Popliteal fossa swelling — the discriminators that separate benign from limb- and life-threatening
- A popliteal mass that is solid on ultrasound is a sarcoma until proven otherwise — refer to a sarcoma unit before any biopsy.
- A Baker's cyst arises between the medial head of gastrocnemius and the semimembranosus tendon, and communicates with the joint — it is medial and it points inferomedially.
- A ruptured Baker's cyst produces a painful, swollen, warm calf that is clinically indistinguishable from DVT ('pseudothrombophlebitis') — ultrasound must exclude both, not one.
- Popliteal artery aneurysm is bilateral in a large proportion of cases and is associated with abdominal aortic aneurysm — examine the other knee and the abdomen.
- In a child, a popliteal cyst is usually primary, does not communicate with the joint in the same way, and resolves spontaneously — do not aspirate it.
- Any popliteal mass with night pain, size increasing, or firmness deep to the fascia warrants MRI with contrast, not reassurance.
- “Foucher's sign: a Baker's cyst becomes tense in full extension and soft in 45 degrees of flexion — a solid tumour does not change.
- “Adult Baker's cyst is a symptom, not a diagnosis — look for the intra-articular cause (meniscal tear, osteoarthritis, inflammatory arthritis).
- “Never aspirate a pulsatile popliteal mass — Doppler before needle.
- “Synovial sarcoma is the classic 'small, slow-growing, painful' periarticular mass mistaken for a cyst for years.
Recognising the Pattern
The fossa. A mass behind the knee is a palpable or imaged swelling within the popliteal fossa, the diamond bounded above by biceps femoris laterally and semimembranosus and semitendinosus medially, and below by the two heads of gastrocnemius. Its contents lie, from superficial to deep and from lateral to medial, as tibial nerve, popliteal vein, popliteal artery, which is also their order from posterior to anterior.
What makes a cyst a Baker's cyst. The gastrocnemio-semimembranosus bursa lies between the medial head of gastrocnemius and the semimembranosus tendon, so a Baker's cyst is medial, points inferomedially and communicates with the joint. On ultrasound it is anechoic and compressible, with a neck between those two structures. Anything solid, heterogeneous or lacking that neck is a different diagnosis, and roughly one in ten popliteal soft tissue masses referred as a "cyst" is a tumour.
Examining it. Examine with the patient prone and the knee in full extension, then at 45 degrees. Foucher's sign is the discriminator: a genuine Baker's cyst is tense in extension and softens with flexion, because the valve-like neck closes, and a solid tumour does not change with position. Then decide whether the mass is medial or central. The bursa is medial; a midline pulsatile mass is arterial until disproven.
Test compressibility, transillumination, pulsatility and fixity to deep fascia: fixed, firm and deep to fascia is the sarcoma triad. Finish with the joint itself, looking for effusion, joint line tenderness, range and signs of inflammatory arthritis, because an adult cyst without an intra-articular cause is unusual and should raise suspicion.
Saying it out loud in a viva. "This is a well-defined mass in the popliteal fossa. I would describe its position relative to the midline, whether it is cystic or solid on examination, whether it is pulsatile, whether it is fixed to deep fascia, and its relationship to the neurovascular bundle. On this radiograph I am looking for calcification, phleboliths, a soft tissue shadow, periosteal reaction on the posterior femoral or tibial cortex, and any bony destruction. My first investigation is an ultrasound with colour Doppler to answer three questions: cystic or solid, vascular or avascular, and is there a coexisting deep vein thrombosis."
What mimics the pattern (false positives).
- Prominent medial head of gastrocnemius or muscle hernia in a lean athlete — reduces with relaxation.
- Meniscal (parameniscal) cyst presenting posteromedially — tracks from the joint line, not the bursa.
- Ganglion of the proximal tibiofibular joint — posterolateral, may cause common peroneal palsy with foot drop.
- Distal femoral or proximal tibial osteochondroma with an overlying bursa — the bone is the mass; the "cyst" is reactive.
- Fabella or a fabella-related bursitis — a sesamoid in lateral gastrocnemius, present in a minority of knees, easily mistaken for a loose body.
- Post-traumatic haematoma or medial gastrocnemius tear ("tennis leg") in a middle-aged squash player.
Next Investigation
Ultrasound with colour Doppler. The universal first test. It answers cystic versus solid and vascular versus avascular, compresses the popliteal and femoral veins for thrombosis in the same examination, and demonstrates the neck of a Baker's cyst. That is what separates the reassure-and-treat-the-joint group from the urgent-referral group. It is operator dependent: a normal ultrasound in a patient with a convincing lump does not close the case.
Plain radiograph of the knee, two views. Cheap and mandatory when the mass is firm or the patient has mechanical symptoms. It shows the osteoarthritis that explains an adult cyst, the calcification patterns listed above, an osteochondroma and an aggressive bone lesion. It does not exclude soft tissue sarcoma.
MRI with intravenous gadolinium. The definitive study for any solid or indeterminate mass. It defines the compartment, the relationship to the neurovascular bundle, the enhancement pattern and any bone involvement, and it distinguishes a simple cyst, thin-walled with no enhancing nodule, from a cystic-appearing myxoid sarcoma with internal enhancement. It changes management by planning the biopsy tract and the resection.
Duplex and CT angiography. For the pulsatile mass. Duplex measures aneurysm diameter and thrombus load; CTA maps the runoff for bypass or endovascular exclusion. The treatment threshold is generally a diameter of about 2 cm, or the presence of thrombus or embolic symptoms regardless of size.
Image-guided core biopsy. Only after MRI, and only at the treating unit, never in the referring hospital. The tract must lie within the planned resection incision. Fine needle aspiration is inadequate for sarcoma subtyping and grading.
When no further imaging is needed. A child aged 4 to 8 years with a painless, soft, transilluminating, medially placed popliteal swelling, a normal knee examination, no effusion and no systemic symptoms has a clinical diagnosis of primary popliteal cyst: reassure and review. Likewise an adult with a classic soft compressible medial cyst, a known osteoarthritic knee and an ultrasound already confirming a simple anechoic collection needs no MRI unless symptoms change.
The Differential
The list is long and the table carries it, with the discriminator and the confirming test for each cause. Three entries drive everything else on this page: the sarcoma that is mistaken for a cyst, the aneurysm that gets aspirated, and the joint disease whose overflow presents as a lump.
Soft tissue sarcoma. A solid popliteal mass is a sarcoma until proven otherwise, and referral to a sarcoma unit comes before any biopsy. The trap is that myxoid sarcomas can look near-cystic on ultrasound and on T2, so contrast is what separates them from a cyst. A percutaneous or open biopsy performed locally through a transverse incision contaminates all three compartments and may convert a limb-sparing resection into an amputation; biopsy is a sarcoma-unit decision.
Unplanned excision. Excise a sarcoma unplanned and salvage is possible but conditional: outcomes matched primary resection only in patients who reached a sarcoma centre and were liberally re-excised, and residual disease found at that re-excision predicted metastasis (Fiore 2006).
Popliteal artery aneurysm. The limb is lost to distal emboli and acute ischaemia, not to the lump. Aspirating an aneurysm, or a vascular sarcoma, is a catastrophe; ultrasound with colour Doppler costs minutes, so the needle comes after imaging, never before.
The joint that overflows. Synovial chondromatosis fills the posterior recess with loose bodies: the joint is the disease and the fossa swelling is overflow.
- Typical age / setting
- Synovial sarcoma 15-40 years; UPS/myxofibrosarcoma 55-80 years
- Discriminating feature
- Solid, firm, deep to deep fascia, greater than 5 cm, enlarging; synovial sarcoma is often painful and may be small with calcification on plain film
- What confirms it
- MRI with gadolinium showing a solid enhancing mass; core biopsy through a planned longitudinal tract at a sarcoma unit
- Typical age / setting
- Men over 60 years, smokers, often with abdominal aortic aneurysm
- Discriminating feature
- Pulsatile and expansile mass in the midline; may present with acute ischaemia or distal embolic 'trash foot' rather than a lump
- What confirms it
- Duplex ultrasound showing focal dilatation greater than 2 cm or 1.5 times the adjacent artery; CT angiography for planning
- Typical age / setting
- Any age; post-operative, immobilised, malignancy, long-haul travel
- Discriminating feature
- Diffuse calf swelling and tenderness rather than a discrete lump; asymmetry of calf circumference measured 10 cm below tibial tuberosity
- What confirms it
- Compression ultrasound of femoral and popliteal veins — non-compressible vein with absent flow
- Typical age / setting
- Osteosarcoma 10-25 years; Ewing 5-25 years
- Discriminating feature
- Bone-centred mass with night pain and systemic upset; aggressive periosteal reaction (Codman triangle, sunburst, onion-skin) on plain film
- What confirms it
- Plain radiograph then MRI of the whole bone; staging CT chest and bone scan before biopsy
- Typical age / setting
- 40-70 years with osteoarthritis, meniscal tear or inflammatory arthritis
- Discriminating feature
- Medial, arises between medial gastrocnemius and semimembranosus, compressible, positive Foucher's sign, accompanied by joint effusion
- What confirms it
- Ultrasound showing anechoic collection with a neck communicating with the joint; MRI if the underlying joint pathology is unclear
- Typical age / setting
- Same population, sudden onset after activity
- Discriminating feature
- Sudden calf pain with a 'crescent sign' of ecchymosis around the medial malleolus a few days later; the popliteal lump may have disappeared
- What confirms it
- Ultrasound showing fluid dissecting into the calf plus a compressible popliteal vein excluding DVT
- Typical age / setting
- Children 4-8 years, boys more than girls
- Discriminating feature
- Painless, no effusion, no intra-articular pathology, often noticed by a parent; does not fluctuate with joint symptoms
- What confirms it
- Ultrasound alone; observation is definitive management as most resolve within a few years
- Typical age / setting
- 20-50 years, sporting population
- Discriminating feature
- Sits at or just above the joint line, tender over the meniscus, associated with mechanical symptoms and a horizontal cleavage tear
- What confirms it
- MRI showing a fluid collection contiguous with a meniscal tear
- Typical age / setting
- Middle-aged recreational athletes, sudden push-off
- Discriminating feature
- Acute audible 'pop' with immediate pain at the musculotendinous junction; the mass is a haematoma below the fossa, not within it
- What confirms it
- Ultrasound showing fluid between medial gastrocnemius and soleus with normal deep veins
- Typical age / setting
- 30-60 years
- Discriminating feature
- Posterolateral position with common peroneal nerve symptoms — foot drop, dorsal foot paraesthesia, Tinel's over the fibular neck
- What confirms it
- MRI showing a lobulated cyst tracking along the peroneal nerve from the tibiofibular joint
- Typical age / setting
- 20-50 years, monoarticular
- Discriminating feature
- Recurrent haemarthrosis with brown-stained aspirate; boggy diffuse swelling rather than a discrete cyst
- What confirms it
- MRI showing blooming artefact on gradient echo from haemosiderin
- Typical age / setting
- 30-50 years
- Discriminating feature
- Multiple similar-sized calcified loose bodies within a distended posterior recess, mechanical locking
- What confirms it
- Radiograph showing multiple ring-and-arc calcifications; MRI confirms intrasynovial origin
- Typical age / setting
- Any age; malformation often present since childhood
- Discriminating feature
- Soft, compressible, enlarges on dependency or Valsalva; phleboliths on plain radiograph are pathognomonic of venous malformation
- What confirms it
- Duplex ultrasound and MRI; venous aneurysm is a source of pulmonary embolism and needs vascular referral
- Typical age / setting
- 10-25 years, may be multiple in hereditary multiple exostoses
- Discriminating feature
- Hard, bony, immobile mass continuous with the femur; may cause a reactive bursa or pseudoaneurysm of the popliteal artery
- What confirms it
- Radiograph showing cortical and medullary continuity; MRI if the cartilage cap exceeds 2 cm in an adult (malignant transformation)
Narrowing It Down
Seven questions, in an order that puts the dangerous answers first.
1. Is it pulsatile? This comes first because it changes the next action from "reassure" to "do not touch, vascular referral". Palpate for expansile pulsation and auscultate, then, in the same examination, feel the contralateral popliteal fossa and the abdominal aorta: popliteal aneurysms are bilateral in 50 to 75 per cent, and 33 to 43 per cent have a coexisting abdominal aortic aneurysm. A pulsatile mass is a popliteal artery aneurysm until duplex says otherwise.
2. Is it cystic or solid? Ultrasound answers this in minutes and is the correct first test. Anechoic, compressible and without internal vascularity is a cyst; internal echoes with vascularity, or a mass that does not compress, is solid. A solid mass deep to deep fascia, larger than 5 cm, or enlarging triggers the sarcoma pathway: MRI with contrast and referral before biopsy.
3. Where exactly is it? Localise the mass against the anatomy rather than calling it "popliteal":
- Medial, in the gastrocnemio-semimembranosus bursa: Baker's cyst
- At the joint line posteromedially: parameniscal cyst
- Posterolateral near the fibular neck with peroneal symptoms: tibiofibular ganglion
- Central and deep: vascular or nerve sheath
- Below the fossa in the calf: gastrocnemius tear
4. How old is the patient, and is there joint disease? Examine the knee itself, not only the swelling behind it. A child with a painless cyst and a normal joint has a primary popliteal cyst: observe. An adult with a cyst almost always has an intra-articular cause, whether effusion, meniscal tear, osteoarthritis, rheumatoid or crystal arthropathy, and treating the cyst without treating the joint guarantees recurrence.
Treat the joint, not the cyst. The cyst is a marker of the joint rather than a source of pain in its own right: it is present in 28 per cent of painless osteoarthritic knees against 33 per cent of painful ones, and it does not track with pain once arthritis severity is accounted for (Hill 2001).
5. Is the calf involved, and is the leg red and swollen? An acute painful swollen calf puts deep vein thrombosis and ruptured Baker's cyst on the table together, and they can coexist. Request an ultrasound that specifically compresses the femoral and popliteal veins as well as imaging the fossa. A negative D-dimer does not exclude a ruptured cyst, and a positive Doppler for thrombosis does not exclude the mass that caused the venous compression: extrinsic compression by a mass is a recognised cause of popliteal DVT. Do not anticoagulate a ruptured cyst; do not discharge an undiagnosed DVT.
6. Are there red flags for malignancy? Ask about night pain, rest pain, systemic upset and weight loss; measure the mass; note whether it is deep to fascia, growing, or recurrent after previous excision. Any one of these mandates MRI with gadolinium of the whole compartment plus staging, regardless of how cystic it looks.
7. Does the plain film add anything? Take orthogonal radiographs and look specifically for the calcification pattern and cortical continuity:
- Phleboliths: venous malformation
- Ring-and-arc calcification: synovial chondromatosis or a chondroid tumour
- Amorphous calcification in a young adult: synovial sarcoma
- Cortical continuity: osteochondroma
- Aggressive periosteal reaction or bone destruction: primary bone sarcoma
A normal radiograph never excludes a soft-tissue sarcoma.
MCQ Practice Points
Q: Between which two structures does a Baker's cyst arise?
A: The medial head of gastrocnemius and the semimembranosus tendon — the gastrocnemio-semimembranosus bursa. This is why the cyst is medial and points inferomedially. A laterally placed cyst is not a Baker's cyst and demands imaging.
Q: What is Foucher's sign and what does a negative sign suggest?
A: A Baker's cyst becomes tense on full knee extension and softens at about 45 degrees of flexion, because the valvular neck closes in extension. A mass whose consistency does not change with knee position is more likely solid — tumour, thrombosed aneurysm or ganglion.
Q: Which four features of a soft tissue mass mandate referral to a sarcoma service?
A: Size greater than 5 cm, deep to the deep fascia, increasing in size, and painful. Any one of these is sufficient. Recurrence after previous excision is a fifth. A mass may be mobile, painless and still malignant.
Q: A 6-year-old boy has a painless 3 cm swelling behind the knee, no effusion, full range of motion. Management?
A: Ultrasound to confirm a simple cyst, then reassurance and observation. Paediatric popliteal cysts are usually primary, not associated with intra-articular pathology, and the majority resolve spontaneously. Excision has a high recurrence rate and is reserved for large, symptomatic or atypical lesions.
Q: Which popliteal mass classically causes foot drop and why?
A: A ganglion arising from the proximal tibiofibular joint. It tracks along the articular branch of the common peroneal nerve, which winds around the fibular neck, producing progressive foot drop and dorsolateral foot numbness. Treatment must include disconnection of the articular branch and the joint connection or it recurs.
Q: An adult presents with a recurrent popliteal cyst after two previous aspirations. What has been missed?
A: The intra-articular cause. Secondary Baker's cysts are driven by joint effusion from meniscal tears, osteoarthritis or inflammatory arthritis. Address the joint — arthroscopic meniscal treatment, joint replacement or disease-modifying therapy — and the cyst resolves. Isolated cyst excision without treating the joint has a high recurrence rate.
A BAD MASSPopliteal Mass — 'A BAD MASS'
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“You are shown a photograph and a lateral knee radiograph of a 32-year-old woman with an 18-month history of a firm, mildly painful swelling behind the right knee. The radiograph shows a soft tissue shadow posterior to the joint with faint amorphous calcification. She has been told twice that it is a Baker's cyst. How do you proceed?”
“You are shown a clinical photograph of a 58-year-old man, three weeks after a knee arthroscopy, with a swollen, warm, tender left calf and bruising around the medial malleolus. What is your differential and what do you do?”
“You are shown a duplex image of the popliteal fossa in a 71-year-old male smoker referred with a 'Baker's cyst' that has been aspirated once unsuccessfully. The vessel measures 2.6 cm with mural thrombus. What now?”
Anatomy you must state
- Boundaries: biceps femoris superolaterally, semimembranosus and semitendinosus superomedially, two heads of gastrocnemius inferiorly.
- Contents from superficial to deep: tibial nerve, popliteal vein, popliteal artery.
- Common peroneal nerve runs along the medial border of biceps femoris to the fibular neck.
- Baker's cyst arises from the gastrocnemio-semimembranosus bursa — medial, points inferomedially.
Three questions in order
- Is it pulsatile? — aneurysm until duplex says otherwise; never aspirate first.
- Is it cystic or solid on ultrasound? — solid equals sarcoma pathway.
- Where exactly is it? — medial equals cyst, joint line equals meniscal cyst, posterolateral with foot drop equals tibiofibular ganglion.
Red flags for sarcoma
- Greater than 5 cm; deep to deep fascia; enlarging; painful; recurrent after excision.
- Night pain, weight loss, systemic upset.
- Amorphous calcification in a young adult — think synovial sarcoma.
- Myxoid sarcomas can look cystic on ultrasound and T2 MRI — contrast is essential.
Clinical signs
- Foucher's sign — tense in extension, soft at 45 degrees flexion: Baker's cyst.
- Crescent sign — medial malleolar ecchymosis days after cyst rupture.
- Pseudothrombophlebitis — ruptured cyst mimicking DVT; scan for both.
- Tinel's at the fibular neck with foot drop — tibiofibular ganglion.
Management principles
- Adult Baker's cyst: treat the joint, not the cyst — recurrence is otherwise the rule.
- Paediatric cyst: ultrasound, reassure, observe; most resolve.
- Popliteal aneurysm: intervene at about 2 cm, or with thrombus or embolic symptoms; check the other leg and the aorta.
- Suspected sarcoma: MRI with contrast, then refer, then biopsy at the treating unit through a planned longitudinal tract.
Evidence Base
Knee Effusions, Popliteal Cysts, and Synovial Thickening - Association with Knee Pain in Osteoarthritis
- MRI in three groups: 381 subjects with knee pain AND radiographic osteoarthritis, 52 with radiographic osteoarthritis but NO pain, and 25 with neither
- Popliteal cysts were present in 33.0 per cent of painful osteoarthritic knees, 28.0 per cent of PAINLESS osteoarthritic knees, and 9.1 per cent of normal knees
- THE KEY NEGATIVE RESULT: after adjusting for radiographic osteoarthritis severity, moderate or larger effusions and synovial thickening were significantly more frequent in painful knees (both p less than 0.001) - but the prevalence of POPLITEAL CYSTS WAS NOT
- Among those with pain, synovial thickening was associated with the SEVERITY of pain; the cyst was not
- The authors' framing: effusions and popliteal cysts are common in middle-aged and elderly people