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 Table of Contents  
Year : 2018  |  Volume : 13  |  Issue : 5  |  Page : 36-42

Ultrasound of the knee in rheumatology

Department of Radiology, Division of Musculoskeletal Imaging; Department of Internal Medicine, Division of Rheumatology, University of Michigan Hospitals, Ann Arbor, Michigan, USA

Date of Web Publication1-Aug-2018

Correspondence Address:
Dr. Girish Gandikota
Department of Radiology, Division of Musculoskeletal Imaging, 1500 E, Medical Center Drive, TC 2910S, Ann Arbor, Michigan 48109
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-3698.238200

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The knee joint, surrounding ligaments, and extensor mechanism (quadriceps and patellar tendon), given their superficial nature, are easily accessible and accurately assessed with high-resolution musculoskeletal (MSK) ultrasound. Its bedside availability, lower cost than magnetic resonance imaging (MRI), and its high sensitivity in the detection of a small effusion, synovitis, and Baker's cyst, together with its ability to be used for needle guidance in invasive procedures, make ultrasound a highly desired diagnostic tool. In this article, we will focus on common rheumatologic indications of MSK ultrasound in the knee and discuss scanning technique and sonographic findings. Transducer positioning and essential sonographic bony landmarks will be reviewed and scanning in two orthogonal planes (both long and short axis of the structure) will be emphasized. With advances in technology, cost reduction, and easy availability, it is expected that more and more clinicians will be utilizing ultrasound in their clinics in management and treatment of rheumatologic diseases. The diagnostic ability of MSK ultrasound is based on an operator's training and experience, the patient's habitus and aptly chosen clinical indications. A good grasp of anatomy, sound ultrasound technique, and knowledge of limitations of ultrasound modality is essential for a safe ultrasound practice. It is also important to keep in mind that ultrasound provides limited information about internal structures such as meniscus and cruciate ligament pathologies and it has a complementary role with MRI.

Keywords: Knee, Rheumatology, Ultrasound, ultrasound-guided

How to cite this article:
Gandikota G. Ultrasound of the knee in rheumatology. Indian J Rheumatol 2018;13, Suppl S1:36-42

How to cite this URL:
Gandikota G. Ultrasound of the knee in rheumatology. Indian J Rheumatol [serial online] 2018 [cited 2022 Aug 14];13, Suppl S1:36-42. Available from:

  Introduction Top

The knee joint, surrounding ligaments, and extensor mechanism (quadriceps and patellar tendon), given their superficial nature, are easily accessible and accurately assessed with high-resolution musculoskeletal (MSK) ultrasound. In many centers around the world, ultrasound is being used as an extension of clinical examination in patients with inflammatory arthritis. Its bedside availability, lower cost than magnetic resonance imaging (MRI), and its high sensitivity in the detection of a small effusion, synovitis, and Baker's cyst, together with its ability to be used for needle guidance in invasive procedures, make ultrasound a highly desired diagnostic tool.[1],[2] In this article, we will focus on common rheumatologic indications of MSK ultrasound in the knee and discuss scanning technique and sonographic findings.

  Indications Top

Common diagnostic rheumatologic indications of MSK ultrasound in the adult knee include:

Anterior knee

  • Joint effusion/synovitis
  • Bursa pathology
  • Quadriceps and patellar tendon pathology
  • Trochlear articular cartilage for crystals.

Lateral knee

  • Gout at the proximal popliteal tendon attachment site
  • Iliotibial band syndrome
  • Parameniscal cyst.

Medial knee

  • Calcification of proximal medial collateral ligament
  • Pes anserine bursa
  • Medial compartment osteoarthritis.

Posterior knee

  • Baker's cyst.

Ultrasound is also an integral part of image-guided joint aspirations, injections, biopsies, bursa injections, and in monitoring treatment response.

  Technical Equipment Top

  • 12 MHz (range 7–15 MHz) linear high-frequency transducer is preferred to evaluate the adult knee
  • Both color and power Doppler techniques can be utilized to identify vascularity and hence assess inflammation. Power Doppler is independent of the direction of flow, hence deemed to be more sensitive to tissue vascularity. However, with recent upgrades in technology, both color and power Doppler appear to have similar sensitivities.

  Scanning Techniques Top

Ultrasound evaluation of the knee can be compartmentalized into anterior, medial, lateral, and posterior regions. Given the reasonably accurate localization of symptoms in the knee, the examination can be targeted to the region of symptoms.

  • The structure of interest should be scanned in both transverse (short axis) and longitudinal (long axis) planes for a comprehensive evaluation
  • The asymptomatic side can be used for anatomic correlation and comparison
  • When assessing for vascularity through Doppler, a copious amount of jell over the skin along with the least amount of transducer pressure is necessary, in order not to compress the smaller vessels. Significantly, stretching the structure being examined also compromises vascularity and reduce sensitivity, so extreme flexion should be avoided.

  Anterior Knee Top

Patient position

  • Supine, 30° knee flexion, and foot flat on the bed [Figure 1]. This position allows the quadriceps and patellar tendons to be taut, thereby eliminating anisotropy artifact. Too much knee flexion should be avoided as it will displace the joint fluid away from suprapatellar recess and compromise Doppler assessment.
Figure 1: Patient positioning (a and b) when evaluating anterior knee (30° knee flexion). Transducer is positioned in Figure b to evaluate main quadriceps tendon and the underlying joint fluid

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Transducer position

  • The transducer is placed delicately (without pressure) along the long axis of the extensor mechanism, then turned 90° to get the short-axis view. The copious gel should be used to ensure complete contact of the transducer to the skin. If the underlying cortex is in the field of view, then the image should be aligned in such a way that entire crisp cortical bright line is appreciated.

Anatomy assessed

Joint, joint capsule, ligaments, bursa, quadriceps, and patellar tendon to look for:

  • Joint effusion
  • Synovitis/synovial pathology
  • Quadriceps and patellar tendon pathology
  • Bursa along the anterior knee
  • Trochlea cartilage.

Joint effusion versus synovitis

  • Effusion is defined by OMERACT as abnormal hypoechoic or anechoic intraarticular material that is compressible and displaceable and does not exhibit Doppler signal [Figure 2] and [Figure 3]
  • Complex effusion can be separated from synovial hypertrophy by compressibility, swirling, and redistribution of the internal contents
  • Synovial hypertrophy or synovitis is defined as an abnormal hypoechoic intraarticular tissue that is nondisplaceable and poorly compressible and may exhibit Doppler signal [Figure 4].
Figure 2: Long-axis ultrasound image of the superpatellar region (probe position is depicted in Figure 1b). (a) Normal joint fluid (arrows) between the superpatellar fat pad (white star) and the prefemoral fat pad (black star), deep to the quadriceps tendon; (b) small joint effusion (between arrows); and (c) large joint effusion. C = Femoral articular cartilage, Pat = Patella

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Figure 3: (a) without compression and (b) with compression. Joint effusion is compressible and displaceable as shown in this case. Synovial hypertrophy is not. Both can appear anechoic

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Figure 4: Synovial hypertrophy (arrows) in the suprapatellar recess demonstrating no change with (b) or without (a) compression, showing no compression or displacement. Sometimes, increased vascularity is noted only in the periphery as shown in (a)

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Knee joint effusion and synovial pathology are evaluated at the three recesses around the anterior knee joint. The suprapatellar recess is identified deep to the quadriceps tendon, between the prefemoral and suprapatellar fat pads [Figure 2]. The medial and lateral patellar recesses are evaluated in the transverse plane by placing the transducer along the medial and lateral aspects of the patella, appreciating the outpouching of the joint capsule over the femoral condyle [Figure 5]. Care should be taken to avoid putting too much pressure on the transducer, so as to not displace the fluid. Intraarticular bodies in the knee can be detected sonographically in recesses as well as within a Baker's cyst.
Figure 5: Patient and transducer position when evaluating the lateral knee (a). Corresponding ultrasound image (b) showing joint effusion (arrows) in the lateral knee joint recess

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Fluid and synovial sampling can be done in any of three recesses. The lateral recess is preferred as it is most easily accessible. The suprapatellar recess can be accessed by placing the transducer along the transverse plane and the needle entering from the lateral side of the quadriceps tendon [Figure 6].
Figure 6: Needle guidance to inject or aspirate knee joint. Patient and transducer positioning (a); arrow demonstrating the needle direction and its orientation to the transducer. Corresponding ultrasound image (b) showing needle (arrows) entering the joint from lateral to quadriceps (Quad). Arrowheads = reverberation artifact from the needle

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  Quadriceps and Patellar Tendon Pathology Top

Commonly encountered pathology in rheumatology clinics that are amenable to ultrasound evaluation include:

  • Tendinosis – thickened hypoechoic tendon ± increased vascularity [Figure 7]
  • Tear – tendon defect with volume loss
  • Gout – Urate deposits/tophus with increased vascularity ± calcification [Figure 8]
  • Calcification [Figure 8]
  • Osgood–Schlatter disease
  • Enthesitis.
Figure 7: Ultrasound image along the long axis (a) and short axis (b) of the patellar tendon demonstrating proximal patellar tendinosis (within the circle)

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Figure 8: Ultrasound image (a and b) along the axis of distal patellar tendon at its attachment over tibial tuberosity (Tib tub), demonstrating gout tophus deposition ( circles and arrows ) containing echogenic foci and vascularity

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Jumper's knee refers to focal tendinosis in the proximal patellar tendon and can mimic enthesitis [Figure 7]. Enthesitis is defined as an inflammation involving the insertion of tendons, ligaments, and joint capsule into bone.[3] Although enthesitis is most commonly seen in patients with spondyloarthropathies, it is also observed in patients with rheumatoid arthritis, psoriatic arthritis, and reactive arthritis.[4] Osgood–Schlatter disease is recognized by sonography as a fragmented tibial tuberosity corresponding to the site of pain in a young individual, at the distal attachment of patellar tendon [Figure 9].
Figure 9: Long-axis ultrasound image overlying the patellar tendon (arrows) attaching distally overfragmented tibial tuberosity, findings compatible with Osgood–Schlatter's disease

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Enthesopathy is defined as an abnormally hypoechoic (loss of normal fibrillar architecture) and/or thickened tendon or ligament at its bony attachment (may occasionally contain hyperechoic foci consistent with calcification), seen in two perpendicular planes that may exhibit Doppler signal and/or bony changes including enthesophytes, erosions, or irregularity.[3]

Anterior knee bursa

  • Prepatellar bursa is located anterior to the patella [Figure 10] and [Figure 11]
  • Superficial infrapatellar bursa is contiguous with prepatellar bursa over the patellar tendon [Figure 11]
  • Deep infrapatellar bursa is deep to the distal patellar tendon, overlying the tibia [Figure 10].
Figure 10: Lateral knee illustration demonstrating the locations of anterior knee bursae

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Figure 11: Long-axis views (a-c) over the patella and patellar tendon (pat tendon) demonstrating prepatellar bursa (arrows) overlying patella, extending inferiorly over the proximal patellar tendon (b). (c) demonstrates increased peripheral vascularity surrounding the prepatellar bursa (star) suggestive of inflammation, infection, or recent trauma

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The deep infrapatellar bursa can accompany patellar tendon pathology of that area although a small amount of fluid in the deep infrapatellar bursa is common in healthy individuals.[5] Bursa may be distended with anechoic fluid or complex fluid. If the patient has focal pain with transducer pressure and color Doppler demonstrates increased vascularity (usually in the periphery), it may indicate underlying synovitis/bursitis [Figure 11]c.

Femoral articular cartilage

With a fully flexed knee, the femoral trochlea and the overlying articular cartilage can be examined [Figure 12]. Ultrasound is very sensitive in detecting hyaline cartilage calcifications [Figure 13]a.[6] Analyzing hyaline cartilage is important in patients with crystal-induced arthropathy to distinguish chondrocalcinosis from gout.
Figure 12: Patient and transducer position (a) when evaluating femoral trochlea, knee is fully flexed. Corresponding ultrasound image (b) demonstrates normal trochlear femoral articular cartilage (arrows)

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Figure 13: Illustration of femoral trochlear articular cartilage. (a) Normal contour, (b) double contour, the inner bright line corresponds to normal cortical outline and the outer bright line (arrow) is formed by gout crystal desposits over the surface of articular cartilage. Note the irregular outline of the outer bright line, and (c) deposition of CPPD crystals within the articular cartilage (arrow) as seen in chondrocalcinosis

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  • Calcium pyrophosphate dihydrate crystals tend to form an echogenic layer, within the substance of hyaline cartilage.[7] Sonographically, this appears as a hyperechoic, irregular line embedded in anechoic appearing hyaline cartilage [Figure 13]c
  • Monosodium urate crystals in gout layer over the surface of the cartilage called the double contour sign [Figure 13]b. Hyaline cartilage of the femoral trochlea can be also accessed for thinning and defects, a sign of osteoarthritis.

  Medial and Lateral Knee Top

Patient position

For examination of the medial and lateral knee, the leg is externally and internally rotated, respectively, while the patient in the supine position with 20° to 30° of knee flexion [Figure 14]a.
Figure 14: Patient (a) and transducer position (white rectangle), when examining the medial knee corresponding ultrasound image (b) at the midline knee demonstrates osteophytes (arrow) and intervening extruded body of medial meniscus (white triangle), indicating medial compartment osteoarthritis and degenerative tear of the meniscus

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Medial knee

  • In advanced osteoarthritis, there is medial compartment joint space narrowing and osteophytosis with extrusion of the body of the medial meniscus [Figure 14]b.

Erosions can be identified close to the articular surface. Erosions are defined as intraarticular discontinuity of the bone surface or cortex greater than 2 mm in diameter, visualized in two perpendicular planes.[3] Ultrasound can underestimate the extent and number of erosions when compared to MRI. However, it has been shown that ultrasound is more sensitive in detection of bony erosions compared to radiographs in patients with rheumatoid arthritis.[8],[9]


  • Every cortical irregularity noticed on ultrasound is not an erosion. In fact, the vast majority of cortical irregularities are not erosions and likely represent ongoing degenerative change. The definition of erosion should strictly adhere too. Often, the erosion is filled with synovitis/fluid. These findings should always be correlated with radiographs.

  Pes Anserine Bursa Top

Ultrasound is helpful in the evaluation of pes anserine bursitis.[10] Inferior to the medial joint line, the pes anserinus bursa can be observed as a defined fluid collection, originating at the distal attachment site of sartorius, gracilis, and semitendinosus tendons, along with the anteromedial proximal tibia, just anterior to the distal medial collateral ligament [Figure 15].
Figure 15: Soft-tissue windowing of the radiograph demonstrates increased soft-tissue thickening (between arrows) at the expected location of pes anserine bursa. Corresponding ultrasound image showing the Pesanserine Bursa (arrows) along the medial side of the proximal tibia

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Lateral knee

  • In the lateral knee, a gout tophus is most commonly seen in the popliteal groove, involving the proximal popliteal tendon
  • Para meniscal cyst originates from a tear in the adjacent meniscus and can be seen both medially or laterally, close to the body of the meniscus.

  Posterior Knee Top

Patient position

  • Knee extended, in a prone position, with a cushion underneath the shin or ankle as needed.

Baker's cyst

Baker's cyst is also known as semimembranosus-gastrocnemius bursa or popliteal cyst. Baker's cyst communicates with the knee joint (usually by a one-way valve mechanism [5]) through the neck between the semimembranosus tendon and the medial head of the gastrocnemius tendon, located along the posteromedial aspect of the knee, best appreciated with a transducer placed in a transverse plane [Figure 16]. The body of the Baker's cyst lies superficial to the gastrocnemius. Size, extension, vascularity, and complexity of a Baker's cyst and its mass effect on the adjacent popliteal neurovascular bundle should be assessed and documented. Rupture of a Baker's cyst can produce clinical symptoms similar to a deep vein thrombosis, which should be distinguished with use of Doppler ultrasound.
Figure 16: Patient and transducer position (a) when evaluating posteromedial knee for Baker's cyst. Corresponding ultrasound image (b) of anechoic Baker's cyst. Its neck should always be identified, as seen here, between semimembranosus and medial head of gastrocnemius (MG) (b and c). Long-axis ultrasound image of Baker's cyst (d), arrow demonstrating the needed path, for aspiration or injection. Med fem condyle = Medial femoral condyle

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Treatment of a symptomatic Baker's cyst with mass effect includes aspiration and corticosteroid injection into the cyst.[11] Interventions, when performed under ultrasound guidance, lower the risk of popliteal neurovascular injury [Figure 16]d. An 18- or 20-gauge needle is normally used as the content of the cyst tends to be viscous. If there is a large associated knee joint effusion, the joint should also be aspirated at the same time, otherwise, the Baker's cyst can recur immediately.

  Future Direction Top

Ultrasound is an inexpensive, portable, and dynamic imaging modality. It can be used to detect a small effusion and early synovitis as well as bedside ultrasound-guided procedures. With advances in technology, it is expected that more and more clinicians will be utilizing ultrasound in their clinics in management and treatment of rheumatologic diseases. It is important to keep in mind that ultrasound provides limited information about internal structures such as meniscus and cruciate ligament pathologies and it has a complementary role with MRI.


The authors would like to thank Danielle Dobbs for the illustrations, Vanessa Allen for her assistance in figure preparation, and Dr. Moe Tun for her assistance with manuscript preparation.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Kane D, Balint PV, Sturrock RD. Ultrasonography is superior to clinical examination in the detection and localization of knee joint effusion in rheumatoid arthritis. J Rheumatol 2003;30:966-71.  Back to cited text no. 1
Walther M, Harms H, Krenn V, Radke S, Faehndrich TP, Gohlke F, et al. Correlation of power Doppler sonography with vascularity of the synovial tissue of the knee joint in patients with osteoarthritis and rheumatoid arthritis. Arthritis Rheum 2001;44:331-8.  Back to cited text no. 2
Wakefield RJ, Balint PV, Szkudlarek M, Filippucci E, Backhaus M, D'Agostino MA, et al. Musculoskeletal ultrasound including definitions for ultrasonographic pathology. J Rheumatol 2005;32:2485-7.  Back to cited text no. 3
Frediani B, Falsetti P, Storri L, Allegri A, Bisogno S, Baldi F, et al. Ultrasound and clinical evaluation of quadricipital tendon enthesitis in patients with psoriatic arthritis and rheumatoid arthritis. Clin Rheumatol 2002;21:294-8.  Back to cited text no. 4
Bianchi S, Martinolli C. Ultrasound of the Musculoskeletal System. New York: Springer Berlin Heidelberg; 2007. p. 637-8.  Back to cited text no. 5
Grassi W, Meenagh G, Pascual E, Filippucci E. “Crystal clear”-sonographic assessment of gout and calcium pyrophosphate deposition disease. Semin Arthritis Rheum 2006;36:197-202.  Back to cited text no. 6
Filippucci E, Riveros MG, Georgescu D, Salaffi F, Grassi W. Hyaline cartilage involvement in patients with gout and calcium pyrophosphate deposition disease. An ultrasound study. Osteoarthritis Cartilage 2009;17:178-81.  Back to cited text no. 7
Grassi W, Filippucci E, Farina A, Salaffi F, Cervini C. Ultrasonography in the evaluation of bone erosions. Ann Rheum Dis 2001;60:98-103.  Back to cited text no. 8
Carter JD, Kedar RP, Anderson SR, Osorio AH, Albritton NL, Gnanashanmugam S, et al. An analysis of MRI and ultrasound imaging in patients with gout who have normal plain radiographs. Rheumatology (Oxford) 2009;48:1442-6.  Back to cited text no. 9
Toktas H, Dundar U, Adar S, Solak O, Ulasli AM. Ultrasonographic assessment of pes anserinus tendon and pes anserinus tendinitis bursitis syndrome in patients with knee osteoarthritis. Mod Rheumatol 2015;25:128-33.  Back to cited text no. 10
Di Sante L, Paoloni M, Ioppolo F, Dimaggio M, Di Renzo S, Santilli V, et al. Ultrasound-guided aspiration and corticosteroid injection of baker's cysts in knee osteoarthritis: A prospective observational study. Am J Phys Med Rehabil 2010;89:970-5.  Back to cited text no. 11


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16]


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  In this article
Technical Equipment
Scanning Techniques
Anterior Knee
Quadriceps and P...
Medial and Later...
Pes Anserine Bursa
Posterior Knee
Future Direction
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