Where Is the MCL and ACL? Location and Function

The ACL (anterior cruciate ligament) sits deep inside the knee joint, running diagonally between the thighbone and shinbone, while the MCL (medial collateral ligament) runs along the inner side of the knee just beneath the skin. Both stabilize the knee, but they occupy different real estate, resist different forces, and behave very differently when injured. Understanding where each one lives inside (or alongside) the joint explains a lot about why an ACL tear usually means surgery and an MCL tear often does not.

Where Exactly Is the ACL?

The ACL is one of two ligaments that cross inside the knee joint itself, making it an intra-articular structure. It attaches to the back of the thighbone (femur), on the inner wall of the lateral femoral condyle, and angles forward and downward to attach to the front of the shinbone (tibia), in a shallow depression between the tibial plateaus. The name “anterior cruciate” comes from that front-facing tibial attachment and from the Latin word for “cross,” because the ACL crosses over the posterior cruciate ligament (PCL) to form an X shape in the center of the knee. The femoral attachment point has a particularly strong influence on how the ligament behaves during movement, which is why surgeons pay close attention to replicating its position during reconstruction.¹

Because the ACL lives inside the joint capsule, it is bathed in synovial fluid rather than surrounded by the blood-rich tissue that wraps around ligaments on the outside of the joint. That detail matters for healing and will come up again later. In its resting state the ACL is roughly the width of a finger, and it consists of two functional bundles: the anteromedial bundle, which tightens when you bend your knee, and the posterolateral bundle, which tightens when you straighten it. Together they keep the shinbone from sliding too far forward relative to the thighbone.

Certain anatomical features of the knee can make the ACL more vulnerable. A narrow intercondylar notch (the groove in the femur where the ACL sits), a steeper tibial slope on the outer side, and a larger angle at which the ligament attaches have all been identified as risk factors for ACL injury.² In other words, the physical dimensions of your knee partly determine how much stress the ACL has to absorb during sudden movements.

Where Exactly Is the MCL?

The MCL lies on the medial (inner) side of the knee, entirely outside the joint capsule. It is the ligament you can almost feel through the skin if you press the inner surface of your knee. On radiographic studies, the superficial MCL’s femoral attachment sits about 6 mm from the bony bump called the medial epicondyle, while its tibial attachment spans a long stretch down the inner shinbone, ending roughly 66 mm below the top of the tibia.³ That broad, fan-shaped footprint gives the MCL a lot of surface area to distribute force, which is one reason it handles stress differently than the compact ACL.

The medial side of the knee is actually a complex of several structures. The superficial MCL is the largest and most clinically important, but behind it sits the posterior oblique ligament (POL), and deeper still is a layer sometimes called the deep MCL that blends into the joint capsule. When clinicians refer to “the MCL,” they almost always mean the superficial layer, because it bears the lion’s share of the stabilizing load on the inner knee.

What Each Ligament Does

Although both ligaments prevent unwanted movement, they resist fundamentally different forces. The ACL’s primary job is stopping the tibia from sliding forward under the femur and controlling rotation during pivoting and cutting movements. It acts as a kind of internal check-rein: every time you plant your foot and change direction, the ACL prevents the shinbone from shifting out of alignment. As a key intra-articular ligament, the ACL has a strong influence on the overall movement patterns of the knee.⁴

The MCL, by contrast, is the knee’s primary guard against valgus force, which is the inward-collapsing motion you would see if someone pushed the outside of your knee while the foot stayed planted. It is the most commonly injured ligament in the knee precisely because that sideways force is so common in sports and everyday falls.⁵ Cadaveric testing has shown that the superficial MCL carries load during both valgus stress and internal rotation, while the posterior oblique ligament contributes mainly during internal rotation, and the ACL contributes during a different phase of the pivot maneuver altogether.⁶ In short, the MCL handles side-to-side stability, while the ACL handles front-to-back and rotational stability.

How ACL Injuries Happen

Most ACL tears are non-contact injuries, meaning no one hits your knee. About 70% of ACL injuries happen during landing or deceleration maneuvers, often in team sports, and they frequently involve a minor disruption (like an unexpected bump or off-balance step) just before the knee gives way.⁷ The classic scenario is a cutting or pivoting move with the foot planted flat on the ground, the knee close to full extension, and body weight shifted over the injured leg.⁸

Research into the biomechanics points to a specific loading pattern: a nearly straight knee combined with a strong quadriceps contraction and high ground-reaction force creates a large forward pull on the tibia through the patellar tendon, which the ACL has to resist. That forward pull is the dominant loading mechanism.⁹ Contrary to what many people assume, side-to-side collapse (valgus) or twisting alone is not usually enough to rupture the ACL in isolation; those forces tend to damage other structures first. The compressive force that drives the tibia into the femur while the knee is nearly straight is what research increasingly identifies as the critical factor. A steeper lateral tibial slope and flat-footed landing both favor a sliding motion at the joint surface instead of a rolling one, which concentrates stress on the ligament.⁷

How MCL Injuries Happen

MCL injuries look different from ACL tears. In a video analysis of consecutive MCL sprains in professional soccer, roughly 62% involved direct contact, while only about 14% were truly non-contact.¹⁰ The three main patterns were a direct blow to the outer knee that forces the joint inward, contact with the leg or foot that creates a lever effect, and sliding tackles. Because the MCL resists valgus stress, any force that pushes the knee inward while the foot is planted stretches it beyond its limits.

ACL and MCL injuries sometimes happen together, particularly when a valgus blow is combined with a rotational force. Managing that combination is a well-studied but tricky clinical problem because the treatment paths for the two ligaments diverge significantly.¹¹

Why the MCL Heals but the ACL Usually Does Not

This is one of the most practically important differences between the two ligaments, and it traces directly back to where they sit. The MCL, lying outside the joint capsule, is surrounded by soft tissue with a rich blood supply. When the MCL is injured, it responds with a substantial spike in blood flow and new blood vessel growth, which delivers the cells and materials needed for scar formation and eventual remodeling.¹² Even though the normal MCL has relatively little vascularity (about 1.5% by area in animal studies), an injured MCL can temporarily double its blood supply, and that vascular response drives healing. Over time the scar remodels back toward normal tissue.¹³

The ACL cannot mount the same response. Sitting inside the joint capsule, bathed in synovial fluid that dissolves blood clots before scar tissue can form, the ACL lacks the scaffold and blood supply needed to bridge the gap after a tear. Research suggests that the MCL’s superior ability to increase blood flow through angiogenesis is the major difference in healing potential between the two ligaments.¹² This biological reality is the main reason surgeons almost never attempt a direct repair of a torn ACL and instead replace it with a graft.

How Doctors Test for Each Injury

Because the two ligaments resist different forces, the physical examination tests for each one apply a different kind of stress to the knee.

For the ACL, the best-known hands-on tests are the Lachman test, the anterior drawer test, and the pivot shift test. In the Lachman test, the examiner stabilizes the thighbone with one hand and pulls the shinbone forward with the other while the knee is bent about 20 to 30 degrees. If the tibia slides forward more than normal, the ACL is likely torn. A meta-analysis of studies on acute injuries found pooled sensitivities of about 79% for the Lachman test, 78% for the anterior drawer, and 55% for the pivot shift, with specificities above 90% for all three.¹⁴ A newer test called the lever sign showed a sensitivity of about 82% in the same analysis. The pivot shift test has the highest positive predictive value, meaning that when it is positive, you can be fairly confident the ACL is torn, but it is harder to perform on an awake patient whose muscles are guarding.¹⁵

For the MCL, the go-to examination is valgus stress testing.¹⁶ The examiner pushes the knee inward at both full extension and about 20 degrees of flexion while feeling for how much the medial joint space opens up compared to the other knee. A grade III (complete) MCL tear is suspected when the inner side of the joint opens more than about 3 mm beyond the opposite knee at 20 degrees of flexion, and opening in full extension suggests deeper structures are involved as well.¹⁷

The Role of MRI

Physical exams are helpful but not perfect, especially in an acutely swollen, painful knee where muscle spasm can mask ligament laxity. MRI fills the gap and also reveals associated damage to cartilage or other ligaments. For ACL tears, 3-Tesla MRI scanners achieve sensitivity and specificity in the range of 83% and 99% for complete tears, and about 77% and 97% for partial tears.¹⁸ Newer diffusion-weighted imaging techniques can push the specificity for distinguishing complete from partial tears even higher. One study found that adding diffusion-weighted MRI raised specificity from 50% with conventional sequences to 94%.¹⁹ Optimizing the imaging planes and pulse sequences used during the scan also makes a meaningful difference in how accurately ACL injuries are graded.²⁰

MCL injuries are also visible on MRI, though low-grade sprains can be subtle. For isolated MCL sprains, imaging is sometimes used to confirm the grade and rule out damage to deeper medial structures or the ACL. When both ligaments are torn, MRI becomes essential for surgical planning.

Treatment Differences

The contrast in treatment approaches flows directly from the healing biology discussed earlier. Most people who sprain an MCL, even a complete tear, can return to their pre-injury activity level with bracing, physical therapy, and time, without surgery.²¹ Surgery becomes more likely only when the MCL tear is combined with other ligament injuries, particularly an ACL tear, because the medial instability can undermine an ACL reconstruction if left unaddressed.

ACL reconstruction, on the other hand, is the standard treatment for active people who want to return to cutting and pivoting sports. The procedure replaces the torn ligament with a tissue graft. The two most common graft sources are a strip of the patellar tendon (bone-patellar tendon-bone graft) and a doubled hamstring tendon graft. A meta-analysis found that both graft types produce similar overall stability results, though patellar tendon grafts tend to score slightly better on some clinical measures of knee stability, while hamstring grafts cause less kneeling pain and less stiffness in extension.²² A randomized trial comparing the two graft types at two years found equal results for both.²³ The choice between them is usually a conversation between patient and surgeon about trade-offs rather than a clear-cut superiority of one over the other.

Proprioception and Rehabilitation After ACL Injury

One of the underappreciated consequences of tearing an ACL is the loss of proprioception, your unconscious sense of where your knee is in space. The ACL contains mechanoreceptors, nerve endings that send position information to the brain. When the ligament tears, that feedback loop breaks. Research on people more than a year after ACL reconstruction has found that proprioceptive deficits persist, and the pattern of sensory loss sometimes extends beyond the injured knee, suggesting the disruption involves the spinal cord or brain rather than just the torn ligament itself.²⁴ These sensory deficits may contribute to the early-onset osteoarthritis that commonly follows ACL injuries.

Balance and proprioceptive exercises are a staple of ACL rehabilitation programs because of this loss. A systematic review found some evidence that proprioceptive training improves joint position sense, muscle strength, and perceived knee function in people with ACL-deficient knees.²⁵ After reconstruction, however, the picture is muddier. A meta-analysis found that higher-quality studies showed few detectable benefits from proprioceptive training beyond conventional rehabilitation, while lower-quality studies were more likely to report an effect, which raises the possibility that study design rather than true benefit explains the positive results.²⁶ In practical terms, proprioceptive work after ACL reconstruction is unlikely to hurt and probably helps with range of motion, but anyone expecting a dramatic boost in proprioceptive accuracy from balance-board drills alone should temper that expectation.

When Both Ligaments Tear Together

Combined ACL and MCL injuries are common enough that a systematic review pooled data from over 50 studies on the topic.¹¹ The management dilemma is real: the MCL usually heals on its own, but if it is too lax when the ACL graft is placed, the reconstruction loses some of its protective benefit. Clinicians typically let a low-grade MCL sprain settle down with bracing for several weeks, then proceed with ACL reconstruction once medial stability has returned. For a high-grade MCL tear, the surgeon may repair or reconstruct the MCL at the same time as the ACL, though the evidence base for that timing is still mixed and the outcomes measured across studies vary widely.

What makes this situation tricky for patients is that the MCL side may feel fine relatively quickly, tempting them to rush the ACL timeline. The two ligaments heal on fundamentally different schedules because of their different biological environments, and respecting that difference is one of the most important parts of managing the combined injury.