Somebody as far back as 1900 tried suturing torn anterior cruciate ligaments back together, but that was unsuccessful because of its poor capacity to heal. Next, surgeons tried all sorts of artificial substitutes, silver wire and silk cord among them, but they all stretched out, frayed away, or caused the body to reject them. Somebody even tried using a kangaroo tendon; and now, for older, less athletic people, human cadaver tendon grafts are available. That collagen-dense material is treated to avoid any infectious or immunity issues, but it is not as strong as a tendon borrowed from somewhere in the patient’s own body. The difficulty is that there are few areas where a sufficiently long and thick strip of tendon can be harvested without leaving a problematic donor site. The two current mainstays are a tendon from either the front of the knee or the back.
On the front, the surgeon may take strip of the dense tendon that attaches the kneecap to the tibia. It includes a chunk of bone on each end and is harvested through an incision running down the front of the knee. Then using small stab incisions, an arthroscope, drill bits, and graspers, the surgeon can see inside well enough inside the knee joint to drill holes in femur and tibia and fix the graft in place. (If you are curious, have a look at the procedure on YouTube.) The blocks of bone on each end help the graft heal faster.
The other graft that is frequently used is part of a hamstring tendon from just behind the knee. It is palpable when you are sitting in a chair with your feet pressed firmly on the floor. By trying to pull your foot under the chair, the hamstring muscles contract and tent the skin behind the knee like banjo strings. The incision to harvest a hamstring tendon graft is on the back side of the knee, so the scar is mostly invisible; but the absence of one of these three muscles may result in considerable weakness and put the knee at risk for another ACL injury. So unless the patient is a wrestler or plumber who would object to tenderness just below their kneecap, the bone-tendon-bone graft from the front is usually the chosen donor.
In a newer and incompletely vetted procedure, the surgeon leaves the torn ends of the ACL in place and spans the gap with a plug of bovine-derived collagen mesh. It is injected with the patient’s own blood, which supplies the plug with some of the same growth stimulants that are present in platelet rich plasma.
One potential benefit of this procedure is that it preserves specialized nerve endings residing in the ACL remnants. They are critical for providing position sense. So even though people with torn ACLs might not be all that unstable walking on level ground, they have a sense of their knee giving way most any time because their sensory receptors are not doing what sensory receptors do–receiving mechanical clues and sending them to the brain so that it knows the levels of stress that the knee is experiencing.
This is potentially a great advantage of using a blood-infused collagen plug–it saves the native sensory receptors. By contrast, for the reconstructions using tendon grafts, the ACL remnants and their imbedded sensory receptors are trimmed out before the graft is placed. Position sense suffers.
Even in the face of a successful ACL repair or replacement, 10-90% of people have some degree of wear-and-tear arthritis 10-20 years later. Despite a successful operation, the joint never glides as precisely as before, so its cartilage surfaces slowly get ground away, sort of like driving a car with the front wheels misaligned–bad for tread wear. Compared to an ACL replacement, it is too soon to know whether an ACL repair will better protect the joint surfaces and offer better position sense with preservation of the sensory receptors.
Rehabilitation after ACL surgery is long and arduous, and the devotion that typically drives elite athletes to succeed is a boon to their recovery. Within a few days after surgery, all the muscles and ligaments in the limb benefit from being moved and mildly stressed to preclude deterioration. A graded strengthening and endurance program extends over at least eight months before the person should try unrestricted activity.
Several tests indicate whether the rehab is complete. The first is known as the timed six-meter (about 20 feet) hop test, first on the recovering leg, then on the sound one. The next is to see how far forward one can get by hopping on one leg and then demonstrating the ability to hop from side to side back and forth over a line on the floor. Try them yourself with your presumably intact ACLs. I did and was pleased that I didn’t fall over.
One last caution for recreational skiers. ACL injuries are more likely with increasing age, icy or new snow conditions, low temperature, and—get this—easy slopes, which is understandable for beginners; but carelessness and higher speeds may account for experienced skiers to catch an edge when they least expect it.