Menisci of the knee

Key points

The two menisci are C-shaped pads of fibrocartilage (a tough, fibrous kind of cartilage) between the femur (thigh bone) and tibia (shinbone)¹.
Standing load squeezes them, and collagen (a strong, rope-like protein) fibres running around each ring turn that squeeze into tension².
Only the outer rim of each meniscus is well supplied with blood, and a review links this to how tears heal².
A 2019 European consensus statement (a report that sets out what a panel of experts agrees on) reports that the inner meniscus is involved in 75% of tears in acutely injured knees in the general population³.
A meta-analysis (a study that pools the results of several earlier studies) of 53 studies found the odds of knee osteoarthritis (a long-term joint disease in which the cartilage lining the joint wears away) about six times higher after a meniscal injury than in an uninjured knee, though results varied widely between studies⁴.

Function

Two pads between the thigh bone and shinbone

The knee has two menisci, an inner (medial) one and an outer (lateral) one¹. Each is thin along its inner edge and thick along its outer edge¹. The upper surface is cupped to fit the rounded femoral condyle (the knob at the end of the thigh bone) above it². The lower surface is flat, to sit on the top of the tibia².

The two menisci differ in shape, cover and movement²:

Feature	Medial meniscus	Lateral meniscus
Shape	Semicircular, about 35 mm front to back, much broader at the back²	Almost circular, about the same width all round²
Share of its side of the tibia covered	About 60%²	About 80%²
Movement across the tibia, one study of 5 cadaver (donated body) knees	5.1 mm on average²	11.2 mm on average²

Engraving of the top of the right tibia seen from above with the femur removed. The C-shaped medial meniscus and the almost circular lateral meniscus sit on the flat tibial surfaces, joined at the front by the transverse ligament, with the cut ends of the anterior and posterior cruciate ligaments and the ligament of Wrisberg between them. — The menisci from aboveHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 349. Public domain, via Wikimedia Commons. (source, Public domain)

How they are anchored

The menisci attach to the tibia, and both can move as the knee moves¹. The medial one moves less, because its outer edge is anchored to the joint capsule (the sleeve of tough tissue that encloses a joint) and the inner collateral ligament¹. Each has a front and a back end, called horns, and both horns of each meniscus attach to the tibia². The medial front horn sits in front of the anterior cruciate ligament (ACL, the front one of two ligaments that cross inside the knee)². Its back horn attaches behind, near the posterior cruciate ligament (the back one of the two)².

A band of fibrous tissue, the transverse ligament, joins the two front horns². The medial meniscus is tied to the tibia by part of the joint capsule². At its midpoint, a thickening of the capsule called the deep medial collateral ligament ties it more firmly to the femur². The lateral meniscus is only loosely attached to the capsule². It has no attachment to the lateral collateral ligament, the ligament on the outer side of the knee².

The back horn of the lateral meniscus is linked to the femur by the meniscofemoral ligaments². Sources differ on them^2,5. One review reports that a knee may have none, one, two or four, and that their function is unknown². Another source reports at least one in more than 90% of knees, working with the posterior cruciate ligament against backward and twisting loads⁵.

What they are made of

Meniscal tissue is about 72% water and 22% collagen, with cells set in it². Most of that collagen is type I, which sets the menisci apart from the hyaline cartilage covering the bone ends². Collagen is mainly what gives the menisci their strength under pulling loads². Proteoglycans, molecules that hold water in the tissue, give much of its resistance to squeezing².

In the deep layers, collagen fibres run around the meniscus, parallel to its outer edge². Near the surface they run more across the meniscus, and “tie” fibres woven between the deep fibres hold the structure together². The cells are called fibrochondrocytes, because they look like a mix of fibre-making cells and cartilage cells².

What they do

A review of meniscus research reports that the menisci steady the knee, spread its load, absorb shock, and help oil and nourish it². When a person stands, the menisci are squeezed, and the ring-shaped fibres turn that squeeze into tension, called hoop stress². The horn attachments stop the menisci being pushed out sideways². A review reports that about 70% of the load on the knee’s outer side passes through the menisci, and 50% on the inner side². In a straight knee, half the load through the menisci passes through the back horns, rising to 85% at a right-angle bend².

Because the front and back horns move by different amounts, the menisci curve more tightly as the knee bends². This matches the tighter curve at the back of the femur and keeps contact with both bones².

A review cites a study finding the normal knee absorbs about 20% more shock than knees that have had a meniscus removed². The review reports that removing the whole lateral meniscus cuts contact area by 40% to 50%². Contact stress on that side rises to 200% to 300% of normal².

A review cites a study finding that, without a working anterior cruciate ligament, the medial back horn is the main brake on the tibia sliding forward². The review reports that removing the medial meniscus in such a knee raised front-to-back movement of the tibia by up to 58% at a right-angle bend². With the ligament intact, the removal had little effect².

Breakdown

How tears happen

European consensus reports describe traumatic tears, which follow a knee injury with sudden pain, and degenerative lesions (areas of damaged tissue), which develop slowly, typically as a horizontal split in the meniscus of a middle-aged or older person^3,6. The meniscus tear page covers these definitions and what trials found.

A review describes how the meniscus becomes stiffer and less elastic with age². At areas of cystic degenerative change, a twisting force from the femur can shear the surface layers off the deep layer, splitting it horizontally². The same review notes that the back inner corner moves least². It proposes that trapping the medial back horn between femur and tibia in full bending may be one way tears happen².

Who tears a meniscus

Studies in different groups report these rates^3,7,8:

In the general population, 0.5 to 0.7 acute injuries per 1,000 people a year, more in men than women³.
In US military personnel from 1998 to 2006, 8.27 acute injuries per 1,000 person-years (people counted times years followed)⁷. Those over 40 were more than four times as likely to be injured as those under 20⁷.
In US high school athletes in 22 sports, 5.1 injuries per 100,000 athlete exposures, over four times higher in competition than practice⁸.

In 392 adults aged 18 to 60 with tears seen at arthroscopy (keyhole surgery inside a joint), about a third came from sport⁹. Most tears from injuries outside sport happened in daily life, and half of those happened while squatting or rising from a squat⁹.

Risk factors studies have linked

A meta-analysis of 11 studies of people over 16 found strong evidence for these risk factors¹⁰:

for sudden tears, playing soccer or rugby¹⁰
for degenerative tears, age over 60, male sex, kneeling and squatting at work, and climbing more than 30 flights of stairs¹⁰

A second meta-analysis linked work-related kneeling, squatting, stair climbing and lifting 10 kg or more with meniscal lesions¹¹. The overall quality of that evidence was moderate to low¹¹. It linked professional football with about five times the risk¹¹. A review of 16 studies linked a steeper backward slope of the top of the tibia with more lateral tears alongside ACL tears¹².

Tears alongside ligament injuries

With a fresh anterior cruciate ligament (ACL) tear, lateral tears are more common than medial ones, apart from one type called ramp tears, the 2019 consensus reports³. With a long-standing ACL tear, medial tears become more common with time and age³.

Tears, pain and osteoarthritis

The 2019 consensus states that traumatic tears can cause pain by acting on nerve endings and raising inflammatory signals in the joint³. A study of a population sample used MRI (a scan that shows soft tissue in detail) in 991 adults aged 50 to 90¹³. Of those with a tear, 61% had had no knee pain, aching or stiffness in the previous month¹³.

A meta-analysis of 53 studies found the odds of knee osteoarthritis about six times higher after meniscal injury⁴. Results varied widely between those studies⁴. The 2016 consensus states it is still unclear whether degenerative lesions cause osteoarthritis or result from it⁶.

Recovery

Blood supply shapes healing

The menisci get their blood mainly from the geniculate arteries, branches of the popliteal artery (the main artery behind the knee)². Only the outer rim is well supplied: about the outer 10% to 30% of the medial meniscus and 10% to 25% of the lateral².

The rest, 65% to 75% of each meniscus, is fed from the synovial fluid (the slippery fluid inside the joint)². A review states that this feeding needs the joint to be loaded and unloaded again and again, as joint cartilage does². In a study of menisci from 20 cadavers, nearly the whole meniscus had a blood supply at birth². After age 50, only the outer quarter of the base did².

A review names the outer rim with blood the red zone and the inner part the white zone². The 2019 consensus advises against sorting tears this way, because blood supply changes through life³. It uses measured zones, 0 to 3, instead³.

What studies found about healing

The 2019 consensus found no results on whether isolated tears heal on their own³. It reports that repaired tears in the outer zone 1 healed at a significantly better rate than those in zone 2³:

Zone of the repaired tear	Healing rate
Zone 1 (outer)	87% to 91%³
Zone 2 (next zone in)	59% to 79%³

A meta-analysis of repairs of traumatic tears, in patients whose bones had stopped growing, found failure rose with time¹⁴. Failure here meant symptoms, no healing on MRI, or another operation¹⁴:

Share of repairs failed (%). Bars start at zero; the table below gives each value with its source.

Time after repair	Share of repairs failed
Up to 1 year	12%¹⁴
2 to 3 years	15%¹⁴
4 to 6 years	19%¹⁴

How the tissue responds to running

One study used ultrasound (a scan made with sound waves) in 100 recreational runners¹⁵. Straight after a marathon, the medial meniscus bulged out of the joint 38% further¹⁵. It stayed raised through day 3 and was back to baseline from day 4¹⁵. A larger rise in the bulge straight after the race was linked with a higher knee adduction (the lower leg tilting toward the body’s midline) load, higher body mass index and more monthly running¹⁵.

A systematic review of 14 MRI studies of healthy marathon runners found changes in knee cartilage and meniscus after long runs¹⁶. Five studies found volume fell at first, and ten found T2 (an MRI measure that changes when tissue is damaged or swollen) returned to baseline in the short term¹⁶. Another MRI measure may stay raised for months¹⁶.

Connections

The menisci sit in the knee joint, attached to the tibia¹. The medial one is also tied to the femur by the deep medial collateral ligament². The medial front horn attaches in front of the anterior cruciate ligament². The back roots of both menisci sit close to the tibial attachment of the posterior cruciate ligament⁵.

The deep part of the medial collateral ligament attaches to the medial meniscus¹⁷. The lateral collateral ligament has no attachment to the meniscus¹⁸. Like the articular cartilage on the bone ends, the parts of the menisci fed from the joint fluid depend on repeated loading and unloading for that feeding².

Related exercise pages: deadlift. A narrative review found few studies linking the deadlift to specific injuries, and knee meniscus injuries were among those reported¹⁹.