General Comments

Ultrasound examination of the majority of the knee structures is completed with the patient supine; the posterior structures are best evaluated with the patient prone. A high-frequency transducer of at least 10 MHz is typically used, with the exception of the posterior knee, for which a transducer of 7 MHz or perhaps 5 MHz may be needed to penetrate the deep soft tissues. Evaluation of the knee may be focused over the area that is clinically symptomatic or that is relevant to the patient’s history. Regardless, a complete examination of all areas should always be considered and is recommended for one to become familiar with normal anatomy and normal variants and to develop a quick and efficient sonographic technique.

Anterior Evaluation

The primary structures evaluated from the anterior approach are the quadriceps tendon, the patella, the patellar tendon, the patellar retinaculum, the suprapatellar recess, the medial and lateral recesses, and the bursa around the anterior knee. Examination is begun in the sagittal plane proximal to the patella (Fig. 7-2A). This plane demonstrates the normal hyperechoic and fibrillar appearance of the quadriceps tendon (see Fig. 7-2B). Slight flexion of the knee with a pad or roll behind the knee is often helpful because this position straightens and tenses the extensor mechanism to reduce tendon anisotropy. Often, the trilaminar appearance of the quadriceps tendon can be appreciated, with the rectus femoris as the anterior layer, the combined vastus medialis and intermedius as the middle layer, and the vastus intermedius as the deepest layer (see Quadriceps Femoris Injury). The quadriceps tendon is also evaluated in short axis (Fig. 7-3A and B). Returning to the quadriceps tendon in long axis, the suprapatellar recess is identified deep to the quadriceps tendon and evaluated for anechoic or hypoechoic joint fluid, which would separate the quadriceps fat pad (located superficial) from the prefemoral fat pad (located deep) (see Fig. 7-2). Slight knee flexion also shifts fluid from other parts of the knee joint into the suprapatellar recess. The transducer is then moved inferiorly below the patella in the sagittal plane to visualize the hyperechoic, fibrillar, and uniform patellar tendon (Fig. 7-4A and B). The Hoffa infrapatellar fat pad appears minimally hyperechoic or isoechoic to muscle deep to the patellar tendon. The transducer should also be floated on a layer of gel over the proximal patellar tendon and patella to evaluate for patellar fracture, as well as prepatellar bursal fluid, because the latter may be easily redistributed out of view with the slightest transducer pressure. The region around the distal patellar tendon is also evaluated for superficial and deep infrapatellar bursal fluid; minimal fluid in the latter is considered physiologic (see Other Bursae). Although long axis is most important in evaluation of extensor mechanism abnormalities, imaging should also be completed in short axis to ensure a thorough evaluation, especially with the patellar tendon, where a focal abnormality may not be located in midline (Fig. 7-5A and B).

FIGURE 7-2 Quadriceps femoris: long axis.

A, Sagittal imaging over anterior knee proximal to the patella shows (B) the quadriceps tendon (arrowheads), quadriceps fat pad (Q), prefemoral fat pad (PF), and collapsed joint recess (curved arrow). F, femur; P, patella.

FIGURE 7-3 Quadriceps femoris: short axis.

A, Transverse imaging over anterior knee proximal to patella shows (B) the quadriceps tendon (arrowheads).

FIGURE 7-4 Patellar tendon: long axis.

A, Sagittal imaging over anterior knee distal to the patella shows (B) the patellar tendon (arrowheads) and Hoffa fat pad (H). P, patella; T, tibia.

FIGURE 7-5 Patellar tendon: short axis.

A, Transverse imaging over anterior knee distal to patella shows (B) the patellar tendon (arrowheads) and Hoffa fat pad (H).

The transducer is then moved to both the medial and lateral margins of the patella in the transverse plane to visualize the thin hyperechoic patellar retinaculum as well as distention of the medial and lateral recesses that are continuous with the suprapatellar recess, which is more apparent when the knee is completely extended (Fig. 7-6A and B). One must be careful not to displace joint fluid from view with transducer pressure (see Joint Effusion and Synovial Hypertrophy). The patellar retinaculum may demonstrate three defined layers.⁸ Within the medial patellar retinaculum, the medial patellofemoral ligament may be identified as hyperechoic with a compact fibrillar echotexture, which extends from the adductor tubercle of the femur to the patella. Finally, with the knee in flexion, the hypoechoic hyaline cartilage that covers the trochlea of the anterior femur can be visualized in the transverse plane superior to the patella (Fig. 7-7A and B), and the hypoechoic hyaline cartilage covering the anterior and central aspects of the femoral condyles can be seen in the parasagittal plane (see Fig. 7-7C).¹³

FIGURE 7-6 Medial and lateral knee joint recesses.

Transverse imaging on each side of the patella shows (A) the medial patellar retinaculum, which contains the medial patellofemoral ligament (arrowheads), and (B) the lateral patellar retinaculum (arrowheads) (curved arrows, collapsed joint recess). LC, lateral femoral condyle; MC, medial femoral condyle; P, patella.

FIGURE 7-7 Trochlear and femoral condyle cartilage.

A, With knee flexion, (B) transverse imaging and (C) parasagittal imaging show hypoechoic hyaline cartilage (arrowheads). LC, lateral femoral condyle; MC, medial femoral condyle.

Medial Evaluation

For medial knee evaluation, the patient remains supine and rotates the hip externally to gain access to the medial structures. The structures of interest include the medial collateral ligament (composed of several layers), the body and anterior horn of the medial meniscus, and the pes anserinus.⁵ To begin, the transducer is placed in the coronal plane along the medial joint line, which is identified by the bone contours of the femoral condyle and the proximal tibia (Fig. 7-8A).¹⁴ The thick hyperechoic and fibrillar superficial layer of the medial collateral ligament (or tibial collateral ligament) is easily identified in long axis (see Fig. 7-8B); it extends proximally from the medial femoral condyle and extends distally and slightly anterior to the proximal tibial metaphysis. With rotation of the transducer short axis to the tibial collateral ligament, the anteroposterior extent of this structure can be appreciated (Fig. 7-9A and B). By toggling the transducer along the long axis of the tibial collateral ligament, the borders of the ligament can be better appreciated because the ligament fibers become hypoechoic as a result of anisotropy and the adjacent soft tissues remain hyperechoic (see Fig. 7-9C). Returning to the coronal plane or long axis to the tibial collateral ligament, the thinner hyperechoic deep layers of the medial collateral ligament, also called the meniscofemoral and meniscotibial ligaments, are identified from the meniscus to the femur and tibia, respectively (see Fig. 7-8B). The fibrocartilage meniscus is identified as a triangular hyperechoic structure between the femur and the tibia. The transducer is then moved anteriorly from the coronal plane to the oblique-sagittal plane to visualize the anterior horn of the medial meniscus.

FIGURE 7-8 Medial knee evaluation: coronal plane.

A, Coronal imaging at the medial joint line shows (B) the superficial (arrows) and deep (arrowheads) layers of the medial collateral ligament (curved arrow, body of medial meniscus). F, femur; T, tibia.

FIGURE 7-9 Medial knee evaluation: transverse plane.

A, Transverse imaging (B) without and (C) with anisotropy shows the medial collateral ligament (arrowheads). F, femur.

Returning back to the coronal plane long axis to the tibial collateral ligament, the transducer is moved distally beyond the joint line along the tibial collateral ligament and slightly anterior to its attachment on the tibia, about 4 to 5 cm beyond the joint line (Fig. 7-10A). Here, the pes anserinus can be seen as three hyperechoic tendons superficial to the tibial collateral ligament that converge onto the tibia. Toggling the transducer is often helpful because this will cause the tendons of the pes anserinus superficial to the tibial collateral ligament to appear hypoechoic from anisotropy and be more conspicuous. By turning the transducer to the oblique-axial plane along the long axis of each pes anserinus tendon, the individual sartorius, gracilis, and semitendinosus tendons can be seen; they extend to their tibial attachment as the pes anserinus (see Fig. 7-10B). The more proximal aspects of the pes anserinus tendons can also be visualized when the posterior knee is evaluated. One potential pitfall in evaluation of the posterior aspect of the medial meniscus body is misinterpretation of the adjacent semimembranosus tendon anisotropy as a meniscal cyst. Identification of a hypoechoic round structure just distal to the meniscus with an associated groove in the tibial cortex represents anisotropy of the semimembranosus tendon at its tibial insertion (Fig. 7-11). The normal semimembranosus tendon may be confirmed with the transducer repositioned long axis and perpendicular to the tendon to demonstrate the normal hyperechoic and fibrillar echotexture.

FIGURE 7-10 Distal medial collateral ligament and pes anserinus.

Coronal imaging distal to knee joint shows (A) the superficial layer of the medial collateral ligament (arrowheads) and the tendons of the pes anserinus (arrows) (T, tibial metaphysis). B, Imaging in long axis to semitendinosus proximal to pes anserinus shows normal tendon (arrowheads).

FIGURE 7-11 Semimembranosus: pseudocyst appearance.

Coronal-oblique imaging at the posteromedial joint line shows (A) a hypoechoic round area (arrows), which represents anisotropy of the distal semimembranosus tendon at its tibial attachment. B, This area (arrows) becomes hyperechoic and fibrillar with repositioning of the transducer. Note characteristic bone contour of tibia (T) at semimembranosus tendon attachment (curved arrow, medial meniscus with intrameniscal abnormality). F, femur.

Lateral Evaluation

For evaluation of the lateral knee structures, the leg is internally rotated, or the patient rolls partly onto the contralateral side. Structures of interest laterally include the iliotibial tract, the lateral (or fibular) collateral ligament, the biceps femoris tendon, the supporting structures of the posterolateral corner of the knee, and the common peroneal nerve. To begin, the transducer may be initially placed over the anterior knee long axis to the patellar tendon. The transducer is then moved laterally (Fig. 7-12A). As one leaves the patellar tendon, the next fibrillar structure identified is the iliotibial tract or band, which inserts on the Gerdy tubercle of the proximal tibia (see Fig. 7-12B). It is important to evaluate the tissues between the iliotibial tract and the distal femur more proximally for disorders related to iliotibial band friction syndrome. Next, the transducer is moved laterally to the coronal plane over the lateral femoral condyle. At this location, an important bony landmark is identified: the groove or sulcus for the popliteus tendon.¹⁴ After this groove is identified, the proximal aspect of the transducer is fixed to the femur while the distal aspect is rotated posteriorly toward the fibular head (Fig. 7-13A). In this position, the hyperechoic and fibrillar echotexture of the lateral collateral ligament is seen, which extends from the lateral femoral condyle to the lateral aspect of the fibular head (see Fig. 7-13B and C). The proximal aspect of the lateral collateral ligament extends over the popliteus tendon located within the femoral groove. The distal insertion on the fibula may appear thickened and heterogeneous owing to the bifurcating distal biceps femoris tendon seen both superficial and deep to the lateral collateral ligament (see Fig. 7-13C).¹⁵ One must be aware that slight valgus angulation of the knee joint may cause a wavy appearance to the lateral collateral ligament and possible anisotropy. This can be minimized with the patient positioned so that the opposite knee is flexed under the knee being examined; this position places the knee in slight varus angulation.

FIGURE 7-12 Iliotibial tract: long axis.

A, Coronal imaging between lateral joint line and patellar tendon shows (B) the iliotibial tract (arrowheads). F, femur; G, Gerdy tubercle of tibia.

FIGURE 7-13 Lateral collateral ligament.

A, Coronal-oblique imaging shows (B and C) characteristic contours (arrows) of the femur (F) adjacent to the popliteus tendon (curved arrow) and proximal lateral collateral ligament (arrowheads). Note superficial (arrow) and deep (open arrow) heads of bifurcating biceps femoris tendon in C. f, fibula; L, lateral meniscus; T, tibia.

After the transducer is moved along the lateral collateral ligament to its fibular attachment, the distal aspect of the transducer is fixed to the fibular head while the proximal aspect is rotated posteriorly to the coronal plane (Fig. 7-14A) to bring the biceps femoris tendon into view; this tendon is differentiated from ligament by the less compact fibrillar echotexture and the associated hypoechoic muscle more proximally (see Fig. 7-14B). Both the lateral collateral ligament and the biceps femoris tendon insert onto the lateral aspect of the proximal fibula. The distal biceps femoris may appear heterogeneous as fibers bifurcate both superficial and deep to the lateral collateral ligament at the fibula, which should not be mistaken for tendinosis (see Fig. 7-13C).¹⁵ As the transducer is then moved posteriorly from the biceps femoris in the coronal plane, the relatively hypoechoic appearance of the common peroneal nerve can be seen in long axis (Fig. 7-15A), although the more proximal aspect is best evaluated from a posterior approach with the patient prone in short axis. Evaluation of the posterolateral aspect of the knee proximal to the fibula demonstrates the relative locations of the lateral collateral ligament, the biceps femoris, and the common peroneal nerve (see Fig. 7-15B).

FIGURE 7-14 Biceps femoris.

A, Coronal imaging shows (B) the biceps femoris (arrowheads). f, fibula.

FIGURE 7-15 Common peroneal nerve.

Coronal imaging posterior to biceps femoris shows (A) the common peroneal nerve (arrowheads) (f, fibula). Transverse imaging proximal to the fibula shows (B) the lateral collateral ligament (arrows), biceps femoris (open arrows), and common peroneal nerve (arrowheads) in short axis (left side of image is posterior).

Returning to the popliteus groove in the lateral femoral condyle in the coronal plane, the popliteus tendon may be followed as it curves posteriorly around the joint. The adjacent hyperechoic fibrocartilage body and anterior horn of the lateral meniscus may also be evaluated. Because of the curved course of the popliteus tendon, this tendon is assessed in segments to avoid misinterpretation of hypoechoic anisotropy as tendon abnormality (Fig. 7-16A). The popliteus muscle is best evaluated from a posterior approach, in which the muscle belly is located between the tibia and the tibial vessels (see Posterior Evaluation). Finally, a hyperechoic extension from the popliteus tendon at the joint line may be seen, which attaches to the fibular styloid, called the popliteofibular ligament (see Fig. 7-16B).¹⁶ Other supporting structures of the posterolateral corner, such as the arcuate ligament and the possible fabellofibular ligament, are difficult to identify.

FIGURE 7-16 Popliteus and popliteofibular ligament.

Imaging long axis to the proximal popliteus tendon shows (A) the popliteus tendon (arrowheads) and lateral collateral ligament (open arrows) (F, femur). Coronal imaging shows (B) the popliteofibular ligament (arrowheads) between the popliteus tendon (P) and the fibula (f). T, tibia.

(B, From Sekiya JK, Jacobson JA, Wojtys EM: Sonographic imaging of the posterolateral structures of the knee: findings in human cadavers. Arthroscopy 18:872–881, 2002.)

Posterior Evaluation

To evaluate the posterior structures of the knee, the patient is turned prone. The structures and pathology of interest include a Baker cyst, the posterior horns of the menisci, the cruciate ligaments, and the neurovascular structures of the posterior knee. Examination begins with evaluation for a Baker cyst. One technique is to initially place the transducer in the transverse plane over the mid-calf (Fig. 7-17A).⁴ At this location, three distinct muscles are identified: the soleus anteriorly and the medial and lateral heads of the gastrocnemius muscle superficially (see Fig. 7-17B). The transducer is then moved superiorly along the medial aspect of the medial head of the gastrocnemius muscle (see Fig. 7-17C). As the transducer approaches the knee joint, the distinct hyperechoic semimembranosus tendon is identified just medial to the medial head of the gastrocnemius tendon and muscle over the medial femoral condyle (see Fig. 7-17D). This is the location where distention of a semimembranosus-medial gastrocnemius bursa or Baker cyst is seen. The smaller round and hyperechoic tendon of the semitendinosus is also seen in short axis directly superficial to the semimembranosus tendon. The course of the medial head of the gastrocnemius tendon is not parallel to that of the semimembranosus tendon; therefore, it may be difficult to have both tendons appear hyperechoic in the same plane. One pitfall is incorrect interpretation of the semimembranosus tendon or the medial head of gastrocnemius tendon anisotropy as a small Baker cyst (Fig. 7-18A and B).⁴ Toggling the transducer while imaging the tendons in short axis can create anisotropy (helping to identify the tendons) and eliminate anisotropy (avoiding the pitfall interpreting anisotropy as a Baker cyst) (Video 7-1). If a Baker cyst is identified, the transducer is then turned in the sagittal plane to evaluate the extent of the Baker cyst and to assess for rupture (see Fig. 7-17E). The semitendinosus can also be imaged from this point distally to its insertion at the pes anserinus.

FIGURE 7-17 Posterior knee evaluation: Baker cyst.

A, Transverse imaging over the mid calf shows (B) the soleus (S), medial head of gastrocnemius (MG), and lateral head of gastrocnemius (LG) muscles. C, Transverse imaging over the knee joint shows (D) the medial head of gastrocnemius muscle (arrowheads) and tendon (curved arrow) as well as the semimembranosus tendon (open arrow) and semitendinosus tendon (arrow) (left side of image is medial). Parasagittal imaging over the posteromedial knee shows (E) the semimembranosus (SM), medial head of gastrocnemius (MG) and semitendinosus (ST). F, femur.

FIGURE 7-18 Semimembranosus anisotropy: pseudo-Baker cyst.

Transverse imaging (A and B) shows anisotropy of the medial head of gastrocnemius tendon (curved arrows), which may simulate a small Baker cyst (arrows, semitendinosus tendon; open arrows, semimembranosus tendon). F, femur; MG, medial head of gastrocnemius muscle.

The transducer is then moved over the medial aspect of the posterior knee in the sagittal plane (Fig. 7-19A). At this location, the posterior horn of the medial meniscus is evaluated; this structure normally appears hyperechoic and triangular (see Fig. 7-19B). It may be important to use a lower-frequency transducer (5 or 7 MHz) to assess the posterior horns of the menisci and cruciate ligaments adequately. Toward the medial aspect of the medial meniscus posterior horn, the semimembranosus can be seen as it inserts on the posteromedial tibial cortex, just beyond the meniscus at a prominent concavity or sulcus in the bone. With anisotropy, the normal semimembranosus tendon may appear hypoechoic and may potentially simulate a meniscal cyst (see Fig. 7-11). The transducer is then moved toward the midline in the sagittal plane, and the posterior cruciate ligament is seen with its attachment to the posterior tibia, identified by characteristic bone contours (see Fig. 7-19C). The normal posterior cruciate ligament may appear artifactually hypoechoic as a result of anisotropy, but its thickness should be uniform and less than 1 cm.¹⁷ Anisotropy of the posterior cruciate ligament may be reduced with the heel-toe maneuver or the use of beam steering (available on some ultrasound machines). The transducer is then moved laterally to assess the posterior horn of the lateral meniscus, although accurate identification of pathology is difficult in this location because the popliteus tendon and sheath cross at the peripheral aspect of the lateral meniscus (see Fig. 7-19D).

FIGURE 7-19 Posterior knee evaluation: menisci and posterior cruciate ligament.

A, Parasagittal imaging over the posterior medial knee shows (B) the posterior horn of the medial meniscus (arrowheads) (h, hyaline articular cartilage). Sagittal imaging in midline shows (C), the posterior cruciate ligament (arrowheads). Lateral parasagittal imaging shows (D) the posterior horn of the lateral meniscus (arrowheads), popliteus tendon (curved arrow), and popliteus tension sheath (arrow). F, femur; T, tibia.

The transducer is then turned to the transverse plane and is positioned over the intercondylar notch (Fig. 7-20A). Normally, this space should be hyperechoic, which contains the anterior cruciate ligament along the lateral aspect and the adjacent hyperechoic fat (see Fig. 7-20B).¹⁸ Identification of the anterior cruciate ligament may be improved by toggling the transducer because the normal ligament becomes hypoechoic relative to the adjacent hyperechoic fat as a result of anisotropy. Finally, the popliteal artery and vein are evaluated in short axis and long axis. The muscle belly of the popliteus is located between these vessels and the tibia (Fig. 7-20C). The tibial nerve can be followed proximally to its junction with the common peroneal nerve at the sciatic nerve, which is evaluated with the posterior thigh. Although the sciatic nerve demonstrates a honeycomb appearance from hypoechoic nerve fascicles and surrounding hyperechoic connective tissue, the smaller peripheral nerve branches may consist of only a few hypoechoic fascicles.

FIGURE 7-20 Posterior knee evaluation: anterior cruciate ligament and popliteus.

A, Transverse imaging over the posterior distal femur shows (B) anterior cruciate ligament in short axis (arrowheads) in the lateral aspect of the intercondylar notch and popliteal artery (a). Transverse-oblique ultrasound shows (C) a long axis view of the popliteus tendon and muscle (arrowheads). LC, lateral femoral condyle; MC, medial femoral condyle; T, tibia.

Joint Effusion and Synovial Hypertrophy

Increased joint fluid in the knee is characterized by anechoic or hypoechoic distention of the suprapatellar recess. With slight knee flexion, joint recess distention preferentially occurs deep to the quadriceps tendon (Fig. 7-21), where fluid extends superiorly from between the patella and the femur. In the setting of a small joint effusion, often joint fluid is only identified superolateral to the patella with the knee in flexion, so this area should always be assessed. In knee extension, joint distention may be seen only medial or more likely lateral to the patella in the transverse plane (Fig. 7-22).¹⁹ When imaging these areas, it is important not to apply too much pressure with the transducer because this can collapse the joint recess and displace the joint fluid out of view (Video 7-2). Joint fluid may also collect in the popliteus tendon sheath or in a Baker cyst when there is a communication with the posterior knee joint. A superior patellar plica, which typically is located in the transverse plane through the suprapatellar recess superior to the patella, may uncommonly completely separate the suprapatellar recess into two compartments (Fig. 7-23). In the setting of an intra-articular fracture, several layers of varying echogenicity within the joint may be visible as a lipohemarthrosis (Fig. 7-24).²⁰

FIGURE 7-21 Joint effusion: knee flexion.

Ultrasound images with knee in slight flexion (A) long axis to the quadriceps tendon and transverse over the (B) medial and (C) lateral patellar retinacula show hypoechoic joint recess distention (arrows). Note preferential distention deep to quadriceps tendon (open arrows) and separation of the quadriceps (Q) and prefemoral (PF) fat pads (arrowheads, patellar retinaculum). F, femur; P, patella.

FIGURE 7-22 Joint effusion: knee extension.

Ultrasound images with knee in full extension (A) long axis to the quadriceps tendon and transverse over (B) the lateral and (C)) medial patellar retinacula show hypoechoic joint recess distention (arrows). Note preferential distention lateral and medial to the patella (P) (arrowheads, patellar retinaculum; open arrows, quadriceps tendon). F, femur; PF, prefemoral fat pad; Q, quadriceps fat pad.

FIGURE 7-23 Superior patellar plica.

Ultrasound image long axis to the quadriceps tendon shows the superior plica (arrowheads) that separates the superior aspect of the suprapatellar recess (curved arrow, distended with hypoechoic complex fluid) from the knee joint (arrows, distended with anechoic simple fluid). P, patella.

FIGURE 7-24 Lipohemarthrosis.

Ultrasound image over lateral aspect of suprapatellar recess shows layering of echogenic fat (F) superficial to intra-articular hemorrhage (arrows) and more dependent hematocrit level separating serum from blood cells (H).

The causes of joint effusion are many; however, ultrasound including color or power Doppler imaging cannot distinguish between aseptic and septic effusion (Figs. 7-25 and 7-26). If joint recess distention is not anechoic but rather hypoechoic, isoechoic, or hyperechoic to muscle, then considerations include complex fluid versus synovial hypertrophy. Compressibility of the joint recess, redistribution of recess contents or swirling of the contents with compression or joint movement, and lack of internal flow on color or power Doppler imaging all suggest complex fluid rather than synovial hypertrophy.²¹ The differential diagnosis for complex fluid includes inflammation (including infection) (see Fig. 7-26) and hemorrhage (Fig. 7-27; see Fig. 7-24). If there is concern for infection, percutaneous aspiration should be considered. Inflammatory synovial hypertrophy may be associated with cortical erosions, characterized by cortical irregularity and discontinuity, often associated with increased blood flow on color or power Doppler imaging. Although synovial hypertrophy may also result from inflammation, such as rheumatoid arthritis and crystal deposition (Fig. 7-28), synovial proliferative disorders such as pigmented villonodular synovitis²² (Fig. 7-29), lipoma arborescens,²³ and synovial osteochondromatosis²⁴ are other considerations, with possible hyperechoic foci seen in the last condition. The differential diagnosis for mixed hyperechoic and hypoechoic tissue associated with the suprapatellar recess with compressible vascular channels is synovial hemangioma (see Vascular Abnormalities).²⁵ Localized nodular synovitis may also occur in the knee joint recesses, and it typically appears hypoechoic and noncompressible with possible increased through-transmission (Fig. 7-30).²⁶ Dynamic imaging may demonstrate snapping of synovial hypertrophy. In the setting of a total knee arthroplasty, abnormal synovial hypertrophy may cause snapping, termed patellar clunk syndrome (Fig. 7-31) (Video 7-3).²⁷ Within joint fluid, hyperechoic and shadowing intra-articular bodies may be identified, commonly in a Baker cyst (see Baker Cyst) or suprapatellar recess (Fig. 7-32). When an intra-articular body is identified, the hyaline articular cartilage should be evaluated for a donor site (Fig. 7-33).

FIGURE 7-25 Joint effusion: infection.

Ultrasound image long axis to quadriceps tendon shows hypoechoic distention of the suprapatellar recess (arrows). Note increased through-transmission. P, patella.

FIGURE 7-26 Joint effusion: infection (Pseudomonas).

Ultrasound images (A and B) long axis to quadriceps tendon show heterogeneous distention of the suprapatellar recess (arrows) from complex fluid and synovial hypertrophy with increased flow on color Doppler imaging (B). F, femur; P, patella; Q, quadriceps femoris tendon.

FIGURE 7-27 Intra-articular hemorrhage.

Ultrasound images (A and B) in the transverse plane over the lateral patellar retinaculum in two different patients show mixed-echogenicity hemorrhagic joint fluid (arrows) (arrowheads, patellar retinaculum, which is abnormally thickened in A). Note increased through-transmission. F, femur; P, patella.

FIGURE 7-28 Pseudogout (calcium pyrophosphate dihydrate deposition disease).

Ultrasound images (A and B) long axis to quadriceps tendon show heterogeneous distention of the suprapatellar recess (arrows) from synovial hypertrophy and complex fluid with increased flow on color Doppler imaging. F, femur; P, patella.

FIGURE 7-29 Pigmented villonodular synovitis.

Ultrasound image in the sagittal plane over the posterior knee shows hypoechoic synovial hypertrophy (arrows) adjacent to posterior cruciate ligament (P). F, femur; T, tibia.

FIGURE 7-30 Localized nodular synovitis.

Ultrasound images from two different patients (A and B) show hypoechoic synovial hypertrophy within the suprapatellar recess (arrows). Note joint effusion (curved arrow in A) and increased through-transmission (open arrow in B). F, femur; Q, quadriceps tendon.

FIGURE 7-31 Patellar clunk syndrome.

Ultrasound image long axis to quadriceps tendon (Q) shows hypoechoic synovial hypertrophy (arrows), which moved and produced a snapping sensation with knee flexion and extension. Note hyperechoic metal component of total knee arthroplasty (M) with hyperechoic posterior reverberation (open arrows), and native femur (F) with shadowing (right side of image is distal).

FIGURE 7-32 Intra-articular body.

Ultrasound image long axis to quadriceps tendon shows hyperechoic and shadowing ossified intra-articular body (arrows) within the suprapatellar recess. Note anechoic joint fluid (curved arrow). P, patella.

FIGURE 7-33 Intra-articular body.

Ultrasound image over the lateral aspect of the suprapatellar recess shows (A) a well-defined hypoechoic noncalcified intra-articular body (arrowheads). Ultrasound image over the anterior aspect of the medial femoral condyle shows (B) cortical irregularity (arrowheads) and a cartilage defect (between open arrows). F, femur; m, medial meniscus; T, tibia.

Cartilage Abnormalities

One common cause of joint effusion is a cartilage abnormality. Meniscal degeneration may appear as heterogeneous or internal hypoechogenicity, whereas meniscal tear appears as a well-defined anechoic or hypoechoic cleft that extends to the articular surface, or possibly meniscal irregularity and truncation (Fig. 7-34). Sensitivity and specificity for diagnosis of meniscal tears using ultrasound have been described as 85% and 86%, respectively.²⁸ Because ultrasound is limited with respect to evaluation of the knee menisci as a result of incomplete or poor visualization, magnetic resonance imaging (MRI) remains the imaging method of choice for evaluation of the menisci.²⁹ However, evaluation of the menisci can be accomplished in minutes, and pathologic features are often seen. The posterior horn of the medial meniscus is the most common site for tears, so evaluation should be at least considered here. Meniscal tears often are associated with pain with transducer pressure directly over the abnormality.

FIGURE 7-34 Meniscal abnormalities.

A, Coronal ultrasound image of medial meniscus body shows intrameniscal abnormality. Ultrasound images from four different patients of the (B) medial meniscus body, (C) lateral meniscus body,(D and E) posterior horns of medial meniscus show meniscal tears as hypoechoic or anechoic clefts (arrows) that extend to the meniscal articular surface. Note macerated or degenerative appearance of the meniscus in E. F, femur; T, tibia.

Parameniscal cysts can be diagnosed at ultrasound with a reported accuracy of 88%.³⁰ They are typically associated with an adjacent meniscal tear, although parameniscal cysts adjacent to the anterior horn of the lateral meniscus are less likely to have an associated meniscal tear.³¹ Parameniscal cysts are usually multilocular and characteristically are located at the joint line at the base of the meniscus (Fig. 7-35).³⁰ Whereas some parameniscal cysts are anechoic with increased through-transmission, others are complex cysts and are hypoechoic (Fig. 7-36).³² Small meniscal cysts may be located within the meniscus; larger parameniscal cysts can be quite extensive and typically are associated with a meniscal tear. A medial parameniscal cyst may extend some distance from the meniscal tear, so the possible diagnosis of parameniscal cyst should be considered with any multilocular cyst around the knee, and possible meniscal extension should always be sought. It is important not to misinterpret the normal semimembranosus tendon insertion on the adjacent tibia as a parameniscal cyst; this tendon often appears oval and hypoechoic in evaluation of the posterior horn medial meniscus as a result of anisotropy given the oblique course of the tendon (see Fig. 7-11).

FIGURE 7-35 Parameniscal cyst: lateral.

Coronal ultrasound image over body of lateral meniscus shows (A) hypoechoic meniscal tear (arrows) in connection with anechoic parameniscal cyst (arrowheads). Ultrasound image posterior to (A) shows (B) hypoechoic heterogeneous appearance of parameniscal cyst (arrowheads). F, femur; T, tibia.

FIGURE 7-36 Parameniscal cyst.

Ultrasound images over the (A) anterior horn medial meniscus and (B) anterior horn lateral meniscus show hypoechoic parameniscal cyst (arrowheads), which is in contact with the base of the meniscus and meniscal tear (arrow). F, femur; T, tibia.

In addition to meniscal tear and degeneration, other meniscal pathology includes meniscal extrusion in the setting of osteoarthritis (Fig. 7-37).³³ Abnormal displacement of the meniscus relative to the tibia is often appreciated in the coronal plane deep to the tibial collateral ligament, where associated edema of the tibial collateral ligament is possible.³⁴ In contrast to medial meniscal extrusion, extrusion of the anterior horn and body of the lateral meniscus may be a variation of normal.³⁵ Abnormal symptomatic meniscal displacement may occur with knee flexion and extension and be visualized dynamically (Video 7-4).³⁶ One hallmark of osteoarthritis is the osteophyte, which can be reliably assessed at the knee with ultrasound.³⁷

FIGURE 7-37 Meniscal extrusion.

Coronal ultrasound image over the medial joint shows abnormal hypoechoic meniscus (arrowheads) with joint space narrowing and meniscal extrusion medial to the tibia (T) (curved arrows, osteophyte). F, femur.

Other cartilage abnormalities may involve the hyaline articular cartilage, such as cartilage thinning or defect (Fig. 7-38; see Fig. 7-33B).^37–39 In the setting of osteoarthritis, femoral articular cartilage thickness is best assessed in knee flexion in the parasagittal plane and correlates with MRI to a better degree than imaging of the trochlear cartilage in the transverse plane.¹³ If an intra-articular body is identified, the hyaline cartilage should be evaluated for a defect as a potential donor site.⁴⁰ Another cartilage abnormality relates to deposition of calcification, which can involve both the fibrocartilage meniscus (Fig. 7-39) and the hyaline articular cartilage (Fig. 7-40). Calcification deposition within cartilage is seen as hyperechoic foci and occurs in pseudogout (calcium pyrophosphate dihydrate crystal deposition disease), among other conditions.^41,42 Unlike pseudogout, the deposition of monosodium urate crystals with gout is on the surface of the cartilage (Fig. 7-41), which can be seen on the meniscus (Video 7-5) and hyaline cartilage; the latter, termed the double contour sign, disappears when serum urate levels are below 6 mg/dL.^42–44

FIGURE 7-38 Osteochondral abnormality.

Ultrasound image over the anterior femoral condyle shows subchondral bone plate irregularity (arrowheads) and thickened hyaline articular cartilage (between cursors). F, femur.

FIGURE 7-39 Chondrocalcinosis: pseudogout.

A and B, Ultrasound images of the medial meniscus in two different patients show hyperechoic and shadowing chondrocalcinosis (arrows) within the meniscus. F, femur; T, tibia.

FIGURE 7-40 Chondrocalcinosis: pseudogout.

Ultrasound images of the (A) anterior femoral condyle and (B) trochlea show hyperechoic calcification (arrows) within the hypoechoic hyaline cartilage.

(Courtesy of R. Thiele, MD, Rochester, NY.)

FIGURE 7-41 Gout.

Ultrasound images show hyperechoic monosodium urate crystals (arrows) on surface of (A) anterior femoral condyle hyaline cartilage, and (B) medial meniscus body.

Quadriceps Femoris Injury

Ultrasound is well suited to evaluate the extnsor mechanism of the knee because the tendons are superficial and relatively large.⁴⁵ With regard to the quadriceps tendon, tendinosis will appear hypoechoic and swollen, with continuous tendon fibers visible and possible calcification and increased flow on color or power Doppler imaging, which often correlates with patient symptoms (Fig. 7-42).⁴⁶ The term tendinosis is used rather than tendinitis because this condition primarily represents a degenerative process without acute inflammatory cells. Partial-thickness tears appear as superimposed, well-defined hypoechoic or anechoic clefts or incomplete disruption of tendon fibers.⁴⁷ Partial-thickness tears of the quadriceps tendon may involve only one or two of the three layers while sparing other layers of the quadriceps tendon (Fig. 7-43). Full-thickness tears are characterized by complete tendon disruption, tendon retraction, joint fluid that tracks from the suprapatellar recess through the tendon defect, and secondary wavy appearance of the patellar tendon resulting from a low-lying patella (Fig. 7-44).⁴⁸ A retracted tendon stump may show posterior refraction shadowing, or it may be associated with a hyperechoic bone avulsion fragment. A displaced avulsion fracture fragment from the superior patellar pole may be seen in both partial-thickness and full-thickness quadriceps tendon tears.⁴⁷ Dynamic imaging during evaluation of the quadriceps tendon is often helpful in the diagnosis of a full-thickness tear because lack of tendon movement or translation across the abnormal segment or movement of the tendon stump or avulsion fragment away from the patella indicates complete tear (Video 7-6). Dynamic imaging can be accomplished by squeezing the patient’s thigh, slightly flexing the knee, or manually pressing inferiorly on the patella. This method is helpful with a subacute tear, when hemorrhage may fill the torn tendon gap with echogenic material, and also with a chronic tear, in which a complete tear may demonstrate partial healing.

FIGURE 7-42 Quadriceps tendinosis.

Ultrasound image long axis to the quadriceps tendon shows hypoechoic thickening of the distal quadriceps tendon (arrowheads) without disruption of tendon fibers. F, femur; P, patella.

FIGURE 7-43 Quadriceps partial-thickness tear.

Ultrasound image long axis to the quadriceps tendon shows (A) absence of the superficial aspect of the quadriceps tendon (arrowheads) with an intact deep layer (arrow). Note echogenic avulsion bone fragment (curved arrow). Ultrasound image proximal to A shows (B) full-thickness tears of the superficial layer (rectus femoris) and middle layer (vastus medialis and intermedius) (arrowheads) with an intact deep layer (vastus intermedius) (white arrows indicate three layers of quadriceps tendon). F, femur; P, patella.

FIGURE 7-44 Quadriceps full-thickness tears.

Ultrasound images (A to C) long axis to the quadriceps tendon from three patients show complete disruption of the quadriceps tendon (arrows). Note superior patellar pole bone avulsion (curved arrow) in B. F, femur; P, patella.

Patellar Tendon Injury

Tendinosis and partial-thickness tears may also involve the proximal patellar tendon, also termed jumper’s knee (Figs. 7-45 and 7-46). At ultrasound, tendinosis appears as hypoechoic swelling with continuous tendon fibers.⁴⁹ The presence of more clearly defined hypoechoic or anechoic clefts suggests a superimposed interstitial tear. Marked hyperemia from neovascularity may be seen with color and power Doppler imaging, which is associated with a higher level of pain.⁵⁰ In the setting of a penetrating injury or laceration, it is important to assess the abnormal tendon in both long and short axis because spared and intact tendon fibers exclude a full-thickness tear (Fig. 7-47, online). With a full-thickness patellar tendon tear, there is complete tendon fiber discontinuity (Fig. 7-48). Similar to quadriceps tendon tears, tendon retraction, refraction shadowing at the torn tendon stump, and a wavy patellar tendon may be present; however, the patella may be high-riding in contrast to quadriceps tear, in which the tendon may be positioned low. Dynamic imaging may also be used to confirm tendon discontinuity and to identify tendon retraction in the setting of a full-thickness tendon tear by minimally flexing the knee or by manually pressing superiorly on the patella. Hypoechoic swelling of the distal patellar tendon, swelling of the nonossified cartilage, and possible fragmentation of the tibial tuberosity are consistent with Osgood-Schlatter disease, a painful condition that affects the distal patellar tendon insertion from repetitive trauma in an adolescent (Fig. 7-49).⁵¹ Similar changes may involve the proximal patellar tendon at the inferior pole of the patella, termed Sinding-Larsen-Johansson disease.⁵¹ A central defect or persistent hypoechoic area in the central patellar tendon may be seen when this segment of the tendon is used for anterior cruciate ligament reconstruction (Fig. 7-50).⁵² After total knee arthroplasty, the patellar tendon may be swollen as an expected finding.⁵³

FIGURE 7-45 Patellar tendon tendinosis.

Ultrasound images (A) long axis and (B) short axis to proximal patellar tendon show hypoechoic tendon swelling with intact fibers (arrowheads). Note the normal distal patellar tendon thickness (open arrows) in A and width of the patellar tendon (curved arrows) in B. Corresponding long axis color Doppler image is shown in (C). P, patella.

FIGURE 7-46 Patellar tendon tendinosis.

A, Gray-scale and (B) power Doppler ultrasound images long axis to proximal patellar tendon show well-defined hypoechoic areas (arrow) within the thickened and hypoechoic tendon (arrowheads) consistent with tendinosis. P, patella.

FIGURE 7-48 Patellar tendon full-thickness tears.

Ultrasound images (A and B) long axis to the patellar tendon from two patients show complete disruption of the patellar tendon (arrows) with refraction shadowing (curved arrows) deep to the torn tendon stumps. Note tendon retraction, heterogeneous hemorrhage, and diffuse thickening of the distal patellar tendon (arrowheads) in B. P, patella; T, tibia.

FIGURE 7-49 Osgood-Schlatter disease.

Ultrasound image long axis to the distal patellar tendon shows hypoechoic thickening (arrowheads) with tibial tuberosity bone fragmentation (arrows). Pain was present with transducer pressure. Note the normal proximal patellar tendon (open arrows). T, tibia.

FIGURE 7-50 Postoperative patellar tendon: anterior cruciate ligament (ACL) reconstruction.

Ultrasound images (A) long axis and (B) short axis to patellar tendon show post-surgical defect in the inferior patella (arrowheads) and absence of the central third of the patellar tendon (arrows) for ACL reconstruction. Note full width of patellar tendon (open arrows) in B. P, patella.

FIGURE 7-47 Patellar tendon partial-thickness tear: laceration.

Long axis (A) and short axis (B) ultrasound images show hypoechoic disruption of the lateral aspect of the patellar tendon (arrows) with intact medial fibers (open arrows). P, patella.

Other Knee Tendon Injuries

Other tendon abnormalities around the knee are less common. However, tendinosis may involve the semimembranosus (Fig. 7-51) or biceps femoris. Normal bifurcation of the distal biceps femoris tendon around the lateral collateral ligament should not be confused with tendinosis (see Fig. 7-13C).¹⁵ One other disorder that deserves mention is iliotibial friction band syndrome.^54,55 In this condition, chronic and repetitive contact between the iliotibial tract and the lateral femoral condyle may produce hypoechoic edema, inflammation, and possibly adventitious bursa formation deep to the iliotibial tract, with a possible thickened iliotibial tract (Fig. 7-52). Acute trauma may also cause injury to the iliotibial tract (Fig. 7-53).

FIGURE 7-51 Tendinosis: semimembranosus.

Ultrasound image long axis to the distal semimembranosus tendon shows hypoechoic thickening (arrowheads) (open arrows, normal proximal semimembranosus tendon). T, tibia.

FIGURE 7-52 Iliotibial band friction syndrome.

Ultrasound images long axis to the iliotibial tract in two different patients show (A) hypoechoic soft tissue thickening (arrows) and (B) heterogeneous but hypoechoic bursa (between cursors) deep to the iliotibial tract (open arrows). Note bone irregularity of the femur in A (curved arrows). F, femur; G, Gerdy tubercle of tibia.

FIGURE 7-53 Iliotibial tract tear.

Ultrasound image long axis to iliotibial tract shows tear (arrows) with heterogeneous hemorrhage. F, femur; G, Gerdy tubercle of tibia.

Gout

In the setting of gout, the patellar tendon (Fig. 7-54) (Video 7-7) and popliteus tendon (Fig. 7-55) (Video 7-8) are prone to involvement; therefore, these sites should be included when evaluating for inflammatory arthritis. Even with asymptomatic hyperuricemia, involvement of the distal patellar tendon is not uncommon.⁵⁶ At ultrasound, a gouty tophus appears hyperechoic and amorphous, often with an anechoic or hypoechoic halo.^57,58 A tophus is often more difficult to delineate in a tendon given that both are hyperechoic; however, real-time imaging shows lack of fibrillar echotexture within the tophus. Shadowing may also be seen deep to the tophus, which is more often due to refraction rather than true shadowing from calcification of the tophus (see Fig. 7-54B). Hyperemia may also be present, as may adjacent cortical erosion (see Fig. 7-55B) and soft tissue extension of the tophus.

FIGURE 7-54 Gout: patellar tendon tophus.

Ultrasound images (A) long axis and (B) short axis to the patellar tendon show hyperechoic tophi (arrows) with posterior acoustic shadowing (open arrows) within the patellar tendon (arrowheads). Shadowing was from sound beam attenuation as tophus was not calcified. P, patella; T, tibia.

FIGURE 7-55 Gout: popliteus tendon tophus.

Ultrasound images (A and B) long axis to the proximal popliteus tendon in two different patients show hyperechoic tophi with hypoechoic halo (arrows). Note erosions of lateral femur (curved arrows) (arrowheads, lateral collateral ligament). F, femur; T, tibia.

Medial Collateral Ligament

Sonographic evaluation of the ligaments around the knee is most effective for the superficially located ligaments, such as the medial collateral and lateral collateral ligaments. Transducer position long axis to a ligament is the most important plane, although any abnormality is also assessed short axis to the ligament as well. With regard to the tibial collateral ligament, a grade 1 sprain is characterized by adjacent hypoechoic or anechoic fluid but an intact ligament (Fig. 7-56A). Edema around the tibial collateral ligament may not be traumatic because it may be secondary to meniscal extrusion and osteoarthritis (see Fig. 7-37).³⁴ With a grade 2 injury or partial-thickness tear, the normally hyperechoic and compact fibrillar echotexture is replaced by abnormal hypoechogenicity and possible adjacent hypoechoic or anechoic fluid. With a grade 3 injury or full-thickness tear, there is complete disruption of the ligamentous fibers with heterogeneous hemorrhage and fluid (see Fig. 7-56B). Overall, a tibial collateral ligament injury is suggested when greater than 6 mm thick at the femoral attachment or greater than 3.6 mm thick at the tibial attachment.⁵⁹ Dynamic imaging may also be used to assess the integrity of the medial collateral ligament because medial joint space widening with valgus stress less than 5 mm represents a grade 1 injury, 5 to 10 mm represents a grade 2 injury, and greater than 10 mm indicates a grade 3 injury.⁵⁹ A segment of the tibial collateral ligament that is thickened with intact fibers yet no symptoms is compatible with a remote injury (see Fig. 7-56C) or prior total knee arthroplasty.⁵³ Because the tibial collateral ligament is a relatively flat structure, it is important to assess the entire anterior to posterior extent in short axis. It is not uncommon to have complete fiber discontinuity involving the anterior fibers, but with intact fibers posteriorly. A bursa may also be found between the superficial and deep layers of the medial collateral ligament.⁶⁰

FIGURE 7-56 Medial collateral ligament injury.

Ultrasound images long axis to the medial collateral ligament in three different patients show (A) anechoic fluid (arrows) superficial to the intact tibial collateral ligament (arrowheads) (grade 1 injury), (B) full-thickness tear (arrows) (grade 3 injury), and (C) hypoechoic thickening of the proximal tibial collateral ligament (arrows) from remote injury with normal distal ligament (arrowheads). F, femur; M, medial meniscus body; T, tibia.

Lateral Collateral Ligament

The lateral collateral ligament is an important structure that is a part of the posterolateral ligamentous complex.¹⁶ Injuries may create a swollen, hypoechoic appearance or complete discontinuity (Fig. 7-57).⁶¹ Distal ligamentous avulsions may be associated with a hyperechoic fibular fracture fragment. In this situation, the ligament is structurally intact but functionally completely torn. Other supporting structures of the posterolateral corner include the popliteofibular ligament. Because this ligament may be difficult to visualize, it is often helpful to use other signs of posterolateral corner injury, such as lateral collateral ligament tear and abnormal widening of the lateral joint space with varus stress, where lateral joint space of more than 10.5 mm can predict those who will require posterolateral corner repair or reconstruction.⁶¹

FIGURE 7-57 Lateral collateral ligament tears.

Ultrasound images long axis to the lateral collateral ligament in two different patients show (A) hypoechoic thickening (arrows) at the fibula (f) consistent with high-grade partial-thickness tear and (B) full-thickness tear (arrows) with proximal ligament edema and laxity (arrowheads). F, femur; T, tibia.

Cruciate Ligaments

With regard to the anterior cruciate and posterior cruciate ligaments, each can be partially visualized at sonography, but MRI is considered the imaging test of choice in their evaluation. At sonography, the posterior cruciate ligament is considered abnormal if it is hypoechoic or anechoic and swollen more than 1 cm thick (Fig. 7-58).¹⁷ A torn posterior cruciate ligament may be focally disrupted or diffusely enlarged.⁶² Hyperechoic bone avulsions from the posterior aspect of the tibia are also possible.⁶³ An anterior cruciate ligament tear is diagnosed at ultrasound when the normally hyperechoic ligament in the lateral aspect of the intercondylar notch, when imaged transversely, is abnormally hypoechoic or anechoic (Fig. 7-59).^18,64 Dynamic stress views have also been used in conjunction with ultrasound to identify abnormal anterior tibial translation as an indirect sign of anterior cruciate ligament tear.⁶⁵ Although evaluation for anterior and posterior cruciate ligament tears is limited with ultrasound, ganglion cysts associated with the cruciate ligaments (discussed later) may extend posteriorly and can be visible at sonography.⁶⁶

FIGURE 7-58 Posterior cruciate ligament tears.

Ultrasound images long axis to the posterior cruciate ligament in two different patients show hypoechoic thickening of the posterior cruciate ligament (arrowheads). F, femur; T, tibia.

FIGURE 7-59 Anterior cruciate ligament tear.

Ultrasound image short axis to the anterior cruciate ligament shows hypoechoic thickening of the anterior cruciate ligament (arrowheads). LC, lateral femoral condyle; MC, medial femoral condyle.

Osseous Injury

With regard to ultrasound of bone, the osseous surfaces have a characteristic contour that is important for orientation, especially when evaluating ligaments. The normal bone cortex is smooth and continuous. Any focal step-off deformity, especially if point tenderness with transducer pressure, should raise concern for fracture (Fig. 7-60).^67,68 With regard to the patella, it is important not to misinterpret a bipartite or tripartite patella, which is a normal variation, as a fracture.⁶⁹ Unlike a fracture, this normal variation is isolated to the upper outer quadrant of the patella, has more irregular osseous margins, and is often asymptomatic; correlation with radiography is also important (Fig. 7-61).

FIGURE 7-60 Fracture: patella.

Ultrasound image transverse over patella shows cortical discontinuity (arrow) and adjacent edema or hemorrhage (arrowheads). P, patella.

FIGURE 7-61 Bipartite patella.

Ultrasound image in the sagittal plane over the lateral patella shows the separate patellar bone segment (curved arrows) and synchondrosis (arrow) with native patella (P). Q, quadriceps tendon.

Baker Cyst

Besides parameniscal cysts described in the preceding section, other cystic abnormalities are often seen around the knee. One of the most common is distention of the semimembranosus-medial gastrocnemius bursa, which results in Baker (or popliteal) cyst.⁴ Although distention of this bursa may occur from local irritation or inflammation, more commonly it becomes distended with joint fluid through communication with the knee joint. Present in 50% of adults who are older than 50 years, this communication is acquired by a combination of degenerative weakening of the intervening capsule and increased intra-articular pressure and joint fluid from internal derangement.⁴ In the pediatric population, Baker cyst may be associated with underlying arthritis or joint hypermobility.⁷⁰ Accurate diagnosis of Baker cyst relies on identification of the characteristic channel or neck between the semimembranosus and the medial head of the gastrocnemius tendon, which connects the bursa to the knee joint via the subgastrocnemius bursa. The result is a C-shaped fluid collection, concave lateral, which wraps around the medial head of the gastrocnemius tendon and muscle (Fig. 7-62).

FIGURE 7-62 Baker cyst.

Ultrasound images transverse (A) and sagittal (B) over the posterior medial knee show predominantly anechoic distention of the semimembranosus-medial gastrocnemius bursa (curved arrows). Note the communication to the knee joint (open arrow) between the semimembranosus tendon (arrowhead) and the medial head of the gastrocnemius tendon (arrows) and muscle (MG) via the subgastrocnemius bursa (SG). F, medial femoral condyle.

A Baker cyst may be distended with anechoic or hypoechoic fluid. The presence of isoechoic or hyperechoic material within a Baker cyst may represent complex fluid, hemorrhage, or synovial hypertrophy (inflammatory or the result of proliferative synovial conditions such as pigmented villonodular synovitis) (Fig. 7-63). Hyperechoic and shadowing intra-articular bodies are also commonly present within a Baker cyst (see Fig. 7-63E). It is important to evaluate the inferior margin of the Baker cyst in the sagittal plane, which is normally well defined and smooth. The presence of hypoechoic fluid beyond the confines of the Baker cyst suggests rupture, which can produce diffuse edema or reactive cellulitis that typically is located superficial to the medial head of the gastrocnemius muscle and may extend to the ankle (Fig. 7-64). A more chronically ruptured Baker cyst may result in a heterogeneous mass-like area in the calf, usually superficial to the medial head of the gastrocnemius muscle (see Fig. 7-64C and D). In this situation, it is important to differentiate a ruptured Baker cyst from a soft tissue neoplasm; identification of the Baker cyst communication to the knee joint between the medial head of the gastrocnemius and semimembranosus tendons is critical in this differentiation. Extension of a Baker cyst deep to the calf musculature is uncommon, and extension within the muscle is rare, so such findings should raise concern for another etiology, such as sarcoma. Ultrasound-guided aspiration and steroid injection of Baker cyst may be considered, although re-accumulation of fluid from the knee joint is possible (see Chapter 9).

FIGURE 7-63 Complex Baker cysts.

Ultrasound images transverse (A) and sagittal (B to E) over the posterior medial knee from five different patients show heterogeneous and variable echogenicity (arrows) within Baker cysts (cursors or arrowheads) from complex fluid, hemorrhage, and synovitis. Note Baker cyst rupture (open arrow) in C and an ossified intra-articular body with posterior acoustic shadowing (curved arrow) in E that was mobile with transducer pressure.

FIGURE 7-64 Ruptured Baker cysts.

Ultrasound images sagittal over the posterior medial knee from two patients show (A) anechoic fluid (arrows) and (B) heterogeneous hypoechoic fluid and hemorrhage (arrows), which extend distal to the Baker cyst (B) with an irregular inferior margin. Sagittal ultrasound images in two different patients show (C) loculated hypoechoic fluid (arrows) and (D) hypoechoic to isoechoic hematoma (arrows) superficial to the medial head of the gastrocnemius. In each case, proximal communication to the posterior knee joint was demonstrated.

Other Bursae

Several other bursae around the knee in addition to Baker cysts may become distended. Medially and anteriorly, the pes anserinus bursa is located deep to the pes anserinus adjacent to the medial tibia, but it may be extensive (Fig. 7-65).² Symptoms referable to the pes anserinus rarely correspond to a tendon or bursal abnormality but rather are associated with knee osteoarthritis.⁷¹ Posterior and superior to the pes anserinus and at the joint line, the semimembranosus-tibial collateral ligament bursa takes the form of an inverted U shape as it wraps around the semimembranosus tendon (Figs. 7-66 and 7-67).³ The prepatellar bursa is located anterior to the patella (Fig. 7-68).⁷² The superficial infrapatellar bursa (Fig. 7-69) and deep infrapatellar bursa (Fig. 7-70) are located around the distal patellar tendon, the latter of which normally contains minimal fluid (see Fig. 7-70A).⁷³ A bursa may be distended with anechoic or hypoechoic fluid, although complex fluid, hemorrhage, or synovial hypertrophy may range from hypoechoic to hyperechoic, with possible increased flow on color or power Doppler imaging. Causes of bursal distention include trauma; inflammation, such as infection, rheumatoid arthritis, and gout; and other synovial proliferative disorders. The presence of pain with transducer pressure and hyperemia on color or power Doppler imaging suggest true inflammation or bursitis rather than mechanical or reactive bursal fluid. Knowledge of these common bursae allows one to distinguish an abnormal bursa from a nonspecific fluid collection or abscess. At points of abnormal mechanical friction or contact, an adventitious bursa may form, such as after knee amputation (Fig. 7-71).⁷⁴ Unlike a Baker cyst, the previously described bursae do not normally communicate to the knee joint; therefore, ultrasound-guided percutaneous aspiration and injection may be warranted.

FIGURE 7-65 Pes anserinus bursa.

Ultrasound images (A) short axis and (B) long axis to the gracilis tendon (G) show anechoic distention of the pes anserinus bursa (arrows). S, sartorius; T, semitendinosus.

FIGURE 7-66 Semimembranosus-tibial collateral ligament bursa.

Ultrasound images (A) long axis and (B) short axis to the semimembranosus tendon (SM) show hypoechoic fluid (arrows), which distends the semimembranosus-tibial collateral ligament bursa. Note location between the semimembranosus and tibial collateral ligament (hypoechoic from anisotropy) (arrowheads). MG, medial head of gastrocnemius.

FIGURE 7-67 Semimembranosus-tibial collateral ligament bursal fluid.

Ultrasound image short axis to the semimembranosus tendon (SM) shows anechoic fluid (arrows), which distends the semimembranosus-tibial collateral ligament bursa. Note location between the semimembranosus and tibial collateral ligament (arrowheads), which is unlike the location of a Baker cyst between the semimembranosus and medial head of gastrocnemius (MG). T, semitendinosus.

FIGURE 7-68 Prepatellar bursa.

Ultrasound images long axis to the proximal patellar tendon over distal patella in three different patients show prepatellar bursal distention (arrows) from (A) anechoic fluid (aseptic), (B) hypoechoic heterogeneous synovial hypertrophy and complex fluid (infection), and (C) isoechoic hemorrhage. P, patella; T, patellar tendon. Note that the transducer is floated on a thick layer of gel in A and C.

FIGURE 7-69 Superficial infrapatellar bursa.

Ultrasound image (A) long axis to the distal patellar tendon (arrowheads) show predominantly hypoechoic distention of the superficial infrapatellar bursa (arrows) with internal linear echoes resulting from sterile complex fluid and hemorrhage. B, Long axis ultrasound image in a different patient shows complex fluid and variable echogenicity synovitis (arrows) with increased blood flow on (C) power Doppler imaging (arrowheads, patellar tendon). T, tibia.

FIGURE 7-70 Deep infrapatellar bursa.

Ultrasound images long axis to the distal patellar tendon (arrowheads) in two different patients show (A) physiologic distention and (B) abnormal fluid distention of the deep infrapatellar bursa (arrows). T, tibia.

FIGURE 7-71 Adventitious bursa.

Sagittal ultrasound image over the distal femur (F) amputation site shows hypoechoic adventitious bursa formation (arrows).

Ganglion Cysts

Ganglion cysts have a propensity to be located at several areas around the knee.⁷⁵ The exact cause of ganglion cysts is not known, but synovial herniation, tissue degeneration, and tissue response to trauma are several possibilities.⁷⁵ One common site is around the cruciate ligaments, where a ganglion cyst may extend into the soft tissue and bone (Fig. 7-72).⁶⁶ Ganglion cysts may also occur posteriorly at the gastrocnemius tendon origins (Fig. 7-73) and anteriorly in the Hoffa infrapatellar fat pad (Fig. 7-74). Most ganglion cysts demonstrate lobular margins and internal septations with a multilocular appearance. Ganglion cysts may be anechoic, with increased through-transmission, or hypoechoic, without increased through-transmission when small. Percutaneous aspiration reveals thick and clear gelatinous fluid. The differential diagnosis of a soft tissue multilocular cyst is a parameniscal cyst, with the latter located around the joint line, which extends from the meniscus. If a large cyst is identified at ultrasound that is not multilocular (such as a ganglion cyst) and not in the expected location of a bursa, then a hypoechoic solid neoplasm must be considered. Intraneural ganglion cysts of the peroneal nerve are discussed later.

FIGURE 7-72 Ganglion cysts: cruciate ligaments.

Sagittal ultrasound images over posterior knee long axis to posterior cruciate ligament (PCL) in two different patients show hypoechoic ganglion cysts (arrows) (arrowheads, PCL). F, femur; T, tibia.

FIGURE 7-73 Ganglion cysts: gastrocnemius tendon.

Sagittal ultrasound images over posterior knee long axis to gastrocnemius tendons in two different patients show (A) hypoechoic and (B) anechoic multilobular gastrocnemius origin ganglion cysts (arrows). Note posterior through-transmission in B. F, femur.

FIGURE 7-74 Ganglion cyst: Hoffa fat pad.

Ultrasound image long axis to patellar tendon shows hypoechoic multilobular cyst (arrows) in Hoffa fat pad (T, tibia) with increased through-transmission.