ACL, MCL and Meniscus Tears: What's the difference?

Knee injuries are often a serious barrier to both exercise and competitive sport depending on the severity. Knee pain can be caused by a variety of factors but 3 of the more prevalent injuries in sport are ACL tears, MCL tears and meniscus injuries, especially in contact sports like football and rugby. But what is the difference between ACL, MCL and meniscus injuries? I'll explain exactly what they are below, looking at the anatomy of the knee and the mechanism of injury that causes each, along with the recovery process and rough timeframe for return to sport.

But to understand the types of knee injury, let's have a look at the knee itself and what structures make up the complex joint.


The anatomy of the knee

The knee is a hinge joint that connects the thigh bone (femur) to the shin bone (tibia) and the smaller bone in the lower leg (fibula). Four main ligaments provide stability to this joint. The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) cross each other within the joint, preventing excessive forward and backward movement. On the sides of the knee, the medial collateral ligament (MCL) and lateral collateral ligament (LCL) provide stability against side-to-side motion. Sitting between the femur and tibia are the menisci—two C-shaped wedges of cartilage that act as shock absorbers, distributing weight and reducing friction. Lastly, the patella, commonly known as the kneecap, is embedded in the patellar tendon, which connects the quadriceps muscles to the tibia. This intricate interplay of ligaments, tendons, and cartilage ensures the knee's flexibility whilst maintaining stability, enabling a wide range of movements while supporting the body's weight.


Cruciate Ligaments

The knee's cruciate ligaments, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL), are major ligaments that play a crucial role in maintaining stability and controlling the movement of the joint. The ACL runs diagonally within the knee, attaching the back of the femur to the front of the tibia. Its primary function is to prevent the tibia from moving too far forward relative to the femur and to stabilise the knee during rotational movements. On the other hand, the PCL, situated opposite the ACL, connects the back of the femur to the back of the tibia. The PCL acts as a stabiliser against backward movement of the tibia in relation to the femur. Together, these two cruciate ligaments form a cross-like configuration, effectively controlling the back-and-forth motion of the knee and contributing to the overall strength and integrity of the joint. Any injuries or disruptions to these ligaments can significantly impact the knee's stability and function.


Collateral Ligaments

The knee's collateral ligaments, comprised of the medial collateral ligament (MCL) and the lateral collateral ligament (LCL), are integral in providing stability to the joint, particularly against side-to-side movements. The MCL runs along the inner side of the knee, connecting the medial aspect of the femur to the medial side of the tibia. Its primary function is to resist forces that push the knee inward, preventing excessive widening of the joint. On the opposite side, the LCL spans the outer portion of the knee, linking the lateral femur to the head of the fibula. The LCL's role is to guard against lateral stress, restricting the knee from moving too far outward. Together, the collateral ligaments work harmoniously to maintain the knee's structural integrity, preventing it from deviating too much in either direction. These ligaments are crucial for overall joint stability, and injuries to them can compromise the lateral and medial support of the knee.


The knee's meniscus

The menisci, two crescent-shaped wedges of fibrocartilage located between the femur and tibia in the knee joint, are essential components that contribute to the joint's stability and functionality. Positioned on the inner (medial) and outer (lateral) aspects of the knee, these structures serve as crucial shock absorbers and play a vital role in distributing weight evenly across the joint. The menisci deepen the articular surface of the tibia, enhancing the fit with the rounded condyles of the femur. This not only aids in load transmission but also helps to stabilise the knee during various movements. Moreover, the menisci facilitate joint lubrication and nourishment by promoting synovial fluid circulation. Their presence minimizes friction between the bones, reducing wear and tear on the articular cartilage. In essence, the menisci are integral to the knee's biomechanics, providing both stability and protection to ensure smooth and efficient movement.


ACL, MCL and Meniscus Injuries

So now I've explained what the 3 structures of the knee are and what role they play, that should hopefully make understanding the injuries slightly easier. So let's discuss what causes them and what it looks like if you suffer one of them.


Anterior Cruciate Ligament Injuries

An ACL (anterior cruciate ligament) injury is a common and often debilitating condition that affects the stability of the knee joint. These injuries typically occur when there is a sudden, forceful twisting or hyperextension of the knee, often during activities like sports or sudden stops and changes in direction. An ACL injury is a type of ligament tear. They are categorised into different grades based on the severity of the tear, ranging from mild sprains to complete ruptures. Grade I involves a mild stretch of the ligament, Grade II is a partial tear, and Grade III is a complete tear. Treatment options for ACL injuries vary, and the decision between conservative (non-surgical) and surgical approaches depends on factors such as the extent of the injury, the individual's activity level, and the presence of associated injuries. Conservative treatment involves physical therapy and rehabilitation to strengthen the surrounding muscles and improve joint stability. ACL reconstruction surgery, often through arthroscopic procedures, may be recommended for more severe cases to reconstruct the torn ligament using grafts by an orthopedic surgeon. Recovery times can vary, with conservative treatments generally requiring several months of rehabilitation, while surgical approaches may extend the recovery period to six months or more, emphasising the importance of a comprehensive rehabilitation program to restore functionality and prevent future complications.


Medical Collateral Ligament Injuries

An MCL injury is a common occurrence, impacting the stability of the inside of the knee joint. This type of injury typically results from a direct blow to the outer knee or a force that causes the knee to buckle inward, placing stress on the MCL. MCL injuries are graded based on severity, ranging from Grade I, which involves mild stretching, to Grade III, indicating a complete tear of the ligament. Grade I and II injuries are often treated conservatively through rest, ice, compression, and physical therapy to promote healing and strengthen the surrounding muscles. Grade III injuries, while less common, may require surgical intervention, especially if there are associated injuries or if the MCL tear cannot heal on its own. Recovery times for MCL injuries vary depending on the grade, with Grade I injuries healing in a few weeks, Grade II injuries taking several weeks to a few months, and Grade III injuries, whether treated conservatively or surgically, requiring several months for full recovery. Physical therapy plays a crucial role in rehabilitation, aiding in the restoration of function and stability to the knee with a structures treatment plan.


Meniscus Tears

A meniscus injury involves damage to the fibrocartilage discs, known as menisci, located within the knee joint. These injuries often occur due to sudden twisting or rotational movements of the knee, especially during activities that involve pivoting or squatting. Meniscus tears are classified into different grades based on their severity. Grade I represents a minor tear with minimal symptoms, Grade II involves a more significant tear, and Grade III indicates a complete tear that can cause the meniscus to become displaced. Treatment options for meniscus injuries depend on the severity, location, and type of tear. Conservative approaches may include rest, ice, compression, and physical therapy to strengthen the surrounding muscles and improve joint stability. In some cases, particularly for larger tears or those in specific locations with poor blood supply, surgical intervention such as arthroscopic meniscus repair or partial meniscectomy may be recommended. Recovery times vary, with smaller tears healing in a few weeks, while more extensive injuries, especially those requiring surgery, may take several months for full recovery and rehabilitation. Physical therapy plays a crucial role in regaining strength, range of motion, and overall joint function post-injury.


The "unhappy triad" of knee injuries

The term "unhappy triad" refers to a specific and unfortunate combination of knee injuries involving three primary structures: the anterior cruciate ligament (ACL), the medial collateral ligament (MCL), and the meniscus. This triad is often associated with sports-related trauma, particularly in scenarios where the knee experiences a forceful blow from the lateral side and results in severe pain. The simultaneous occurrence of these injuries can significantly compromise the stability and function of the knee joint. The ACL is typically torn due to excessive forward or backward movement, the MCL is affected by lateral impact, and the meniscus can be injured by the rotational forces involved in the traumatic event. The unhappy triad represents a complex challenge in orthopedics, requiring comprehensive evaluation and a multi-faceted treatment approach to address the unique aspects of each injury within the triad.


In conclusion, any knee ligament injury and meniscus are distinctive yet interconnected challenges that individuals may face, particularly those engaged in physical activities. ACL injuries, often caused by sudden twisting motions, entail varying recovery times with conservative and surgical treatments available, the latter often requiring a longer rehabilitation period. MCL injuries, commonly resulting from lateral impacts, may be treated conservatively or surgically, with recovery times contingent on the severity of the tear and its associated recovery times allowing the initial healing process to do its thing. Meniscus injuries, stemming from rotational movements, present a spectrum of tears, each demanding specific interventions. While conservative measures and physical therapy are viable for smaller tears, surgical procedures might be necessary depending on the severity of the injury, especially for complete tears. Notably, the recovery time for meniscus injuries span a wide range, influenced by the tear's size and location. Returning to sports post-injury is a gradual process for all three, emphasising the importance of comprehensive rehabilitation to ensure not only recovery but also a safe and sustained return to physical activities. It is important a patient or athlete regains their joint strength to keep the knee stable and able to cope with the demands of daily activities before progressing gradually into more vigorous exercise.

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Arun Gray sports therapist strength and conditioning coach skegness

The author

Arun Gray is a sports therapist and strength & conditioning coach with over 15 years experience in the industry. He also has a personal history with chronic shoulder and back pain along with a range of other sporting injuries.

Arun writes about common injuries and aims to help people understand and manage their pain to prevent having to rely on national healthcare.

Read more about Arun