Muscle, joint & pain

Sprains on the Sides of the Knee

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The knee has ligaments on both sides acting like guy-wires, and a hard sideways force can sprain either one: the MCL on the inside, or the LCL on the outside. The two injuries mirror each other in mechanism and symptoms but differ in how often they happen and how they tend to be treated. Here is how to recognize a collateral ligament sprain, how it is graded, and what separates it from an ACL tear or meniscus injury.

Last updated: July 2026

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What an MCL or LCL sprain is, and how it happens

The medial collateral ligament (MCL) and lateral collateral ligament (LCL) are the two straps of tissue that stabilize the sides of the knee, resisting sideways bending much the way a hinge's side supports keep a door from swinging too far. A sprain means one of these ligaments has been overstretched or partially torn, most often by a sideways force at the knee rather than the twisting motion that usually injures the ACL or meniscus.

An MCL sprain typically follows a valgus force — something pushing the knee inward, such as a direct hit to the outside of the leg in a contact sport, or the leg planting while the body twists the opposite way. An LCL sprain follows the mirror-image mechanism, a varus force pushing the knee outward, and it happens less often because the outer side of the leg is naturally more protected by the other leg during most falls and collisions 1.

What the symptoms typically feel like

Pain and tenderness that trace a specific line along the inner (MCL) or outer (LCL) side of the knee, right over the ligament itself, are the hallmark symptom, often reproducible by pressing directly on the injured spot. Swelling tends to be more localized to that side of the knee rather than filling the whole joint, and bruising may appear over the following days.

In more significant sprains, the knee can feel loose or unstable specifically when a sideways force is applied — for example, feeling the knee give slightly when pivoting or cutting during activity — rather than the sudden buckling more typical of an ACL tear. Localized swelling and pain along one side of the knee, without the joint filling up entirely, is generally a reassuring sign that points toward a collateral ligament sprain rather than a more extensive injury. That is because the MCL in particular sits partly outside the joint capsule, so it does not bleed into the joint the way an ACL tear often does.

How collateral ligament sprains are graded

Like most ligament sprains, an MCL or LCL injury is graded on a three-point scale that reflects how much the ligament has been stretched or torn, similar to the ankle sprain grades used for a rolled ankle. A Grade I sprain is a mild stretch with tenderness but no looseness on exam; Grade II involves partial tearing with some looseness; Grade III is a complete tear with obvious instability when the joint is stressed sideways.

This grading, along with a focused ligament exam, is central to how clinical guidelines recommend classifying and managing knee ligament injuries, since the knee sprain grades largely determine how much bracing or protection is needed and how quickly activity can safely resume 1. Higher-grade sprains generally take longer to heal and are more likely to involve some detectable looseness even after the acute pain resolves.

How this differs from an ACL tear, a meniscus tear, or a PCL injury

A collateral ligament sprain is usually distinguishable from these other knee injuries by mechanism, location, and how the knee behaves on exam. An ACL tear typically follows a twisting or pivoting injury, often with a popping sound and swelling that fills the whole joint within hours, and is checked with the ACL exam Lachman test rather than a side-to-side stress test.

A meniscus tear more often causes catching, locking, or pain that is worse with squatting or twisting rather than pain along one clean line at the side of the knee. Injury to the ligament behind the knee, the posterior cruciate ligament, is less common and usually follows a direct blow to the front of the shin, such as in a dashboard injury during a car accident, producing pain deep in the back of the knee rather than along a side.

How MCL and LCL sprains are typically treated

Most MCL sprains, even higher-grade ones, are managed without surgery, since the ligament has a strong blood supply and heals well with bracing, protected weight-bearing, and a progressive rehabilitation program. LCL sprains are treated similarly when isolated, though a complete LCL tear is watched more closely, since it heals less reliably on its own than the MCL and is more often associated with injury to nearby structures.

Unlike the ACL, the MCL usually heals well on its own — most collateral ligament sprains do not need surgery. The ACL has a poor capacity to heal on its own and more often needs reconstructive surgery when a person wants to return to pivoting sports 2. Physical-therapy rehabilitation guidelines for knee ligament injuries reflect a similar, nonoperative-first logic for the collateral ligaments, reserving surgery mainly for combined injuries or a persistently unstable knee 1. Recovery from a straightforward Grade I or II sprain commonly proceeds over several weeks, and clinicians sometimes track progress with a validated outcome measure such as the KOOS, which scores pain, symptoms, and daily function over time 3.

Recovery timeline and when to be concerned about something more

A Grade I sprain often improves substantially within two to four weeks, while a Grade II sprain can take around six weeks, and a Grade III tear may need two to three months of protected rehabilitation before a return to full activity. These are general ranges; the exact timeline depends on the grade, whether other structures were also injured, and how consistently the rehabilitation program is followed.

An LCL injury deserves closer attention than its MCL counterpart because it is more often part of a combined injury pattern, sometimes involving the posterolateral corner of the knee or, less commonly, the peroneal nerve that runs close to it, which can cause numbness or weakness in the foot. Persistent instability, numbness below the knee, or a sprain that is not improving on the expected timeline are reasons to have the knee reassessed rather than continuing rehabilitation on the original plan.

Common questions

An MCL sprain usually causes pain focused on the inner side of the knee, often from a direct blow to the outside of the leg, with swelling limited mostly to that side. An ACL tear more often follows a twisting or pivoting injury, sometimes with an audible pop and swelling that fills the whole knee within a few hours. A clinician's exam, checking each ligament separately, distinguishes them reliably.

Usually not. Most MCL sprains, even higher-grade ones, heal well with bracing, protected activity, and rehabilitation because the ligament has a strong blood supply. Surgery is reserved mainly for a complete tear that is also part of a more complex, multi-ligament injury, which is uncommon for an isolated MCL sprain.

Often, yes, mainly because an LCL injury is more likely to occur alongside damage to nearby structures, such as the posterolateral corner of the knee or, less commonly, the peroneal nerve. An isolated, low-grade LCL sprain is managed much like an MCL sprain, but a complete LCL tear generally gets a closer look for these associated injuries.

A mild, Grade I sprain often improves substantially within two to four weeks. A Grade II sprain, with some partial tearing, can take around six weeks, and a full Grade III tear may need two to three months of protected rehabilitation before a return to full activity. The exact timeline depends on the grade and how consistently rehabilitation is followed.

A Grade II sprain means the ligament has partially torn, producing some detectable looseness on exam along with pain and swelling, but the knee is not completely unstable. It sits between a Grade I stretch, which has no looseness, and a Grade III complete tear, which causes obvious instability when the knee is stressed sideways.

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When a knee sprain needs more than rest and a brace

  • —The knee feels grossly unstable or buckles when bearing weight
  • —Numbness, tingling, or weakness in the foot after an outer-knee (LCL) injury
  • —Significant swelling that fills the entire joint within a few hours, suggesting more than an isolated collateral ligament sprain
  • —No improvement at all after several weeks of bracing and rehabilitation

New numbness, weakness, or a foot that cannot be lifted after a knee injury needs prompt same-day evaluation in urgent care or an emergency department, since it can mean a nerve or blood vessel is involved.

This article is general education, not a diagnosis. Knee injuries vary widely in severity and often involve more than one structure. A clinician's exam, and imaging when needed, can determine exactly what is injured and how it should be treated.

References

  1. 1.Logerstedt DS, Scalzitti D, Risberg MA, et al. (2017). Knee Stability and Movement Coordination Impairments: Knee Ligament Sprain Revision 2017. Journal of Orthopaedic & Sports Physical Therapy. doi:10.2519/jospt.2017.0303Clinical practice guideline for knee ligament sprains covering examination, grading, and criteria-based, largely nonoperative rehabilitation for isolated collateral ligament injuries.
  2. 2.American Academy of Orthopaedic Surgeons (AAOS) (2022). Management of Anterior Cruciate Ligament Injuries — Clinical Practice Guideline. AAOS. link ✓ACL injuries have a poor capacity for self-healing and more often require reconstructive surgery, particularly for a return to pivoting sports.
  3. 3.Roos EM, Roos HP, Lohmander LS, Ekdahl C, Beynnon BD (1998). Knee Injury and Osteoarthritis Outcome Score (KOOS)—Development of a Self-Administered Outcome Measure. Journal of Orthopaedic & Sports Physical Therapy. doi:10.2519/jospt.1998.28.2.88The KOOS is a validated self-administered outcome measure with subscales for pain, symptoms, activities of daily living, sport/recreation, and knee-related quality of life.

3 sources, numbered by first appearance. General health information, not medical advice. AI-assisted editorial content — citations link their sources. Editorial policy

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