Most anatomy references count around eleven to fourteen bursae in and around the knee, though the exact number depends on which structures an individual person actually develops and how you define a distinct bursa versus a pocket of an existing one. A bursa is a small, fluid-filled sac that sits between tissues that would otherwise grind against each other, cushioning the contact between skin and bone, tendon and bone, or tendon and ligament. The knee has more bursae than almost any other joint, which makes sense given the forces it absorbs and the number of tendons, ligaments, and bones converging in a tight space. Some of these bursae are well-known clinical landmarks, while others are small enough that even surgeons encounter them only incidentally.
What Bursae Actually Do
Every time you bend, straighten, or twist your knee, multiple layers of soft tissue slide across each other and across bone. Without some kind of buffer, that motion would cause constant irritation. Bursae solve the problem by providing a thin, lubricated cushion right at the friction point. The inner lining of a bursa secretes a small amount of synovial-like fluid that allows the sac’s walls to glide almost frictionlessly against each other. In a healthy knee, you would never notice them. They become relevant only when they get inflamed, infected, or distended with excess fluid, a condition broadly called bursitis.
Bursae around the knee are typically grouped by their position relative to the kneecap: those in the front (anterior), on the inner side (medial), on the outer side (lateral), and in the back (posterior).1PubMed Central. Bursae around the knee joints That geographic classification is useful because bursae in different zones tend to cause different symptoms and respond to different activities.
Bursae at the Front of the Knee
The front of the knee is bursa-dense territory. The most clinically familiar is the prepatellar bursa, the one that sits right over the kneecap, just beneath the skin. It is the bursa responsible for “housemaid’s knee,” the swelling that develops from prolonged kneeling on hard surfaces. But the prepatellar area is not a single sac. A cadaver study of the soft tissue in front of the kneecap identified three distinct prepatellar spaces that function as bursae: a subcutaneous bursa closest to the skin, a subfascial bursa beneath the fascial layer, and a subaponeurotic bursa deeper still, beneath the quadriceps tendon expansion.2Journal of Bone and Joint Surgery. Soft-Tissue Anatomy Anterior to the Human Patella In everyday clinical practice, these three are often treated as a single unit, but they are anatomically separate layers, which is why prepatellar bursitis can look slightly different on imaging depending on which layer is inflamed.
Below the kneecap, between the patellar tendon and the tibia, sits the deep infrapatellar bursa. A cadaver study using MRI-guided bursography found this bursa in all nine specimens examined, with average dimensions of roughly 25 mm tall by 29 mm wide. A shallower superficial infrapatellar bursa, located between the patellar tendon and the overlying skin, was present in only about half the specimens, averaging about 20 mm across when it did appear.3PubMed. Deep and superficial infrapatellar bursae: cadaveric investigation of regional anatomy using magnetic resonance after ultrasound-guided bursography The deep version is reliably present; the superficial one is not, which is one reason bursa counts for the knee vary from source to source.
The Suprapatellar Bursa and Its Disappearing Wall
Above the kneecap lies the suprapatellar bursa, a large pouch that sits between the lower end of the thigh muscle and the femur. What makes this bursa unusual is that in most adults, it is not truly a separate structure at all. During fetal development, a sheet of tissue called the suprapatellar septum completely separates the suprapatellar bursa from the main knee joint cavity. Around the fifth month of fetal life, that septum normally perforates and breaks down, allowing the bursa to open directly into the joint space.4PubMed Central. New insights about suprapatellar cyst 5PubMed. Classification of the suprapatellar septum considering ontogenetic development
In most people, then, the suprapatellar bursa is effectively part of the knee joint itself, acting as its largest recess. But in a minority of adults, the septum persists fully or partially, keeping the bursa as a walled-off pocket. Whether or how much the septum remains intact affects how far knee-joint fluid can track upward and can influence the appearance of effusions on imaging.6PubMed. Morphologic variants of the proximal knee-joint cavity. An anatomical and radiological study When radiologists see a large fluid collection above the kneecap, the question of whether it is inside the joint or trapped in a persistent separate bursa depends on the status of that fetal septum.
Bursae on the Inner Side of the Knee
The medial knee has at least two well-recognized bursae. The pes anserinus bursa lies on the inner-lower part of the knee, beneath the tendons of three muscles (sartorius, gracilis, and semitendinosus) that converge and insert on the upper tibia in a fan-shaped arrangement.7PubMed Central. Pes anserinus and anserine bursa: anatomical study This bursa cushions the tendons as they glide over each other and over bone. Pes anserinus bursitis is common in runners, in people with osteoarthritis, and in anyone whose inner-knee tendons are under repetitive stress. The pain typically sits a few centimeters below the joint line on the inner shin, and it can mimic a meniscus tear if the clinician is not looking for it.
Deeper and less obvious is the medial collateral ligament (MCL) bursa. The MCL is not one solid strap; it has a superficial layer and a deep layer, and between those two layers lies a thin bursa. Cadaver studies and MRI have confirmed that this bursa typically has separate femoral and tibial compartments, meaning it is essentially two connected pockets running along the length of the ligament.8PubMed. MR imaging of the medial collateral ligament bursa: findings in patients and anatomic data derived from cadavers When inflamed, the MCL bursa can produce medial knee pain that is easily mistaken for a ligament sprain or meniscus injury.9Journal of Diagnostic Medical Sonography. Distension of the Medial Collateral Ligament Bursa of the Knee With Possible Bursitis Animal research has explored injecting corticosteroids or hyaluronic acid directly into the MCL bursa; steroid injections reduced bursa size and suppressed the growth of new blood vessels and pain-associated nerve fibers, though they also caused some tissue thinning.1PubMed Central. Bursae around the knee joints
Bursae at the Back of the Knee
The posterior knee is home to the gastrocnemius-semimembranosus bursa, which sits in the popliteal fossa, the soft hollow behind your knee. This bursa lies between the medial head of the calf muscle (gastrocnemius) and the semimembranosus tendon, one of the hamstrings. When this bursa becomes enlarged with fluid, it forms what is commonly known as a Baker’s cyst.10PubMed Central. Baker’s Cyst Filled with Hematoma at the Lower Calf
What makes this bursa interesting, and explains why Baker’s cysts become more common with age, is that it does not always communicate with the knee joint. In children, the bursa is almost always a sealed pocket with no connection to the joint cavity. Over the decades, a one-way communication gradually develops: the passageway between the joint capsule and the bursa opens in roughly 10 percent of people in their teens, about 20 percent in their twenties, and continues to climb to around 50 to 60 percent in older adults.11ReumatologÃa ClÃnica. Clinical Anatomy of the Knee Once that channel exists, any condition that produces a knee effusion, such as arthritis, a meniscus tear, or gout, can push fluid into the bursa and inflate it into a cyst. The connection functions somewhat like a valve: fluid enters the bursa more easily than it drains back out.
The arthroscopic anatomy of this region varies considerably between individuals. The angle of the oblique popliteal ligament, its crossing point relative to the calf muscle tendon, and the local capsular and synovial structure all influence how freely the bursa communicates with the joint.12PubMed Central. Arthroscopic patterns of the poster-medial aspect of the knee joint: classification of the gastrocnemius-semimembranosus gateway and its relationship with Baker’s cyst This variability is part of the reason Baker’s cysts are unpredictable: two patients with identical-looking arthritis may have very different outcomes behind the knee, depending on their individual anatomy.
Bursae on the Outer Side of the Knee
The lateral knee is where bursa anatomy gets genuinely contested. Older textbooks describe a bursa beneath the iliotibial band (ITB) at the point where it crosses the lateral femoral epicondyle, the bony bump on the outer side of the knee. ITB syndrome, a common overuse injury in runners and cyclists, was long attributed to the band snapping back and forth over that bump and irritating the bursa underneath.
A detailed cadaver and MRI study challenged that model. Researchers found that in every specimen, the ITB was firmly anchored to the femur by fibrous strands, and no discrete bursa was present beneath it. Instead, a layer of richly innervated and vascularized fat occupied the space deep to the band. In patients with ITB syndrome, MRI signal changes showed up in that fat pad, not in a bursal sac.13PubMed Central. The functional anatomy of the iliotibial band during flexion and extension of the knee: implications for understanding iliotibial band syndrome On the other hand, ultrasound studies of patients with ITB-related pain do sometimes show discrete fluid collections between the band and the epicondyle that look like bursitis on imaging.14PubMed Central. Ultrasound of iliotibial band syndrome Whether that fluid represents a true anatomical bursa or a reactive, adventitious sac that forms in response to chronic irritation remains a point of debate. The practical result is that some knee bursa counts include an ITB bursa and others do not.
Less controversial is the popliteus bursa, a recess associated with the popliteus tendon as it wraps around the back of the lateral knee. This bursa is really an extension of the knee joint cavity itself, and it plays a role in the arthrographic appearance of the lateral meniscus and its peripheral attachments.15PubMed. An anatomic-arthrographic study of the relationships of the lateral meniscus and the popliteus tendon Because it communicates freely with the joint, it fills with contrast during arthrography and can occasionally be mistaken for a lateral meniscal tear if the anatomy is not well understood.
Why the Count Is Never Quite Settled
If you read five orthopedic textbooks, you will encounter five slightly different lists of knee bursae. A comprehensive review identified eleven distinct bursae as the clinically relevant set for evaluation and treatment purposes. But that number could reasonably go higher or lower depending on how you handle several recurring gray areas.
One issue is communicating versus non-communicating bursae. The suprapatellar bursa communicates with the joint in most adults. The popliteus bursa communicates with the joint by definition. Some anatomists count these as bursae; others classify them as recesses of the joint capsule and exclude them from the bursa tally. A second issue is variable bursae. The superficial infrapatellar bursa appears in only about half of cadavers studied.3PubMed. Deep and superficial infrapatellar bursae: cadaveric investigation of regional anatomy using magnetic resonance after ultrasound-guided bursography The ITB bursa may not exist as a native structure in some people. A third factor is adventitious bursae, structures that are not present at birth but develop over a lifetime of mechanical stress. A person who spends decades kneeling on one side might develop a bursa-like sac in a location where the textbooks say there is none.
For practical purposes, most clinicians work with a mental map that includes the following named bursae: the three prepatellar layers (often lumped together clinically), the suprapatellar, the deep infrapatellar, the superficial infrapatellar (when present), the pes anserinus, the MCL bursa, the gastrocnemius-semimembranosus (Baker’s cyst origin), the popliteus, and the ITB bursa (when recognized). A few additional small bursae around the tendons of the biceps femoris and the lateral collateral ligament bring some lists up to fourteen or more.
How Bursae Are Diagnosed When They Cause Pain
Bursitis around the knee can mimic a surprising range of other problems: meniscus tears, ligament sprains, tendinitis, even referred pain from the hip. The location of tenderness is the first clue. Prepatellar bursitis produces swelling you can see and touch right over the kneecap. Pes anserinus bursitis causes pinpoint tenderness a few centimeters below the joint line on the inner knee. Baker’s cysts create a palpable lump behind the knee that may feel tight when the knee is fully extended.
For cases that are not obvious on physical exam, ultrasound is a reliable and accessible tool. A study comparing ultrasound to MRI for detecting knee bursitis found that ultrasound correctly identified bursitis in about 87 percent of confirmed cases, with perfect specificity, meaning it did not flag bursitis when none was present.16PubMed Central. Knee bursitis: a sonographic evaluation Its weak spot was the suprapatellar bursa, where sensitivity dropped to about 71 percent compared with MRI. MRI remains the gold standard when precise characterization is needed, particularly for distinguishing an inflamed bursa from a solid mass or a complex cyst with internal debris.
Treatment and What to Expect
Most episodes of knee bursitis resolve with rest, ice, compression, and avoidance of the activity that provoked the irritation. Prepatellar bursitis in someone who kneels for work often responds to knee pads and modified technique. Pes anserinus bursitis may improve with stretching the inner-thigh muscles and addressing any underlying biomechanical issue like overpronation.
When conservative measures fail, corticosteroid injections are the traditional next step. They work quickly for most people, but recurrence rates are not trivial, sitting in the range of 20 to 40 percent depending on the bursa and the underlying cause. Newer approaches using platelet-rich plasma alone or combined with hyaluronic acid have shown promise in reducing pain and lowering recurrence in selected cases, though treatment protocols are still not standardized across clinics. For Baker’s cysts, treating the underlying joint problem (such as a meniscus tear or arthritis flare) is often more effective than draining the cyst itself, because the cyst will refill as long as the valve-like communication keeps funneling fluid into the bursa.
Surgical removal of a bursa, called a bursectomy, is reserved for chronic cases that have failed everything else, or for bursae that have become infected (septic bursitis). The prepatellar bursa is the most common site for septic bursitis, because its superficial position makes it vulnerable to skin breaks from scrapes or puncture wounds. Septic bursitis is a different animal from overuse bursitis and typically requires antibiotics and sometimes surgical drainage.
Bursae You Never Knew You Had
Beyond the named bursae that appear in anatomy atlases, the knee can develop additional bursae over time. These adventitious bursae form wherever the body decides it needs more friction protection, often at sites of chronic mechanical irritation that the original anatomy did not anticipate. A person who develops an unusual gait after a hip replacement, for instance, might load the lateral knee in a way that produces a new fluid-filled cushion in a non-standard location. These structures are histologically identical to congenital bursae, with a synovial-like lining producing lubricating fluid, but they do not appear in textbook diagrams because they are unique to the individual’s mechanical history.
This adaptability is part of why the knee’s bursa count resists a definitive answer. The named bursae represent the typical human blueprint, but the body adds to the blueprint as needed. If you went through life without ever kneeling, crawling, running, or twisting, your knee might get by with fewer functional bursae than the person who spent thirty years laying tile. The number is partly genetic, partly developmental, and partly a biography of how you have used your knees.