From hip to hoof, a lot can go wrong in a horse’s hind limb. A multitude of small bones, ligaments, tendons, and muscles, work in tandem to propel a horse forward, jump, turn quickly, or even just allow them to remain standing while they rest. The stifle is a complex joint in the horse’s upper hind-limb and plays an important role in his ability to move with smooth impulsion and stand for long periods of time.

“Stifle lameness is relatively common, occurring in all types of horses and disciplines,” says Ellen Law, DVM, ECVDI, a large animal resident in the Diagnostic Imaging Clinic at the University Animal Hospital, in Uppsala, Sweden. “It is especially common in jumping, barrel racing, cutting, and high-level dressage horses.”

Stifle Anatomy

While often thought of as a single joint, the stifle consists of three joints: two femorotibial joints and the femoropatellar joint. Within the stifle, the end of the femur (thigh bone) divides into two fistlike, round bulbs of bone. The front part of these “fists” consists of the medial (inner) and lateral (outer) trochleas, which do not directly bear any weight and do not articulate (move) with the tibia (the inner of the two bones that extends down from the knee to the hock). Rather, the trochleas articulate with the patella (a small, cartilage-covered bone colloquially referred to as the kneecap), forming the femoropatellar joint. The medial and lateral condyles make up the bottom of the fists, which articulate with the top surface of the tibia, bearing all the horse’s weight.

A flat disc of flexible, strong fibrocartilaginous tissue lies between each condyle and the tibia, providing cushioning during weight bearing. This joint is therefore referred to as the femorotibial joint. To complicate things slightly, the medial and lateral femorotibial joints do not communicate (connect so that fluid can flow between the joints) in many horses, making the stifle three separate joints.

Each joint in the stifle connects to important soft-tissue structures, such as the collateral ligaments, muscles, tendons, and meniscal fibrocartilages and their associated ligaments that attach the menisci (the cartilaginous discs between the femur and the tibia that facilitate frictionless stifle joint movement) to both the tibia and femur.

Sue Dyson, MA, VetMB, PhD, an independent consultant from the U.K., describes how these structures work in motion. “In the femoropatellar joint, the patella glides up and down over the cranial (toward the head) aspect of the trochleas,” she says. “The three patellar ligaments arise from the base of the patella and are effectively the insertions of the quadriceps muscle, an important extensor muscle of the stifle.”

The three patellar ligaments course medially, laterally, or straight down and converge on the front of the tibia.

Common Stifle Pathology

Dyson says common conditions causing lameness in the stifle of horses include:

1. Osteoarthritis (OA) of the femorotibial joint, with the medial compartment affected more commonly than the lateral;
2. Meniscal injury either with or without simultaneous injury of the cranial or caudal (toward the tail) meniscal ligaments;
3. Upward fixation/delayed release of the patella, occurring more often in young rather than mature horses; and
4. Chronic osteochondrosis of the femoropatellar joint, more often in the lateral rather than medial trochlea. Ostochondrosis is a relatively common developmental abnormality characterized by a defect in cartilage and the underlying bone. Osteochondrosis. Affected joints often have effusion and can result in lameness.

“The medial femorotibial joint and related structures, such as the medial collateral ligament and medial meniscus, are most commonly injured,” says Law. “This is due to the biomechanics and loading of the limb during exercise.”

A Closer Look at Stifle Lameness in Horses

Osteoarthritis of the femorotibial joint

The femorotibial joints bear weight directly, so trauma, inflammation, and other factors that lead to OA cause lameness by gradually breaking down the cartilage on these weight-bearing surfaces.

“OA can be secondary to meniscal injury, anatomical variants/developmental anomalies, flattening of the medial femoral condyle, concavities in the articular surface of the distal (further away from the body) aspect of femur, and subchondral bone cysts,” says Dyson.

Treatment for this type of OA resembles others and might include oral nonsteroidal anti-inflammatory drugs (NSAIDS, e.g., phenylbutazone) or joint injections with corticosteroids, orthobiologics such as interleukin-1 receptor antagonist protein/IRAP, or synthetic hydrogels.

Meniscal injury either with or without simultaneous injury of the cranial or caudal meniscal ligaments

“Meniscal injuries and injuries to their associated ligaments are common,” says Dyson. “The meniscal cartilages move a little during flexion and extension, but those movements are restricted by the meniscal ligaments. They are therefore subject to large loads, shear forces, and torsional strain that predisposes them to injury.”

Horses can sustain different types and locations of meniscal tears, and only some can be addressed arthroscopically.

Upward fixation/delayed release of the patella

“Delayed release is probably more common in all types of horses,” says Dyson. “In this case the affected limb(s) is (are) not stuck in extension, but the movement of the patella is jerky, not smooth.”

Veterinarians and owners might only observe delayed release when the horse moves laterally (side to side, e.g., asking the horse to step away from you on the ground) or when transitioning from trot to walk. It occurs repetitively but often intermittently. “This condition unquestionably causes chronic pain and reduced performance,” Dyson adds.

Upward fixation is much easier to recognize because the patella does lock—near the tops of the trochleas—with the limb in extension. This condition mostly self-rectifies or can be by pushing the horse backward.

Conservative care involves improving the horse’s muscling by instituting a controlled exercise program and ensuring the horse’s appropriate nutrition for his age and workload. When this condition causes persistent lameness and femoropatellar joint effusion (swelling), veterinarians might opt to transect (cut) the medial patellar ligament, also called ligament splitting.

“If the medial patellar ligament is split in multiple sites, it is still intact, so the stay apparatus is completely intact,” says Dyson, referring to a network of muscles, tendons, and ligaments that allows a horse to stand with minimal effort. “Transection is done less commonly now because of potential postoperative complications and is followed by repair of the ligament in a lengthened, thickened form.”

Chronic osteochondrosis of the femoropatellar joint

In this painful condition the patella applies pressure to the femoral condyles, causing painful cartilage dysfunction and subchondral bone damage. Dyson says loose pieces of cartilage can become detached and progressively ossify, resulting in movable pieces in the joint. Due to its poor quality, the cartilage might fail to properly absorb shock for the underlying bone.

Treatment usually consists of surgery, although very early instances of the condition in young foals might self-repair. “Chronic cases in mature sports horses are difficult to manage,” Dyson says.

Focus on the Patellar Ligaments

Law recently studied the patellar ligaments in more detail, hoping to understand if horses experience chronic pain originating from these three ligaments and the infrapatellar fat pad (a soft tissue pad behind the kneecap)—a condition similar to “jumper’s knee” in human athletes.

After scanning the 116 horses’ patellar ligaments on ultrasound, Law and colleagues found a large percentage of horses in full work had ultrasonographic changes in the patellar ligaments.

“These changes were very similar to lesions described in previous literature, such as bleeding and inflammation in the ligaments (desmitis),” says Law. “This highlights the importance of thorough clinical examination and blocking when investigating stifle lameness to confirm that findings on ultrasound are clinically significant.”

Take-Home Message

Stifle lameness is common in horses and can be a result of more than just these four conditions; however, they are the most frequently diagnosed. Veterinarians should perform a careful clinical examination, block nerves with local anesthetics, and conduct imaging using radiography, ultrasonography, or computed tomography (CT) to identify the underlying cause of the lameness.