POL Function

For Medical Students

The Principle

A structure running obliquely across the back of a joint is stretched when the joint straightens and slackens when it bends.

That makes it a restraint near full extension and little use in flexion.

If a joint has one restraint for most of the range and another near extension, testing in the first structure's range is how you isolate it.

At the Knee

The posterior oblique ligament is a thickening of the posteromedial capsule running from the femur obliquely downwards and forwards to the tibia and meniscus. 2.9.3 covers the anatomy.

It is taut near full extension and slack in flexion, which is the reciprocal of the superficial ligament's behaviour. 3.9.3 covers that ligament.

Near extension it contributes substantially to valgus and to rotational restraint. 3.9.9 covers the division through range.

That is why valgus testing is performed at 20 to 30 degrees: flexing the knee slackens this structure and isolates the superficial ligament.

What Changes It

Flexion angle, which it responds to more sharply than the superficial ligament.

Whether the posteromedial corner is injured as a unit. 3.9.6 covers the corner.

Semimembranosus activity, since the muscle tensions the capsule dynamically. 2.9.5 covers it.

Chronicity, since a chronically lax medial side has stretched this structure.

Clinical Relevance

Valgus opening in full extension implicates this structure and the corner, which is why testing in extension is informative. Section 4 covers the tests.

It is the structure most often implicated when a medial injury behaves worse than an isolated superficial tear should.

Reconstruction of the medial side usually includes it, because rotation and near extension control depend on it. 3.9.8 covers the construct.

Knowing it is slack in flexion explains the whole examination sequence.

MRCS Level

The Principle

The reciprocal behaviour is what gives the medial side continuous coverage across the arc, in the same way the cruciate bundles do for their directions. 3.7.4 covers that comparison.

It also means a single structure's apportionment figure is uninformative without an angle, and that the examination sequence is a sampling strategy rather than a ritual. 3.9.9 covers the division.

Its capsular nature matters: it is a thickening rather than a discrete cord, so its boundaries are defined by convention as much as by dissection.

At the Knee

Conventional teaching describes three arms of the posterior oblique ligament, superficial, central and capsular, with the central arm the most substantial. 2.9.3 covers the anatomy.

Its contribution to valgus restraint is greatest at or near full extension and falls rapidly with flexion.

It contributes substantially to restraining external rotation near extension, which is why it appears in the anteromedial rotatory instability account. 3.9.7 covers it.

The semimembranosus inserts into and tensions the posteromedial capsule, making this a dynamically reinforced structure. 2.9.6 covers dynamic stabilisation.

What Changes It

Flexion angle, as the dominant variable.

Whether the corner is injured as a unit rather than in isolation.

Semimembranosus function.

Chronic stretching in a long standing medial laxity.

How It Is Measured

Sectioning studies with valgus and rotational loads at several angles apportion its contribution.

Magnetic resonance imaging demonstrates the posteromedial capsule and its injury. 5.9 covers imaging.

Valgus stress radiography in full extension measures the functional consequence of its loss.

Its capsular nature makes selective sectioning harder than for a discrete ligament, which is a limitation in the figures.

Clinical Relevance

Test valgus in full extension as well as at 30 degrees.

Read opening in extension as implicating the posteromedial structures.

Expect apportionment figures here to be less precise than for a discrete ligament.

Consider semimembranosus function as part of the dynamic restraint.

FRCS Level

At the Knee

Treat the posterior oblique ligament as the structure whose involvement converts a medial injury from non operative to operative, and treat the examination in extension as the way that is established. 3.9.1 covers valgus stability.

Conventional teaching includes it in anatomic medial reconstruction, with a separate limb or a separate graft, because reconstructing the superficial ligament alone leaves the near extension range and the rotation less controlled. 3.9.8 covers the construct.

In a chronic medial injury it is the structure most likely to have stretched, which is why a chronic reconstruction addresses it even where the acute injury appeared to spare it.

What Changes It

Acute or chronic presentation, which changes its state.

Whether the corner was injured as a unit.

Graft availability for a second limb.

Alignment, which loads the reconstruction. 3.13 covers alignment.

How It Is Measured

Valgus stress radiography in extension measures the consequence of its loss.

Examination under anaesthetic at several angles is the practical assessment.

Robotic testing compares reconstructions that include and exclude it.

No measurement confirms a reconstructed posterior oblique limb is functioning.

Clinical Relevance

Establish its involvement by examining in extension before deciding on surgery.

Include it in anatomic reconstruction, as a separate limb or graft.

Address it in a chronic reconstruction even where the acute injury spared it.

Measure the result with stress radiography in extension as well as at 30 degrees.