Patellar Luxation in Dogs and Cats

TL;DR. Patellar luxation occurs when a pet's kneecap slips out of its natural groove, causing a characteristic skipping gait, bow-legged appearance, or lameness that may require surgical correction depending on its severity.

A classic "skipping" gait is a common early sign of patellar luxation.

What is it?

Patellar luxation is one of the most common orthopedic conditions diagnosed in veterinary medicine. To understand this condition, it helps to look at the anatomy of your pet's knee, which veterinarians call the "stifle" joint. The patella, or kneecap, is a small bone embedded within the tendon of the large quadriceps muscle group at the front of the thigh. In a healthy joint, the patella slides smoothly up and down within a deep, vertical depression at the end of the thigh bone (femur) called the trochlear groove.

When a pet suffers from patellar luxation, the kneecap slips sideways out of this groove. This displacement can occur toward the inside of the leg (medial luxation) or toward the outside of the leg (lateral luxation). When the kneecap is out of place, the mechanical alignment of the limb is disrupted. The pet cannot properly extend the knee joint, leading to mechanical lameness, abnormal posture, and progressive wear on the joint cartilage.

Veterinarians classify patellar luxation into four distinct grades based on how easily the kneecap slips out of place and whether it can return to its normal position:

• Grade I: The patella can be manually slipped out of the groove by a veterinarian during an exam, but it immediately pops back into its normal position on its own when released.
• Grade II: The patella occasionally slips out of the groove on its own, especially during exercise. It may remain out of place until the pet stretches the leg or a veterinarian manually pushes it back into position.
• Grade III: The patella sits permanently out of the groove. It can be manually pushed back into the groove during an exam, but it immediately slips back out as soon as the pressure is released.
• Grade IV: The patella is permanently out of the groove and cannot be manually pushed back into place. This grade is associated with severe bony deformities of the hind limb.

In a luxating patella, the kneecap slips out of its natural groove on the femur.

Causes & risk factors

Patellar luxation can be congenital (present from birth) or traumatic (caused by an injury). By far, the congenital form is the most common. It is caused by a complex combination of inherited skeletal abnormalities, including an abnormally shallow trochlear groove, misalignment of the muscles and tendons of the thigh, or abnormal curvature of the femur and tibia (shin bone).

As noted in a leading veterinary surgical reference:

"Medial patellar luxation is one of the most common patellar problems presented to the veterinary practitioner. This disorder can be either congenital or traumatic. The congenital form, which is more common, usually is observed in small breed dogs and may cause minimal to severe gait abnormalities."

While medial patellar luxation (MPL) is highly prevalent in small and toy breed dogs, it can also affect large breeds and cats. Conversely, lateral patellar luxation (LPL) is less common overall and is most frequently diagnosed in large and giant breed dogs. In these larger dogs, the condition is often linked to other developmental orthopedic diseases. According to the same reference:

"Lateral patellar luxations, which are not as common as medial patellar luxations, occur most often in large breed dogs. The disorder can be congenital of traumatic. In the congenital form, this condition is often associated with hip dysplasia, and thus simple correction of the resulting patellar alignment may eliminate the signs and symptoms but may not correct [the underlying hip dysplasia]."

In cats, the condition is less common than in dogs, but it follows a very similar pattern. Feline cases are typically medial and congenital, involving a combination of underlying bony abnormalities that alter the mechanics of the hind limb.

Signs to watch for

Symptoms of patellar luxation can vary widely depending on whether one or both knees are affected, the grade of the luxation, and the presence of secondary arthritis.

Common Signs

• Skipping or hopping gait: This is the classic presentation. A pet will run or walk normally, suddenly hold one hind leg up for a few steps (often taking a "skipping" step), and then return to using all four legs as if nothing happened. This occurs when the patella slips out of place and then pops back in.
• Lameness: Persistent or intermittent limping on one or both hind legs.
• Varus (bow-legged) deformity: A noticeable outward bowing of the hind limbs, which is especially common in small dogs with medial patellar luxation.
• Crouched stance: A low-to-the-ground, "crab-like" posture caused by the inability to fully extend the knees.
• Toes pointed inward: A pigeon-toed walking style often seen in pets with medial luxation.

Occasional Signs

• Toes pointing outward: An outward rotation of the paws, which is more common in cases of lateral luxation.
• Valgus (knock-knee) deformity: An inward collapse of the knees, occasionally seen in large dogs with lateral luxation.

Rare Signs

• Acute lameness: A sudden, severe inability to bear weight on the leg. While patellar luxation is usually a chronic, gradual condition, sudden lameness can occur if a traumatic event causes the kneecap to luxate, or if a chronic luxation leads to a sudden tear of the cranial cruciate ligament (CCL).

A bow-legged (varus) appearance is a common structural sign of medial patellar luxation.

How vets diagnose it

Diagnosing patellar luxation is a straightforward process that begins with a physical examination. Your veterinarian will perform a physical exam and orthopedic evaluation, gently feeling (palpating) your pet's hind legs while they are standing and lying down. By gently manipulating the stifle joint through its range of motion, the vet can determine if the patella is stable, if it can be displaced manually, and whether it returns to the groove on its own. This palpation is the primary tool used to assign a grade (I through IV) to the luxation.

Once patellar luxation is identified on palpation, your vet will recommend radiographs (X-rays) of the hips and stifles. Radiographs are essential for several reasons:

1. They allow the vet to evaluate the depth of the trochlear groove and the overall alignment of the femur and tibia.
2. They help identify the presence and severity of osteoarthritis (degenerative joint disease) within the knee joint.
3. They allow the vet to screen for concurrent orthopedic conditions, such as hip dysplasia, which is particularly important in large breed dogs diagnosed with lateral patellar luxation.

Radiographs help veterinarians assess bone alignment and check for concurrent conditions like hip dysplasia.

Treatment options

Treatment plans for patellar luxation are highly individualized. They depend on the grade of the luxation, the pet's size, whether they are experiencing pain or lameness, and whether both legs are affected.

Conservative Management

For pets with Grade I or mild, asymptomatic Grade II patellar luxation, conservative (non-surgical) management is often the first step. This approach focuses on protecting the joint and managing discomfort:

• Weight Management: Keeping your pet at a lean body weight is the single most important factor in reducing stress on the knee joints.
• Exercise Modification: Low-impact exercises, such as swimming or controlled leash walking, help maintain muscle mass around the joint without causing excessive wear.
• Joint Support: Veterinarians frequently recommend joint supplements containing glucosamine, chondroitin, and omega-3 fatty acids to support cartilage health and reduce inflammation.

Surgical Management

Surgery is generally recommended for pets with symptomatic Grade II, Grade III, or Grade IV patellar luxation. The goal of surgery is to restore normal alignment of the kneecap and the quadriceps muscle group. Because the underlying cause is often structural, several surgical techniques may be combined during a single procedure:

• Trochleoplasty (Trochlear Groove Deepening): The surgeon deepens the shallow trochlear groove in the femur, allowing the patella to sit securely inside it.
• Tibial Tuberosity Transposition (TTT): The bony point on the shin bone where the patellar tendon attaches (the tibial tuberosity) is cut and moved to align it properly with the groove. It is then secured in its new position using surgical pins or wires.
• Soft Tissue Reconstruction: The tissues on the side of the joint toward which the patella was slipping are loosened (released), while the stretched tissues on the opposite side are tightened (imbricated) to help hold the kneecap in place.
• Corrective Osteotomies: In severe Grade IV cases where the femur or tibia is significantly bent or twisted, the surgeon may need to cut, realign, and stabilize the bones using plates and screws to restore a straight leg column.

Prognosis

The long-term outlook for pets with patellar luxation is highly dependent on the severity of the condition at the time of treatment:

• Grades I to III: The prognosis is good to excellent, with up to a 90% success rate following surgical correction. Most pets return to normal, pain-free activity. However, owners should be aware that some degree of degenerative joint disease (arthritis) may still progress over time, requiring lifelong joint support.
• Grade IV: The prognosis is guarded to poor. Correcting a Grade IV luxation requires highly complex bone surgeries. Even with successful surgery, these pets may experience persistent mild lameness, reduced range of motion, or accelerated arthritis.

Prevention

Because congenital patellar luxation is primarily an inherited developmental defect, it cannot be prevented through lifestyle changes alone. The most effective preventative measure is responsible breeding. Dogs and cats diagnosed with patellar luxation should not be used for breeding.

For pet owners, the best way to minimize the impact of this condition is to maintain your pet at an ideal body condition from puppyhood or kittenhood onward. Excess body weight significantly increases the mechanical forces acting on a misaligned kneecap, accelerating cartilage damage and the onset of painful arthritis.

When to call your vet

If you notice your dog or cat performing the classic "skip" while running, or if they appear bow-legged or stiff when rising, you should schedule a non-emergency appointment with your veterinarian.

However, you should seek immediate veterinary care if you observe any of the following emergency signs:

• Sudden, complete inability to bear weight on one or both hind legs.
• Signs of severe, acute pain, such as vocalizing, panting, trembling, or refusing to eat.
• A sudden, dramatic worsening of a pre-existing limp.
• Inability or refusal to stand up or walk.

For specific breeds

While patellar luxation is widely known as a canine disease, certain feline breeds have a suspected genetic predisposition to the condition. Specifically, the Abyssinian and the Devon Rex are breeds where medial patellar luxation is suspected to be inherited. If you own one of these breeds, pay close attention to their gait during their developmental months, and ask your veterinarian to check their stifles during routine wellness exams.

Sources

• Current Techniques in Small Animal Surgery, pages 1084, 1085, 1090.