dog dislocated knee cap

Medial Patellar Luxation (MPL) in Pets

Do you have a small dog? Do they skip or hop on 3 legs occasionally, holding up a hind leg? Your dog may have a “trick knee” or an “MPL” as we call it in veterinary medicine. Keep reading to learn about the causes, diagnosis, and treatment options for this common cause of limping in dogs.

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What is Medial Patellar Luxation?

Medial means towards the middle of the body (towards the other leg), the patella is the knee cap bone, and luxation means displacement from its appropriate position. This is a hereditary condition where the patella doesn’t develop properly.

When the patella is displaced, dogs and cats have trouble unbending or extending their leg, which explains the hopping. MPLs may or may not be significant and may or may not require any medication or medical intervention. MPLs are a hereditary condition that is often associated with multiple other deformities of the bones of the hind leg.

Medial patellar luxation is very common in toy and small dog breeds, but it can also occur in cats and larger dogs. Larger dogs tend to have lateral (not medial) luxation of the patella. This can occur in dogs and cats of any age but tends to be present from birth and progress/worsen throughout life. Approximately 50% of dogs have a medially luxating patella in one knee, while the other 50% have it in both.

What causes medial patellar luxation in dogs?

Contributing factors to the kneecap being displaced:

  • Frequently accompanied by internal rotation of the tibia (the shin bone)
  • Frequently accompanied by a shallow groove in the trochlea (the top of the femur where the patella is supposed to sit)

What are the signs of medial patellar luxation in pets?

Clinical signs depend on severity. There are 4 grades we talk about when we define MPLs:

  • Grade I MPL: mild and infrequent clinical signs; patella can be manually pushed out of the trochlear groove but snaps back into place on its own
  • Grade II MPL: the pet skips once in a while or holds up a hind leg; when the knee is bent, the patella luxates but goes back into place when the leg is extended again
  • Grade III MPL: consistent lameness; patella is more often dislocated than in place in the trochlear groove; bone deformities present
  • Grade IV MPL: severe lameness and deformed limbs

How are MPLs diagnosed?

MPLs can typically be diagnosed during a physical orthopedic exam performed by your vet.

  • X-rays are helpful to evaluate all bones of the hind legs and determine if there are any other deformities or ligament injuries. Untreated, severe MPLs can cause deformities and arthritis of the long bones and hips.
  • MPL may be accompanied by cranial cruciate ligament rupture or other injuries to ligaments and structures in the knee such as the meniscus.

What treatment options are available for pets with MPL?

Treatment depends on the severity of the condition. Options may include:

1. Glucosamine/chondroitin supplements (for life) may be recommended by your vet to support joint health regardless of grade/severity.

2. Anti-inflammatory pain medications to reduce pain and swelling of the stifle (knee joint) are often prescribed for Grade II-IV MPLs.

3. Surgery (advised for some Grade III and almost all Grade IV MPLS) can include both orthopedic and soft tissue procedures, some of which are:

  • Increase depth of trochlear groove so patella stays in place
  • Correction of the angle of the ligament attached to the patella that is pulling the patella out of place
  • Tension releasing incisions around the patella

Prognosis after surgery:

  • Excellent in cats
  • Good in dogs with mild-to-moderate severity of the disease
  • Best results with appropriate post-operative care, rehabilitation and hydrotherapy (water treadmill), and other physical therapies

Read more:

Medial Luxating Patella in Dogs

Patellar Luxation in Dogs and Cats

Joint Disorders in Cats

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