Posterior Oblique Ligament

For Medical Students

Structure and Location

The posterior oblique ligament is a fan of fibres running backwards and downwards from the femur to the posteromedial corner of the knee. It is layers two and three merged, which is why it is a ligament behind the collateral ligament and a capsule at the same time.

Its femoral attachment is just below and in front of the gastrocnemius tubercle, behind and above the attachment of the superficial collateral ligament. Conventional teaching places its centre about 8 mm distal and 3 mm anterior to the gastrocnemius tubercle.

It spreads into three arms. The superficial arm runs down beside the collateral ligament, the central arm attaches to the tibia and the posterior horn of the medial meniscus, and the capsular arm blends into the posterior capsule.

The central arm is the largest and the one most descriptions mean when they say the ligament.

Its lower end is reinforced by the semimembranosus tendon, which 2.9.5 covers.

Relations

In front of it: the superficial medial collateral ligament, with a clear interval between the two at the femur that closes as they descend.

Behind it: the posterior capsule and the oblique popliteal ligament.

Below it: the posterior horn of the medial meniscus, to which the central arm attaches.

Around its femoral attachment: the adductor tubercle above and the gastrocnemius tubercle behind, the two landmarks 2.9.7 uses.

Function

The ligament tightens as the knee extends and slackens as it flexes. That is the reverse of the superficial collateral ligament and it is the key fact about it.

It is the primary restraint to internal rotation of the tibia with the knee near extension.

It is a secondary restraint to valgus, and the share it carries rises as the knee straightens. 3.9.5 covers its contribution through range.

Clinical Relevance

A knee that opens to valgus at full extension has lost this ligament as well as the collateral. A knee that opens only at 20 degrees has not.

Because it tightens in extension, it is injured by a blow to a straight knee rather than to a bent one.

Its attachment to the posterior horn of the medial meniscus means an injury here moves the meniscus as well as loosening the knee.

MRCS Level

Structure and Location

Locate the femoral attachment from the gastrocnemius tubercle rather than from the epicondyle. Conventional teaching places it about 10 mm posterior and proximal to the superficial collateral ligament's femoral attachment, which is near enough that an attachment found from the epicondyle alone will usually be the wrong one.

The central arm attaches to the tibia immediately below the joint line and to the posterior horn of the medial meniscus on the way. It is therefore a meniscal attachment as well as a bony one.

The superficial arm has no separate bony attachment. It runs into the sheath of semimembranosus and into the fibres of the collateral ligament.

The ligament is a sheet rather than a cord, so a description of its length is less useful than a description of its borders.

Relations

The interval between this ligament and the superficial collateral ligament is a real surgical plane proximally and disappears distally. Its presence at the femur is what lets the two be addressed separately there.

Semimembranosus reinforces the ligament from below and tensions it in flexion, which partly offsets the slackening that flexion would otherwise cause.

The oblique popliteal ligament arises from semimembranosus and runs laterally across the back of the knee. It is a neighbour of this ligament rather than part of it.

The medial head of gastrocnemius arises just behind the femoral attachment, which is why the gastrocnemius tubercle is the landmark.

Blood Supply and Innervation

Supply is from the medial superior and medial inferior genicular arteries, reaching it through the capsule it is part of.

Being capsular, it shares the vascularity of the posteromedial capsule, which is better than that of an intra-articular ligament and less than that of the superficial collateral ligament.

Nerve supply is from the posterior and medial articular branches, with free nerve endings accounting for posteromedial pain in an extension injury.

Function

Through the last 30 degrees of extension it carries a rising share of the valgus restraint and the large majority of the internal rotation restraint.

Its reciprocal relationship with the superficial collateral ligament means the medial side is restrained across the range by two structures that are never both tight.

With semimembranosus tensioning it, the ligament is partly dynamic. A knee with a working semimembranosus behaves better than the static anatomy predicts. 2.9.6 covers that.

Clinical Relevance

Test valgus at 0 degrees and at 20 degrees and compare. The pair of findings separates this ligament from the collateral, and 4.12 covers the examination.

Expect a knee with an injury here to feel unstable going into extension rather than in mid flexion, because that is when the structure should be tight.

Consider the meniscal attachment when an injury here is found. The posterior horn moves with this ligament.

Identify the gastrocnemius tubercle before the ligament in any posteromedial approach. The landmark is reliable and the tissue planes are not.

FRCS Level

Relations

The ligament has no posterior border of its own. It becomes the posteromedial capsule, so the question at operation is not where it ends but how far forward the useful tissue extends.

Its femoral attachment and the medial head of gastrocnemius share the same small area of bone. A fixation point placed for one can compromise the other.

The meniscal attachment of the central arm means a repair that advances the ligament also moves the posterior horn, which is a second effect of a single step.

Function

Treat it and the superficial collateral ligament as a reciprocal pair, in the way 2.8.2 and 2.8.3 treat the cruciate bundles. Restoring one restores restraint over part of the range.

Anteromedial rotatory instability depends on this ligament more than on the collateral, because rotation near extension is its job. 3.9.7 carries the mechanics.

Its dynamic tensioning by semimembranosus means a static reconstruction reproduces less than the whole function, which is an anatomical statement rather than an argument against doing it.

Clinical Relevance

Assess rotation at full extension when a medial injury is being staged. This structure is invisible to a valgus test at 20 degrees.

Plan the femoral point from the gastrocnemius tubercle, and check it against the collateral ligament's point rather than measuring from the epicondyle alone.

Consider the posterior horn when advancing or reattaching the ligament, because the central arm carries it.

Protect the medial head of gastrocnemius origin when working at the femoral attachment. The two share the bone.

Avoid treating a posteromedial repair as equivalent to a reconstruction of this ligament. The tissue is capsular, and capsular tissue stretched once has already failed.