This is a congenital, heritable condition in which the kneecap dislocates out of its normal anatomic position when physical stress is applied to the knee joint. Although luxation may not be present at birth, the anatomic deformities that cause the knee to dislocate are. In the normal knee joint, the patella glides up and down the groove in the lower femur known as the trochlear groove. This movement guides the action of the large quadricep muscle that is present on the front of the thigh and connects to the tibia in the lower leg. This muscle is responsible for extending or straightening the leg. In patellar luxation, the edges of the groove are too shallow and can allow the patella to slip out either partially or completely. In addition, this condition can be attributed to an abnormal laxity or looseness of the medial collateral ligament. When too loose, the ligament is unable to hold the patella in place. This condition is most frequently seen in toy and miniature breeds and Shar-Peis. It may also occur in larger breeds including the Flat-Coated Retriever.
Reproduced from Hill's Atlas of Veterinary Clinical Anatomy
This condition can manifest itself in a number of ways. An affected dog may appear to be knock-kneed (genu valgum) or cow-hocked (external tibial rotation). The dog may be unwilling to bear weight on the involved hind leg. The foot may twist outward as weight is placed on it. With normal physical exercise or manual flexion of the stifle, the patella may come out of the trochlear groove and stay out or it may rotate to the edge of the groove and then "pop" back into place on its own. Some dogs have been diagnosed with patella(s) that are clinically subluxated constantly, yet there is no evidence of limping or pain. Other dogs may refuse to go up stairs or jump. Dogs who experience a complete dislocation may be able to "pop" the patella back into place themselves by extending the stifle backwards toward the tail. Lameness may be evident. The leg may be carried with the stifle joint flexed, but may be touched to the ground every third or fourth step at fast gaits. In more severe disease, the dog may exhibit a crouching, bowlegged stance with the feet turned inward. Most of the dog's weight will be borne on its front legs. Permanent luxation renders the quadriceps muscle ineffective in extending the stifle.
Diagnosis is accomplished by palpating the knee joint to ascertain extent of movement of the patella. This may require sedation or light anesthesia. The severity of the condition will be assigned one of four grades.
Grade 1: The patellar luxation is intermittent. The dog may occasionally carry the affected limb. The patella easily luxates manually at full extension of the stifle joint, but returns to the trochlear groove when released. No crepitation will be felt. There will be only minimal lateral deviation of the tibial crest and patella and very slight rotation of the tibia. Flexion and extension of the stifle is in a straight line with no abduction of the hock away from the body.
Grade 2: Patellar luxation occurs frequently and may become permanent. The affected limb may be carried. If able to bear weight, the stifle will be slightly flexed. If both knees are involved, the dog will bear most of its weight on its forelimbs. Dogs with this grade of patella luxation may live reasonably well for many years. However, over time, the constant luxation of the patella over the edge of the trochlear groove can cause erosion of the groove. This will result in a bone-on-bone grating that yields a grinding noise known as crepitation when the patella is manually luxated.
Grade 3: The patella will be permanently luxated along with tibial torsion and deviation of the tibial crest as much as 50 degrees from its normal plane. Many dogs may continue to use the limb with the stifle held in a semi-flexed position. Flexion and extension of the joint causes the hock to move away or in toward the body. The trochlear groove will be very shallow or even flattened.
Grade 4: The patella will be permanently luxated. The tibia will be twisted in toward the body. The tibial crest may be deviated as much as 90 degrees from its normal position. The patella will lie just above the medial condyle of the tibia and a space between the patellar ligament and the femur can be palpated. The trochlear groove will be absent or possibly convex shaped. The limb will either be carried or the dog will move in a crouched position with the limb partly flexed.
Treatment of patella luxation involves surgical repair. There are several procedures available depending on the extent of disease.
Mildly affected: Lateral luxation is present, but without marked rotational deformity of the femur.
Tibial tubercle transposition.
Markedly affected: Lateral luxation is present with marked rotation of the femur.
Corrective osteotomy of the femur.
Soft-tissue reconstruction versus bone reconstruction:
Surgery of the knee joint to stabilize the patella can involve either soft-tissue repair or reconstruction of the bony constituents of the joint or a combination of both techniques. Judgment and experience are necessary to decide the best procedure. Some cases of patellar luxation may prove difficult to categorize as grade 1, 2, 3 or 4 and determining the best surgical procedure may also be difficult. A Board-Certified or otherwise highly skilled veterinary orthopedic surgeon can help you make the best decision. Stability of the joint is the desired outcome. A cardinal rule is that skeletal deformity such as deviation of the tibial tuberosity and a shallow trochlear groove should be corrected by bone reconstruction techniques. Attempting to overcome skeletal malformation by soft-tissue reconstruction alone is the most frequent cause of failure. Soft-tissue reconstruction must be limited to obvious grade 1 cases. In cases of bilateral knee involvement, surgical success can be achieved when both knees are operated on at the same time. This can reduce the difficulties of undergoing two separate operations and two separate recovery periods.
Surgical repair of grades 1, 2 and 3 patellar luxation can have as high as a 90-95% success rate and render the limb pain-free. This is applicable regardless of the age of the dog. Grade 4 cases involving severe bony deformity and contraction of the quadriceps muscle will carry a poor prognosis. Post-operatively, external splinting of the leg(s) is usually not needed. One or two days of complete crate rest will be recommended, then mild exercise. This exercise might begin with brief leash walks, gradually building up in duration over a period of three weeks. Brief periods of swimming can be very helpful. Jumping should be avoided If your dog is prone to jumping, external splints may provide more support during the healing process. Analgesics to control post-operative pain, especially for bilateral surgical procedures may be administered. Passively flexing and extending the knee twenty to thirty times four times a day may be advised. After six weeks of consistent limb strengthening, a slight limp may or may not be evident. This limp should disappear when full conditioning of the limb has been achieved. With proper surgical technique, the long-term prognosis for these animals is good to excellent.
MODE OF INHERITANCE:
Patellar luxation is genetically transmissible. According to the 1997 FCRSA Health Survey, 4.2% of male Flat-Coated Retrievers and 3.2% of females had been diagnosed with this condition. Breeding of affected dogs is not recommended. The OFA does maintain a patella registry. Contact the OFA for the appropriate form. Your veterinarian can palpate your dog's knees and then complete the form. For purposes of data collection, you are encouraged to send the form in to the OFA regardless of normal or abnormal findings.
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