Pain behind the knee can stem from a surprisingly wide range of structures, from fluid-filled cysts and torn cartilage to strained tendons, damaged ligaments, and even blood vessel problems. The space behind your knee, called the popliteal fossa, is a crowded intersection of muscles, tendons, nerves, and blood vessels, and trouble in any of them can produce aching, stiffness, or sharp pain in that area. Because so many different tissues converge there, pinning down the cause usually requires thinking about when the pain started, what makes it worse, and whether you have other symptoms like swelling or instability.
Baker’s Cysts and How They Form
The single most recognized cause of a noticeable lump or fullness behind the knee is a Baker’s cyst, also called a popliteal cyst. This is a fluid-filled sac that develops in the popliteal fossa, most commonly in a bursa that sits between two muscles at the back of the knee. The cyst forms when excess joint fluid gets pushed through a one-way valve-like connection between the knee joint and the bursa. Fluid flows into the bursa but has difficulty draining back out, so the sac gradually swells.1PubMed. Popliteal cysts in adults: a review
Baker’s cysts rarely appear on their own. In adults, they almost always accompany some other problem inside the knee joint, such as osteoarthritis, a meniscus tear, or an inflammatory condition like rheumatoid arthritis.2PubMed. Popliteal cysts: a current review That means the cyst itself is often more of a symptom than a root cause. You might feel tightness or a dull ache behind the knee, especially when you fully bend or straighten it. Small cysts sometimes go unnoticed. Larger ones can be felt as a soft, grape-sized or even golf-ball-sized bulge that becomes more prominent when you stand.
Treatment usually focuses on whatever is driving the extra fluid production rather than on the cyst itself. If you address the underlying arthritis or meniscal tear, the cyst often shrinks on its own. Draining the cyst with a needle can provide temporary relief, but it tends to refill if the source of inflammation remains.
Meniscal Tears at the Back of the Knee
Your knee contains two crescent-shaped pieces of cartilage, the menisci, that act as shock absorbers between the thighbone and shinbone. Tears in the posterior horn of the medial meniscus are a common source of pain that localizes to the back or inner-back part of the knee. These tears are more frequent than clinicians once assumed, and they become increasingly common with age.3PubMed. Radial tears of the posterior horn of the medial meniscus
Most people with posterior horn tears can point to a specific moment the pain started. In one study, roughly 85% of patients recalled a distinct event, often described as a sudden click or a shock-like sensation, followed by sharp pain or a feeling that the knee might give way.4PubMed. Clinical features of the posterior horn tear in the medial meniscus Other common complaints include catching or locking when bending the knee and pain that worsens with deep squatting or twisting. Because a torn meniscus can also cause excess fluid to accumulate in the joint, it is one of the conditions that frequently leads to a Baker’s cyst forming behind the knee.
Hamstring, Calf, and Popliteus Tendon Problems
Several tendons cross the back of the knee, and irritation or injury to any of them can produce posterior knee pain. The distal hamstring tendons, specifically the semimembranosus and semitendinosus, insert near the inner-back side of the knee. When these tendons are thickened or inflamed, they correlate strongly with posteromedial knee pain. Ultrasound studies have found that patients with this type of pain show measurably thicker tendons compared to pain-free individuals, and thicker tendons are associated with higher pain scores.5PubMed Central. Ultrasonographic Evaluation of the Distal Medial Hamstring Tendons and their Association with Posteromedial Knee Pain Common ultrasound findings include areas of abnormal darkness within the tendon, small tears, and even new blood-vessel growth inside the tissue, all signs of tendinopathy or chronic overload.
The gastrocnemius, the large calf muscle, originates at two points just above the back of the knee. When the upper portion of this muscle or its tendon is strained, the pain tends to settle right behind the knee or at the very top of the calf. This is sometimes confused with a Baker’s cyst or a more serious injury. Gastrocnemius strains are especially common in sports that involve sudden pushing off or jumping, and they tend to hurt most during activities that load the calf, like walking uphill or pushing off the ground.6PubMed Central. Gastrocnemius vs. soleus strain: how to differentiate and deal with calf muscle injuries
Less well known is the popliteus, a small muscle and tendon tucked deep behind the knee that helps control rotation. Injuries to the popliteus tendon are uncommon, but case reports have documented unusual tears and subluxation of this tendon causing persistent posterior knee pain that can be difficult to diagnose.7PubMed Central. Two cases of chronic knee pain caused by unusual injuries to the popliteus tendon If conventional imaging and examination have not explained chronic pain behind the knee, popliteus injuries are worth considering.
Posterior Cruciate Ligament Injuries
The posterior cruciate ligament (PCL) runs along the back of the knee and prevents the shinbone from sliding too far backward relative to the thighbone. A PCL tear can result from a direct blow to the front of the bent knee, such as hitting the dashboard in a car accident, or from a hyperextension injury. When other ligaments are also damaged, the diagnosis is usually straightforward. But isolated PCL tears are a different story: they tend to produce subtler symptoms and are frequently missed or misdiagnosed.8PubMed Central. Rupture of posterior cruciate ligament: diagnosis and treatment principles
The hallmark of an isolated PCL injury is posterior knee pain that worsens when the knee is bent past about 90 degrees, such as when kneeling, walking downstairs, or crouching. If you have this pattern and can recall a relevant impact or fall, it is worth specifically asking a clinician to test for PCL laxity, because standard knee exams do not always catch it unless the examiner is looking for it.
Posterolateral Corner Injuries
The posterolateral corner (PLC) is a group of ligaments and tendons on the outer-back side of the knee that together resist rotational forces. Injuries here typically happen during athletic collisions, car accidents, or falls, and they are both rare and frequently overlooked. In one documented case, the only positive clinical finding was a specific rotation test; all other knee tests came back normal.9PubMed Central. Delayed diagnosis of an isolated posterolateral corner injury: a case report This illustrates why PLC injuries earn a reputation as one of the most commonly delayed diagnoses in knee trauma.
Missing a PLC injury matters because untreated instability in this region can undermine surgical repairs of other ligaments and lead to chronic pain and giving way of the knee.10PubMed. Current concepts in the recognition and treatment of posterolateral corner injuries of the knee If you had a significant injury, still feel instability or pain on the outer-back aspect of the knee weeks later, and initial imaging was unremarkable, a follow-up evaluation specifically targeting the PLC structures is reasonable to request.
Blood Vessel Problems in the Popliteal Fossa
Not all pain behind the knee comes from muscles, tendons, or cartilage. The popliteal artery and vein pass directly through the space behind the knee, and problems with either vessel can mimic musculoskeletal injuries.
Popliteal artery entrapment syndrome (PAES) is an uncommon but important vascular cause of posterior knee and calf pain, particularly in young, athletic people who have no typical risk factors for artery disease. In PAES, the popliteal artery is compressed by surrounding muscle or fibrous tissue, restricting blood flow during exercise. The result is cramping or aching in the calf and behind the knee that comes on during activity and fades with rest.11PubMed Central. Popliteal artery entrapment syndrome Because these patients are young and otherwise healthy, the diagnosis is easy to miss. An athlete who develops exertional calf and posterior knee pain that does not respond to typical rest-and-rehab approaches should have PAES considered.12PubMed Central. About a rare cause of calf pain in an athlete: the popliteal artery entrapment syndrome (a case report)
Popliteal artery aneurysms, where the artery wall bulges outward, are another vascular cause. These can present as a palpable mass behind the knee and may easily be confused with a Baker’s cyst on initial examination. In one reported case, a man with rheumatoid arthritis was found to have a popliteal artery aneurysm that had been mistaken for a cyst until ultrasound clarified the diagnosis.13PubMed. Popliteal aneurysm simulating a Baker’s cyst in a patient with rheumatoid arthritis: a case presentation Masses in the popliteal fossa can also turn out to be popliteal vein clots, fatty tissue growths, or even tumors, all of which are rarer but clinically significant. The takeaway is that any persistent lump behind the knee that does not clearly behave like a simple cyst deserves imaging.
Nerve-Related Pain Behind the Knee
Several nerves pass through or near the popliteal fossa, and when they are compressed or irritated, the resulting pain can be felt behind the knee, down the calf, or into the foot. The tibial nerve and common peroneal nerve both travel through this area and can become entrapped by fibrous bands, cysts, or swelling. Causes of nerve entrapment in the lower leg generally fall into two categories: compression at fixed anatomical tunnels and dynamic irritation during certain limb positions.14Radiographics. MR imaging of entrapment neuropathies of the lower extremity. Part 2. The knee, leg, ankle, and foot The pain from nerve entrapment often has a burning or electric quality and may be accompanied by tingling or numbness further down the leg.
Pain behind the knee can also be referred from the lower back. Lumbar disc herniations and spinal stenosis can compress nerve roots that supply the area behind the knee, producing pain that seems to originate in the knee itself. If your posterior knee pain comes with lower-back stiffness or radiates up or down the leg, a spinal source is worth investigating.
Inflammatory and Arthritic Conditions
Osteoarthritis is one of the most common underlying drivers of posterior knee pain, especially in people over 50. As the joint surfaces wear down, the knee produces excess synovial fluid, which can cause a generalized feeling of fullness and aching that extends to the back of the knee. This excess fluid is also one of the main triggers for Baker’s cyst formation, which explains why osteoarthritis and posterior knee swelling so often go hand in hand.
Inflammatory forms of arthritis can produce similar symptoms through different pathways. Rheumatoid arthritis causes the synovial lining to thicken and inflame, which generates fluid and swelling that can extend into the popliteal fossa. Crystal-deposit diseases like gout can do the same: in one case report, a patient with both rheumatoid arthritis and gout was found to have knee effusion, synovial thickening, and bilateral popliteal cysts, with needle-shaped crystals confirmed in the joint fluid.15PubMed Central. Recognition of gout in rheumatoid arthritis: A case report This overlap is a reminder that multiple conditions can coexist in the same knee, and treating only one may not resolve your symptoms.
When a Ruptured Baker’s Cyst Mimics Something Worse
A Baker’s cyst that ruptures deserves its own mention because it creates a clinical scenario that can alarm both patients and physicians. When the cyst wall breaks, fluid leaks into the surrounding calf tissues, producing sudden pain, swelling, and redness in the calf that closely imitates a deep vein thrombosis (DVT). This resemblance is more than cosmetic: there are documented cases where a ruptured cyst was misdiagnosed as a DVT and the patient was started on blood thinners, which then caused bleeding and led to compartment syndrome, a dangerous buildup of pressure inside the leg’s muscle compartments.16Journal of Urgent Care Medicine. Ruptured Baker Cyst is an Uncommon Complication of a Common Diagnosis: A Case Report
Other complications from a ruptured cyst include infection and compression of nearby nerves or blood vessels. These outcomes are rare, but they highlight why sudden-onset pain and calf swelling in someone with a known Baker’s cyst should prompt an ultrasound to distinguish between a cyst rupture and a true blood clot before starting treatment.
Pain Behind the Knee After Knee Replacement
Persistent pain after total knee replacement is more common than many patients expect. Studies report that somewhere between 10% and 34% of people who undergo the surgery experience ongoing adverse pain, and about one in five actually report more pain after the operation than before it.17PubMed Central. Literature review of the causes of pain following total knee replacement surgery: prosthesis, inflammation and arthrofibrosis Pain that settles behind the knee specifically can arise from scar tissue formation (arthrofibrosis), component malalignment, nerve damage during surgery, or infection. In rare instances, trauma to the popliteal artery during surgery can produce a false aneurysm that presents as a pulsatile swelling behind the knee, sometimes without the classic signs clinicians expect.18PubMed Central. Popliteal artery pseudoaneurysm following primary total knee arthroplasty
If you have had a knee replacement and develop new or worsening pain behind the knee, the possible causes span from relatively benign stiffness to urgent vascular complications. Imaging and a thorough clinical exam can usually sort out which category you fall into.
Posterior Knee Pain in Young Athletes
Children and teenagers who play sports present a somewhat different set of concerns when it comes to knee pain. Their skeletons are still growing, which means they have open growth plates and developing cartilage that are vulnerable in ways adult tissues are not. Overuse injuries to the posterior knee area in young athletes can involve the growth plates, tendon insertion sites, or the joint surfaces themselves, and the diagnostic approach needs to account for these age-specific vulnerabilities.19PubMed Central. Narrative Review – Knee Pain in the Pediatric Athlete While many of the same structures can be involved, a stress fracture at a growth plate or an osteochondral injury in a teenager has different treatment implications and a different urgency than the same symptom in a 45-year-old.
Parents and coaches should be aware that young athletes often underreport pain or push through it, and that knee pain that persists beyond a few days of rest warrants a clinical evaluation rather than a wait-and-see approach. Early identification of overuse injuries in growing bones can prevent more complicated problems down the road.
How Clinicians Sort Through the Possibilities
Given how many different structures can cause pain in the same general area, diagnosis often starts with the details of your story. A clinician will want to know whether the pain came on suddenly after an injury or gradually over weeks, whether it is sharp or dull, whether it changes with bending or straightening the knee, whether there is visible swelling, and whether you have any calf symptoms. That history alone can narrow the field considerably. A sudden pop during a pivoting sport points toward a meniscal or ligament injury. Gradual onset in a middle-aged person with stiffness and swelling leans toward arthritis and a possible Baker’s cyst. Exertional cramping in a young athlete with no clear injury raises the question of a vascular cause like PAES.
Physical examination adds another layer. Specific tests can stress the PCL, the posterolateral corner, and the menisci individually. Palpation can identify tender tendons or a fluctuant cyst. Ultrasound is often the first imaging step because it can quickly distinguish a fluid-filled cyst from a solid mass or an aneurysm, and it can evaluate tendons and detect blood clots. MRI comes into play when internal structures like ligaments, menisci, or nerves need more detailed visualization. The goal is not just to name the pain but to identify which structure is driving it, because the treatment for a Baker’s cyst caused by a meniscal tear is fundamentally different from the treatment for a gastrocnemius strain or a PCL injury, even though all three can make the back of your knee hurt.