Once ruptured, the posterior cruciate ligament, highlighted, was typically not repaired because the repairs were risky and often did not work. (Courtesy photo)

The curious case of the posterior cruciate ligament

The posterior cruciate ligament was long the forgotten stepchild of knee ligaments. When ruptured — which occurs most often in car accidents, when the knee hits the dashboard and the tibia (shin bone) is forced backward — the ligament was usually not repaired. Why? Because the repairs were risky and often did not work. Here is what’s new:

The PCL connects the femur (thigh bone) to the back of the tibia in the knee joint. It originates as a wide, fan-shaped insertion; narrows into a strong ligament the size of your index finger; then inserts itself on the down-sloping part of the underside of the tibia. Each of these characteristics makes it hard to rebuild.

Let’s focus first on the insertion on the femur. Any replacement ligament must have the same fan shape as the PCL — and none really do. In the past, when surgeons inserted round ligaments into the femur, the circular shape failed to control the femoral rotation properly. Today, the solution is to use two grafts simultaneously to more closely match the expanse of the normal PCL.

The insertion of the PCL on the back of the tibia presented problems for orthopedic surgeons as well. First of all, the major nerves and arteries that feed the leg are only one centimeter (about half an inch) away, hidden just behind the capsule of the joint. When a surgeon drills a hole for the replacement ligament into the front of the tibia, exiting out the back, there is only a small margin of error before bad things begin to happen. The ideal exiting point for the drill hole is well below the back surface of the tibia, so looking at it from the front with the arthroscope made it even more challenging for the surgeon.

The techniques and tools have evolved to protect those structures and to see the insertion site more clearly. Better guides, X-ray control, and preoperative 3D imaging of the anatomy have all made this possible.

The materials used to replace the PCL have improved as well. In the U.S., high-quality allograft — donor tissue from young donors who most often die in motorcycle accidents — provides the best options for rebuilding the PCL. In Europe, pig tissue called Z-Lig is now available. It’s younger, stronger and safer than other choices. To date, no artificial materials have worked. Dacron, polyesters, carbon fibers and Gore-Tex have been tried, but all eventually break.

Left unrepaired, the PCL-deficient knee rotates abnormally and sags backwards. This mal-alignment leads to rapid wear of the inside surfaces of the knee joint and a lifetime of arthritis. When repaired properly, though, patients can return to full sports. The key is in the technique, the materials and the rehabilitation.

So the days of leaving the PCL ruptured after injury, dooming the victim to a lifetime of instability and arthritis, are over — as long as the right diagnosis, the optimal choice of tissues, perfect anatomic placement of the grafts and great rehab are utilized.

Dr. Kevin R. Stone is an orthopedic surgeon at The Stone Clinic and chairman of the Stone Research Foundation in San Francisco.

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