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Medial Patella Luxation in Dogs

By: Dr. Nicholas Trout

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Related Symptoms or Diseases

There are many other causes of lameness referable to the knee joint, and your veterinarian considers these during the history taking and the physical examination. Some examples include the following:

  • Lateral patella luxation. This will present with exactly the same clinical signs as a medial patella luxation but the displacement is to the outside rather than the inside of the knee. This disorder occurs more frequently in larger breeds of dog.

  • Cranial cruciate ligament rupture. Like patella luxation this injury may result from an acute traumatic event or develop as a chronic lower grade lameness. It is not uncommon to have combinations of patella luxation and a torn cranial cruciate ligament.

  • Collateral ligament injury. The collateral ligaments are located on the inner (medial) aspect and outer (lateral) aspect of the knee joint. They are most commonly injured in major trauma to the whole joint called stifle luxation or a deranged stifle. The medial collateral ligament and the cranial cruciate ligament are often both torn together with damage to the cartilage shock absorber, the meniscus, that lies between the two major bones of the knee joint. The patella is not normally affected in this injury.

  • Patella fractures are uncommon and would present as an acute traumatic lameness, usually as a result of a direct blow.

  • Patella tendon rupture is even less common and results in the patella being displaced above the knee joint, not out to one side or the other.

    Diagnosis In-depth

    Following a general medical history your veterinarian will ask questions concerning the nature of your pet's lameness. This will include:

  • What is the duration?

  • Does the lameness vary or stay the same? Worse in the morning or at night? Worse when your pet first gets up after lying down or worse after exercise?

  • Is there a hopping or skipping component to the gait? Perhaps your dog runs, then hitches up the leg for a few steps and then places it down again and carries on as normal.

  • Is one side worse than the other?

  • Is the problem getting worse?

  • Does the problem improve with rest?

    With an acute patella luxation the lameness is usually the result of trauma and so this questioning is not so important as the physical examination. With a chronic patella luxation, the lameness usually varies, is often worse when first getting up after lying down, may produce some skipping during exercise or low grade partial weight bearing lameness that is slowly becoming more noticeable and does not change much despite rest.

    Your veterinarian will ask you to walk and then trot your dog, in order to observe the lameness. With your pet in a standing position, the affected side will be compared to the opposite side for muscle wasting, patella position while standing, swelling of the knee joint, thickening of the joint, position and thickness of the patella tendon and its attachment to the tibia, the bone below the joint.

    The stifle joint will be put through a normal range of motion, paying particular attention to the tracking of the patella through its groove and the ability to push the knee cap in or out of its normal position manually.

    The knee will be assessed for abnormal motion such as side to side looseness or laxity due to collateral damage, and front to back laxity or "drawer" due to a damaged cranial cruciate ligament.

    The patella luxation will be assigned a grade, from 1, being mild and without clinical signs, through 4, being severe with a permanently displaced patella. Dogs with grades 2, 3 and 4 are often candidates for surgery.

    All other joints, bones and soft tissues on the affected side are also palpated and put through a normal range of motion to ensure the problem is isolated to the knee. X-rays of the affected knee are not always essential but may be helpful to evaluate bony deformity and arthritic change as a result of the luxation. X-rays are important following traumatic luxation of the patella, to ensure that the bones around the joint are not fractured.

    Treatment In-depth

  • Dogs with low grade MPL should be managed conservatively with a period of rest and a short course of non-steroidal anti-inflammatory agents. If the lameness persists or worsens they should be re-evaluated.

  • Some dogs will fall into a gray zone, with grade 2 or even 3 MPL but minimal clinical signs. If the lameness is occasional and not progressive then surgery is probably not indicated. But where this grade is associated with a more marked lameness or serial X-rays that show significant progression of arthritis, surgery would be appropriate.

  • There are a variety of surgical techniques available, but they all aim to restore the patella to tracking permanently in its correct plane within the groove (sulcus) of the femur. To achieve this goal the groove may be artificially deepened either by abrading the existing surface or cutting a v-shaped wedge into the bone. The joint capsule surrounding the knee may be too tight on the inside, pulling the patella in that direction, and so may need to be loosened while the opposite side of the capsule is tightened. Finally, the attachment of the patella itself may deviate to the medial or inner aspect of the tibia, and therefore need to be restored to a straight attachment. This is usually achieved by cutting the tendon at its bony interface and fixing this with pins and wire into a new straighter location on the tibia.

    Home Care In-depth

    A soft-padded bandage is usually placed on the leg following surgery to offer comfort, reduce some of the normal postoperative swelling, and provide some support. Because it is not possible to observe the surgical incision for problems, you need to check the toes on a daily basis for swelling, excessive heat or pain. If your pet tries to chew the bandage and is not behaving normally in any other respect, a bandage change might be helpful to evaluate the surgical site.

    Some dogs just do not tolerate a bandage well, despite the absence of complications at the surgical site. Elizabethan collars can be helpful, but in some cases the bandage should be removed if it becomes more trouble than it is worth.

    Your dog should have the bandage and stitches removed at 10 to 14 days following surgery and have strict rest for four to six weeks. This would require, in most cases, confinement to a single room, no going up or down stairs, avoiding slippery surfaces such as tile, hardwood floors or linoleum, and leash walk for bathroom duty only.

    Your veterinarian may demonstrate how to put the knee through a passive range of motion, that is gentle flexion and extension of the knee to encourage earlier weight bearing.

    The prognosis for dogs with grade 2 or 3 MPL is generally good following corrective surgery. As anticipated, the more severe the deformity, the grade 4 MPL's, the more guarded the prognosis, because there is only so much correction that can be achieved. Severe MPL will be associated with more cartilage damage, more arthritis and subsequently a poorer long term outlook for the knee.

    It is not possible to prevent developmental MPL. Certain breeds of small dogs commonly have MPL. At the first sign of lameness or gait abnormality, have your dog evaluated by a veterinarian ti assess the position and tracking of the patella. As discussed earlier, not all MPL's necessitate surgical intervention. The problem may only need to be monitored and evaluated periodically during your pet's development. In cases of more severe MPL, the prognosis is improved with early detection and surgical correction, as this improves joint function and alignment while the animal is still developing.

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