Medial Patella Luxation
The Patella is the anatomical term for the kneecap. It sits in the ligament of the quadriceps muscles as this structure crosses the knee and runs through a groove in the joint surface of the thigh bone. When it falls out of this groove, we term it luxating patella.
Medial luxation of the patella is a condition that we tend to see in small breed dogs such as Yorkshire Terriers, Westies, Jack Russell Terries as well as in slightly larger breeds such as Staffordshire Bull Terriers. Most cases appear to be related to the development and growth of the dog, whilst occasionally we will see dogs damage the joint traumatically.
When the kneecap slips out of its normal location in a groove, it slips towards the inside surface of the joint. This changes the direction of the pull of the thigh muscles. Rather than these thigh muscles straightening the limb, their contraction has the effect of bending the knee. This means that the limb will bend at the wrong time and the result is seen as the dog limping. The x-ray on the right shows the patella as an oval white shape.
Affected dogs will often be seen to run along and then suddenly pick up the affected leg for several paces before putting it back down. The condition is rarely painful and when dogs are examined it is often possible to move the kneecap in and out of position quite easily without the dog showing any signs of discomfort at all.
Nevertheless, this condition can affect the way these dogs get around and really does interfere with their quality of life. It has also been implicated as one of the factors that contributes to cruciate ligament rupture.
Repair of this abnormality is generally completed by deepening the groove in which the kneecap should sit and then changing the attachment of the patella ligament to the shin bone so as to move the direction of pull. Surgery involves literally cutting the bone on the front of the shin bone, rotating it towards the outer surface and then pinning it back into place shown on the right.
This bone will then heal over a 6 week period, after which we will occasionally need to remove the pin or pins used.
Recent years and advances in the types of plates available have led us to start addressing this abnormality in more ambitious ways.
Basically, we look at the shape of the leg, and potentially straighten out some of the curves which can be involved in why the kneecap moves out of position.
The picture on the left is taken from a CT scan of a dog with a kneecap that is significantly displaced from where it needs to be.
The white arrow shows where the kneecap is positioned and the blue arrow is where it should be.
To correct this , we have to look at cutting the thigh bone to straighten it and help get the kneecap running in the right orientation.
The picture below shows the same bone once we have straightened the bone and it has been repaired with a plate.