• Discuss the normal and variant musculoskeletal anatomy of the elbow, knee and ankle
• Describe the meaning and appearance of common joint and tendon injuries
• Compare radiographic and sonographic imaging for diagnosis of musculoskeletal injury

The written request for musculoskeletal ultrasound should provide sufficient information to demonstrate the medical necessity of the examination and allow for the proper performance and interpretation of the requested examination. The request should cover the signs and symptoms and /or relevant history. The study may be a full evaluation of the entire joint or a partial study tailored to the area of interest. During the ultrasound study patients can often elaborate on their clinical symptoms. Most patients, especially sports injury patients, will be only too glad to describe the mechanisms of injury, areas of point tenderness and symptomatic changes with motion. By listening and handling patients with care, especially these patients with a history of trauma or focal pain, a more accurate diagnosis can be more readily achieved. Generally, patients will try to help with the study and gaining their cooperation can be the key to successful examination, especially if it takes several tries to get the best images. A physician should be available for case–by- case consultation and review either on site or on a regular basis.

Comparisons are useful. If a tendon or other structure appears to be abnormal, check the asymptomatic side for changes in structure and size. Such comparisons are a quick learning tool and document the range of normal variation. Correlate ultrasound findings with other imaging modalities, especially conventional radiography that often is available at the time of the ultrasound study. A second look at the radiograph may help confirm an ultrasound diagnosis. Unlike general ultrasound, in musculoskeletal ultrasound the scanning planes are longitudinal and transverse to the specific structure undergoing evaluation. Abnormalities should be measured in both of these planes. Since ultrasound planes may appear unfamiliar, liberal use of an anatomic atlas or textbook may be needed for comparison during the examination. Ultrasound is operator dependent and it is better to take the time to correctly identify a structure during the examination than try to figure it out from the images later. Before performing a patient examination on a new anatomic region, it might be useful to scan yourself or a colleague to become familiarized with the anatomy before you have to work with a patient and the alterations of pathology.

Tear of anterior tibial tendon (arrow) in a soccer player. Normal for comparison with the tendon labeled. Both views are in the longitudinal plane of the tendon.

Equipment Selection: Musculoskeletal ultrasound evaluation is best with high-quality equipment including high frequency linear transducers (10mhz and above), color flow, and split screen capabilities, although in some cases less than optimal equipment can be adequate. Checking for hyperemia with color flow helps distinguish fluid from pannus or scar from acute injury. A cine loop with color flow makes for a great workup of tendon dynamics including tendon stability.

Panoramic longitudinal view of a ruptured distal biceps tendon with 6 cm of retraction (arrows)

Panoramic views document the extent of tendon pathology as seen above. Good scanning practices also achieve better results. If a structure is superficial or tender, float the transducer lightly along the skin with lots of gel. Graded compression views should be gently performed in these areas as when trying to distinguish between pannus and fluid. Tears should have the location documented as well as the gap measured between the torn ends.

Beware of anisotropy especially when imaging tendons. Tendons are strongly anisotropic reflectors. Since tendons are composed of multiple collagen fibers in bundles, they are seen optimally when the ultrasound beam is at right angles to the reflective fibers. If the tendon is curving away or the transducer is angling away, the reflective echogenic fibers are not perpendicular to the beam. The tendon will appear hypoechoic with loss of architectural definition. Since hypoechoic areas and loss of fiber definition within tendons are the changes associated with pathology, real-time scanning should be carefully performed perpendicular to the long axis of the tendon. Careful perpendicular scanning in both longitudinal and transverse planes is generally not difficult to do even when tendons curve around other structures especially if small high frequency transducers can be used. However, at insertion sites such as the biceps tendon on the radial tuberosity or the posterior tibial tendon on the navicular, the tendon fibers twist and fan out. Anisotropy then can produce a hypoechoic, poorly defined appearance of a normal tendon.

Example of anisotropy (arrows) at the normal insertion of posterior tibial tendon on the navicular with an accessory navicular (*)

Normal Anatomy

In the normal elbow joint, the capsule folds over the ulno-humeral and radio-capitellar articulations without noticeable fluid. Two hyperechoic anterior fat pads fill the radial and coronoid fossae. Fluid is associated with intra-articular pathology in the elbow and the smallest amounts can be seen in the posterior elbow between the olecranon process and the posterior fat pad. The radiographic findings are often negative in the case of small elbow effusions.

Normal anterior elbow appearance at the radio-capitellar joint (wide short arrow) with the fat pad at the radial fossa demonstrated (thin arrow).

Anterior Elbow:
The distal biceps tendon is approximately 7 cm long and crosses the elbow to insert on the medial aspect of the radial tuberosity. The distal muscle and tendon course superficially to the brachialis muscle group with the pronator teres and common flexors medially, and the brachioradialis and common extensors laterally. In the antecubital area the biceps tendon is adjacent to the median nerve and the vessels which can be used as landmarks. The biceps aponeurosis or lacertus fibrosus connects the musculotendinous junction of the tendon to the medial deep fascia of the forearm. These thin fibers which generally are not seen on ultrasound examination lie superficial to the brachial artery and median nerve. There is no sheath around the distal biceps tendon.

Normal distal biceps tendon (arrows) with insertion deep to vein (longitudinal) .

Position: Start the anterior elbow examination with the patient facing the examiner and the extended forearm slightly flexed on the examination table. It can be useful to have both arms in this position for comparison between symptomatic and asymptotic sides. Scan in both the long axis of the arm and the transverse planes angling to follow the tendons.

The distal biceps tendon insertion on the proximal radius sometimes requires slight alterations because the fibers of the distal tendon fan out and rotate as they approach the radial tuberosity. The resulting anisotropy makes the distal tendon difficult to see. In the longitudinal plane, the elbow is extended and the forearm supinated. The radial tuberosity is scanned through the flexor compartment twisting to follow the fibers as they fan out. The transducer is moved medially through the brachial vessels and then angled laterally and slightly inferiorly to follow the fibers of the distal tendon. Since the tendon is adjacent and deep to the vessels, following the vessels helps to locate the tendon.

Finding the distal biceps tendon in the longitudinal plane.

Normal lateral common extensor tendon origin (arrows) with normal hyperechogenicity of the longitudinal tendon

Lateral Elbow
Normal Anatomy
The examination of the common extensor tendon origin on the lateral epicondyle is best performed in the lateral coronal projection. The four muscles of the common extensor tendon are superficially the extensor digitorum with the smaller extensor carpi ulnaris and extensor digiti minimi in the center and the thicker extensor carpi radialis brevis fibers as the deepest layer. It is the deep fibers of the extensor carpi radialis brevis that tend to be involved with lateral epicondylitis. The radial collateral ligament is deep to the common extensor tendon and can be seen as a separate thin echogenic band.

Medial Elbow
On the medial epicondyle, the origin of the common flexors tendons for the wrist and hand forms a hyperechoic reflective triangle when scanned in the medial coronal plane. Just inferior and deep to the common flexor origin is the anterior band of the ulnar collateral ligament running from the distal humerus to the coronoid process of the ulna. It provides the main medial stability of the elbow.

Normal medial epicondyle, common flexor tendon origin (large arrows on hyperechoic longitudinal tendon ) and ulnar collateral ligament (small arrows on hypoechoic ligament).

Position:
Start in the same position used for the anterior elbow with the patient facing the examiner and the extended forearm slightly flexed on the examination table. Again for comparison it is useful to have both arms in this position. Scan in the long axis of the arm and the transverse planes as well as the long and transverse planes of the individual structures. For evaluation of the lateral epicondyle area, better positioning is obtained when the patient is pronated slightly into a “thumbs up” position. Also both arms can be extended and the palms placed together.

Positioning for the medial elbow (left) and the lateral epicondylar area with a "thumbs up" position (right) both shown for longitudinal planes.

Posterior Elbow:
The olecranon fossa, triceps muscle, and posterior joint space can be easily seen. Fluid within the joint is best distinguished between the posterior aspect of the distal humerus and the triangular hyperechoic posterior fat pad. The ulnar nerve can be traced around the joint as it changes appearance due to anisotropy.

Position:
With the arm in 90 degrees of flexion, place the palm on the examination table and rotate the posterior elbow laterally. Scan in both the longitudinal and transverse planes with varying degrees of flexion. If loose bodies are suspected, flexing the elbow may help to show the mobility of the loose bodies. If because of elbow pain or stiffness, the patient can not tolerate this position, then have them place their hand across their chest or abdomen and scan the elbow from the back and side.

Position for the posterior elbow and the longitudinal triceps tendon on the left and following the ulnar nerve longitudinally (right)

Normal ulnar nerve at the joint in longitudinal (left) and transverse (right) planes (arrows).

Pathology

Rupture of the distal biceps tendon usually occurs when one tries to lift a heavy object and hears a popping sound, and feels a sudden pain. A palpable defect is not always present. This history generally is adequate for the diagnosis but if there is a limited physical examination due to pain or soft tissue swelling, ultrasound evaluation is appreciated since delayed repair can result in impaired function. If the elbow is slightly flexed and the forearm maximally supinated, the tendon tear should be seen on the longitudinal scan.

If there is a complete distal biceps tendon rupture, the tendon can retract above the antecubital fold, particularly if the aponeurosis, the lacertus fibrosus, also is torn. If the lacertus fibrosus is intact the clinical presentation may be confusing. Even though the tendon is completely torn the fibers of the aponeurosis keep the stump from retracting. The location of the stump can be seen by following the empty track superiorly. The defect between the stump ends can be filled with fluid or hematoma. The surgeon needs to know the location of the stump if an acute tear is to be reattached.

Panoramic view of a ruptured distal biceps tendon demonstrating the longitudinal track with trace fluid (arrows) from a rupture with 6 cm of retraction.

Transverse view of a ruptured biceps tendon (arrows).

Partial tears are more difficult to diagnosis and tend to occur closer to the radial tuberosity where the tendon swings between the proximal radius and ulna. The tendon may be thickened and echogenic like severe tendinosis but if there is a wavy contour or some of the fibers are wavy, lax and hypoechoic there is probably a partial thickness tear. Longitudinal tears may be seen as well. If fluid is seen between the distal biceps tendon and the radial tuberosity it is not tenosynovitis, since the biceps does not have a sheath, but a bicipital bursa formed from repetitive trauma and compression.

	Ruptured and retracted biceps tendon with avulsion bone fragment near the surface of the antecubital fold more than 6 cm from insertion site (arrow) seen transverse plane on ultrasound.

A painful lateral elbow is the common presentation of lateral epicondylitis or “tennis elbow”. It probably is caused by degeneration of the common extensor tendon from repetitive cycles of trauma and repair. Most patients with lateral epicondylitis do not play tennis but activities of daily living can contribute to symptoms. Lateral epicondylitis usually involves the deep fibers of the common extensor tendon which appear hypoechoic from focal degeneration. With more diffuse tendon involvement, the tendon is hypoechoic, less reflective, and loses the normal tendon’s fibrillar pattern.

Lateral epicondylitis with thick hypoechoic tendon insertion on the left compared to normal longitudinal plane on the right.

As the severity increases there may be associated tendon swelling, foci of calcification in the tendon, thickening of the adjacent tissues, and even hyperostosis of the epicondyle cortex. Bursae may form in reaction to the degenerative changes. In a partial tear of the common extensor tendon, anechoic clefts may be seen in between the fibers of the tendon. When there is a complete tear, hypoechoic fluid is seen between the torn fibers and the stumps identified. Even the underlying thin echogenic band of the radial collateral ligament can be thickened. With severe lateral epicondylitis, in particular after release surgery, the band may become partially or completely torn.

On the medial side of the elbow, the common flexor tendon origin can become de-generated by overuse and develop medial epicondylitis, known as “golfer’s elbow”. The appearance is similar to the lateral epicondylitis with focal or diffuse hypoechoic areas, tendon swelling, and changes in the adjacent tissues as well as partial or complete tears.

Medial epicondylitis with cortical irregularity and hyperostosis on the left with normal for comparison on the right, both on longitudinal planes.

Deep to the common flexor tendon is the ulnar collateral ligament, a major stabilizer of the elbow which can be injured by valgus forces that do not damage the overlying common flexor tendon.

Thick anterior band of the ulnar collateral ligament (arrows) with slight opening of the joint by valgus stress in a baseball player. Compare with the following image.

Disruption of the anterior band of the ulnar collateral ligament is often near the origin on the medial epicondyle. The ligament can be torn from repeated valgus stress as happens in baseball pitchers or in other throwing athletes. Partial tears within the substance of the ligament can be seen as hypoechoic clefts with loss of compact echogenic structure. Complete ruptures can have anechoic fluid in the gap and in between the rounded edges of the torn tendon. Dynamic valgus stress of the ligament can make the tears more visible during examination.

Complete disruption of the ulnar collateral ligament deep to the intact common flexor with fluid around the torn fibers (arrows) as seen in the longitudinal plane.

In children the apophysis of the medial epicondyle may avulse with the ulnar collateral ligament attachment resulting in a functional rupture. Cartilage can be injured or sheared off, especially from the capitulum with valgus stresses.

Chronic functional tear of the ulnar collateral ligament (ucl) with chronic medial epicondyle avulsion (arrow). Ulnar collateral ligament remains intact but motion occurs at the medial epicondyle avulsion site acting like a tear.

Anterior Knee

Normal Anatomy:
The quadriceps tendon is formed from the fibers of the four quadriceps muscles: the rectus femoris central in the thigh with the vastus intermedius deep to it and vastus medialis and the vastus lateralis located medially and laterally. The conjoined tendon fibers insert on the superior patella.

Normal quadriceps tendon insertion (arrows) on superior pole of patella

Normal patellar tendon with its origin on the patellar inferior surface (arrow). Distal insertion not included on this longitudinal view.

Some fibers of the quadriceps tendon continue over the patella to fuse with the patellar tendon that inserts on the tibial tuberosity. The patellar tendon is slightly wider at its origin on the inferior patella, and then narrows in the body of the tendon before expanding again at the tibial tuberosity insertion. The quadriceps and patellar tendons form the extensor mechanism of the knee. Neither of these tendons has a tendon sheath.

Superior to the quadriceps tendon and the patellar tendon is subcutaneous fat. Deep to the quadriceps tendon is the suprapatellar fat, the suprapatellar recess or bursa and the pre-femoral fat. Normally there is up to 2mm of fluid within the suprapatellar recess which is the largest recess or bursa around the knee. The suprapatellar recess communicates with the knee joint and articular pathology produces an effusion within it. A prepatellar bursa can develop in the subcutaneous tissues over the patella. Hoffa’s fat pad is deep to the patellar tendon. There are two potential bursae at the insertion of the patellar tendon on the tibial tuberosity, the superficial and the deep infrapatella bursae. Normally there is little to no fluid within these structures.

Chronic small suprapatellar recess or bursa effusion (arrows) deep to the quadriceps tendon.

Bursae are sacs with a small amount of fluid providing protection at points of friction. The classic synovial bursae are generally constant in location and like the suprapatellar recess or bursa, communicate with the joint. A small amount of fluid may be normal. Adventitial bursae form between friction surfaces and are more variable, usually indicating unusual stress or friction points. The suprapatellar recess, and less common, popliteal or Baker’s cyst, are extensions of the joint space and can give some indication of conditions within the knee joint. When a bursa is seen, note the general amount and character of the fluid: anechoic, hypo or hyperechoic, speckled or filled with debris. Loose bodies, especially around the knee, can become entrapped within bursae.

Superficial to the patellar tendon is the prepatellar bursa (arrows) here swollen and inflamed and scanned with lots of gel and light pressure on the transducer.

Position:
The patient is supine with the knee comfortably bent about 15 to 20 degrees over a rolled pillow or triangular cushion. Scan all structure under examination in both transverse and longitudinal planes making sure to include the entire width of the tendons and of the underlying bursae. Bursae can decrease in size or disappear in full extension or with a heavy hand on the transducer. Try floating the transducer over a gel covered knee to find small amounts of fluid. Moderate knee flexion is best to fill the bursae. The suprapatellar bursa normally has 1 to 2 mm of fluid.

Knee over a cushion	Patellar tendon longitudinal scan

To better visualize the patellar tendon, the knee can be further flexed. Longitudinal scanning can start to the inferior apex of the patella and slide distally. Be careful with anisotropy in these tendons and hold the transducer 90 degrees to them, rocking as needed to achieve maximum results. Compare the symptomatic to the asymptomatic side.

Tip: Try to get at least one panoramic picture of the full extent of the quadriceps and patellar tendons from musculotendinous junction to the tibial tuberosity with their associated bursa. At least document the full extent of the tendons.

Pathology:

The quadriceps tendon often tears transversely within 2 cm of its insertion on the superior portion of the patella. Most tears are incomplete, often involving the central fibers which are the rectus femoris contribution to the tendon. While trauma is a frequent cause, systematic conditions like gout, diabetes, chronic renal failure contribute to ruptures. The tendon may be stronger than the bone and cause an avulsion fracture of the patella with a partial quadriceps tear. Check the quadriceps musculotendinous junction and into the quadriceps muscle bellies to rule out additional tears and bleeds. A mass in the distal thigh or “pseudo-tumor” can be formed from a partial rectus femoris muscle or proximal tendon tear that is often unrecognized. Clinical diagnosis is often delayed which can complicate repair.

Quadriceps tendinosis produces hypoechoic enlargement of the tendon often at the insertion on the superior portion of the patella. These entheseal changes reveal the tendon’s loss of normal architecture. Since there is no sheath around the tendon, focal fluid collections are rarely seen. Chronic repetitive strain can produce spur formation, fibrotic bands and micro tears. Tendinosis can precede failure of the tendon in some cases.

Quadriceps tendon tear by radiography and ultrasound demonstrating extent of tear (* *) above the patella.

Scarring in the rectus femoris from previous partial tear (arrow).

Patellar tendon tendinosis can cause chronic pain and swelling. Focal as well as diffuse involvement may be seen. When located along the inferior pole of the patella, the tendon appears hypoechoic and thick. The pathology can be complicated by small tears, fluid collections, calcification and mucoid degeneration. In adults these entheseal changes are often called jumper’s knee, an overuse injury. In children the changes are associated with chronic traction on the inferior pole of the patella is called Sinding-Larsen-Johansson disease with small avulsed pieces of cartilage or bone present distal to the apex of the patella. Occasionally an entire sleeve of cartilage will acutely avulse.

Medial Knee

Normal Anatomy:
The medial collateral ligament is the broad flat band that runs from the medial femoral condyle about 12 cm distally to the proximal tibia. It can look like a tendon because of its many long bright fibers. It is one of the stabilizers of the knee and it is often damaged with trauma. The deep fibers of the medial collateral ligament insert onto the medial meniscus. Normally the meniscus is hyperechoic and uniform in echotexture. In the coronal, coronal oblique and sagital planes, the body of the medial meniscus is triangular in shape with the thin inner margin pointing into the joint. On the transverse plane it is more ribbon-like. The adjacent hyaline cartilage is hypoechoic. The pes anserinus is slightly posterior and inferior to the insertion of the medial collateral ligament on the tibia.

Position:
Start with the patient supine with the knee flexed over a cushion or pillow, with slight flexion of the knee and hip and slight external rotation of the hip. Scan to either side of the midline looking for the triangular shape of the meniscus which will be hyperechoic in the joint space. If the meniscus is not seen, try placing the patient on their side, lateral decubitus with the symptomatic side down or the patient can be supine with external rotation of the leg. The knee should be slightly flexed and externally rotated (rolled out laterally).

Scanning with the knee over the edge of the table to open the medial joint

Start with the transducer at the midline of the joint and identify the medial collateral ligament, following it superiorly and inferiorly across its width. Increase the gain to see the meniscus. Mild valgus or varus stress will open up the joint space and allow better visualization of the inner margin of the meniscus. Place the knee at the edge of the table or over a triangular cushion and apply light pressure. Normally there is more motion on the lateral than on the medial side of the knee and in patients older than 50 years. The menisci may be difficult to see in large patients and where there is degenerative change. Also, both menisci are rarely seen completely in both compartments. MRI is the imaging method for meniscal evaluation.

Pathology

The medial collateral ligament is commonly injured near is femoral origin. Generally the deep fibers of the ligament are damaged and the resulting fluid can displace the superficial layer laterally. Partial tears or even ligamentous strain will appear hypoechoic with thickening of the ligament. Complete tears can present as structure change and irregular clefts with hypoechoic or anechoic fluid. Hemorrhage can be seen in the tears. The tears may be easier to see with the application of varus stress to the knee during scanning. The medial collateral ligament may calcify or ossify at its proximal femoral attachment after injury. This lesion is known from radiography as Pellegrini-Stieda. In general, chronic injury may result in calcifications within the ligament or associated bursa formation with fluid and hyperemia.

Ossification (thick arrow) of the medial collateral ligament (thin arrows) attachment on the femoral origin. Pellegrini-Stieda. Ultrasound on the left, radiography on the right. ( MFC - medial femoral condyle, JS - medial knee joint space)

Medial collateral ligament tear at the superior aspect near the femoral condyle origin (arrows on ligament, thick arrows at tear).

Some of the deep fibers of the medial collateral ligament join the medial meniscus. If the medial collateral ligament is damaged, particularly the deep fibers, be sure to check the medial meniscus. While MR is the modality of choice for meniscal injury, when there is point tenderness at the joint line, evaluating for focal defects can be worthwhile. Medial collateral ligament damage is also seen with anterior cruciate ligament tears, but anterior cruciate ligament tears are more often diagnosed by MR.

For meniscal injury, MR is the modality of choice, but meniscal tears may be seen with ultrasound, medial collateral ligament (thin arrows) and meniscus with hypoechoic tear (thick arrow).

Medical meniscal tears are more common than lateral damage. On the longitudinal plane the inner margin may be blunted. Look for radial tears on the transverse scans.

Meniscal cysts can form after a tear and are especially common with the degenerative horizontal complex tears. The joint fluid extends through the torn meniscus and forms a bugle near the surface tear. Because of the attachment of the medial collateral ligament to the medial meniscus, the classic medial meniscal cyst may track along the anterior medial joint line.

Transverse scan of medial meniscal cyst (arrows) at the femoral condyle

Lateral Knee

Normal Anatomy:
There are several structures along the lateral femoral condyle parallel to the long axis of the leg in the longitudinal plane. The fibular or lateral collateral ligament is slightly posterior to the midline of the condyle and forms a hypoechoic thin long cord running in an anterior to posterior course as it descends to the fibular head. Angle the transducer to follow its path. Unlike the medial collateral ligament, the lateral collateral ligament does not give fibers to the adjacent meniscus and is damaged less often when the lateral meniscus is torn. The other tendon structure inserting on the lateral fibular head is the distal biceps femoris tendon. It is more posterior and forms a “V” shape with the lateral collateral ligament as it inserts.

Lateral collateral ligament (arrows) running from the lateral femoral condyle to the fibular head (not seen). Because of the angle of the transducer, the meniscus is not seen.

The “V” insertion of the lateral collateral ligament ( superior) on the fibular head with the distal biceps femoris tendon ( more inferior tendon).

Lateral meniscus in normal position (arrow) on the left and a torn slightly exuded lateral meniscus on the right also demonstrating the femoral articular cartilage . np – notch for popliteus

Just anterior to the origin of the lateral collateral tendon, the popliteal tendon inserts in a pronounced groove on the lateral femoral condyle. The popliteal tendon swings inferiorly and posteriorly around the posterior lateral corner of the knee in a short tendon within the capsule before exiting the knee capsule when the muscle belly is seen. The muscle arises from the posterior proximal tibia. Fluid can track along the tendon path when there is a large effusion or if there is posterior knee trauma. More anterior is the iliotibial tract or band which extends to insert on Gerdy’s tubercle of the anterior lateral tibia.

Normal popliteal tendon insertion (small arrows) with lateral collateral ligament (large arrows).

At tip popliteal tendon (arrows) curving around the lateral femoral condyle. Normal iliotibial band (arrows) bottom image.

Position:
Start with the patient supine with the knee flexed over a cushion or pillow and slightly internally rotated to better expose the lateral structures. Again, a decubitus position can be tried. Scan for the lateral meniscus as described in the section for the medial meniscus. Identify the iliotibial band, lateral collateral ligament, popliteus tendon and biceps femoris tendon. Follow the lateral collateral ligament and the biceps femoris tendon to their insertion on the fibular head.

Position for the lateral knee. Checking the meniscus (left) and the iliotibial band in the longitudinal plane (right)

Pathology:

Lateral ligament tears are uncommon and there is less association between ligament and meniscal damage. Meniscal injury is evaluated as discussed with the medial meniscus. Partial tears, as elsewhere, will appear hypoechoic with thick structures and clefts. Complete tears of the lateral collateral ligament are generally due to direct varus trauma and are less common than tears of the medial collateral ligament. Complete tears of the popliteus tendon will produce disruption although the popliteus tendon tears can be partially obscured within the knee capsule. Fluid can track down the path of the popliteus tendon if there is an effusion within the knee and is not a sign of tendon tear or a meniscal cyst. Tears of the biceps femoris are often in the distal tendon.

Because it is immediately adjacent to the lateral femoral condyle the iliotibial band can become irritated as it rubs over the bone. This is the iliotibial band syndrome or runner’s knee. It is associated with thickening of the band and possible bursa formation between the band and the lateral femoral condyle. At the superior tibiofibular joint ganglia can form which are usually asymptotic but can compress the peroneal nerve sheath as it swings around the proximal fibula. Because of its superficial position overlying the bony fibular, the peroneal nerve can because compressed or damaged with fibular neck fractures. The peroneal nerve can be seen as it circles lateral around the fibular neck and alternations in its appearance can be appreciated.

Normal peroneal nerve (arrows) adjacent to the fibular in the proximal lateral calf.

Panoramic view of superior tibiofibular joint ganglion, these can compress the peroneal nerve that runs adjacent to them.

Posterior Knee

Normal Anatomy:
Along the posterior lateral knee the major muscle is the lateral head of the gastrocnemius arising on the lateral femoral condyle. The popliteus muscle separates it from the femoral condyle at the joint line. The common peroneal nerve runs along the lateral border of the gastrocnemius before it curves around the neck of the fibula. The posterior horn of the lateral meniscus can be best seen with dynamic scanning. On flexion and extension it will move up to ten millimeters and is not adhered tightly to the capsule since the popliteal tendon arcs in its sheath between the capsule and the meniscus. It is easy to mistake a tear in the popliteal tendon for a posterior lateral meniscal tear. The posterior horn of the medial meniscus is attached to the joint capsule and even with flexion and extension of the knee, moves only a few millimeters out of the joint. On the medial side of the posterior knee is the medial head of the gastrocnemius muscle and the semi membranous muscle.

Position: For the posterior knee a prone position is best with the transverse plane generally the most useful although the longitudinal plane can give an overview of a large Baker’s cyst between the medial head of the gastrocnemius and the semi-membranous muscle. Panoramic views may be needed if the cyst is large. Color flow is useful in the posterior knee to document normal blood flow in the popliteal vessels. It can also be useful to evaluate hyperemic in suspected synovial thickening or pannus formation and for infection.

Scanning the posterior medial knee as part of an evaluation for Baker’s cyst.

Pathology:

Posterior knee complaints are a common referral for ultrasound examination and often the request specifies the clinical suspicion of a popliteal cyst also called a Baker’s cyst. A Baker’s cyst is a fenestration of a normal bursa towards the joint synovium, a communication that develops with age. The bursa forms between the semi membranous tendon and the tendon of the medial head of the gastrocnemius muscle. The Baker’s cyst may be a single component or have a superficial and a deep limb wrapped around the medial head of the gastrocnemius. To make the diagnosis of Baker’s cyst the neck of the cyst must be demonstrated between these two tendons. Otherwise an erroneous diagnosis will be made and rare cystic masses such as cystic synovial sarcoma may be missed.

Neck of a Baker’s Cyst (arrows) Infected Baker’s Cyst in Septic Knee (arrows)

The cyst fluid can be complicated by hemorrhage, chronic inflammation or infection. Patients such as those with rheumatoid arthritis can have synovial thickening and pannus formation. Check the inferior pole of the cyst, which should appear smooth with a convex contour. If it is irregular, or forms a thin rat-tail, the cyst probably has ruptured. Scan further down into the calf to check for fluid beneath the fascia of the gastrocnemius muscle or between the soleus and gastrocnemius muscles. This leaking fluid can inflame the overlying tissue and produce pain, swelling and signs of pseudothrombophlebitis. Comparison with the contralateral posterior knee can sometimes find an asymptomatic Baker’s cyst.

Large Baker’s cyst (arrows) demonstrated with panoramic views.

Large leaking Baker’s Cyst (small arrows) with the irregular distal “rat tail” consistent with rupture (large arrow)

Lobulated Baker’s cyst palpable as a large mass.

Much less common than a Baker’s cyst, a mass in the popliteal fossa may be a popliteal artery aneurysm. While this usually presents as a pulsating mass, thrombosis can obscure its clinical presentation. Color flow and careful correlation are needed in a suspicious clinical setting such as direct trauma or dislocation. Popliteal vein thrombosis can be excluded by observing the compression of the vessel with normal color venous flow.

In the posterior knee the most common muscle tear is the medial head of the gastrocnemius. Known as “tennis leg”, the tear is often at the aponeurosis with the soleus muscle. Clinical symptoms may mimic a leaking Baker’s cyst.

Normal appearance of the medial head of the gastrocnemius on the right (arrows) and an old healed tear on the left with hyperechoic scar distal to the rounded and shorted distal fibers (large arrows).

Normal Anatomy:
Normally the capsule is against the cartilage of the anterior talar dome and the anterior tibia will have no more than 3 mm of fluid. The anterior extensor tendons include the anterior tibial tendon, the extensor hallucis longus and the extensor digitorum longus. Most medially is the largest of the extensor tendon, the anterior tibial tendon. It is twice the diameter of the adjacent extensor hallucis longus and the extensor digitorum longus. The anterior tibial tendon inserts on the first or medial cuneiform while the hallucis extends to the great toe. The digitorum divides more distally into 4 tendons for the 2nd to 5th digits. Generally in traumatic injury to the anterior ankle, the tendons are spared and the ligaments are torn.

In the anterior ankle there are the important anterior lateral ligaments which are often torn with inward rotation of the foot. The anterior talofibular ligament is about 3 mm wide and runs from the anterior distal fibula at the epiphysis to the lateral talus. The anterior tibiofibular ligament extends from the distal tibia to the distal fibula.

Normal anterior talofibular ligament (arrows) longitudinal plane

Normal anterior tibiofibular ligament which is damaged in a “high ankle sprain”.

Position:

The patient should be supine with the knee flexed and the foot on the table. Scan along the anterior ankle in the transverse and longitudinal planes to evaluate the joint for synovitis and effusion. The three extensor tendons are readily seen in the anterior lateral ankle. The anterior lateral ligament complex is best seen with the transverse views.

Anterior ankle: transverse and longitudinal planes

Lateral Ankle

Normal Anatomy:

The peroneal tendons, peroneus longus and brevis, can be traced from their supramalleolar musculotendinous origins posterior to the distal fibula around and down to their inframalleolar path. They are held in place against the fibula by the band-like superior peroneal retinaculum. Along the fibula the peroneus brevis is anterior against the bone and the peroneus longus is more posterior, sliding against the center of the brevis tendon. The largest amount of fluid seen within the peroneal tendon sheath is just inferior to the lateral malleolus where up to 3 mm can be normal. Otherwise only a trace is allowed. The peroneus brevis courses along the plantar surface of the foot to the base of the 5th metatarsal. The longus enters the plantar groove in the cuboid, turns medially and swings over to insert onto the plantar aspect of the medial cuneiform and the first metatarsal. It is can be followed as it obliquely crosses the sole of the foot.

Normal appearance of peroneal tendons superior to the malleolus (pl – peroneus longus, pb – peroneus brevis)

Pathology:

Because of the peroneal tendons unprotected course along the lateral ankle against the irregular surface of the fibula and because of their normal range of excursion with ankle motion, they are subject to acute and chronic tears. The peroneus brevis is more often torn trapped between the peroneus longus tendon and the fibula. This tendon can be directly torn with distal fibular fractures. More often though, as the peroneus longus slides over it, the brevis can become chronically flatten and develop longitudinal partial tears with anechoic clefts and fluid.

Torn peroneus brevis tendon with intact peroneus longus at the level of the malleolus

Known as the peroneal split syndrome, this can be a common source of lateral ankle pain. The peroneal brevis can be cleaved into two components so that three tendons are seen in the sheath. More distal degeneration of the peroneal longus along the plantar surface often is seen in diabetics. Another weak link occurs at the os peroneum, the sesamoid in the distal peroneus longus tendon. A complete tear of the tendon occasionally manifests itself by fracture or displacement of that sesamoid. Yet another weak point of the tendon is in the cuboid groove along the plantar surface of the foot where degenerative tears can occur.

With trauma to the superior peroneal retinaculum, the tendons can sublux from their position behind the fibula. Often the more posterior peroneus longus tendon swings lateral and anterior around the brevis to move over the lateral malleolus. This type of subluxation can be clinically obvious. Even if the peroneal tendons are normal in appearance, excessive fluid in the sheath can be a sign of adjacent damage in the lateral ligament complex, particularly to the calcaneofibular ligament. The calcaneofibular ligament can be seen in the lateral ankle between the peroneal tendon sheath and the underlying calcaneus. Dorsiflexion stretches the ligament. The normal ligament pulls away from the bone surface. It is thicker and brighter than the anterior talofibular ligament.

Subluxation of peroneus longus tendon anterior to brevis

Normal calcaneofibular ligament (arrow) deep to the peroneal tendons transverse on the left and torn calcaneofibular ligament (arrowhead) deep to the peroneal tendons longitudinal at the malleolus on the right.

Position:

To evaluate for peroneal subluxation, place the transducer in the transverse plane inferior to the lateral malleolus. With gentle passive dorsiflexion and eversion of the foot, watch the behavior of the two tendons.

Lateral ankle transverse and longitudinal in slight plantar flexion

Posterior Ankle

Normal Anatomy:

The Achilles tendon is make up by the combined tendon fibers of the gastrocnemius and soleus muscles spiraling down to insert over an area of 1- 2.5 cm of the posterior calcaneus. Some tendon fibers wrap around to the plantar surface. The plantaris muscle has thin tendon fibers along the medial Achilles tendon. Kager’s fat pad lies just anterior to the Achilles tendon. A small bursa extends between the distal Achilles tendon and the superior posterior calcaneus, the retrocalcaneal bursa, with up to 1mm thick layer of normal fluid. This bursa can become irritated and its chronically irritated synovium can lose the normal fat/fluid contrast seen with ultrasound.

Normal Achilles tendon insertion without bursa formation (arrows) longitudinal and transverse planes.

The small deeply located flexor hallucis longus tendon can be seen along the posterior talus in a groove between two small tubercles before it swings inferior and medially around the sustentaculum tali. Because the tendon sheath can communicate with the ankle joint, fluid is often seen surrounding the tendon but that alone is not an indicator of pathology. At the posterior ankle joint the posterior talofibular ligament forms a short horizontal band from the fibular to the posterior talus. It is rarely ruptured but if injured it can indicate significant trauma. The ligament can also tear from degenerative change.

Position:

The patient is prone with the foot dangling over the edge of the table. The tendon is scanned from the insertion on the calcaneus into the muscle belly of the gastrocnemius in both transverse and longitudinal planes. Floating the transducer with lots of gel helps especially near the insertion. Gentle plantar and dorsiflexion can help uncover otherwise occult injury and demonstrate the dynamic gap between torn tendon edges.

Longitudinal scanning over the distal Achilles tendon

Dorsiflexion (left) and plantar flexion (right)

Pathology:
Chronic posterior ankle and heel pain are common presentations of Achilles tendon pathology. With chronic overuse the tendon can undergo degenerative changes as a result of repetitive damage and repair. Both the tendon and its surrounding tissues can become hypoechoic and thickened. The normal contour of the distal Achilles can change and become more rounded with a “beer belly” shape at the site of tendinosis.

Achilles tendon (at) thickening or “beer belly’’ from a weekend basketball game seen with radiography and as hypoechoic thick tendon with ultrasound (arrows)

There can be focal changes along the dorsal surface. Dynamic flexion of the foot can help distinguish between changes of the tendon and of the surrounding paratendon. Classic degenerative changes involve the proximal 2/3 of the tendon and the medial more than the lateral side. Distal enthesopathic changes are often associated with plantar fasciitis and adjacent bursitis. With bursitis there is a hypoechoic or anechoic bean-shaped structure showing between the Achilles and the posterior superior calcaneus.

Evaluation of possible Achilles tear is a common request for ultrasound. Ruptured Achilles tendons usually result from trauma or sports related injuries. If the tear is not treated quickly and appropriately, permanent loss of function can result. The tear should be located, the extent determined, and then the reducibility of the tendon gap should be evaluated.

The most common site for Achilles tears is in the mid portion of the tendon at the level of the posterior malleolus about 5 to 6 cm proximal to the calcaneus. The typical patient is in their 30’s to 50’s and often gives a history of sudden sharp pain and snap in the lower leg while playing sports. Another site of injury is at the musculotendinous junction and can involve the medial gastrocnemius muscle belly as well as the aponeurosis. Distal avulsion is not common unless there is predisposing weakness of the bone as with diabetics, steroid use, renal disease and inflammatory arthritis.

Ruptured proximal Achilles tendon at medial gastrocnemius musculotendinous junction with shadowing at the torn tendon ends (ps – proximal stump, ds – distal stump) and fat from the pre-Achilles fat pad herniating into the defect

Distal avulsion in diabetic with radiography and ultrasound (at – Achilles tendon, bf – bony fragment)

A complete tear can be obscured by debris within the gap formed by the retracted proximal stump. The stumps often have noticeable acoustic shadowing at their edges which is a pathognomic finding of a tear. The distance between the two stumps should be measured at the rent and then one should attempt to approximate the stumps by plantar flexion. For the dynamic evaluation, actively but gently plantar flex and dorsiflex the ankle and measure the change in the resulting gap. These observations enable the surgeon to determine the type of treatment for tendon recovery, choosing between operative repair and casting. This motion can visualize a previous occult complete tear as well. The tendon stumps may come back into contact with the foot in plantar flexion. Such a patient is ideally suited for cast treatment.

Complete Achilles tear during a weekend basketball game. Dynamic scanning demonstrates the changing gap between the torn ends with dorsiflexion (top) and plantar flexion (bottom) at the level of the posterior malleolus.

Sometimes there is a complete tear of the Achilles tendon but the foot appears clinically to function as in a partial tear because the small weak plantaris tendon remains intact. The plantaris tendon can be identified by ultrasound. In a chronic rupture the gap may be filled with granulation tissue that is echogenic like the tendon but that lacks the fibrillar pattern of a normal tendon.

Partial tears should be carefully documented as well since they can go on to become complete if appropriate therapy is neglected. The partial tear should be tested with gentle flexion to avoid missing an occult complete tear. Clinically, tears within the muscle of the gastrocnemius can present with similar symptoms to an Achilles tear. The musculotendinous junction is a susceptible site for tendon tears so check the tendon proximally as it merges with the muscle tissue. Tears of the posterior tibial tendon can also clinically mimic Achilles tendon tears.

The flexor hallucis longus tendon can be impinged upon by prominent posterior talar tubercles or by an os trigonum. It develops chronic irritation and tendinosis from this impingement syndrome. Impingement is especially common with ballet dancers and others who routinely hyperextend the foot, like soccer players.

Flexor hallucis longus tendon tear in ballet dancer (arrows).

This presentation is a short review of the sonographic appearance of the elbow, knee and ankle. There are numerous excellent books and articles on musculoskeletal ultrasound so when you do grab the transducer there is a vast amount of information available. The following two books were used in the preparation of this review.

McNally E: Practical Musculoskeletal Ultrasound, Philadelphia, Elsevier Churchill Livingstone, 2005.

van Holsbeeck M, Introcaso J: Musculoskeletal Ultrasound. St Louis, Mosby-Year Book, 2001.

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