Medial collateral ligament

Function

Three parts on the inner knee

The MCL runs from the inner epicondyle of the femur to a ridge at the back-inner edge of the tibia¹. An epicondyle is a bony bump on the side of a condyle, where muscles or ligaments attach. A condyle is a rounded knob at the end of a bone.

The MCL is 8 to 10 cm long⁵. It is a complex with three parts⁴:

The superficial MCL is the largest ligament on the inner side of the knee⁴. It is broad, flat and triangular, and widest in its middle⁶.
The deep MCL is a thickening of the inner joint capsule, the sleeve of tough tissue that encloses a joint⁶. It attaches to the inner meniscus (a pad of cartilage in the knee)⁶.
The posterior oblique ligament is made of fibrous extensions of the tendon of the semimembranosus, one of the three hamstring muscles⁴. They strengthen the back-inner part of the capsule⁴.

Engraving of the right knee from the front. The quadriceps tendon comes down the femur to the kneecap, and the patellar ligament (ligamentum patellae) runs from the kneecap to the tibia, with the tibial collateral ligament on the inner side. — Knee joint from the frontHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 345. Public domain, via Wikimedia Commons. (source, Public domain)

Where it attaches

The superficial MCL joins the femur and tibia through one attachment on the femur and two on the tibia⁴. Anatomy studies have measured these sites:

What was measured	What was found
Length of the superficial MCL	Averaged 94.8 mm in eight cadaver knees (knees from bodies donated for study)⁷
Attachment on the femur	Wrapped around the inner epicondyle, centred about 1 mm above it, in 22 knees⁸
Upper attachment on the tibia	Reaches the front arm of the semimembranosus tendon, 12.2 mm below the joint on average⁶
Lower attachment on the tibia	61.2 mm below the knee joint in eight cadaver knees⁷

What it holds steady

The MCL is a broad, strong band whose main job is to steady the knee from side to side¹. It resists valgus and helps hold the knee against external rotation (turning a limb outward, away from the middle of the body)¹. Its parts share this work by knee angle^6,9:

In a cutting study of 18 cadaver knees, the superficial MCL controlled valgus at each angle tested⁹. It was the dominant restraint from 30° to 90° of bend⁹.
In 28 cadaver knees tested by robot, the superficial MCL was the most important restraint to external rotation and valgus at each angle tested¹⁰. The same held for anteromedial rotation (a twist of the shinbone in which its inner side moves forward)¹⁰.
The deep MCL takes about 20% of the load resisting outward turning of the tibia at 0–30° of bend⁶.
In the 18-knee cutting study, the deep MCL was a secondary restraint to valgus⁹.
The posterior oblique ligament is tight with the knee straight and goes slack as soon as it bends⁶.

In one cadaver study at 60° of knee bend, the lower superficial MCL took 45% more valgus load than the upper portion⁶. In 28 cadaver knees, cutting the upper tibial attachment caused no significant rise in laxity while the lower one stayed intact¹⁰. Laxity is looseness in a joint or ligament.

How little it stretches in landing

When a robot moved 18 cadaver knees through a recorded drop-jump landing, the ACL’s peak strain far exceeded the MCL’s¹¹. Strain is stretch as a share of resting length. The peaks were 6.1% for the ACL and 0.4% for the MCL¹¹.

Breakdown

How it gets hurt

MCL injury most often comes from a blow to the outside of the knee, pushing it inward². It can also come from valgus force combined with the tibia turning outward². The ligament is especially exposed with the knee bent about 20–30°, when the superficial MCL is taut⁶. Minor trauma can tear the superficial part, while higher-energy injuries can disrupt both layers⁶.

Studies of professional men’s football show how this plays out^2,12:

In video of 37 MCL injuries, 62% were direct contact, 24% indirect contact and 14% non-contact¹².
Each of the indirect and non-contact injuries showed the knee loaded into valgus¹². Most also showed the foot turned outward and hip abduction (the thigh moving out to the side)¹².
In a prospective study (one that follows people forward in time) of 51 teams, 75% of 130 MCL injuries came through contact². The commonest were being tackled (29%) and tackling (12%)².

Who injures it, and how often

Most MCL injuries come from valgus loading, outward turning or both, in sports such as football, skiing and ice hockey⁴. They occur mainly in young athletes⁴. A 2010 review describes the superficial MCL, deep MCL and posterior oblique ligament as the knee’s most commonly injured ligaments³. One 10-year series recorded 19,530 sports injuries, 7,769 of them to the knee¹³. MCL lesions made up 7.9% of the injuries documented, against 20.3% for the ACL¹³.

That series linked MCL injury with judo and skiing¹³.

Group studied	What was found
27 European professional football teams over 11 seasons	346 MCL injuries, 0.33 per 1000 hours, nine times higher in matches than in training¹⁴
Cadets at the United States Military Academy, 2005 to 2009	128 had isolated MCL injuries (no other structure hurt)¹⁵
College athletes at the same academy	Wrestling, judo, hockey and rugby had the highest rates of isolated MCL sprain (a stretched or torn ligament)¹⁵
College athletes at the same academy, by sex	Men had about 2.9 times the women’s rate of isolated MCL sprain¹⁵
US high school and college athletes	Female athletes had the higher rate in high school, but the lower rate in college¹⁶

How injuries are graded

MCL injuries are graded I to III by how far the inner side of the joint opens under valgus⁵. On MRI (a scan that shows soft tissue in detail), each grade has its own look²:

Grade	Inner joint opening⁵	On MRI²
I	No laxity; 3–5 mm⁵	Fibres intact, with oedema (fluid swelling) around them²
II	Opens at 30° of bend; 6–10 mm⁵	Some fibres torn²
III	Opens at 0° and 30°; more than 10 mm⁵	Fully torn, or pulled off the bone (avulsion)²

In 130 MCL injuries in European professional men’s football, the upper third of the ligament was the most common site (54%)². The middle third followed (31%), then the lower third (15%)². Of these, 88% were isolated². The inner meniscus was the most frequent injury alongside it².

When the ACL tears too

Studies report different figures for how often the MCL is hurt alongside an ACL tear^17,18. A 2019 review puts MCL injuries at about 30% of ACL ruptures¹⁷. ACL reconstruction is surgery that rebuilds a torn ACL using a graft. Among 100 people having ACL reconstruction for what seemed an isolated ACL rupture, 67% had MCL complex injury on MRI¹⁸. Biomechanical studies show that partial or complete MCL tears raise the load on the ACL⁶.

Recovery

How well it heals: two views

Sources differ on how completely the MCL heals^2,4. One says it has greater potential to heal than the ACL, often to full recovery, because it lies outside the joint with enough blood supply².

Another source says injured ligaments heal by forming scar rather than regrowing, and the MCL heals fairly well but cannot be fully restored⁴. One source says that such scar is worse, biologically and mechanically, than the tissue it replaces¹⁹.

Tears of the lower superficial MCL have shown poorer healing than upper tears⁶. This has been linked to a poorer blood supply⁶. It has also been linked to soft tissue, such as the pes anserinus, getting caught between the torn ends⁶. The pes anserinus is the spot on the inner upper shinbone where the tendons of three thigh muscles attach together.

What animal studies show

Animal studies have tracked healing over weeks and months^4,20:

In a rabbit MCL model, looseness seemed restored by 14 weeks⁴. Yet about 65% of the ligament’s strength had returned by 40 weeks, compared with the other side⁴.
In dog models, the MCL’s mechanical properties had not fully recovered by 48 weeks after injury²⁰.

A systematic review (a study that searches for all the research on a question) looked at seven studies of surgically injured rat or rabbit MCLs²⁰. It found early evidence that loading after injury raised the ligament’s breaking force and stiffness, compared with unloading it, such as by keeping it still²⁰. Stiffness is how much force it takes to stretch something by a given amount.

Yet three of those studies consistently reported more laxity in the loaded groups²⁰. The review judged these changes slow, with large, precise effects not clear until 14 weeks²⁰. Data from two studies suggest that strength was about 40% of uninjured controls after 12 weeks of loading²⁰.

Time lost from sport

Days lost to MCL injuries, by study and grade^2,5,15:

Days lost. Bars start at zero; the table below gives each value with its source.

Group and grade	Days lost
Professional men’s football, grade I (average)	10²
Professional men’s football, any grade (average)	24²
Military academy, grade 1 (median)	13.5¹⁵
Military academy, higher grades (median)	29¹⁵
Review of 26 reports, grade I (average)	10⁵
Review of 26 reports, grade II (average)	35⁵

In the study of 130 football injuries, 11 (8.5%) were re-injuries of the same knee within 2 months of return to play².

Nine years on

One study followed people about nine years after isolated MCL sprains that had no surgery²¹. The 54 people with grade II sprains generally did well, though some leftover looseness was common²¹. The 27 with grade III sprains fared much worse²¹. Their problems included lasting instability, weakness and osteoarthritis (a long-term joint disease in which the cartilage lining the joint wears away) after the injury²¹.

Connections

The joint and bones

The MCL runs down the inner side of the knee joint, from the femur to the tibia¹. Both it and its outer partner, the lateral collateral ligament, sit outside the joint capsule²². In 10 cadaver knees from older donors, the fan-shaped MCL was longer, much wider and thinner than the cord-like lateral collateral ligament²³.

The meniscus and other ligaments

The MCL is firmly joined on its deep side to the joint capsule and the inner meniscus²². The deep MCL holds the inner meniscus firmly at its midpoint²⁴. The anchor to the MCL is one reason the inner meniscus moves less than the outer one²². The menisci page covers them. The anterior cruciate ligament takes more load when the MCL is torn⁶.

The gastrocnemius, the inner hamstrings and the MCL work together to steady the inner knee against valgus⁵. In a biomechanical study of knees with a complete MCL tear, simulated hamstring pull brought the valgus angle close to that of the intact knee⁶.

Exercises that load it

Related exercise pages: drop landing, single-leg squat. One study tracked 16 people with ACL deficiency during a single-leg squat, using a two-angle X-ray system²⁵. ACL deficiency means a knee whose ACL is torn or no longer working. At 0–30° of bend on the way down, the MCL was longer before ACL reconstruction than 18 or more months after²⁵.