Skeletal Radiol DOI 10.1007/s00256-014-1841-6

REVIEW ARTICLE

Ultrasound of the knee with emphasis on the detailed anatomy of anterior, medial, and lateral structures Michel De Maeseneer & Stefaan Marcelis & Cedric Boulet & Mimoun Kichouh & Maryam Shahabpour & Johan de Mey & Erik Cattrysse

Received: 23 December 2013 / Revised: 24 January 2014 / Accepted: 3 February 2014 # ISS 2014

Abstract Objective To describe the detailed ultrasound anatomy of the anterior, medial, and lateral aspects of the knee and present the ultrasound examination technique used. Materials and Methods We present ultrasound using images of patients, volunteer subjects, and cadaveric specimens. We correlate ultrasound images with images of anatomical sections and dissections. Results The distal quadriceps tendon is made up of different laminas that can be seen with ultrasound. One to five laminas may be observed. The medial retinaculum is made up of three anatomical layers: the fascia, an intermediate layer, and the capsular layer. At the level of the medial patellofemoral ligament (MPFL) one to three layers may be observed with ultrasound. The medial supporting structures are made up of the medial collateral ligament and posterior oblique ligament. At the level of the medial collateral ligament (MCL), the superficial band, as well as the deeper meniscofemoral and meniscotibial bands can be discerned with ultrasound. The posterior part, corresponding to the posterior oblique ligament (POL), also can be visualized. Along the posteromedial aspect of the knee the semimembranosus tendon has several insertions including an M. De Maeseneer : C. Boulet : M. Kichouh : M. Shahabpour : J. de Mey Department of Radiology, University Hospital Brussels, Brussels, Belgium M. De Maeseneer : E. Cattrysse Department of Experimental Anatomy, Free University Brussels, Brussels, Belgium S. Marcelis Department of Radiology, Sint Andries Ziekenhuis, Tielt, Belgium M. De Maeseneer (*) Department of Radiology, Vrije Universiteit Brussel, Laarbeeklaan 101, 1090 Brussels, Belgium e-mail: [email protected]

anterior arm, direct arm, and oblique popliteal arm. These arms can be differentiated with ultrasound. Along the lateral aspect of the knee the iliotibial band and adjacent joint recesses can be assessed. The fibular collateral ligament is encircled by the anterior arms of the distal biceps tendon. Along the posterolateral corner, the fabellofibular, popliteofibular, and arcuate ligaments can be visualized. Conclusion The anatomy of the anterior, medial, and lateral supporting structures of the knee is more complex than is usually thought. Ultrasound, with its exquisite resolution, allows an accurate assessment of anatomical detail. Knowledge of detailed anatomy and a systematic technique are prerequisites for a successful ultrasound examination of the knee. Keywords Ultrasound . Knee . Ligaments . Tendons

Introduction Ultrasound, with its exquisite resolution, is the technique of choice for depicting anatomical detail of superficial structures such as tendons and ligaments. High-resolution ultrasound allows visualization of the internal fibrillar structure of the ligaments and tendons. Assessment of knee injuries is typically performed using MR imaging. MRI remains the method of choice for intraarticular knee structures, such as menisci, cartilage, and cruciate ligaments. Ultrasound is an excellent imaging modality for evaluation of superficial soft tissue structures [1–6]. Direct correlation with the site of pain, and comparison with the contralateral normal side are some of the advantages of ultrasound. Ultrasound allows a ready evaluation of joint effusions, superficial ligaments and tendons, and superficial nerves. In the imaging literature the anatomy of many supporting structures of the knee is presented in a simplified way, while in

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reality it may be more complex. Evolving reconstruction techniques described in recent surgical literature illustrate the clinical importance of a detailed assessment of the medial patellofemoral ligament (MPFL), medial collateral ligament (MCL), posterior oblique ligament, posteromedial corner, and posterolateral corner [7–9]. The precise location of injuries and identification of the specific components involved may be important in a preoperative setting. In this article we describe the ultrasound anatomy and examination technique used for the quadriceps tendon, medial retinaculum, medial collateral ligament, posteromedial corner, lateral supporting structures, and posterolateral corner. With our work we sought to complement descriptions provided in textbooks [3, 4]. We present ultrasound using images of patients, volunteer subjects, and cadaveric specimens. We correlate ultrasound images with images of anatomical dissections.

Ultrasound examination technique The assessment of the fine details of tendons and ligaments requires state of the art ultrasound equipment. Linear transducers with a frequency of up to 18 MHz are ideal [3, 4]. Anisotropy is an inherent problem in ultrasound imaging of tendons and ligaments. The hyperechoic fibrillar appearance is best demonstrated when the ultrasound probe is positioned perpendicular to the structure being imaged. A systematic approach is a prerequisite for a complete evaluation of knee ligaments and tendons. Bony landmarks are helpful for identification of the most medial and lateral supporting structures [10]. Ultrasound examination of the knee may be performed with the patient lying supine on the examination table. The extensor mechanism is first examined with the knee flexed. The knee is then extended and the medial and lateral supporting structures are assessed. We recommend completing the examination with the patient in the prone position. The direct and oblique popliteal arm of the semimembranosus tendon, cruciate ligaments, posterolateral corner ligaments, and Baker’s cyst can only be examined using a posterior approach. Imaging of the contralateral knee may be helpful for comparing the thickness of the ligaments and tendons of the symptomatic side with that of the asymptomatic knee. Ultrasound allows a dynamic examination and is of value in the assessment of snapping knee syndrome related to distal biceps tendon, pes anserinus, and popliteus tendon snapping [11–14].

Quadriceps tendon The quadriceps tendon is rarely injured. Injuries may occur following trauma or in the setting of a knee arthroplasty.

Corticosteroid injections and diabetes also are risk factors for a tear [15–19]. Tears can usually be diagnosed clinically but swelling and hematoma may limit the sensitivity of clinical examination. Partial tears are more difficult to diagnose clinically because some degree of function is usually maintained [15]. The quadriceps muscle is made up of four components including the rectus femoris, vastus medialis and lateralis, and the vastus intermedius. Their tendons converge to form the multiple laminas of the quadriceps tendon. The laminas fuse 1–5 cm above the patella, but may remain separate up to the patellar insertion [19]. The layered configuration of the quadriceps tendon is significant for partial tears. Although tears are commonly transverse, they may involve only a portion of the laminar structure. Although a three-layered pattern is most commonly observed, one to five laminas may be present (Figs 1, 2) [19, 20]. From medial to lateral the number of laminas may not be uniform, and laterally laminas tend to converge [19, 20]. The anterior lamina consists of the rectus femoris tendon. These fibers form a continuation over the patella, designated the prepatellar quadriceps continuation [21]. The middle lamina is made up of fibers from the vastus medialis and vastus lateralis tendons. The posterior lamina is made up of fibers from the vastus intermedius tendon. Importantly, the four components of the quadriceps tendon not only contribute fibers to their respective laminas, but also to the other laminas [19]. Ultrasound can be used to diagnose partial and complete tears of the quadriceps tendon. On ultrasound, the quadriceps tendon is assessed in the sagittal plane with the knee flexed. Dynamic scanning of the tendon during flexion and extension of the knee is important in the evaluation of partial tears [6]. The ultrasound appearance of the quadriceps tendon is variable. The tendon fibers have a characteristic fibrillar appearance, and anisotropy artifact is evident. When the laminas are closely opposed, they are differentiated only by a subtle

Fig. 1 Anatomical dissection. Normal quadriceps. Quadriceps tendon sectioned 2 cm above patella. Note distinct lamina made up of vastus intermedius fibers (deep), vastus lateralis and medialis fibers (intermediate), and rectus femoris fibers (superficial). a anterior aspect of the knee, Vm vastus medialis, Vi vastus intermedius, Vl vastus lateralis, R rectus femoris

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Fig. 2 Sagittal anatomical slice. The quadriceps tendon is made up of different laminas (arrows)

hyperechoic interface (Fig 3). When the intervening tissue planes are wider they are seen as hyperechoic, band-like areas without the fibrillar pattern that is characteristic of tendinous fibers (Fig 4 a, b).

Medial retinaculum Patellar dislocations represent a common knee injury in pediatric and adolescent athletes. Retinacular tears may occur at the patellar or femoral insertion. Most authors consider both types of tears equally prevalent [22–24]. The medial retinaculum is an all inclusive term that designates all the medial retaining structures [25, 26]. The MPFL is the most important medial static stabilizer of the patellofemoral joint, contributing more than 50 % to medial stability (Fig 5) [26]. The MPFL inserts at the superior 2 cm of the medial aspect of the patella, and has a length of 4 cm. At the femoral insertion, the width of the MPFL is 1–2 cm [26]. Adjacent

Fig. 3 A 28-year-old female volunteer. Normal quadriceps tendon (along the lateral aspect of the tendon). Longitudinal ultrasound shows the fibrillar pattern of the tendon (arrowheads). Laminas are difficult to differentiate, especially along the lateral aspect of the tendon. s superior, F femur, Pat patella. Insert positioning for the longitudinal image

Fig. 4 A 52-year-old male patient with medial knee pain. a. Normal quadriceps tendon. Longitudinal ultrasound image. Three distinct laminas are seen (arrows). Laminas are separated by hyperechoic septa. s superior, Pat patella. Insert positioning for the longitudinal image. b Normal quadriceps tendon. Corresponding MRI to a Sagittal proton densityweighted image shows three distinct laminas in the quadriceps tendon (arrows)

to its patellar insertion the MPFL is meshed with fibers of the vastus medialis obliquus (VMO), and the two structures are difficult to separate [22]. The VMO is the distal part of the vastus medialis (VM). It is usually not separated from the VM by a clear tissue plane, but the fibers of the VMO have a more horizontal orientation [19, 27]. The femoral insertion of the MPFL consists of a transverse band, also termed “outer fibers,” and an oblique band, also termed “inner fibers.” The transverse fibers cross superficial to the proximal MCL and insert onto an area between the medial epicondyle and the adductor tubercle, known as Nomura’s point (Figs 6, 7) [28, 29]. The smaller oblique part attaches to the anterosuperior aspect of the MCL. The medial patellomeniscal ligament (MPML) courses from the lower pole of the patella toward the medial meniscocapsular area. It is deeper with respect to the MPFL and medial patellotibial ligament (MPTL). The MPTL courses from the lower pole of the patella, toward an area on the tibia 1.5 cm below the joint line (Fig 8). The MPTL can typically be seen with ultrasound, but in our experience the MPML cannot be seen [26].

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Fig. 5 Normal structures of the medial retinaculum. The medial patellofemoral ligament (Mpfl) courses from the superior aspect of the patella to Nomura’s point. The medial patellomeniscal ligament (Mpml) courses from the inferior aspect of the patella to the meniscocapsular area and the medial patellotibial ligament (Mptl) courses towards the tibia below the joint line

Ultrasound has been shown to be of value in the assessment of acute MPFL injuries [23]. On ultrasound, the medial

Fig. 6 Anatomical dissection. Anteromedial aspect of the knee. By exerting traction on the inferior half of the retinaculum (instrument) the fibers of the medial patellofemoral ligament (MPFL; arrowheads) become well demarcated. Adjacent to the patella (Pat) the MPFL is closely attached to the vastus medialis obliquus (Vmo). Posteriorly, the MPFL inserts (arrow) onto the area between the femoral insertion of the medial collateral ligament (Mcl) and adductor tubercle. Add adductor magnus tendon

Fig. 7 Transverse anatomical section. Medial aspect of the knee at the level of the MPFL. The MPFL (arrows) appears bilayered in its anterior aspect. It wraps over the femoral insertion of the medial collateral ligament (Mcl) just superior to the medial femoral epicondyle. More superficially, the deep fascia (layer 1) is seen (arrowheads). Anteriorly, it is not distinguishable from the deeper layers. sa sartorius, gr gracilis, sm semimembranosus, st semitendinosus

retinaculum is examined with the knee flexed, and the probe placed in the transverse plane between the superior half of the patella and the femoral epicondyle. The number of layers that can be seen on ultrasound at the level of the MPFL is variable and ranges from 1 to 3 (Figs. 9, 10, 11). There is some disagreement regarding the precise nature of the visualized layers [28, 30]. The most superficial and delicate layer (Warren’s layer 1) corresponds to the fascia, and cannot always be seen. It may not exist as a separate layer inferior to the level of the VMO,

Fig. 8 Anatomical dissection. Medial aspect of the knee at the level of the inferior aspect of the medial retinaculum. The medial patellotibial ligament (arrows) is seen coursing from the inferior aspect of the patella (Pat) to the tibia (Tib). Slightly more superiorly, note the smaller medial patellomeniscal ligament (arrowheads). Men meniscus

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Fig. 9 A 23-year-old male volunteer. Normal MPFL with a single layer is seen. Transverse ultrasound image. Note one distinct layer in the medial retinaculum at the level of the MPFL (arrowheads). Fem femur, Pat patella. Insert ultrasound positioning

where it may be fused to layer 2. The intermediate layer (Warren’s layer 2) corresponds to the MPFL [27], and is usually well seen at ultrasound. Layer 2 may appear bilaminar at ultrasound and this is in agreement with the finding in an MR study of an additional layer of fibers originating from the VMO (Fig. 10) [28]. The deepest layer (Warren’s layer 3) corresponds to the capsule. It can only be seen with ultrasound when the joint is distended [30]. The patellar insertion of the MPFL is thicker and more clearly identifiable than the femoral insertion, which is thin and difficult to visualize. The transverse fibers of the MPFL can be seen wrapping over the proximal aspect of the MCL, and attaching in between the medial epicondyle and the adductor tubercle (Fig. 11).

Fig. 11 A 28-year-old female volunteer. Normal MPFL. Transverse ultrasound image. The thin femoral insertion of the MPFL (arrows) is seen wrapping over the femoral insertion of the medial collateral ligament (Mcl). Compare with Fig. 6. a anterior, Fem femur, arrowhead adductor tubercle. Insert ultrasound positioning

The supporting structures along the medial aspect of the knee consist of the MCL and posterior oblique ligament (POL) (Fig. 12). Three layers can be identified along the medial aspect of the knee at the level of the anterior part. Layer 1 consists of the deep fascia, layer 2 corresponds to the superficial band of the MCL, and layer 3 to the deep meniscofemoral and meniscotibial bands. Both deep bands are tightly attached to the meniscus, but loosely attached to the superficial MCL [27, 31, 32]. In contrast, more posteriorly, at the level of the POL, layers 2 and 3 are fused [27]. Three fascial attachments of the distal semimembranosus form the POL. These attachments include the superficial, central (tibial), and capsular arms. The POL is a fibrous expansion between the fascial attachments of the

Medial collateral ligament Medial collateral ligament (MCL) injuries are common. They may be isolated or associated with other knee injuries. Isolated MCL injuries are not typically repaired; however, recent research suggests that injuries involving multiple components of the medial supporting structures may benefit from surgical reconstruction [8].

Fig. 10 A 34-year-old male volunteer. Normal bilayered MPFL. Transverse ultrasound image. Note two distinct layers in the MPFL (arrowheads, arrow), that fuse posteriorly (curved arrow). Fem femur, Pat patella. Insert ultrasound positioning

Fig. 12 Anatomical dissection. Normal medial aspect of the knee. The medial supporting structures consist of an anterior part, the medial collateral ligament (arrowheads) and a posterior part, the posterior oblique ligament (arrows). Also, note the adductor magnus tendon (Add) inserting onto the adductor tubercle, and the pes anserinus (Pes)

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semimembranosus posteriorly and the fibers of the MCL anteriorly (Fig. 12) [8, 33]. On ultrasound, the medial supporting structures are examined with the knee extended. The medial epicondyle is a bony protrusion on the medial femoral condyle. When the probe is moved inferiorly along the distal aspect of the femur the medial epicondyle comes into view (Figs. 13, 14). The femoral insertion of the MCL is seen adjacent to the epicondyle. In the transverse plane, both the thicker anterior MCL and thinner more posterior POL can be seen (Fig. 14). With the probe turned in the coronal plane, a longitudinal image of the MCL, with its superficial and deep bands, is obtained (Figs. 15, 16a). The deep fascia (layer 1) cannot be seen. The superficial and deep parts of the MCL can be identified as anisotropic fibrillar structures separated by a more hyperechoic intervening layer. The gutters formed between the deep bands and the femur and tibia should be examined as they are a common location of meniscal fragments in the setting of meniscal tears. More posteriorly, a longitudinal image of the POL can be obtained. At this level only one layer is seen, in contrast to the two layers at the level of the MCL (Fig. 16b).

Fig. 14 A 28-year-old female volunteer. Normal medial collateral ligament and posterior oblique ligament. Transverse ultrasound. Note the anteriorly located medial collateral ligament (arrow) inserting at the femoral epicondyle (E), and the posterior oblique ligament (arrowheads). Fem femur, a anterior, p posterior. Insert ultrasound positioning

Finally, from the position for a longitudinal image of the MCL, the probe is moved inferiorly along the proximal diaphysis of the tibia, to examine the pes anserinus (Fig. 17). The pes anserinus tendons are arranged in a linear fashion with the sartorius tendon superiorly, the semitendinosus tendon inferiorly, and the gracilis tendon in between. The tendons may remain distinct up to the insertion at the anteromedial aspect of the tibia. A dynamic evaluation is important in the assessment of snapping pes anserinus syndrome [11, 13].

Posteromedial corner Injuries involving the posteromedial corner of the knee and the oblique popliteal ligament have been associated with pain,

Fig. 13 Transverse anatomical slice. Normal medial aspect of the knee at the level of the femoral epicondyle. The medial collateral ligament consists of an anterior part, the medial collateral ligament (curved arrow) inserting onto the femoral epicondyle (arrow), and the posterior oblique ligament (arrowhead). Sa sartorius, Gr gracilis, St semitendinosus, Sm semimembranosus

Fig. 15 Coronal anatomical slice. Normal medial collateral ligament. Note superficial band (arrowheads) and a deeper band consisting of a meniscofemoral (Mf) and meniscotibial part (Mt). Note the intervening medial collateral ligament (MCL) bursa (not distended) between the two layers

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Fig. 18 Anatomical dissection. Normal anatomy of the posteromedial aspect of the knee. The common semimembranosus tendon (Sm) is seen and divides into an oblique popliteal ligament arm (OPLa), a direct arm (Da), and an anterior arm (Aa). Gc medial gastrocnemius tendon, a anterior, p posterior, Mcl medial collateral ligament

Fig. 16 a A 26-year-old male volunteer. Normal medial collateral ligament. Longitudinal ultrasound image shows the fibrillar structure of the superficial band of the MCL (arrowhead), and meniscofemoral (Mf) and meniscotibial (Mt) parts of the deeper band. Connective tissue, which is more hyperechoic, is seen intervening between the two bands. A bursa may also be seen in this area (not shown). Fe femur, Ti tibia. Insert ultrasound positioning for longitudinal image of MCL. b A 26-year-old male volunteer. Normal posterior oblique ligament. Longitudinal ultrasound image slightly posterior to that for the MCL (Fig. 16a). Note that only one layer is seen at the level of the posterior oblique ligament (POL; arrowheads). M meniscus, S superior, i inferior

functional genu recurvatum instability, and failed anterior cruciate ligament (ACL) reconstructions [8, 33–35]. The distal semimembranosus has several attachments distal to the main common tendon, the most important being an extension to the oblique popliteal arm, the direct arm, the anterior arm, and attachments to the components of the posterior oblique ligament (capsular arm) (Fig. 18) [36].

Fig. 17 A 28-year-old female volunteer. Normal pes anserinus. Three distinct tendons are seen corresponding to the sartorius, gracilis, and semitendinosus tendons (arrows). Tendons can remain distinct nearly up to the insertion. s superior. Insert ultrasound positioning for the image along the short axis of the pes anserinus

The oblique popliteal extension attaches to the oblique popliteal ligament. The oblique popliteal ligament consists of a 5-cm long fascial sheath located over the posterior aspect of the knee [36]. The direct arm inserts at the posteromedial tibia below the joint line. The anterior arm attaches to the tibia deep to the MCL. The semimembranosus bursa is J-shaped and is wrapped around the direct and anterior arms of the semimembranosus. Its inner part is between the tendon and the tibia, while the outer part is superficial to the tendon [37]. On ultrasound, the groove in the posteromedial aspect of the tibia is an important landmark for identification of the insertion of the direct arm of the semimembranosus tendon. It is easiest to examine the direct arm with a posterior sagittal approach, using this bony landmark (Fig. 19). The anterior arm of the semimembranosus tendon can be visualized with an anterior approach, just posterior to the plane used to obtain a longitudinal image of the MCL (Fig. 20). Along the posteromedial aspect of the knee, the area where the semimembranosus tendon is seen adjacent to the medial

Fig. 19 A 28-year-old female volunteer. Normal insertion of the direct arm of the semimembranosus. Longitudinal ultrasound image. Note the fibrillar structure of the tendon (arrowheads). It is seen inserting into the groove (arrow) at the posterior tibia (Tib). Hypoechogenicity in the insertion is normal anisotropy. s superior, i inferior, p posterior. Insert probe position for the longitudinal image of the direct arm

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Fig. 20 A 28-year-old female volunteer. Normal insertion of the anterior arm of the semimembranosus. This ultrasound imaging plane is similar to that for the medial collateral ligament, but slightly more posterior. Note the anterior arm of the semimembranosus (arrows). The tibial groove is less distinct than more posteriorly at the level of the direct arm. The anterior arm is covered by fibers of the posterior part of the MCL (arrowheads). Fe femur, Ti tibia, s superior, i inferior

gastrocnemius tendon is identified in the transverse plane (Fig. 21). With different degrees of angulation of the probe one tendon may appear hyperechoic and the other hypoechoic (Fig. 22). Hypoechoic tendons should not be mistaken for evidence of a Baker’s cyst, since this is the area where Baker’s cyst typically occurs. Slightly above this level the semitendinosus tendon is seen overlying the semimembranosus muscle belly. The appearance resembles a “cherry on a pie” (Figs. 23, 24). Slightly below the level where the semimembranosus tendon is seen adjacent to the gastrocnemius tendon, the oblique popliteal arm of the semimembranosus tendon can be visualized as a thick fibrillar structure extending from the common semimembranosus tendon to the posterior knee capsule (Figs. 25, 26).

Lateral supporting structures The lateral retinaculum is rarely injured, although it may be sectioned in the treatment of femoropatellar instability. It is not

F i g . 2 1 Tr a n s v e r s e a n a t o m i c a l s l i c e . N o r m a l c o m m o n semimembranosus and gastrocnemius tendons. Semimembranosus tendon (Sm) is immediately adjacent to the medial gastrocnemius tendon (Gc). Also note the sartorius (Sa), gracilis (Gr), and semitendinosus (St). Curved arrow medial gastrocnemius muscle belly

Fig. 22 A 49-year-old female patient with anterior knee pain. Transverse ultrasound images at the level of Fig. 21. In a, obtained with superior tilting of the probe, note the hyperechoic semimembranosus tendon (Sm) and the hypoechoic medial gastrocnemius tendon (Gc). Note the inverse relationship in b, with inferior tilting of the probe. Hypoechoic tendon should not be mistaken for a Baker’s cyst. Ti tibia

typical to observe a distinct lateral patellofemoral ligament (LPFL), in contrast to the MPFL [19, 38]. The lateral supporting structures of the knee have also been described using a three-layer approach [39]. The superficial layer contains the iliotibial band and the biceps femoris tendon. The middle layer contains the retinacular structures. The deepest layer corresponds to the joint capsule and encompasses the fibular collateral ligament (FCL) and the posterolateral corner ligaments. The iliotibial band inserts onto Gerdy’s tubercle. Fibers of both the iliotibial band and the anterior oblique band of the

Fig. 23 Transverse anatomical slice. Normal posteromedial aspect of the knee. Note the muscle belly of the semimembranosus (Sm) with fatty atrophy in this specimen, and the overlying semitendinosus tendon (St)

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Fig. 24 A 28-year-old female volunteer. Transverse ultrasound image at the same level as in the anatomical image of Fig. 23. Note the semitendinosus tendon (St) overlying the semimembranosus muscle belly (Sm). This results in a “cherry on a pie” appearance. a anterior. Insert probe positioning for the transverse ultrasound image of the posteromedial knee

fibular collateral ligament (also termed anterolateral ligament) extend toward an area on the tibia anterior to the fibular head and posterior to Gerdy’s tubercle. This common insertion may explain the occurrence of a Segond fracture in this area [40]. The FCL is a cord-like structure that originates at the lateral femoral condyle and courses toward the lateral aspect of the fibular head, where it converges with the biceps tendon (Fig. 27) [41]. The popliteal tendon inserts into the popliteal groove of the femur. It is an intra-articular, extrasynovial tendon that is strongly adherent to the joint capsule. The popliteus tendon is rarely injured at its femoral insertion. More commonly, the musculotendinous junction is injured. Ultrasound, with its dynamic capability, can be used to evaluate a snapping popliteus tendon [11]. The biceps femoris is composed of a short and a long head. The short head arises from the posterior aspect of the femur, while the long head arises from the ischial tuberosity. Distally, both the short and long heads terminate in capsular,

Fig. 25 Transverse anatomical slice. Normal posteromedial aspect of the knee. Section slightly below the level depicted in Fig. 21. At this level the semimembranosus tendon (arrow) is seen giving off the oblique popliteal extension (arrowheads). Also, note the semitendinosus tendon (St) and the tendon of the medial gastrocnemius (curved arrow)

Fig. 26 Cadaver. Transverse ultrasound image at the same level as the anatomical image in Fig. 25. The semimembranosus tendon (arrow) is seen to give off its oblique popliteal extension (arrowheads). Note the tendon of the medial gastrocnemius (curved arrow) and the muscle (Gc). Fe femur, m medial, l lateral

aponeurotic, and tendinous insertions. Both heads have two tendinous insertions termed the direct and anterior arms [36, 42–44]. The direct arm of the short head inserts just medial to the FCL. It is continuous in an anterior arm that inserts onto the lateral aspect of the tibia approximately 1 cm posterior to Gerdy’s tubercle [36, 42–44]. This insertion is located inferior to the insertion site of the anterior oblique band of the FCL. The direct arm of the long head inserts at the posterolateral aspect of the fibula. The anterior arm of the long head is located along the lateral aspect of the fibular head, crossing over the lateral aspect of the FCL, and continues in an anterior aponeurosis. The fibular collateral ligament bursa is found in between the medial aspect of the anterior arm of the long head

Fig. 27 Sagittal anatomical section. Normal lateral aspect of the knee. The fibular collateral ligament (arrowheads) is seen to join the biceps tendon (Bi) to form a common insertion at the fibular head (Fib). The popliteus (Po) tendon is seen deep and slightly anterior to the femoral insertion of the fibular collateral ligament. In this specimen also note the anteriorly directed bundle of fibers originating from the fibular collateral ligament (FCL), corresponding to the anterior oblique band (arrow)

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Fig. 30 A 22-year-old male patient with mild joint effusion. Normal iliotibial band. With proximal probe displacement from the position in Fig. 29 the iliotibial band (arrows) is seen adjacent to the femoral condyle (Fe). Note the normal joint recess beneath the iliotibial band that should not be mistaken for a bursa. s superior, i inferior

Fig. 28 Coronal anatomical slice. Normal iliotibial band. Iliotibial band (arrowheads) is seen to insert onto Gerdy’s tubercle (arrow) at the anterolateral tibia

and the FCL. The so-called bifurcating distal biceps femoris refers to the anterior extension of the distal biceps [45]. Different variations of this arrangement are predisposing factors for snapping biceps femoris tendon syndrome. The most important factors are a thick cord-like anterior arm or absence of the direct arm [11]. On ultrasound, the iliotibial band is examined in the coronal plane, with the distal end of the probe placed over Gerdy’s tubercle. Gerdy’s tubercle is seen as a bony protrusion at the anterolateral aspect of the tibia, and represents a bony landmark for identification of the insertion of the iliotibial band (Figs. 28, 29) [46]. A joint recess may be seen deep to the iliotibial band, adjacent to the insertion (Fig. 29). The normal joint recess should not be mistaken for a meniscal cyst, since this is an area

Fig. 29 A 46-year-old female patient with mild joint effusion. Normal iliotibial band. Coronal ultrasound image. Iliotibial band (arrowheads) is seen to insert onto Gerdy’s tubercle (G). Normal joint recess (R), which may be variably distended, is seen deep to the iliotibial band. This should not be mistaken for a meniscal cyst, also common in this area. s superior. Insert probe positioning for the longitudinal image of the iliotibial band

where meniscal cysts typically occur. Continuity of the recess with the remainder of the joint can be demonstrated with ultrasound. The lateral joint recess extends superiorly deep to the iliotibial band and adjacent to the femoral condyle (Fig. 30). In this area, the joint recess should not be mistaken for evidence of bursitis, as may be seen in iliotibial band friction syndrome. On ultrasound images, the popliteal sulcus is seen at the posterolateral aspect of the femoral condyle and helps to localize the femoral insertion of the popliteal tendon and the FCL. Although this bony landmark may be visualized with an anterior coronal approach, the visualization of the FCL may

Fig. 31 Coronal anatomical section. Normal fibular collateral ligament. The fibular collateral ligament is seen (arrowheads). Note insertion of the popliteus tendon at the popliteal sulcus (curved arrow). Distally, note the fibers of the anterior arm of the long head (arrow) superficial to the fibular collateral ligament, and of the anterior arm of the short head (short arrow) deep to the fibular collateral ligament

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Fig. 32 A 28-year-old female volunteer. Normal fibular collateral ligament (proximal). Longitudinal ultrasound image. The FCL (arrowheads) is seen overlying the popliteus tendon located in the popliteal sulcus of the femur (arrow). Fe femur. Insert probe positioning for the longitudinal image of the FCL

sometimes be improved with a posterior approach (Figs. 31, 32). To assess the FCL in the coronal plane, the inferior side of the probe is slightly tilted posteriorly. Slight knee flexion may also improve visualization of the FCL. With longitudinal scanning along the distal aspect of the FCL, the anterior arms of the short and long heads of the biceps tendon can be seen wrapped around the FCL (Figs. 33, 34). With transverse scanning, one or both of the anterior arms can be seen to course toward a common insertion at the lateral aspect of the tibia.

Posterolateral corner Posterolateral corner injuries are frequently seen in combination with other injuries such as PCL tear [39]. Untreated injuries may lead to chronic instability and cruciate graft failure. Wijdicks et al. showed that surgical reconstruction in grade 3 injuries improved clinical outcome [8]. The anatomy of the posterolateral corner is complex. There is considerable anatomical variability. The prevalence of the different ligaments on dissection and imaging is not agreed upon [47–52]. The fabellofibular ligament courses from the fabella to the styloid process of the fibula (Figs. 35, 36, 37). When the

Fig. 33 A 33-year-old male volunteer. Normal FCL (distal aspect). Longitudinal ultrasound image. The FCL (arrow) is seen. Note the anterior arm of the long head laterally and the anterior arm of the short head medially (arrowheads), relative to the FCL. s superior, i inferior, Fi fibula

Fig. 34 A 47-year-old male patient with medial knee pain. Normal biceps tendon with a prominent anterior arm. Sagittal proton densityweighted MR image. Note the prominent anterior arm of the long head (arrow) of the biceps (Bi). The direct arm is also seen (arrowhead). Fcl fibular collateral ligament, Fib fibula

fabella is absent, the fabellofibular ligament inserts onto the posterior aspect of the femoral condyle. The popliteofibular ligament is consistently present and consists of a short attachment of the popliteal tendon to the fibular head [39, 47]. The arcuate ligament has a Y-shaped configuration and corresponds to a thin layer of connective tissue fanning from the fibular styloid (Fig. 37). It courses over the musculotendinous junction of the popliteus to divide into a medial and lateral limb. The lateral limb attaches to the lateral capsule, whereas the medial limb attaches to the posterior capsule. The fabellofibular ligament and arcuate ligament have been reported to be inversely related [39, 47].

Fig. 35 Normal fibular insertion sites of posterolateral corner structures. Note the insertion site of the long (lB) and short (sB) heads of the biceps, the fibular collateral ligament (Fcl), the popliteofibular ligament (Pop), and the fabellofibular and arcuate ligament (Fab-Arc). Anterior extension of the biceps tendon is also seen (aB). Ti tibia

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Fig. 36 Anatomical dissection. Normal anatomy of the posterolateral corner. Note the fabellofibular ligament (arrowheads) coursing from the fibula (Fi) toward the fabella (Fa) over the popliteus tendon (Po). The popliteofibular ligament (instrument) is pulled from beneath the fabellofibular ligament. Note the anatomical variation in this dissected specimen, with a few delicate fibers from the fabellofibular ligament attaching to the popliteus tendon. Note the more anterior fibular collateral ligament (arrow). Gc gastrocnemius, a anterior, p posterior

The plantaris muscle is a small vestigial muscle that is absent in 10 % of the population [49, 50]. Superiorly, it inserts at the medial aspect of the linea aspera of the femur and the posterior capsule [53–57]. It is located posterior to the

Fig. 37 Coronal anatomical slice. Normal posterolateral corner. Note the fabellofibular ligament (arrow) coursing from the fibula to the posterior aspect of the femoral condyle. No fabella is seen in this specimen. The arcuate ligament is located more medially (arrowhead). Bi biceps, Pop popliteal tendon

Fig. 38 Anatomical dissection of the posterolateral aspect of the knee. Normal plantaris muscle. The plantaris (Pla) is seen to insert (arrow) medial to the lateral gastrocnemius tendon (Gc). a anterior, p posterior

popliteal muscle and anteromedial to the lateral gastrocnemius muscle (Fig. 38). The tendon courses inferiorly between the medial head of the gastrocnemius muscle and the soleus muscle. Distally, it attaches to the calcaneus medial to the Achilles tendon. The plantaris may also insert onto the aponeurosis, separating the medial gastrocnemius and soleus, or more inferiorly onto the medial aponeurosis of the ankle, or even the plantar fascia [58]. Identification of the posterolateral corner ligaments on ultrasound is considered to be difficult. Sekiya et al. were successful in identifying the ligaments in a cadaver study [51]. Probe positioning is only slightly different for the posterolateral corner ligaments, since they all insert into a small area on the fibula.

Fig. 39 A 40-year-old male patient with medial knee pain. Normal fabellofibular ligament. Slightly hypoechoic fibrillar band-like structure (arrows) is seen coursing from the fibula (Fib) to the fabella (Fab). Tib tibia, s superior. Insert ultrasound position for the longitudinal image

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Fig. 40 A 22-year-old male volunteer. Normal popliteofibular ligament. Note the hypoechoic band-like structure (arrows) coursing from the fibular head (Fib) toward the popliteus tendon (Pop), immediately adjacent to the tibia. Inferior geniculate vessels are seen superficial to the ligament (arrowhead). Tib tibia, Fib fibula, s superior. Insert probe position for the longitudinal image

To visualize the fabellofibular ligament the inferior side of the probe is placed along the fibular head, while the superior side of the probe is placed along the fabella. When the fabella is absent the superior side of the probe is placed along the posterior aspect of the lateral femoral condyle. The fabellofibular ligament is generally the longest ligament, and is seen as a hyperechoic fibrillar band (Fig. 39). The ligament may be thin or present a curvilinear shape, and then it is difficult to visualize with ultrasound. To visualize the popliteofibular ligament, the probe is slightly displaced medially and the superior side of the probe is tilted inferiorly, starting from the initial position for the fabellofibular ligament. The popliteus tendon is then brought into view. The popliteofibular ligament is seen as a hypoechoic band coursing immediately adjacent to the tibia, between the fibula and the musculotendinous junction of the popliteus (Fig. 40). The inferior geniculate artery courses superficial to the popliteofibular ligament. As an alternative visualization technique, the course of the popliteus tendon may be followed from its femoral insertion inferiorly toward the musculotendinous junction.

Fig. 41 A 33-year-old male volunteer. Normal arcuate ligament. Note the delicate hypoechoic band (arrows) coursing from the fibular head toward the posterior capsule. Ti tibia, Fi fibula. Probe position is located between those for the fabellofibular and popliteofibular ligaments

Fig. 42 Normal plantaris muscle. Transverse anatomical section. At the posterolateral aspect of knee, note the plantaris (Pla) along the medial aspect of the lateral gastrocnemius (Gc). Bi biceps

The arcuate ligament is the most delicate structure. Probe positioning is located between those described for the previous two ligaments. The arcuate ligament is located more superficially than the popliteofibular ligament, but deeper than the fabellofibular ligament. It is seen as a delicate hypoechoic band (Fig. 41). The plantaris muscle may best be visualized with transverse scanning (Figs. 42, 43). The insertion of the plantaris muscle can be seen medial to the insertion of the lateral head of the gastrocnemius muscle. The muscle can then be followed inferiorly, where it obtains a triangular shape in cross-section. At the level of the proximal tibia the muscle is situated between the lateral head of the gastrocnemius muscle posteriorly and the

Fig. 43 A 28-year-old female volunteer. Normal plantaris muscle. Transverse ultrasound scan just distal to the femoral insertion. The lateral gastrocnemius (Gc) is seen anterolateral to the plantaris (Pla). Fe femoral condyle, l lateral. Insert probe position for the transverse image of the plantaris muscle

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popliteus muscle anteriorly. Inferiorly, the plantaris muscle is in continuity with its tendon.

Conclusion Ultrasound, with its exquisite resolution, allows a detailed assessment of the supporting structures of the knee. Ultrasound shows anatomical detail of the superficial structures that is difficult to obtain with other imaging modalities. Knowledge of the anatomical complexity of the different knee areas is required for an accurate ultrasound interpretation. We discussed the ultrasound anatomy of the knee with a focus on the quadriceps tendon, medial retinaculum, MCL, posteromedial corner, lateral supporting structures, and the posterolateral corner. The radiologist should be aware of the normal anatomy of these areas, and the proper examination technique. Acknowledgements Annemieke Milants, MD, Eddy Broodtaerts, Dominique Doms, Jo De Neef, Jan Moerman (Vrije Universiteit Brussel, Belgium), Leon Lenchik, MD (Wake Forest University, Winston Salem, NC, USA). Conflict of interest The authors declare that they have no conflict of interest.

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