Medial Patellar Luxation (MPL)

The patella or knee cap is a small bone buried in the tendon of the quadriceps muscles. The patella normally rides in a groove in the distal femur. The patellar tendon attaches on the tibial crest. This is a boney prominence at the top of the tibia, just below the knee. The patellar tendon, patella, and quadriceps muscles are to be in alignment with one another. Patellar luxation is a when the patella is outside the groove when the knee is flexed. A patellar luxation can be characterized as medial (to the inside of the knee) or lateral (to the outside of the knee).
Incidence

Patellar luxation has been described as the most common congenital anomaly in dogs, diagnosed in 7% of puppies. Patellar luxation affects both knees in 50% of all cases, resulting in discomfort and bilateral lameness. In most studies, small-breed dogs are 10 times more commonly affected than large dogs, especially the Boston terrier, Chihuahua, Pomeranian, miniature poodle and Yorkshire terrier. A recent study of breed susceptibility for developmental orthopedic diseases found a predisposition for patellar luxation in 32 breeds, including larger dogs such as Chinese Shar Pei, flat-coated Retriever, Akita and Great Pyrenees.

Epidemiology of patellar luxation

  1. Dogs with patellar luxation are born with the disease (congenital) in 82% of cases
  2. Both knees affected in 50% of cases
  3. The luxation is medial in 98% of small dogs, around 80% in medium and large-breed dogs and 67% of giant-breed dogs
  4. Females are 1.5 times more likely to be affected than males

Causes


Although the cause of patellar luxation remains unclear, early diagnosis of bilateral diseases in the absence of trauma and breed predisposition are suggestive of a congenital or developmental misalignment of the extensor mechanism. Possible causes include:
  1. ?Abnormal conformation of the hip joint, such has hip dysplasia
  2. Malformation of the femur, with angulation and torsion
  3. Malformation of the tibia
  4. Deviation of the tibial crest, the boney prominence onto which the patellar tendon attaches below the knee
  5. Tightness/atrophy of the quadriceps muscles, acting as a bowstring
  6. A patellar tendon that may be too short or too long

Signs and Symptoms


Clinical signs associated with patella luxation vary greatly with the severity of the disease: this condition may be an, incidental finding detected by your veterinarian on a routine physical examination or may cause your pet to carry the affected limb up all the time. Most dogs affected by this disease will suddenly carry the limp up for a few steps, and may be seen shaking or extending the leg prior to regaining its full use. As the disease progresses in duration and severity, this lameness becomes more frequent and eventually becomes continuous. In young puppies with severe patella luxation, the rear legs often present a bow-legged appearance that worsens with growth. 
When to Seek Veterinary Advice

You should seek veterinary surgical advice if you have any concern about the gait of your pet or if your primary care veterinarian advises you to do so. The severity of patella luxation has been graded on a scale of 0 to 4, based on orthopedic examination of the knee. Surgical treatment is typically considered in grades 2 and over:
  1. Grade 1: Can be luxated, but reduces without manipulation
  2. Grade 2: Can be reduced by manipulation
  3. Grade 3: Spontaneous luxation found at least once in standing-reducible
  4. Grade 4: Patella cannot be reduced

What Will Happen If Patella Luxation is Left Untreated?


Every time the knee cap rides out of its groove, cartilage is damaged, leading to osteoarthritis and associated pain. The knee cap may ride more and more often out of its normal groove, eventually exposing areas of bone. In puppies, the abnormal alignment of the patella may also aggravate the shallowness of the femoral groove and lead to serious deformation of the leg. In all dogs, the abnormal position of the knee cap destabilizes the knee and predisposes affected dogs to rupture of their cranial cruciate ligament, at which point they typically stop using the limb.
What Options are Available for Treating Patellar Luxation?

Patellar luxations that do not cause any clinical sign should be monitored but do not typically warrant surgical correction, especially in small dogs. Surgery is considered in grades 2 and over. One or several of the following strategies may be required to correct patella luxation:
  1. Reconstruction of soft tissues surrounding the knee cap to loosen the side toward which the patella is riding and tighten the opposite side
  2.  Wedge Recession - Deepening of the femoral groove so that the knee cap can seat deeply in its normal position
  3. Tibial tuberosity transposition - Transposing the tibial crest, bony prominence onto which the tendon of the patella attaches below the knee. This will help realign the quadriceps, the patella and its tendon.
  4. Correction of abnormally shaped femurs is occasionally required in cases where the knee cap rides outside of its groove most or all the time. This procedure involves cutting he bone, correction its deformation and immobilizing it with a bone plate.

Prognosis:


Over 90% of owners are satisfied by the progress of their dog after surgery.

Complications:


Osteoarthritis is expected to progress on radiographs. However, this does not necessarily mean that your dog will suffer or be lame as a result. Keeping your pet trim and encouraging swimming/waling rather than jumping/running will help prevent or minimize clinical signs of osteoarthritis.

Some degree of knee cap instability will persist in up to 50% of cases. This does not cause further lameness in the majority of cases. Migration or breakage of surgical implants used to maintain bones in position occurs rarely. Infection is a rare complication.
Prevention:

Because some breeds are predisposed to this condition, dogs diagnosed with patella luxation should not be bred.
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