Knee — Reference values

Knee — Reference values: Minimum knee protocol, Effusion — suprapatellar recess, Patellar tendon — thickness, Quadriceps tendon — thickness, Meniscal extrusion.

Knee ultrasound is a compartment-based targeted examination. It is strong for effusion, synovium, bursae, tendons, superficial collateral ligaments, Baker cyst and periarticular masses; it is limited for deep cartilage, cruciate ligaments and the central menisci. The colors below separate consensus normality, borderline/divergent zones and abnormal or high-risk findings.

Measurements and reference values

Measurement	Usual value	Note
Minimum knee protocol	anterior, medial, lateral, posterior and dynamic	Document the suprapatellar recess, patellar and quadriceps tendons, collateral ligaments, accessible menisci, bursae, Baker cyst, popliteal fossa and dynamic comparison when useful.Source: ESSR knee technical guidelines / RadioGraphics knee US
Effusion — suprapatellar recess	3,6–6,0 mm	There is no single cutoff: 3.6 mm increases sensitivity, 6 mm increases specificity; some services use 4 mm. Prefer longitudinal measurement with the knee around 30 degrees flexed.Green: <2 mm without inflammatory contextYellow: 2-5.9 mm or cutoff disagreementRed: ≥6 mm with fever, trauma, postoperative status or complex fluidSource: J Diagn Med Sonogr 2020 / Arthritis Care & Research 2012
Patellar tendon — thickness	4–5 mm	Thickness alone does not diagnose tendinopathy; correlate with hypoechogenicity, fibrillar loss, Doppler, enthesis and focal pain.Green: 4-5 mm with preserved echotextureYellow: >5 mm or focal thickeningSource: AJR 2001
Quadriceps tendon — thickness	no universal cutoff	Measurements vary with technique, sex, activity and measurement point. Use the calculator as a contextual alert, not as a standalone diagnosis.Green: continuous fibers and no focal painYellow: thickening, hypoechogenicity or DopplerRed: complete discontinuity or retractionSource: Radiographics knee US / MSK ultrasound reviews
Meniscal extrusion	3 mm	Three millimeters is a practical but not absolute cutoff; loading, age, osteoarthritis, technique and MRI change interpretation.Green: <2 mmYellow: 2-4.9 mm or degenerative contextRed: ≥5 mm, especially with root-tear concern or advanced osteoarthritisSource: Orthopedic Reviews 2023 / AJR meniscal extrusion / BLOKS
Medial collateral ligament — thickness	3,3–5,6 mm	Range described at the femoral attachment. Thickening is more meaningful with edema, focal pain, fibrillar loss or instability.Green: up to 5.6 mm at the femoral point with preserved fibersYellow: >5.6 mm or edema without tearRed: complete discontinuity or avulsionSource: Skeletal Radiology 1996
Iliotibial band — thickness	~1,1 mm	Values above 2-3 mm, with edema or deep fluid near the lateral femoral condyle, favor iliotibial band syndrome.Green: ≤2 mm without edemaYellow: >2-3 mm or dynamic lateral painRed: >3 mm with typical deep edema/fluidSource: Skeletal Radiology 2010 / RadioGraphics knee US
Baker's cyst	neck between semimembranosus and medial gastrocnemius	A posterior cyst may drain knee fluid; absence of suprapatellar fluid does not exclude effusion when a Baker cyst is present.Yellow: simple or septated, without rupture signsRed: ruptured, dissecting into calf or mimicking thrombosisSource: ESSR knee guideline / RadioGraphics knee US / J Diagn Med Sonogr 2020
Prepatellar and pes anserine bursae	normally not visible	Simple distension is usually contextual; complex content, gas, intense hyperemia or wound raises priority.Source: RadioGraphics knee US / MSK ultrasound reviews

Classifications and calculators

Interactive assistant — knee ultrasound

Input	Output	How to use
Measurements in range and no clinical alert	Green	Use when the minimum protocol was documented and a deep structure is not the main question.
Small effusion, mild synovitis, tendinopathy, simple bursitis or borderline meniscal extrusion	Yellow	Describe as contextual, compare with the opposite side when useful and suggest clinical correlation or MRI if the question is internal.
Possible infection, complete tear, fracture/avulsion, thrombosis/aneurysm, ruptured cyst or significant effusion in a risk context	Red	Communicate priority and route to orthopedics, rheumatology, vascular care or emergency according to the dominant pattern.

Fill measurements, synovium, tendons, menisci, ligaments, Baker cyst, posterior region and warning signs to generate a color reading.

Source: ESSR / EULAR-OMERACT / J Diagn Med Sonogr / RadioGraphics

Anatomic roadmap by compartment

Compartment	What to assess	Pitfalls
Anterior	Suprapatellar recess, quadriceps tendon, patellar tendon, Hoffa fat pad and prepatellar bursa.	Fluid redistributes with flexion; also scan medial and lateral to the quadriceps tendon.
Medial	Medial collateral ligament, peripheral medial meniscus, pes anserine region and osteophytes.	Meniscal extrusion depends on loading, osteoarthritis and technique; do not diagnose root tear by ultrasound alone.
Lateral	Iliotibial band, lateral complex, peripheral lateral meniscus, fibular head and common peroneal nerve.	Anisotropy may mimic ligament injury; use long- and short-axis views.
Posterior	Baker cyst, popliteal fossa, artery, vein, tibial nerve and common peroneal nerve.	Ruptured cyst may mimic thrombosis; with calf swelling, perform compression venous assessment.

Source: ESSR knee technical guidelines / RadioGraphics

Effusion and synovitis — color reading

Finding	Green	Yellow	Red
Suprapatellar recess	<2 mm and no synovitis	2-5.9 mm or divergent cutoff	≥6 mm with fever, trauma, postoperative status or complex fluid
Greyscale synovium	no hypertrophy	grade 1-2	grade 3 or exuberant synovial mass
Synovial Doppler	no flow	few signals or moderate flow	intense flow or associated infection concern

Effusion and synovitis are not synonyms: fluid may be mechanical/traumatic; synovial hypertrophy with Doppler suggests inflammatory activity.

Source: J Diagn Med Sonogr 2020 / EULAR-OMERACT

Simplified EULAR-OMERACT synovitis score

Grade	Greyscale	Doppler	Practical reading
0	no synovial hypertrophy	no Doppler signal	no active synovitis by score
1	minimal hypertrophy	few punctate signals	mild or early activity
2	moderate hypertrophy	confluent flow in less than half of synovium	probable inflammatory activity
3	marked hypertrophy	flow in more than half of synovium	important activity; correlate with inflammatory arthritis or infection according to context

Source: EULAR-OMERACT / RMD Open

Extensor mechanism and tendons

Structure	Normal/low risk	Caution	High risk
Patellar tendon	4-5 mm, fibrillar and no Doppler	>5 mm, hypoechogenicity, enthesopathy or Doppler	complete tear or avulsion
Quadriceps tendon	continuous fibers	thickening or partial tear	complete discontinuity, retraction or extension inability
Hoffa fat pad and prepatellar bursa	no distension or hyperemia	edema, simple bursitis or anterior impingement	complex content, gas, wound or possible infection

Source: RadioGraphics knee US / AJR patellar tendon

Menisci, collateral ligaments and iliotibial band

Structure	Green	Yellow	Red
Meniscal extrusion	<2 mm	2-4.9 mm; 3 mm is practical, not absolute	≥5 mm or root-tear/advanced osteoarthritis concern
Medial collateral ligament	continuous fibers; up to 5.6 mm at the described femoral point	thickening, edema or partial tear	complete tear or avulsion
Lateral complex	continuous fibers and no focal pain	sprain, thickening or partial tear	complete tear, avulsion or posterolateral instability
Iliotibial band	≤2 mm without deep edema	>2-3 mm or dynamic pain	>3 mm with typical deep edema/fluid

Source: Orthopedic Reviews / Skeletal Radiology / RadioGraphics

Posterior knee and differential diagnoses

Finding	Ultrasound clue	Teaching action
Baker's cyst	typical neck between semimembranosus and medial gastrocnemius	measure, describe simple/complex and look for effusion/synovitis
Ruptured Baker's cyst	fluid dissecting into calf	differentiate from thrombosis, hematoma and cellulitis
Popliteal thrombosis	noncompressible vein or thrombus	activate venous protocol; do not treat as isolated cyst
Aneurysm or pseudoaneurysm	pulsatile mass, yin-yang flow or arterial neck	use Doppler and communicate vascular finding
Ganglion or intraneural cyst	relationship with common peroneal or tibial nerve	document neural course and relation to fibular head

Source: ESSR / RadioGraphics knee US

Knee report checklist

Item	Question the report should answer
Technique	Were transducer, compartments assessed, position/flexion and dynamic maneuvers described?
Measurements	Were effusion, tendons, meniscal extrusion, ligament thickening or cyst measured when present?
Limitation	Does the conclusion avoid excluding central meniscus, deep cartilage and cruciate ligaments when not assessable?
Alert	Is there a communication phrase for possible infection, complete tear, thrombosis/aneurysm or fracture/avulsion?

Source: Best-practice synthesis from ESSR / EULAR-OMERACT / RadioGraphics

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