Musculoskeletal ultrasound of the knee

INTRODUCTION — The extra-articular structures of the knee are readily accessible for detailed sonographic evaluation. While intra-articular evaluation is incomplete, many structures can be partially visualized, and the examination often provides useful clinical information.

This topic will describe a systematic approach to complete sonographic evaluation of each knee quadrant. Topics devoted to knee pain and specific knee conditions are found separately. (See "Approach to the adult with knee pain likely of musculoskeletal origin" and "Anterior cruciate ligament injury" and "Posterior cruciate ligament injury" and "Medial (tibial) collateral ligament injury of the knee" and "Meniscal injury of the knee" and "Knee bursitis" and "Osgood-Schlatter disease (tibial tuberosity avulsion)".)

USES, ADVANTAGES, AND LIMITATIONS OF KNEE ULTRASOUND — Below, the advantages and limitations of the ultrasound examination of the knee are reviewed; the general advantages and limitations of musculoskeletal ultrasound are discussed separately. (See "Musculoskeletal ultrasound of the shoulder", section on 'Uses, advantages, and limitations of shoulder ultrasound' and "Musculoskeletal ultrasound of the ankle and hindfoot".)

Ultrasound evaluation of the knee offers many benefits. Its superior spatial resolution allows for detailed evaluation of tendon, ligament, and nerve disorders that may not be possible with other imaging modalities. Mechanical complaints of snapping or clicking can be assessed dynamically in real time, and the location of pain can be correlated with precise anatomic structures by palpation with the ultrasound transducer (ie, sonopalpation). Orthopedic hardware does not produce artifacts in ultrasound images, allowing for evaluation of surrounding structures for signs of impingement, irritation, or injury. However, although many extra-articular knee structures are easily accessible for sonographic examination, intraarticular structures are not, and this inherent limitation of ultrasound technology means that other imaging modalities may be required to assess the knee joint thoroughly.

KNEE ANATOMY — For the purposes of sonographic evaluation, the knee is divided into four anatomic quadrants: anterior, lateral, medial, and posterior. These quadrants are divided by the long axis of the femur and tibia:

●Anterior – Structures evaluated in the anterior knee quadrant include (picture 1 and picture 2 and figure 1): quadriceps muscles and tendon (figure 2 and figure 3); suprapatellar recess of knee joint; patella (figure 4); prepatellar bursa; patellar tendon (ligament) (figure 4); superficial infrapatellar bursa; deep infrapatellar bursa; medial patellar retinaculum and medial patellofemoral ligament (figure 2); lateral patellar retinaculum (figure 2); distal femoral cartilage.

●Lateral – Structures evaluated in the lateral knee quadrant include (figure 5 and picture 3): iliotibial band; lateral meniscus (figure 6); fibular (peroneal) collateral ligament (figure 7); biceps femoris muscle and tendon (picture 4); popliteus muscle and tendon (picture 5); lateral patellar retinaculum (figure 2); proximal tibiofibular joint (picture 6); common fibular (peroneal) nerve (figure 8 and picture 4).

●Medial – Structures evaluated in the medial knee quadrant include: tibial collateral ligament (picture 7); medial meniscus (figure 6); pes anserine tendons and bursa (picture 8); medial patellar retinaculum (figure 2).

●Posterior – Structures evaluated in the posterior knee quadrant include (picture 9 and picture 6 and picture 4): semimembranosus muscle and tendon; semitendinosus muscle and tendon; medial gastrocnemius muscle and tendon; semimembranosus-medial gastrocnemius bursa; popliteal artery and vein (figure 9); sciatic, tibial, and common fibular nerves (picture 4); posterior horns of medial and lateral meniscus; posterior cruciate ligament (picture 6); lateral gastrocnemius muscle and tendon (figure 9).

Knee anatomy is discussed in greater detail separately. (See "Physical examination of the knee", section on 'Anatomy'.)

ULTRASOUND EXAMINATION OF THE KNEE

Guidelines, structures to image, and positioning — The sonographic examination of the knee is organized by quadrants: anterior, lateral, medial, and posterior. It is often appropriate to focus the sonographic examination on the region of the patient’s complaints, but other structures may need to be evaluated as clinically indicated, and examinations may involve more than one quadrant. A list of structures to be evaluated in each quadrant is provided above. These lists are drawn from the American Medical Society for Sports Medicine (AMSSM) Recommended Sports Ultrasound Curriculum for Sports Medicine Fellowships and the American Institute of Ultrasound in Medicine (AIUM) Guidelines for Performance of the MSK US Examination [1,2]. The approach described is consistent with that of the European Society of Musculoskeletal Radiology [3,4].

A high frequency (eg, 12 to 5 MHz) linear array transducer is preferred for the diagnostic evaluation of the knee, unless a large body habitus makes this infeasible. All structures should be imaged in both short and long axis, and any possible pathology confirmed with orthogonal imaging. Doppler evaluation may be needed to evaluate possible synovitis, bursitis, and intratendinous neovascularization, as is commonly seen in chronic tendinosis.

Anterior knee — To examine the anterior knee, the patient lies supine with their knee flexed to approximately 30 degrees.

The transducer is placed in the transverse plane at the anterior aspect of the mid-thigh (picture 10). Within this scanning plane, the hyperechoic deep curvilinear structure is the femur. Superficial to the femur lies the vastus intermedius, and superficial to it the rectus femoris (image 1). Medial to these two structures is the vastus medialis (image 2), and lateral is the vastus lateralis (image 3). The transducer is then moved in the transverse plane inferior to the musculotendinous junction of each muscle. In this image, the merging of the vastus medialis and lateralis muscles into the middle layer of the quadriceps tendon is appreciated, with the superficial layer of the tendon (rectus femoris) and deep layer (vastus intermedius) also visualized (image 4). The transducer is moved inferior in a systematic fashion to visualize the insertion of the quadriceps tendon onto the patella. (See "Quadriceps muscle and tendon injuries".)

At this point, the transducer is rotated to the sagittal plane to assess the three layers of the quadriceps tendon in the long axis (image 5). The transducer is moved medially and laterally to visualize the entire width of the quadriceps tendon. The quadriceps fat pad can be visualized proximally, deep to the insertion of the quadriceps tendon onto the patella (image 6). In the same image, the prefemoral fat pad can be visualized just superficial to the hyperechoic line representing the femur and proximal to the patella. Between the quadriceps tendon and prefemoral fat pad lies the potential space of the suprapatellar recess (image 6). This recess should be assessed for an effusion medially and laterally in the sagittal plane and then superiorly and inferiorly in the transverse plane. If no fluid is detected, the knee can be flexed further in an attempt to move intraarticular fluid into the suprapatellar recess. Parapatellar pressure and patient-initiated quadriceps contraction can also be used to improve detection of knee joint effusion [5].

Next, the transducer is moved inferiorly over the patella in the sagittal plane (image 7). In this plane, the patella is visualized as a hyperechoic line. The transducer is moved medially and laterally to visualize the entire width of the patella. A discontinuity in the hyperechoic surface of the patella suggests a bipartite patella or patellar fracture, a distinction that is made clinically. The patella should also be imaged in the transverse plane, moving the transducer superior and inferior, and then medially and laterally, to visualize the entire structure (image 8). (See "Patella fractures".)

The same maneuvers should be repeated in the sagittal and transverse planes while floating the transducer over a layer of acoustic coupling gel to evaluate for prepatellar bursitis (superficial to the patella). Even light pressure from the transducer can compress bursal fluid resulting in a false negative examination, thus careful technique is required. One should note the most superficial fibers of the quadriceps tendon, and follow them inferior in the sagittal plane, appreciating continuity with the patellar periosteum and then with the proximal patellar tendon (image 7 and image 8).

While in the transverse plane over the patella, the transducer should be moved medially so that the lateral edge of the transducer is fixed on the patella, and the medial edge is fixed on the adductor tubercle of the medial femoral condyle (picture 11). In this image, the fibrillar structure connecting both bony structures would be the medial patellofemoral ligament (image 9). The medial recess is deep to the medial patellofemoral ligament and should be assessed for fluid, ensuring minimal pressure is applied. The cartilage of the medial patellar facet can be visualized at this point by applying a medially directed force with one finger on the lateral border of the patella.

Although the lateral patellar facet cannot be imaged in a similar manner, the lateral patellar retinaculum (relatively thinner fibrillar structure) and lateral recess can be imaged by sliding the transducer laterally (image 10). The cartilage of the femoral condyles and trochlea can be visualized by placing the transducer in the transverse plane, proximal to the proximal pole of the patella and fully flexing the knee (image 11).

To begin imaging the infrapatellar region, the transducer is rotated to the sagittal plane and positioned so the superior edge of the transducer is fixed on the distal pole of the patella, while the inferior edge is fixed on the tibial tubercle if possible (picture 12). The fibrillar structure connecting both bony structures is the patellar tendon (image 12). It is often difficult to image the patella tendon in its entirety due to the limited field of view of the ultrasound transducer, and distal sliding of the transducer from the patella is required to image the patellar tendon attachment at the tibial tubercle. Hoffa’s infrapatellar fat pad is visualized deep to the proximal patellar tendon. The potential spaces of the superficial infrapatellar bursa and the deep infrapatellar bursa lie superficial and deep to the distal patellar tendon, respectively, and should be assessed for fluid. The transducer is then moved medially and laterally to visualize the entire width of the patellar tendon in its long axis, appreciating its continuity with the patellar periosteum and the superficial layer of the quadriceps tendon (image 12). The transducer is then rotated into the transverse plane to image the entire width of the patellar tendon and moved superior and inferior to visualize along its entire length (image 13).

Lateral knee — To examine the lateral knee, the patient lies on their side with a pillow or bolster between their knees (picture 13).

The transducer is placed in the long axis of the patellar tendon (sagittal plane) and then moved laterally over the tibia until Gerdy’s tubercle is visualized with the inferior end of the transducer. With the inferior end of the transducer fixated on this point, the proximal edge is pivoted to optimize the image of the fibrillar structure, the iliotibial band (image 14). Deep to the iliotibial band the joint line can be visualized, including a portion of the lateral meniscus. The iliotibial band should be scanned in its long axis cranially over the lateral femoral condyle. At this position, the transducer is rotated into the transverse plane to image the iliotibial band over the lateral femoral condyle (image 15).

Ensuring that the lateral femoral condyle is optimally viewed (hyperechoic line) allows the iliotibial band to be identified easily in the short axis and any fluid within the iliotibial band bursa or lateral joint recess to be identified. Note should be made of any pain with direct sonopalpation, as this is the common site for iliotibial band friction syndrome. Dynamic evaluation of patients with complaints of lateral knee snapping can be performed by asking the patient to flex and extend the knee while observing for abrupt movement of the iliotibial band over the posterior aspect of the lateral femoral condyle. The iliotibial band can then be scanned superior and inferior from this point. (See "Iliotibial band syndrome".)

If the transducer is moved over the distal iliotibial band in the long axis, the joint line and lateral meniscus can be visualized deep to it, and both structures should be examined along their entire width by sliding the transducer anteriorly and posteriorly (image 14). Proximal to the joint line is a concave fossa in the femoral condyle, which houses the popliteus tendon (image 16). The popliteus tendon can be visualized best in a coronal-oblique plane using a piecemeal approach, given the tendon’s tortuous course, but its muscular portion is best visualized as part of the posterior examination of the knee (image 17). The other lateral structures of the knee can be visualized in the transverse plane with the iliotibial band seen in short axis at the anterior edge of the tibia (image 18).

Posterior to the iliotibial band (from anterior to posterior) are the fibular collateral ligament, biceps femoris tendon, and common fibular nerve. Each of these structures should be visualized in both long and short axis. The distal biceps femoris tendon splits into superficial and deep sections, with the former running superficial to the fibular collateral ligament, and the latter deep to the fibular collateral ligament, before the tendon and ligament insert onto the fibula. This can be visualized in long axis, but the course of the distal biceps femoris tendon and its division is best visualized in short axis (image 19).

Note that the biceps femoris insertion onto the tibia can vary, which may introduce anisotropy that should not be confused for the changes of tendinosis. When imaging the fibular collateral ligament inserting onto the fibula in long axis, the tibiofibular joint can be seen cranial to the insertion point. Although the lateral patellar retinaculum can be imaged from this position, the supine position is preferred as effusions in the lateral recess are more apparent.

Medial knee — To examine the medial knee, the patient lies on their ipsilateral side with the hip and knee on that side in slight flexion and the contralateral hip in neutral position (picture 14).

The medial joint line is first palpated, and the transducer is placed in the coronal plane of the medial knee (image 20). The medial joint line should be aligned with the middle of the screen. Between the femur and tibia, a hyperechoic structure is visible, which is the body of the medial meniscus. The transducer can be moved anteriorly to view the anterior horn of the medial meniscus. Orienting the transducer in the coronal plane over the body of the medial meniscus, a fibrillar structure comes into view superficially, which is the medial collateral ligament in long axis (image 20). The medial collateral ligament has a superficial layer, the tibial collateral ligament, which can be scanned in long axis from its origin at the adductor tubercle to its insertion on the proximal tibial metaphysis. Deep to this structure lie the deep layers of the medial collateral ligament, the meniscofemoral and meniscotibial ligaments, which attach to the medial meniscus of the femur and tibia, respectively. (See "Meniscal injury of the knee" and "Medial (tibial) collateral ligament injury of the knee".)

If one has difficulty identifying the medial collateral ligament, its borders, or extent in long axis, relative anisotropy of this fibrillar structure with the adjacent fatty tissue can be used to accentuate the structure. Between the deep and superficial layers of the medial collateral ligament lies the potential space for Voshell’s bursa. Both layers of the medial collateral ligament should also be scanned in short axis from their origin to insertion (image 21). Although the medial patellofemoral ligament can be appreciated in this plane, anterior to the mid-portion of the medial collateral ligament, it is advised to scan this structure with the patient in the supine position, to optimally assess the medial recess for an effusion.

The pes anserine tendons are, from anterior to posterior, the sartorius, gracilis and semitendinosus. To view these structures, the transducer is first aligned in the long axis of the distal medial collateral ligament at the proximal tibial metaphysis where the pes anserine tendons are seen in a short-axis oblique plane, superficial to the ligament. Given their oblique course relative to the medial collateral ligament, the tendons are often hypoechoic due to anisotropy. This relative anisotropy can be helpful in identifying the tendons but should not be confused for pathology (image 22). If one encounters difficulty identifying the tendons at this location, they can be identified more easily individually in the posterior knee and traced distally in both short and long axis to their insertions. Between the pes anserine tendons and medial collateral ligament lies the pes anserine bursa (image 23).

Posterior knee — To examine the posterior knee, the patient lies in a prone position with the feet supported by a bolster, which flexes the knee slightly.

The transducer is placed in the transverse plane in the medial aspect of the posterior distal thigh (picture 15). In this plane the superficial semitendinosus tendon and deep semimembranosus muscle are identified in short axis (image 24). The transducer is moved distally over the musculotendinous junction of the semimembranosus, and further distally to identify the semimembranosus tendon in short axis as it inserts onto the tibia (image 25). The semimembranosus tendon should also be scanned in the long axis (image 26). (See "Hamstring muscle and tendon injuries".)

To identify a Baker’s cyst, the semimembranosus tendon is scanned in short axis, proximal to its insertion onto the tibia, where it lies superficial to the articular cartilage of the posterior femoral condyle (image 27). At this location, a hypoechoic structure is noted lateral to the semimembranosus tendon, which is the medial gastrocnemius tendon. The medial gastrocnemius tendon appears hypoechoic (due to anisotropy) as it courses in an oblique plane relative to the semimembranosus tendon at this level. Tilting the transducer superiorly optimizes visualization of the medial gastrocnemius tendon but introduces anisotropic hypoechogenicity to the semimembranosus tendon. Careful scanning technique is required to accurately identify the borders between the two tendons and not mistake anisotropy for a small Baker’s cyst. A true Baker’s cyst must demonstrate a neck arising between the semimembranosus and medial head of the gastrocnemius tendons (image 28). If a Baker’s cyst is identified in the transverse plane, the transducer should be rotated to the sagittal plane to identify its superior and inferior extent. (See "Popliteal (Baker's) cyst".)

To assess both heads of the gastrocnemius muscle, the transducer is placed in the transverse plane over the proximal posterior lower leg, where the medial and lateral heads of the gastrocnemius muscle are identified in short axis (picture 16). Both heads of the muscle should be evaluated from origin to insertion, taking care to evaluate the entire width (image 29). After scanning both heads of the muscle in short axis, the transducer should be oriented in the sagittal plane for long-axis evaluation. The most common site of pathology is the myotendinous junction of the medial gastrocnemius where it inserts onto the aponeurosis that eventually forms a portion of the Achilles tendon (image 30). (See "Calf injuries not involving the Achilles tendon".)

The transducer is then placed in the transverse plane, on the posterior aspect of the knee in the popliteal fossa (picture 17). In this plane, moving from superficial to deep, the tibial nerve, popliteal vein, and popliteal artery can be visualized in their short axis (image 31). The popliteal and tibial veins should be compressible in this region. If not, a deep vein thrombosis should be suspected. (See "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)

Once identified, the tibial nerve can be scanned proximally until it adjoins the common fibular nerve to form the sciatic nerve. Deep to the neurovascular structures, the popliteus muscle can be identified in its long axis when the transducer is oriented in the transverse-oblique plane in the popliteal fossa (image 32). The popliteus muscle can be scanned laterally to its insertion as described above. (See 'Lateral knee' above.)

With the transducer again in the transverse plan in the popliteal fossa, a limited portion of the anterior cruciate ligament can be visualized in short axis in the lateral aspect of the intercondylar notch (image 33). The transducer is then rotated to the sagittal plane, midline in the popliteal fossa. In this plane, the posterior cruciate ligament is seen attaching to the characteristic bony contour of the tibia (image 34). The posterior horns of the lateral and medial menisci are visualized by moving the transducer laterally and medially, respectively, from this midline position (image 35). (See "Anterior cruciate ligament injury" and "Posterior cruciate ligament injury".)

ADDITIONAL ULTRASOUND RESOURCES — Instructional videos demonstrating proper performance of the ultrasound examination of the knee and related pathology can be found at the website of the American Medical Society for Sports Medicine: anterior knee US examination, medial knee US examination, lateral knee US examination, posterior knee US examination, sports US knee pathology, US guided interventional procedures of the knee. Registration must be completed to access these videos, but no fee is required.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Musculoskeletal ultrasound" and "Society guideline links: Knee pain".)

SUMMARY AND RECOMMENDATIONS

●Use and limitations of ultrasound knee examination – The knee is susceptible to a variety of traumatic and nontraumatic injuries. Due to the relatively superficial location of the knee and its surrounding soft tissues, many of these pathologic conditions can be diagnosed or assessed using ultrasound. The advantages of ultrasound include portability, absence of radiation, and dynamic imaging capability. Incomplete evaluation of intraarticular structures is an inherent limitation of ultrasound, and other advanced imaging modalities may be required for complete evaluation. (See 'Uses, advantages, and limitations of knee ultrasound' above.)

●Anatomy – An understanding of the anatomy of the knee and surrounding structures is essential for interpreting ultrasound images. A brief outline of the structures assessed as part of the fundamental knee examination is provided above; detailed descriptions of knee anatomy and biomechanics are provided separately. (See 'Knee anatomy' above and "Physical examination of the knee", section on 'Anatomy' and "Physical examination of the knee", section on 'Biomechanics'.)

●Transducer selection – A high frequency linear array transducer is preferred for evaluation of the superficial structures of the knee. However, a low frequency curvilinear array transducer may be required for adequate imaging of the deeper structures or in larger patients. (See 'Guidelines, structures to image, and positioning' above.)

●Anatomy-based examination – Sonographic evaluation of the knee is organized into a quadrant system as described in the text. The major quadrants and structures examined include the following:

•Anterior – Structures evaluated in the anterior knee quadrant include (picture 1 and picture 2 and figure 1): quadriceps muscles and tendon (figure 2 and figure 3); suprapatellar recess of knee joint; patella (figure 4); prepatellar bursa; patellar tendon (ligament) (figure 4); superficial infrapatellar bursa; deep infrapatellar bursa; medial patellar retinaculum and medial patellofemoral ligament (figure 2); lateral patellar retinaculum (figure 2); distal femoral cartilage.

•Lateral – Structures evaluated in the lateral knee quadrant include (figure 5 and picture 3): iliotibial band; lateral meniscus (figure 6); fibular (peroneal) collateral ligament (figure 7); biceps femoris muscle and tendon (picture 4); popliteus muscle and tendon (picture 5); lateral patellar retinaculum (figure 2); proximal tibiofibular joint (picture 6); common fibular (peroneal) nerve (figure 8 and picture 4).

•Medial – Structures evaluated in the medial knee quadrant include: tibial collateral ligament (picture 7); medial meniscus (figure 6); pes anserine tendons and bursa (picture 8); medial patellar retinaculum (figure 2).

•Posterior – Structures evaluated in the posterior knee quadrant include (picture 9 and picture 6 and picture 4): semimembranosus muscle and tendon; semitendinosus muscle and tendon; medial gastrocnemius muscle and tendon; semimembranosus-medial gastrocnemius bursa; popliteal artery and vein (figure 9); sciatic, tibial, and common fibular nerves (picture 4); posterior horns of medial and lateral meniscus; posterior cruciate ligament (picture 6); lateral gastrocnemius muscle and tendon (figure 9).