The ligament on the inside of your knee is the medial collateral ligament, commonly called the MCL. It runs along the inner edge of the knee joint, connecting your thighbone (femur) to your shinbone (tibia), and it’s the most commonly injured ligament in the knee. If you’re feeling pain on the inner side of your knee, this ligament is often the reason.
What the MCL Does
The MCL is the principal stabilizer of the inner (medial) side of your knee. Its main job is resisting valgus force, which is any pressure that tries to push the knee inward or bend it sideways away from the midline of your body. Think of a direct hit to the outside of your knee during a football tackle: the MCL is what keeps the inner side of the joint from opening up.
Beyond that sideways stability, the MCL also helps control rotation. The upper portion of the ligament is the primary stabilizer against side-to-side forces, while the lower portion resists excessive twisting of the shin. Together, these two roles make the MCL essential for cutting, pivoting, and any movement where your knee absorbs lateral contact or changes direction quickly.
How the MCL Gets Injured
MCL injuries typically happen when a force hits the outside of the knee while the foot is planted, driving the knee inward. This is common in contact sports like football, soccer, and hockey, but it can also happen during skiing or simply from an awkward landing. Sometimes the ligament is stretched by a sudden twist rather than a direct blow.
When the MCL is damaged, you’ll usually feel sharp pain along the inner edge of the knee. Swelling tends to develop quickly on the medial side, and the knee may feel unstable or wobbly, especially when you try to change direction. In mild injuries, you might still be able to walk with some discomfort. In severe tears, the knee can feel like it’s giving way with every step.
Grades of MCL Tears
MCL injuries are classified into three grades based on severity:
- Grade 1 (mild): The ligament is stretched but not torn. You’ll have pain and tenderness along the inner knee, but the joint still feels stable. This typically heals within one to three weeks.
- Grade 2 (moderate): The ligament is partially torn. There’s more swelling, more pain, and some looseness when the knee is stressed sideways. Recovery generally takes four to six weeks.
- Grade 3 (severe): A complete tear of the ligament. The inner side of the knee feels noticeably unstable, and there’s often significant swelling and bruising. Healing takes six weeks or more, and recovery time increases further if surgery is needed.
Treatment and Recovery
The MCL has a good blood supply compared to other knee ligaments, which means it heals relatively well on its own. Most grade 1 and grade 2 tears are managed without surgery. Treatment typically involves rest, ice, compression, and a hinged knee brace that allows the knee to bend while preventing the sideways motion that stresses the ligament. Physical therapy focuses on gradually restoring range of motion, then rebuilding strength in the muscles around the knee, particularly the quadriceps and hamstrings, which help compensate for ligament laxity during healing.
Surgery is more likely when a severe MCL tear occurs alongside damage to other structures in the knee, such as the anterior cruciate ligament (ACL) or meniscus. A multi-ligament injury creates instability that bracing and rehab alone may not resolve. In those cases, the MCL may be repaired or reconstructed, and recovery can extend well beyond the six-week baseline for a grade 3 tear alone.
Other Structures on the Inner Knee
While the MCL is the primary ligament on the medial side, it’s not the only structure there. The medial meniscus, a crescent-shaped piece of cartilage that cushions the inner portion of the joint, sits nearby and can be injured at the same time as the MCL. The pes anserinus, a group of three tendons that attach just below the inner knee, can also be a source of medial knee pain, particularly from overuse rather than acute injury. If your inner knee pain came on gradually without a specific incident, these tendons or the meniscus could be contributing rather than the MCL itself.