You do have lymph nodes near your knees, and they are called popliteal lymph nodes. They sit in the popliteal fossa, the soft hollow behind your knee, embedded in the fat that cushions the blood vessels and nerves running through that space. Most people have one or two on each side, though the exact number varies from person to person. These nodes are part of the same network that includes the larger, better-known inguinal lymph nodes in your groin, and together the two groups handle lymphatic drainage for your entire leg.
Where Exactly They Sit
The popliteal fossa is that diamond-shaped depression you can feel when you bend your knee slightly and press into the back of the joint. It is bordered by the hamstring tendons above and the calf muscles below. Within that space, popliteal lymph nodes tend to cluster a few centimeters above the knee joint line itself. An MRI study of a Western population found that the average popliteal lymph node sat about 5.8 centimeters above the lowest point of the outer thigh bone, with individual nodes appearing anywhere from roughly 2 to 12 centimeters above that landmark.1PubMed Central. Features of the popliteal lymph nodes seen on musculoskeletal MRI in a Western population In other words, they live behind your knee but slightly above the crease, tucked among blood vessels and fat.
These nodes are small. In people without inflammatory disease, a typical popliteal lymph node measures only a few millimeters across. Because they are buried in the popliteal fat pad rather than sitting close to the skin surface, you usually cannot feel them the way you might feel a swollen lymph node in your neck or armpit. That is partly why so many people are surprised to learn they exist at all.
What They Actually Do
Lymph nodes act as biological filters. Fluid that has leaked out of blood capillaries into surrounding tissues gets picked up by a network of thin-walled lymphatic vessels, and those vessels route the fluid through one or more lymph nodes before returning it to the bloodstream. Along the way, immune cells inside the nodes inspect the fluid for bacteria, viruses, damaged cells, or other threats.
In the leg, the drainage route follows a general upward pattern. Lymphatic vessels in the foot, ankle, and lower leg collect fluid from the skin and deeper tissues and carry it toward the popliteal nodes behind the knee, then onward to the inguinal nodes in the groin.2Plastic and Reconstructive Surgery. Superficial Lymphatic Drainage of the Lower Extremity: Anatomical Study and Clinical Implications Not all vessels pass through the popliteal nodes on the way; some superficial vessels bypass them entirely and travel straight up the inner thigh to the groin. But for a meaningful fraction of the deep lymphatic drainage from the lower leg and foot, the popliteal nodes are the first checkpoint. The human leg’s lymphoid system was described as consisting of popliteal and inguinal lymph nodes as far back as classic anatomical dissection studies.3British Journal of Surgery. The normal anatomy of the lymphatic system in the human leg
Lymphatic Vessels Inside the Knee Joint
Beyond the popliteal lymph nodes themselves, the knee joint is threaded with its own network of tiny lymphatic vessels. Research in both animal models and human tissue has mapped lymphatic vessels running through the soft tissues surrounding the joint, particularly around the joint capsule, ligaments, fat pads, and nearby muscles.4PubMed Central. Distribution and alteration of lymphatic vessels in knee joints of normal and osteoarthritic mice The synovial membrane, the thin lining that produces the lubricating fluid inside your knee, also contains lymphatic vessels that have been identified in both healthy and arthritic tissue using specialized staining techniques.5Annals of the Rheumatic Diseases. Distribution of lymphatic vessels in normal and arthritic human synovial tissues
These intra-articular lymphatics matter because they help clear waste products and excess fluid from the joint space. When you bend and straighten your knee throughout the day, the mechanical motion helps push lymphatic fluid through those vessels and ultimately out toward the popliteal and inguinal nodes. This drainage system is not just plumbing; it also helps regulate the inflammatory environment inside the joint. When it works well, immune cells and inflammatory molecules are efficiently shuttled out. When it breaks down, fluid accumulates and inflammation can worsen.
How Popliteal Nodes Change in Arthritis
Doctors and researchers have discovered that popliteal lymph nodes respond differently depending on the type of arthritis affecting the knee, and these differences may be useful for distinguishing between conditions on imaging.
In a study comparing knees affected by osteoarthritis and rheumatoid arthritis on MRI, popliteal lymph nodes were visible in all of the rheumatoid arthritis cases but in fewer than half of the osteoarthritis cases. The nodes were also larger in people with rheumatoid arthritis, averaging about 5.4 millimeters compared to roughly 3.6 to 4.1 millimeters in the various grades of osteoarthritis. Another telling difference was that the nodes in the rheumatoid arthritis group were less likely to show the normal fatty center seen in healthier nodes.6PubMed Central. The role of popliteal lymph nodes in differentiating rheumatoid arthritis from osteoarthritis by using CE 3D FSPGR MR imaging: relationship of the inflamed synovial volume The researchers found that both the number and size of popliteal nodes correlated with the volume of inflamed synovial tissue in the knee, suggesting that the nodes swell in proportion to the inflammatory load they are processing.
Work on rheumatoid arthritis flares has taken this observation further. Imaging of flaring knees in people with early rheumatoid arthritis showed enlarged, expanding popliteal lymph nodes next to highly vascular inflamed tissue. In contrast, people with long-standing rheumatoid arthritis who flared showed the opposite pattern: their popliteal nodes appeared collapsed and shrunken, sitting beside tissue that contained both active and dead regions.7Arthritis & Rheumatology. A127: Validating Popliteal Lymph Node Phenotype and Bin Expansion as Biomarkers of Rheumatoid Arthritis Knee Flare in Clinical Pilot Studies The idea is that early in the disease, the lymph nodes ramp up their activity to deal with inflammation, but after years of chronic disease, the nodes may burn out and lose their ability to drain effectively.
Osteoarthritis engages the lymphatic system differently. Research into the synovial lymphatic system has identified compromised lymphatic drainage as a contributor to osteoarthritis progression, not just a bystander.8PubMed. An injectable thermosensitive hydrogel delivering M2 macrophage-derived exosomes alleviates osteoarthritis by promoting synovial lymphangiogenesis When lymphatic vessels in and around the joint fail to clear inflammatory molecules efficiently, the joint environment becomes more destructive over time. This has opened a research avenue exploring whether boosting lymphatic function could slow osteoarthritis down.
Why Surgeons Care About Popliteal Nodes in Melanoma
One of the most clinically significant roles of popliteal lymph nodes involves cancer staging for melanoma of the lower leg and foot. When a melanoma develops below the knee, cancer cells that spread through the lymphatic system may reach the popliteal nodes first, before traveling onward to the groin. Identifying whether cancer has reached these nodes can change the treatment plan substantially.
Sentinel lymph node biopsy is a procedure where surgeons identify the first node or nodes that drain a tumor site and check them for cancer cells. For melanomas on the calf, heel, ankle, or foot, the popliteal fossa sometimes lights up on lymphoscintigraphy scans as a drainage destination. One study of over 460 patients who had sentinel node biopsies for melanomas below the knee found that about 3 percent had drainage to the popliteal region. Among those patients, more than half had cancer cells in their popliteal sentinel node.9PubMed Central. Management of Popliteal Sentinel Nodes in Melanoma All of the patients with popliteal drainage also had drainage to the groin, so both basins needed to be evaluated.
A separate study of over a hundred patients with lower extremity melanoma found a similar pattern: about 9 percent had drainage to the popliteal nodes, with all of them also draining to the groin. In one patient, cancer was found in the popliteal sentinel node while the groin sentinel node was clean, meaning a surgeon who had checked only the groin would have missed the spread entirely.10JAMA Surgery. Lymphatic Drainage to the Popliteal Basin in Distal Lower Extremity Malignant Melanoma This is why guidelines for melanoma below the knee now emphasize looking for popliteal drainage on preoperative imaging rather than assuming all lymph flows straight to the groin.
Research into dual-basin drainage (where both popliteal and groin basins show up on imaging) has explored which basin to biopsy when resources or surgical access are limited. In one series of 29 patients with dual drainage, about a quarter showed similar uptake in both basins and had both biopsied. The remainder had the groin identified as the dominant basin, and only the groin was sampled.11Korean Journal of Clinical Oncology. Management of sentinel lymph nodes in lower extremity melanoma with dual-basin (inguinal and popliteal) drainage on lymphoscintigraphy The decision involves balancing the surgical complexity of accessing the popliteal fossa against the risk of missing a positive node.
Knee Surgery and Lymphatic Disruption
Total knee replacement is one of the most common major orthopedic surgeries performed worldwide, and the swelling that follows it can be remarkably persistent. Part of the reason is that the surgery itself disrupts the lymphatic drainage around the knee. Cutting through the joint capsule, repositioning tissues, and inserting the implant all damage the delicate lymphatic vessels that normally clear fluid from the area. Total knee replacement has been identified as the most common cause of non-cancer surgery-related lymphedema.12J Orthopaedic Experience & Innovation. Mitigating the Post-operative Swelling Tsunami in Total Knee Arthroplasty: A Call to Action
When lymphatic drainage stalls, the resulting tissue swelling creates a pro-inflammatory environment that can delay healing and contribute to stiffness. This is not simply cosmetic puffiness; the fluid buildup carries inflammatory molecules that, if not cleared, can keep the surgical site in a heightened state of irritation longer than necessary. Functional lymphatic and venous drainage are both needed to remove those inflammatory mediators from the tissues around the knee.
Does Manual Lymphatic Drainage Help After Knee Replacement?
Given the lymphatic disruption that follows knee surgery, it is natural to wonder whether manual lymphatic drainage, a gentle massage technique designed to encourage lymph flow, might speed recovery. The evidence here is surprisingly mixed.
One randomized trial found that manual lymphatic drainage reduced both swelling and pain in the first few days after total knee replacement compared to standard care, with moderate-to-large effect sizes. However, those benefits faded over the following weeks, and by six weeks after surgery, the groups looked the same.13PubMed. Manual lymphatic drainage and Kinesio taping applications reduce early-stage lower extremity edema and pain following total knee arthroplasty Another study found that adding manual lymphatic drainage to an exercise program reduced a blood marker associated with pain and muscle damage but did not significantly reduce leg volume compared to exercise alone.14PubMed. Can Lymphatic Transport Impaired by Total Knee Arthroplasty be Managed with Manual Lymphatic Drainage?
A systematic review and meta-analysis pooling data from multiple randomized controlled trials concluded that manual lymphatic drainage did not improve knee edema after total knee replacement when measured by lower extremity circumference.15PubMed Central. Effects of manual lymphatic drainage on total knee replacement: a systematic review and meta-analysis of randomized controlled trials The overall picture is that manual lymphatic drainage may provide modest, short-term comfort but does not appear to meaningfully reduce the total amount of swelling over the recovery period. Standard rehabilitation exercises, compression, and elevation remain the mainstays of managing post-surgical swelling.
Infections That Cause Popliteal Node Swelling
Because the popliteal lymph nodes filter drainage from the lower leg and foot, infections in those areas can cause the nodes to enlarge. A wound on the foot, an infected insect bite on the calf, or cellulitis of the lower leg may all trigger a noticeable swelling behind the knee as the popliteal nodes mount an immune response. In most cases, this resolves once the infection is treated.
A more specific example comes from Lyme disease. In children with Lyme arthritis of the knee, MRI studies have found that regional lymph node swelling (adenopathy) was present in every case, along with inflammation of surrounding muscles. This pattern of joint swelling plus regional adenopathy helped distinguish Lyme arthritis from other causes of an infected knee in that study population.16PubMed. MRI features of Lyme arthritis in children If a child develops a swollen, painful knee and lives in or has visited a tick-endemic area, the presence of enlarged popliteal nodes on imaging can be a useful clue pointing toward Lyme disease rather than a bacterial joint infection.
Why Popliteal Nodes Are Easy to Overlook
Several factors conspire to keep popliteal lymph nodes out of the public awareness. First, they are small and deep. Unlike the nodes in your neck, which sit close to the skin and become obviously swollen during a cold, popliteal nodes are buried under layers of fat and muscle. Even when they enlarge, the surrounding tissues often mask the change. A person might notice swelling behind the knee but attribute it to a Baker’s cyst or a muscle strain rather than a lymph node.
Second, medical education and primary care practice tend to focus lymph node examinations on the more accessible sites: cervical (neck), axillary (armpit), and inguinal (groin). Popliteal nodes are examined less routinely, and many clinicians only investigate them when imaging for another reason reveals an unexpected finding or when staging a lower-leg cancer. This creates a gap in public knowledge. People generally know they have lymph nodes in their neck, armpits, and groin but have no idea there is a set behind each knee.
Popliteal Nodes in Laboratory Animal Research
If you have ever read about immunology or toxicology experiments in mice or rats, you may have encountered the popliteal lymph node assay. Researchers inject a substance into the hind footpad of a rodent and then examine the draining popliteal lymph node for signs of immune activation. This assay has been used for decades as a screening tool to predict whether chemicals might trigger autoimmune reactions in humans.17PubMed. Morphology of popliteal lymph node responses in Brown-Norway rats The popliteal node is ideal for this purpose because it drains a well-defined anatomical region (the foot and lower leg), making it easy to track exactly which node received the injected material.
Studies in mice have also used popliteal lymph nodes to investigate how the immune system retains memory of foreign substances. Injecting a labeled protein into the hind footpad and then examining the draining node revealed that specialized immune cells in the node held onto the antigen for extended periods, helping the body mount a faster response upon re-exposure.18PubMed Central. Long-term antigen retention by dendritic cells in the popliteal lymph node of immunized mice Work in dogs has likewise explored popliteal lymph node drainage patterns for veterinary cancer staging, finding that the node consistently drains to the internal lymph nodes higher in the pelvis, with about a quarter of dogs having an additional accessory drainage pathway.19PubMed. Frequency of an accessory popliteal efferent lymphatic pathway in dogs The popliteal lymph node, in short, is a workhorse of experimental immunology across species.
When a Lump Behind the Knee Is Not a Lymph Node
Not every swelling in the back of the knee is a lymph node. The popliteal fossa is a busy intersection of anatomical structures, and several other things can produce a palpable lump or feeling of fullness in that area. The most common is a Baker’s cyst (also called a popliteal cyst), which is a fluid-filled sac that forms when excess joint fluid pushes backward through a weak point in the knee capsule. Baker’s cysts feel smooth and fluctuant and tend to enlarge when the knee is extended and shrink when it is bent. They are especially common in people with osteoarthritis or meniscal tears.
Other possibilities include ganglion cysts, enlarged bursae, popliteal artery aneurysms, and soft-tissue tumors, all of which occupy the same real estate behind the knee. An enlarged popliteal lymph node can sometimes be felt as a firm, round mass in the fossa, but it is difficult to distinguish from these other structures by touch alone. Ultrasound is typically the first imaging step, as it can quickly differentiate a fluid-filled cyst from a solid lymph node and assess whether blood flow patterns suggest something that warrants further workup. If you notice persistent swelling behind your knee that does not resolve with rest or that is accompanied by other symptoms like night sweats, unexplained weight loss, or a skin lesion on your lower leg, it is worth having it evaluated rather than assuming it is a harmless cyst.