What Can Cause Pain Behind the Knee?

Some of the most common causes of pain behind the knee (posterior knee pain) include, Baker's cyst, arthritis, infection, injury, tumor, or deep vein thrombosis.

Since the knee is the largest and most complex joint in the body, it makes sense that it might hurt sometimes. Although knee pain is a common complaint, it is less common behind the knee.

Pain in the back of the knee is called posterior knee pain, and it can have a variety of causes. Occasionally, posterior knee pain is "referred" from the front of the knee or the spine.

Since several conditions can cause pain behind the knee, the symptoms can vary. The most common symptoms include:

Varying types of pain

The pain can be sharp, dull, or burning. It may come on suddenly or gradually. It may be constant, or it may occur when you put weight on the leg or when you bend the knee. This information can help a doctor diagnose your knee problem.

Swelling or stiffness

The knee may look swollen or misshapen. You may be unable to bend the knee, or your knee may pop, lock up, or collapse when you put weight on it. These symptoms usually indicate that you have sustained an injury, but there are other possibilities as well.

Redness or warmth

Under certain circumstances, the back of your knee could feel hot to the touch, or redness could be visible. You might also have a fever. These symptoms would point to a different cause than if you only have pain.

Pain behind the knee can be simple or difficult to diagnose, depending upon the cause. Here are some of the most common causes of posterior knee pain:

Baker's cyst

A lump-like swelling behind the knee is characteristic of Baker's cyst, making it fairly easy to diagnose. This type of cyst is also called a popliteal cyst because it is located in the popliteal fossa, a small hollow at the back of the knee.

It forms when synovial fluid gathers at the back of the knee. Injury or stress from arthritis can trigger the accumulation of fluid.

Different forms of arthritis

Arthritis is the generic name given to diseases that affect the joint. Arthritis can cause pain anywhere in the knee joint, including the back.

Knee pain is most often associated with osteoarthritis, the type that is related to aging or overuse. Knee pain can also occur with rheumatoid arthritis, but it will occur in both knees at once.

Gout is a type of arthritis that first attacks the big toe, but later attacks may strike the knees. Septic or infectious arthritis is caused by an infection that lodges in a joint, usually a knee.

Infection

Besides infection in the knee joint (infectious arthritis), posterior knee pain could result from several other infections, including:

• Skin infection (cellulitis)
• Infection in the bone (osteomyelitis)
• Infection of one of the fluid-filled sacs called bursae (septic bursitis)

These infections have different causes and symptoms, although pain, redness, heat, and swelling are typical of most infections.

Injury

Damage to a muscle, tendon, ligament, or other connective tissue could cause posterior knee pain. Such injuries can be acute or caused by overuse. Hamstring injuries, meniscus tears, and injuries to the posterior cruciate ligament (PCL) are three injuries that may cause pain in the back of the knee.

Tumor

It is rare that knee pain is caused by a tumor, but both malignant and benign tumors can occur near the knee. Three types of cancer that could cause posterior knee pain are:

• Liposarcoma, cancer that grows in fat tissue and can occur behind the knee
• Osteosarcoma, cancer that can appear in the long bones of the leg
• Synovial sarcoma, a misnamed cancer occurring in large joints that doesn't actually occur in synovial fluid

Deep vein thrombosis

Deep vein thrombosis (DVT) is caused by a blood clot deep in a vein. In most cases, the clot is in the pelvis, thigh, or calf, but it can cause pain anywhere in the leg, including the back of the knee.

DVT can be a life-threatening condition. If part of the blood clot breaks loose and travels to the lungs, it can block blood flow. If the clot stays in place, it can damage the valves in the veins, causing pain, swelling, ulcers, and serious symptoms.

Most posterior knee pain calls for a visit to the doctor. If you have signs of an infection or DVT, you should go immediately. You should also go immediately for severe pain.

If your symptoms are less urgent, your doctor is still the best one to guide your recovery. In the meantime, you can temporarily manage your pain with over-the-counter medications like ibuprofen. For knee injuries, you may use the RICE protocol: rest, ice, compress, and elevate.

A physical exam is of primary importance in diagnosing posterior knee pain from injury or arthritis. The doctor will manipulate the legs to check rotation, flexion, and stability and to see which movements cause pain.

The doctor may also use imaging procedures such as ultrasound, X-ray, CT, arthrography, and MRI. Sometimes, the doctor will draw fluid from the knee for laboratory examination.

The doctor will use physical examination, lab studies, and imaging if they suspect an infection, tumor, or DVT. A doctor can usually diagnose a Baker's cyst by sight, but they may confirm the diagnosis with an imaging procedure.

Treatment for posterior knee pain depends on the diagnosis. Damage from injury, wear and tear, or arthritis may be treated with rest, medications, physical therapy, injections, or surgery.

Some Baker's cysts go away on their own, and some require treatment, possibly including draining the cyst.

Other causes of pain behind the knee, such as infection, tumors, or DVT, will require highly individualized treatment following diagnosis.

Although knee injuries can happen to anyone, some activities and demographics are higher risk than others, such as: 

• Athletes, especially in high-impact sports
• Runners, especially if wearing improper footwear 
• People who are overweight
• People who sit too long in uncomfortable positions

Knee injuries left untreated can potentially lead to a number of complications, like: 

• Ligament instability
• Meniscus injury
• Osteoarthritis 
• Gait problems

Understanding the signs of a serious knee injury can help you know when it’s time to seek treatment from a health professional.

The first sign of a knee injury that most people feel is pain. Though the pain may be severe, often there is only a very mild stiffness or discomfort around the knee joint, especially while making certain repetitive motions with it. 

Other important warning signs of a serious knee injury include: 

Pain while climbing stairs

Pain while climbing up the stairs is indicative of a torn meniscus, whereas pain while going down the stairs is a sign your kneecap is pushing painfully against the femur bone in the leg. 

Swelling

Some kinds of swelling can stop you from bearing weight on your knee or bending it at all, whereas you may have no problem whatsoever walking in other cases. Either way, a swollen knee shouldn’t be ignored. 

Immediate pain

Sometimes the injury shows itself right away, such as a sharp pain in the middle of exercising. In this case, the pain and swelling will happen immediately, suggesting a torn ligament or even a bone fracture has occurred. 

Gradual pain

Some injuries appear a few hours or even days after the injury actually took place. An overuse injury, for example, develops little by little in response to prolonged pressure and is often a sign of cartilage or meniscal tearing. 

Some more signs to look for are: 

• Redness
• Bruising
• Popping sounds
• Feeling unstable

Seek medical attention if you notice any of these symptoms. 

Some kinds of knee pain go away with a little rest, but some injuries are more serious. The most common types of knee injuries are: 

Stress fractures

Fractures can happen with any bone in the knee joint. They can occur as a result of trauma, but osteoporosis can also cause fractures. 

Dislocation

The bones of the knee joint may sometimes fall completely or partially out of alignment. Some people, due to their specific bone structure, may be more prone to this kind of injury than others. The most common cause of knee dislocation is trauma.

Anterior cruciate ligament (ACL) injury

Athletes who often quickly change direction or jump will often have ACL injuries. Many hear it "pop" when the ligament is either torn or strained.

Meniscal tearing

The menisci are two cartilages that protect the joint from stress as you walk. Though the menisci are often damaged during sports activity, they can also wear down as a result of aging or arthritis. 

Tendonitis

Tendonitis in the knees is sometimes known as"jumper’s knee" because it often affects athletes who jump regularly. There are two tendons involved in the knee joint, around the quadriceps and patellar bones, and both can be stretched, torn, or worn down with time.

Doctors have many tools at their disposal to aid in the diagnosis of knee injuries. For instance, they’ll often collect your medical history and ask you questions about your activity level. X-rays, ultrasounds, and MRI scans can also provide a clear picture of your knee joint for an accurate diagnosis.

Some mild knee injuries are treated using the R.I.C.E. method, which stands for rest, ice, compression, and elevation. 

Other cases may require physical therapy to strengthen the joint and restore its range of motion. Surgery is reserved for the most serious knee injuries and may be either minimally invasive or involve a total replacement of the joint.