What Is Behind Your Knee? Anatomy and Pain Causes

The soft, diamond-shaped hollow behind your knee is called the popliteal fossa, and it contains a surprisingly dense collection of structures: a major artery and vein, several nerves, tendons from the hamstring and calf muscles, a small stabilizing muscle called the popliteus, fat, lymph nodes, and the joint capsule itself. Pain in this area can stem from any of these tissues, which is partly why it can be tricky to pin down. The most common culprits range from Baker’s cysts and muscle strains to ligament injuries and, less often, vascular problems that demand urgent attention.

What the Popliteal Fossa Actually Contains

Think of the back of your knee as a narrow corridor that everything important has to pass through on its way between the thigh and the lower leg. The two largest structures running through it are the popliteal artery and the popliteal vein, which carry blood to and from the lower leg. Right alongside them runs the tibial nerve, a major branch of the sciatic nerve that controls much of the calf and foot. The common fibular (peroneal) nerve splits off at the upper outer edge of the fossa and wraps around the head of the fibula just below the knee. Cadaver mapping studies have shown that these structures sit close together, with the artery, vein, and tibial nerve clustered roughly 33 to 39 millimeters from bony landmarks on the inner side of the knee, while the common fibular nerve runs closer to the outer side, about 8 to 13 millimeters from the lateral femoral epicondyle depending on the person’s sex.¹

The walls of this space are formed by muscles. The upper boundaries come from the hamstring tendons: the biceps femoris tendon on the outer side and the semitendinosus and semimembranosus tendons on the inner side. The lower boundaries are the two heads of the gastrocnemius, the large calf muscle that gives the back of your lower leg its shape. Sitting deep within the fossa is the popliteus muscle, a small, flat muscle that helps unlock the knee from a fully straightened position so you can begin to bend it. The small saphenous vein also runs through the area just under the skin, draining blood from the outer part of the foot and calf. Buried within the fat pad behind the knee are a handful of popliteal lymph nodes, which filter lymph from the lower leg.

All of these structures are packed into a space that is only a few centimeters wide when the knee is straight, and that changes shape dramatically as the knee bends. That close packing explains why a problem in one structure can produce symptoms that feel like they’re coming from another, and why swelling in this area tends to press on neighboring tissues quickly.

Baker’s Cysts

If you’ve ever noticed a squishy, grape-sized lump at the back of your knee, the most likely explanation is a Baker’s cyst (also called a popliteal cyst). This is an enlargement of the gastrocnemius-semimembranosus bursa, a small fluid-filled sac that normally sits between those two tendons behind the knee.² When the knee joint produces excess fluid, whether from arthritis, a meniscus tear, cartilage damage, or simple overuse, that fluid can be pushed through a one-way valve-like connection into the bursa, inflating it like a water balloon.

Baker’s cysts can be caused by both inflammatory conditions like rheumatoid arthritis and non-inflammatory problems like osteoarthritis or meniscal tears.³ Many are painless and are found incidentally. When they do hurt, it’s usually a feeling of tightness or fullness behind the knee that worsens when you fully straighten or fully bend the leg. The real trouble comes if a cyst ruptures: fluid leaks down into the calf, causing sudden pain, swelling, and redness that can closely mimic a deep vein thrombosis (DVT). This mimicry is a well-known diagnostic trap. Clinicians see it frequently enough that it has its own informal name, “pseudothrombophlebitis.”

Baker’s Cysts in Children

Baker’s cysts in children behave quite differently from those in adults. In adults, there is almost always an underlying joint problem feeding fluid into the bursa. In children, cysts tend to appear around the middle of the first decade of life without any associated joint disease at all, suggesting that the cause is different in the two age groups.⁴ Pediatric popliteal cysts are usually painless, noticed by a parent during bath time, and the majority resolve on their own without any treatment. The key clinical distinction matters because children with a popliteal cyst rarely need the same workup for intra-articular damage that an adult would.

Muscle and Tendon Injuries Around the Back of the Knee

Several muscles and tendons converge at the back of the knee, and injury to any of them can produce pain in or near the popliteal fossa. The hamstring tendons (semimembranosus, semitendinosus, and biceps femoris) all cross the back of the knee on their way to their attachment points on the tibia and fibula. Strains at their lower ends can cause pain that the person localizes to the back of the knee rather than the thigh, especially during deep bending or deceleration.

The semimembranosus tendon deserves a special mention because it inserts on the posteromedial corner of the knee and is surrounded by its own bursa. Overuse of this tendon, particularly in runners, can cause a condition called distal semimembranosus tendinopathy, which produces localized pain on the inner-back aspect of the knee. Both the tendon and the bursa around it can be the pain source, and the overlap with other causes of medial knee pain sometimes makes the diagnosis elusive.

“Tennis Leg” and Gastrocnemius Injuries

“Tennis leg” is the common name for sudden, sharp pain in the upper calf and behind the knee that strikes during explosive movements, usually when the knee is extending and the ankle is being forced into dorsiflexion at the same time. For decades, this was blamed on a rupture of the plantaris tendon, a thin, vestigial tendon that runs alongside the gastrocnemius. That turns out to be wrong in most cases. In a review of ultrasound findings in 141 patients diagnosed with tennis leg, a partial tear of the medial head of the gastrocnemius was the cause in about two-thirds of cases, while actual plantaris rupture accounted for only about 1.4%.⁵ The typical patient is middle-aged and active, not necessarily a tennis player. The injury happens at the distal muscle-tendon junction of the medial gastrocnemius, which is the area closest to the back of the knee.

The clinical presentation of tennis leg overlaps with several other conditions, including Achilles tendon rupture, DVT, and a ruptured Baker’s cyst, so imaging is often needed to sort things out.⁶ Research into why the medial head of the gastrocnemius is the weak link has found that dorsiflexion of the ankle markedly increases its stiffness, concentrating stress at the muscle-tendon junction and making it more vulnerable to tearing.⁷

Popliteus Tendon Injuries

The popliteus is a small but functionally important muscle. Its tendon attaches to the outer (lateral) femoral condyle, and the muscle itself wraps behind the knee to insert on the back of the tibia. Injuries here are far less common than meniscal or ligament tears, but they do occur and can be a source of chronic, hard-to-diagnose knee pain. Reported injury patterns include tears at the muscle-tendon junction and avulsion tears where the tendon attaches to the femur, usually associated with trauma to other parts of the knee.⁸ Isolated popliteus tendon injuries are uncommon enough that they can go unrecognized on initial evaluation.

Ligament and Meniscus Problems That Cause Posterior Knee Pain

The posterior cruciate ligament (PCL) sits deep inside the knee and prevents the shin bone from sliding backward under the thigh bone. Unlike the more commonly injured ACL, PCL tears often result from a direct blow to the front of the upper shin, such as hitting the dashboard in a car crash or falling onto a bent knee. The hallmark symptom is severe pain behind the knee, especially when bending past about 90 degrees.⁹ The posterior drawer test, in which the examiner pushes the tibia backward and checks for abnormal movement, is the standard bedside check. MRI studies have confirmed that PCL tears produce measurable increases in how far the tibia shifts backward, even in a relaxed, unloaded position.¹⁰

Tears of the posterior horn of the meniscus can also cause pain that the person describes as “behind the knee.” This is especially true for the medial meniscus, whose posterior horn sits close to the back of the joint. Obesity is a significant risk factor: one retrospective study found that obese patients were considerably more likely to have posterior horn tears, and the risk jumped further for tears at the meniscal root.¹¹ Root tears matter because they effectively untether the meniscus, reducing its ability to distribute weight across the joint and accelerating cartilage wear.

Vascular Problems in the Popliteal Fossa

The popliteal artery is the continuation of the femoral artery as it passes behind the knee, and it is the sole source of blood flow to the lower leg. Problems here tend to be less common than muscular or joint causes of posterior knee pain, but they can be far more serious.

Popliteal Artery Aneurysms

A popliteal artery aneurysm (PAA) is a ballooning of the artery wall behind the knee. Most are found in older men with a history of cardiovascular disease. Many PAAs are asymptomatic and discovered incidentally. The danger comes from clot forming inside the aneurysm sac, which can break off and block blood flow to the foot, or from rupture. Rupture is uncommon, occurring in roughly 2.5% of PAAs that come to surgery, but it is a surgical emergency.¹² Patients with ruptured PAAs tend to be older and have larger aneurysms than those treated electively. A major clinical problem is misdiagnosis: in one systematic review, the initial diagnosis was wrong in up to about 79% of recorded ruptured PAA cases, with DVT and ruptured Baker’s cyst being the most common incorrect first impressions.¹³

Even without rupture, PAAs can cause limb-threatening events through clot buildup. A higher percentage of thrombus lining the aneurysm wall is strongly associated with symptomatic disease and acute events like sudden limb ischemia.¹⁴ If you’ve been told you have a popliteal aneurysm, the amount of clot inside it matters more than the diameter alone when it comes to predicting whether it will cause trouble.

Popliteal Artery Entrapment Syndrome

Popliteal artery entrapment syndrome (PAES) is a very different vascular problem, and it tends to affect young, athletic people rather than older adults. In PAES, the popliteal artery gets compressed by surrounding muscle or tendon tissue, usually because of an anatomical variant in how the gastrocnemius muscle developed. The syndrome affects an estimated 0.17% to 3.5% of the population, though the wide range reflects how often mild cases go undiagnosed.¹⁵ It can be structural (caused by an abnormal muscle or tendon path) or functional (caused by muscle hypertrophy compressing a normally positioned artery). Common symptoms include cramping or fatigue in the calf during exercise, numbness or tingling in the foot, and sometimes coolness or pallor of the foot that resolves with rest.¹⁶

PAES is worth keeping in mind for younger athletes who develop exercise-related calf and posterior knee pain that doesn’t fit the usual pattern of a muscle strain or overuse injury. The key red flag is that symptoms come on predictably with exertion and go away completely at rest, often accompanied by changes in pulse at the ankle during provocative maneuvers like forced plantar flexion.

Nerve Compression Behind the Knee

Because the tibial nerve and the common fibular nerve both travel through or along the borders of the popliteal fossa, anything that takes up space in this region can compress them. Masses such as Baker’s cysts, ganglion cysts, hematomas, or tumors can all press on these nerves and cause pain, tingling, or weakness in the lower leg and foot. One documented example involved a patient with rheumatoid arthritis who developed an organized hematoma (a clot that had become walled off into solid tissue) in the popliteal fossa, which compressed both the popliteal artery and the tibial nerve simultaneously, causing vascular and neurological symptoms in the lower leg.¹⁷

Schwannomas, which are benign tumors that grow from the sheath of a nerve, can also appear in the popliteal fossa. When they arise from the common peroneal nerve, they present as a palpable mass at the outer edge of the knee and can mimic a cyst, a vascular abnormality, or other soft-tissue lesions, often leading to delayed diagnosis.¹⁸ The takeaway is that any persistent lump behind the knee that doesn’t behave like a typical Baker’s cyst, especially one associated with nerve symptoms like foot drop or numbness along the outer shin, deserves imaging rather than watchful waiting.

How Posterior Knee Problems Are Diagnosed

Because so many different structures occupy such a small space, clinical examination alone often cannot pinpoint the exact source of posterior knee pain. Two imaging tools dominate the workup: ultrasound and MRI. Ultrasound is quick, inexpensive, and excellent for superficial and peri-articular problems. Studies comparing the two have found near-perfect agreement between ultrasound and MRI for detecting Baker’s cysts, meniscal cysts, and collateral ligament tears.¹⁹ Where ultrasound falls short is in evaluating deep intra-articular structures: it has very low sensitivity for ACL tears and only moderate sensitivity for PCL tears and degenerative meniscal changes. For those, MRI remains the standard.

Both modalities are useful and often complementary: ultrasound is a reasonable first-line tool, particularly when the clinical suspicion is a Baker’s cyst, fluid collection, or superficial tendon problem, while MRI is reserved for situations where deep ligament or meniscal pathology needs to be ruled in or out.²⁰ In cases where a vascular problem is suspected, Doppler ultrasound or CT angiography enters the picture.

Why Surgeons Worry About the Back of the Knee

The dense packing of neurovascular structures in the popliteal fossa creates real hazards during knee surgery. During arthroscopic repair of the posterior horn of the lateral meniscus, for example, the popliteal neurovascular bundle sits dangerously close to the surgical field. Research using upright MRI has shown that bending the knee to 90 degrees increases the distance between the meniscus and the neurovascular bundle compared to a straight leg, making surgery safer in flexion.²¹

The proximity issue is even more dramatic during PCL reconstruction, which requires drilling a tunnel through the tibia toward the back of the knee. Cadaver studies have shown that at full extension, a drill path aimed at the PCL attachment passes directly through the popliteal artery in every specimen tested. Increasing knee flexion reduces but does not eliminate this risk.²² Detailed anatomical measurements place the popliteal artery as close as 3 millimeters from the upper PCL attachment site, with an average distance of about 10 millimeters.²³ These margins are thin enough that surgeons operating in the posterior compartment of the knee treat the popliteal artery as a constant concern, not a theoretical one.

When Pain Behind the Knee Needs Urgent Attention

Most causes of posterior knee pain are musculoskeletal and, while uncomfortable, are not dangerous. But a few warrant prompt evaluation. Signs that point toward a vascular emergency include sudden, severe swelling of the entire lower leg, especially if the skin looks dusky or pale; a palpable, pulsating mass behind the knee; loss of pulses at the ankle; or new coldness and numbness in the foot. As noted earlier, ruptured popliteal aneurysms are frequently mistaken for DVT or a ruptured Baker’s cyst, and the delay in correct diagnosis can cost the limb. DVT itself is another urgent diagnosis: if swelling, warmth, and calf tenderness appear behind the knee without a clear musculoskeletal explanation, particularly in someone with risk factors like recent immobility or surgery, an ultrasound to check for clot is standard practice.

Nerve compression symptoms that come on suddenly or progress, such as foot drop, loss of sensation over the top of the foot, or difficulty lifting the toes, also need timely evaluation. These can result from masses in the popliteal fossa pressing on the common peroneal nerve, and early decompression often produces better outcomes than waiting.

For the majority of people, posterior knee pain turns out to be a Baker’s cyst, a hamstring or gastrocnemius strain, or referred pain from a meniscal tear or mild arthritis. But the popliteal fossa’s role as a narrow corridor for the leg’s major artery, vein, and nerves means that the less common vascular and neurological causes carry disproportionately high stakes, which is why persistent or atypical symptoms behind the knee deserve more than a wait-and-see approach.