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What is Medial Patellar Luxation - MPL?

What Is The Patella?
The patella is your pet’s knee cap. While differently shaped and sized, it is designed to work very similarly to your own. The patella in pets is a small bone, buried in the tendon portion of the quadriceps muscle – those big muscles that run down the front of the thigh. The patella sits in a groove in the femur – your pet’s thigh bone – and works in combination with the quadriceps and hamstring muscles to provide a mechanism that allows for flexion and extension of the knee.

What Is Patellar Luxation? 
Patellar luxation occurs when the patella slips out of its normal position and fails to glide properly in the groove of the femur. The patella can slip either to the inside or to the outside of the knee. Medial patellar luxation occurs when the knee cap improperly slides to the inside of your pet’s knee, or stifle joint. If it slides to the outside, it is called a lateral patellar luxation. Medial patellar luxation is the most common type of luxation seen in dogs and cats, occurring 75 to 80 percent of the time. Patellar luxation occurs in both knees 25 to 50 percent of the time. Patellar luxation can be seen in all breeds of dogs, but some breeds, such as toy breeds, are particularly prone to this disorder. 

What Causes Patellar Luxation?
Patellar luxation is most commonly caused by a congenital abnormality. Patellar luxation is usually secondary to a conformational abnormality, such as abnormal bending or twisting of the femur or tibia, a too shallow groove in the femur, or hip dysplasia. It is usually present early in a pet’s life. Luxation can also be caused by injury or trauma to the knee.

Predisposed Breeds
The condition primarily affects small breeds, although its incidence in larger dogs is on the rise.

Smaller breeds:
• Boston Terrier
• Chihuahua
• Pomeranian
• Miniature Poodle
• Yorkshire Terrier

Larger breeds:
• Akita
• Chinese Shar Pei
• Flat-coated Retriever
• Great Pyrenees

What Are The Signs? 
Clinical signs associated with patellar luxation may be a "skipping" gait, occasionally holding the leg up, or lameness of varying degrees. Luxation of the patella may cause acute pain when the patella comes out of its groove. A hallmark of patellar luxation, however, is that the pet usually has a normal gait when the patella is not luxated and no lameness is noted when the patella is sitting properly in its groove. Patellar luxation is graded I, II, III or IV, with higher grades representing more severe luxation. 

  • Grade I – The patella occasionally pops out of its groove and then immediately pops back in. The patella is in the groove more than it is out of it. It may be that the only time the patella is out of the groove is when it is manipulated out of place by a veterinarian during an examination. Signs may range from none to occasional skipping or kicking out of the leg.
  • Grade II – Pets may show a more persistent skipping gait, may intermittently hold the leg up, and may have a mild degree of lameness. The patella tends to be out of the groove more than it is in the groove. It is easily manipulated back into the groove.
  • Grade III – The patella is always out of the groove, can be manipulated back into the groove, but will immediately pop out again. Pets have a more persistent lameness with increasing conformational abnormalities.
  • Grade IV – These pets have a patella that rides out of the groove all the time and cannot be manipulated back into place. They have significant lameness, show outward signs of physical deformity and may walk with a "bow-legged" appearance. The knee usually cannot be extended.

How Is Patellar Luxation Diagnosed? 
Patellar luxation is diagnosed through orthopedic examination and radiographs. The condition may be first detected, however, during a routine examination. 

How Can Patellar Luxation Be Treated? 
Pets with patellar luxation that exhibit no clinical signs should be monitored and do not typically warrant surgical correction. Surgery is dependent on clinical signs, age, and grade of patellar luxation. Surgery is usually considered once the condition is a Grade II or above. Surgical treatment seeks to realign the patella in its groove through basic surgical techniques. More advanced techniques can be employed to correct conformational abnormalities as needed, depending on the grade of patellar luxation. 

Surgical corrections include:

  • Deepening the groove over which the patella slides so that it can glide in its proper position.
  • Changing the alignment of the patellar ligament attachment to prevent the pull of the quadriceps either to the inside or to the outside of the leg.
  • Tightening or loosening the joint capsule attachments depending on the pull of the joint capsule on the patella.
  • Cutting and properly aligning one or both bones to correct any abnormal bending or twisting action of the femur or tibia. (Usually required only for more severe cases.)

What To Expect After Surgery
When your pet returns home from surgery, confine the pet to a small area or room. For the first six weeks after your pet’s surgery, restrict exercise to short leash walks only. Increases in the duration of leash walks will depend on how quickly your pet recuperates from surgery. Pain medications for your pet will be provided as needed by your veterinarian. Physical therapy is generally recommended in order to provide the best post-surgical outcome.

Does Patellar Luxation Need To Be Treated?
If left untreated, the patella will continue to damage the cartilage of the joint, leading to the development of osteoarthritis. Patellar luxation can also contribute to the development of other conditions, such as cranial cruciate ligament rupture.

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