Valgus Stability

For Medical Students

The Principle

Pushing a joint sideways opens it on one side and compresses it on the other. The side that opens is held by whatever crosses it.

A structure on the outside of a joint resists that opening with a long lever arm, so it does the job efficiently.

The compressed side helps too. A joint pushed open on one side is held partly by the bones meeting on the other.

At the Knee

A valgus load pushes the knee inwards, opening the medial side and compressing the lateral. 3.2.6 covers valgus motion.

The superficial medial collateral ligament is the primary restraint. Conventional teaching gives it the large majority of the resisting force. 2.9.1 covers the anatomy.

The lateral compartment's bony contact resists too, which is why valgus opening is limited even when the ligament is divided.

The posterior oblique ligament takes a larger share near full extension, where it is taut. 3.9.5 covers it.

What Changes It

Flexion angle, which changes which structure is the restraint. 3.9.9 covers the contributions through range.

Load, since compression across the lateral compartment adds to the restraint.

The state of the posteromedial corner, which shares the job near extension. 2.9.4 covers the corner.

Muscle activity, since the medial muscles resist valgus dynamically. 2.9.6 covers dynamic stabilisation.

Clinical Relevance

Valgus testing is performed at 20 to 30 degrees of flexion rather than in extension, because extension recruits the other structures and hides the ligament. Section 4 covers the tests.

Opening in full extension suggests injury beyond the superficial ligament alone.

Valgus opening is graded in millimetres by convention, which is a translation describing a rotation. 3.2.1 covers the degrees of freedom.

This ligament heals well, which is why most isolated injuries are managed without surgery. Section 6 covers management.

MRCS Level

The Principle

Coronal restraint is a moment problem. A valgus moment has to be resisted by a tensile force at a distance from the joint centre, and the medial structures provide it. 3.4.6 covers external moments.

The further a restraint sits from the centre, the less force it needs for a given moment, which is why a superficial structure is the primary one.

Compression on the opposite side contributes a couple as well, so the restraint is shared between tension medially and contact laterally.

At the Knee

Conventional teaching gives the superficial medial collateral ligament roughly 50 to 80 per cent of the valgus restraint at 20 to 30 degrees of flexion, which is where it is the clear primary.

Near full extension the posterior oblique ligament and the posteromedial capsule contribute substantially, so the share falls. 3.9.9 covers the division.

The deep medial collateral ligament contributes less to valgus and more to rotational control, which is a division of labour between the layers. 3.9.4 covers it.

Lateral compartment contact provides the counterforce, which is why a valgus load in a loaded knee opens less than in an unloaded one.

What Changes It

Flexion angle, as the dominant variable in the share.

Axial load, which increases lateral compartment compression.

Posteromedial corner integrity, particularly near extension.

Previous injury and healing, since this ligament commonly heals with some residual length.

How It Is Measured

Sectioning studies with an applied valgus moment apportion the restraint by angle, with the usual sectioning order limitation. 3.2.9 covers that.

Valgus stress radiography measures joint line opening against bony landmarks and is the reproducible clinical measurement.

Clinical grading in millimetres of opening is the practical method and depends on technique and relaxation.

Examination under anaesthetic removes muscular contribution and gives the structural figure.

Clinical Relevance

Test at 20 to 30 degrees to isolate the superficial ligament.

Read opening in full extension as implicating the posteromedial structures.

Use valgus stress radiography where the grade decides the treatment.

Expect a healed ligament to be functional at a greater length than the original.

FRCS Level

At the Knee

Treat valgus laxity as the finding that is easy to measure and easy to over-read in isolation. The question that matters is whether the posteromedial structures and the cruciates are involved. 3.9.6 covers the corner.

Conventional teaching holds that opening in full extension, or opening with external rotation laxity, indicates injury beyond the superficial ligament and changes the management from non operative to operative consideration. 3.9.7 covers rotatory instability.

The ligament's healing capacity is the reason the medial side is treated differently from the lateral. It heals with length rather than failing to heal, which leaves a functional but lax structure. 3.9.3 covers the ligament.

What Changes It

Whether the posteromedial corner is involved, which is the decision point.

Concurrent cruciate injury, which changes the loads the medial side carries. 3.9.10 covers combined injury.

Alignment, since a valgus limb loads a healed medial side differently. 3.13 covers alignment.

Time, since healing changes the measured laxity over weeks to months.

How It Is Measured

Valgus stress radiography quantifies the opening and is the measurement that settles the grade.

Examination under anaesthetic separates structural from muscular contribution.

Magnetic resonance imaging localises the injury by layer and by site, which bears on healing. 5.9 covers imaging.

Serial examination captures the healing that defines this ligament's natural history.

Clinical Relevance

Test in extension as well as at 30 degrees, since extension findings change the plan.

Assess rotation, because rotatory laxity implicates the corner.

Reassess serially before committing to surgery, given the healing capacity.

Account for alignment when a healed medial side remains symptomatic.