Knee Swelling


Knee swelling happens when fluid builds up within or outside the knee joint, usually after an injury. The knee joint is a closed capsule similar to a balloon. When fluid collects inside the joint, the swelling is called an effusion (Figure 1).

  • Knee effusions following trauma almost always represent a serious injury to an important structure within the knee joint.
  • Swelling on the outside of the knee capsule usually occurs directly over the knee structure that is injured.
  • Swelling can build up slowly or happen suddenly.
  • Swelling can come and go, but persistent swelling is usually associated with a more serious injury
Figure 1.


  • Swelling inside the joint occurs most often after injury, but can sometimes happen without an obvious injury.
  • Injuries can be caused by direct impact in contact sports such as football and by activities such as cutting, pivoting or changing direction or speed rapidly when running, jumping or landing.
  • Swelling not resulting from injury can be caused by a medical condition such as infection or inflammatory arthritis.

If swelling occurs inside the knee joint after injury, you may have:

  • An anterior cruciate ligament (ACL) tear: The ACL is a strong, fibrous band of tissue in the center of the knee. It holds the main bones above and below the knee together and gives the knee stability. An ACL tear is the most common cause of swelling within the joint after a knee injury.
  • Torn cartilage: Cartilage is tissue that lines and cushions joints. There are two kinds of cartilage in the knee joint, rubbery and firm. The half-moon-shaped, flexible pads in the inner and outer sides of the knee joint are called meniscus cartilage. The shiny, smooth, firm cartilage on the end of the knee bones is called articular cartilage.
  • A fracture within the knee joint: If any bone is broken or if articular cartilage is broken through, significant swelling occurs.
  • A kneecap dislocation: When you bend or straighten your leg, your kneecap slides through a groove in the end of your thigh bone (femur). The kneecap can come out of the groove (become dislocated) and move toward the outer side of the knee with strong contractions of the thigh muscles, such as during a forceful takeoff. It can also happen when twisting, cutting or landing on a bent knee when stopping or pivoting, or after jumping from a height.

If you have swelling inside the knee joint not following injury it may be caused by:

  • Gouty arthritis: In this type of inflammatory arthritis, crystals are deposited in the knee joint causing rapid swelling.
  • Degenerative or osteoarthritis: This kind of arthritis usually occurs after age 40, but it can happen earlier, especially if you have injured your knee. It involves a slow breakdown of knee bones from wear and tear.
  • Patellofemoral pain syndrome: This condition causes pain over, under, around or behind the kneecap. It results from overuse or repetitive activities such as running, jumping, kneeling and squatting. It is the most common knee pain syndrome.
  • Baker’s cyst: A Baker’s cyst is a fluid-filled swelling at the back of the knee that often occurs with degenerative arthritis or an injury.

Knee swelling outside the joint can result from an injury or from a condition that develops over time. Common causes include:

  • Collateral ligament injury: Collateral ligaments are strong, fibrous bands of tissue on the inner and outer sides of the knee that that connect the thigh bone (femur) to the lower leg bones (tibia and fibula).
  • Bursitis: Bursitis is inflammation of a bursa, a fluid-filled sac located between bone and tendons or muscle that serves to prevent friction between tissues, allowing them to glide easily over each other.
  • Tendonitis: Tendonitis is irritation of a tendon, which is strong, fibrous tissue that connects the muscles around the knee to the bones.
  • Growth plate injury: Growth plates are specialized areas of growing bone in children and teenagers that can become irritated with injuries and sports activities.


Swelling inside the joint

  • You usually have generalized pain and tightness of your entire knee.
  • You will have difficulty bending your knee due to the fluid buildup, and pain may worsen when you try to bend it.
  • You often will notice pain around your knee when you walk.
  • If you have injured cartilage or a ligament (a strong band of tissue connecting one bone to another), your knee may feel unstable. It may buckle or give out, lock (get stuck in a certain position) or catch (something within the joint snags or rubs roughly). With a fracture of joint cartilage or bone, there may be loud crunching at the moment of injury and/or whenever you try to move it after an injury.
  • Your knee may feel warm and look red.
  • You may have a lot of bruising around your knee.

Swelling outside the joint

  • There will be pain directly over and around the swollen area.
  • There may be warmth and redness over the swelling.
  • If you have a ligament injury, your knee may buckle or give way when you try to cut, pivot or twist.
  • Activities that stress injured structures may increase pain and swelling.
  • Kneeling or squatting can worsen pain and swelling over the kneecap or below the front of the joint.
  • You may have bruising directly over or below an injured knee structure.


Call 911 if:

  • You have a knee dislocation (the bones above and below your knee do not line up normally).
  • Your lower leg and foot are pale and cool following a severe knee injury. This could mean that you have torn a major blood vessel.

Call your doctor right away (day or night) if:

  • You have severe pain, swelling, redness or warmth in your knee, especially after an injury.
  • You can’t bend your knee after an injury.
  • You can’t put weight on your knee after an injury.
  • You heard or felt a loud pop during an injury.
  • Your knee feels swollen like a balloon.
  • You hear crunching and have pain when you try to move your knee or to walk.
  • Your knee gives out, buckles or locks, or something catches in the joint when you try to walk after an injury.
  • You have numbness, tingling or loss of sensation at or anywhere below your knee after an injury.
  • You have a fever along with knee swelling, which could signal an infection within the joint.

Call your doctor during regular office hours if you have injured your knee and:

  • You have mild to moderate pain and swelling.
  • You have no warmth or redness around your knee.
  • You can bend your knee well.
  • You can put weight on your knee and leg without significant discomfort.

You should also consult your doctor if you were not injured but you notice that your knee occasionally buckles, gives out or locks, or something catches in the joint.


You do not need to see a doctor right away for mild knee swelling if:

  • Swelling within the joint builds up slowly and does not follow an injury.
  • Swelling outside the knee joint gets progressively better over a week or two.
  • You have minimal discomfort on the inside or outside of your knee.
  • You have no significant bruising.
  • You have no locking, catching or buckling

All other knee swelling should be evaluated by a doctor because serious injuries can be missed easily, especially with swelling inside the knee joint.

  • Once the soreness and swelling starts to get better, you may try riding a stationary bike using low resistance for 10 to 20 minutes daily or every other day.
  • If your knee starts to feel normal within one or two weeks, you may try functional drills to see if you are ready to resume sports or regular exercise. You may progress through a series of drills by completing each one pain-free. If you get symptoms when you do a drill, stop and try it again in a day or two.
    • Jog a straight, 50-yard line at about 50-percent intensity.
    • Run a straight, 50-yard line at 75-percent, then 100-percent intensity.
    • Using the same intensity progressions, run a straight line and progress through 45-degree and then 90-degree cuts to the right and to the left.
    • Then, run a large or “loose” figure-of-eight path. Gradually make it smaller or “tighter” and faster with repeat attempts.

When you can do all of these drills pain free, you can try sports-specific activity.When you return to specific sports or exercise activity, at first try sports-specific drills or exercise at only 50-percent intensity and in a noncompetitive setting. Progress in intensity and sports participation only if you can do so without pain.

  • Avoid running, jumping, twisting and kicking until your knee has been examined by a doctor or until it is completely better.
  • Use crutches to take all of the weight off your injured knee if you have pain or a sense of buckling when you try to walk or put weight on your knee or leg.
  • If it hurts when you try to bend your knee, hold it as straight as possible or in the most comfortable position you can find until you see your doctor.
  • For the first few days after injury, elevate your knee on a couch or chair as often as possible and for as long as you can. Place an ice pack or cold pack directly over the injured area of your knee or, if the pain is not localized, around your knee. You can use an elastic wrap to hold the ice pack or cold pack in place.
    • To minimize pain, swelling and bruising, use the cold pack for 20 minutes every one to two hours for the first 24 to 36 hours.
    • After the first day or two, use cold packs at least three times daily, more often if your symptoms require it.
    • You may place a thin washcloth between the cold pack and your skin to minimize the risk of frostbite.
    • If you are busy, you may choose to ice during meals so as to save time and avoid interrupting other activities.
  • Use a snug elastic wrap or sleeve around your knee to minimize swelling..
  • Because most over-the-counter knee braces do not help stabilize your knee very well, don’t use them without advice from your doctor.
  • You may take ibuprofen (Advil®, Motrin®) or naproxen (Aleve®) for swelling and pain. (See labels for dose and risks.)


  • Regular exercise that strengthens the muscles around your knee and keeps your weight down can help prevent knee injuries and swelling.
  • A regular stretching program that focuses on the hips, legs and ankles can help prevent some knee injuries.
  • Avoid sudden changes or increases in exercise or sporting activities.



Last reviewed: November 2009

Last revised: November 2009