Lameness in the horse’s stifle joint can result in shortened stride length, reluctance to work or a rough canter. While lameness is more commonly attributed to problems with the hock joint, stifle lameness is seen frequently in performance horses.
The stifle is considered the most complex joint in the horse’s body with a similar function to the human knee. Stifle injuries can result from repetitive stress, trauma, excessive use, changes in direction and rapid deceleration. Horses engaged in jumping and barrel racing are most at risk of these injuries.
Stifle lameness does not always mean the end of a horse’s athletic career, although it may mean the animal will never again perform at its previous level. The good news is that diagnosis and treatment of stifle lameness have improved significantly in recent years. 
Advancements in imaging technology have made a significant difference in the ability of veterinarians to make accurate diagnoses of what formerly may have been deemed an “unknown” lameness.
Prevention of stifle lameness and other stifle injuries starts with ensuring your horse is not overworked and supporting overall joint health. It is important to keep excess weight off of your horse and to be vigilant for any signs of over-use.
The Horse’s Stifle Joint
The stifle is one of the most complicated parts of the equine body. The stifle is designed for flexion and extension of the hind leg.
The stifle is the largest joint in the horse’s body and the site where the femur meets the tibia in the hind end. The former is the long bone between the hip joint and the stifle, while the latter is the bone running between the stifle and hock.
Also involved in the stifle joint is the patella, or kneecap, situated on the femur’s lower end. This small, flat, round bone is attached to the thigh’s quadriceps muscle.
The patella sits in the trochlear groove on the femur. From this groove, it slides up and down as the rear leg moves. The patella’s movement lets the joint move like a hinge. The leg is able to extend and flex, but it cannot move sideways.
Three shock-absorbing joints are involved with these bones:
- Medial femorotibial joint
- Lateral femorotibial joint
- Femoropatellar joint
On the outside edges of the joint, the short, thick, medial and lateral collateral ligaments join the femur and tibia bones. Deep within the cleft area are the anterior and posterior cruciate ligaments (ACL and PCL). These ligaments aid in joint stabilization.
In total, 14 ligaments hold the joints together. There are also the two menisci, discs cushioning the femur and tibia.
Stifle vs. Human Knee
The stifle is the equivalent of the human knee. However, when people stand still, their knees are upright. The equine “knee” is angled.
Although anterior or cranial cruciate ligament ruptures are common injuries in human and dog knees, this injury seldom occurs in horses.
In people, just one patellar ligament comes off the kneecap. Equines have three such ligaments. If a horse does suffer a severe cruciate ligament rupture, options for the animal are limited.
Types of Stifle Disorders
For instance, intermittent upward fixation of the patella frequently afflicts young horses. The condition is sometimes referred to as “sticky stifles” or “catchy stifles”. In some pony breeds, this is an inherited condition.
The stifle has a locking joint that allows the horse to stand while sleeping. That is a normal process. A sticky stifle means that this mechanism is keeping the leg in a locked position.
In many cases, regular exercise can strengthen muscle and ligament tone sufficiently to prevent the hitchiness inherent in upward fixation of the patella.
Signs and Symptoms of Stifle Lameness
Initially, signs of stifle lameness are often subtle. Horses may seem off when taken out of their stalls, but get better as they continue working. A long period of stall rest and subsequent loss of muscle and ligament tone may exacerbate the problem.
Reluctance to work may prove an early indicator of a stifle issue. Other signs of weak stifles in horses include:
- Dragging the toe
- Canter resistance
- Very rough canter
- Difficulty backing up
- Shortened stride
- Issues going up and down hills
- Drifting to one side over fences
- Problems transitioning from trot to canter and vice versa.
A sore back or croup often accompanies stifle lameness. If your horse seems sensitive when these areas are groomed or when saddled, suspect a possible stifle issue.
Because minor stifle weakness or lameness is hard to pinpoint, especially if the stifle is not swollen, identifying the underlying problem often involves a process of elimination.
Clicking noises may emanate from some horses’ stifles. Although alarming, the noise itself may not indicate a serious stifle issue. However, the clicking can mean a rupture of the cartilage or ligaments of the joints. In such cases, the horse is severely lame.
Causes of Stifle Lameness
The stifle is a complex joint and there are many different causes that can lead to stifle lameness. The most common include:
- Arthritis: As with any joint, the stifle is prone to arthritis as the horse ages. Arthritis development in the stifle is often secondary to a long-term injury.
- Bone cysts beneath cartilage: Large cysts may develop in the stifle – as well as the fetlock – although the cause is not always clear. Trauma may play a role, as may osteochondritis dissecans or another disorder of the bone or cartilage.
- Meniscal tears: Among the most common causes of stifle lameness, a meniscal tear can range from low-grade lameness to a severe situation if the tear is severe. 
- Osteochondrosis: These lesions most often develop in the foal’s first six months. Signs of lameness may not become apparent until the animal begins work. 
- Stifle trauma: While horses used for certain disciplines or with various conformation issues are at greater risk for stifle lameness, any horse can step in a hole, get kicked, or slip in the mud.
Risk Factors for Stifle Injuries
Stifle injuries are more common in certain disciplines. These include:
- Barrel racing
- Harness racing
- Upper-level dressage
Basically, any activity in which a horse makes sudden stops, changes direction quickly, or travels at speed can increase the odds of a stifle lameness.
Disciplines such as dressage, which at the higher levels require upper body rotation and bending, can increase the risk of a stifle injury.
Horses with straight hind limbs are more prone to stifle lameness. So are equines whose hooves have low heels and long toes.
Diagnosing Stifle Lameness
An acute stifle injury will generally have swelling associated with it. The veterinarian will check for swelling and perform a flexion test.
After viewing the horse moving in hand or under saddle, the vet will block the three joints if a stifle injury is suspected. All three joints are usually blocked at once to narrow down a lameness in that area.
If the horse is sound after blocking, the next stage is using radiographs and ultrasound to examine the stifle. These imaging technologies are generally used in conjunction, as certain areas of the stifle are better viewed via X-ray and others with ultrasound.
X-rays should reveal the existence of arthritis, bone cysts, or fractures. Ultrasound will show ligament or muscle injury.
Horses with a mild stifle lameness may respond to rest, the use of non-steroidal anti-inflammatory drugs (NSAIDs), or injections of a corticosteroid and hyaluronic acid to battle inflammation.
The vet will discuss a timeframe after the rest period for resuming work with the focus on building conditioning.
Candidates for arthroscopic surgery are those for whom soft tissue abnormalities have been ruled out. This minimally invasive procedure is a useful tool for both diagnostic and therapeutic purposes.
This once common surgery in which the medial patellar ligament is severed is not used frequently today, but it is an option when horses do not respond to more conservative treatments.
This hormone, in the form of estradiol cypionate, is injected into the stifle to increase ligament tension, including the distal patellar ligament tone.
Interleukin-1 receptor antagonist protein (IRAP)
This natural anti-inflammatory, derived from the horse’