Patellar Luxation in Small Breed Dogs
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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.(1) In addition, females were 1.5 times as likely to be affected.1 Some researchers have suggested a recessive method of inheritance,(2),(3) and the higher incidence in females could possibly be related to X-linked(4) 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(5) 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.

Drawing A






































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.(6) 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.

Drawing B


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(7),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(8) 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.(9) 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.

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A Little More About Patellas
Teri Dickinson, DVM

In the last issue of this magazine (Volume XXXIX Number 1) we reprinted an article about luxated patellas which had been previously published. The information in the original article is still relevant, as the disease process itself has not changed. At this time, I would like to add a little updated information on the topic.

The previous article stated that no data existed about the incidence of luxating patellas in Italian Greyhounds. While no serious scientific study has even been undertaken, the Italian Greyhound Club of America did conduct a health survey in 1993. A total of 2174 adult dogs were included in the survey, and of those, 38 (1.75%) were reported as afflicted with luxating patellas. In reality, the percentage of affected dogs may well be higher than that, as affected dogs may go undiagnosed.

The Orthopedic Foundation for Animals (OFA) has begun a registry to certify the patellas of dogs which are not affected with luxating patellas. The procedure is quite simple. A veterinarian must palpate the dog's patellas and fill out a form indicating that the patellas do not luxate. During the exam, the veterinarian will extend (straighten) the stifle joint, and attempt to luxate (dislocate) the patella. If the patella luxates (leaves the groove) the veterinarian will assess what degree of luxation is present. The OFA recognizes and describes in their written material the four grades of luxation presented in the last article.

The DVM will then fill out the OFA form indicating that either the dog's patellas do not luxate, or that they do luxate, and to what degree. The OFA will issue a registry number only to dogs whose patellas do not luxate.

If you are interested in having your dogs certified by the OFA, you can download the necessary forms from the OFA web site www.offa.org.  Any licensed veterinarian can perform the evaluations. If you, or your veterinarian, are not comfortable having a general practitioner evaluate your dogs, then ask to be referred to a board certified orthopedic specialist. If you live in an area where there are no board certified specialists, there still may be a veterinarian in your area who is particularly interested in this field, and may have a lot of experience in orthopedics. Many IG owners already have the name of someone who specializes in this field, in the event they have a dog suffer a broken leg. Generally speaking, these veterinarians should be good candidates to evaluate patellas.

The OFA requires that a dog be 12 months of age or older in order to be entered in the breed registry. There is a $15.00 charge.Veterinarians are encouraged to submit the results of all evaluations, whether normal or abnormal, for the purpose of completeness of data. There is NO FEE for entering an abnormal evaluation of the patella in the data base.

As a review of the symptoms one might see with an affected dog, I've included an excerpt from the OFA publication.

The following descriptions were written by Donald L. Piermatti, DVM, PhD and Steven Arnoczky, DVM and obtained with permission from "OFA Patellar Luxation Registry: General Procedures."

MEDIAL LUXATION IN TOY, MINIATURE, AND LARGE BREEDS

These luxations are often termed "congenital" because they occur early in life and are not associated with trauma. Although the luxation may not be present at birth, the anatomical deformities that cause these luxations are present at that time and are responsible for subsequent recurrent patellar luxation. Patellar luxation in these breeds should be considered an inherited disease.

Medial (to the inside) luxation is far more common than lateral (to the outside) luxation in all breeds, representing 75 to 80 percent of cases, with bilateral (both legs) involvement seen 20 to 25 percent of the time.

CLINICAL SIGNS

Three classes of patients are identifiable:

1. Neonates and older puppies often show clinical signs of abnormal hind-leg carriage and function from the time they start walking, these generally present with grades 3 and 4.

2.Young to mature animals with grade 2 to 3 luxations usually have exhibited abnormal or intermittently abnormal gaits all their lives but are presented when the problem symptomatically worsens.

3. Older animals with grade 1 or 2 luxations may exhibit sudden signs of lameness because of further breakdown of soft tissues as result of minor trauma, or because of worsening of degenerative joint disease pain.

Signs vary dramatically with the degree of luxation.  In grades 1 and 2, lameness is evident only when the patella is in the luxated position. The leg is carried with the stifle joint flexed, but may be touched to the ground every third or fourth step. Grade 3 and 4 animals exhibit a crouching , bowlegged stance (genu varum) with most of the weight transferred to the front legs.

LATERAL LUXATION IN TOY AND MINIATURE BREEDS


Lateral luxation in small breeds is most often seen late in the animal's life, from 5 to 8 years of age. The heritability is unknown. Skeletal abnormalities are relatively minor in this syndrome, which seems to represent a breakdown in soft tissue in response to, as yet, obscure skeletal derangement. Thus, most lateral luxations are grades 1 and 2, and the bony changes are similar, but opposite to those described for medial luxation. The dog has more functional disability with lateral luxation than with medial luxation.

Copyright The Italian Greyhound

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