Posterior cruciate ligament

Function

Where it runs

The ligament is named for its attachment to the back of the tibia¹. The name also refers to the cross it forms with the anterior cruciate ligament inside the joint capsule¹. The capsule is the sleeve of tough tissue that encloses the knee¹.

It starts on the inner condyle (rounded knob) of the femur, in the intercondylar notch⁵. The notch is the groove between the two condyles at the lower end of the thigh bone⁵. It runs down to the back of the tibial plateau (the flat top of the shinbone, where the thigh bone rests)⁵.

It averages 36 to 38 mm long³. Its cross-sectional area (the size of a slice cut straight across it) is 40 to 60 mm² at its middle³. It sits inside the joint but outside the synovial lining (the thin inner lining of a joint capsule)³. It is about 1.3 to 2 times as thick as the anterior cruciate ligament and about twice as strong⁵.

Its two footprints (the patches of bone where it attaches) differ in shape and size³:

Attachment	Shape and place	Area
Femur	Half-moon shaped, on the side of the inner condyle facing the notch, reaching the notch’s roof³	About 190 to 230 mm²³
Tibia	Trapezoid shaped, in a groove below the joint surface, midway across, near the back roots of both menisci (knee cartilage pads)³	About 160 to 220 mm²³

Engraving of the right knee from the front with the kneecap and capsule removed. The rounded lateral and medial condyles of the femur sit on the tibia, the anterior and posterior cruciate ligaments cross between them, and the C-shaped lateral and medial menisci rest on top of the tibia. — Inside the knee joint from the frontHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 347. Public domain, via Wikimedia Commons. (source, Public domain)

Two bundles and two helpers

The ligament is made of two bundles, the anterolateral (front-outer) and the posteromedial (back-inner)¹. The anterolateral bundle is larger and stronger in tension³. The posteromedial bundle is weaker, and its anatomy varies more from person to person³.

The bundles tighten in different patterns as the knee moves, so they resist backward sliding and rotation at different knee angles². Recent studies call them codominant, meaning neither is the main restraint at a given angle³. Both bundles do more work with the knee bent than straight, and both lengthen as it bends³.

Two meniscofemoral ligaments run beside it³. They link the back of the outer meniscus to the inner femoral condyle, starting near this ligament⁶. At least one is present in more than 90% of knees³. With the two bundles, they make four fibre bundles that work together against backward and twisting loads on the tibia³.

What it holds back

The ligament stops the tibia sliding backwards under the femur¹. One review reports that it provides 95% of the force holding the tibia against backward movement⁷. Recent studies suggest it also helps resist twisting loads, mainly with the knee bent beyond 90 degrees³. In the bent, weight-bearing knee, as in walking downhill, it keeps the femur from sliding forward off the top of the tibia⁸.

How its load changes as the knee bends

Laboratory studies found that the force in the ligament changed with knee angle and load^9,10:

A robot pushed the tibia backwards with 22 to 110 N (newtons, a unit of force)⁹. Force in the ligament ranged from about 6 N (knee straight) to about 112 N (90 degrees, largest load)⁹.
In nine donated knees, with a simulated hamstrings (back-of-thigh muscles) pull, the force rose as the knee bent, from about 13 N straight to about 86 N at 90 degrees¹⁰.

In the same nine knees, adding a quadriceps (front-of-thigh muscles) pull to the hamstrings pull significantly lowered the force in the ligament¹⁰.

What it is made of

Most of both cruciate ligaments is made of bundles of collagen (a strong, rope-like protein)¹¹. Long cells lie between its parallel fibrils (very thin strands of collagen)¹¹.

An anatomical study that injected the blood vessels found the ligament’s blood comes mainly from the middle genicular artery¹². That is an artery at the back of the knee that supplies the cruciate ligaments. The injection study found three areas without blood vessels: both bone attachments and the centre of the middle third¹².

A review reports that the ligament holds several kinds of nerve ending that can signal movement and position to the brain and spinal cord⁷.

Breakdown

How it gets torn

Injury can come from a force that pushes the tibia backwards, most commonly with the knee bent². Because the ligament is larger and stronger than the anterior cruciate, its injuries are associated with high-energy trauma such as vehicle and sport injuries³. Over-bending or over-straightening the knee can also tear it³. Over-straightening injuries are associated with tears near its upper end and crush fractures at the front of the tibial plateau³.

In 494 people with a loose, torn ligament, dashboard injuries (35%) and falls onto the bent knee with the foot pointed (24%) led the causes¹³.

Who tears it

Males are injured more often than females, which a review links to their greater involvement in trauma³. The same review puts the average age at injury at 28 to 34³. The share of injuries caused by sport differs between studies^13,14:

Share caused by sport, %. Bars start at zero; the table below gives each value with its source.

Group studied	Share caused by sport, %
494 people with a loose, torn ligament	40¹³
1,000 reconstructions (surgery to rebuild the ligament) at one centre	38.8¹⁵
1,287 reconstructions in Scandinavia	35.4¹⁶
Swedish registry reconstructions of this ligament without the ACL	64¹⁴

A population study of 48 people put the yearly incidence (new cases) of isolated, complete tears at 1.8 per 100,000¹⁷. Of 2,285 players at the NFL (National Football League) Combine from 2009 to 2015, 69 (3%) showed signs of an injury to it¹⁸. The NFL Combine is a yearly event where college American football players are examined and tested before the professional draft.

A case-control study (comparing people with a condition to similar people without it) matched 104 people with tears to 104 controls¹⁹. The injured group had a flatter backward slope of the tibial plateau, 5.7 against 8.6 degrees¹⁹. In that group, 87.5% of tears came from contact¹⁹. Another study linked a shallower inner tibial plateau and a narrower notch with isolated tears and with avulsion fractures at the tibia²⁰. An avulsion is a tear in which a ligament pulls away from the bone it attaches to.

How injuries are graded

Injuries are graded by how far the tibia moves backwards³:

Grade I: a partial tear, under 5 mm³.
Grade II: a partial tear, 5 to 10 mm³.
Grade III: a complete rupture, over 10 mm³.

Often torn with other injuries

A review reports that over 60% of injuries come with other capsule or ligament damage³. The most common pairing, in 15% to 42% of patients, is with the posterolateral corner (the ligaments and tendons at the outer back of the knee)³. The same review reports that over 80% of patients with acute (recent) injuries, alone or with the posterolateral corner, have had a bone bruise³. A bone bruise is an injury inside a bone, without a break, that shows up on an MRI scan.

Other groups also show frequent combined injuries^15,18:

In 1,000 reconstructions at one centre, combined injuries outnumbered isolated ones in sport (58.5%) and traffic (71.7%)¹⁵. Handball was the one activity where isolated tears were more common¹⁵.
In that group, a posterolateral corner tear was the most common extra injury, except in skiing and cycling, where a medial collateral ligament injury led¹⁵. This is the ligament on the inner side of the knee.
In 69 NFL Combine players, the medial collateral ligament was also injured in 42%, the anterior cruciate in 11.6% and cartilage in 31.8%¹⁸.

A 2019 review reports that this ligament is generally uncommon among the injuries that come with an ACL tear²¹.

Recovery

It often heals back into one piece

A systematic review of MRI follow-up studies found that 77% to 93% of partial or complete tears regained continuity⁴. One study followed 48 acutely injured knees²². Its authors concluded that many of these ligaments grow slack but continuous tissue, which might partly hold the tibia²². MRI studies have followed it^22,23:

In 40 knees followed for 3.2 years on average, each low- or mid-grade injury healed with continuity, as did 19 of 22 high-grade ones²³.
In 48 knees after acute injury, 75% showed a continuous ligament²². Those knees had less front-to-back tibial movement (mean 7.6 mm) than knees with a disrupted one (11.4 mm)²².
In 46 complete tears scanned more than two months after injury, 72% showed continuity, more often when scanned after six months²⁴.

In the 40-knee study, most injuries managed without surgery could heal with continuity even when other knee ligaments were also damaged²³. In the 46-tear study, other ligament injuries made regaining continuity less likely²⁴.

What the scans can and cannot show

The same systematic review found a possible link between a ligament regaining continuity on MRI and a stable knee on examination⁴. But in long-standing injuries, scar or healing in a stretched shape can make the ligament look intact on MRI³.

After surgery

In 22 isolated injuries, surgeons rebuilt the ligament with hamstring tendon grafts (pieces of tissue moved to rebuild a damaged part)²⁵. Repeated MRI scans followed the grafts²⁵. Their look on MRI matured gradually from 6 months, and the part inside the joint matured more slowly²⁵. The grafts needed more than a year to reach a low signal on the scan²⁵.

Connections

Its partner and the bones it joins

With the anterior cruciate ligament, it forms a cross inside the knee that stops the tibia moving too far forward or backward on the femur²⁶. It joins the femur to the tibia⁵.

The posterolateral corner includes the lateral collateral ligament (the ligament on the outer side of the knee), the popliteus tendon and the popliteofibular ligament³. The popliteus is a small muscle at the back of the knee. The popliteofibular ligament joins the popliteus tendon to the top of the fibula. These structures resist backward movement, inward bending and outward twisting of the tibia, and work with this ligament³. In loaded knees tested in the lab, force in an intact posterior cruciate rose when these structures were deficient³.

On the inner side, the medial collateral ligament complex and nearby structures add to backward and twisting looseness when this ligament is deficient³.

In donor knees, force in it rose as the knee bent under a simulated hamstrings pull, and adding a quadriceps pull lowered it¹⁰.

Exercises that load it

A 2001 review of the squat (see back squat) reports low to moderate backward shear forces at the knee throughout the lift²⁷. These forces, which push surfaces to slide across each other, are held mainly by this ligament²⁷.

A modelling study (one that estimates forces with a model) looked at 18 people doing the wall sit²⁸. Peak force in the ligament came at 80 to 90 degrees of knee bend²⁸. It averaged about 723 N with the feet farther from the wall, 786 N with them closer, and 414 N in the one-leg squat²⁸. Across 0 to 90 degrees, though, the feet-farther version generally produced greater force²⁸.

A narrative review (a summary without a set search method) ordered exercises from least to most load on this ligament²⁹. The order was level walking, the one-leg squat (see single-leg squat), lunges, the wall squat, the leg press and the two-leg squat²⁹.