LCL Function

For Medical Students

The Principle

A discrete cord ligament resists whatever stretches the straight line between its two attachments.

If flexing a joint brings those two attachments closer, the ligament slackens with flexion.

A ligament that is slack in flexion is not a restraint in flexion, whatever its strength.

At the Knee

The lateral collateral ligament runs from the lateral femoral epicondyle to the head of the fibula, outside the joint capsule. 2.10.1 covers the anatomy.

It is the primary restraint to varus near full extension. 3.10.1 covers varus stability.

It slackens with flexion, because the femoral attachment moves closer to the fibula as the knee bends.

It is not attached to the meniscus and sits outside the capsule, which is the opposite arrangement from the medial side's deep layer. 3.9.4 covers that layer.

What Changes It

Flexion angle, which it slackens with.

Varus load, which is what tensions it.

Whether it is avulsed from the fibula or the femur, which affects the repair.

Alignment, since a varus limb loads it in stance. 3.13 covers alignment.

Clinical Relevance

Varus testing near extension loads this ligament, and flexing the knee takes it out of the test. Section 4 covers the tests.

It heals less reliably than the medial collateral, which is part of why lateral injuries are treated differently. Section 6 covers management.

Being extrasynovial and extracapsular, it has a poorer blood supply than some other ligaments, which bears on healing.

Isolated injury is uncommon. A significant lateral collateral injury usually comes with corner involvement. 3.10.6 covers the corner.

MRCS Level

The Principle

Its mechanical character follows from being a discrete extracapsular cord: it is loaded by coronal opening, slackens with flexion, and contributes modestly to rotation because it runs close to the rotational axis.

That is a different arrangement from the medial side, where a long superficial ligament and a short capsular layer divide the coronal and rotational jobs. 3.9.3 covers that ligament.

The lateral equivalent of the medial deep layer is the corner, which contains a muscle and is not a simple capsular sheet. 3.10.6 covers the corner.

At the Knee

Conventional teaching gives the lateral collateral its greatest share of varus restraint at or near full extension and a reducing share through flexion as it slackens.

Its femoral attachment is slightly proximal and posterior to the lateral epicondyle, which matters for reconstruction tunnel placement. 2.10.7 covers the surgical anatomy.

Its fibular attachment is on the lateral aspect of the fibular head, distal to the popliteofibular ligament's attachment at the styloid. 2.10.3 covers that ligament.

It contributes to external rotation restraint, less than the popliteus complex does. 3.10.2 covers external rotation.

What Changes It

Flexion angle, through slackening.

Avulsion site, femoral or fibular, which affects repair options.

Corner integrity, since the structures are usually injured together.

Alignment and load.

How It Is Measured

Sectioning studies with varus and rotational loads apportion its share by angle.

Varus stress radiography measures the functional consequence of its loss.

Magnetic resonance imaging demonstrates it and identifies the avulsion site. 5.10 covers imaging.

Its discrete cord structure makes selective sectioning easier here than for the posteromedial capsule.

Clinical Relevance

Test varus near extension, where this ligament is the restraint.

Identify the avulsion site, since it affects whether repair is possible.

Expect corner involvement alongside a significant injury.

Place a reconstruction's femoral attachment proximal and posterior to the epicondyle.

FRCS Level

At the Knee

Treat this ligament as the easier half of a lateral reconstruction and the corner as the harder half. Its attachments are discrete and identifiable, which makes anatomic placement achievable. 3.10.8 covers the construct.

Conventional teaching holds that reconstructing it alone leaves rotational laxity, which is the lateral equivalent of reconstructing the superficial medial ligament alone. 3.9.3 covers that comparison.

Its poor healing is the reason lateral injuries are repaired or reconstructed more often than medial ones, and the reason a chronic lateral laxity rarely improves with time. Section 6 covers management.

What Changes It

Attachment positions in a reconstruction, which set the length change pattern.

Whether the corner is reconstructed alongside.

Alignment, which loads the construct. 3.13 covers alignment.

Graft choice and fibular bone quality for fixation.

How It Is Measured

Varus stress radiography before and after measures the coronal result.

Robotic testing compares reconstructions for restored varus and rotational restraint.

Postoperative imaging locates the tunnels against footprint maps.

No measurement confirms the graft reproduces the native length change pattern.

Clinical Relevance

Place the attachments anatomically, since they are identifiable here.

Reconstruct the corner alongside, since this ligament alone leaves rotation lax.

Correct varus alignment where it would load the construct.

Expect a chronic lateral laxity not to improve with time.