Pain behind the knee when straightening the leg and applying pressure often points toward a mechanical issue within or surrounding the knee joint. This area, the popliteal fossa, is densely packed with nerves, blood vessels, tendons, and ligaments. Full extension and weight-bearing compress these structures, making a subtle problem suddenly noticeable. Understanding the potential causes is the first step toward finding relief.
Structural Causes of Posterior Knee Swelling
A frequent source of posterior knee pain aggravated by full extension and pressure is a Baker’s Cyst, also known as a popliteal cyst. This is a fluid-filled sac that forms when excess synovial fluid collects in the back of the knee. The cyst often presents as a lump or a feeling of tightness or fullness in the popliteal space.
The pain intensifies when the knee is straightened because the cyst is compressed in the limited space between the bones and soft tissues. Weight-bearing further increases this compression by pushing the joint surfaces together. A Baker’s cyst is a secondary condition, resulting from an underlying knee problem like arthritis or a meniscus tear that causes chronic inflammation and overproduction of joint fluid.
Treating the underlying cause is necessary for long-term resolution.
Pain Originating from Muscle and Tendon Stress
Soft tissue structures like tendons and muscles that cross the back of the knee can become painful when put under maximum tension during full extension and loading. Hamstring tendinopathy, particularly near the tendon’s attachment points, is a common culprit. Overuse or repetitive stress can lead to inflammation or tiny tears in the tendon.
This pain is often described as a deep ache or localized tenderness that worsens with physical activity. Full extension stretches the hamstring tendons to their limit, and when combined with weight-bearing, the strained tissue is intensely loaded, causing sharp discomfort.
Another potential source is a strain in the gastrocnemius muscle, one of the main calf muscles, near its attachment just above the back of the knee. A strain here will similarly protest when the leg is fully straightened and carries weight.
Internal Joint Damage and Instability
Sharp pain, accompanied by mechanical symptoms and worsened by weight-bearing extension, can indicate damage to the cartilage structures inside the joint. Posterior horn meniscus tears are a specific concern because the rear portion of the cartilage is directly pinched between the femur and tibia when the leg is fully extended and loaded. This mechanical compression causes sharp, localized pain.
These tears can also cause symptoms like clicking, catching, or a sensation that the knee is locking, indicating a fragment of torn cartilage is interfering with normal joint movement. A Posterior Cruciate Ligament (PCL) injury can also cause posterior knee pain.
The PCL prevents the shinbone from sliding too far backward relative to the thighbone. An injury, often resulting from a direct blow to a bent knee, can lead to chronic instability and a deep aching sensation, especially when bearing weight or walking down an incline.
The loss of PCL stability alters joint mechanics, which can lead to pain and increased pressure on the cartilage. Even a subtle, isolated PCL injury can be a source of persistent posterior discomfort.
Initial Steps and Urgent Warning Signs
For immediate, non-severe discomfort, initial self-care can help manage symptoms and prevent further irritation. The R.I.C.E. protocol—Rest, Ice, Compression, and Elevation—is recommended to reduce pain and swelling. Rest involves modifying activity to avoid painful full extension and weight-bearing. Applying ice to the back of the knee for 15 to 20 minutes several times a day can help reduce local inflammation.
Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, can provide short-term relief from pain and swelling. However, self-treatment is temporary, and persistent or worsening symptoms require a professional diagnosis to identify the underlying cause.
It is important to know that certain symptoms, known as red flags, warrant immediate medical attention, as they may indicate a serious condition. These urgent warning signs include:
- Fever or severe warmth and redness around the joint, which could signal an infection.
- The possibility of Deep Vein Thrombosis (DVT), a blood clot in the leg veins, which presents with sudden, significant swelling, redness, and warmth in the calf, sometimes mimicking a ruptured Baker’s cyst.
- Any sign of neurological compromise, such as numbness, tingling, or foot weakness.
- The inability to bear any weight on the leg after an injury.