Articular cartilage

Function

What it is made of

Articular cartilage is hyaline cartilage, between 2 and 4 mm thick¹. Hyaline cartilage, the most common kind in the body, is smooth, strong and flexible². It covers the ends of bones where they meet in movable joints².

The tissue is a dense framework, or matrix, made mostly of water, collagen (a strong, rope-like protein) and proteoglycans (large protein-and-sugar molecules)¹. Cartilage cells, called chondrocytes, are scattered thinly through it¹. Each sits in a small space in the matrix called a lacuna². The cells make up about 2% of the tissue’s volume¹.

Water makes up 65% to 80% of its weight¹. Collagen is about 60% of the dry weight, and 90% to 95% of that collagen is one type, type II¹. The largest and most common proteoglycan, aggrecan, gives the tissue properties that are key to resisting squeezing¹.

Two labelled drawings of the right knee. On the left, a side-on cut through the joint shows the thigh bone resting on the shin bone, with the kneecap in front, the cruciate ligaments and menisci between the bones, and the joint capsule around them. On the right, the top of the shin bone seen from above shows two grey oval patches labelled articular cartilage on the medial and lateral tibial condyles, ringed by the C-shaped menisci. — Articular cartilage in the kneeBetts JG, Young KA, Wise JA, et al. Anatomy and Physiology, Figure 9.19. OpenStax, 2013. Access for free at openstax.org. (source, CC BY 4.0). Cropped to panels (a) and (b), removing panel (c), the front view of the knee; resized to 1600 pixels wide.

Articular cartilage has no blood vessels, lymph vessels or nerves¹. In adults, the subchondral plate (the bone plate just beneath the cartilage) separates it from the blood spaces in that bone¹. Instead, it is fed by substances seeping in from the synovial fluid (the slippery fluid inside a joint that oils its moving surfaces)¹.

Four layers, from the surface down

The tissue is built in four zones, or layers, from the surface down¹:

Layer	Share of the tissue	Collagen fibres run	Main job
Superficial	About 10% to 20% of thickness¹	Parallel to the surface, tightly packed¹	Shields deeper layers from forces that push surfaces to slide across each other¹
Middle	40% to 60% of volume¹	At a slant¹	First line of resistance to squeezing¹
Deep	About 30% of volume¹	Upright, at right angles to the surface¹	Gives the most resistance to squeezing¹
Calcified	—	Deep-zone fibres are anchored here¹	Fixes the cartilage to the bone beneath¹

How it carries load

Its main job is to give a joint a smooth, slippery surface and pass loads across it with low friction¹. It passes load to the bone beneath and can bear high, repeated loads with little or no sign of damage or wear¹.

When a joint takes load, pressure in the cartilage’s fluid rises and fluid flows out of the matrix¹. When the load comes off, the fluid flows back in¹. The tissue lets fluid through slowly, which stops it being squeezed out quickly¹. This pressurised water carries a significant share of the load and eases the stress on the solid matrix¹.

Together, that resistance to flow and the water pressure let the cartilage bear loads often several times body weight¹. Under a steady squeeze, the tissue keeps slowly deforming until it settles, a behaviour called creep¹. Water moving through and across the cartilage also helps carry nutrients to the cells and oil the joint¹.

Why thickness differs

In 11 sets of ankle, knee and hip joints from cadavers (bodies donated for study), knee cartilage was on average the thickest⁵. Thick cartilage was found in joints whose surfaces fit together less closely, like the knee⁵.

In 372 adults aged 26 to 61, men had 33% to 42% more knee cartilage volume than women⁶. Allowing for body and bone size cut the gap to 8% to 18%, and the gap was larger over age 50⁶.

Breakdown

Three kinds of injury

Impact and twisting loads on a joint can injure articular cartilage⁷. A review sorts these injuries into three classes⁷:

damage inside the tissue while the surface stays whole⁷
a break that stays within the cartilage⁷
a break through both the cartilage and the bone beneath⁷

Injuries to the anterior cruciate ligament (ACL, a ligament inside the knee) are linked with damage to the joint cartilage⁸. A meta-analysis (a study that pools the results of several earlier studies) covered 37,423 ACL reconstructions⁹. Cartilage damage was over three times as common at a repeat reconstruction as at the first one⁹.

How often damage is found

In 25,124 knee arthroscopies (keyhole surgery inside a joint, using a small camera), cartilage damage was found in 60% of patients¹⁰. Of the damage recorded, 67% was local patches in the cartilage, or in cartilage and bone¹⁰. Another 29% was osteoarthritis (a long-term joint disease in which joint cartilage wears away)¹⁰. In the same study, 70% of the damage came alongside other knee injuries¹⁰. The most common were a tear of the inner meniscus (a pad of cartilage in the knee) (37%) or an ACL injury (36%)¹⁰.

A systematic review of 11 studies covered 931 athletes³. It put full-thickness patches of damage in athletes’ knees at 36% overall, from 2.4% to 75% between studies³. Its authors concluded this damage is more common in athletes than in the general population³. Of the athletes, 14% had no symptoms when the damage was found³. More than half of the athletes without symptoms had full-thickness damage³.

The two studies recorded where the damage sat^3,10:

Where in the knee	25,124 arthroscopies	Athletes’ knees
Kneecap (back of it, or its joint)	36%¹⁰	37%³
Condyles (rounded knobs) of the femur (thigh bone)	34%, inner condyle¹⁰	35%, both condyles³
Top of the tibia (shinbone)	—	25%³

Osteochondritis dissecans

Osteochondritis dissecans is a disorder of the cartilage and the bone just beneath it¹¹. In a US health system of over 1 million patients, it occurred in the knee at 9.5 per 100,000 in 6- to 19-year-olds¹¹. The rate was 15.4 per 100,000 in males and 3.3 in females¹¹. In that group, 12- to 19-year-olds had 3.3 times the risk of 6- to 11-year-olds¹¹.

In adults aged 20 to 45 in one US health system, the knee rate was 1.21 per 100,000 person-years (a count of people multiplied by the years each was followed)¹². Men had 3.6 times the risk of women¹².

Wear and ageing

One review notes that osteoarthritis comes with sharp changes in how cartilage is built and broken down¹. These changes happen when the cells’ breakdown and building of matrix fall out of balance¹. It adds that with age, cartilage holds less water and grows stiffer under squeezing¹. The number of cells stays about the same¹.

Recovery

Why it heals slowly

Articular cartilage has a limited capacity to heal and repair itself¹. Each cell is trapped in its own matrix and cannot move to other parts of the cartilage¹. The cells also have a limited ability to divide, which adds to the tissue’s poor healing after injury¹.

One review gives the half-life (the time for half of it to be replaced) of its collagen as several decades to up to 400 years¹. One study calculated it at 117 years from age-related changes in human tissue, against 15 years for skin collagen¹³.

How damage changes over time

In 325 adults with an average age of 45, MRI (a scan that shows soft tissue in detail) was done two years apart¹⁴. Over about 2.3 years, knee cartilage damage got worse in 33% and got better in 37%¹⁴. Worsening was linked with female sex, older age, higher body mass index (a measure of weight for height) and bony spurs at the start¹⁴.

What surgery studies found

A 2009 systematic review of 28 studies covered 3,122 patients who had microfracture (a cartilage repair technique)¹⁵. Knee function improved in the first 24 months, but reports on whether that improvement lasted were conflicting¹⁵. The repair tissue held little hyaline cartilage and varied in amount, and function might decline¹⁵. At the time, there was too little data on long-term results¹⁵.

Squashing and bouncing back

A systematic review and meta-analysis of 40 studies covered 653 adults¹⁶. In healthy adults, knee cartilage got thinner or smaller on MRI right after each loading activity studied, though the evidence was of very low certainty¹⁶. Kneecap cartilage fell by 5.0% after body-weight knee bends¹⁶. The review suggested knee cartilage regains its shape within 30 minutes of walking and 90 minutes of 100 knee bends¹⁶.

In an MRI study of 12 volunteers, kneecap cartilage volume fell after each activity¹⁷:

Fall in kneecap cartilage volume, %. Bars start at zero; the table below gives each value with its source.

Activity	Fall in kneecap cartilage volume, %
Knee bends	5.9¹⁷
Running	5.0¹⁷
Squatting	4.7¹⁷
Cycling	4.5¹⁷
Normal walking	2.8¹⁷

Does training make it thicker?

One review notes that joint motion and load matter for keeping cartilage normal, and joint inactivity has been shown to break it down¹. In one study, knee cartilage thickness did not differ significantly between 18 triathletes training over 10 hours a week and 18 people never regularly active⁴. The authors suggested that joint size can be shaped during growth, but that cartilage thickness does not adapt to load, unlike muscle and bone⁴.

One systematic review looked at randomised controlled trials (studies that assign people to groups by chance, then compare the groups) of exercise¹⁸. In people with knee osteoarthritis, six study comparisons reported no effect of exercise on cartilage thickness, volume or damage on MRI¹⁸. Kneecap cartilage squashing after knee bends did not differ significantly between 14 athletes (seven weight lifters and seven sprinters) and 14 untrained volunteers¹⁷.

In 74 children aged 9 to 18, measured about 1.6 years apart, knee cartilage volume rose at all sites¹⁹. Boys gained cartilage faster than girls¹⁹. Those above the median (middle value) for sport intensity gained about twice as much at the tibia as those below, though not at the kneecap¹⁹.

What happens without load

In 20 people with ankle fractures who bore partial weight for 7 weeks, knee cartilage thinned²⁰. The loss ranged from 2.9% at the kneecap to 6.6% at the inner tibia²⁰. The other knee showed no significant change²⁰. In people with a complete spinal cord injury, knee cartilage was thinner than in healthy volunteers and kept thinning²¹. By 12 months after injury, it was 21% to 24% thinner at the kneecap and inner tibia, and 23% to 25% thinner by 24 months²¹.

Connections

The joints and bones it covers

In the knee, each bone surface in the joint has a thin coat of hyaline cartilage that lets the bones move smoothly². The hip and knee, at each end of the femur, are covered by cartilage that cuts friction²². The back of the patella (kneecap) was a common damage site in the arthroscopy study¹⁰. In the athletes’ review, the kneecap joint and the top of the tibia were³. In 10 healthy adults, single-leg hopping thinned ankle cartilage on the tibia and talus (the ankle bone under the shinbone) by about 3% and 2%²³.

The structures injured with it

The two menisci are C-shaped pads of fibrocartilage (a tough, fibrous kind of cartilage) between the femur and tibia². In knee arthroscopies, cartilage damage often came with a meniscus tear or ACL injury, the subjects of the meniscus tear and ACL tear pages¹⁰. In one review of athletes’ knees, 37% of cartilage damage was in the kneecap joint³. Patellofemoral pain (pain around or behind the kneecap) has its own page.

Exercises that load it

In 20 healthy people, the modelled peak force at the kneecap joint in a single-leg squat was 6.9 times body weight²⁴. In 16 experienced men doing drop jumps, force on the inner half of the knee rose with drop height²⁵. Related exercise pages: back squat, single-leg squat, drop landing.