Pain in the back of the knee, known as the popliteal fossa, is a common complaint that frequently occurs when the joint is bent (flexion). This discomfort often signals an issue involving soft tissues, such as tendons or fluid-filled sacs, or deeper mechanical problems within the joint structure. Understanding the nature of the pain—whether it is a dull ache, a sharp sensation, or accompanied by stiffness—can help pinpoint the underlying cause.
Primary Causes: Baker’s Cysts and Tendon Strain
A frequent cause of posterior knee discomfort is a Baker’s cyst, also called a popliteal cyst, which is essentially a fluid-filled sac that develops behind the knee joint. This cyst forms when the knee produces an excessive amount of synovial fluid, often in response to an underlying issue like arthritis or a cartilage tear. The fluid then pools in the back of the knee, where it creates a noticeable bulge.
The pressure from this fluid accumulation causes a feeling of tightness or fullness, and the pain typically worsens when the knee is fully bent or fully straightened because of the compression or stretching of the cyst. While the cyst itself is benign, its presence is a sign that there is another problem inside the knee joint generating the excess fluid. If the cyst ruptures, the fluid can leak into the calf, causing a sudden, sharp pain, along with swelling and bruising down the leg.
Another common source of pain when bending the knee involves soft tissue injuries, specifically tendon strain or tendinitis affecting the hamstrings or the popliteus muscle. The hamstring muscles run down the back of the thigh and insert near the knee, making them susceptible to strain during sudden, forceful movements or overstretching. A hamstring strain often presents as a sharp, muscular pain that is felt high up behind the knee and is aggravated by bending the knee against resistance.
The popliteus muscle, a small triangular muscle located deep in the back of the knee, plays an important role in “unlocking” the knee joint from a fully straightened position. Overuse, particularly in activities involving downhill running or repetitive knee flexion, can lead to tendinopathy or strain in this muscle. Pain from a popliteus injury is often felt on the outer side of the back of the knee and becomes worse when walking down stairs or hills.
Deeper Joint and Structural Problems
Pain upon bending can originate from deeper structural issues within the knee joint, involving the menisci, ligaments, or cartilage. A meniscal tear, particularly one affecting the posterior horn, causes mechanical symptoms noticeable during flexion. The torn cartilage can get pinched within the joint space when the knee is bent deeply, leading to sharp pain, catching, or a locking sensation. These symptoms are frequently triggered by actions like squatting or twisting while bearing weight.
Injury to the Posterior Cruciate Ligament (PCL), one of the four main stabilizing ligaments, is a less common but serious cause of posterior knee pain. The PCL connects the thigh bone to the shin bone and prevents the shin bone from shifting too far backward. An injury usually results from a powerful force, such as a direct blow to the front of a bent knee, and often causes pain felt deep in the back of the knee, especially when kneeling or attempting to decelerate.
Osteoarthritis, typically known for causing pain in the front or sides of the knee, can also manifest as pain in the back of the joint during flexion in advanced stages. This posterior discomfort occurs due to the formation of bone spurs (osteophytes) or severe joint space narrowing that crowds the structures in the back of the knee when it bends.
Immediate Steps and Warning Signs
For immediate management of acute posterior knee pain, initial steps focus on protecting the joint and reducing inflammation.
- Resting the affected knee by avoiding activities that trigger the pain is important for the healing process.
- Applying a cold pack to the back of the knee for periods of 10 to 20 minutes can help to minimize swelling and dull the pain sensation.
- Elevating the leg slightly above the level of the heart while lying down can further aid in reducing any fluid build-up or swelling in the area.
- If the pain is severe or makes walking difficult, using a cane or crutch to temporarily limit the amount of weight placed on the leg can prevent further aggravation.
It is important to recognize specific warning signs that signal the need for urgent medical evaluation to rule out more serious conditions.
- Inability to bear weight on the leg or if the knee appears deformed or twisted.
- Sudden, severe swelling, fever, or signs of infection, such as increased warmth or red streaks.
- Calf pain, tenderness, and swelling, which could indicate a deep vein thrombosis (DVT) and is sometimes mistaken for a ruptured Baker’s cyst.
- Significant pain accompanied by breathlessness or chest pain, which could indicate a pulmonary embolism and requires emergency services.