The knee contains four major ligaments that hold the joint together and control its movement: the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). Beyond these four, several smaller ligaments play supporting roles in keeping the kneecap and outer joint stable. Here’s how each one works and what happens when they’re damaged.
The Four Major Knee Ligaments
Ligaments are tough bands of connective tissue that link one bone to another. In the knee, they connect the thighbone (femur) to the two lower leg bones (the tibia and the smaller fibula), creating a joint that can bend and straighten while resisting forces that would push it sideways or twist it out of alignment.
These four ligaments work in pairs. The two cruciate ligaments sit deep inside the joint, crossing over each other in an X shape. The two collateral ligaments run along the inner and outer sides of the knee. Together, they give the knee stability in every direction.
Cruciate Ligaments: ACL and PCL
The cruciate ligaments control forward and backward movement of the shinbone relative to the thighbone. They cross each other inside the center of the knee joint, which is where the name “cruciate” (meaning cross-shaped) comes from.
The anterior cruciate ligament (ACL) connects the femur to the tibia toward the front of the knee. Its primary job is to prevent the tibia from sliding forward under the thighbone. It also resists rotational forces, which is why it’s so vulnerable during cutting, pivoting, and sudden deceleration movements common in sports like soccer, basketball, and skiing. ACL tears are one of the most well-known knee injuries, affecting an estimated 200,000 people per year in the United States.
The posterior cruciate ligament (PCL) connects the femur to the tibia toward the back of the knee. It does the opposite job of the ACL: it prevents the tibia from sliding backward. The PCL is thicker and stronger than the ACL, so it tears less often. When it does, the cause is typically a direct blow to the front of the bent knee, like hitting the dashboard in a car accident or landing hard on a bent knee.
Collateral Ligaments: MCL and LCL
The collateral ligaments protect the knee from side-to-side forces. They run vertically along the inner and outer edges of the joint.
The medial collateral ligament (MCL) runs along the inner side of the knee, connecting the femur to the tibia. It resists valgus force, which is pressure that pushes the knee inward toward the midline of your body. A hit to the outside of the knee (common in contact sports like football) stretches or tears the MCL. It’s the most frequently injured knee ligament, but it also heals well on its own because it has a good blood supply. Most MCL injuries recover with bracing and physical therapy rather than surgery.
The lateral collateral ligament (LCL) runs along the outer side of the knee, connecting the femur to the fibula (the smaller bone in the lower leg). It resists varus force, meaning pressure that pushes the knee outward, away from the body’s center. LCL injuries are less common than MCL injuries because the opposite leg naturally shields the inner knee from direct blows.
Supporting Ligaments You May Not Know About
Beyond the big four, several lesser-known ligaments contribute to knee stability. Two of the most clinically significant are the anterolateral ligament and the medial patellofemoral ligament.
The anterolateral ligament (ALL) runs along the outer front of the knee, from the outer knob of the thighbone to the upper tibia. It functions as a backup to the ACL, helping resist forward translation and inward rotation of the shinbone. Research into the ALL has intensified over the past decade because damage to it may explain why some patients continue to experience knee instability even after successful ACL reconstruction. Some surgeons now repair or reinforce the ALL alongside ACL surgery in patients with significant rotational instability.
The medial patellofemoral ligament (MPFL) doesn’t connect the thighbone to the shinbone at all. Instead, it connects the inner edge of the kneecap (patella) to the thighbone. It acts like a leash, keeping the kneecap centered in its groove on the femur as you bend and straighten your leg. When the MPFL tears, the kneecap can dislocate or slide out of position, usually toward the outer side of the knee. This is a common cause of patellar instability, especially in teenagers and young adults.
How Knee Ligament Injuries Feel
Different ligament injuries produce slightly different sensations, but they share some common features. A sudden pop at the moment of injury is the classic sign of a complete ligament tear, particularly an ACL tear. Rapid swelling within the first few hours usually indicates bleeding inside the joint, which points to a cruciate ligament injury rather than a collateral one.
Collateral ligament injuries tend to cause localized pain and tenderness along the inner or outer edge of the knee. You may feel the knee “open up” or shift to one side when standing or walking. Cruciate injuries are more likely to cause a feeling of the knee giving way or buckling, especially when changing direction or going downstairs. PCL injuries can be surprisingly subtle at first, with deep aching behind the knee that worsens over time.
Treatment Depends on the Ligament
Not all ligament injuries require surgery, and the treatment path varies significantly depending on which ligament is torn and how active you are.
MCL tears heal well without surgery in the vast majority of cases. The ligament has a rich blood supply that supports natural repair, so treatment typically involves a hinged brace and a progressive physical therapy program over six to eight weeks for moderate sprains.
ACL tears are more complex. Surgery has long been considered the standard for active people who want to return to pivoting sports, with reconstruction achieving return-to-sport rates as high as 83%. However, recent systematic reviews have found that structured rehabilitation can produce comparable outcomes to reconstruction for primary ACL tears in some patients. The decision often depends on your activity level, your tolerance for modifying how you move, and whether you experience ongoing instability during daily activities. Even after successful reconstruction, some athletes experience decreased performance or reinjury, with revision surgery carrying up to a 20% rate of a third injury.
PCL tears are often managed without surgery unless combined with other ligament damage. Strengthening the quadriceps muscle group, which helps compensate for the PCL’s function, is the cornerstone of rehabilitation. LCL tears, when isolated, may heal with conservative treatment, but they frequently occur alongside other structural damage that complicates the picture.
MPFL reconstruction has become a reliable option for people with recurrent patellar dislocations. The surgery involves replacing the torn ligament with a graft to restore the kneecap’s natural tracking within its groove.