The meniscus functions primarily as a shock absorber between the thigh bone (femur) and the shin bone (tibia). This structure helps distribute the body’s weight evenly across the joint surface, preventing excessive stress on the underlying bone and articular cartilage. The integrity of the meniscus is important for the long-term health and stability of the knee, and a tear to this tissue represents one of the most frequently occurring orthopedic injuries.
Anatomy and Function of the Meniscus
The knee contains two distinct menisci: the medial (inside) and the lateral (outside). Both are crescent-shaped; the medial meniscus resembles a larger “C” and the lateral is more circular. These pads contribute significantly to joint stability by deepening the shallow surface of the tibia, creating a more congruent fit with the femoral condyles.
Beyond shock absorption, the menisci transmit a significant portion of the load across the knee joint, up to 50% when extended and 85% when flexed. They also assist in joint lubrication and proprioception. A key anatomical difference is mobility: the medial meniscus is firmly anchored to the joint capsule and the medial collateral ligament (MCL), making it far less able to move than the lateral meniscus.
Identifying the Most Common Tear Type
The reduced mobility of the medial meniscus makes it significantly more susceptible to injury, resulting in medial meniscal tears being more common than lateral tears. The most frequent site of injury is the posterior horn of the medial meniscus, which bears substantial load and often sustains damage during rotational stress.
The most common pattern of tear observed overall, particularly in older individuals, is the horizontal (cleavage) tear. This degenerative tear runs parallel to the tibial plateau, splitting the meniscus into upper and lower layers. These tears are associated with tissue wear and degeneration, typically beginning in the posterior horn and extending forward.
In contrast, younger, active patients who sustain acute trauma often experience vertical or longitudinal tears, sometimes resulting in a displaced bucket-handle tear. However, the prevalence of age-related degenerative changes means the horizontal cleavage tear of the medial meniscus posterior horn is the most frequently encountered injury in the general population.
Typical Mechanisms of Injury
Traumatic tears typically occur in younger, athletic individuals and involve a sudden, high-energy event. The most common action is a non-contact, rotational force, such as a sharp pivot or twist while the foot is planted and the knee is bent. These rotational forces, combined with compression, can shear or pinch the meniscal tissue, leading to an acute tear.
In contrast, degenerative tears are common in individuals over the age of 40 and result from cumulative wear on the tissue. These tears can occur with minimal trauma, such as an awkward twist while getting out of a chair or performing a deep squat. Since tissue in older knees is less resilient, ordinary daily activities can be sufficient to cause a tear without a specific, high-impact event. The most common overall mechanism is the gradual wear that leads to a degenerative injury.