The ACL is not a tendon. It is a ligament. The anterior cruciate ligament connects your thighbone (femur) to your shinbone (tibia) inside the knee joint, and that bone-to-bone connection is exactly what makes it a ligament rather than a tendon. The confusion is understandable because both tissues look and feel similar, and tendons are actually used to replace the ACL when it tears. But structurally and functionally, they are different types of connective tissue.
Ligaments vs. Tendons
The distinction comes down to what each tissue connects. A tendon attaches muscle to bone. Its job is to transmit force so your muscles can move your skeleton. A ligament attaches bone to bone. Its job is to hold structures together and keep them stable.
Both are made of tough, fibrous connective tissue, and both are primarily composed of collagen. In the ACL specifically, Type I collagen makes up about 85% of the total collagen content, with the remaining 15% consisting of other collagen types. Tendons have a similar collagen-heavy composition, which is one reason the two tissues are easy to confuse. The key difference is in their arrangement and purpose: tendons are built to handle pulling forces in one direction, while ligaments like the ACL are designed to resist forces from multiple angles.
What the ACL Actually Does
The ACL sits deep inside the knee joint, running diagonally from the back of the thighbone to the front of the shinbone. Its primary job is preventing the shinbone from sliding forward relative to the thighbone. It also helps control rotational movement of the knee, which is why it so often tears during sudden pivots or direction changes in sports.
The ligament is made up of multiple fiber bundles, each attaching at slightly different points on the bones. On the femur side, the bundles attach along a bony ridge on the inner wall of the knee joint. On the tibia side, they anchor between the two bony spines at the top of the shinbone. This multi-bundle design lets the ACL resist stress across a range of knee positions rather than just one.
A Built-In Sensor
Beyond providing mechanical stability, the ACL plays a surprisingly important role in how your brain senses your knee’s position. The ligament contains four types of specialized nerve endings called mechanoreceptors, each tuned to different kinds of information. Some detect the knee’s static position. Others fire only when movement starts or stops. A third type activates at extreme ranges of motion when the joint is under heavy stress. The fourth type is simply a pain receptor.
Together, these nerve endings feed your brain a constant stream of data about where your knee is in space, how fast it’s moving, and how much force it’s absorbing. This is why people who tear their ACL often describe a lasting sense of instability even after swelling goes down. The mechanical support is gone, but so is a chunk of the knee’s sensory wiring.
Why the ACL Heals Poorly on Its Own
One of the ACL’s defining features is its location inside the joint capsule, bathed in synovial fluid. Its blood supply comes indirectly through surrounding soft tissues like the synovial lining and the infrapatellar fat pad, which relay blood from a network of arteries around the knee. This indirect supply gives the ACL significantly less blood flow than external structures like tendons or skin.
That poor blood supply is the main reason a torn ACL rarely heals on its own. Tendons outside the joint, like the Achilles or patellar tendon, have a more direct blood supply and can often repair themselves with rest and rehabilitation (though not always well). The ACL doesn’t get that advantage. A complete tear typically stays torn, which is why surgical reconstruction is so common.
Why Tendons Are Used to Replace It
Here’s where the tendon-ligament confusion gets reinforced: when surgeons reconstruct a torn ACL, they almost always use a piece of tendon as the replacement graft. The most common options include the patellar tendon (the band running from your kneecap to your shinbone), hamstring tendons from the back of the thigh, and the quadriceps tendon above the kneecap. Sometimes an Achilles tendon from a donor is used instead.
The bone-patellar tendon-bone graft, for example, involves harvesting the middle third of the patellar tendon along with small blocks of bone from each end. This graft is threaded through tunnels drilled in the femur and tibia, positioned where the original ACL was. Over time, the transplanted tendon tissue gradually remodels to become more ligament-like in structure, a process surgeons call “ligamentization.” The graft never becomes identical to a native ACL, but it adapts enough to provide functional stability.
The quadriceps tendon graft is used in roughly 10% of ACL reconstructions. Some techniques harvest only one of the two hamstring tendons (the semitendinosus), folding it into four strands for strength while preserving the other hamstring tendon (the gracilis) to minimize donor-site weakness.
How Common Are ACL Injuries?
ACL tears are one of the most frequent serious knee injuries in sports. Data from Sweden’s national knee ligament registry, which captures over 90% of all ACL reconstructions in the country, recorded more than 52,700 primary reconstructions and over 4,000 revision surgeries between 2005 and 2020. The average age at injury was about 22 years old.
Women face a substantially higher risk. Among competitive athletes in Sweden, female athletes had a 3.3 times higher rate of ACL reconstruction compared to males. The gap was largest in basketball, where women tore their ACLs at nearly six times the rate of men per exposure. These differences are likely driven by a combination of anatomical factors, hormonal influences on tissue stiffness, and movement patterns during cutting and landing.
Interestingly, athletes competing at the second-highest division level had higher ACL injury rates than those at the top division, possibly reflecting differences in training resources, conditioning programs, or playing surfaces.