The meniscus is a C-shaped pad of tough, rubbery cartilage in your knee that absorbs shock, distributes weight, stabilizes the joint, and helps keep the surfaces lubricated. You have two in each knee: the medial meniscus on the inner side and the lateral meniscus on the outer side. Together, they handle a remarkable amount of mechanical work, and losing even part of one changes how your knee functions.
How the Meniscus Absorbs Shock
The meniscus is made of fibrocartilage, a material that’s about 72% water, 22% collagen, and small amounts of large water-attracting molecules called glycosaminoglycans. That high water content is key to how it works. Under light pressure, those water-attracting molecules give the tissue a springy, elastic quality. Under heavier loads, like running or jumping, fluid begins to flow through tiny channels in the tissue, and the meniscus starts behaving more like a hydraulic cushion, absorbing and dissipating energy through fluid resistance.
This two-phase system is a hallmark of a healthy meniscus. Research modeling fluid flow through the meniscus has found that below a certain pressure threshold (around 1.7 bar), fluid moves extremely slowly through the tissue, providing elastic cushioning. Above that threshold, the flow shifts to a faster, pressure-driven pattern that handles bigger impacts. Damaged or degraded meniscus tissue loses this ability to switch between the two modes, which is one reason worn-out menisci protect the knee so poorly.
Spreading the Load Across the Joint
Without menisci, the full weight passing through your knee would concentrate on a small contact patch where the rounded end of the thighbone meets the flat top of the shinbone. The menisci dramatically increase that contact area by acting as a wedge between the two bones, spreading force over a much wider surface.
The collagen fibers inside the meniscus run in loops around its circumference, almost like hoops in a barrel. When your body weight pushes down, those fibers convert the vertical compression into tension that circles outward through the tissue. This “hoop stress” mechanism is what lets the meniscus flatten slightly and spread load evenly rather than getting crushed. A radial tear, which cuts across those fibers, can reduce the contact area by as much as 70% and triple the stress on the underlying bone. That’s why even a partial tear can accelerate cartilage wear.
The two menisci don’t share the job equally. The lateral meniscus, on the outer side of the knee, carries up to 70% of the load on its side of the joint. The medial meniscus handles about 50% of the load on the inner side. The difference is partly structural: the lateral meniscus is more circular, more uniform in width, and considerably more mobile than its medial counterpart, giving it more range to adapt under pressure.
Stabilizing the Knee
The menisci also act as secondary stabilizers, particularly for front-to-back motion. The primary restraint against the shinbone sliding forward is the anterior cruciate ligament (ACL), but the medial meniscus plays a surprisingly large supporting role. At deeper angles of knee bend, around 60 to 90 degrees, the force carried by the medial meniscus in resisting forward translation is larger than that of every other structure in the knee except the ACL itself.
This is why meniscus tears and ACL injuries so often travel together. When the medial meniscus is damaged, the ACL has to pick up more of the stabilizing work, and vice versa. Surgeons monitoring patients with a torn meniscus root, for example, pay close attention to the ACL because of the extra strain it’s absorbing. In an ACL-deficient knee, the medial meniscus becomes even more critical as a backstop against instability.
Lubricating the Joint
Your knee joint is bathed in synovial fluid, a viscous liquid that reduces friction between moving surfaces. The meniscus helps distribute this fluid across the cartilage surfaces as you move, functioning like a squeegee that spreads lubricant with every bend and straighten of the knee. Because the meniscus tissue itself is porous and saturated with synovial fluid, it also releases small amounts of fluid under compression, creating a thin film between the bones. This squeeze-film effect reduces friction further and protects the smooth articular cartilage that caps the ends of the bones.
Sending Signals to the Brain
The meniscus isn’t just a passive cushion. It contains nerve receptors that detect tension and compression during movement and relay that information to the brain. These receptors function as an early-warning system: they sense how much force is passing between the thighbone and shinbone and help your nervous system make real-time adjustments to muscle activation and joint alignment. Receptors in the medial meniscus, in particular, detect compression between the two bones and contribute to proper alignment during movement. This is why people who lose meniscus tissue sometimes report that the knee feels “unreliable” even after the pain resolves. The joint has lost some of its ability to sense its own position.
What Happens When the Meniscus Is Damaged
Because the meniscus does so many things at once, a tear doesn’t just cause pain. It disrupts load distribution, increases stress on the bone surfaces, reduces stability, and compromises lubrication. Over time, this combination accelerates wear on the articular cartilage, the smooth coating on the bone ends, which is the pathway to osteoarthritis.
Healing depends largely on where the tear occurs. The outer third of the meniscus has a blood supply, which gives it the raw materials to repair itself or to heal after surgical repair. The inner two-thirds has almost no blood flow, relying instead on nutrients diffusing from the synovial fluid. Tears in this inner zone heal poorly on their own, which is why surgeons sometimes trim the damaged tissue rather than attempting a repair. Preserving as much meniscus as possible is always the goal, because every bit of tissue removed means less shock absorption and faster joint wear down the road.