Reasons You Have Pain in the Back of Your Knee When Straightening Your Leg

Medically reviewed by Anita C. Chandrasekaran, MDMedically reviewed by Anita C. Chandrasekaran, MD

Pain in the back (posterior) portion of the knee is relatively common. Many things could be to blame, whether it’s constant soreness in the area or simply knee pain when extending the leg. Tendon issues, cysts, injuries to the ligaments, and nerve concerns are all possible causes.

Causes of Back of Knee Pain When Extending

Baker’s (Popliteal) Cyst

Baker’s cysts are a relatively common occurrence. They arise when an underlying knee issue, like osteoarthritis, meniscal degeneration, or a ligament tear, causes excess synovial fluid to be produced in the joint. This fluid eventually begins to seep out of the joint’s capsule and collect in the back of your knee joint. Baker’s cysts can be slow to develop or may come on rapidly and typically cause the following symptoms:

• A firm lump in the back of your knee when you press on it

• Pain when extending the leg

• Swelling in the knee or lower leg

• Stiffness or tightness in the posterior knee joint

These can rupture or leak and cause symptoms mimicking deep vein thrombosis (DVT).

Tendon Strains or Tears

Strains or tears in several different muscles can lead to posterior knee pain when extending your leg.  The most common ones include:

• Gastrocnemius: A large calf muscle that aids in bending the knee and pointing (plantarflexing) the foot

• Hamstrings: A group of three muscles in the back of the thigh that help bend the knee and extend the hip

• Popliteus: A muscle situated in the back of the knee that helps “unlock” the joint as it bends (flexes)

Strains or tears typically occur during exercise or other strenuous activities.  People involved in sports involving sprinting or cutting (making sudden changes of direction), such as basketball and tennis, are particularly at risk, especially if they lack flexibility or fail to warm up properly.

Cutting or jumping movements are also commonly to blame for strains or tears of the gastrocnemius muscle.

Ligamentous Injuries

Injuries to several knee ligaments can also cause pain in the back of the knee. This includes the posterior cruciate ligament (PCL) and a group of ligaments on the outer corner of the back of the knee (called the posterolateral corner).

Injuries to these structures are typically sudden and occur as the result of a traumatic injury, like a car accident or a blow to the leg. In addition to posterior knee pain, injuries to the ligaments also frequently lead to leg instability and difficulty walking.

Nerve Injuries

Injuries to one of the nerves in the back of the knee can also cause pain in this area. The common peroneal nerve is the most frequently affected structure.

Traumas, like a blow to the knee, are the most common cause of injuries to this nerve and frequently also cause damage to multiple other muscles and ligaments at the same time. In addition to pain in the back of the knee, common peroneal nerve injuries may also cause:

• Difficulty walking

• Foot drop (trouble lifting the top part of the foot)

• Numbness and tingling in the outer portion of the leg or top
  of the foot

• Weakness in the ankle or foot muscles

Meniscal Tear

The meniscus is a C-shaped ring of cartilage that helps cushion and stabilize the knee. While meniscal tears commonly cause pain along the outer or inner part of the knee, occasionally the back area is also affected. In these situations, the posterior meniscus (called the posterior horn) is usually torn. Meniscal tears may occur gradually or as the result of a sudden twisting injury. In addition to pain, this condition can also cause:

• Instability while walking

• Joint stiffness and swelling

• Locking or catching
  

• Reduced range-of-motion

Blood Clots

In rare instances, a blood clot or deep vein thrombosis (DVT) may be the cause of your posterior knee pain. Some groups of people are more at risk than others, including:

• Individuals taking hormone therapy or birth control
  

• Older individuals

• People who are overweight or have obesity

• People who are pregnant or who recently had surgery

• Smokers

Blood clots are a medical emergency and can cause the following symptoms:

• Pain or tenderness in the back of the knee or calf

• Skin redness or discoloration

• Swelling

• Thick or distended veins

Unfortunately, many people with a blood clot experience no symptoms at all. If you suspect you have a blood clot, it is crucial to be evaluated by a medical provider immediately.

How to Treat Minor Back-of-Knee Pain

Many types of posterior knee pain can be treated noninvasively. Your medical provider will typically recommend starting with conservative interventions if they suspect a minor muscle strain, a degenerative meniscal tear, a low-grade ligamentous tear, or a Baker’s cyst. Common at-home and noninvasive treatments include:

• Anticoagulant drugs (blood thinners) for blood clots

• Applying ice to the painful area

• Avoiding aggravating activities

• Nonsteroidal anti-Inflammatory drug (NSAID) medication, such as Advil or Motrin (ibuprofen) or Aleve (naproxen)

• Physical therapy

• Splinting or immobilization

Assistive Devices

If you are experiencing difficulty walking because of your knee pain, the following devices may help improve your mobility:

• Cane

• Crutches

• Rollator

• Walker

• Walking Stick
  

When Knee Pain Warrants a Provider Visit

Your knee pain may warrant a visit to a medical provider in
the following situations:

• After a traumatic injury or car accident

• If you are experiencing knee instability, difficulty walking,
  or weakness in the leg

• If you suspect a blood clot or deep vein thrombosis
  

• If your knee symptoms are getting worse or not responding to conservative treatments

Contact Your Healthcare Provider

It is always better to be safe than sorry. Be sure to communicate with your physician if you have any concerns about your knee pain.

Diagnosis

When evaluating your knee, a medical provider will usually ask about the origins of the pain and any corresponding symptoms. In addition, several tools may be used to make a diagnosis, including:

• Electromyography (EMG) to assess the function of your nerves

• Magnetic resonance imaging (MRI) to evaluate for a ligament, tendon, or meniscal damage
  

• Ultrasound to check for blood clots or Baker’s cysts

• X-rays to evaluate for bone injury
  

Understanding Basic Knee Anatomy

The knee joint is where the femur (thigh), the tibia, and the patella (kneecap) bones meet. Surrounding the knee are various stabilizing structures, including the joint capsule and ACL, PCL, LCL, and MCL ligaments. In addition, muscles like the quadriceps, gastrocnemius, hamstrings, and popliteus aid in moving the knee and provide additional stability.

Treatment for More Complex Back of Knee Pain Causes

In some cases, more invasive interventions may be needed for your posterior knee pain. More severe ligament and muscle tears often must be surgically repaired or reconstructed. Meniscal tears that do not respond to conservative treatments also frequently require a meniscectomy operation.

Baker’s cysts are also occasionally surgically removed or aspirated (drained of fluid), though they typically reform if the underlying cause of the cyst is not addressed.

If anticoagulant medications are not effective, a vena cava filter may need to be placed in your veins to prevent a blood clot from traveling to your lungs (called a pulmonary embolism).

When to Resume Normal Activity

The timeline to return to normal activity varies widely, depending on the cause of your knee pain. Daily activities like walking, stair climbing, and household chores should be attempted first and, only then, if they do not increase your pain significantly. After successfully reintroducing these tasks, you may be able to return to an exercise regimen gradually.

Be sure to start slowly and avoid pushing through pain. It is important to work with your medical provider before restarting strenuous activities. Beginning them too early may worsen your condition and prolong the pain.

Summary

A variety of conditions can cause posterior knee pain that worsens when you straighten your leg. This includes muscle strains, ligament tears, nerve or meniscus damage, Baker’s cysts, and blood clots. Many of these issues are well treated with conservative interventions like medications, ice, and physical therapy.

Others require more invasive surgical procedures to improve the pain. Getting your symptoms diagnosed by a medical provider is an important first step if you are experiencing pain in the back of your knee.

Read the original article on Verywell Health.