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.Â 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 located in 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.Â 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).Â 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.Â 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 stifle lameness 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 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 â€“ as well as the fetlock â€“ 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’s blood, is injected into the affected joint.
Platelet-rich plasma (PRP)
PRP is used to promote healing in soft tissue injuries. The horse’s blood is harvested and the PRP removed in a centrifuge.
It is then injected into the horse via ultrasound guidance. One benefit of PRP therapy is that there is no potential for allergic reaction, as the horse’s own blood is used.
Stem cell treatments
The vet obtains stem cells from the animal’s fat or bone marrow and injects them into the joint. A study published in the March 2014 edition of Veterinary Surgery looked at the efficacy of this treatment.
Researchers found that horses undergoing arthroscopy for localized stifle lameness followed by intra-articular administration of autologous bone marrow-derived mesenchymal stem cells (BMSCs), 75 percent of those with meniscal damage returned to their previous level of work. 
The study consisted of 33 horses, 24 of which had meniscal damage. Previous studies using arthroscopy alone had no more than 63 percent of horses returning to work.
The study concluded that BMSC administration was safe and that some horses could return to work with BMSC treatment alone in lieu of arthroscopy.
In the study published in Veterinary Surgery, researchers performed a two-year follow-up on 82 Western performance horses that had undergone arthroscopic stifle surgery.
Thirty-two, or 40 percent, of the horses had returned to their intended use. The disciplines included cutting, Western pleasure, and reining.
Factors affecting the likelihood of a horse to return to its intended use include:
- Age â€“ Younger animals were more likely to fully recover than older equines.
- Degree of lameness
- Duration of lameness â€“ The longer the horse was lame, the lesser the odds of recovery.
- Presence of partial-thickness cartilage lesions
Horses whose radiographs show severe change are less likely to return to their intended use. When making a decision regarding stifle surgery, owners must take these factors into consideration.
For many horses with stifle lameness, a cure is unlikely. What the horse owner can hope for is successful symptom management.Â
Successful management may preclude a horse from competing at its former level, but that does not mean the animal cannot have a useful life in a less demanding job.
The good news is that more treatment options are available than ever before, with additional therapies in the pipeline.
Preventing Stifle Lameness
The adage, “An ounce of prevention is always worth a pound of cure,” was probably not coined for horses,Â but it is apropos.
- Provide your horse with plenty of turnout. If you can put him in a field with gently rolling hills, so much the better. That regular movement will benefit his stifles. For best results, pair him with companionable equines to encourage even more movement.
- Perform strengthening exercises either in hand or under saddle. Have someone hold your horse while you stand by his hip and gently pull on his tail. Keep pulling it toward you until you feel the horse pull back.
Hold the pull for 10 seconds before releasing as the horse braces against the pull, in the process tightening his back, abdominal, and quadriceps muscles. Repeat 10 times per session on each side.
- Regular hill work can strengthen the muscles supporting the stifles and pelvis. Make sure the horse remains straight and does not move the haunches in either direction. When riding in fields, perform serpentines, so the horse uses his quadriceps and other inside and outside leg muscles during bending.
- Keep your horses at a healthy weight, not allowing them to get fat. An overweight horse lacking in conditioning is among the most vulnerable to stifle injury or lameness.
- When horse shopping, make sure to ride any horse you are considering at every gait to detect subtle signs of potential stifle issues. Ride the horse in circles and straight lines, and try to ride up and down hills.
- Bring your trainer or another experienced horse person with you to watch for gait regularity or for any telltale signs of stifle discomfort. If you like the horse but are worried about any low-level stifle lameness indicators, ask the veterinarian conducting the pre-purchase exam about your concerns.
Supporting Joint Health through Diet
There are many different factors that affect joint health and your horse’s feeding program is an important one. The right diet will keep your horse at a healthy body weight, prevent excessive inflammation and support injury healing.
If your horse is affected by stifle lameness or other injuries or joint problems, submit their diet for analysis online and Mad Barn’s equine nutritionists can help you with personalized recommendations.
You should also read our article on the Top 8 Principles for Joint Health in Horses.
Nutritional supplements can play a role in helping to support your horse’s joint function and mobility. A joint supplement is not necessarily going to reverse the problem but may improve comfort, help to reduce inflammation or support the health of connective tissue.
There are several common ingredients used in joint supplements with efficacy and safety supported by clinical research. These supplements include methyl-sulfonate-methane (MSM), hyaluronic acid, and omega-3 fatty acids.
Other supplements like avocado/soybean unsaponifiables, resveratrol and green-lipped mussels have preliminary data suggesting they may work, but require further study.
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- Smith Thomas, Heather. New ways to aid healing of horse stifle injuries. The Horse Owner’s Resource. Equus Magazine. Oct 9, 2020.
- Boswell, Jane C. Disorders of the Stifle in Horses – Musculoskeletal System. Merck Veterinary Manual. Sep 2015.
- Boswell, Jane C. Meniscus and Meniscal Ligament Injuries in Horses – Musculoskeletal System. Merck Veterinary Manual. Sep 2015.
- Boswell, Jane C. Osteochondrosis of the Stifle in Horses – Musculoskeletal System. Merck Veterinary Manual. Sep 2015.
- Equinews. Corrective Shoeing Can Alleviate Locking Stifles. American Farriers Journal.
- Ferris, Dora J. Clinical outcome after intra-articular administration of bone marrow derived mesenchymal stem cells in 33 horses with stifle injury. Vet Surg. 2014 Mar.
- Marcella, Kenneth L. 3 Steps to Stronger Stifles. Practical Horseman. Oct. 4, 2017.