Rotational Stability

For Medical Students

The Principle

A long ligament running down a joint resists the joint opening. A short one attached close to the joint line resists the bones turning on each other.

Length is the reason. A long structure changes length little when the bones rotate, so it is not loaded by rotation.

So a joint needing both coronal and rotational control needs structures of both kinds.

At the Knee

The deep medial collateral ligament is short and attaches to the meniscus and the tibia close to the joint line. It resists rotation. 2.9.2 covers the anatomy.

The superficial medial collateral ligament is long and spans from the femoral epicondyle to the tibia below the joint line. It resists valgus. 3.9.3 covers it.

The posterior oblique ligament and the posteromedial capsule add rotational restraint near extension. 3.9.5 covers the posterior oblique ligament.

External rotation of the tibia is the motion the medial structures resist, and internal rotation is resisted more laterally. 3.2.4 covers external rotation.

What Changes It

Flexion angle, which changes which structure is taut.

Which layer is injured, since the layers do different jobs.

The anterior cruciate, which shares rotational restraint. 3.7.7 covers that.

Muscle activity, particularly the semimembranosus and the medial hamstrings. 2.9.5 covers the semimembranosus.

Clinical Relevance

Rotational laxity on the medial side is the finding that matters, and it is easier to miss than valgus laxity. Section 4 covers the tests.

Anteromedial rotatory instability is the pattern it produces, and it has its own mechanics. 3.9.7 covers it.

A superficial ligament injury alone does not produce rotational laxity, which is why the finding implicates the deeper structures.

Repairing or reconstructing the medial side has to address rotation as well as valgus. 3.9.8 covers the construct.

MRCS Level

The Principle

Rotational restraint depends on a structure's orientation relative to the axis of rotation. A structure running parallel to the axis is not loaded by rotation about it.

The medial structures lie at different distances and orientations, so their rotational contributions differ even where their valgus contributions are similar.

Rotatory laxity is a coupled motion, so it is harder to apportion than a pure one. 3.2.9 covers coupling.

At the Knee

Conventional teaching gives the deep medial collateral ligament and the posteromedial capsule the main medial contribution to restraining external rotation, with the superficial ligament contributing less.

The posterior oblique ligament's contribution is greatest near extension, where it is taut and the capsule with it. 3.9.5 covers it.

The anterior cruciate contributes to rotational restraint, so a combined injury produces a larger rotatory laxity than either alone. 3.9.10 covers combined injury.

Anteromedial rotatory instability is the clinical pattern: anterior translation of the medial tibial plateau with external rotation. 3.9.7 covers it.

What Changes It

Flexion angle, through which structures are taut.

Which layer and which structure is injured.

Cruciate integrity, which shares the restraint.

Meniscal attachment integrity, since the deep layer attaches to the meniscus. 3.6.9 covers roots.

How It Is Measured

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

The anteromedial drawer and the dial test assess it clinically, with the second also testing the lateral side. Section 4 covers the tests.

Rotatory laxity has no instrument with the reproducibility of a valgus stress radiograph, which limits quantification.

Examination under anaesthetic is where the rotational findings are most reliably obtained.

Clinical Relevance

Assess external rotation as well as valgus on the medial side.

Expect a larger rotatory laxity where a cruciate is also injured.

Treat the absence of a reproducible instrument as a reason to examine under anaesthetic.

Consider the meniscal attachment, since the deep layer attaches to it.

FRCS Level

At the Knee

Treat rotational laxity as the finding that decides whether a medial injury is operative, and valgus laxity as the finding that is easier to obtain.

Conventional teaching holds that a medial injury with rotatory laxity, or one that fails to heal, is the operative subgroup, and that isolated valgus laxity usually is not. Section 6 covers management.

In reconstruction, addressing rotation means reconstructing the posterior oblique ligament or the deep layer in addition to the superficial, which is why anatomic reconstruction is specified. 3.9.8 covers the construct.

What Changes It

Which structures are injured and whether the corner is involved.

Concurrent cruciate reconstruction, which shares the rotational restraint.

Alignment, which loads the reconstruction. 3.13 covers alignment.

Chronicity, since a chronically rotationally lax knee has stretched its remaining restraints.

How It Is Measured

Examination under anaesthetic with the knee at several angles is the practical assessment.

Valgus stress radiography quantifies 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 medial rotatory laxity reproducibly.

Clinical Relevance

Decide the operative question on rotational rather than coronal laxity.

Reconstruct the posterior oblique ligament or deep layer where rotation is lax.

Examine under anaesthetic, since no instrument substitutes.

Correct alignment where it would load the reconstruction.