A knee sprain is an injury of the ligaments, tough bands of fibrous tissue that connect the bones of the upper and lower leg at the knee joint. The knee joint has four major ligaments.
- Anterior cruciate ligament (ACL) — The ACL and the posterior cruciate ligament (PCL) bridge the inside of the knee joint, forming an “X” pattern that stabilizes the knee against front-to-back and back-to-front forces. The ACL typically sprains during one of the following knee movements: a sudden stop; a twist, pivot or change in direction at the joint; extreme overstraightening (hyperextension); or a direct impact to the outside of the knee or lower leg. These injuries are seen among athletes in football, basketball, soccer, rugby, wrestling, gymnastics and skiing.
- Posterior cruciate ligament (PCL) — The PCL works with the ACL to stabilize the knee. It most often sprains because of a direct impact to the front of the knee, such as hitting the knee on the dashboard in a car crash or landing hard on a bent knee during sports. In athletes, PCL injuries are most common among those who play football, basketball, soccer and rugby.
- Medial collateral ligament (MCL) — The MCL supports the knee along the inner side of the leg. Like the ACL, the MCL can be torn by a direct sideways blow to the outside of the knee or lower leg, the kind of blow that can happen in football, soccer, hockey and rugby. The MCL can be injured by a severe knee twist during skiing or wrestling, particularly when a fall twists the lower leg outwards, away from the upper leg.
- Lateral collateral ligament (LCL) — The LCL supports the outer side of the knee. It is the least likely knee ligament to be sprained because most LCL injuries are caused by a blow to the inside of the knee, and that area usually is shielded by the opposite leg.
Like other types of sprains, knee sprains are classified according to a grading system:
- Grade I (mild) — This injury stretches the ligament, which causes microscopic tears in the ligament. These tiny tears don’t significantly affect the overall ability of the knee joint to support your weight.
- Grade II (moderate) — The ligament is partially torn, and there is some mild to moderate instability (or periodic giving out) of the knee while standing or walking.
- Grade III (severe) — The ligament is torn completely or separated at its end from the bone, and the knee is more unstable.
When one knee ligament suffers a serious sprain, there is a good chance that other parts of the knee may also be injured. For example, because the MCL helps to protect the ACL from certain types of extreme knee forces, the ACL can become vulnerable to injury when the MCL is torn. In more than half of moderate or severe MCL sprains, the ACL also is sprained.
Knee sprains are very common. ACL sprains tend to cause more significant symptoms compared to MCL injuries. Many MCL sprains are so mild that they don’t result in a visit to a doctor.
More than any other group, competitive athletes have a very high risk of knee sprains and other types of knee problems. In U.S. high schools, the knee is the most frequently injured joint among athletes who compete in football, soccer or wrestling.
Symptoms of a knee sprain vary depending on the specific ligament that is torn:
- A pop inside your knee at the moment of injury
- Significant knee swelling within a few hours after injury
- Severe knee pain that prevents you from continued participation in your sport
- Black-and-blue discoloration around the knee
- Knee instability — the feeling that your injured knee will buckle or give out if you try to stand
- Mild knee swelling, with or without knee instability
- Mild difficulty in moving the knee
- Mild pain at the back of the knee that worsens when you kneel
- Knee pain and swelling
- Knee buckling toward the outside
- An area of tenderness over the torn MCL (at the inner side of the knee)
- Knee pain and swelling
- Knee buckling toward the inside
- An area of tenderness over the torn LCL (at the outer side of the knee)
Your doctor will want to know exactly how you hurt your knee. He or she will ask about:
- The type of movement that caused the injury (sudden stop, twist, pivot, hyperextension, direct contact)
- Whether you felt a pop inside your knee when the injury happened
- How long it took for swelling to appear
- Whether severe knee pain sidelined you immediately after the injury
- Whether your knee immediately felt unsteady and could not bear weight
The doctor will examine both your knees, comparing your injured knee with your uninjured one. During this exam, the doctor will check your injured knee for signs of swelling, deformity, tenderness, fluid inside the knee joint and discoloration. If you don’t have too much pain and swelling, the doctor will evaluate your knee’s range of motion and will pull against the ligaments to check their strength. During the exam, you will bend your knee and the doctor will gently pull forward or push backward on your lower leg where it meets the knee.
If the results of your physical exam suggest you have a significant knee injury, you will need diagnostic tests to further evaluate your knee. These may include standard X-rays to check for ligament separation from bone or fracture. Tests may also include a magnetic resonance imaging (MRI) scan or camera-guided knee surgery (arthroscopy).
How long a knee sprain lasts depends on the type of knee sprain, the severity of your injury, your rehabilitation program and the types of sports you play. In general, milder Grade I and Grade II MCL or LCL sprains heal within 2 to 4 weeks, but other types of knee sprains may take 4 to 12 months.
To help prevent sports-related knee injuries, you can:
- Warm up and stretch before you participate in athletic activities.
- Perform exercises to strengthen the leg muscles around your knee, especially the quadriceps.
- Avoid sudden increases in the intensity of your training program. Never push yourself too hard, too fast. Increase your intensity gradually.
- Wear comfortable, supportive shoes that fit your feet and fit your sport. If you have problems in foot alignment that may increase your risk of a twisted knee, ask your doctor about shoe inserts that can correct the problem.
- If you play football, ask your sports medicine doctor or athletic trainer about specific types of shoe cleats that may help reduce your risk of knee injuries.
- If you ski, use two-mode release bindings that are properly installed and adjusted. Make sure that the binding mechanism is in good working order and that your boots and bindings are compatible.
If you have a Grade I or Grade II knee sprain, your doctor probably will recommend that you follow the RICE rule:
- Rest the joint.
- Ice the injured area to reduce swelling.
- Compress the swelling with an elastic bandage.
- Elevate the injured knee.
Your doctor may suggest that you wear a knee brace for a short period of time and that you take a nonsteroidal anti-inflammatory drug (NSAID), such as ibuprofen (Advil, Motrin and others), to relieve pain and ease swelling. As your knee pain gradually goes away, your doctor will prescribe a rehabilitation program to strengthen the muscles around your knee. This program should help to stabilize your knee joint and prevent you from injuring it again.
If you have a Grade III knee sprain or if multiple ligaments are injured, treatment depends on the specific type of sprain:
- Grade III ACL or PCL sprain — Your torn ligament may be reconstructed surgically using either a piece of your own tissue (autograft) or a piece of donor tissue (allograft). Almost all knee reconstructions use camera-guided (arthroscopic) surgery.
- Grade III MCL sprain — This injury usually is treated conservatively with RICE, NSAIDs (such as ibuprofen) and physical therapy. In certain cases, surgery may be used to repair a torn MCL.
- Grade III LCL sprain — In a severe LCL sprain, the torn ligament often is repaired surgically.
- Simultaneous injury of multiple ligaments — Your doctor will discuss the various surgical options available.
When To Call a Professional
If you injure your knee, call your doctor to request an urgent evaluation if the knee:
- Becomes very painful or swollen
- Cannot bear weight
- Feels as if it will buckle or give out
About 90% of people with ACL injuries and 80% with PCL injuries can expect a full recovery after proper treatment and a good physical therapy program. Almost all MCL sprains and most LCL sprains have an excellent prognosis.
As a long-term complication, some people with ACL or PCL sprains eventually develop pain from osteoarthritis in the injured knee joint. This symptom may not start until many years after the initial knee injury.