The menisci, also known as articular discs, are specialized cushions of fibrocartilage found within various joints. This tough, yet flexible, tissue allows them to withstand significant mechanical stress. While articular discs exist in joints like the temporomandibular joint in the jaw, they are most recognized for their location and function within the knee. In the knee, two wedge-shaped structures sit between the thigh bone (femur) and the shin bone (tibia), acting as biological spacers.
Anatomical Structure and Placement
Menisci are crescent-shaped structures made of fibrocartilage, known for its high tensile strength due to a dense network of collagen fibers. They have a thick peripheral rim that tapers to a thin inner edge, creating a wedge-like profile. This shape allows them to conform precisely to the rounded femur and the flat tibia.
Each knee contains two menisci: the medial meniscus (inner side) and the lateral meniscus (outer side). The medial meniscus is C-shaped and tightly anchored, making it less mobile and more susceptible to injury. The lateral meniscus is more O-shaped, and its looser attachments allow for greater movement, offering protection during knee rotation. The outer third of the menisci receives a direct blood supply from the surrounding capsule, which is important for healing.
Essential Roles in Joint Biomechanics
The menisci perform multiple functions essential for the health and movement of the knee joint. Their primary role is shock absorption, dampening forces transmitted during activities like walking and running. By increasing the contact area between the femur and the tibia, the menisci spread the weight across a broader surface, a mechanism called load transmission. This significantly reduces concentrated pressure on the underlying articular cartilage.
Force distribution is maintained by “hoop stress,” where circumferential collagen fibers resist the menisci’s outward expansion under compression. This action prevents forces from focusing on small spots, which would lead to rapid wear of the articular surfaces. The menisci also contribute to joint stability by improving the mechanical fit between the bones, preventing excessive sliding of the femur on the tibia. Finally, they assist in joint lubrication by spreading synovial fluid across the joint surfaces during movement.
Common Causes and Types of Meniscal Injuries
Meniscal injuries are a frequent cause of knee pain, typically falling into two main categories based on their cause. Traumatic tears occur in younger, active individuals from a sudden, forceful twisting or pivoting motion while the foot is planted. These acute injuries often happen during sports involving rapid changes in direction, such as soccer or basketball. Patients may report hearing a distinct popping sound at the moment of injury, followed by swelling and pain.
The second type is a degenerative tear, common in older adults due to chronic wear and tear. As the fibrocartilage ages, it becomes less resilient and can tear with minimal force, sometimes from a simple squatting motion.
Tears are classified by their pattern, which influences treatment. Common patterns include radial tears (extending from the inner edge outward), horizontal tears (splitting the meniscus into layers), and flap tears. A severe injury is the bucket-handle tear, a large longitudinal tear that displaces a segment of the meniscus into the joint center. This displaced cartilage can physically block movement, causing mechanical symptoms like catching or locking. General symptoms of a meniscal tear include pain along the joint line and stiffness.
Treatment and Recovery Pathways
Managing a suspected meniscal injury begins with a physical examination and diagnostic imaging, typically magnetic resonance imaging (MRI). The treatment path depends on the tear’s location, size, pattern, and the patient’s age and activity level. Tears in the outer, well-vascularized “red zone” have a greater capacity for healing and are often managed conservatively.
Conservative treatment involves rest, icing to control swelling, and anti-inflammatory medication, followed by physical therapy. Therapy focuses on restoring range of motion and strengthening the muscles supporting the knee joint.
If the tear causes mechanical locking or is located in the avascular “white zone,” surgical intervention, usually arthroscopy, may be necessary. Surgical options include meniscal repair, where torn edges are stitched together to preserve tissue function, favored for younger patients and specific tear types. The alternative is a partial meniscectomy, where only the damaged portion of the cartilage is trimmed away. Recovery from a meniscal repair is longer, often taking three to six months, while recovery from a partial meniscectomy is typically much shorter.