Medial Patellar Luxation in Dogs

Do you have a toy breed of dog that is running on three legs? Is the leg that is being held up a rear one? Then you are possibly dealing with Medial Patellar Luxation (MPL), the most common congenital abnormality seen in dogs. In one study >7% of puppies were affected. MPL is most frequently seen in small breeds of dogs, however the frequency in larger breeds is increasing. Small breeds commonly affected include miniature poodles, Yorkshire terriers, Cairn terriers, toy poodles, Chihuahuas, Pomeranians, Pekingese, Boston terriers, and the Maltese. MPL is one and one-half times more common in female as compared to male dogs.

Several bone abnormalities are possible with luxation of the patella. The groove that normally houses the patella on the ventral femur (trochlear groove) is typically shallow. The femur, or thighbone, bows laterally while the tibia bows medially. The crest of the tibia is often shifted medially causing the patella to shift medially as well, thereby resulting in a varus (medial bowing) deformity. In addition there may be medial displacement of the quadriceps muscle, instability of the stifle joint, and secondary degenerative joint disease. Since the degree of pathology to the joint may vary from mild to severe, a system for classifying the severity of the deformity has been developed: Grade I is considered the mildest form, and Grade VI the most severe.

MPL often occurs bilaterally, frequently resulting in lameness of both rear legs. The abnormalities occurring with MPL cause progressive cartilage wear and osteoarthritis. It may also contribute to concurrent cranial cruciate ligament degeneration in dogs.

The two most commonly detected skeletal defects are a shallow trochlear groove and medial displacement of the tibial tuberosity. The trochlear groove is the sulcus in which the patella moves up and down, while the tibial tuberosity is a prominent groove on the front side of the proximal, or upper, portion of the tibia.

The patella is actually a small sesamoid that is embedded in the tendon of the quadriceps muscle. The quadriceps, patella, trochlear groove, patellar tendon, and tibial tuberosity should be straight for proper functioning of the stifle or knee joint. The patella should rise smoothly up and down the trochlear groove as the rear leg is flexed and extended.

Most abnormalities occurring with MPL may be corrected surgically through trochleoplasty and tibial crest transposition, respectively. Additional soft tissue reconstructive procedures include a lateral joint capsule imbrication and medial release of the joint capsule. Reluxation occurs in 8% to 50% of surgically-treated cases and is more common in large breed than small breed dogs.

Trochleoplasty is a surgical procedure in which the trochlear groove is deepened to better accommodate the patella and prevent its movement across the groove medially. Tibial crest transposition is a procedure where the tibial crest is moved to better align all structures in a straight line for improved stifle performance.

The end result of untreated medial patellar luxation is often degenerative joint disease or arthritis. Arthritis will result from increases wear and tear, lack of normal articulation, and abnormal stress across the articular surface of the joint.

References:

Bojrab, Joseph. Pathophysiology in Small Animal Surgery. 1981. Lea & Febiger. P. 631-637

Morgan, Joe. Radiology in Veterinary Orthopedics. Lea and Febiger. 1972. Pp. 356-357.

Swiderski, Jennifer and Ross Palmer. “Long-term outcome of Distal Femoral Osteotomy for Treatment of Combined Distal Femoral Varus and Medial Patellar Luxation:” 12 cases (1999-2004). JAVMA. Vol 231, No. 7, October 1, 2007. Pp. 1070-1075.