Patellar Luxation in Small Breed Dogs

Teri Dickinson, DVM

Luxated patellas or “slipped stifles” are a common orthopedic problem in small dogs. A study of 542 affected individuals revealed that dogs classified as small (adult weight 9 kg (20 lbs) or less) were twelve times as likely to be affected as medium, large or giant breed dogs. In addition, females were 1.5 times as likely to be affected.1 Some researchers have suggested a recessive method of inheritance, and the higher incidence in females could possibly be related to X-linked factors or hormonal influences.

Luxated patellas are a congenital8 (present at birth) condition. The actual luxation may not be present at birth, but the structural changes which lead to luxation are present. Most researchers believe luxated patellas to be heritable (inherited) as well, though the exact mode of inheritance is not known. The condition is commonly seen in Italian Greyhounds, although no published data regarding the incidence in IG’s exists at this time. Researchers1 have suggested that due to the high risk factor in toy breeds, breeding trials or retrospective pedigree analyses should be undertaken by national breed clubs to answer some of these questions.

The stifle is a complicated joint which is the anatomical equivalent of the human knee. The three major components involved in luxating patellas are the femur (thigh bone), patella (knee cap), and tibia (calf or second thigh). See drawing A. In a normal stifle, the femur and tibia are lined up so that the patella rests in a groove (trochlea) on the femur, and its attachment (the patellar tendon) is on the tibia directly below the trochlea.

The function of the patella is to protect the large tendon of the quadriceps (thigh) muscle as it rides over the front of the femur while the quadriceps is used to extend (straighten) the stifle joint. Placing your hand on your patella (knee cap) while flexing and extending your stifle (knee) will allow you to feel the normal movement of the patella as it glides up and down in the trochlea.

Luxation (dislocation) of the patella occurs when these structures are not in proper alignment. Luxation in toy breeds most frequently occurs medially (to the inside of the leg). See drawing B. The tibia is rotated medially (inward) which allows the patella to luxate (slip out of its groove) and ride on the inner surface of the femur.

While the patella is luxated, the quadriceps is unable to properly extend the stifle, resulting in an abnormal gait or lameness. In addition, the smooth surface of the patella is damaged by contact with the femur, rather than the smooth articular (joint) cartilage present in the trochlea. With time this rubbing will result in degenerative joint disease (arthritis). Furthermore, while the patella is luxated, the quadriceps puts a rotational force on the tibia, which over time will increase the rotation of the tibia, thereby increasing the severity of the problem. The additional strain caused by the malformation of the bones may also lead to later ligament ruptures. Many individuals are affected bilaterally (both legs).

Signs of luxation may appear as early as weaning or may go undetected until later in life. Signs include intermittent rear leg lameness, often shifting from one leg to the other, and an inability to fully extend the stifle. The leg may carried for variable periods of time. Early in the course of the disease, or in mildly affected animals, a hopping or skipping action occurs. This is due to the patella luxating while the dog is moving and by giving an extra hop or skip the dog extends its stifle and is often able to replace the patella until the next luxation, when the cycle repeats.

Several grades of luxation have been defined,5. In simple terms they are:

  • Grade I. Patella can be luxated manually (by the examiner) but returns to the trochlea when released. Occasional luxation occurs causing the animal to temporarily carry the limb. Tibial rotation is minimal
  • Grade II. Patella can be easily luxated manually and remains luxated until replaced. Luxation occurs frequently for longer periods of time, causing the leg to be carried or used without full extension. Tibial rotation is present.
  • Grade III. The patella is permanently luxated, but can be replaced manually. The dog often uses the leg, but without full extension. Tibial rotation is marked.
  • Grade IV. The patella cannot be replaced manually, and the leg is carried or used in a crouching position. Extension of the stifle is virtually impossible. Tibial rotation is quite severe, resulting in a “bow legged” appearance.

While no data has been published, personal observation reveals most affected IG’s appear to have Grade I or II luxations. I have also encountered puppies born with no trochlea and severe tibial rotation causing permanent luxation from birth (Grade IV), and adult dogs so severely affected they were non-weight bearing in both hind legs and merely dragged their rear legs along in a frog-like position (Grade IV).

Diagnosis is relatively simple for a veterinarian familiar with orthopedics. It involves palpation of the joint and manual luxation of the patella. X-rays may also be used to determine the degree of rotation. Motivated owners may be trained by veterinarians to palpate the stifles, but care must be exercised in order to avoid injuring the joint, or making an incorrect diagnosis.

Diagnosis in severe cases may be possible at weaning, but in most cases the joints should be tight enough at 4 to 6 months to allow reliable palpation. Screening of puppies at this age will help prevent large expenditures training and showing dogs which later prove unsound. Screening of breeding stock and culling of affected individuals should, over time, reduce the incidence of the condition.

Treatment involves surgical correction of the deformities. Many techniques are available depending on the severity of the condition. Satisfactory results are usually obtained if the joint degeneration has not progressed too far. Once the condition is repaired, most affected individuals make satisfactory pets.

Bibliography

1. Priester WA: Sex, Size and Breed as Risk Factors in Canine Patellar Luxation. J Am Vet Med Assoc. 160:740, 1972.

2. Hutt FB: Genetic Defects of Bones and Joints in Domestic Animals. Cornell Vet. 58:104, 1968.

3. Kodituwakku GE: Luxation of the Patella in the Dog. Vet. Rec. 74:1499, 1962.

4. present on the X chromosome, of which females have two, XX and males one, XY

5. Miller, ME: Anatomy of the Dog. WB Saunders Co., Philadelphia, PA 1964.

6. Putnam RW: Patellar Luxation in the Dog. M.Sc. Thesis. Presented to the faculty of graduate studies, University of Guelph, Ontario, Canada, January 1968.

7. Singleton WB: The Surgical Correction of Stifle Deformities in the Dog. J Small An Pract 10:59, 1969.

8. Archibald J: Canine Surgery. American Veterinary Publications, Santa Barbara, CA, 1974.

9. Brinker WO: Handbook of Small Animal Orthopedics & Fracture Treatment. WB Saunders Co., Philadelphia, PA, 1990.