A torn ACL and a torn MCL share several symptoms, including sudden pain, swelling, and sometimes an audible pop at the moment of injury. The key difference is where you feel it: an MCL tear causes pain along the inside of the knee, while an ACL tear is often more tender on the outside. Both injuries can happen during sports or from everyday falls, and they occasionally occur together in the same injury.
Symptoms They Share
Both ACL and MCL tears tend to happen suddenly. You might feel or hear a “pop” at the moment of injury, followed quickly by pain and swelling. A loud pop with immediate pain typically signals damage to a ligament or the meniscus. Both injuries can make walking difficult right away, though some people with partial tears are able to bear weight with discomfort.
Swelling develops rapidly with both injuries, though the pattern differs. ACL tears often cause the knee joint itself to fill with fluid (because the ACL sits inside the joint capsule and bleeds into that space). MCL tears tend to produce swelling along the inner edge of the knee rather than deep inside the joint. In either case, the knee feels stiff, tight, and hard to bend or straighten within the first few hours.
ACL Tear Symptoms
The ACL sits in the center of your knee and prevents the shinbone from sliding forward relative to the thighbone. When it tears, the most distinctive symptom is a feeling that the knee has “given way” or buckled. Many people describe feeling unstable the moment they try to stand or change direction, as though the knee can no longer hold itself together during movement.
Pain with an ACL tear is often more noticeable on the outer side of the knee, which surprises people who expect a centrally located ligament to hurt in the middle. The explanation is that when the ACL ruptures, the shinbone briefly shifts forward and the outer parts of the knee bones can collide, bruising the outside of the joint. You’ll also likely have difficulty fully straightening your knee, a hallmark of ACL injuries that helps distinguish them from other knee problems.
During a physical exam, a doctor checks for an ACL tear by gently pulling the shinbone forward while stabilizing the thighbone. If the shin slides more than about 5 mm further than it does on the uninjured side, and there’s no firm stopping point, the ACL is likely torn. That lack of a firm endpoint is the clinical hallmark of a complete rupture.
MCL Tear Symptoms
The MCL runs along the inner edge of the knee, connecting the thighbone to the shinbone. Its job is to prevent the knee from bending inward. When it tears, pain is concentrated along the inside of the knee, and pressing on that area will feel noticeably tender. The most sensitive spot is usually near the top of the ligament, at the bony bump on the inner side of the thighbone.
MCL tears are classified into three grades, and the symptoms escalate with each one:
- Grade 1 (mild): Less than 10% of the ligament fibers are torn. You’ll have tenderness and mild pain along the inner knee, but the joint still feels stable.
- Grade 2 (moderate): The ligament is partially torn. Pain and tenderness along the inner knee are more intense, and the knee may feel loose or wobbly when pressure is applied from the side.
- Grade 3 (severe): The ligament is completely torn. The knee feels very unstable and loose, with significant pain and tenderness. You may feel the knee buckle when you try to walk or stand.
A key difference from an ACL tear: MCL injuries don’t always cause the same deep joint swelling. The swelling tends to stay along the inner border of the knee rather than ballooning the entire joint. Range of motion may be limited by pain, but the inability to straighten the knee fully is more characteristic of ACL injuries.
How the Injury Happens
The mechanism of injury is one of the best clues for figuring out which ligament is involved. ACL tears commonly happen during non-contact movements: planting your foot and pivoting, landing from a jump awkwardly, or suddenly decelerating while running. Sports like soccer, basketball, and skiing are frequent culprits.
MCL tears are more often caused by a direct blow or force that pushes the knee inward, like a tackle hitting the outside of the knee. This “valgus stress” stretches or tears the MCL on the opposite side. It’s common in contact sports like football and hockey, though it can also happen from a simple stumble where the knee buckles inward.
Both ligaments can tear in the same incident, particularly when a valgus force is combined with rotation. These combined injuries tend to cause widespread swelling, limited range of motion, and tenderness across multiple areas of the knee rather than one isolated spot.
When a Meniscus Tear Is Also Involved
ACL and MCL tears frequently occur alongside meniscus damage, and the combination adds its own set of symptoms. The meniscus is a C-shaped piece of cartilage that acts as a shock absorber in the knee. When it tears, you may notice symptoms that aren’t typical of a ligament injury alone.
The hallmark signs of a concurrent meniscus tear include a catching or locking sensation, where the knee feels stuck and won’t straighten all the way. You might also feel the knee “catch” during movement, as though something is mechanically blocking it. Meniscus tears can take longer to declare themselves: pain and swelling sometimes don’t fully develop until 24 hours or more after the injury, especially with smaller tears. If you have a ligament injury and notice locking or catching in the days that follow, a meniscus tear is a likely explanation.
Telling the Two Apart
The simplest way to begin distinguishing the two at home is by location. Press gently along the inner edge of your knee: if that’s where the pain concentrates, the MCL is the more likely culprit. If the pain is more diffuse or concentrated on the outer side, and the knee feels unstable or gives way when you try to walk, an ACL injury is more probable.
Instability is the other major dividing line. ACL tears create a front-to-back instability, where the knee feels like it could slip forward or buckle during pivoting. MCL tears create side-to-side instability, where the knee feels like it could collapse inward. Both can feel like the knee is “giving out,” but the direction of that looseness differs.
That said, these injuries overlap enough that imaging is almost always needed for a definitive answer. An MRI can show exactly which structures are damaged and how severely. If you felt a pop, have significant swelling, or can’t bear weight comfortably, getting evaluated promptly helps ensure the right structures are identified and treated appropriately.