Luxating patella Classification and external resources
ICD-10	M21.1

Luxating patella (or trick knee, subluxation of patella, or floating patella), is a condition in which the patella, or kneecap, dislocates or moves out of its normal location.

Patellar luxation is a common condition in dogs, particularly small and miniature breeds. The condition usually becomes evident between the ages of 4 to 6 months. It also occurs in cats, especially the Domestic Shorthair.¹

It also occurs in humans, where it can be associated with damage to the anterior cruciate ligament.²

Presentation

Most cases of patellar luxation are medial and this is frequently a congenital problem in toy and miniature breed dogs. Breeds showing a predisposition for medial patellar luxation include miniature and toy poodles, Yorkshire terriers, Pomeranians, Pekingese, Chihuahuas and Boston Terriers. Large breed dogs are also affected and the Labrador retriever seems particularly predisposed.Patellar luxation is less common in cats than in dogs. Predisposed breeds include the Devon Rex and the Abyssinian. Although the specific cause of patellar luxation is unknown in these cases, it is generally agreed that a defect in hind limb conformation is the underlying cause. ³

Causes

It can be caused by some form of blunt trauma, or may be a congenital defect. In congenital cases, it is usually bilateral.

Diagnosis

Diagnosis is made through palpation of the knee. X-rays are necessary in some cases. The luxating patella often causes no or very slight symptoms. There may be intermittent limping in the rear leg. Osteoarthritis can develop secondarily.

There are four diagnostic grades of patellar luxation⁴, each more severe than the previous:

• Grade I - the patella can be manually luxated but is reduced (returns to the normal position) when released;
• Grade II - the patella can be manually luxated or it can spontaneously luxate with flexion of the stifle joint. The patella remains luxated until it is manually reduced or when the animal extends the joint and derotates the tibia in the opposite direction of luxation;
• Grade III - the patella remains luxated most of the time but can be manually reduced with the stifle joint in extension. Flexion and extension of the stifle results in reluxation of the patella;
• Grade IV - the patella is permanently luxated and cannot be manually repositioned. There may be up to 90¼ of rotation of the proximal tibial plateau. The femoral trochlear groove is shallow or absent, and there is displacement of the quadriceps muscle group in the direction of luxation.

Treatment

Grades III and IV, as well as most grade II cases, require surgery to correct, if the animal has difficulty walking. The surgery involves a sulcoplasty, a deepening of the trochlear sulcus that the patella sits in.

Additional help can be given with the use of pet ramps, stairs, or steps. These can help the animal travel from one place to another, especially up and down, without adding any pain or damage to the patella.

External links

• The KNEEguru - educational site packed with knee content with sections on patellar luxation

References

1. ^ Ettinger, Stephen J.;Feldman, Edward C. (1995). Textbook of Veterinary Internal Medicine, 4th ed., W.B. Saunders Company. ISBN 0-7216-6795-3. 
2. ^ "The Knee and Shoulder Centers - [PRINTABLE Anterior Cruciate Ligament Surgery]". Retrieved on 2007-11-27.
3. ^ Patellar Luxation, canine and feline (cat and dog) veterinary factsheets
4. ^ OFA. "Patellar Luxation" (text/html). Orthopedic Foundation for Animals. Retrieved on 2007-09-04.

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