A lateral collateral ligament (LCL) injury of the knee is less common than an injury to the medial ligament — but it can be just as serious for an athlete. An LCL injury usually follows trauma, twisting, or a force that drives the knee inward. Recognising and treating it promptly is essential for a safe return to sport. So how do we diagnose and manage an LCL tear?
Anatomy of the LCL
The lateral collateral ligament runs along the outside of the knee, connecting the thigh bone (femur) to the fibula. Unlike the medial ligament, the LCL doesn’t attach to the joint capsule or the meniscus, which makes it a distinct stabiliser of the outer knee. Its main job is to resist inward (varus) forces and provide side-to-side stability.
The LCL is part of the posterolateral corner (PLC) — a complex of structures that also includes the popliteus tendon, the popliteofibular ligament, the arcuate ligament, and the lateral capsule. This matters because severe LCL tears often involve the PLC, which significantly affects the outlook.
Another important nearby structure is the common peroneal nerve, which wraps around the neck of the fibula. This nerve is at risk in severe LCL injuries — and during surgery — which is why it’s always checked.
Causes of an LCL sprain
- A direct blow to the inside of the knee during sport (for example, a football tackle)
- Sudden twisting with the foot planted
- Hyperextension combined with an inward force
- High-energy trauma, such as a skiing accident or a contact-sport collision
Symptoms of an LCL injury
People with an LCL sprain or tear usually report:
- Sharp pain on the outer side of the knee
- Tenderness and swelling along the ligament
- Instability, or a sense of the knee “giving way” when changing direction
- Difficulty weight-bearing, in more severe injuries
If there’s numbness or tingling on the top of the foot, or weakness lifting the foot, that suggests the peroneal nerve is involved — an important sign to report.
Assessing an LCL injury
Assessment combines careful inspection and palpation with specific tests:
- Tenderness over the ligament, between the femoral epicondyle and the fibular head
- Varus stress test — at 30° of knee bend, pain or laxity suggests an isolated LCL injury; laxity in full extension points to additional injury to the cruciate ligaments or the PLC.
- Joint-line tenderness — which may suggest an associated meniscal injury
- Neurovascular assessment — the peroneal nerve is always tested (sensation over the top of the foot, and the strength of pulling the foot up)
For higher-grade injuries, we add specific posterolateral-corner tests, such as the external recurvatum and dial tests.
Associated injuries
An isolated LCL sprain is possible, especially in lower-grade injuries. But higher-grade trauma often damages nearby structures — most importantly the posterolateral corner and the cruciate ligaments (ACL or PCL), and sometimes the lateral meniscus or the peroneal nerve. Identifying these matters matters because the management and outlook change significantly when the PLC or cruciates are involved.
Imaging of a lateral collateral ligament sprain
- MRI — the gold standard, diagnosing the LCL tear and assessing for PLC or cruciate involvement
- Ultrasound — provides a dynamic assessment of ligament integrity.
- X-ray — to exclude a fracture at the fibular head or femur (and, in adolescents, an avulsion)
Treatment of an LCL injury
Conservative treatment
Most isolated grade I and II LCL sprains are treated without surgery:
- Rest, ice, compression, and elevation (RICE) in the acute phase
- A knee brace for 2–6 weeks to protect against varus stress, starting in full extension and gradually allowing more bend
- Physiotherapy — restoring range of motion, progressive strengthening, and balance (proprioceptive) training
- A return to sport once pain-free stability and function are restored
Injection therapy
In selected cases of a low-grade LCL injury with persistent pain, a PRP (platelet-rich plasma) injection may support the healing of a partial tear.
Surgery
- Grade III LCL tears usually need surgical repair (if acute) or reconstruction with a graft.
- Combined injuries involving the PLC, ACL, or PCL almost always need surgery.y
- Surgery is also considered where chronic instability persists despite rehabilitation.
Prognosis and return to sport
- Grade I sprain — return within about3- 66 weeks
- Grade II tear — recovery typically 6–8 weeks with structured rehabilitation
- Grade III tear, with or without PLC involvement — surgical recovery of 4–6 months or longer, depending on the associated repairs
Frequently asked questions about an Lateral collateral ligament sprain
How do I know if my LCL is sprained or torn?
Both cause outer knee pain, tenderness, and swelling. A sprain (grade I–II) tends to remain stable, whereas a full tear (grade III) causes instability and a sensation of the knee giving way, especially when changing direction. A clinical examination — including the varus stress test — and an MRI grade the injury and check for associated damage.
Why is the posterolateral corner so important in an LCL injury?
Because severe LCL tears often involve the posterolateral corner, a complex of structures that stabilises the outer and back of the knee. Missing a PLC injury leads to ongoing instability and poorer results, and can compromise a cruciate ligament reconstruction. That’s why higher-grade LCL injuries are assessed carefully for PLC involvement.
Can an LCL injury affect a nerve?
It can. The common peroneal nerve wraps around the fibular neck, close to the LCL, so it’s at risk in severe injuries. Numbness or tingling on the top of the foot, or weakness lifting the foot, suggests nerve involvement — which is why the nerve is always checked, and any such symptoms should be reported.
Do all LCL injuries need surgery?
No. Most isolated grade I and II sprains heal well with a brace and rehabilitation. Surgery is generally reserved for full (grade III) tears, combined injuries involving the cruciates or posterolateral corner, and cases of chronic instability that haven’t settled with rehabilitation.
Final word from Sport Doctor London about a lateral collateral ligament sprain
An LCL injury is less common than an MCL injury, but it shouldn’t be underestimated. Accurate assessment matters — not only to grade the injury, but to identify associated damage to the cruciate ligaments or posterolateral corner. Most isolated LCL injuries respond well to conservative treatment, whereas high-grade tears often require surgical reconstruction to restore stability and enable a safe return to sport.
If you have an outer knee injury or instability, Dr Masci can assess you in London, including in-clinic ultrasound. Contact the team here or call +44 (0) 203 488 0350.
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