Gross Anatomy

For Medical Students

Structure and Location

The anterior cruciate ligament runs inside the intercondylar notch, from the tibia upwards, backwards and laterally to the femur. It is intracapsular but covered in synovium, so it sits inside the joint and outside the synovial cavity.

Conventional teaching gives its length as about 32 to 38 mm and its width as about 10 mm, with the ligament broader at both ends than in its middle.

Its tibial attachment is in the anterior intercondylar area. Its femoral attachment is on the medial surface of the lateral femoral condyle, which means on the lateral wall of the notch. Students routinely get that the wrong way round.

It is conventionally described as having two bundles, anteromedial and posterolateral, named for where their fibres attach on the tibia.

Relations

The posterior cruciate ligament crosses it inside the notch. The two give the notch its contents and the knee its cruciate name.

Above and laterally: the lateral wall of the notch and the roof, with Blumensaat's line marking the roof on a lateral radiograph. The ligament runs roughly parallel to that roof in extension.

In front: the anterior horns of both menisci and the fat pad, with the transverse intermeniscal ligament crossing anteriorly.

Behind: the posterior cruciate, then the posterior capsule, then the popliteal vessels.

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

Function

The ligament resists the tibia translating forwards on the femur. That is its primary job and it is the one a Lachman test examines.

It also resists internal rotation of the tibia, and it contributes to resisting varus and valgus at the extremes. Those are secondary.

It carries mechanoreceptors and contributes to joint position sense, which 2.7.10 covers.

Clinical Relevance

Rupture produces anterior instability and a positive Lachman test, and the knee gives way on turning rather than on walking straight.

The ligament is intra-articular with a single main blood supply, so it does not heal reliably in the way an extra-articular ligament does. That single anatomical fact underlies most of the surgery in Section 7.

Because it runs parallel to the notch roof, its orientation on a sagittal image is assessable, and a ligament that has lost that orientation has usually failed.

MRCS Level

Structure and Location

Describe it as a band of fibres whose fibre length and orientation differ across its width, rather than as a cord. The fibres fan out at both ends, so the cross sectional area is smallest in the mid substance and larger at each footprint.

Conventional teaching gives the mid substance cross sectional area as of the order of 40 square millimetres, with the footprints several times larger.

The two bundle description is a functional convention rather than a dissectable separation. The bundles are named by their tibial attachment: anteromedial fibres attach anteromedially and arise proximally on the femoral wall; posterolateral fibres attach posterolaterally and arise distally. 2.7.8 and 2.7.9 cover each.

The fibres twist as they run, because the femoral attachment is vertical in extension while the tibial attachment is horizontal. That twist is the reason the whole ligament cannot be isometric.

Relations

Within the notch: the posterior cruciate medially and behind, with the two ligaments crossing. The anterior cruciate is anterolateral to the posterior cruciate throughout.

The lateral wall of the notch carries the femoral footprint, with the lateral intercondylar ridge marking its anterior limit. 2.7.6 covers the landmarks.

The anterior intercondylar area carries the tibial footprint, which overlaps the anterior root of the lateral meniscus. 2.7.7 and 2.6.4 cover the overlap.

Anteriorly the ligament can impinge on the roof of the notch in full extension if the notch is narrow or if a graft is placed too anteriorly.

The synovial covering is continuous with the synovium of the notch and carries the vessels.

Blood Supply and Innervation

The middle genicular artery is the main supply. It arises from the popliteal artery, pierces the posterior capsule and reaches the ligament through its synovial covering, which 2.7.4 covers.

There is a small contribution from the inferior genicular arteries through the fat pad and the synovium at the tibial end.

Innervation is from the posterior articular branches of the tibial nerve, reaching the ligament through the posterior capsule alongside the artery. Mechanoreceptors are concentrated near both insertions and 2.7.5 covers them.

One artery, arriving through one route, is the anatomical reason this ligament behaves as it does after injury.

Function

Primary restraint to anterior tibial translation at all flexion angles, with the contribution of each bundle varying through the range. 3.7.5 and 3.7.6 own the bundle function.

Secondary restraint to internal rotation, which is why a deficient knee shows a pivot shift rather than only a forward drawer.

A tertiary contribution to varus and valgus restraint, relevant only when the collaterals have also failed.

A sensory function through its mechanoreceptors, which is why reconstruction restores mechanics faster than it restores control.

Clinical Relevance

The ligament's intra-articular, extrasynovial position with a single arterial supply explains both the poor healing of mid substance tears and the better behaviour of proximal avulsions with intact synovial sleeve.

Its orientation parallel to the notch roof is the reference for assessing it on imaging and for judging graft obliquity afterwards.

Notch narrowing reduces the space available, and a graft that fills the notch completely will impinge in extension.

The dimensions on this page are standard teaching values, registered and unchecked. They vary with sex, height and measurement method, and the architecture asked for dimensions on this page specifically.

FRCS Level

Relations

The notch is a fixed volume shared with the posterior cruciate, the meniscofemoral ligaments and, after surgery, a graft and its fixation. Everything added competes for it.

The relation that constrains reconstruction most is the lateral wall. Its obliquity determines the trajectory any femoral tunnel can take, and its anteroposterior extent determines how much of the native footprint a tunnel can cover.

Function

Treating the ligament as a single structure predicts the wrong things, because its fibres are not isometric and no single band reproduces them. Treating it as two discrete bundles also predicts the wrong things, because they are not discrete.

The useful framing is a continuum of fibres with differing length patterns, of which a graft reproduces a part. Which part depends on where it is attached, and that is the whole of the tunnel position argument.

3.7 owns the quantitative account and Section 7 owns the surgical argument.

Clinical Relevance

Graft obliquity, tunnel position and notch volume are three expressions of the same anatomy, and a decision about one constrains the other two.

In a narrow notch the options are a smaller graft, a notchplasty, or accepting impingement. All three have costs and the anatomy says so before any trial does.

In revision the native footprints may be destroyed, so the lateral wall and the intercondylar ridge are the references that survive. 2.7.6 covers them.

The ligament's dimensions set the ceiling on what a graft can reproduce. A graft substantially larger than the native ligament does not restore anatomy, it fills a notch.