Patella Luxation: A Hop, Skip and Jump.
Our little Chihuahua Squirt loved to run and chase the cats. He would dive under the furniture, on the sofa, behind the cushions and play, play, play. Another favorite past-time of his was to chase his rubber toy dumbbell. We would throw it and he would fetch it and bring it back and we would throw again. This could go on for hours, Then one day, at only a few years of age, Squirt started to
yelp out when running and playing. He would hold up his right rear leg and skip. Then, after a bit of time, he would put his leg back down and continue to play. This happened on and off for sev- eral days, but progressively got worse. Each time he would yelp, he seemed to be fright- ened by what was occurring. It was time for me to perform a physical examination and determine what was affecting our little boy. After a complete and
thorough examination, it was determined that Squirt was suffering from a right medial patella luxation. In layman’s terms, he had a knee cap dislocation. I took radiographs to be sure there were no other issues present and none were found. The patella is the medical term for the knee-cap. The patel-
la normally rides and sits in a space known as the femoral trochlear groove. When the patella moves outside of this groove, then the patella is said to be luxated. This popping in and out of the groove was what was causing Squirt to yelp out in pain and hold up his leg. Patella luxation occurs frequently in dogs and is uncommon
in cats. The patella can luxate medially (towards the middle of the body) or lateral (towards the side of the body). However most are medial. Most of the time, patella luxations are secondary to anatomical abnormalities and are considered genetic, but heri- tability is unknown. Luxations in either direction may occur when associated with trauma. A medial patella luxation is abbre- viated as MPL. A lateral patella luxation is abbreviated as LPL. Medial patella luxation (MPL) is considered a congenital dis-
order and signs occur early in life. Most dogs are not born with luxations, but with anatomical deformities that eventually cause luxation. It is suspected that abnormal angles of the bony struc- tures, displacement of the muscles, age of the pet, where the growth plates are not closed, shallow grooves and abnormal bone rotation may be underlying factors.
72 THE NEW BARKER
–by Timothy D. Hodge, DVM MPL is a common abnormality and found in about 7% of all
puppies. MPL accounts for about 75% of all patellar luxations. MPL affects primarily small breed dogs, especially Boston Terrier, Yorkshire Terrier, Chihuahua, Pomeranian, Affenpinscher, Brussels Griffon, English Toy Spaniel, Japanese Chin, Maltese, Manchester Terrier, Miniature Pincher, Papillon, Pekingese, Pug, and Shih Tzu. MPL also appears to be increasing in some large breeds such as Chinese Shar Pei, Flat- Coated Retriever, Akita and Great Pyrenees. Dogs with MPL may
have only one knee affected or both can be affected and even different grades per knee. Lameness may wax and wane.
Lateral patella luxation
(LPL) occurs most commonly in large breed dogs, but can occur in any breed. It is also suspected that skeletal defor- mities, rotation of the bones, abnormal formation of the lateral side of the femoral
groove and hip position may be underlying factors. LPL can occur in small breed dogs of mature age (5 to 8 years), but is pre- dominantly in medium to large breed dogs. Mean age of onset in large breed dogs was from 10 months to 1.3 years. The younger the dog, generally the more severe the grade of luxation. 60% of LPLs occur in male dogs — most commonly in the Great Dane, St. Bernard, Irish Wolfhound and Newfoundland. Dogs with LPL are more functionally disabled as compared to dogs with MPLs. Most dogs will have LPL on both sides and walk with a knock-knee posture. If an LPL is present in a small breed dog, the onset of lameness may be precipitated by exercise or mild trauma. In dogs with LPL, the foot commonly turns out when weight is placed on the leg. The medial side of the joint may palpate thickened. Patella luxations are graded according to the severity
of the luxation: Grade 1 – The patella can be manually luxated, but immediately returns to normal position when released. This may be an incidental finding. Grade 2 – The patella luxates and returns to normal position spontaneously. Manual luxation of the patella can be performed when the knee is in extension and the foot rotated towards the direction of the luxation. It then resolves when the knee joint is flexed. A Grade 2 can become a Grade 3 if the medial side of the groove wears down over time.
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