Congenital luxation of the patella represents one of the most common orthopedic conditions in small animal practice. Patellar luxation is the result of anatomic abnormalities involving the entire hind limb. The patella (kneecap) is a small bone located in the tendon of the muscles of the thigh. The tendon is a band of tough, inelastic tissue that connects a muscle with its bony attachment. The kneecap may slip (luxate) out of the tendon and then slip back. Although the overwhelming majority of patellar luxation are congenital and certainly hereditary, traumatic cases do occur when a blow is sustained to the lateral side of the stifle joint. Females have been reported to be 1.5 times more likely to be affected than males.
Patellar luxation is graded I to IV based on the severity of the defect, 1 being occasional mild lameness. Newborn and older puppies with noticeable pelvic limb gait abnormalities usually suffer from grade II to III. Older animals occasionally show signs of acute lameness following a minor trauma which worsens preexisting degenerative joint disease of the stifle. Often these dogs have a grade I or II of patellar luxation.3 As the disease progresses in duration and severity, this lameness becomes more frequent. In young puppies with severe patellar luxation, the rear legs often present a "bow-legged" appearance that worsens with growth. In severe cases, the limb may cease to function or cause other degenerative joint diseases such as osteoarthritis. It is still unclear what causes this orthopedic problem. Possible causes have been suggested including hip dysplasia and deviation of muscles and bones to which patellar attaches. A study of 1,679 puppies sold in pet stores, conducted over a 2-year period, found that over 7% of 6- to 18-week-old puppies had congenital patellar luxation.
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Signs of patellar luxation vary depending on the animal's size, age, and severity of the disease. Puppies and young adult dogs may have intermittent “skipping” gait. The owners describe the dog as pulling up the leg for several steps before returning it to the ground and then resuming normal ambulation with no sign of lameness. The skipping gait may be widely intermittent or may be almost continuous, depending on the severity of the luxation. Middle-aged or older dogs may have a more constant hind limb lameness. The owner may comment that the dog has had a problem with the leg from time to time for as long as they can remember, but the symptoms seem to be getting worse. A significant number of dogs with patellar luxation may show few or no clinical signs. Fewer large breed dogs with patellar luxation would fall into this category.
Surgical intervention is generally required to prevent the skeletal deformities and to improve limb function. Surgical techniques involve bony structures and those that involve only soft tissues. Most patients will receive some combination of bony and soft tissue surgical techniques. Surgery is indicated only in those dogs that are experiencing significant clinical signs or in young dogs where soft tissue techniques might be utilized in an attempt to mitigate the negative effects of the condition on growing bone. In the asymptomatic adult dog, despite the risk of degenerative joint disease and rupture of the cranial cruciate ligament, there is no evidence that surgery is beneficial prophylactically.2
- Patellar luxation
- Patellar luxation: Pathogenesis and surgical correction
- Diagnosing Lameness in Dogs. Leo Brunnberg, Ken Johnson