What the LCL does
The lateral collateral ligament (LCL), also called the fibular collateral ligament (FCL), is a cord-like structure running from the outer side of the femur down to the head of the fibula, just below the knee joint. Unlike the broad, flat MCL on the inner knee, the LCL is thin, round, and sits slightly proud of the joint line — which is part of why it is easy to miss on a rushed examination.
Its main job is resisting varus stress — the force that pushes the knee outward, bowing it away from the other leg. It also acts as a secondary restraint against excessive rotation of the shin bone. The LCL rarely works alone: it is one of three primary stabilisers of the posterolateral corner (PLC) of the knee, alongside the popliteus tendon and popliteofibular ligament. This anatomical relationship matters clinically, because an LCL injury can be an isolated, self-contained problem, or it can be the visible part of a larger PLC injury — and the two are managed quite differently.
How LCL injuries happen
The LCL is injured by a varus force — a blow or load to the inside of the knee that bows it outward, or a non-contact twisting injury with the knee in a vulnerable position. Compared with ACL and MCL injuries, isolated LCL injuries are uncommon, reported in the literature as making up roughly 1 in every 100 significant knee ligament injuries when truly isolated. They occur more often in high-velocity pivoting and contact sports — soccer, rugby, skiing, and basketball are frequently cited — and less often as part of everyday low-energy trauma.
When an LCL injury may be more than a simple sprain
- The knee feels like it "gives way" on the outer side when pushing off a straight leg, not just when it's tender
- There was a high-energy mechanism — a fall from height, a motor vehicle or motorcycle accident, a direct blow while the knee was planted
- Numbness or weakness affecting the foot or lower leg (the peroneal nerve runs close to the LCL and can be stretched in more severe injuries)
- The knee was dislocated, or felt like it briefly "went out of place"
- Pain and swelling are out of proportion to what feels like a routine sprain
Any of these features should prompt a proper clinical assessment rather than a wait-and-see approach, because they raise the possibility of a combined posterolateral corner or multi-ligament injury rather than an isolated LCL sprain.
How LCL injuries are graded
Like other knee ligament sprains, LCL injuries are graded I through III based on the degree of laxity found on examination, usually performed with the knee at both full extension and 30° of flexion.
| Grade | What's injured | Clinical finding |
|---|---|---|
| Grade I | Ligament fibres stretched, structurally intact | Tenderness over the ligament; minimal to no increased opening compared with the other knee; firm endpoint |
| Grade II | Partial tear | Noticeably increased opening under varus stress, but still a discernible endpoint |
| Grade III | Complete tear | Gross laxity with no firm endpoint; more likely to involve the wider posterolateral corner |
An important and somewhat counter-intuitive point from recent research: how an LCL injury looks on MRI and how unstable it feels on clinical examination don't always agree. In a 2025 case series of 55 professional athletes with MRI-confirmed isolated LCL injuries, MRI grading and clinical (hands-on) grading showed only weak correlation. In practice, this means the exam performed by the treating clinician — not the MRI report alone — usually drives the treatment decision.
How the diagnosis is made
Assessment starts with a history of the injury mechanism, followed by varus stress testing at 0° and 30° of knee flexion, comparing the injured side to the uninjured one. Because the LCL is a PLC structure, a thorough exam also checks for signs of combined injury: a "dial test," which compares external rotation of the shin at 30° and 90° between the two knees, helps distinguish an isolated LCL sprain from a broader PLC injury with rotational instability.
MRI is the standard imaging study to confirm the diagnosis, grade the extent of injury, and — critically — check for associated damage to the ACL, PCL, popliteus tendon, or peroneal nerve, since isolated LCL tears are the exception rather than the rule in more severe injuries. When the clinical picture is unclear or a complete tear needs to be objectively confirmed, varus stress radiographs (X-rays taken while a controlled varus force is applied) can be used; a meaningful side-to-side difference in how much the joint opens up is a validated way of identifying a complete, grade-III tear.
What the evidence shows: A biomechanical and clinical analysis of varus stress radiography (Kane et al., Orthopaedic Journal of Sports Medicine, 2018) found that a measurable side-to-side difference in lateral joint opening under applied varus stress is a reliable, reproducible way to objectively confirm a complete fibular collateral ligament tear — useful when clinical exam and MRI findings don't tell the full story on their own.
Treatment: why most low-grade sprains don't need surgery
Grade I and II LCL sprains are generally treated non-surgically: a period of protected weight-bearing or bracing, activity modification, and a structured rehabilitation programme focused on restoring motion and then strengthening the muscles that support the knee. Isolated, low-grade LCL injuries have a strong track record of healing well with this approach.
What the evidence shows: A 2025 case series of 55 elite athletes with MRI-confirmed, isolated LCL injuries — 87% of which were clinically grade I or II — found that non-surgical treatment resulted in a 100% return to play, at a median of 76 days, with no significant residual varus laxity at follow-up (Haslhofer et al., Orthopaedic Journal of Sports Medicine, 2025). An older but frequently cited long-term study similarly found that grade II sprains generally did well with non-operative treatment, though some residual looseness of the ligament was common even in good outcomes (Kannus, American Journal of Sports Medicine, 1989).
When it's a different conversation: grade III and combined injuries
Complete (grade III) LCL tears behave differently — particularly when they occur alongside injury to the other posterolateral corner structures, or alongside the ACL or PCL, which is common in higher-energy trauma. Left untreated, a complete tear with persisting instability places abnormal load on the rest of the knee and on any cruciate ligament reconstruction performed alongside it.
What the evidence shows: The same long-term study that found good results for grade II sprains also followed grade III sprains for an average of 8 years and found a markedly different picture — persisting instability, muscle weakness, and posttraumatic osteoarthritis were common (Kannus, American Journal of Sports Medicine, 1989). This is one reason grade III and combined posterolateral corner injuries are generally evaluated for surgical reconstruction rather than managed the same way as a lower-grade sprain, particularly in younger or more active patients.
This is the point at which an LCL injury stops being a "wait and rehab" situation and becomes a surgical discussion — typically framed around reconstruction of the posterolateral corner as a whole, not the LCL in isolation. That evaluation and the reconstruction options involved are outside the scope of this particular column.
What recovery looks like for a low-grade sprain
For grade I–II injuries managed non-operatively, early treatment focuses on controlling swelling and protecting the ligament — often with a hinged brace and a period of limited weight-bearing — followed by progressive range-of-motion work and then strengthening of the quadriceps, hamstrings, and hip muscles that share the load with the LCL. Return to straight-line activity often begins within a few weeks; return to cutting, pivoting, or contact sport is guided by functional testing and typically takes longer, in the range of two to three months for many athletes, though timelines vary with the severity of the sprain and the demands of the sport.
If you have outer-knee pain after an injury, the most useful first step is a focused clinical examination — including varus stress testing and a dial test — rather than assuming either extreme: that it's "just a sprain" that will resolve on its own, or that it necessarily needs surgery. The exam, more than the MRI alone, is what tells the difference.
References (PubMed)
Kannus P. Nonoperative treatment of Grade II and III sprains of the lateral ligament compartment of the knee. Am J Sports Med. 1989;17(1):83–88. DOI: 10.1177/036354658901700114
Kane PW, Cinque ME, Moatshe G, Chahla J, DePhillipo NN, Provencher MT, LaPrade RF. Fibular Collateral Ligament: Varus Stress Radiographic Analysis Using 3 Different Clinical Techniques. Orthop J Sports Med. 2018;6(5). DOI: 10.1177/2325967118770170
Chahla J, Moatshe G, Dean CS, LaPrade RF. Posterolateral Corner of the Knee: Current Concepts. Arch Bone Jt Surg. 2016;4(2):97–103. DOI: 10.22038/abjs.2016.6435
Haslhofer DJ, Jaggard MKJ, Abdul W, Jones M, Mitchell A, Lee J, Ball SV, Williams A. Isolated Clinically Diagnosed Grades 1-2 Lateral Collateral Ligament Injuries in Elite Athletes Do Not Require Surgery. Orthop J Sports Med. 2025;13(12). DOI: 10.1177/23259671251391357