Lateral Collateral Ligament (LCL) Injuries

The lateral collateral ligament is the primary static stabiliser of the knee against varus stress, and it works in concert with the structures of the posterolateral corner. Injury to it is uncommon in isolation but frequently occurs as part of a complex multi-ligament knee injury.

Who and how. The typical patient is a male aged 20-40 hurt in a contact sport (rugby, AFL, American football) or skiing. The mechanism is a varus force to the weight-bearing knee, often with rotation.

Why the corner matters. The LCL cannot be considered in isolation. Truly isolated injuries are rare; injury to the lateral structures almost always involves the posterolateral corner, and most lateral-sided injuries occur with PLC and/or cruciate damage. Failing to identify and treat the PLC leads to residual instability, and untreated posterolateral corner injury is the most common cause of ACL and PCL reconstruction failure.

The ligament. The LCL runs from the lateral femoral epicondyle, where its origin lies anterior and distal to that of popliteus, to the lateral aspect of the fibular head, where its insertion is conjoint with the biceps femoris tendon. It is a round, cord-like structure roughly 60mm long and 5-8mm wide.

Extra-articular. Unlike the MCL, the LCL is not attached to the capsule or the meniscus: the popliteus tendon separates it from the joint, and the lateral meniscus has popliteomeniscal fascicles instead. This is why an isolated LCL injury does not cause meniscal damage, whereas the MCL is intimately related to the medial meniscus.

Neighbours at risk. The common peroneal nerve wraps around the fibular neck, directly adjacent to the LCL insertion, and passes 10mm posterior to the biceps tendon insertion. The lateral inferior genicular artery runs beneath the ligament.

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The posterolateral corner. The PLC is described in three layers:

• Layer 1, superficial: iliotibial band and biceps femoris tendon
• Layer 2, middle: lateral retinaculum and patellofemoral ligaments
• Layer 3, deep: LCL (fibular collateral ligament), popliteus muscle and tendon, popliteofibular ligament, arcuate ligament complex, fabellofibular ligament (when a fabella is present) and the lateral capsule

The triad. The core of the corner is the LCL, popliteus and the popliteofibular ligament. The LCL is the static varus restraint; popliteus is a dynamic stabiliser that resists external rotation and anchors the corner; the popliteofibular ligament connects popliteus to the fibular styloid and is critical to PLC stability. Injury to all three produces significant posterolateral rotatory instability.

Biomechanics. The LCL is the primary varus stabiliser at all flexion angles, providing 55% of varus restraint at 5° and 69% at 25°, with its maximum constraint at 30°, where the cruciates relax. The secondary stabilisers are the cruciates (especially the PCL) at full extension, the popliteus complex, the ITB and biceps femoris.

Why the test is done at two angles. At 0° the ACL and PCL are taut and provide secondary varus restraint; at 30° they relax and the LCL becomes the isolated primary restraint. Opening at 0° therefore indicates a combined LCL plus cruciate injury, usually involving the PCL, while opening only at 30° suggests an isolated LCL injury.

Mechanism. Ask what direction the force came from and whether there was contact. The patterns that tear the lateral side:

• Varus force to the weight-bearing knee (a tackle from the medial side)
• Non-contact hyperextension with a varus moment
• Dashboard injury with the knee flexed and externally rotated
• Twisting injury with the foot planted

Symptoms. Lateral knee pain that is worse with varus stress, a feeling of instability especially on pivoting or cutting, and the knee giving way in combined injuries. A pop or snap at the time of injury is reported, though less commonly than with an ACL tear.

Questions that change management. Was there immediate swelling, which suggests cruciate involvement? Could the patient weight-bear afterwards? Any previous knee injury, and what are the sport and activity demands?

Inspection. Look for lateral ecchymosis; bruising that is posterolateral suggests the corner. An effusion is an intra-articular sign and indicates cruciate involvement. Watch the patient walk.

Palpation. Work along the structures in turn:

• The LCL along its course from epicondyle to fibular head
• The fibular head, where tenderness suggests avulsion
• The lateral joint line
• The peroneal nerve at the fibular neck
• The popliteal fossa for popliteus

Special tests.

1. Varus stress test, the most important. Stabilise the femur, apply a varus force at the ankle and test at 0° and 30° of flexion, comparing with the other knee. Grade the opening as I (under 5mm), II (5-10mm) or III (over 10mm) and note the endpoint: firm means a partial tear, soft or absent means a complete one.
2. Dial test. Test external rotation at 30° and 90° of flexion and read the asymmetry against the contralateral side; performing it prone as well as supine (the heel-height / prone versus supine nuance) makes the foot-thigh angle easier to read. Over 10° of asymmetry at 30° only indicates an isolated PLC injury; over 10° at both 30° and 90° indicates a combined PLC plus PCL injury.
3. Posterolateral drawer. At 90° of flexion, apply a posterior force with external rotation. A positive test indicates PLC injury.
4. Reverse pivot shift. Extend the knee from flexion with valgus and external rotation. A reduction clunk indicates PLC laxity and posterolateral rotatory instability.
5. External rotation recurvatum test (Hughston). With the patient supine and the knees extended, lift both feet off the bed by the great toes. In a PLC-deficient knee the tibia drops into hyperextension, varus and external rotation relative to the other side. It is most positive when the PLC injury is combined with an ACL injury and is a useful screening sign for significant posterolateral and combined instability.
6. Varus thrust on gait. A dynamic sign of functional PLC deficiency: the knee thrusts laterally into varus at heel-strike and stance. Its presence, especially in chronic injury, signals that ligament reconstruction alone will fail without addressing alignment, and it flags the chronic, malaligned knee that needs an osteotomy.

Neurovascular examination. Peroneal nerve assessment is mandatory in every lateral knee injury. Test and record:

• Motor: ankle dorsiflexion (tibialis anterior, deep peroneal), great toe extension (EHL, deep peroneal) and ankle eversion (peronei, superficial peroneal)
• Sensory: the first web space (deep peroneal) and the dorsolateral foot (superficial peroneal)
• Vascular: dorsalis pedis and posterior tibial pulses, capillary refill, and an ABI if there is any concern about vascular injury

Peroneal nerve injury complicates around 25% of knee dislocations with posterolateral disruption (Niall et al, 2005), and recovery is frequently incomplete.

Differential Diagnosis of Lateral Knee Instability/Pain

Radiographs. Start with AP, lateral and skyline views; add bilateral weight-bearing views if the injury is chronic and stress views if the diagnosis is uncertain. The findings to look for:

• Arcuate sign - avulsion of the fibular styloid. It is highly specific for posterolateral corner injury (if you see it, the corner is torn) but not sensitive, and most PLC injuries show no avulsion at all. A normal radiograph is not reassurance: Pacheco's series missed 72% of these injuries at first presentation.
• Segond fracture - a lateral tibial avulsion of the anterolateral capsule that suggests ACL injury
• Lateral capsular avulsion
• Fibular head fracture
• Varus alignment in chronic deficiency

Stress radiographs. A varus stress view at 20° of flexion with side-to-side comparison is the objective measure of lateral compartment gapping, and the one investigation that separates an isolated ligament tear from a whole corner. There are two thresholds, not one (LaPrade 2008, clinician-applied stress): an increase of about 2.7mm indicates an isolated fibular collateral ligament tear, and about 4.0mm a complete grade III posterolateral corner injury. Quote only the 4mm figure and a complete isolated FCL rupture gets reported as normal. Under a standardised 12-Nm moment rather than a clinician's hand the increases are smaller, 2.1mm and 3.4mm, so record which method produced the number.

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MRI. The gold standard for soft-tissue assessment, with a sensitivity of 90-95% for complete LCL tears and somewhat lower for partial tears. The LCL itself shows increased T2 signal (oedema or tear), discontinuity in a complete tear, a bony avulsion at the fibula or epicondyle, or a wavy appearance in chronic elongation. Read the rest of the scan for the company the ligament keeps:

• PLC structures: popliteus tendon and muscle, popliteofibular ligament, arcuate ligament and lateral capsule, and the bone bruise pattern (lateral femoral condyle and medial tibial plateau = a PLC mechanism)
• Associated injuries: PCL and ACL status, the menisci, cartilage, and signal change in the peroneal nerve

Vascular and nerve studies. Where there is concern about the circulation, an ankle-brachial index under 0.9 is abnormal, CT angiography follows any suspicion of vascular injury, and duplex ultrasound is the alternative. EMG and nerve conduction studies are indicated if peroneal nerve injury is suspected, timed at 3 weeks after injury to allow for wallerian degeneration.

Examination under anaesthesia. Indicated when pain precludes an adequate examination, and often combined with arthroscopy for intra-articular assessment. Document every stability finding carefully while the patient is asleep.

Who. Non-operative treatment suits all grade I injuries, most grade II injuries, selected isolated grade III injuries, elderly or low-demand patients, and those with significant medical comorbidities.

Treatment decisions rest on injury severity, the associated structures and the patient's demands. A knee that opens at 0° has a combined LCL and cruciate injury and cannot be managed non-operatively.

Decision TreeTiming Principles

Grade I Isolated

Initial Assessment

Under 5mm varus, firm endpoint

Management

Functional treatment, early ROM

Timeline

Return 2-4 weeks

Grade II Isolated

Initial Assessment

5-10mm varus, endpoint present

Management

Hinged brace 4-6 weeks, PT

Timeline

Return 6-12 weeks

Grade III Isolated

Initial Assessment

Over 10mm varus at 30° only

Management

Trial bracing 6-8 weeks, reassess

Timeline

Surgery if persistent laxity

Grade III + PLC

Initial Assessment

Varus + external rotation asymmetry

Management

Surgical reconstruction (LCL + PLC)

Timeline

Surgery within 2-3 weeks ideal

Combined + Cruciate

Initial Assessment

Varus at 0° AND 30°, + cruciate tests

Management

Multi-ligament reconstruction

Timeline

Staged or single-stage, surgeon preference

LCL Injury Management Decision Tree

Injury Pattern	Initial Assessment	Management	Timeline
Grade I Isolated	Under 5mm varus, firm endpoint	Functional treatment, early ROM	Return 2-4 weeks
Grade II Isolated	5-10mm varus, endpoint present	Hinged brace 4-6 weeks, PT	Return 6-12 weeks
Grade III Isolated	Over 10mm varus at 30° only	Trial bracing 6-8 weeks, reassess	Surgery if persistent laxity
Grade III + PLC	Varus + external rotation asymmetry	Surgical reconstruction (LCL + PLC)	Surgery within 2-3 weeks ideal
Combined + Cruciate	Varus at 0° AND 30°, + cruciate tests	Multi-ligament reconstruction	Staged or single-stage, surgeon preference

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Acute, under 3 weeks. The best window for primary repair: tissue quality still allows direct suturing, and augmentation with an internal brace can be considered. Acute repair or reconstruction within 2-3 weeks has better outcomes than delayed surgery.

Subacute, 3-6 weeks. Repair is still possible if the tissue quality is adequate, but scarring increasingly interferes with reduction, so assessing tissue viability becomes critical.

Chronic, over 6 weeks. Primary repair is no longer possible and reconstruction with a graft is required, staged if several ligaments are involved.

Non-operative. Grade I injuries return to sport in 100% of cases with no residual laxity, and grade II in 95% or more with minimal residual laxity. Isolated grade III injuries do satisfactorily with bracing in 70-80%, and the remaining 20-30% need delayed surgery.

Surgical. By procedure:

What shifts the result. Acute surgery, an isolated injury, a young patient and anatomic technique favour a good outcome; chronic injury, multi-ligament involvement, varus alignment and nerve injury work against it.

Global epidemiology. Isolated LCL injury is rare; lateral-sided injuries usually involve the posterolateral corner and are frequently part of multiligament injury or frank knee dislocation. PLC injury is commonly missed at first presentation - Pacheco et al found it was unrecognised in 72% of referred cases, with a mean diagnostic delay of 30 months (J Bone Joint Surg Br, 2011). In knee dislocation, common peroneal nerve palsy complicates around 25% and is associated with posterolateral disruption (Niall et al, 2005). Sport-related cohorts confirm that lateral and bicruciate patterns carry a worse return-to-play prognosis than medial-sided injuries (Bakshi et al, Sports Health, 2018).

Guidance across major bodies. No society has published a high-level (Level I) standalone guideline for isolated LCL/PLC injury; recommendations are consensus-based and consistent internationally.

Registry evidence. Joint-replacement registries (AOANJRR, NJR, AJRR) do not capture isolated ligament reconstruction, so there is no national LCL/PLC implant-survival dataset; the evidence base remains cohort studies and biomechanical work. Multiligament and dislocation cohorts (including military and professional-sport series) provide the best available outcome data.

Practice variation. Surgeons increasingly favour anatomic two-graft reconstruction (after LaPrade) over older single-graft (Larson) techniques and over primary repair, reflecting Stannard's finding of higher failure with repair. Timing is convergent internationally: acute injuries are best addressed within roughly 2-3 weeks while primary repair/augmented repair remains feasible, with reconstruction reserved for chronic deficiency.