Medial Patellar Luxation (MPL)

Medial Patellar Luxation (MPL), also called “dislocating kneecaps”, is a common cause of hind leg lameness in dogs. Small breeds are most commonly diagnosed with MPL, however recent studies have found MPL is also becoming common in medium and large breeds. Patellar luxation is a developmental condition caused by the quadriceps muscles and bones being out of alignment, allowing the kneecap to dislocate. Some of the underlying structural abnormalities which lead to MPL include: a shallow trochlear groove, bow legged femur bone, and abnormally shaped tibia.

Dogs with MPL will have occasional lameness or limping when the kneecap dislocates out of the groove. They will sometimes hold the leg up or stretch their leg out when the kneecap is dislocated. Dogs may also show reluctance to jump or may sit differently than usual. These signs are commonly observed after certain activities such as running or playing with other pets. Many dogs will begin showing symptoms of MPL when they are puppies, but in some dogs these signs are not observed until later in life.

Medial patellar luxation is graded on a 4 point scale. Dogs with Grade 1 MPL may show infrequent and more mild signs, where dogs with Grade 4 MPL have more consistent and severe limping. Some dogs with low grade MPL can use their legs quite normally and may not require surgery to correct their MPL. Dogs with higher grade MPL or dogs with consistent limping greatly benefit from surgery.

Corrective surgery is usually recommended when limping is being frequently observed at home or when lameness has become more severe. An orthopedic examination will identify discomfort in the knee joint and the kneecap can be moved in and out of the groove. Prior to surgery, x-rays will be performed to evaluate the shape of your pet’s bones and knee joints. If surgery is indicated to treat their MPL, a surgical plan will be tailored to each patient’s individual abnormalities.

The goals of surgery are to realign the kneecap, quadriceps muscles, and underlying skeleton to prevent the kneecap from dislocating. Correcting MPL also slows the progression of arthritis and helps to decrease the chances of tearing the Cranial Cruciate Ligament (CrCL); it has been shown that MPL is a predisposing factor for CrCL tears.

Depending on the underlying musculoskeletal abnormalities causing your pet’s MPL, a combination of procedures within the same surgery may be necessary to properly correct their MPL. Surgical procedures may involve: trochlear recession, tibial tuberosity transposition, balancing the soft tissues, and distal femoral osteotomy. Trochlear recession is a procedure where the trochlear groove is deepened. This allows the kneecap to glide deeply within the groove where it should be normally located. Tibial tuberosity transposition (TTT) is where a cut is made in the crest of the shin bone, and the piece of bone is repositioned. The piece of bone is held in its new location with pins and wire. This procedure helps to realign the patellar ligament and kneecap, so that it glides within the groove. Distal femoral osteotomy (DFO) is necessary when the femur bone is abnormally curved (bow legged). DFO involves making a cut in the femur bone, straightening it, and securing its new shape with a bone plate and screws. This procedure is more frequently indicated in medium or large breeds with high grade MPL.