Varus Stability

For Medical Students

The Principle

Pushing a joint outwards opens the lateral side and compresses the medial. The side that opens is held by what crosses it.

A structure further from the joint centre resists that opening with less force, because it has a longer lever arm. 3.4.4 covers lever arms.

The compressed side helps as well, so the restraint is shared between tension on one side and bone contact on the other.

At the Knee

A varus load pushes the knee outwards, opening the lateral side. 3.2.5 covers varus motion.

The lateral collateral ligament is the primary restraint. Conventional teaching gives it the large majority near full extension. 2.10.1 covers the anatomy.

Its contribution falls with flexion, because the ligament slackens as the knee bends. 3.10.3 covers the ligament.

The popliteus complex and the posterolateral capsule take a larger share in flexion, which is why the corner matters here. 3.10.6 covers the corner.

What Changes It

Flexion angle, which changes which structure is the restraint.

Load, since medial compartment compression adds to the restraint.

The state of the posterolateral corner. 2.10.4 covers the corner.

Alignment, since a varus limb loads the lateral structures differently. 3.13 covers alignment.

Clinical Relevance

Varus testing is performed near extension and at 30 degrees, because the structures differ between the two. Section 4 covers the tests.

Varus opening is graded in millimetres of joint line opening by convention, as on the medial side. 3.9.1 covers valgus stability.

The lateral side heals badly compared with the medial, which is why isolated lateral collateral injuries are treated more readily. Section 6 covers management.

Varus alignment and lateral laxity reinforce each other, which makes the combination progressive.

MRCS Level

The Principle

Coronal restraint is a moment problem solved by tension at a distance, as on the medial side. 3.4.6 covers external moments.

What differs laterally is the geometry beneath. The lateral tibial plateau is convex, so the compressed medial compartment provides the counterforce and the lateral surfaces offer less help. 3.5.1 covers contact area.

The other difference is that the lateral corner contains a muscle, so part of the restraint is dynamic rather than purely passive. 3.10.4 covers the popliteus.

At the Knee

Conventional teaching gives the lateral collateral ligament roughly 55 to 70 per cent of the varus restraint near full extension, with its share falling through flexion.

The popliteus tendon, the popliteofibular ligament and the posterolateral capsule contribute increasingly with flexion. 3.10.5 covers the popliteofibular ligament.

The iliotibial band contributes to varus restraint through much of the arc, and more in flexion. 2.11.1 covers the band.

Medial compartment compression provides the counterforce, so a varus load opens a loaded knee less than an unloaded one.

What Changes It

Flexion angle, as the dominant variable in the share.

Axial load, through medial compartment compression.

Corner integrity, particularly in flexion.

Alignment, since varus loads the lateral structures in stance. 3.13 covers alignment.

How It Is Measured

Sectioning studies with an applied varus moment apportion the restraint by angle, with the usual order dependence. 3.2.9 covers that.

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

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

Examination under anaesthetic matters more laterally, because the findings are subtler.

Clinical Relevance

Test varus near extension and at 30 degrees, since different structures dominate.

Use varus stress radiography where the grade decides the treatment.

Examine under anaesthetic, because lateral findings are subtler than medial ones.

Assess alignment, since varus loads the lateral structures in stance.

FRCS Level

At the Knee

Treat lateral laxity as the finding that is harder to detect and more consequential than its medial counterpart, because the structures heal badly and the alignment usually works against them.

Conventional teaching holds that isolated lateral collateral injuries are uncommon and that most clinically significant lateral injuries involve the corner, which changes both the assessment and the operation. 3.10.6 covers the corner.

Varus alignment is the environmental factor here, equivalent to slope in anterior cruciate failure. A lateral reconstruction in a varus limb carries the load that stretched the original. 3.13 covers alignment.

What Changes It

Whether the corner is involved, which is usually the case.

Alignment, which loads a deficient or reconstructed lateral side.

Concurrent cruciate injury, particularly posterior. 3.10.9 covers combined injury.

Chronicity, since the lateral structures stretch rather than heal.

How It Is Measured

Varus stress radiography quantifies the opening and settles the grade.

Examination under anaesthetic separates structural from muscular contribution.

Long leg standing radiography measures the alignment that loads the side.

Magnetic resonance imaging identifies which structures are injured. 5.10 covers imaging.

Clinical Relevance

Assume corner involvement in a clinically significant lateral injury until shown otherwise.

Measure alignment before planning a lateral reconstruction.

Use varus stress radiography rather than grading by hand where the treatment turns on it.

Expect the lateral structures to stretch rather than heal.