While there are four bones that come together at the knee, only the femur (thighbone) and the tibia (shinbone) form the joint itself. The head of the fibula (strut bone on the outside of the leg) provides some stability, and the patella (kneecap) helps with joint and muscle function. Movement and weight-bearing occur where the ends of the femur called the femoral condyles match up with the top flat surfaces of the tibia (tibial plateaus).

There are two major muscle groups that are balanced and allow movement of the knee joint. When the quadriceps muscles on the front of the thigh contract, the knee extends or straightens. The hamstring muscles on the back of the thigh flex or bend the knee when they contract. The muscles cross the knee joint and are attached to the tibia by tendons. The quadriceps tendon is special, in that it contains the patella within its fibers. The patella allows the quadriceps muscle/tendon unit to work more efficiently. The quadriceps tendon is renamed the patellar tendon from the kneecap to its attachment in the tibia.

The stability of the knee joint is maintained by four ligaments, thick bands of tissue that stabilize the joint. The medial collateral ligament (MCL) and lateral collateral ligament (LCL) are on the sides of the knee and prevent the joint from sliding sideways. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) form an “X” on the inside of the knee and prevent the knee from sliding back and forth. These limitations on knee movement allow the knee to concentrate the forces of the muscles on flexion and extension.

Inside the knee, there are two shock-absorbing pieces of cartilage called menisci (singular meniscus) that sit on the top surface of the tibia. The menisci allow the femoral condyle to move on the tibial surface without friction, preventing the bones from rubbing on each other. Without this cartilage covering, the friction of bone on bone would cause inflammation, or arthritis.

Bursas surround the knee joint and are fluid-filled sacs that cushion the knee during its range of motion. In the front of the knee, there is a bursa between the skin and the kneecap called the pre-patellar bursa and another above the kneecap called the supra-patellar bursa (supra means above).

Each part of the anatomy needs to function properly for the knee to work. Acute injury or trauma as well as chronic overuse may cause inflammation and its accompanying symptoms of pain, swelling, redness, and warmth.

If the knee is stressed from a specific direction, then the ligament trying to hold it in place against that force can stretch or tear. These injuries are called sprains. Sprains are graded as first, second, or third degree based upon how much damage has occurred. Grade-one sprains stretch the ligament but don’t tear the fibers; grade-two sprains partially tear the fibers, but the ligament remains intact; and grade-three tears completely disrupt the ligament.Twisting injuries to the knee put stress on the cartilage or meniscus and can pinch them between the tibial surface and the edges of the femoral condyle, potentially causing tears.

Injuries of the muscles and tendons surrounding the knee are caused by acute hyperflexion or hyperextension of the knee or by overuse. These injuries are called strains. Strains are graded similarly to sprains, with first-degree strains stretching muscle or tendon fibers but not tearing them, second-degree strains partially tearing the muscle tendon unit, and third-degree strains completely tearing it.

There can be inflammation of the bursas (bursitis) of the knee that can occur because of direct blows or chronic use and abuse.

Anatomically, many of the structures that support the knee are interconnected. A knee that is injured may cause damage to one or more structures depending upon the mechanism.

Almost all of these strains are treated with ice, elevation, and rest. Sometimes compression with an Ace wrap or knee sleeve is recommended, and crutches may be used for a short time to assist with walking. Ibuprofen (Advil, Motrin) or naproxen can be used as an anti-inflammatory medication. (As with all over-the-counter medications, it is important to check with your health care provider or pharmacist regarding safety and drug interactions.)

The mechanism of injury is either hyperextension, in which the hamstring muscles can be stretched or torn, or hyperflexion, in which the quadriceps muscle is injured. Uncommonly, with a hyperflexion injury, the patellar or quadriceps tendon can be damaged and rupture. This injury is characterized by the inability to extend or straighten the knee and a defect that can be felt either above or below the patella (kneecap). Surgery is usually required to repair this injury.

Except for elite athletes, tears of the hamstring muscle are treated conservatively without an operation, allowing time, exercise, and physical therapy to return the muscle to normal function.

These ligaments can be stretched or torn when the foot is planted and a sideways force is directed to the knee. This can cause significant pain and difficulty walking as the body tries to protect the knee, but there is usually little swelling within the knee. The treatment for this injury may include a hinged knee brace that helps stabilize the knee and control range of motion. RICE (rest, ice, compression, and elevation) are the mainstays of treatment. Surgery may be an option for completely torn ligaments or when the knee joint remains unstable after completing a course of rest and physical therapy. Injuries to these ligaments may also be associated with damage to a meniscus or cruciate ligament.

If the foot is planted and there is force applied from the front or back to the knee, then the cruciate ligaments can be damaged. Swelling in the knee occurs within minutes, and attempts at walking are difficult. The definitive diagnosis may be difficult in the emergency department or doctor’s office because the swelling and pain make it hard to test knee stability and determine whether the ligament is loose. Long-term treatment may require surgery and significant physical therapy to return good function of the knee joint. Recovery from these injuries is measured in months, not weeks.

The cartilage of the knee can be acutely injured or can gradually tear due to chronic inflammation or arthritis. Acutely, the injury is a twist; the cartilage that is attached to and lays flat on the tibia is pinched between the femoral condyle and the tibial plateau. Pain and swelling occur gradually over many hours (as opposed to an ACL tear which swells much more quickly). Sometimes the injury seems trivial and no care is sought, but chronic pain develops over time. There may be intermittent swelling, pain with walking uphill or climbing steps, or giving way of the knee that results in near falls. Squatting may be painful. History and physical examination often can make the diagnosis, and MRI may be used to confirm it. MRI is also able to define where within the meniscus that the tear has occurred and how much of the cartilage is involved in the tear. This may help guide the decision as to whether surgery, physical therapy, or both would be appropriate.

Accidents happen, and injuries occur on the football field, on the basketball court, on the soccer pitch, and in daily life. Often knee injuries cannot be prevented. Maintaining a healthy weight, keeping fit, staying strong, and stretching may be helpful in minimizing the risk of many injuries, including those that involve the knee.

Imbalances of muscles that surround and support the knee can lead to knee injuries. If either the quadriceps or hamstring muscle groups become weak, the stability of the knee and ability to withstand an injury is decreased.

Similarly, an imbalance between muscles within the quadriceps muscles of the thigh may cause the kneecap (patella) to track improperly, causing patellofemoral syndrome or increasing the risk of patellar dislocation.

Pregnant women are at higher risk for knee injury and pain because of increased weight, the body’s shifted center of gravity, and hormonal changes that can weaken ligaments and make them more lax.

The knee joint absorbs a significant amount of the force that is generated with each step with walking or running. People who are significantly overweight may develop knee pain because of the excess weight that needs to be supported.

Acute knee injuries can cause pain and swelling with difficulty bending the knee and weight-bearing. Acute knee injuries often may be considered as falling into two groups: those where there is almost immediate swelling in the joint associated with the inability to bend the knee and bear weight, and those in which there is discomfort and perhaps localized pain to one side of the knee, but with minimal swelling and minimal effects on walking.

If the swelling occurs immediately, it may suggest a ligament tear or fracture. If the swelling arises over a period of many hours, meniscal or cartilage injuries may be the cause. However, injuries to the knee may involve more than one structure and the symptoms may not present classically.

Longer-term symptoms that point to knee problems will include pain and swelling in addition to other complaints. Inflammation in the joint may be caused by even minor activity. Swelling may be intermittent, brought on by activity, and may gradually resolve as the inflammation decreases.

Pain, too, may come and go. It may not occur right away with activity but might be delayed as the inflammation develops. Pain can also be felt with specific activities. Pain while climbing stairs is a symptom of meniscus injury, where the cartilage is being pinched in the joint as the joint space narrows with knee bending. Pain with walking down stairs suggests patellar pain, where the kneecap is being forced onto the femur.

Giving way, or a feeling of instability of the knee, or popping or grinding in the knee is associated with cartilage or meniscus tears. Locking is the term used when the knee joint refuses to completely straighten, and this is almost always due to torn cartilage. In this situation, the torn piece of cartilage folds upon itself and doesn’t allow the knee to extend.