Why ACL Repairs Fail
For decades there have been unsuccessful efforts to repair the anterior cruciate ligament (ACL) after rupture. Why? Because the forces required to tear the complex weave of the ACL’s collagen fibers involve both tearing and pulling apart. Think of a climbing rope: When overstretched the rope fails within its sheath, not just at a single clean edge. Here is why the ruptured anterior cruciate ligament is most often replaced (reconstructed) rather than repaired.
The ACL is a strong band of collagen fibers about the size of your index finger. It stretches from the femur to the tibia, guiding the knee through its range of motion. The fibers are interspersed with blood vessels, nerves, and cells that provide life and sensation to the ligament. To rupture it, the tibia must shift forward—often with abnormal excessive rotation.
Imagine if you were inside this ligament during a ski fall. You would first see and feel the majority of the fibers around you tightening rapidly, and stretching to their limit. Then—at different areas along the length of the taut ligament—breaks would occur: first at the shortest fibers, then twisting up through the rope-like structure, with each sequential fiber giving way. Depending on the position of the tibia at the moment of impact, and the relative strength of the bone versus the ligament, the majority of the torn fibers might be close to the femur, close to the tibia, or in the middle. In any case, most of the fibers would have experienced forces that come close to or exceed their stretching limits.
With milder injuries the sheath surrounding the ACL may stay intact, holding the unfortunate ends of the once beautiful, interwoven fiber linkages. But if the sheath ruptures, you may see (from your viewpoint inside this ligament) the joint fill with blood. The ruptured blood vessels then leak their fluid into the injury, causing swelling and pushing apart the native structure.
Almost as fast as the rupture occurs, the body’s defenses respond to the alarm. The brain sends protective signals to splint the joint with the surrounding structures. Stem cells—those that live within the joint, and those called into action by the chemicals and proteins released by the cells of the ACL—rush to the site. Other cells, designed to remove damaged fibers and lay down scar, also race to the injury. But just as the ends of a frayed rope shrink when a lit match is held to them, the fibers of the ACL retract. And without the natural tension of being strung from one end to the other, the body’s ineffective repair process leads to shortened, scarred tissue—often attached to the nearby posterior cruciate ligament-like gum stuck to a wall.
Enter the young surgeon into this bloody field of disarray. He or she arrives with the knowledge that ACL reconstruction is often performed by using part of the patient’s own knee, the patellar tendon, or the hamstrings. Though these procedures are often relatively successful, harm is done to the donor—and follow-up visits show a disappointing level of re-injury and arthritis down the line. This leads to the unfortunate decision to simply repair the frayed “rope.” Sutures are put in place, while clots of blood or combinations of resorbable collagen are stuffed between the ruptured ends of the ligament and femur from which the ACL originates—all in the hope that this complex structure will magically re-form itself.
Yet study after study, for the past 100 years, has shown that while certain very small tears of the ACL (generally in women over the age of 40) can be healed effectively with sutures and scaffolds, all the others stretch out the repair over time—leaving the knee vulnerable to re-injury when the patient returns to sports. My own experience with suturing ligaments, in the enthusiasm of my early career, led to exactly the same results. A few do very well. Most do not.
The reason is obvious. The forces involved in rupturing the ACL—a key linkage between very large bones—do damage to the entire ligament. The human body’s repair process cannot effectively re-create this ligament, except under the perfect tension and biologic conditions. But suturing a cruciate ligament, even with an additional scaffold, does not solve the majority of the joint’s problems. The solution lies in better replacement tissues, better surgery, and better rehabilitation techniques.