Gross Anatomy

For Medical Students

Structure and Location

The posterior cruciate ligament runs inside the intercondylar notch, from the back of the tibia upwards, forwards and medially to the femur. Like the anterior cruciate it is intracapsular and covered in synovium, so it sits inside the joint capsule and outside the synovial cavity.

Its tibial attachment is in the posterior intercondylar area, in a shallow depression between the two tibial plateaus. That depression is called the posterior intercondylar fossa, and the attachment does not stop at the joint line: it runs down the back of the tibia for roughly 1 cm below it.

Its femoral attachment is on the lateral surface of the medial femoral condyle, which means on the medial wall of the notch. This is the first thing students get backwards. The anterior cruciate attaches to the lateral wall, the posterior cruciate to the medial wall, and each is named for its tibial end rather than its femoral one.

Conventional teaching gives its length as about 38 mm and its mid substance width as about 13 mm. It is the thicker of the two cruciates, with a cross sectional area roughly one and a half times that of the anterior cruciate.

It is described as having two bundles, anterolateral and posteromedial, each named for where it sits at both ends. 2.8.2 and 2.8.3 cover each.

Relations

The anterior cruciate crosses it inside the notch. The two cross near their mid substance, which is what gives the pair their name.

Medially: the medial wall of the notch and the articular cartilage of the medial condyle, which the femoral footprint comes close to in front.

Behind: the posterior capsule, then the popliteal fat, then the popliteal artery and vein. The artery lies close behind the tibial attachment. That proximity is the single anatomical fact behind most of the care taken in Section 7, and 2.8.6 returns to it.

Flanking it: the two meniscofemoral ligaments, running from the lateral meniscus to the medial condyle, one in front and one behind. 2.6.3 covers them.

The middle genicular artery enters the notch from behind to supply both cruciates.

Function

The ligament resists the tibia translating backwards on the femur. That is its primary job and it is what a posterior drawer test examines.

It also resists external rotation of the tibia, and it contributes at the extremes of varus and valgus. Those are secondary, and 3.8 covers how each changes through range.

It carries mechanoreceptors and contributes to joint position sense, which 2.8.4 covers alongside its nerve supply.

Clinical Relevance

Rupture produces posterior instability. The knee tends to trouble the patient going downhill and down stairs rather than on turning, which is the complaint pattern that distinguishes it from anterior cruciate rupture in the history.

The second thing students get backwards is the direction of the drawer. A posterior cruciate injury lets the tibia sag backwards, so the examiner pushes and the tibia moves; the starting position itself is already abnormal, and a posterior sag can be mistaken for an anterior drawer if the resting position is not checked first.

It is better supplied with blood than the anterior cruciate, so partial injuries can heal in continuity, often lengthened. 2.8.4 covers why.

Because the tibial attachment continues below the joint line, an avulsion can take a fragment of cortex with it, which is a bone injury rather than a ligament one.

MRCS Level

Structure and Location

Describe the femoral footprint as a crescent on the medial wall of the notch, its long axis running front to back, with its anterior border close to the articular margin of the medial condyle. Conventional teaching gives the footprint a long axis of about 32 mm and an area of the order of 200 square millimetres, which is larger than the tibial footprint and larger than either anterior cruciate footprint.

The tibial footprint sits in the fossa, bounded by the two plateaus and sloping down the posterior cortex, its centre about 1 cm below the joint line by conventional teaching.

The two bundle description is a functional convention. The fibres between them form a continuum, and the anterolateral bundle carries roughly two thirds of the area.

The ligament is not straight. It runs with a gentle forward curve, convex anteriorly, and that curve is what gives it a recognisable shape on a sagittal image.

Relations

Inside the notch, the anterior cruciate lies anterolateral to it and the two cross obliquely. In a cadaveric notch the posterior cruciate is the one you meet first from behind.

The posterior capsule separates it from the popliteal vessels. Conventional teaching places the popliteal artery about 1 cm behind the posterior tibial cortex at the level of the attachment with the knee extended, and further back as the knee flexes.

The anterior meniscofemoral ligament passes in front of it and the posterior meniscofemoral ligament behind it, both running to the medial condyle. Either can be large enough to be mistaken for part of the ligament.

The posteromedial and posterolateral corners anchor either side, and their state changes what a deficient posterior cruciate does. 2.9 and 2.10 cover them.

Blood Supply and Innervation

The middle genicular artery is the main supply. It arises from the popliteal artery, pierces the posterior capsule and enters the notch from behind.

Periligamentous vessels run in the synovial sheath and reach the ligament along its whole length. Branches from the inferior genicular arteries and periosteal vessels at the tibial attachment add to the supply.

Nerve supply comes from the posterior articular branches of the tibial nerve, which accompany the vessels into the notch. 2.8.4 covers both in detail.

Function

At 90 degrees of flexion the ligament provides most of the restraint to posterior translation. Near extension the restraint is shared with the capsule and the corners, which is why a posterior drawer is tested in flexion.

Its contribution to resisting external rotation rises with flexion, and an isolated injury produces less rotational laxity than a combined one. 3.8.2 covers the quantitative picture.

It acts as a hinge for the femur rolling back on the tibia, so losing it changes the path of the joint as well as its laxity.

Clinical Relevance

A posterior drawer at 90 degrees is the test, with the resting step off between the medial plateau and the femoral condyle read first. A reduced step off is the finding that gives the grade, and 4.11 covers the examination.

Grading rests on anatomy. A plateau still in front of the condyle means the ligament is partly intact; a plateau level with it means the restraint has gone; a plateau behind it means the corners have gone too.

An injury that looks isolated often is not, because the ligament anchors the whole posterior side. Examining the corners is part of examining the ligament.

FRCS Level

Relations

The anatomy that constrains treatment is posterior, not intra-articular. The artery, the vein and the tibial nerve lie behind the tibial attachment in that order from front to back, separated from it by the capsule and a variable layer of fat.

That distance is not fixed. It increases with flexion and falls in a knee whose posterior capsule is injured, because the plane holding the vessels away has gone.

The corners share the load through the arc of extension, and the anatomy predicts which pattern a combined injury produces: the posterolateral corner for external rotation, the posteromedial for valgus and rotation together.

Function

Treat the ligament as a restraint whose share of the load changes through range rather than as a fixed stop. Near extension it carries a minority of the posterior restraint; by 90 degrees it carries the large majority.

The two bundles load reciprocally. The anterolateral bundle tightens as the knee flexes and the posteromedial tightens as it extends, which is why a single bundle restoring only the anterolateral is a partial reconstruction of the function. 3.8.3 to 3.8.5 carry the detail.

The meniscofemoral ligaments carry a measurable share of the posterior restraint, so in a deficient knee they are part of what remains.

Clinical Relevance

Assess the corners before accepting that a posterior cruciate injury is isolated. The anatomy makes a combined injury more common than an isolated one in high energy trauma.

Document the resting step off as well as the translation, because the two together say whether the ligament or the capsule and corners have failed.

Where a graft is planned, the footprint near the articular margin and the one below the joint line are the two facts that shape the route. 2.8.6 covers the landmarks.

Avoid treating the arterial distance as a constant. In a knee with posterior capsular disruption the protective plane is gone, and the anatomy that applies is the injured anatomy rather than the textbook one.