LCL

For Medical Students

What It Shows

The lateral collateral ligament is a discrete cord running from the lateral femoral epicondyle to the fibular head. 2.10.1 covers the LCL anatomy.

It is superficial and separate from the capsule, so ultrasound shows it along its whole length. 5.4.3 covers the lateral knee.

It matters because it runs obliquely, so following it requires continuous reorientation and an oblique beam darkens it. 5.2.10 covers anisotropy.

How It Is Acquired

Start at the fibular head and follow the cord proximally to the femoral epicondyle. 5.4.3 covers the lateral knee.

Reorient the probe continuously, because the ligament's direction changes along its course. 5.2.10 covers anisotropy.

Distinguish it from the biceps femoris tendon, which converges on the same landmark. 2.10.5 covers the biceps femoris.

Apply gentle varus while scanning where laxity is the question. 5.4.15 covers dynamic instability.

Assess the common peroneal nerve at the same sitting, since it is adjacent. 2.10.6 covers the common peroneal nerve.

What Normal Looks Like

A discrete bright cord from the femoral epicondyle to the fibular head. 2.10.1 covers the LCL anatomy.

A consistent thickness along its course.

Separation from the biceps femoris tendon at the fibular head. 2.10.5 covers the biceps femoris.

No gapping on gentle varus. 5.4.15 covers dynamic instability.

What Abnormal Looks Like

Thickening with loss of the fibrillar pattern. 5.10.1 covers lateral collateral injuries.

Discontinuity, which indicates a complete tear. 5.10.1 covers lateral collateral injuries.

A bony fragment at the fibular head attachment. 5.3.14 covers avulsion injuries.

Gapping on varus with the cord not restraining. 5.4.15 covers dynamic instability.

MRCS Level

How It Is Acquired

In a viva this is the ligament to name when asked which structure anisotropy affects most on the lateral side.

The expected points are the fibular head start, the reorientation, the biceps distinction, the dynamic varus, and the nerve.

Say that an isolated injury to this ligament is uncommon. 5.10.3 covers posterolateral corner injuries.

Say that anisotropy along the oblique course is the main pitfall. 5.2.10 covers anisotropy.

Say that the corner as a whole needs magnetic resonance imaging. 5.10.3 covers posterolateral corner injuries.

What Abnormal Looks Like

The reasoning expected is that this ligament is the primary varus restraint in extension, so an isolated injury produces varus laxity without rotational laxity. 3.10.3 covers the lateral collateral function.

Varus laxity with rotational laxity indicates the corner rather than the ligament, which is the distinction that changes the operation. 3.10.6 covers the posterolateral corner function.

An isolated injury is uncommon, so finding one should prompt a search for the rest of the corner. 5.10.3 covers posterolateral corner injuries.

The oblique course means the probe angle changes along the ligament, and a constant angle produces a falsely dark segment. 5.2.10 covers anisotropy.

How Good It Is

Ultrasound images this ligament completely and does not establish whether the corner is intact.

Its best contribution is confirming or excluding a discrete injury to this specific cord. 5.10.1 covers lateral collateral injuries.

It is limited because the clinically important question is usually about the corner, whose deeper components are not all reachable. 5.10.3 covers posterolateral corner injuries.

No figures appear for accuracy on this page, under decision 028, and no thickness or gapping values appear either: those are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level this ligament is rarely the whole question.

A discrete isolated injury with varus laxity and no rotational laxity is the case where this assessment is sufficient. 4.13.2 covers the LCL assessment.

Varus laxity with external rotation laxity means the corner, which needs a magnetic resonance study and changes the operation. 5.10.3 covers posterolateral corner injuries.

A fibular head avulsion is a direct finding with a surgical answer and is visible here. 5.3.14 covers avulsion injuries.

The adjacent nerve should be assessed whenever a lateral injury is found. 2.10.6 covers the common peroneal nerve.

Where It Misleads

The commonest error is reading anisotropy along the oblique course as injury. 5.2.10 covers anisotropy.

The biceps femoris tendon is mistaken for the ligament. 2.10.5 covers the biceps femoris.

An isolated injury is reported without the corner being considered. 5.10.3 covers posterolateral corner injuries.

The nerve is not assessed. 2.10.6 covers the common peroneal nerve.

A normal study is taken to exclude rotational instability. 3.10.6 covers the posterolateral corner function.

Fellowship Level

Where It Misleads

At fellowship level this ligament demonstrates that imaging a structure completely is not the same as answering the clinical question.

Ultrasound sees this cord end to end, better than it sees almost anything else on the lateral side, and the decision rests on the corner's rotational function rather than on this cord's continuity. 3.10.6 covers the posterolateral corner function.

That makes a complete and confident assessment of the wrong variable, which is a subtler failure than a missed finding. 5.10.3 covers posterolateral corner injuries.

Dynamic varus assessment is available and unquantified, so the functional observation that would bridge the gap is an impression. 5.4.15 covers dynamic instability.

Magnetic resonance imaging assesses the corner's components directly and still performs inconsistently on the deeper ones, so neither modality closes the question. 5.10.3 covers posterolateral corner injuries.

The useful conclusion is to use this study for a named structure and to put the functional question to the examination. 4.13.2 covers the LCL assessment.