Knee Ultrasound Made Easy: Step-By-Step Guide

Primary Authors: Katienne Yau, Nathaniel Kim, Johnna Torres, Vi Dinh.

A critical part of the Musculoskeletal (MSK) exam, knee ultrasound can be used for a range of musculoskeletal complaints from tendon tears to effusions. While MRI remains superior for some internal pathologies (deep meniscal, ligament tears), ultrasound proves to be a powerful tool for diagnosing many common knee problems.

In this article we will systematically perform an ultrasound examination of the knee. We will cover: 

1. Anatomy of the knee
2. Step-by-step maneuvers to obtain the desired views 
3. Pathology seen at each view 
4. Therapeutic ultrasound-guided injection/aspiration procedures that are commonly performed 

Knee Ultrasound Anatomy

The knee is the complex joint formed where the distal femur approximates to the proximal tibia. The stability of the joint is provided by numerous tendons, ligaments, and cartilaginous structures, many of which are easily identified using ultrasound imaging.

Bones

There are three main bony structures that contribute to the knee joint. The superior aspect of the joint is made by the distal femur. The most distal ends of the femur’s lateral and medial condyles articulate with the superior aspect of the lateral and medial menisci respectively. Superior to the lateral and medial condyles lies the lateral and medial epicondyles which serve as the insertion point for various ligaments. On the posterior aspect of the femur, the intercondylar fossa is a groove located between the lateral and medial condyles. Between these two condyles on the anterior side lies the femoral trochlear groove, an important path for the movement of the patella.

The bone that forms the inferior part of the knee joint is the proximal tibia. The most superior aspect of the tibia, the tibial plateau, articulates with the inferior aspect of the lateral and medial menisci. A bony prominence on the anterior tibia, named the tibial tuberosity, serves as the insertion point for the patellar tendon.

Finally, a small bone sits anterior to the femoral condyles that provides protection and decreased friction for the joint. The patella, which is buried within the quadriceps tendon, glides within the femoral trochlear groove, tracking superiorly with knee extension and inferiorly with knee flexion. The posterior patella contains the lateral and medial facet, which are directly responsible for articulating with the femoral trochlear groove.

Muscles and Tendons

Several muscles are important to allow for smooth and strong flexion and extension of the knee joint.

The quadriceps femoris muscle group serves as the primary knee extensors and are positioned on the anterior thigh. There are four muscles that make up the quadriceps muscle group: the rectus femoris, the vastus medialis, the vastus lateralis, and the vastus intermedius. These four muscles come together inferiorly to form the quadriceps tendon which then connects to the patella. The tendon continues on past the patella, inserting onto the tibial tuberosity. After passing the patella, the tendon is renamed to be called the patellar tendon. This means that the tendon superior to the patella is the quadriceps tendon and inferior to the patella is the patellar tendon.

Knee flexion is controlled primarily by the hamstring muscle group located on the posterior thigh. Three muscles make up this group: the biceps femoris, the semitendinosus, and the semimembranosus. These originate from the ischial tuberosity of the pelvis (with the short head of the biceps femoris originating from the posterior femur). Both heads of the biceps femoris insert on the lateral side of the proximal fibula. The semitendinosus and semimembranosus insert on the medial aspect of the proximal tibia.

Two other muscles play smaller roles in the function and stability of the knee joint. The gastrocnemius originates from the posterior aspect of the femoral condyles and inserts on the calcaneus of the foot as the Achilles tendon. Because this muscle crosses both the knee and ankle joints, it plays a role in knee flexion, particularly when the leg is extended, providing important strength for pushing off while walking. 

The popliteus muscle is a small muscle that originates on the lateral femoral condyle and inserts on the posterior, proximal tibia. It is responsible for medial rotation of the tibia on the femur, a crucial step in unlocking the knee from full extension.

Ligaments

Ligaments play a significant role in maintaining the stability of the knee joint and are some commonly injured knee structures. The four most important ligaments include the lateral and medial collateral ligaments and the anterior and posterior cruciate ligaments.

The lateral and medial collateral ligaments are positioned on the lateral and medial aspects of the knee joint. The lateral collateral ligament (LCL) connects the lateral aspect of the lateral femoral epicondyle with the head of the fibula. It plays a crucial role in stabilizing the lateral knee and resisting varus stress. The medial collateral ligament (MCL) originates on the medial aspect of the medial epicondyle of the femur and inserts on the medial proximal tibia. The MCL serves to stabilize the medial knee and resist valgus stress. The MCL also has fibers that attach to the medial meniscus and joint capsule.

Though the cruciate ligaments are not well visualized on ultrasound, they play a key role in maintaining knee stability. These ligaments cross deep within the knee to provide resistance to anterior and posterior translocation of the tibia on the femur. The anterior cruciate ligament (ACL) originates from the posteromedial aspect of the lateral femoral condyle, within the intercondylar notch of the femur. It inserts on the anterior intercondylar area of the tibial plateau. The ACL serves to limit anterior translation and internal rotation of the tibia. The posterior cruciate ligament (PCL) originates from the anterolateral aspect of the medial femoral condyle, within the intercondylar notch of the femur. It inserts on the posterior intercondylar area of the tibial plateau. The PCL prevents posterior translation of the tibia.

The Menisci

The knee joint is cushioned by two crescent shaped fibrocartilaginous structures named the lateral and medial menisci. These sit on top of the tibial plateau and provide cushion and support between the femoral condyles and the tibia. The lateral meniscus is similar in shape to the letter O. As the name implies, the lateral meniscus lies between the lateral femoral condyle and the lateral aspect of the tibial plateau. The medial meniscus is shaped like the letter C. It lies between the medial femoral condyle and the medial aspect of the tibial plateau. The MCL and semimembranosus tendon have deep fibers that connect them to the medial meniscus. 

Both the menisci are thicker at the peripheral edge and become thinner toward the center. The thicker, outside edge receives the majority of the blood flow, with the thinner zones receiving very little blood. They both have anterior and posterior horns that anchor to the intercondylar area of the tibia and are attached to the tibial plateau and joint capsule via the coronary ligaments. In addition to providing mechanical cushioning, the menisci serve to create a deeper articular surface for the femoral condyles. They also provide joint lubrication via synovial fluid distribution and contain mechanoreceptors that aid in proprioception of the knee joint.

Bursae and Fat Pads

Several fluid filled sacs, called bursae, are strategically placed within the knee joint to reduce friction and enable smooth gliding of the knee. The prepatellar bursa is located anterior to the patella and cushions the patella from the skin and subcutaneous tissue while kneeling on the patella. The superficial and deep infrapatellar bursa lie superficial and deep to the patellar tendon respectively. They provide protection for the patellar tendon and reduce friction during kneeling. The suprapatellar bursa, also known as the suprapatellar synovial recess, is located between the quadriceps tendon and the femur and reduces friction during knee extension. It also serves as a reservoir for synovial fluid, allowing the knee to quickly adapt to changes in joint volume during flexion and extension. The popliteal bursa is located in the posterior knee between the medial head of the gastrocnemius and the tendon of the semimembranosus muscle. It supports smooth functioning of the posterior knee muscles.

Fat pads are made of highly vascularized, innervated adipose tissue and serve to provide cushioning, joint lubrication, and proprioception to the knee joint. The infrapatellar (Hoffa’s) fat pad is located between the patella, the patellar tendon, and the anterior tibial plateau and serves to absorb shock and fill space allowing for joint movement. The suprapatellar fat pad is located deep to the quadriceps tendon, cushioning the suprapatellar bursa.

Patient Preparation

• The knee exam is performed with the patient lying supine for Steps 1-5 (anterior, medial, and lateral views) and prone for Step 6 (posterior view).
• When lying supine, the knee should be flexed 20-30 degrees (a pillow or rolled towel under the knee may help support).
• Exception: When assessing the femoral trochlea in the anterior knee exam, the knee should be flexed to 90 degrees for optimal visualization.
• When scanning the posterior knee, position the patient prone with knees fully extended.
• Expose the patient’s knee joint. Use a gown if necessary.

Machine Preparation

• Transducer: Linear probe
• Preset: Musculoskeletal
• Indicator: Towards the patient’s head (longitudinal view), towards the patient’s left (transverse view posterior knee)
• Machine Placement: Place the machine on the patient’s right side so you can scan with your right hand and manipulate ultrasound buttons with your left hand.

Knee Ultrasound Exam

Our 6-step systematic approach to knee ultrasound begins with the anterior aspect, scanning cranially to caudally. The suprapatellar, prepatellar, and infrapatellar regions are imaged in succession.

Step 1: Suprapatellar View (Longitudinal)

• Position: Begin with the patient lying supine, flexing the knee 20-30 degrees to assess its anterior aspect. Tightening the extensor tendons with slight knee flexion will make them easier to view and reduce anisotropy, which can mimic tendon tears. 
• Probe: Place the probe oriented longitudinally (indicator to the patient’s head) just above the patella

• Identify and fan through the quadriceps tendon (QT). Visualize its attachment onto the superior portion of the patella. Ensure that the probe remains perpendicular to the tendon to prevent anisotropy, which can lead to a false positive for tendon injury.
• Identify the suprapatellar recess, which is located deep to the QT and superior to the patella. An anechoic region extending superiorly from the physiologic recess may represent an effusion that could benefit from drainage.

Step 2: Prepatellar View (Longitudinal)

• Probe: slide the probe downward in the longitudinal orientation until positioned just over the patella.

• Identify the prepatellar bursa, which is located in the subcutaneous region superficial to the patellar tendon. It is usually indistinguishable from the subcutaneous tissues unless there is fluid within the bursa.
  • Note: Be sure to use a lot of gel, and don’t compress too much. The bursa is easily obliterated, so you can get a false negative by applying too much pressure/not enough gel.
• Identify the patellar tendon (PT). Ensure that you examine the entire length of the PT by moving your probe up and down along the patella

Step 3: Infrapatellar View (Longitudinal)

• Probe: slide your probe down to the infrapatellar portion of the anterior knee. Keep the knee bent at 20-30 degrees.

• Identify the patellar tendon and its attachment to the inferior portion of the patella. Sweep through the entirety of the PT to ensure consistent echogenicity and size. Trace the PT inferiorly to its attachment on the tibial head.
• Identify the infrapatellar (Hoffa’s) fat pad, which has a hypoechoic appearance and is located deep to the patellar tendon.

Step 4: Lateral View (Longitudinal)

• Position: To examine the lateral knee, you may rotate the patient’s leg internally while maintaining 20-30 degrees of knee flexion (slight lateral decubitus positioning may help with this).
• Probe: Place the probe longitudinally over the lateral joint space (hint: many assume this at the level of the mid-patella, but it is actually lower).

• Identify the lateral meniscus (LM), which will appear as a hyperechoic triangle between the patella and proximal tibia.
  • Note: To increase the sensitivity of your image, zoom in on the LM and add a chroma map. This will allow for improved identification of small tears in the LM, which will appear as hypoechoic fissures.

• Identify the lateral collateral ligament (LCL) and iliotibial band (ITB), which both appear superficial to the lateral meniscus. The IT band is normally more hyperechoic and thinner than the LCL.
  • Note: The LCL attaches proximally at the lateral femoral epicondyle and distally at the fibular head. The ITB attaches proximally at the iliac crest and distally at the tibial head (Gerdy’s tubercle).

Step 5: Medial View (Longitudinal)

• Position: To examine the medial knee at the level of the knee joint, it may help to have the patient externally rotate their hip slightly. If doing this, be sure to provide support to limit subtle varus force on the knee, which may create MCL laxity.
• Probe: Place the probe longitudinally over the medial joint space

• Identify the medial meniscus (MM), which is the echogenic triangle located between the inferior part of the patella and the proximal tibia.
  • Utilize the zoom and chroma features to increase the sensitivity of your scan.
• Identify the medial collateral ligament (MCL), which is located superficial to the medial meniscus and has linear striations. Assess its proximal attachment to the medial epicondyle of the femur and its distal attachment to the proximal tibia.

Step 6: Posterior View (Transverse)

• Position: Assess the posterior knee in the popliteal fossa by having the patient lie prone.
• Probe: Point your indicator toward the patient’s left.

• Identify the popliteal artery (PA), which appears as an anechoic circular structure deep to the muscle.
  • Add color Doppler for further characterization of the artery. Assess for aneurysm and plaque.
• Identify the popliteal vein (PV), which is normally an oval structure located adjacent to the popliteal artery.
  • Assess for thrombosis by applying pressure to the vein.
• Identify the medial gastrocnemius (MG), which overlies the PA and PV.

Common Pitfalls

• Applying too much pressure. This may result in missed effusions, as the increased pressure can displace fluid.
• Applying too little gel. The knee is an irregularly-shaped joint with many bony prominences. Applying too little gel will reduce the viewing window and may lead to missed effusions if pressure is applied directly to skin.
• Interpreting anisotropy as a tear or cyst. Slight angulation of the probe from perpendicular can produce hypoechoic/anechoic structures, which can mimic tears and cysts.

Major Knee Ultrasound Pathology

Medial Collateral Ligament (MCL) Injury

The MCL is the ligament most commonly injured in the knee, and mechanisms include turning, cutting, or twisting. Direct trauma or hits to the lateral knee causes valgus stress, which puts the MCL at great risk for tears (Naqvi & Sherman, 2025).

On ultrasound:

• Thickened or hypoechoic MCL
• Hypoechoic interruption in MCL fibers

Lateral Collateral Ligament (LCL) Injury

The LCL is injured with excessive varus stress and hyperextension of the knee, most commonly seen in high-energy impacts to the anteromedial knee. Sports that require frequent pivoting and jumping or involve high-energy physical contact increase the likelihood of injury (Yaras et al., 2024).

On ultrasound:

• Thickened and hyper-/hypoechoic LCL
• Hypoechoic interruption of LCL fibers

Quadriceps Tendon Tear

Patients with quadriceps tendon tears will usually present with a swollen knee and may be unable to ambulate. Complete tears typically have a hematoma present on ultrasound. Partial tears may be distinguished from complete if there is only a focal hypoechoic defect in the tendon. Gentle traction on the patella may result in better visualization of the tendon defect if patients are restricted in their knees’ range of motion (Secko et al., 2011).

On ultrasound:

• Tendon fibers complete or partial separation from attachment points
• Hypoechoic or anechoic region between tendon fibers

Patellar Tendinopathy (Jumper’s Knee)

Athletes who engage in frequent jumping place repeated stress on the extensor tendon complex during both the takeoff and landing phases. This repetitive loading is considered a contributing factor in the development of patellar tendinopathy. Affected individuals typically report anterior knee pain, which is most commonly centered in the infrapatellar area. However, discomfort may also present in the suprapatellar region, involving the quadriceps tendon, or at the distal patellar tendon near its insertion at the tibial tuberosity (Jacobson, 2018).

On ultrasound:

• Hypoechoic tendon enlargement/thickening
• Tendon fibers remain visible
• Hyperemia from neovascularization on color and power Doppler imaging

Patellar Tendon Rupture/Tear

Patellar tendon rupture is more common in younger populations than older, especially those who suffer from trauma or with chronic tendinopathy, also known as jumper’s knee. It occurs most often at the superior attachment site on the patella. 

On Ultrasound for Tear:

• Hypoechoic area that interrupts the fibers

On Ultrasound for Rupture:

• Complete tendon fiber discontinuity
• Refraction shadowing at retracted torn tendon stumps

Patellar Fracture

A fracture should be suspected when there is a disruption in the normal continuous, smooth cortex of the bone and point tenderness with direct pressure (Jacobson, 2018).  Also, a hematoma may be present, which is typically visualized as a hypoechoic (dark) collection (Carter et al., 2016). When using lipohemarthrosis, a multi-layered collection of fluid in the subquadricipital recess, as an indicator for fracture in acute knee trauma, MSK US has been shown to have increased sensitivity, 94% versus 84%, over plain radiography in the diagnosis of fractures (Bonnefoy et al., 2006).

On ultrasound:

• Disrupted bone cortex
• Hematoma or lipohemarthrosis 

Medial/Lateral Meniscus Tears

Tears of the meniscus can occur as a result of increased axial load plus rotational or shearing forces, especially during squatting, lifting heavy weights, rapid acceleration or deceleration, changing directions, or jumping. Males over the age of 40 are at increased risk for meniscal tears as well as individuals who play football, basketball, soccer, skiing, and wrestling (Raj & Bubnis, 2023).

On ultrasound:

• Irregular edge(s)
• Hypo- or anechoic areas within the meniscus 

Medial Head of Gastrocnemius Tear

Tears of the medial head of the gastrocnemius occur during running, jumpings, or sports involving rapid push off. Additional risk factors include old age and prior history of posterior calf strains (Coffey & Khan, 2023).

On ultrasound:

• Focal hypo-/anechoic edema within isoechoic muscle tissue
• Possible local hematoma and signs of inflammation

Knee Effusion

A joint effusion is defined as an increased amount of fluid within the synovial compartment of a joint. Although there is normally a small amount of physiological intra-articular fluid, abnormal fluid accumulation can result from inflammation, infection, or trauma. Affected individuals commonly endorse pain around the joint with feeling of fullness and/or warmth. Assessing the knee joint for effusions requires patients to slightly flex the knee, placing the probe over the suprapatellar, lateral, and medial regions of the anterior knee. The ultrasonographic appearance of an effusion differs depending on what type of fluid collection is present.

Simple Knee Effusion

On ultrasound for simple knee effusion:

• Simple joint effusions appear anechoic without any internal echoes, indicating clear, serous fluid.

Complex Knee Effusion

Complex joint effusions, which include hemarthrosis and lipohemarthrosis, have a heterogeneous appearance unlike simple joint effusions. 

Additionally, color Doppler will demonstrate lack of internal flow, and compression will result in redistribution or swirling of contents. 

Furthermore, hemarthrosis will appear hypoechoic with internal echoes, while lipohemarthrosis will have a fat-fluid level. Either of these findings suggest a possible occult intra-articular knee fracture. Of note, ultrasound has been shown to be significantly more sensitive for the evaluation of lipohemarthrosis in the suprapatellar bursa than radiographs (97% versus 55%, respectively) (Alves et al., 2016).

Baker Cyst

A Baker (or popliteal) cyst is a swelling of the popliteal bursa in the posterior compartment of the knee. This typically results from secondary causes of joint space inflammation including osteoarthritis, rheumatoid arthritis, or meniscal tears. Structural communication between the inflamed joint space and popliteal bursa leads to a visible pocket of fluid that can be swiftly identified on ultrasound. Patients commonly endorse posterior knee fullness or tightness that worsens with knee extension or activity. 

On ultrasound:

• Anechoic or hypoechoic defect between the semimembranosus and medial head of the gastrocnemius tendon. 
• Larger cysts will spill out into the space immediately superficial to the gastrocnemius tendon. 
• The presence of isoechoic or hyperechoic material within a Baker cyst may represent complex fluid, hemorrhage, or inflammatory changes to the synovium (Jacobson, 2018). 

The presence of hypoechoic fluid beyond the bounds of the Baker cyst suggests rupture. In the case of cyst rupture, blood and edema can extend as far distally as the ankle.

Gout

Gout is the most common crystal-induced arthritis and is caused by monosodium urate crystal deposition in joints and soft tissue. The ultimate result of gout is hyperuricemia caused by either overproduction or underexcretion. Gout flares may be triggered by alcohol, red meat, seafood, dehydration, trauma, and/or surgery. Patients with gout often present acutely with severe pain, redness, swelling, and warmth of the joint. Even before symptoms of hyperuricemia arise, ultrasound detection of distal patellar tendon involvement is not uncommon. Dynamic ultrasound examination is more effective in differentiating gout from other crystal arthritis as the double contour sign of gout moves with cartilage in the same direction (Cipolletta, 2023).

On ultrasound: 

• Double contour sign: hyperechoic line over superficial margin of articular cartilage, parallel to subchondral bone interface (Parvanescu, 2024).
  • Deep hyperechoic line = normal subchondral bone
  • Superficial hyperechoic line = urate crystal deposition on cartilage 

Osgood-Schlatter and Enthesophyte

Osgood-Schlatter presents most commonly in teenage males as bone formation over the tibial tuberosity and is painful with repetitive mechanical stress such as jumping and running (Vaishya et al., 2016). An enthesophyte is also a bony proliferation within the patellar tendon at its insertion site on the tibia that results from repetitive movements or inflammatory conditions (Hacking et al., 2019).

On ultrasound for Osgood-Schlatter:

• Thick hyperechoic strip at the patellar insertion on the tibial tuberosity with bone shadow dorsal to the cortex
• Acute phase may have edema or swelling of the tendon

On ultrasound for Enthesophyte:

• Thin hyperechoic strip within the tendon
• No bone shadow dorsal to the cortex (ie able to visualize tendon fibers).

Ultrasound-Guided Knee Procedures

Knee Arthrocentesis

Indications

• Indications include those of traditional arthrocentesis, such as diagnosing septic arthritis, evaluating inflammatory arthropathies, administering medications, evaluating synovial integrity after injury.

Probe placement

• Use long axis initially to identify the desired location
• Switch to short axis for needle insertion

Supplies

1. Antiseptic solution
2. Sterile gloves
3. Sterile drapes
4. Sterile probe cover
5. Surgical marker
6. Local anesthetics (eg. lidocaine)
7. 27-gauge needle for anesthetic
8. 18-20 gauge needle for joint aspiration
9. Collection tubes for synovial fluid 
10. High-frequency linear transducer

Procedure

1. Obtain a long axis view of the desired location
2. Identify appropriate anatomy for orientation
3. Identify lesion of interest (eg. suprapatellar effusion)
4. Rotate probe 90° for short axis view with probe indicator to patient’s right side
5. Freeze image and measure on screen the distance between the probe and desired location
6. Insert needle from lateral to medial using a flat, in-plane approach at the measured depth
7. Advance needle with continuous visualization

Adapted from Puebla & Farrow, 2025

Injections

Indications

• Indications for knee injection include therapeutic management of inflammatory or degenerative conditions after failure of conservative measures

Probe placement

• Use long axis initially to identify the desired location
• Switch to short axis for needle insertion

Supplies

• Antiseptic solution
• Sterile gloves
• Sterile drape
• Two 10-mL syringes
• Two 21-gauge, 1-inch needles
• Intrarticular medication

Procedure

1. Obtain a long axis view of the desired location
2. Identify appropriate anatomy for orientation
3. Rotate probe 90° for short axis view with probe indicator to patient’s right side
4. Freeze image and measure on screen the distance between the probe and desired location
5. Insert needle from lateral to medial using a flat, in-plane approach at the measured depth

Adapted from Zuber, 2002; Puebla & Farrow, 2025

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