NEW CONCEPTS IN DISLOCATION OF THE PATELLA

by Robert N. Steensen, M.D.

The patella (or kneecap) normally moves up and down in a groove on the front of the femur (thigh bone) as we bend and straighten our knee. The patella will sometimes come out of its groove during an injury. This is known as a dislocation of the patella or a patellar dislocation. It occurs most commonly in teenagers when the knee twists a specific way. Patients who sustain this injury may recover and do well or may have dislocations continue to occur from time to time. On some occasions, as the patella dislocates it scrapes cartilage off the side of the groove creating a loose piece that needs to be removed.

There are several new concepts that have emerged lately regarding this injury.

One is the importance of a ligament of the knee, the medial patellofemoral ligament or MPFL. This ligament runs from the inside border of the patella to an attachment on the femur. It has been found to be the main soft tissue structure that holds the kneecap in place. When a patellar dislocation occurs this ligament is injured. When patients have continued episodes of dislocation, surgery is often considered. With the knowledge of the importance of the MPFL, many surgeries now focus on reconstructing this ligament.

At the Cardinal Orthopaedic Institute, research has helped define the anatomy of the MPFL and a procedure based on this information has been developed and results have been published in orthopedic journals. The procedure takes a portion of the large quadriceps tendon and re-routes it to the same position as the MPFL to duplicate the function of the ligament.

Another new concept in patellar dislocation is identifying anatomy that patients have that may contribute to dislocation. Some patients have a groove that is not as deep as normal, some have a groove that is flat and others may have a groove that is not a groove at all but convex. This can make it fairly easy for a patella to dislocate particularly a second time when the MPFL was injured with the first dislocation. If a groove is flat or convex, a procedure may be performed that can deepen the groove making it harder for the patella to dislocate.

These concepts help us understand that all patellar dislocations are not the same. We can better appreciate the individual nature of a patellar dislocation to help determine if the optimal treatment may be rehab with no surgery, surgery to reconstruct the MPFL (ligament) or more involved surgery to correct a flat or convex groove.