The anterior cruciate ligament is a cord about the width of your little finger that runs diagonally through the centre of the knee. It is one of two cruciate ligaments, so called because they cross, and its job is stability: it stops the shin bone sliding forward under the thigh bone, and it stops the knee rotating too far when the foot is planted and the body turns. Walking, cycling and running in a straight line barely use it. Cutting, pivoting, landing and tackling depend on it.
How it tears
About three quarters of ACL tears happen without contact. The pattern is familiar to anyone who plays or watches pivoting sport: a sudden deceleration, a change of direction or a landing from a jump, with the knee nearly straight and collapsing inward while the body rotates over the planted foot. Most people feel or hear a pop, cannot continue, and have a knee that swells over the following hours as the joint fills with blood.
The rest are contact injuries, a blow to the outside of the knee in a tackle, which often damage the medial collateral ligament and the meniscus as well.
The numbers in Australia are high and rising. Between 2000 and 2015 the annual rate of ACL reconstruction rose by 43%, and by 74% in people under 25, with the fastest growth in children aged five to fourteen. In 2014-15 the peak was men aged 20 to 24 and women aged 15 to 19. In the same sports, female athletes tear the ligament at about three times the rate of male athletes per hour of play, which is why the warm-up programs covered in chapter 5 matter most for girls and women.
What else is damaged
An ACL tear is rarely an isolated injury, and what else is damaged often matters more for the long-term health of the knee than the ligament itself.
- The meniscus. The two menisci are the crescent-shaped cartilage cushions between the bones. In the Norwegian registry, 47% of knees coming to reconstruction had a meniscal tear. The lateral meniscus tears at the moment of injury; the medial meniscus tends to tear later, in a knee that keeps giving way. Whether a meniscal tear can be repaired rather than trimmed is one of the strongest predictors of arthritis decades on.
- The joint surface. About a quarter of knees have a cartilage injury at the time of surgery, and the proportion rises with delay and with each episode of instability.
- Bone bruises. The MRI almost always shows bruising in the bone on the outer side of the knee, where the bones collided as the knee gave way. In a large review, bruising was present on the outer shin bone in 83% of acute tears. It is the footprint of the injury, it is painful for some weeks, and it heals on its own.
- The collateral ligaments. The medial collateral ligament on the inner side of the knee is sprained in a proportion of tears, particularly contact ones, and usually heals in a brace without surgery.
How the diagnosis is made
The history is often diagnostic on its own: a non-contact twist, a pop, swelling within hours, and a knee that does not feel trustworthy afterwards.
The examination comes first. The Lachman test, done with the knee bent to about 20 degrees, detects about 85% of ACL tears and about 96% of complete ones, and it is almost never positive in an intact knee. The pivot-shift test reproduces the giving-way itself; it is positive in only about a quarter of tears in an awake patient, because the muscles guard against it, but when it is positive the diagnosis is certain. The examination is harder in the first week, when the knee is swollen and guarded, and is often repeated once the knee has settled.
The MRI confirms and surveys. MRI detects ACL tears with about 87% sensitivity and 93% specificity, so a small proportion of tears are missed and a small proportion of normal ligaments are over-called, which is why the examination matters. The more useful job of the MRI is the survey: the menisci, the cartilage, the collateral ligaments and the bone bruises, which together decide how urgent the next step is. X-rays are taken to exclude a fracture and, in a younger patient, to see how much growth is left.
Partial tears, and whether the ligament can heal
A complete tear does not reliably heal into a working ligament. The torn ends retract and bathe in joint fluid, and the scar that forms is rarely strong or tight enough to stop the knee giving way. Some partial tears, where enough of the ligament is intact, do settle and function. There is active research into early bracing of complete tears in a bent-knee position to bring the ends together, with healing visible on MRI in a large proportion; the next chapter covers what that evidence does and does not show.
What happens next
The first days are about settling the knee: ice, elevation, crutches while it is painful, and getting it straight. Within the first week or two the question becomes the one that matters, whether this knee needs a reconstruction, and that depends on the knee, the sport and the person. That is chapter 2.