Anterior cruciate ligament

Function

Where it runs

The anterior cruciate ligament (ACL) is a band of collagen fibres inside the knee¹. It starts on top of the tibia (shinbone), at the front inner part of the area between its two joint surfaces¹. It runs backward and outward to the inner face of the outer condyle of the femur (thigh bone)¹. A condyle is a rounded knob at the end of a bone, where it meets another bone at a joint. The ACL measures about 32 mm long and 7 to 12 mm wide¹.

It has two bundles, separated where they meet the femur by a bony ridge called the bifurcate ridge¹:

Bundle	Tightest when	Main role
Anteromedial (front, inner side)	The knee is bent1	Resisting forward movement of the tibia1
Posterolateral (back, outer side)	The knee is straight1	Side-to-side and turning stability, as a backup restraint1

What it holds back

The ACL has long been considered the main passive restraint stopping the tibia sliding forward on the femur³. Together with the posterior cruciate ligament, it forms a cross inside the knee¹. The pair stop the tibia moving too far forward or backward as the knee bends and straightens¹. The ACL tightens as the knee straightens, and so resists the knee bending backward past straight². Its direction also helps it steady the knee against turning and side-to-side movement³.

Blood and nerve supply

The ACL gets its blood mainly from the middle genicular artery, one of the arteries of the knee¹. In one study of a cadaver (a body donated for study), the ends of the ACL looked better supplied than its middle⁶. Its attachments to bone added little to that supply⁶.

The posterior articular nerve, a branch of the tibial nerve (the main nerve at the back of the lower leg), supplies the ACL¹. A review with a systematic search reported that mechanoreceptors, nerve endings that sense movement, sit mainly near the ligament’s attachments to bone⁷. They are believed to add to the sense of where the knee is⁷.

Breakdown

How it tears

A position statement describes ACL rupture as loading beyond the tissue’s current strength⁸. It says that strength varies with age, sex, training and injury⁸. A 2014 review puts more than 70% of ACL injuries as happening without contact, typically when landing from a jump or cutting sideways³. A 2022 review of 45 team-sport studies put the share at 55%: 63% in females and 50% in males⁴.

The 2014 review says studies have named a mix of three loads as the worst case and main way non-contact tears happen³. One is a shear force (a force that pushes two surfaces to slide across each other) moving the tibia forward³. The others are the knee falling inward and the tibia twisting inward³.

The position statement lists these typical body postures at the moment of injury⁸:

• a nearly straight knee⁸
• the knee collapsing inward, with the hip turned in⁸
• a flat foot⁸
• the trunk leaning sideways over the standing leg⁸

The position statement estimates that the ligament ruptures less than 50 milliseconds after the foot first touches the ground⁸. It says that is too fast for muscles to switch on, whether by choice or by reflex⁸. In 8 people imaged during a jump landing, ACL strain (stretch as a share of resting length) peaked 55 milliseconds before touchdown, on average⁹.

A systematic review (a study that searches for all the research on a question in a set, checkable way) studied bone bruises after ACL tears¹⁰. It found bruises on the outer top of the tibia in 82.8% of cases and on the outer femoral condyle in 71.4%¹⁰. Where the bruises sat, front to back, showed the tibia shifting forward on the femur at the moment of injury¹⁰.

Who tears it

A systematic review converted injury studies into yearly rates¹¹:

Group	ACL injuries per person per year
National population studies in Australia	Up to 0.05%11
Amateur sport	Between the two11
Professional basketball, soccer and other football codes	0.15% to 3.7%11

The professional figures exclude small studies, and amateur rates were generally in between¹¹.

One population study counted 1,841 people newly diagnosed with isolated ACL tears (the ACL torn on its own) between 1990 and 2010⁵. Adjusted to the US population, the yearly incidence (new cases in a set time) was 68.6 per 100,000 person-years⁵. It was higher in males (81.7) than in females (55.3)⁵. In that study, tears peaked at ages 19 to 25 in males and 14 to 18 in females⁵.

In athletes, a meta-analysis (a study that pools the results of several earlier studies) found female athletes more likely than males to injure the ACL¹². Its authors noted it is unknown whether this holds in every sport or level¹². Another meta-analysis put the female rate at about 3 times the male rate in contact sports¹³. It found similar rates between the sexes in collision, limited-contact and noncontact sports¹³.

Risk factors in the body

A position statement drew on a systematic review of prospective studies (studies that follow people forward in time)⁸. It names a previous ACL injury as the most consistent risk factor inside the body⁸. A meta-analysis of 19 studies found a 15% rate of second ACL injury after reconstruction (surgery to rebuild the ligament)¹⁴. That was 7% to the same knee and 8% to the other¹⁴. Other factors have been studied:

• Knee shape. The position statement reports strong evidence tying risk to a smaller ACL, a steeper backward tibial slope and a narrower notch between femoral condyles⁸. It notes these are largely not changeable⁸.
• Family history. In a nationwide Korean group of 5,184,603 people, risk was 1.79 times higher with an affected parent, sibling or child¹⁵. With an affected twin, it was 4.49 times higher¹⁵.

A study followed 880 elite female handball and football players¹⁶. Trained raters’ single-leg squat scores did not pick out those who later had a non-contact ACL injury¹⁶. In 17 healthy adults imaged during a single-leg jump, the knee barely angled inward or outward, under 1 degree on average¹⁷. No significant link was found between ACL strain and that angle¹⁷.

Damage that comes with it

ACL injuries are associated with torn menisci (pads of cartilage in the knee), damage to joint cartilage and a higher risk of early osteoarthritis³. Osteoarthritis is a long-term joint disease in which the cartilage lining the joint wears away. A meta-analysis of 53 studies found the odds of knee osteoarthritis were about 4 times higher after an ACL injury than in an uninjured knee¹⁸.

In 100 patients having surgery for what seemed a lone ACL rupture, MRI (a scan that shows soft tissue in detail) was checked¹⁹. In 67%, it also showed injury to the medial collateral ligament area¹⁹. The ACL tear page covers how the injury is defined, its course and what trials found; the menisci page covers the torn pads.

Recovery

Why it heals poorly

The ACL has long been thought to heal poorly³. Failure rates of 40% to 100% were reported even after surgeons stitched it back together³. A review contrasts this with the high healing ability of tissues outside the joint, such as the medial collateral ligament³. The reasons it reports from earlier research include the joint fluid, the injury response, differences in the cells, blood supply and loading³.

In torn ligaments outside the joint, a blood clot forms a temporary scaffold for healing³. After an ACL tear, that clot is missing, mainly because joint fluid keeps flowing and spreads the blood away³. Cell and tissue studies show the ACL and the medial collateral ligament grow new vessels and nerves after injury in a comparable way³. Collagen production in the ACL carries on for up to a year after injury³.

What scans and tests show after a tear

A systematic review took in 9 studies of 734 people whose complete ACL rupture was managed without surgery²⁰. All 9 reported some ligaments regaining continuous fibres on MRI²⁰. In the two studies that recorded where the tear was, most of the ligaments that healed had torn near the femur²⁰. The studies were of low quality²⁰.

After a rupture, the number of mechanoreceptors in the ACL falls quickly, according to a review with a systematic search⁷. The fall is slower where the torn stump sticks to the posterior cruciate ligament⁷.

How a replacement graft changes

In ACL reconstruction, a tendon (a tough cord of tissue that connects a muscle to a bone) graft takes its place²¹. A systematic review of human biopsy studies found the graft survives in the joint²¹. It goes through a series of changes called ligamentization²¹. Under the microscope, a mature graft may come to resemble a normal ACL, though differences remain in how its finest collagen fibres are laid out²¹. There is no agreement on how long each stage takes²¹.

A systematic review of 23 biopsy and imaging studies, with 706 patients, described three phases: early healing, remodelling and ligamentization²². The process was still going on a year after surgery²².

Connections

The joint, bones and ligaments around it

The ACL sits inside the knee joint, joining the tibia to the femur^1,2. It crosses the posterior cruciate ligament in an X². A review of meniscus research describes one study in a knee without a working ACL²³. It found the back end of the inner meniscus was the most important structure resisting forward movement of the tibia²³.

Muscles that load or unload it

A position statement explains that muscles can raise or lower the load on the ACL, depending on their line of pull and the knee’s position⁸. It says that in landing and cutting, the quadriceps and gastrocnemius pull the tibia forward⁸. The hamstrings and soleus pull it back⁸. A narrative review (a summary of research written without a set, checkable search method) adds detail on each group²⁴:

• The quadriceps and gastrocnemius raise the load, especially when the knee is straight²⁴.
• The hamstrings and soleus appear to unload it, the hamstrings only with the knee bent past about 20 to 30 degrees²⁴.

The position statement names the gluteus medius, gluteus maximus and piriformis as the biggest contributors against the knee-inward turning force⁸. The piriformis is a small muscle deep in the buttock.

Exercises that load it

A narrative review of exercise studies found the ACL is loaded less with the knee bent 50 to 100 degrees²⁵. It found more forward trunk lean in squats and lunges unloads it²⁵. In 12 recreationally active people, modelled peak ACL force was 24% lower in single-leg squats with moderate forward lean than with little lean²⁶. In 17 healthy adults imaged during a single-leg jump, ACL strain rose as the knee straightened¹⁷. Related exercise pages: back squat, single-leg squat and drop landing.

Sources

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Every source the library cites

Changes

• September 30, 2026 A supporting claim was narrowed to match its source; the page text is unchanged.