Pain in the back of the knee can be a perplexing issue, ranging from minor discomfort to a sign of a more serious underlying condition. This common complaint can arise from various sources, including muscle or tendon injuries, issues within the knee joint itself, or even problems with blood vessels and nerves. Understanding the potential origins of this pain is important for seeking appropriate care and managing symptoms effectively.
Muscle and Tendon Related Pain
Pain in the back of the knee often originates from the muscles and tendons surrounding the joint. The hamstring muscles, located at the back of the thigh, are a frequent source. These three muscles—semitendinosus, semimembranosus, and biceps femoris—can experience strains or tears where their tendons attach near the knee. A hamstring strain causes sudden, sharp pain in the back of the thigh, extending behind the knee, and may involve swelling, bruising, or leg weakness.
Calf muscle strains also contribute to posterior knee pain. The gastrocnemius muscle, a large calf muscle crossing both the knee and ankle joints, is particularly susceptible to injury. A gastrocnemius strain causes sudden pain at the back of the lower leg or behind the knee, sometimes with a tearing sensation, swelling, or bruising. The soleus muscle, located deeper in the calf, can also be strained due to overuse, causing pain and tightness behind the knee, especially with activities like running.
Popliteus tendinopathy is another specific tendinopathy affecting the back of the knee. The popliteus is a small muscle behind the knee that aids in stabilization and initial knee flexion. When its tendon becomes irritated or overused, it leads to pain, often felt on the outer side of the back of the knee, worsening with movements like walking downhill or descending stairs. Swelling, tenderness, and stiffness can also accompany this condition.
Joint and Ligament Conditions
Conditions affecting the internal structures of the knee joint are frequent causes of pain behind the knee. A posterior meniscus tear, involving the C-shaped cartilage pads that cushion the knee, can result in pain, especially with deep squatting or twisting motions. Swelling, stiffness, and a sensation of catching or locking are common symptoms associated with meniscal tears. Pain in the back of the knee can indicate a tear in the posterior horn of the medial or lateral meniscus.
A Baker’s cyst, also known as a popliteal cyst, is a fluid-filled lump that forms at the back of the knee. While it can cause pain, tightness, or a noticeable bulge, a Baker’s cyst is often a symptom of an underlying knee problem rather than a primary cause of pain itself. Conditions like arthritis or a meniscus tear can lead to the knee producing excess synovial fluid, which then forms the cyst.
Injury to the Posterior Cruciate Ligament (PCL) can also cause pain and other symptoms. The PCL is a strong ligament connecting the thigh bone to the shin bone, preventing the shin bone from sliding too far backward. PCL injuries typically result from a powerful force to the front of a bent knee, such as a direct blow during a fall or car accident. Symptoms include pain, swelling, tenderness behind the knee, stiffness, and instability.
Vascular and Nerve Concerns
Less common but serious causes of pain behind the knee involve blood vessels and nerves. Deep Vein Thrombosis (DVT) occurs when a blood clot forms in a deep vein, commonly in the leg, including the popliteal vein. Symptoms often include leg swelling, cramping pain or tenderness, warmth, and a reddish or discolored skin appearance. DVT is serious because a clot can break loose and travel to the lungs, leading to a life-threatening pulmonary embolism.
A popliteal artery aneurysm is an irregular bulge in the artery wall behind the knee joint. While small aneurysms might not cause symptoms, larger ones can lead to knee pain, lower leg pain, swelling, or a pulsating feeling. A serious complication is blood clot formation within the aneurysm, which can restrict blood flow to the lower leg and potentially cause severe lack of blood flow, numbness, coldness, or inability to move the foot.
Nerve entrapment, such as of the tibial nerve, can also cause pain in the back of the knee. This occurs when a nerve is compressed or irritated, leading to pain, numbness, or tingling sensations. While direct nerve entrapment in the popliteal space is less common, it can be a source of discomfort.
Other Potential Causes
Pain in the back of the knee can sometimes be referred from other body areas, such as the hip or lower back. Sciatica, where the sciatic nerve is irritated or compressed, can cause pain radiating down the back of the leg, potentially reaching behind the knee. This pain often presents with numbness or tingling.
Infections, while rare, can also lead to posterior knee pain. Osteomyelitis, a bone infection, or septic arthritis, an infection within the joint, can cause severe pain, swelling, warmth, and redness around the knee. These conditions often present with systemic symptoms like fever and chills, and require prompt medical attention.
Extremely rare causes include tumors, which may be benign or cancerous. These growths can cause pain, swelling, or other symptoms in the knee area. They are a consideration in cases of persistent, unexplained pain.
Knowing When to Get Help
Recognizing when back of knee pain warrants medical evaluation is important for proper diagnosis and treatment. Seeking immediate medical attention is advisable if the pain is sudden and severe, or accompanied by rapid, significant swelling. Other concerning symptoms include warmth or redness around the knee or calf, which could indicate inflammation or a serious condition like DVT.
A fever, particularly when combined with knee pain, swelling, and redness, suggests a possible infection like septic arthritis, requiring prompt medical intervention. Inability to bear weight on the affected leg, numbness or tingling in the leg or foot, or a “popping” sound at the time of injury are also indicators for prompt medical consultation. If a pulsating mass is felt behind the knee, especially with associated pain or changes in leg color or temperature, a popliteal artery aneurysm should be considered, requiring immediate medical assessment.