Popliteal lymph nodes sit in the fat-filled space behind each knee, tucked into the soft tissue of the popliteal fossa. They are small and usually not palpable in healthy adults, which is why many people are surprised to learn they exist at all. But these nodes play a genuine role in immune surveillance for the lower leg and foot, and they become clinically important in situations ranging from skin infections to melanoma staging.
Where Exactly They Sit and How They Are Arranged
The popliteal fossa is the diamond-shaped hollow you can feel when you slightly bend your knee. Within it, the popliteal lymph nodes are organized into two layers: a superficial group, located just below the deep fascia that separates skin-level tissue from deeper structures, and a deep group, nestled closer to the blood vessels running behind the knee joint.1European Society of Radiology. Understanding Upper and Lower Extremity Lymph Node Anatomy For PET-CT Utilizing Clay Models A perforating lymph vessel connects the two groups: it rises from the superficial popliteal node and terminates in the deep popliteal node. From there, the deep node’s outgoing vessel travels alongside the femoral blood vessels up into the thigh, eventually relaying to the inguinal lymph nodes in the groin.2PubMed. Perforating and deep lymphatic vessels in the knee region: an anatomical study and clinical implications
Multiple lymph vessels from the lower leg converge on these nodes. The anterior tibial, posterior tibial, and peroneal lymph vessels all travel alongside their corresponding blood vessels and feed into the deep popliteal node. Some of the anterior tibial vessels first pierce the membrane between the tibia and fibula before joining the posterior tibial or peroneal channels, then enter the node together. This arrangement means the popliteal nodes serve as a way station for fluid draining from multiple compartments of the lower leg.
Not All Lower-Leg Lymph Flows Through the Knee
One common misconception is that every lymph vessel in the lower leg routes through the popliteal nodes on its way to the groin. In reality, only a specific subset does. A cadaver study using CT lymphangiography mapped the drainage pathways of the lower leg and found that only the posterolateral group of lymphatic vessels drained to the popliteal nodes. The anteromedial, anterolateral, and posteromedial groups bypassed the knee entirely and connected directly to inguinal nodes in the groin.3PubMed. Lower-Limb Lymphatic Drainage Pathways and Lymph Nodes: A CT Lymphangiography Cadaver Study This has practical consequences: a wound or infection on the back or outer side of the calf is more likely to cause popliteal node swelling than one on the inner shin.
This selective drainage also matters in cancer staging. When a melanoma sits below the knee, whether it drains to the popliteal nodes depends partly on its position. A tumor on the posterior calf may route through the popliteal fossa, while one on the anterior shin could skip it entirely and drain straight to the groin.
How Often They Show Up on Imaging
Popliteal lymph nodes are not always visible on MRI, and whether they appear depends heavily on age. In a study of 150 patients who had knee MRIs for various reasons, about 37% showed at least one visible popliteal node. But the age distribution was striking: roughly 94% of patients aged 5 to 15 had visible popliteal nodes, while only about 20% of those aged 66 to 76 did.4PubMed Central. Features of the popliteal lymph nodes seen on musculoskeletal MRI in a Western population The average node measured just under 6 millimeters in its shortest dimension and about 8 millimeters in its longest, sitting roughly 6 centimeters above the lowest border of the outer femoral condyle.
The age-related decline mirrors what happens at other lymph node sites throughout the body. Lymph nodes tend to become smaller and fattier with age, replaced by tissue that is harder to distinguish from surrounding fat on imaging. In children and teenagers, their prominence is normal and rarely a sign of anything worrying. In older adults, a suddenly visible or enlarging popliteal node is more likely to warrant investigation.
Their Role in Immune Defense
Like lymph nodes elsewhere, popliteal nodes filter lymph fluid, trap pathogens, and serve as a meeting ground where immune cells encounter foreign material. Animal research has shown concretely what happens when they are removed. In one experiment studying a parasitic skin infection, removing the popliteal lymph node before infection caused the disease to worsen considerably, delaying the body’s ability to mount an effective immune response.5PubMed Central. Mechanisms of immunity to leishmaniasis. IV. Significance of lymphatic drainage from the site of infection Separate research found that chronic intestinal infection in mice impaired the popliteal node’s ability to expand and respond to a new bacterial challenge delivered to the footpad, with fewer dendritic cells migrating to the node and fewer pathogen-specific immune cells accumulating there.6PLOS Pathogens. Atrophy of skin-draining lymph nodes predisposes for impaired immune responses to secondary infection in mice with chronic intestinal nematode infection
These findings underscore that popliteal nodes are not vestigial or irrelevant. They provide localized immune protection for the foot and lower leg, and their loss or dysfunction can leave that territory more vulnerable to infection.
When Popliteal Nodes Swell from Infection
You are most likely to notice your popliteal lymph nodes when they swell in response to a nearby infection. Any wound, cellulitis, or infected insect bite on the foot, ankle, or calf can cause reactive enlargement. But one of the more distinctive causes is cat-scratch disease. A case report described a child who presented with a swollen mass behind the knee after cat flea bites on the leg. Ultrasound confirmed popliteal lymphadenopathy, and blood tests revealed active Bartonella henselae infection, the bacterium responsible for cat-scratch disease.7PubMed. The flea’s knees: A unique presentation of cat scratch disease The presentation is uncommon enough that popliteal involvement was considered a rare finding in children.
Other infections that can enlarge popliteal nodes include staph and strep skin infections, tuberculosis, and infections caused by nontuberculous mycobacteria.8Consultant. Cat-Scratch Disease Presenting as a Popliteal Mass The broad clinical message is that any persistent lump behind the knee deserves a look at the skin of the foot, ankle, and lower calf for a potential source of infection, including subtle scratches or bites that the person may not have noticed.
The Popliteal Node and Melanoma Staging
The popliteal fossa is most discussed in surgical oncology in the context of melanoma on the lower leg and foot. Sentinel lymph node biopsy, the procedure used to check whether melanoma has begun to spread, depends on identifying which nodes receive drainage from the tumor’s location. For melanomas below the knee, the popliteal nodes are sometimes the first stop.
A study of more than 3,900 below-knee melanoma cases found that lymphoscintigraphy showed drainage to a popliteal sentinel node in about 4.5% of them. Among patients who had the node biopsied, roughly 17% tested positive for metastatic melanoma, and in about 10% of those cases it was the only positive node found.9PubMed. Clinical importance and surgical management of sentinel lymph nodes in the popliteal fossa of melanoma patients A positive popliteal node was linked to higher recurrence risk and worse survival, though biopsy of the popliteal node itself did not appear to improve regional control or survival in that study.
A systematic review looking at popliteal involvement across below-knee melanomas found that popliteal nodes were the primary lymphatic station in about 5.6% of cases. The overall rate of finding melanoma metastases in popliteal nodes was roughly 1.5%. Most often the nodes acted as “interval” or “in-transit” stations, meaning melanoma cells passed through them on the way to inguinal nodes in the groin.10Indian Journal of Surgery. The Lymphatic Drain of Below-Knee Malignant Melanoma: Is the Popliteal Fossa a Ghost Station?
Another series examined patients who did undergo popliteal sentinel node biopsy for melanomas below the knee. Among 15 patients who had the procedure, more than half had metastatic disease in their popliteal node. In half of those positive cases, the popliteal node was the only site of metastatic involvement, meaning the cancer would have been missed entirely if only the groin nodes had been checked.11PubMed Central. Management of Popliteal Sentinel Nodes in Melanoma That finding underscores why surgeons need to include the popliteal basin in lymphatic mapping for any melanoma arising below the knee.
Popliteal lymph node dissection, in which the nodes are surgically removed, is performed when metastatic disease is confirmed there. It remains rare because the popliteal fossa is an anatomically complex space with major nerves and blood vessels running through it, but it can be done successfully.12PubMed Central. Popliteal lymph node dissection for metastases of cutaneous malignant melanoma
Popliteal Nodes and Rheumatoid Arthritis
Lymph nodes respond to inflammation, not just infection and cancer. In rheumatoid arthritis, the chronic inflammation inside the knee joint can drive changes in the nearby popliteal nodes that are visible on imaging. A study comparing knee MRIs in patients with rheumatoid arthritis versus osteoarthritis found that every single rheumatoid arthritis patient had visible popliteal nodes, compared to fewer than half of osteoarthritis patients. The nodes were also larger in rheumatoid arthritis, averaging about 5.4 millimeters compared to roughly 3.6 to 4.1 millimeters in osteoarthritis, and they were less likely to show the fatty center that characterizes a normal, quiet lymph node.13PubMed Central. The role of popliteal lymph nodes in differentiating rheumatoid arthritis from osteoarthritis by using CE 3D FSPGR MR imaging
This finding suggests that popliteal node characteristics on MRI could help distinguish inflammatory joint disease from degenerative joint disease when the clinical picture is ambiguous. Larger, more numerous popliteal nodes lacking a fatty hilum point more toward an inflammatory process like rheumatoid arthritis.
Mistaking a Popliteal Node for a Baker’s Cyst
When someone notices a lump behind the knee, the first assumption is often a Baker’s cyst, the fluid-filled pouch that forms when knee joint fluid bulges into the popliteal fossa. But an enlarged popliteal lymph node can produce a similar-feeling mass in the same location. Other structures that can mimic a popliteal lump include arterial aneurysms, deep vein thrombosis, fatty tumors, and other soft-tissue masses.14Journal of Pathology and Translational Medicine. Angiomyomatous Hamartoma of Popliteal Lymph Node: An Unusual Entity
Ultrasound is typically the first imaging step and can usually distinguish a fluid-filled cyst from a solid lymph node. A normal reactive lymph node will appear as a small, oval, solid structure with a bright fatty center, while a Baker’s cyst appears as a well-defined fluid pocket that often communicates with the joint space. If ultrasound reveals a solid mass or a lymph node without a normal fatty hilum, further evaluation with MRI or biopsy may follow.
Lymphedema and the Consequences of Popliteal Node Removal
Whenever lymph nodes are surgically removed or damaged by radiation, the risk of lymphedema enters the picture. Secondary lymphedema occurs when disruption to the lymphatic system causes fluid to accumulate in the affected limb.15PubMed. Lower extremity lymphedema update: pathophysiology, diagnosis, and treatment guidelines Because popliteal node dissection is relatively rare and usually involves only a small number of nodes, the lymphedema risk is less studied than for groin dissection, where removal of the inguinal nodes is a well-known cause of chronic leg swelling. Still, any disruption to the lymphatic chain behind the knee has the potential to impair drainage from the lower leg, especially when combined with prior surgery or radiation in the same limb.
If you have had popliteal nodes removed and notice gradual swelling in the foot or lower leg, that warrants attention. Compression garments, manual lymphatic drainage, and exercise-based therapy are the standard first-line approaches for managing lower-extremity lymphedema. Early intervention tends to produce better outcomes than waiting until the swelling becomes entrenched.
Popliteal Nodes and Bone Tumors Near the Knee
Osteosarcoma, the most common primary bone cancer in young people, often arises near the knee. Radiologists reviewing MRIs of patients with distal femoral or proximal tibial osteosarcoma sometimes spot popliteal lymph nodes on the scan, raising the question of whether these nodes represent metastatic spread. Researchers have examined the prevalence, imaging appearance, and significance of popliteal nodes in osteosarcoma patients to help determine when an enlarged node is reactive versus cancerous.16Skeletal Radiology. Popliteal lymph nodes in patients with osteosarcoma: are they metastatic? The key imaging features radiologists look for include whether the node retains its normal fatty center, whether it shows restricted diffusion on MRI, and whether it lights up on PET or bone scans. Osteosarcoma spreads to lymph nodes far less commonly than melanoma does, but when a popliteal node looks abnormal on imaging in this context, it needs to be tracked carefully.
Primary lymphoma can also present around the knee. A reported case involved a young woman whose knee lymphoma with synovial involvement initially appeared to be simple monoarthritis before imaging and biopsy revealed the true diagnosis.17PubMed Central. Non-Hodgkin’s Lymphoma of the Knee: A Case Report Such cases are extremely rare but serve as a reminder that persistent, unexplained knee swelling without a clear mechanical cause deserves thorough workup.
When to Worry About a Lump Behind the Knee
Most popliteal lumps turn out to be Baker’s cysts or reactive nodes from minor skin injuries and infections that resolve on their own. The features that should prompt a visit to your doctor include a node that is growing steadily over weeks, a firm or fixed mass that does not move under the skin, any lump accompanied by unexplained weight loss or night sweats, or a mass that appears in someone with a history of melanoma or other skin cancer on the lower leg or foot. Redness, warmth, and tenderness in a popliteal node typically point to infection and are usually less concerning than a painless, hard node, which is more consistent with malignancy.
If you recently had a cat scratch, insect bite, or wound on the lower leg and then notice a tender lump behind the knee, the most likely explanation is a reactive node doing its job. Give it a couple of weeks to settle. If it persists, grows, or you develop fever, your doctor can order an ultrasound and blood work to sort out whether something more is going on.