The knee has four major ligaments, but the total count is higher when you include the smaller, secondary ligaments that also contribute to stability. Depending on how you count, the knee contains roughly a dozen distinct ligaments. The four that get the most attention are the ones most commonly injured and most critical for keeping the joint stable during everyday movement and sports.
The Four Major Knee Ligaments
Two ligaments sit on the outer edges of the knee, and two cross through the center. Each one prevents a specific type of unwanted movement.
The medial collateral ligament (MCL) is a wide, flat band running along the inner side of your knee, connecting the thigh bone to the shin bone. The lateral collateral ligament (LCL) does the opposite job on the outer side, connecting the thigh bone to the smaller lower leg bone called the fibula. It’s thinner and rounder than the MCL. Together, these two collateral ligaments act like straps on each side, preventing the knee from buckling inward or outward.
The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) form an X shape inside the joint. The ACL resists forward sliding and twisting of the shin bone relative to the thigh. The PCL prevents the shin from shifting too far backward. The PCL is thicker and stronger than the ACL, which is one reason ACL injuries are far more common.
Secondary Ligaments You Rarely Hear About
Beyond the big four, several smaller ligaments reinforce the knee from different angles. These don’t get discussed as often, but they play real roles in joint stability.
The anterolateral ligament (ALL) runs along the outside front of the knee. References to it appeared in anatomy literature over a century ago, but it wasn’t formally named until 2012. It works as a backup to the ACL, helping resist forward translation and inward rotation of the shin. Surgeons now recognize it as a factor in the “pivot shift” sensation some people experience after ACL injuries.
The ligament of Humphrey and the ligament of Wrisberg are two small bands that sit near the PCL, one in front of it and one behind. They connect the meniscus cartilage to the thigh bone and help fine-tune how the knee tracks during deep bending. The popliteofibular ligament, located at the back outside corner of the knee, acts as a restraint against outward rotation and backward shifting of the shin. Several other minor ligamentous structures reinforce the joint capsule itself, though their exact classification varies between anatomy textbooks.
Why Some Ligaments Heal and Others Don’t
One of the most practically important differences between knee ligaments is their blood supply. The MCL has a rich network of blood vessels running through its outer layer, with some vessels reaching deep into the tissue. When it’s injured, blood flow to the area increases roughly eightfold, triggering a strong inflammatory healing response and scar formation. That’s why most MCL sprains heal without surgery.
The ACL tells a very different story. Its blood supply comes from small branches of a single artery that stay mostly on the ligament’s surface, with almost no vessels penetrating the interior. After injury, blood flow barely doubles and the tissue tends to shrink rather than repair itself. This limited ability to generate new blood vessels is the primary reason torn ACLs rarely heal on their own and often require surgical reconstruction.
The LCL and PCL fall somewhere in between. PCL tears are sometimes managed without surgery depending on severity, while isolated LCL tears are uncommon enough that treatment decisions are made case by case.
Which Ligaments Get Injured Most Often
A ten-year study tracking over 6,400 patients with knee-related sports injuries found that ACL tears accounted for 20.3% of all knee injuries, making the ACL by far the most frequently damaged ligament. MCL injuries came next at 7.9%. LCL injuries were much rarer at 1.1%, and PCL tears were the least common at just 0.65%.
These numbers reflect the mechanics of most sports. Cutting, pivoting, and landing from jumps all load the ACL heavily. The MCL is vulnerable to blows that push the knee inward, which is common in contact sports. The PCL and LCL typically require more unusual forces, like a direct hit to the front of the shin or an awkward hyperextension.
How Ligament Injuries Are Detected
Doctors use specific hands-on tests to figure out which ligament is involved. For the ACL, the Lachman test is considered the most reliable: with your knee slightly bent, the examiner pulls your shin forward. If it slides more than it should, the ACL is likely compromised. A similar test called the anterior drawer does the same thing with the knee bent at a sharper angle, though it’s slightly less sensitive.
For the collateral ligaments, the examiner pushes the knee inward and outward (called valgus and varus stress tests) while your leg is straight. Movement beyond a few centimeters on either side points to a tear. These physical exams are usually confirmed with an MRI, but the bedside tests alone are often enough for an experienced clinician to identify the damaged structure.
How the Ligaments Work as a System
While it’s useful to think of each ligament individually, the knee functions as an integrated unit. The ACL and PCL work as a pair to control front-to-back motion. The MCL and LCL work as a pair to control side-to-side stability. The secondary ligaments, like the anterolateral ligament and popliteofibular ligament, fill in the gaps by limiting rotational movements that the major four don’t fully cover.
This is why injuring one ligament can put extra stress on the others. A torn ACL, for example, shifts more rotational demand onto the anterolateral ligament and meniscus. Over time, that added load can lead to further damage if the original injury isn’t addressed. It also explains why some surgical repairs now target multiple structures rather than just the single torn ligament.