External Rotation Stability

For Medical Students

The Principle

Resisting a joint turning needs structures placed away from the axis it turns about, so they are stretched by the turn.

A structure running obliquely across a joint is stretched by rotation in one direction and slackened by rotation in the other.

So rotation in each direction is resisted by a different set of structures, and the two have to be tested separately.

At the Knee

External rotation of the tibia stretches the lateral structures, because it carries the fibula backwards relative to the femur. 3.2.4 covers external rotation.

The popliteus complex and the popliteofibular ligament are the main restraints, with the lateral collateral contributing. 3.10.4 and 3.10.5 cover them.

The posterior cruciate contributes at greater flexion angles, which is why the dial test is performed at two angles. 3.10.7 covers the test.

The medial structures resist internal rotation instead, which is the mirror arrangement. 3.9.2 covers medial rotational stability.

What Changes It

Flexion angle, which changes which structures are taut.

Which lateral structures are injured.

Posterior cruciate integrity, which contributes in deeper flexion. 3.8.1 covers that ligament.

Popliteus activity, since the muscle resists external rotation actively. 2.10.2 covers the tendon.

Clinical Relevance

External rotation laxity is the hallmark of a posterolateral corner injury, and it is tested rather than inferred. Section 4 covers the tests.

Testing at 30 and 90 degrees separates a corner injury from a combined corner and posterior cruciate injury. 3.10.7 covers the mechanics.

Patients describe the knee giving way into hyperextension and varus rather than into rotation, which is the corner's characteristic account.

The peroneal nerve runs close by, so a corner injury can present with a foot drop. 2.10.6 covers the nerve.

MRCS Level

The Principle

Rotational restraint is shared between structures that are taut at different angles, so the share changes through the arc and a single figure is uninformative. 3.9.9 covers the medial equivalent.

Here the sharing is between the lateral corner near extension to mid flexion and the posterior cruciate in deeper flexion, which is the basis of a two angle test.

The clinical inference is a subtraction. If rotation is lax at 30 degrees and not worse at 90, the corner alone is injured; if it is worse at 90, the posterior cruciate is involved too.

At the Knee

Conventional teaching gives the popliteus complex and the popliteofibular ligament the main restraint to external rotation at 30 degrees of flexion. 3.10.5 covers the popliteofibular ligament.

The posterior cruciate's contribution to external rotation restraint increases with flexion, which is why an increase at 90 degrees implicates it. 3.8.1 covers the ligament.

The lateral collateral contributes to rotational restraint less than to varus restraint, because it runs closer to the axis in rotation. 3.10.3 covers it.

The popliteus is also the muscle that internally rotates the tibia, so its injury removes an active restraint as well as a passive one. 3.3.6 covers the reverse screw home mechanism.

What Changes It

Flexion angle, as the dominant variable and the basis of the two angle test.

Which lateral structures are injured.

Posterior cruciate integrity.

Popliteus muscle function, which is the active component.

How It Is Measured

Sectioning studies with an applied external rotation torque apportion the restraint by angle and structure.

The dial test measures it clinically at 30 and 90 degrees. 3.10.7 covers the mechanics.

External rotation stress radiography and computed tomography based measurement quantify it in research settings.

No instrument in routine clinical use quantifies rotatory laxity, as on the medial side.

Clinical Relevance

Test at 30 and 90 degrees and read the difference rather than either value alone.

Read an increase at 90 degrees as implicating the posterior cruciate.

Expect a popliteus injury to remove an active restraint as well as a passive one.

Accept that no routine instrument quantifies this laxity.

FRCS Level

At the Knee

Treat external rotation laxity as the finding that defines the corner injury and drives the operation, and the two angle comparison as the finding that defines its company.

Conventional teaching holds that a corner injury missed alongside a cruciate reconstruction is a leading cause of graft failure, on both cruciates, which makes this assessment a routine part of cruciate surgery rather than a specialist extra. 3.10.9 covers combined injury.

Chronic corner laxity in a varus limb is the combination that defeats reconstruction, and correcting alignment is part of the operation. 3.13 covers alignment.

What Changes It

Which corner structures are injured and whether they are reconstructable.

Cruciate integrity and whether it is being reconstructed.

Alignment, particularly varus.

Chronicity, since the lateral structures stretch.

How It Is Measured

The dial test under anaesthetic at 30 and 90 degrees is the practical assessment.

Varus stress radiography measures the coronal component and not the rotational one.

Robotic sectioning work apportions the rotational restraint and is where the reconstruction targets come from.

No clinical measurement quantifies the rotatory component reproducibly.

Clinical Relevance

Perform a dial test under anaesthetic in every cruciate reconstruction.

Read the 30 and 90 degree comparison to establish what else is injured.

Correct varus alignment where it would load a corner reconstruction.

Accept that the rotatory component cannot be quantified clinically.