Assessment at 0 Degrees

For Medical Students

What It Tests

Valgus stress applied with the knee in full extension tests whether anything beyond the superficial medial ligament is torn.

In full extension the medial restraints are joined by the posterior oblique ligament, the posteromedial capsule and both cruciates, all of which resist valgus with the knee straight. 3.9.6 covers the posteromedial corner.

A knee with an isolated superficial ligament tear therefore feels firm in extension, because the structures that take over in extension are intact. 3.9.3 covers that ligament.

How It Is Performed

Lay the patient supine with the leg flat and the knee fully straight, and make sure it is actually straight rather than resting in a few degrees of flexion.

Hold the ankle in one hand and place the other along the lateral side of the knee as the fulcrum, with your fingers on the medial joint line.

Apply a valgus force and feel the joint line open under your fingers.

Control rotation of the limb, because an externally rotated leg opens medially without any ligament being torn and is the commonest false positive here.

Compare with the other knee in full extension before interpreting anything. 4.1.2 covers the principle.

What a Positive Means

Opening in full extension indicates injury beyond the superficial medial ligament.

In practice that means one of two things: the posteromedial corner, including the posterior oblique ligament, or a cruciate alongside the medial injury. 4.12.5 covers the corner.

A knee that opens in extension and also in flexion is a severe medial injury, and that combination is what takes the medial side from a healing problem to a surgical one. 4.12.3 covers the flexed test.

A knee that is firm in extension and lax at thirty degrees is the common injury, and the one that is treated in a brace. 4.12.1 covers the pair of tests.

Gross opening in extension with a valgus deformity on standing is a different severity again and raises the question of whether the knee was dislocated. 4.11.7 covers that assessment.

MRCS Level

How It Is Performed

In a viva this is the angle candidates omit, and omitting it is the difference between describing a test and performing an assessment.

The expected points are a genuinely extended knee, controlled rotation, the fingers on the joint line, the endpoint, and the comparison.

Say that you would confirm the knee is in full extension rather than assume it, because a few degrees of flexion relaxes the posterior structures and reintroduces the laxity the test is designed to exclude.

Say that a knee that cannot reach full extension cannot have this test performed, and that the finding is then absent rather than negative.

Say that you would examine both cruciates in the same sitting, since a positive result at this angle makes a cruciate injury likely. 4.10.4 covers the Lachman test.

What a Positive Means

The reasoning expected is that valgus restraint in extension is shared, so opening in extension requires more than one structure to be deficient. 3.9.9 covers the angles.

The posterior oblique ligament is the structure that tightens in extension and the one whose loss converts a firm endpoint into a soft one there. 3.9.5 covers its function.

An anterior cruciate rupture contributes to opening in extension, which is why the classic combined injury presents with laxity at both angles rather than at thirty degrees alone. 3.9.10 covers it.

Opening in extension with increased external rotation of the tibia at thirty degrees of flexion describes an anteromedial rotatory pattern rather than simple valgus laxity, and that pattern is graded differently. 4.12.6 covers it.

How Good It Is

The extension test is the single most informative part of the medial examination and the part most often left undone.

It is specific rather than sensitive. A firm endpoint in extension reliably excludes the severe injury, and a soft one reliably indicates it, but a mild posteromedial injury can pass unnoticed.

It is more reproducible than the rotational tests and less reproducible than the thirty degree test, because full extension is harder to guarantee than thirty degrees of flexion.

It is reliable in the acute knee, which matters because the acute knee is when the decision about bracing is made.

No figures appear for any of these comparisons, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level this angle is the finding that sorts the medial injuries into the ones that heal and the ones that need an operation.

A knee that is firm in extension is a brace and a rehabilitation programme, whatever the thirty degree grade says, and that is most medial injuries.

A knee that opens in extension needs the posteromedial corner assessed and the cruciates assessed, and the treatment follows from what that assessment finds rather than from the valgus grade. 4.12.5 covers the corner.

Where a cruciate reconstruction is planned alongside a medial injury, the examination in extension repeated after six weeks of bracing is what decides whether the medial side is addressed at the same operation. That repeat examination is the single most useful sequence in this chapter.

Stress radiographs in extension quantify what the hands grade, and they are worth obtaining where the plan turns on whether the medial side is being left alone.

Where It Misleads

The commonest error is not performing the test, which leaves the severe injury indistinguishable from the common one.

The second is performing it on a knee resting in slight flexion, which reintroduces the posterior restraint and reads as firm.

The third is reading opening in extension as a severe collateral injury rather than as a sign that something else is torn. The finding points away from the medial side as often as it points at it.

The fourth is examining the knee once. A medial side that opens in extension at two weeks and is firm at six weeks has healed, and that trajectory is the information the decision needs. 4.1.8 covers the record that makes it possible.

Fellowship Level

Where It Misleads

In referred practice opening in extension is usually found in a knee that has already been treated as a simple medial injury, and the question is what was missed.

The commonest answer is the posterior oblique ligament, which does not heal as reliably as the superficial ligament and is the structure a persistent valgus complaint sits on. 3.9.5 covers it.

The second commonest is a cruciate reconstruction performed into an uncontrolled medial side, where the graft takes load it does not resist and stretches. The examination in extension is what should have prevented it.

A knee with a valgus thrust in gait and opening in extension is a mechanical problem as much as a ligament one, and alignment is assessed before any further soft tissue surgery. 4.11.5 covers dynamic assessment.

In a chronically lax medial side the endpoint in extension may be firm because of scar rather than ligament, which holds under examination and fails under load. A patient whose knee is firm on the couch and gives way on the pitch is describing that. 4.23.2 covers elite athletes.

The test is also the one most often recorded without the knee having been in extension, so a documented firm endpoint from elsewhere is worth repeating rather than trusting. 4.23.7 covers the medicolegal setting.