The ligament on the inside of your knee is the medial collateral ligament, commonly called the MCL. It runs along the inner edge of your knee joint, connecting your thighbone to your shinbone, and its primary job is to keep the knee from bending inward. It’s one of the most commonly injured knee ligaments, especially in sports that involve cutting, pivoting, or direct contact to the outside of the knee.
What the MCL Looks Like and Where It Sits
The MCL is actually a layered structure with two main components: a superficial layer and a deep layer. The superficial MCL is the larger, stronger band that most people picture when they think of this ligament. It attaches to the thighbone (femur) just above the bony bump you can feel on the inside of your knee, then runs downward and attaches to the shinbone (tibia) at two points. The first attachment is just below the joint line, and the second is about 6 centimeters further down the shin. That long span gives the ligament leverage to resist forces that push the knee inward.
The deep MCL sits underneath the superficial layer and connects directly to the meniscus, the rubbery cartilage disc inside your knee. A third structure called the posterior oblique ligament wraps around the back of the inner knee. Together, these three structures form the “posteromedial corner,” a reinforced zone that stabilizes the entire inner side of the joint.
What the MCL Does
The MCL’s main role is resisting valgus force, which is any force that tries to push your knee inward while your lower leg angles outward. Think of a football player getting hit on the outside of the knee, or a skier whose ski catches an edge and the knee collapses inward. Without a functioning MCL, the inner side of the knee joint opens up under stress, creating instability.
Beyond resisting that side-to-side force, the MCL also helps control rotation of the shinbone. When the MCL is torn completely, the lower leg can rotate more freely than normal. This rotational instability is one reason complete MCL tears in athletes sometimes require more aggressive treatment.
How MCL Injuries Happen
Most MCL injuries come from a blow to the outside of the knee or a sudden twist. Contact sports like football, soccer, and hockey are common culprits, but skiing, basketball, and any activity involving quick direction changes can do it too. The classic scenario is a hit to the outer knee while the foot is planted, which forces the inner knee joint open and stretches or tears the MCL.
You’ll typically feel a pop or sharp pain on the inner side of the knee. Swelling usually develops within hours, and it may hurt to straighten or fully bend the knee. Depending on severity, you might feel like the knee is loose or could give way when you try to walk or change direction.
Grade 1, 2, and 3 Tears
MCL injuries are classified into three grades based on how many fibers are damaged and how unstable the knee becomes.
- Grade 1 (mild): Only a few fibers are torn. You’ll have tenderness along the inner knee, but the joint feels stable. These typically heal within one to three weeks.
- Grade 2 (moderate): More fibers are damaged, the tenderness covers a wider area, and there may be slight looseness when the knee is stressed. Recovery generally takes four to six weeks.
- Grade 3 (severe): A complete tear with clear instability. The inner side of the knee opens up noticeably under stress. Healing takes six weeks or more, and the timeline can extend significantly if other structures are also damaged.
Within grade 3, doctors further classify severity by how much the joint opens during a physical exam. With the knee bent to about 30 degrees, 3 to 5 millimeters of extra opening is mild instability, 6 to 10 millimeters is moderate, and more than 10 millimeters suggests a severe complete tear, often with damage to surrounding structures.
How It’s Diagnosed
The primary test is called a valgus stress test. Your doctor bends your knee slightly (around 20 to 30 degrees) and pushes the lower leg outward while stabilizing your thigh. If the inner side of the joint opens up more than the other knee, that indicates MCL damage. The test is also performed with the knee fully straight. If the joint gaps open in full extension, it usually means the damage extends beyond the MCL itself into the deeper structures or the posterior oblique ligament.
Research on cadaver knees has shown that a complete MCL tear produces about 3.2 millimeters of extra gapping compared to a healthy knee when tested at 20 degrees of flexion. When all three medial structures are torn, that number jumps to nearly 10 millimeters. MRI is often used to confirm the diagnosis and check for damage to the meniscus or cruciate ligaments, which frequently occur alongside MCL tears.
Treatment and Recovery
The good news is that most MCL injuries heal without surgery. The MCL has a relatively good blood supply compared to ligaments inside the joint (like the ACL), which allows it to repair itself. Treatment for grade 1 and 2 injuries typically involves a hinged knee brace, ice, anti-inflammatory measures, and a gradual return to activity guided by physical therapy. Most people with isolated grade 1 or 2 tears return to full activity within three to six weeks.
Grade 3 tears also heal conservatively in many cases, though the brace is worn longer and rehabilitation is more structured. The knee is usually protected from valgus stress for several weeks while the ligament scars down and regains strength.
Surgery becomes necessary in specific situations. If the MCL tears cleanly off the bone and pulls a fragment with it, that bone piece can be fixed back in place with a screw. If the torn end of the ligament gets trapped under the meniscus (which prevents it from healing in the right position), surgical repair is needed to restore both the ligament and meniscal function. Combined injuries are the most common reason for operating. When the MCL tears alongside the posterior cruciate ligament, or in a knee dislocation involving both cruciate ligaments, surgeons typically reconstruct the cruciate ligaments and repair or reconstruct the medial side within the first three to four weeks.
Complete MCL tears on the tibial (shin) side have a somewhat worse track record for healing on their own, particularly in athletes. Some surgeons recommend surgical repair for these injuries, especially when the patient needs to return to high-level sports and can’t afford lingering rotational instability.
Rehabilitation Priorities
Regardless of the grade, early physical therapy focuses on restoring range of motion and preventing the quadriceps from weakening. The muscles around the knee act as dynamic stabilizers, so keeping them strong directly compensates for ligament laxity during healing. As the ligament heals, therapy progresses to balance work, lateral movement drills, and sport-specific exercises.
One thing that often surprises people is how quickly grade 1 injuries allow return to activity. Many athletes with mild MCL sprains are back to practice within two weeks, wearing a brace for protection. Grade 3 injuries are a different story. Even after the ligament heals, rebuilding confidence in the knee’s stability and restoring full strength can take three months or longer, especially if other structures were involved.