A Focus on Knee PoCUS

Resident Pearl by Dr. Iain McPhee


Dalhousie University

Reviewed by Dr. D Lewis

Copy Edited by Dr. J Vonkeman


Ultrasonography is a non-invasive imaging modality that has numerous uses in the Emergency Department, including assessment of musculoskeletal injuries. This Resident Pearl includes an approach to ultrasound examination of the knee, some high yield knee POCUS findings, as well as an approach to ultrasound-guided knee arthrocentesis.


A patient presents to your emergency department with a painful, swollen knee.

Part 1: What is your approach? 


  1. History of Presenting Illness
    • Have a discussion with the patient and discern key elements of the history including;
      • Mechanism?
        • If traumatic or mechanical in nature, obtaining a step by step, walk through will allow you to better understand the mechanism. Typically, the more specific the story, the better diagnostic value.
        • Determine how the injury took place will help narrow your differential. Was this trauma?  Sport related? Idiopathic? Any infectious symptoms?, etc.
    • Medical history
      • Gather the past medical history, medications, allergies, habits, and a review of systems
  2. Location of pain
    • The knee is a complex joint with several potential structures that can be damaged. As such, determining where the patient is experiencing the most pain will also help narrow the differential and will guide you on where to begin your POCUS investigations in a specific quadrant.

PoCUS Knee Quadrants (1) – Anterior Knee Approach


  • Patient positioning: Supine position
  • Knee positioning: 20-30° of knee flexion (can place pillow under the knee if needed)
  • Transducer positioning: transverse and longitudinal views


Relevant structures:


a. Quadriceps Tendon

  • Observe the multilevel appearance to the tendon, representing the distal union of the three tendon layers arising from the quadriceps muscles.


Video 1 Quadriceps Tendon – Short Axis View


Video 2 Quadriceps Tendon – Long Axis View


Figure 1 Quadriceps Tendon


b. Suprapatellar Recess

  • Observe the suprapatellar fat pad – found under distal 1/3 of quadriceps tendon superior to the patella, and the pre-femoral fat pad
  • The suprapatellar recess lies deep to the quadriceps tendon and the suprapatellar fat pad and superficial to the pre-femoral fat pad. This recess is a common site of effusion.


Video 3 Suprapatellar Recess in the Long Axis


Figure 2 Suprapatellar Recess


c. Parapatellar Recess

  • Assess lateral and medial sides of the quadriceps tendon as fluid can accumulate on lateral and medial sides of the suprapatellar recess.

Video 4 Medial Parapatellar Recess in the Short Axis


Figure 3 Parapatellar Recess (medial)


d. Femoral Trochlea

  • Examine in transverse plane with knee in full flexion


Video 5 Trochlea visualized in the Short Axis


Figure 4 Trochlea


e. Patellar Retinacula and Patellar Medial Articular Facet

  • Visualized in transverse plane on either side of the patella (medial shown below)
  • Extend the knee, apply pressure with your hand to the lateral aspect of the patella and hold the transducer in the transverse plane to view the Medial articular facet. Stress movement is seen in video. The lateral articular facet cannot be visualized.

Video 6 Patellar Retinaculum – Medial Aspect. With stress and no stress


Figure 5 Patellar Retinaculum


f. Patellar Tendon

  • Examine tendon, from cephalad origin to distal insertion, in both long and short axis.
  • Deep to the patellar tendon, inferior to the patella, is the Hoffa fat pad
  • Appreciate the deep infrapatellar bursa that is deep to the distal patellar tendon and anterior aspect of the tibial epiphysis


Video 7 Patellar Tendon viewed in the Long Axis


Figure 6 Patellar Tendon in the Long Axis


Video 8 Patellar Tendon viewed in the Short Axis


Figure 7 Patellar Tendon in the Short Axis


PoCUS Knee Quadrants (1) – Medial Knee Approach


  • Patient positioning:  Supine
  • Knee positioning: Maintain 20-30° of knee flexion, rotate leg externally
  • Transducer positioning: Oblique- long axis of ligaments


Relevant structures:


a. Medial Collateral Ligament (MCL) and Pes Anserinus

  • Examine entire length. Add valgus stress to assessment of MCL integrity
  • Follow to distal MCL, then rotate the transducer counter clockwise by 5-10 degrees to visualize the pes anserinus tendon.


Video 9 MCL visualized in Long Axis


Video 10 Pes Anserinus visualized in the Long Axis



Figure 8 a) Medial Collateral Ligament (MCL) and b) Pes anserinus


PoCUS Knee Quadrants (1) – Lateral Knee Approach


  • Patient positioning:  Supine
  • Knee positioning: Maintain 20-30° of knee flexion, rotate leg internally
  • Transducer positioning: Oblique- long axis of ligaments


Relevant structures:


a. Iliotibial Band

  • Located between the anterior and middle third of the lateral aspect of the knee and oriented along the major axis of the thigh


Video 11 Iliotibial Band visualized in the Long Axis


Figure 9 Iliotibial Band


b. Lateral Collateral Ligament (LCL)

  • Obtain long axis view. Transducer should be oriented with the major axis of the lower leg


Video 12 LCL in the Long Axis of the Lower Leg


Figure 10 Lateral Collateral Ligament (LCL)


PoCUS Knee Quadrants (1) – Posterior Knee Approach

  • Patient positioning:  Prone
  • Knee positioning: knee extended
  • Transducer positioning: Transverse and longitudinal


Relevant structures:


a. Medial Tendons

  • Examine in Transverse (Short axis)
  • From medial to lateral  Sartorius (muscle), gracilis tendon and semitendinosus tendon


Video 13 Medial Tendons in the Short Axis


Figure 11 Medial Tendons


b. Semi-Membranous-Gastrocnemius Bursa

  • Site where popliteal (Baker’s) cyst arises
  • Envelopes the Semitendinosus tendon and Sartorius muscle

Video 14 The Semi-Membranous-Gastrocnemius Bursa in the Short Axis


Figure 12 Semi-Membranous-Gastrocnemius Bursa


c. Popliteal Neurovascular Bundle and Intercondylar Fossa

  • The popliteal neurovascular bundle is superficial to the popliteal artery (deep) and popliteal vein (intermediate)
  • Can appreciate the Posterior Cruciate Ligament (PCL) here with transducer in oblique longitudinal plane


Video 15 The Popliteal Neurovascular Bundle in the Short Axis


Figure 13 Popliteal Neurovascular Bundle


Video 16 The Posterior Cruciate Ligament in the Long Axis


d. Posterolateral Corner of the Biceps Femoris

  • In longitudinal plane, can visualize the biceps femoris tendon



Video 17 Posterior Biceps Femoris viewed in the Long Axis


Figure 14 Posterior Biceps Femoris


e. Peroneal Nerve

  • Examine in short axis. Begin on posterolateral aspect of leg and advance around fibular head.
  • The Peroneal nerve is found posterior to the biceps femoris


Figure 15 Various views of the Peroneal Nerve


Videos 18-21 Various views of the Peroneal Nerve 


Part 2: High Yield Knee PoCUS Findings


1. Knee Effusion


  • A knee effusion refers to an increased volume of fluid in the synovial compartments of the knee and can arise from acute or chronic conditions 2,3.
  • Small, asymptomatic effusions can occur in healthy individuals, while larger joint effusions usually indicate underlying pathology 3.

What to look for:

  • Knee effusions are best visualized with a linear transducer, and placed in the long axis over the medial joint line, positioned over the suprapatellar recess 4.
  • An effusion will appear as an anechoic fluid that displaces but remains constrained by the hyperechoic joint capsule 4. Of note, It is important to assess both knees to establish true asymmetry and thus abnormality between knees 4.


2. Haemarthrosis


  • Haemarthrosis is an articular bleeding into the joint space and represents a type of joint effusion.
  • The most common mechanism of knee haemarthrosis is forced twisting in a loaded joint resulting in ligamentous and/or meniscal damage 5.
  • Tearing the anterior cruciate ligament (ACL) represents ~70% of post traumatic haemarthrosis of the knee 5.

What to look for:

  • A haemarthrosis may be more hypoechoic due to potential clotting when compared to other effusion fluids.


3. Medial Collateral Ligament Tear


  • The Medial Collateral Ligament (MCL) is the most commonly damaged ligament of the knee in situations of direct trauma and is usually a result of valgus stress. Using POCUS to evaluate soft tissue of the knee has become more common 6, however, ligaments are small in size and usually in close proximity to bone, making visualization difficult 4.

What to look for:

  • MCL’s are generally dense, hyperechoic in appearance, however may appear darker given their proximity to bone cortex and fascia.
  • An abnormal exam includes a thickened MCL and is typically more heterogeneously hypodense in appearance 17


Figure 17 Longitudinal view of the MCL. a) normal MCL. b) abnormal MCL. F: Femur, T: Tibia, s: superficial portion of MCL, d: deep portion of MCL. 18

4. Patellar Tendonitis


  • Tendons are best viewed with a linear transducer in both the long and short axis, and appear hyperechoic. Tendons are particularly susceptible to anisotropic artefact, that being, the tendon will become more hypoechoic as the transducer angulation changes 4.
  • Patellar tendinitis accounts for 10% of clinical knee diagnoses 9, and is particularly common in athletes whose sport involves frequent jumping ex. basketball, volleyball or running 10.

What to look for:

  • In patellar tendonitis, a clinician can expect to see a thickened patellar tendon with a loss of normal fibrillar pattern, characteristic of a normal tendon 4.
  • There may also be signs of hyperechoic calcifications and/or patchy hypoechoic zones 10.
  • Comparison with the other (asymptomatic) knee is often helpful in visualizing this change.

Figure 18  (a) and (c) represent normal patellar tendons, where (b) and (d) represent patellar tendinopathy. 10,11

5. Osgood Schlatter


  • Apophysitis is a term used to indicate inflammation and stress injury where a tendon attaches to a growth plate of bone 12.
  • Osgood-Schlatter disease (OSD) is a traction apophysitis of the tibial tuberosity (site of patellar tendon insertion on the tibia) due to repetitive strain.
  • Contraction of the quadriceps muscle is transmitted through the patellar tendon which results in the avulsion of the anterior surface of the tibial tuberosity 13.
  • This usually occurs in young boys between the ages of 10-15 14.

What to look for:

  • Best visualized with a linear probe, look first at the region of the tibial tuberosity.
  • A fragmented hyperechoic lesion surrounded by hypoechoic cartilage will be visualized.
  • There will also be a thickening of the distal patellar tendon and evidence of infrapatellar bursitis – represented by an anechoic collection deep to the patellar tendon 13.

Figure 19 a) Osgood-Schlatter anatomy. b) Ultrasound image of Osgood-Schlatter. Arrow: highlighting fragmented lesion of the tibial tuberosity. Ultrasound image obtained from ultrasoundcases.info.

6. Meniscal Injury


  • The meniscus of the knee is a wedge shaped piece of fibrocartilage that plays the role of shock absorber between the distal femur and proximal tibia.
  • There are two menisci, lateral and medial, and they play an important role in knee stabilization 15.
  • An injury to the meniscus typical occurs with internal or external twisting of the upper leg while the foot is securely planted.
  • Ultrasound has been shown to be an effective initial diagnostic modality 16.

What to look for:

  • Best observed with the linear probe in the long axis of the affected side of the knee (medial or lateral side).
  • The meniscus will be a triangular “wedge” between the hyperechoic tibia and femur.
  • Pertinent positive for a torn meniscus include anechoic regions of the wedge representing a tear or fluid accumulation.

Figure 20 a) Normal Medial Meniscus. (b/c) Arrow: Medial Meniscus Tear. Ultrasound images from ultrasoundcases.info.

Part 3: Ultrasound Guided Arthrocentesis of the Knee – A Suprapatellar Approach (7,8)



  • A basic understanding of knee anatomy is required
  • Knee – composed of 4 major bones: the distal femur, proximal tibia, proximal fibula and the patella.
  • Three articulations: Femorotibial, patellofemoral and tibiofibular

Common pathologies

  • Arthritic conditions (Osteoarthritis, rheumatoid arthritis, crystal arthropathies)
  • Trauma (sports injuries and MVA)
  • Infectious (synovitis)


  • Ultrasound machine with linear array transducer (<4cm depth)
  • 22-18 gauge 1.5-2 inch needle for aspiration (Can consider spinal needle- 20 gauge)
  • 10-20ml syringe – for small effusions
  • 30-60ml syringe – for large effusions
  • Local anesthetic – Lidocaine
  • Sterile transducer cover
  • Sterile ultrasound gel
  • Sterile towel
  • Sterile gloves


  • Patient position – Supine
  • Knee positioning – Knee flexed to 30° with either a towel/pillow under the affected knee
  • Clinician position – positioned on the side of the affected knee
    • Note: Ensure the ultrasound machine screen and the knee are in the same line of sight
  • Needle position – Long axis – In-plane, lateral to medial approach


  • Prep and clean the skin
  • Drape the knee, leaving the suprapatellar region exposed
  • Numb the skin and needle track with local anesthetic
  • Prep the U/S machine for sterility
  • Place the transducer superior to the patella in transverse orientation (short axis to the quadriceps tendon)
  • Advance your aspiration needle from lateral to medial until you reach the anechoic suprapatellar recess.


USK Medial Arthrocentesis


Video 22, 23 Medial approach to suprapatellar ultrasound guided arthrocentesis in the long axis.




  1. Martinoli C. Musculoskeletal ultrasound: technical guidelines. Insights Imaging. 2010;1 (3): 99-141. doi:10.1007/s13244-010-0032-9
  2. Li TY. Sonography of Knee Effusion. Journal of Diagnostic Medical Sonography. 2020;36(6):545-558. doi:1177/8756479320944848
  3. Gerena, L. A., Mabrouk, A., & DeCastro, A. (2022). Knee Effusion. In StatPearls. StatPearls Publishing.
  4. Atkinson, Paul, and others (eds), Point of Care Ultrasound for Emergency Medicine and Resuscitation, Oxford Clinical Imaging Guides(Oxford, 2019; online edn, Oxford Academic, 1 Apr. 2019), https://doi.org/10.1093/med/9780198777540.001.0001
  5. Lombardi, M., & Cardenas, A. C. (2023). Hemarthrosis. In StatPearls. StatPearls Publishing.
  6. Ahmadi O, Heydari F, Golshani K, Derakhshan S. Point-Of-Care Ultrasonography for Diagnosis of Medial Collateral Ligament Tears in Acute Knee Trauma; a Diagnostic Accuracy Study. Arch Acad Emerg Med. 2022 Jun 9;10(1):e47. doi: 10.22037/aaem.v10i1.1480. PMID: 35765618; PMCID: PMC9206825.
  7. Josh Hackel, Todd Hayano, John Pitts, Mairin A. Jerome, 21 – Knee Injection Techniques, Editor(s): Christopher J. Williams, Walter. L Sussman, John Pitts, Atlas of Interventional Orthopedics Procedures, Elsevier, 2022, Pages 366-427, ISBN 9780323755146, https://doi.org/10.1016/B978-0-323-75514-6.00021-2.
  8. Avila, Jailyn. Knee arthrocentesis and injections. Core ultrasound. 2023. https://www.youtube.com/watch?v=tf5hF0QflDI
  9. Rosen, A.B., Wellsandt, E., Nicola, M., and Tao, M.A., 2022. Clinical Management of Patellar Tendinopathy.Journal of athletic training, 57(7), pp.621–631.
  10. Sports Medicine Ultrasound Group. Case study – patellar tendinopathy. 2023. https://www.ultrasoundtraining.co.uk/case-study-patella-tendinopathy/
  11. Abat, F., et al. “Patellar tendinopathy: diagnosis by ultrasound and magnetic resonance imaging. Conservative and surgical management alternatives.” Rev Esp Artrosc Cir Articul En1 (2022): 13-20.
  12. Mark Riederer, MD and Neeru Jayanthi, MD. American medical society for sports medicine
  13. Siddiq MAB. Osgood-Schlatter Disease Unveiled Under High-frequency Ultrasonogram. Cureus. 2018 Oct 4;10(10):e3411. doi: 10.7759/cureus.3411. PMID: 30538899; PMCID: PMC6281446.
  14. Indiran, V., & Jagannathan, D. (2018). Osgood-Schlatter Disease. The New England journal of medicine378(11), e15. https://doi.org/10.1056/NEJMicm1711831
  15. Alaia M, Wilkerson R, Fischer S. Meniscus tears. American Academy of Orthopaedic Surgeons: 202. https://orthoinfo.aaos.org/en/diseases–conditions/meniscus-tears/
  16. Ahmadi, O., Motififard, M., Heydari, F. et al.Role of point-of-care ultrasonography (POCUS) in the diagnosing of acute medial meniscus injury of knee joint. Ultrasound J14, 7 (2022). https://doi.org/10.1186/s13089-021-00256-0
  17. Lee, J. I., Song, I. S., Jung, Y. B., Kim, Y. G., Wang, C. H., Yu, H., Kim, Y. S., Kim, K. S., & Pope, T. L., Jr (1996). Medial collateral ligament injuries of the knee: ultrasonographic findings. Journal of ultrasound in medicine : official journal of the American Institute of Ultrasound in Medicine15(9), 621–625. https://doi.org/10.7863/jum.1996.15.9.621
  18. Chiang, Yi-Pin & Wang, Tyng-Guey & Lew, Henry. (2007). Application of High Resolution Ultrasound for Examination of the Knee Joint. Journal of Medical Ultrasound. 15. 203-212. 10.1016/S0929-6441(08)60038-7.
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