Preventing Kneecap Dislocations

The patella (more commonly known as the “kneecap”) moves up and down in front of the knee joint along a built-in track called the patellofemoral groove. It is held in place by several ligaments on either side and by the patellar tendon (attached to the quadriceps muscle). The quadriceps muscle is the large, four-part muscle along the front of the thigh.

Although you can take your hands and passively move the kneecap from side to side, this is not an active movement you can make your patella do without assistance. We call that side-to-side (medial-to-lateral) movement accessory motion. The up-and-down and side-to-side accessory motions are referred to as patellar glide.

As part of the patellar tendon, there are slips of ligamentous fibers that help hold the patella in place and keep it from moving too far to one side or the other. On the inside of the kneecap is the medial patellofemoral ligament. On the outside is the lateral patellofemoral ligament.

Without the medial patellofemoral ligament, the kneecap dislocates laterally (in a direction sideways away from the other knee). Because the medial patellofemoral ligament is connected with other ligamentous structures, complete rupture will likely damage other areas as well. The medial patellofemoral ligament attaches above to the femur (thigh bone) and below to the tibia (lower leg bone).

Most ruptures occur at the femoral attachment. But the ligament can tear away from the tibial attachment or even in the middle (not at either bone attachment). This type of tear is called an intrasubstance tear.

A medial patellofemoral ligament injury can be treated conservatively without surgery. The knee may have to be immobilized in a splint for a number of weeks to allow for healing. Physical therapy, taping, and a home program of exercises prescribed by the therapist begin after the period of immobilization. The rehab program must be given the good old college try: in other words, for more than a few days or weeks. It can take months to rehab this injury.

But if nonoperative care fails and the patella dislocates again, then surgery to repair or reconstruct the ligament may be the next step. The surgical approach that works best depends on the underlying damage and specific patient factors.

The surgeon will use an arthroscope to look inside the joint and assess the damage before performing the actual repair. In this study, there was an equal number of patients with medial patellofemoral tears at the femoral, tibial, and intrasubstance locations.

Results were viewed in terms of final outcome (patellar stability or instability with another dislocation) but also included clinical data. Range of motion for the hip and knee was measured.

X-rays were taken to look at the position and angle of the patella over the femur. Knee function and disability were also measured.

Nearly three-fourths (72 per cent) of the patients had a stable patella with no episodes of redislocation. A majority of the patients (88 per cent) were able to go back to playing sports at the same level as before the injury. One-third of the group was involved in multiple sports so they were very happy to be able to continue participation as before the injury.

One important finding was the way in which sutures were placed during the repair. When the surgeon reattached the ligament where it belonged, there were no recurrent or repeat dislocations. When the ligament was sutured in a nonanatomical place, the risk of recurrent dislocation went up considerably.

A second bit of information gleaned from this study was the importance of the ligament repair tension. Too loose or too tight and the risk of failure increased because of the effect of the patellofemoral ligament on patellofemoral joint forces.

The surgeon must carefully test the ligament tension during the operation. A “too tight” repair will eventually cause patellar overload and damage to the cartilage along the backside of the patella. The patient experiences knee pain and eventual arthritic (degenerative) changes.

Surgeons reading this report will find their comments about the need for a tibial tubercle transfer for some patients. The tibial tubercle is a bump on the front of the tibia where the patellar tendon (with some of the medial patellofemoral ligament) attaches.

The placement of this bit of bone (if off to the side) can cause too much torque or pull on the ligament. It also changes how the patella tracks up and down along the patellofemoral groove. Most of the failures in this study had an abnormal tibial-tubercle trochlear groove (TT-TG) distance.

A TT-TG distance of more than 20 millimeters is suggestive of the need to perform a tibial tubercle transfer. The TT-TG distance is not something that can be measured on X-ray or with a clinical exam. It must be evaluated with CT scans. More study is needed to further explore this concept before recommending CT scans for everyone with recurrent medial patellar dislocation.

In conclusion, the 72 per cent success rate is an encouraging statistic for athletes with chronic medial patellar dislocation. Equally exciting is the fact that so many were able to return to full participation in the sports of their choosing. The authors suggest that success rates and satisfaction are higher when the damage isn’t just repaired (ligaments reattached) but when graft tissue is used to reconstruct the torn structures.

And one final parting piece of advice: these surgeons recommend using fluoroscopy. This type of imaging (real time, 3-D radiographs) used while performing the procedure makes it possible to get the anatomical repair needed for optimal results.