How to identify an MCL tear

Written by Dr Sean Leo, a consultant orthopaedic surgeon whose clinical focus is the knee, sports injuries and the lower limb.

Last updated: 9 October 2026.

An MCL tear (medial collateral ligament tear) is an injury to the ligament on the inner side of the knee. The usual features of an MCL tear are pain and tenderness on the inner side of the knee, swelling, stiffness or reduced movement, and, in a complete tear, a sense that the knee is unstable.

What is the short answer on MCL tears?

• An MCL tear (medial collateral ligament tear) is an injury to the ligament on the inner side of the knee. It is graded Hughston I, II or III ^[1]. Grade I involves a few fibres, grade II is a partial tear, and grade III is a complete tear with instability ^[1].
• The main check is a physical examination called the valgus stress test, which gently stresses the inner knee and compares how much it opens against the other knee. MRI (a detailed scan of the knee) is kept for severe or combined injuries.
• Symptoms on their own do not confirm the diagnosis, so a knee that feels unstable or swells after an injury is worth having assessed.

What does this page cover?

• What are the grades of an MCL tear?
• How is an MCL tear diagnosed?
• Is pain always present in a complete (grade III) tear?
• What does the MCL actually do?
• Do ACL and MCL tears happen together?
• What should I do next?

What are the grades of an MCL tear?

Knee ligament injuries are common in athletes, and the MCL is one of the most common to be injured ^[1]. Doctors describe how much of the ligament is torn using the Hughston scale:

• Grade I: a few fibres are torn. There is tenderness along the inner knee, but the knee is not unstable ^[1].
• Grade II: more fibres are torn, tenderness is more widespread, and the knee is still not unstable ^[1].
• Grade III: the ligament is completely torn, and the knee is unstable ^[1].

The grade tells you how much of the ligament is torn. It does not, by itself, tell you whether you will need an operation.

How is an MCL tear diagnosed?

The main check is a hands-on examination called the valgus stress test. In plain words: the clinician gently pushes the knee inward from the outer side, first with the knee bent to about 30 degrees and then with it straight. They compare how much the inner side of the knee opens, and how firm it feels at the end of the movement, with your other knee.

If the inner side also opens when the knee is held straight, that can point to a second ligament, most often the ACL, being injured as well, and it prompts a check for that.

More on scans

MRI is used to assess the extent of the injury and to look for other injuries inside the knee. It is most useful for grade III tears, and it is often not needed for a straightforward grade I or II tear. Stress X-rays (X-rays taken while the knee is gently stressed) are sometimes used, for example in adolescents, to look for a growth-plate injury ^[1].

Is pain always present in a complete (grade III) tear?

A complete (grade III) tear can still be painful. In one reported series, pain was worse in partial tears than in complete ones ^[1], so less pain does not mean a minor injury. The usual features of an MCL tear are pain and tenderness on the inner side of the knee, swelling, stiffness or reduced movement, and, in a complete tear, a sense that the knee is unstable.

Two swelling patterns are worth knowing. Swelling that appears within about two hours of the injury can suggest bleeding into the joint, which is also seen with an ACL injury ^[1]. A torn joint capsule may mask swelling, so less swelling than expected does not rule out a significant tear ^[1]. Neither pattern is a diagnosis on its own, because these symptoms overlap with other knee injuries, which is why a proper assessment matters.

What does the MCL actually do?

The MCL is a band of tissue on the inner side of the knee. It is the main restraint that stops the knee from opening inward when sideways force pushes on it. A typical injury happens when a blow to the outer side of the knee forces it inward, or during a sudden twist or change of direction without any contact.

Do ACL and MCL tears happen together?

Yes, the two injuries are associated, and athletes can tear the ACL and MCL at the same time. The ACL is a ligament that runs through the middle of the knee. You can read how an ACL tear is identified or how ACL injuries are managed. When both ligaments are involved, the assessment checks each one.

What should I do next?

An inner-knee injury with pain, swelling, stiffness or a knee that feels unstable is a reason to have it assessed. Being assessed does not commit you to surgery, because most MCL injuries can be managed without surgery with good results ^[1]. For how MCL injuries are assessed and treated, including when surgery is considered, see the MCL reconstruction page. You can also explore the OrthoKinetics Knee Hub.

• Read about ACL tear symptoms.
• Read about sports injuries.

Some signs need help sooner than a routine appointment, and each one says where to go.

This article gives general information and cannot diagnose an individual knee injury. Website or messaging contact is not an emergency-response service. For a medical emergency, use your local emergency service.

References

1. Phisitkul P, James SL, Wolf BR, Amendola A. MCL injuries of the knee: current concepts review. Iowa Orthop J. 2006;26:77-90. PMID 16789454. PMC1888587. Open access.