A stifle is the knee joint of a four-legged animal. It connects the thigh bone (femur) to the shin bone (tibia) and includes the kneecap (patella), making it structurally equivalent to the human knee. You’ll most often hear the term from a veterinarian discussing a dog or horse, since “stifle” is the standard veterinary name for this joint across all quadrupeds.
Stifle Anatomy
The stifle sits on the hind leg, roughly where you’d expect a knee to be, though in many animals it’s positioned higher up the leg than people assume. The joint is formed by five bones: the femur, tibia, fibula, patella, and small sesamoid bones that help tendons glide smoothly. Holding everything together is a network of ligaments, including two cruciate ligaments that cross inside the joint, two collateral ligaments on the sides, and a patellar ligament that anchors the kneecap.
The internal structure mirrors the human knee almost exactly. The cranial cruciate ligament (CCL) in animals serves the same role as the ACL in humans. It prevents the shin bone from sliding forward relative to the thigh bone, stops the knee from overextending, and limits rotational twisting. Cartilage pads called menisci cushion the space between the bones, just as they do in a human knee. This similarity is so close that dogs and rabbits are commonly used in research to model human knee injuries and diseases.
Why the Stifle Matters for Dog Owners
Stifle injuries are among the most common orthopedic problems in dogs. Swedish insurance data covering large populations found an overall incidence of about 55 cases per 10,000 dogs per year. Of those, roughly 44% involved a torn cruciate ligament and 30% involved a dislocating kneecap. If your vet has brought up the stifle, there’s a good chance it’s related to one of these two conditions.
A cruciate ligament tear is the big one. Unlike in humans, where ACL tears usually happen from a single traumatic event like a sports injury, dogs often experience a gradual weakening of the ligament over time. The fibers slowly deteriorate until the ligament partially or fully ruptures. This makes it a degenerative condition in many cases, not just a freak accident. Large breeds, overweight dogs, and certain breeds like Labrador Retrievers and Rottweilers face higher risk.
Patellar luxation, the second most common stifle problem, occurs when the kneecap slips out of the groove it normally rides in. Veterinarians grade it on a scale from 1 to 4. A Grade 1 means the kneecap can be manually pushed out of place but pops back on its own. Higher grades involve the kneecap slipping out more frequently or staying displaced permanently. Small breeds are especially prone to this condition.
Signs of a Stifle Problem
The most obvious sign is limping or favoring one hind leg. Dogs with a cruciate tear often suddenly become lame, sometimes yelping during a play session or walk, and then refusing to bear full weight on the leg afterward. With partial tears, the lameness may come and go for weeks before the ligament fully gives out.
Swelling around the joint is another telltale sign. In a healthy stifle, you can feel the distinct edges of the patellar ligament running from the kneecap down to the shin. When the joint is injured, fluid buildup and tissue thickening blur those edges. Over time, a characteristic firm swelling called a “medial buttress” can develop on the inner side of the knee, a sign of chronic cruciate damage. You might also notice that the muscles on the affected leg look thinner than the other side, since dogs naturally shift their weight off a painful limb, causing muscle loss within weeks.
How Stifle Injuries Are Diagnosed
Veterinarians use a hands-on test called the cranial drawer test to check for cruciate ligament tears. With the dog lying on its side, the vet holds the thigh bone steady with one hand and attempts to slide the shin bone forward with the other. If the shin shifts forward like a drawer being pulled open, the cruciate ligament is torn. A second test, the tibial compression test, works on the same principle from a different angle. Both tests are sometimes performed under light sedation, since pain and muscle tension can mask the instability in an awake dog.
X-rays help confirm the diagnosis and reveal how much damage has already occurred. They can show joint swelling, abnormal bone positioning, and the early stages of arthritis. In some cases, advanced imaging like CT or MRI provides a more detailed look at the soft tissue structures inside the joint.
Long-Term Joint Health
An unstable stifle doesn’t just cause immediate pain. It sets off a chain of degenerative changes that worsen over time. A prospective study tracking dogs with stifle injuries found significant progression of bone spurs (osteophytosis) around the affected joint, and 40% of dogs also developed progressive changes in the opposite knee. This happens because dogs compensate by overloading the other hind leg, stressing its cruciate ligament in turn. Roughly 30 to 50% of dogs that tear one cruciate ligament will eventually tear the other.
This is why early treatment matters. Whether surgical or managed conservatively, addressing stifle instability sooner helps slow the cascade of arthritis and protects the opposite leg from excessive strain.
Treatment and Recovery
For cruciate tears in dogs, surgery is the most common recommendation, especially for dogs over about 30 pounds. Several surgical techniques exist, but they all aim to restore stability so the shin bone no longer slides forward during movement. Smaller dogs and those with partial tears sometimes improve with strict rest, weight management, and rehabilitation, though they remain at higher risk for full rupture later.
Recovery from stifle surgery typically involves 8 to 12 weeks of restricted activity. Early weeks focus on controlled leash walks that gradually increase in duration, along with range-of-motion exercises. Most dogs begin bearing meaningful weight on the leg within 2 to 4 weeks after surgery, with a return to normal activity around the 3- to 4-month mark. Rehabilitation exercises that rebuild muscle strength and improve balance play an important role in recovery outcomes.
Stifle Problems in Horses
Horses develop stifle issues too, though the common conditions differ. Bone and cartilage lesions (called OCD, or osteochondritis dissecans) are among the more frequent stifle problems in young horses. These are often treated with arthroscopic surgery, a minimally invasive approach using a small camera and instruments inserted through tiny incisions. Recovery for horses after stifle arthroscopy involves about a week of stall rest, followed by hand-walking sessions of around 10 minutes twice daily. Depending on the specific problem, horses typically return to work somewhere between 9 weeks and 4 months after surgery.
Horses can also experience a condition where the kneecap temporarily locks in an upward position, causing the hind leg to stick in extension. This “locking stifle” is particularly common in young, unfit horses and often improves with conditioning and fitness work.