The first step in treating a sore stifle is identifying which part is involved in the disease process; the second step is identifying the disease.

Dealing With Stifle Disease

Trauma and several different diseases can affect the stifle. "We usually divide stifle diseases into two main categories–acquired disorders and developmental disorders," states John F. Peroni, DVM, MS, Assistant Professor of Large Animal Surgery at the University of Georgia.

Acquired disorders include arthritis, degenerative joint disease (DJD), fractures, and trauma such as cruciate ligament injury. These are abnormalities caused by infection, bacteria, trauma, etc. Clinical signs include acute and severe onset, with sudden, pronounced lameness and obvious swelling.

Developmental stifle diseases are congenital (present at birth) and might or might not be caused by genetic abnormalities. They generally affect foals and young horses, which usually display very subtle clinical signs initially. "Then, as the horse ages and begins training, stifle swelling and persistent lower-grade lameness develop," Peroni says. "The lameness and swelling may become extremely obvious with continued exercise, but decrease when exercise activities are reduced." Developmental disorders include osteochondritis dissecans (OCD), subchondral bone cysts, and patellar luxation.

Finding the exact cause of a stifle disorder can be challenging. The diagnostic process requires a complete physical and lameness examination that might include palpation of the stifle, observation of the horse in-hand and under saddle, imaging, flexion tests, and intra-articular (within the joint) anesthesia to localize the lameness.

"Intra-articular anesthesia of the stifle is often the most important aspect of the diagnostic process because imaging can be challenging, especially in cases of soft tissue trauma," states Mark J. Martinelli, DVM, PhD, Dipl. ACVS, Associate Orthopedic Surgeon at the San Luis Rey Equine Hospital in Bonsall, Calif. "Radiology can diagnose fractures and OCD lesions, but does little to detect soft tissue lesions. Nuclear scintigraphy may be beneficial in some of these cases, especially those of collateral ligament injury. Ultrasound may help detect some soft tissue damage, including collateral ligament damage and meniscal injuries. Exploratory arthroscopic surgery has become very important to the diagnosis of occult stifle lameness."

Treatment varies depending upon the injury or disease process. "Mild soft tissue strains or traumas may respond to rest of several days to several months," explains Martinelli. "Injuries with an inflammatory component may respond to intra-articular medication. Surgery is usually carried out if an obvious radiographic lesion is present or if there has been no resolution of clinical signs after a period of extended rest or intra-articular medication. As with all articular maladies, systemic treatment with anti-inflammatory medication or oral glycosaminoglycan products may be of benefit and should be considered with a stifle lameness." A very gradual return to exercise, when tolerated, is recommended.

Specific diseases and treatment modalities are as follows:

Osteochondrosis, the most common category of stifle disorder, is characterized by an abnormal development of bone and cartilage in the stifle joint, says Martinelli. This disorder of growing cartilage can lead to OCD or subchondral bone cysts.

"The lesions are usually found in specific anatomical locations: Bone cysts occur in the middle of the medial condyle (prominent area of bone in the joint) of the femur, while osteochondral fragments or erosions are usually found along the lateral trochlear ridge of the femur," explains Martinelli.

Traditionally, horses are treated via arthroscopic surgery. In OCD cases, abnormal cartilage is surgically cleaned out from the joint surface and any loose fragments of cartilage or bone are removed. In cyst cases, cyst contents are surgically debrided. MacDonald notes that arthroscopic surgery does not restore normal articular cartilage in OCD-affected horses, but stimulates growth of fibrocartilage, which doesn’t function as well under athletic movement stress. However, there is usually a great deal of improvement from debriding the lesion.

Early detection and subsequent rest in lieu of surgery can achieve good results in some horses. Peroni cites one study where restricted activity for three to six months yielded success in about 50% of cases when OCD was detected early in weanlings.

In such cases, "The OCD fragments never formed the characteristic flap, and this resulted in appropriate joint function," he says. Periodic radiographic examination is used to monitor progress.

Several reports also indicate a success rate of up to 65% in bone cyst cases where horses were rested and treated with anti-inflammatories.

"I’d opt for surgery in a younger horse because surgical response and regeneration is much greater than in an older horse," Peroni says. "In an older horse, I’d treat conservatively at first and see how it goes."

Patellar luxation is a congenital condition in which the patella (kneecap) becomes dislocated. Typically, the patella luxates laterally or to the outside of the limb, MacDonald says, and is not often seen in horses. "Horses are usually comfortable, but are unable to fix the stifle in extension. They stand with a characteristic crouched posture in the affected hind limb."

Treatment involves cutting the tissues that pull the patella to the outside and tightening up the tissues on the inside in order to hold the patella in a normal position.

"In some cases, it’s necessary to deepen the groove in the end of the femur to keep the patella in its normal track," MacDonald says. Horses with partial luxation fare better than ones with more serious complaints.

Fractures of the stifle are not very common, and usually result from a kick, fall, or other direct trauma. Fractures in which fragments are not displaced might heal with rest; fractures that are more serious require surgical treatment that can involve fixation with screws and plates, says Martinelli. Prognosis is best for milder cases.

Cruciate tears, meniscus tears, and meniscal ligament tears can be partial or complete, and can result from a kick or high-speed exercise including jumping and racing. Notes Martinelli, "The degree of lameness appears to be related to the severity of the tear and the amount of time it has been present."

Ligament reconstruction is not yet possible in the horse due to the weight and forces applied to the stifle, so treatment is either conservative or consists of arthroscopic debridement of the torn fibers of the affected ligament.

"If cruciate ligaments are completely torn, nothing will help, and the prognosis for performance is very poor," says Peroni. "If there is partial tearing, about 50% of horses will return to previous levels of performance after arthroscopic debridement, intra-articular medication, and rest."

Septic arthritis and infectious/bacterial joint disease of the femoropatellar (and other) joints are often associated with umbilical remnant infections in foals, in which bacteria enter from the navel, into the bloodstream, and then end up in the joint, MacDonald reports. Older horses can suffer puncture wounds or wire cuts that penetrate the joint, opening the area to infection. Treatment usually consists of antibiotics, draining the infected joint fluid, and flushing with a balanced salt solution, followed by prolonged rest. Prognosis is guarded.

Traumatic lesions of the femoral medial condyle are becoming more frequently recognized in the stifle joint and seem to be similar to the bone cysts that occur in young horses.

"The lesion occurs in older performance horses (usually large sport horse types) and appears to be traumatic in nature," states Martinelli. "The lesion is present in the medial condyle of the femur and usually presents as a shortened cranial (towards the head) phase to the stride, especially when the limb is on the outside of the circle. Although lameness may present in-hand or on the longe line, it is often easiest to see while the horse is being ridden.

"Affected horses don’t always have stifle swelling, but will improve significantly with intra-articular stifle anesthesia," he continues. "Diagnosis is best made by arthroscopic examination of the joint. Treatment usually requires debridement of the abnormal cartilage and/or bone in the region of the medial condyle. Recovery can be prolonged."

Upward fixation of the patella is a common problem associated with a characteristic gait deficit in which the patella literally appears to get caught during the stride with the stifle and hock extended, MacDonald says. Subtle cases will respond to anti-inflammatory drugs, changes in shoeing/trimming, and a progressive strengthening program for the quadriceps muscles.

"In more severe cases or for patients that don’t respond to conservative management, we usually recommend an internal blister with 2% iodine," MacDonald says. "The blistering agent is injected into the middle and medial patellar ligaments and into the quadriceps muscle. Surgical management is reserved for those horses that have not responded to conditioning or internal blisters." Prognosis is generally good.

Osteoarthritis or degenerative joint disease (DJD) of the stifle joint can result from arthritis and is often secondary to collateral ligament, cruciate ligament, or meniscal injuries.

Treatment consists of addressing primary causes and reducing pain. Rest and oral or intravenous administration of non-steroidal anti-inflammatories might suffice in mild cases, or serve as a first step for more severe conditions.

"A more aggressive way of treating is to inject the joint and place anti-inflammatory medications within the joint," Peroni says. "Steroids are very potent anti-inflammatories. Medications such as hyaluronic acid placed in the joint may ameliorate the conditions of the synovial environment and have beneficial effects on the joint, but do not yield the anti-inflammatory results that steroids do. There have been some successes obtained with arthroscopic debridement of cartilage lesions on the articular surface of the femur."

Prognosis depends on the severity of the disease. If the arthritis is substantial, response is limited. In moderate cases, the horse might continue to perform well.

Risk Reduction

Stifle disorders are not often recognized in the horse, and because the stifle is a very complex joint, treatment success is not as high as for problems in other joints. Therefore, owners should minimize the risk of stifle problems by working closely with a veterinarian or equine nutritionist to develop an appropriate ration for young, growing animals. Owners also should provide controlled, appropriate exercise. Learn to recognize changes in stride, and monitor horses for stifle tissue swelling, fluid swelling, or lameness. If a stifle injury is suspected, contact your veterinarian right away, as early treatment is the key to successful recovery.