Lateral collateral ligament

Key points

The lateral collateral ligament (LCL) is a band on the outer side of the knee, running from the femur (thigh bone) to the head of the fibula (the thin outer bone of the lower leg)¹.
A review describes it as the knee’s main restraint against varus load, a sideways force that pushes the lower leg toward the body’s midline and opens the outer side of the knee².
In a 10-year series of 19,530 sports injuries, LCL injuries made up 1.1% of the injuries documented³.
The LCL is part of the posterolateral corner, the group of ligaments and tendons at the outer back of the knee¹. A review reports that 28% of injuries to this corner happen on their own².
In one study of 55 elite athletes with an LCL injury alone, managed without surgery, all returned to play, after a median (middle value) of 76 days⁴.

Function

Where it runs

The LCL is a fibrous band¹. Its top end is on the lateral epicondyle, a bony bump on the outer femur, and its lower end on the fibular head¹. Its accepted anatomical name is the fibular collateral ligament, and lateral collateral ligament is an accepted second name⁵. It sits outside the joint capsule, the sleeve of tough tissue that encloses the knee⁶. It is about 70 mm long, 4 to 5 mm wide and 2 to 3 mm thick¹.

The medial collateral ligament on the inner side is fan-shaped, but the LCL is cord-like¹. It has no attachment to the meniscus (a pad of cartilage in the knee) or to the joint capsule¹. In 10 cadaver knees from older donors (bodies donated for study), the medial ligament was significantly longer, much wider and thinner than the LCL⁷.

Where it attaches

In 10 cadaver knees, the top end attached to the femur on average 1.4 mm above and 3.1 mm behind the lateral epicondyle⁸. The lower end attached to the head of the fibula, on average 8.2 mm behind its front edge⁸.

Engraving of the right knee from behind. The cord-like fibular collateral ligament runs down the outer side from the femur to the head of the fibula, the popliteus tendon passes beneath it, the tibial collateral ligament is on the inner side, and the oblique popliteal ligament crosses the back of the joint between the two heads of gastrocnemius. — Knee joint from behindHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 346. Public domain, via Wikimedia Commons. (source, Public domain)

Its blood comes from branches of the popliteal artery, the main artery behind the knee¹. Its nerve supply comes from the common fibular (or common peroneal) nerve, a nerve that wraps around the outer side of the knee¹. In a study of 36 cadavers, the ligament held three types of nerve ending⁹.

What it holds back

A review describes the LCL as the knee’s primary restraint against varus load². The same review says it also resists outward turning of the shin up to 30° of knee bend, and helps less against inward turning². In 10 cadaver knees, cutting it showed it mattered for resisting varus with the knee straight and at 30° and 60° of bend¹⁰. In a cadaver study, its load under varus was fairly steady at 0°, 30° and 60°, and fell significantly at 90°¹¹.

In a cadaver study, it carried load under varus and under turning in either direction, most when the straight knee was turned outward¹¹. In another study of 10 cadaver knees, it mattered for resisting outward rotation with the knee straight, but not at 30° or 60°¹⁰. That differs from the review, which gives a role up to 30°^2,10.

How it changes as the knee bends

In 10 cadaver knees, the LCL slackened significantly as the knee bent, while the popliteofibular ligament complex did not¹². The popliteofibular ligament runs from the tendon of popliteus (a small knee muscle) to the fibular head’s tip, behind the LCL’s attachment⁵. The authors concluded that the popliteofibular ligament takes over when the knee is bent, because the LCL slackens¹². In 10 cadaver knees, the LCL passed through vertical at 70° of bend, leaving it poorly angled to resist outward rotation¹².

In a cadaver study, the LCL took more of an outward-rotation load than the popliteus complex at lower knee angles¹¹. The popliteus complex took more at 60° and 90°¹¹.

In 12 cadaver specimens moved passively and through loaded squats, it stayed close to constant length from 0° to 70° of bend¹³. Its strain (stretch as a share of resting length) stayed under 2%¹³.

How strong it is

Two cadaver studies measured how much pull the LCL takes before it fails, in newtons (a unit of force)^7,12:

Study	Structure	Force at failure (N)
10 knees from older donors	LCL	392⁷
10 cadaver knees	LCL	309¹²
10 cadaver knees	Popliteofibular ligament	186¹²

In the older donors, the LCL had half the strength of the medial collateral ligament⁷. The study found no significant difference in how stiff the two were⁷. Pulled to failure in 10 cadaver knees, it tore at its fibular attachment in 6 and in its middle in 4⁷.

Breakdown

How often, and in which sports

A 10-year series of 19,530 sports injuries recorded these shares of the injuries documented³:

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

Ligament injured	Share of injuries documented (%)
Anterior cruciate (ACL)	20.3³
Medial collateral	7.9³
Lateral collateral	1.1³

In that series, tennis and gymnastics were linked with injury to the LCL³. At the NFL Combine, 23 of 2,285 players checked (1%) showed signs of a past posterolateral corner injury¹⁴.

Part of a group: the posterolateral corner

The LCL belongs to the posterolateral corner, with the tendon of biceps femoris (a hamstring muscle) and three other ligaments, including the popliteofibular¹.

One prospective study (following people forward in time) used MRI (a scan that shows soft tissue in detail)¹⁵. Of 331 acute (recent) knee injuries with bleeding into the joint, posterolateral corner injuries were 16% of the ligament injuries¹⁵. In that study, 26 of 30 patients (87%) with a posterolateral corner injury had injured more than one ligament¹⁵. A review reports that 28% of these injuries happen alone². The rest usually come with tears of the cruciate ligaments, which cross inside the knee².

A review reports isolated corner injuries in under 2% of people with a knee ligament injury². Among people with significant knee ligament injuries, trauma centres have reported corner injuries of any kind in 16% to 28%². A 2019 review of ACL rupture calls LCL injuries generally uncommon alongside an ACL tear¹⁶.

How it gets hurt

A review names sports injuries, falls and vehicle accidents as the common causes of injury to the outer back of the knee¹⁷. Reviews describe these ways the corner is injured^2,17:

a direct varus force to the knee²
hyperextension (straightening the knee past its normal limit), with or without contact²
a twist in an injury to several ligaments, such as a knee dislocation (the bones forced fully out of place)²
a direct hit on the upper shin with the knee straight¹⁷

In video of NFL (US professional American football) games, 35 knee injuries involved more than one ligament¹⁸. ACL plus LCL was the second most common pattern (15%), after ACL plus medial collateral (65%)¹⁸. Combined ACL and PCL, or ACL and LCL, injuries happened most often with varus and inward-turning forces on a straight knee¹⁸.

Grades, and when scans and examination disagree

A review describes the usual grading of outer-knee injuries by how much extra the joint opens under varus load²:

Grade	Extra joint opening
I	0 to 5 mm²
II	5 to 10 mm²
III	Over 10 mm²

In 55 professional athletes with an LCL injury alone on MRI, 76% had grade 1 looseness on examination, 11% grade 2 and one grade 3⁴. Of those 55, 27 had grade 3 injuries on MRI, but just one had grade 3 looseness on examination⁴.

Injury to a nearby nerve

In 303 patients with knee dislocations, injury to the posterolateral corner went with 42-fold higher odds of injury to the common fibular nerve¹⁹. In a review of the records of 61 corner injuries that had surgery, 26% had a peroneal (fibular) nerve injury when first seen²⁰.

Recovery

How ligaments heal

Ligaments are not inert: they respond to many local and whole-body factors that affect how they work²¹. An injured ligament heals by forming scar²². The scar is weaker and bulkier than normal ligament, and stretches more under load²². Its collagen (the main building material of ligaments) has a different make-up²².

Healing is affected by the size of the gap between the torn ends²². It is also affected by motion in a stable joint, and by whether other ligaments are injured too²². A review describes torn structures at the outer back of the knee turning into fibrous scar after 3 weeks¹⁷. From that point, an injury is counted as chronic (long-lasting)¹⁷.

Athletes with an LCL injury alone

One study followed 55 elite athletes with an LCL injury alone, managed without surgery⁴. All returned to play, after a median of 76 days, with a mean of 103 and a range of 0 to 422⁴. At 2 years, 93% were still playing elite sport, and all had returned to their pre-injury level⁴. None stopped because of the injury⁴.

In the same study, 21 of the athletes came back for a second examination at a mean of 44 days⁴. None had grade 2 or 3 looseness⁴. None reported the knee feeling unstable⁴.

Injuries to several ligaments

One study followed 50 NFL athletes with injuries to several knee ligaments²³. Of those with ACL plus posterior cruciate (PCL) or LCL tears, 55.6% returned to play²³. Of all the athletes with ACL plus PCL or LCL tears, 18.5% returned to their prior level of performance²³. Of those with ACL plus medial collateral tears, 43.5% did²³.

Connections

The joint and bones it joins

The LCL links the femur to the fibula across the outer side of the knee joint⁶. The outer meniscus does not attach to it²⁴.

Other structures act as secondary restraints to varus load¹:

the popliteus tendon¹
the iliotibial band, a long band of tough tissue down the outer thigh¹
the anterior cruciate and posterior cruciate ligaments¹
the biceps femoris tendon and the tendon of the lateral head of gastrocnemius (the large, two-headed calf muscle)¹

The popliteus tendon lies deep to the LCL, and the surface layer of the iliotibial band lies over it¹. The outer head of gastrocnemius starts about 14 mm behind the LCL’s attachment on the femur⁵.

A review describes the biceps femoris tendon, from one of the hamstrings, sending one arm to fix beside the LCL⁵. A second, sheet-like arm joins its outer back surface⁵. In cadaver work, the iliotibial band steadies the outer knee together with the LCL, biceps femoris and the popliteus tendon²⁵. The outer knee kept its passive stability as long as at least two of these stayed intact²⁵.

With ligaments cut one by one in cadaver knees, the popliteus complex, LCL and outer back capsule worked as a unit²⁶. No single structure was the primary restraint to the movements studied²⁶. That differs from the review that calls the LCL the primary varus restraint^2,26.

Links with the cruciate ligaments

The LCL, popliteus tendon and popliteofibular ligament work together with the posterior cruciate ligament²⁷. In laboratory testing of loaded knees lacking these structures, force in the intact posterior cruciate ligament rose²⁷. In cadaver knees with a rebuilt ACL, cutting the LCL significantly raised force on the new ACL under varus load at 0° and 30°²⁸.

Exercises studied

One study filmed 16 people with ACL deficiency doing a single-leg squat, using a two-plane X-ray system²⁹. No difference was found in LCL length before and after ACL reconstruction²⁹. Related exercise pages: single-leg squat.