Core Curriculum, The: Ultrasound
1st Edition

13
Musculoskeletal Ultrasound
US evaluation of the musculoskeletal system is a rapidly expanding area of sonographic diagnosis. Indications include assessment of soft tissue infections, detection and characterization of soft tissue masses, and the evaluation of muscle, tendon, and joint abnormalities [1,2,3]. This chapter highlights some of these areas of musculoskeletal sonography [4].
Imaging Technique
Most structures imaged are superficial; therefore, high-frequency (7-10 MHz) linear array transducers are most useful [4]. Interaction between the patient and the examining physician is essential to accurate examination. Consider the history and precise location of symptoms. Correlate the examination with point tenderness and location of pain with motion. Real-time observation during compression with the transducer yields vital additional information about the nature of visualized structures. Comparison with normal structures on the opposite side of the body may be extremely helpful. Color flow and spectral Doppler provide vital information about the vascularity of masses and inflammatory processes. Anisotropic artifact (see Chapter 1) is a prominent feature of US examination of tendons and ligaments. Because of their longitudinal fibrillar structure, tendons and ligaments appear echogenic when imaged perpendicular to the US beam and appear hypoechoic when imaged at an angle to the US beam [5].
Anatomy
Subcutaneous Fat
Subcutaneous fat is found just below the covering layer of skin.
  • Subcutaneous fat is hypoechoic and interspersed with thin linear septations of connective tissue. Thickness is related to the patient’s state of obesity (Fig. 13.1).
Figure 13.1 Normal Subcutaneous Fat. Transverse image of the abdominal wall near the midline shows the rectus abdominis muscle (R), the linea alba (arrow), the parietal peritoneum (large white arrowhead) and the peritoneal cavity (P). Superficial to the rectus muscle is the subcutaneous fat (F), which appears hypoechoic and septated. The skin (dermal layer) is seen as a thin echogenic layer (small white arrowheads) in contact with the transducer.
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Skeletal Muscle
Skeletal muscle fibers are grouped into bundles defined by fibroadipose septa [4]. Dense connective tissue covers the entire muscle and additional thick fascia separates individual muscles.
  • On longitudinal scans, muscles are hypoechoic with a pattern of fine echogenic strands in an oblique orientation. These strands correspond to fibroadipose septations (Fig. 13.2).
  • On transverse scans, the septa appear punctate or linear and are diffusely scattered over the hypoechoic background of muscle tissue (Fig. 13.2).
  • Doppler shows moderate blood flow within muscle tissue.
  • Dynamic imaging during contraction and relaxation illustrates function and aids in the detection of abnormalities.
  • Diffuse fat infiltration of muscle occurs with obesity and diffusely increases the echogenicity of muscle [6].
Tendons
Tendons in all areas of the body have a fairly uniform, clearly recognizable appearance.
  • In longitudinal plane, tendons have a fine, linear, fibrillar internal pattern of parallel hyperechoic lines (Fig. 13.3). Higher frequency US shows more detail with finer and more numerous fibrillar echoes [7].
  • On transverse section, tendons are hyperechoic and round or oval in shape.
  • Synovial sheaths cover many tendons and appear as a thin (1-2 mm) hypoechoic rim surrounding the echogenic tendon (Fig. 13.3B). The long biceps tendon of the shoulder and the tendons of the wrist and ankle have synovial sheaths. The rotator cuff, Achilles, patellar, gastrocnemius, and semimembranous tendons do not have sheaths [4].
  • Tendons will routinely demonstrate the anisotropic effect (Fig. 13.3A) [5].
Figure 13.2 Normal Muscle. A. Longitudinal image of the calf shows three muscles (M) with oblique muscle fibers oriented in different directions. The dermal layer (white arrowhead) and a thin layer of subcutaneous fat (black arrowhead) are also evident. Layers of thick fascia (arrows) separate the individual muscles. B. Longitudinal image of the thigh shows longitudinal muscle fibers in two muscles (M) of differing echogenicity. The surface of the femur (arrow) is brightly echogenic and casts a dense acoustic shadow. The dermal layer (white arrowhead) and subcutaneous fat (black arrowhead) are identified. C. Transverse image of the thigh in the same location as B shows the more speckled appearance of the fibroadipose septations in the two visualized muscles (M). The round surface (arrows) of the mid-shaft of the femur is evident. The arrowheads identify the dermal (white arrowhead) and subcutaneous fat (black arrowhead) layers.
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Ligaments
Ligaments attach bone to bone to provide stabilization.
  • Ligaments appear similar to tendons but have a more compact hyperechoic fibrillar pattern. Collagen fibers are most interweaved and more irregular in appearance than tendons.
Peripheral Nerves
Larger peripheral nerves can be accurately seen by US [8,9].
Figure 13.3 Normal Tendons. A. Longitudinal image of a flexor tendon of the hand shows the characteristic pattern of linear echogenic strands (black arrowheads) where the tendon is imaged perpendicular to the US beam. Where the tendon curves away and the US beam strikes the tendon at an angle, the tendon (white arrowheads) appears hypoechoic, and the linear pattern of echogenic strands is lost. B. Longitudinal view of the long head of the biceps tendon (black arrowhead) in the bicipital groove of the humerus shows the characteristic fibrillar pattern. A small volume of fluid (white arrowhead) is seen within the biceps tendon sheath. C. Transverse view of the biceps tendon (arrow) near the humeral head shows the characteristic “dot” pattern of tendons when imaged transverse to their long axis. The bicipital groove (arrowhead) separates the greater (G) and lesser (L) tuberosities of the humerus. D. Transverse view of the left shoulder shows the supraspinatus tendon (black arrowheads) coursing to its attachment on the greater tuberosity (G). The hyaline cartilage of the humerus is seen as a thin hypoechoic line (white arrows) covering the bone surface.
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  • Nerves appear as tubular, echogenic structures slightly less echogenic than tendons and ligaments. Multiple, parallel, linear internal echoes are characteristic.
Synovial Bursa
Bursa are potential spaces than normally contain only a tiny volume of fluid.
  • Normal bursa appear as flattened hypoechoic structures 1-4 mm in thickness in characteristic locations.
Bone Cortex
Because bone avidly absorbs sound energy, only the superficial surface of bone is evaluated by US.
  • Bone cortex appears as a bright echogenic surface with prominent posterior shadowing (Figs. 13.2B, C; 13.3C, D).
Hyaline Articular Cartilage
Hyaline cartilage covers the articular cortical surface of bone.
Figure 13.4 Normal Cartilage. Coronal plane image of the hip in a newborn infant shows the characteristic appearance of the cartilaginous head (white arrowheads) and greater trochanter (black arrowhead) of the femur. The iliac bone (larger arrow) and roof of the normal acetabulum (smaller arrow) are also seen. US is an excellent imaging method to diagnose developmental dysplasia of the hip in infants.
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  • Cartilage is seen as a thin hypoechoic rim that covers the echogenic bone cortex (Fig. 13.3D).
Hyaline Epiphyseal Cartilage
Sound transmits well through hyaline cartilage, allowing US evaluation of developing bone such as the infant hip [10,11].
  • In the newborn infant the femoral head and trochanters consist entirely of cartilage. The hyaline cartilage appears diffusely hypoechoic with a pattern of echogenic dots scattered throughout its substance (Fig. 13.4). A pattern of echogenic vertical or spiral columns may also be seen. The nucleus of ossification appears as a highly echogenic focus in the center of the hypoechoic head. This focus progressively enlarges as ossification advances [12].
Musculoskeletal Masses
US can characterize the cystic or solid nature and vascularity of a mass but, with few exceptions, US cannot determine whether a solid mass is benign or malignant. US is effectively used to guide biopsy of soft tissue masses [13,14]. Soft tissue masses are described as to location, size, shape, margins, vascularity, deformability, and number of lesions.
Lipoma
Lipomas are a common mass of the superficial soft tissues. The tumor is benign and consists entirely of fat bounded by a thin capsule. The tumor may be lobulated in contour and divided by fibrous septa.
  • Knowing that fat in the abdomen is usually quite echogenic, it is somewhat surprising to recognize that subcutaneous lipomas appear moderately hypoechoic. Echogenicity is homogeneous. Lipomas may be isoechoic or mildly hyper- or hypoechoic compared to subcutaneous fat (Fig. 13.5) [15].
  • Lipomas appear well defined when surrounded by fibrous tissue or muscle but their margins are often indistinct when surrounded by fatty tissue.
  • Lipomas are oval in shape with the long axis of the tumor parallel to the skin [15].
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  • Correlation with physical examination is useful to define the borders of the mass and to confirm the soft fluctuant nature of fat.
  • Tumor heterogeneity, large size, and prominent lobulations are signs that suggest malignancy (liposarcoma).
Figure 13.5 Lipomas. A. This lipoma (straight arrows) is well defined by surrounding echogenic fibrous tissue. It is oval in shape with long axis parallel to the skin. Its echogenicity is homogeneous and equal to subcutaneous fat (curved arrow). Palpation revealed a fluctuant mass that was easily compressible by gentle transducer pressure. B. This lipoma (arrows) is slightly echogenic compared to adjacent fat and is well marginated by a thin, but distinct, fibrous capsule. C. Because it is surrounded by isoechoic fat, this lipoma (arrows) is difficult to differentiate from surrounding tissues. Correlation with simultaneous physical examination confirms its size and nature.
Hemangioma
Hemangioma is the second most common benign tumor of muscle and subcutaneous tissues. Hemangiomas consist of endothelial-lined vascular spaces of varying size with a variable amount of intervening fibrofatty tissue [16].
  • Hemangiomas are heterogeneous masses with tortuous blood vessels often visible coursing through the mass (Fig. 13.6).
  • Color flow Doppler shows prominent blood flow. A high vessel density (more than 5 vessels/cm2) and high flow velocity is characteristic [17].
  • Shadowing punctate echogenic foci represent phleboliths, which are characteristically present.
Nerve Tumors
Tumors arising from peripheral nerves are usually classified as schwannomas (also called neurinoma or neurilemoma) or neurofibromas. Malignant tumors are sarcomas that arise
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from neurofibromas. Von Recklinghausen neurofibromatosis is characterized by widespread neurofibromas that present as skin nodules [9,18].
Figure 13.6 Hemangioma. A soft tissue mass in the thigh proved to be a hemangioma. The mass (black arrowheads) is well defined but heterogeneous in echogenicity. Several small blood vessels (white arrows) are visible within the mass.
  • Nerve tumors appear as well-defined, fusiform, hypoechoic masses. Lesions may be solitary, multiple, or elongated plexiform masses. A neurogenic origin is confirmed if careful scanning confirms that the soft tissue mass is connected to a nerve bundle at its proximal and distal poles [9].
  • Schwannomas tend to be more eccentric to the nerve and appear homogeneous with posterior acoustic enhancement. Doppler shows prominent vascularity. Cystic changes may occur.
  • Neurofibromas are more echogenic and coarse in appearance. Doppler shows low vascularity. A sonographic target appearance with peripheral low echogenicity surrounding central higher echogenicity has been described [19]. This sonographic finding corresponds to the MR target sign of high intensity peripherally and low intensity centrally seen on T2-weighted images.
  • US cannot reliably differentiate benign from malignant nerve sheath tumors. Findings that suggest malignancy include rapid growth, indistinct tumor margins, and invasion of adjacent tissue. Fine needle aspiration biopsy can be performed with US guidance. Needle insertion into the nerve causes intense pain.
  • Morton’s neuroma is not a true neuroma but is a benign mass of perineural fibrosis arising along the plantar interdigital nerve. The characteristic location is between the second and third, or third and fourth metatarsals. They appear as well-defined, ovoid, hypoechoic nodules. Doppler shows prominent vascularity [20].
Superficial Metastases
  • Superficial melanoma metastases are usually well-defined hypoechoic nodules with smooth or lobulated borders. The lesions have mild to moderate heterogeneity and demonstrate accentuated through-transmission. Doppler shows internal flow in most lesions [21].
  • Kaposi’s sarcoma produces superficial nodules, which are hypoechoic with ill-defined margins (Fig. 13.7) [22].
Figure 13.7 Kaposi’s Sarcoma. Multiple small subcutaneous nodules were palpated in this patient with AIDS. US revealed small, ill-defined hypoechoic masses (arrow) corresponding to the palpable nodules. Biopsy confirmed Kaposi’s sarcoma. (This color Doppler image is shown here in gray scale.)
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Soft Tissue Sarcomas
Soft tissue sarcomas develop most commonly in the extremities. The two most frequent sarcomas are liposarcoma and malignant fibrous histiocytoma.
  • Liposarcomas are predominantly echogenic and heterogeneous.
  • All other soft tissue sarcomas are hypoechoic (Fig. 13.8). They may be relatively well circumscribed or ill defined and infiltrative. Normal muscle and subcutaneous structure is disrupted. Areas of necrosis, cystic change, and calcification may be evident.
  • Sarcoma recurrences appear as small, round or oval, hypoechoic nodules in the area of surgical resection [23].
Lymphoma
  • Cutaneous lymphoma appears as diffuse echogenic thickening of both dermal and subcutaneous layers with ill-defined margins [22].
  • Lymph nodes involved by lymphoma show diffuse low echogenicity. Nodes may appear almost cystic because of uniform cellularity and commonly demonstrate accentuated through-transmission (Fig. 13.9).
Figure 13.8 Soft Tissue Sarcoma. An 85-year-old woman presented with a tender enlarging mass in the right thigh. She noticed the mass after bumping her leg on a table. A. Gray scale US reveals a relatively well-defined, but heterogeneous mass (arrows). B. Color Doppler US shows prominent internal vascularity indicating this mass is a tumor, not a hematoma (see Color Figure 13.8).
Figure 13.9 Lymphoma Nodes. Enlarged lymph nodes in the axilla show accentuated through-transmission (arrowheads). Biopsy confirmed non-Hodgkin’s lymphoma.
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Foreign Bodies
US is excellent for determining the presence and location of foreign bodies, especially those that are non-radiopaque. Fragments of glass, wood, or metal may be identified with US [24].
  • All foreign bodies are echogenic compared to surrounding tissue (Fig. 13.10). Shape will obviously depend on nature of the object. Examination concentrates on the area of the wound. Acoustic shadowing, comet tail artifacts, and reverberation echoes may be seen depending on the nature and shape of the foreign body [24].
  • Bleeding or inflammatory changes may be present adjacent to the foreign body. Air in the soft tissues may obscure detection of the foreign body.
Cystic Lesions
Popliteal Cyst
Popliteal (Baker’s) cysts are common findings associated with internal derangement of the knee. A weakening in the posteromedial wall of the joint capsule allows a synovial communication
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with the gastrocnemius-semimembranosus bursa, forming a cystic mass filled with joint fluid. Any disease process that increases fluid volume in the joint space may cause a Baker’s cyst. The cyst causes symptoms by internal hemorrhage, rupture, or pressure effects on adjacent structures.
Figure 13.10 Foreign Body. A wood splinter embedded in the soft tissues of the calf appears as a well-defined, linear, brightly echogenic object (white arrows). A hypoechoic granulomatous reaction (black arrowheads) has developed around the foreign body that had been in place several weeks.
Figure 13.11 Baker’s Cysts. A. An uncomplicated Baker’s cyst (arrowheads, between cursors, +) appears as a cystic mass containing anechoic fluid in popliteal fossa. A component of the cyst (arrow) extends toward the knee joint space. B. A large Baker’s cyst (arrows) in a patient with rheumatoid arthritis contains echogenic debris and pannus. C. An osteochondral fragment (arrow) from traumatic knee injury has migrated into a Baker’s cyst appearing as an echogenic shadowing mass within the cyst.
  • The cyst occurs in a characteristic anatomic location in the popliteal fossa between the medial head of the gastrocnemius and the distal semimembranosus muscles (Fig. 13.11).
  • Uncomplicated Baker’s cysts contain anechoic joint fluid.
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  • Complications of Baker’s cysts include hemorrhage, pannus formation, dissection, rupture, and loose bodies [25]. Complicated Baker’s cysts may be septated and contain echogenic fluid and debris. The cyst may dissect into the muscles of the calf. Osteochondral fragments form as loose bodies in the knee joint that may migrate into the Baker’s cyst. Synovial proliferation (pannus) is seen with rheumatoid arthritis and pigmented villonodular synovitis.
  • US provides clear distinction from other masses in the popliteal fossa, such as aneurysm of the popliteal artery or soft tissue mass.
Figure 13.12 Ganglion. A “bump” on the ventral surface of the great toe is shown by US to be a ganglion (straight arrow) attached to the tendon sheath of the extensor hallucis longus tendon (curved arrow) shown on a transverse image.
Figure 13.13 Sebaceous cyst. A “bump” on the forearm is shown by US to be a sebaceous cyst (arrow) extending from the dermal layer. An anechoic standoff pad (**) improves visualization of superficial structures. L, left.
Ganglion Cyst
Ganglion cysts are the most common soft tissue mass in the wrist and hand [26]. Ganglions are mucin-filled cysts lined by fibrous tissue. Their etiology is uncertain; however, some may be caused by trauma. The cysts are attached to tendon sheaths, muscles, or cartilage. Unlike synovial cysts, they are not lined by synovium and rarely communicate with the joint space [27].
  • Ganglion appear as well-defined anechoic masses with acoustic enhancement (Fig. 13.12). They are firm masses that do not compress.
  • Some contain layering echogenic debris. They may be multiloculated.
Skin Cysts
Skin cysts are best visualized by US with a standoff pad (Fig. 13.13).
  • Sebaceous cysts are very superficial within the dermal layer. They are round, well-defined and contain diffusely echogenic fluid [22].
  • Mucinous cysts are oval, well-defined, and located superficially between subcutaneous fat and the dermal layer. Fluid is anechoic or hypoechoic [22].
Infection, Inflammation, and Trauma
Cellulitis
Bacterial infections of the soft tissues show a spectrum of abnormalities that range from cellulitis to tissue necrosis to liquefaction to a well-formed abscess [28,29]. The most common causative organisms are Staphylococcus aureus and Streptococcus pyogenes [30]. US-guidance is utilized to aspirate any fluid collections for analysis and culture.
Figure 13.14 Cellulitis. Cellulitis causes diffuse and irregular thickening of the skin (arrow) and edematous disruption of the pattern of subcutaneous tissue. Compare to Figure 13.1.
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  • Cellulitis appears as diffuse thickening and increased echogenicity of the involved soft tissues with numerous hypoechoic edematous strands that create a network pattern through the tissues (Fig. 13.14). The edematous strands represent distended lymphatic channels [30,31].
  • Diffuse skin edema may have a similar appearance [30].
Abscess
Soft tissue abscesses have a variable appearance [30]. Any discrete fluid collection is suspicious in a patient with cellulitis.
  • Abscesses range in appearance from anechoic fluid to complex, heterogeneous, echogenic masses. Internal debris, septations, and loculations are commonly evident (Fig. 13.15).
  • Demonstration of an aspiratable or drainable fluid collection is the key to US examination. Echogenic fluid is best recognized by observation of swirling motion of suspended material during ballottement with the US transducer [32].
  • Doppler will usually show a peripheral rim pattern of prominent blood flow [33].
Necrotizing Fasciitis
Necrotizing fasciitis is a rapidly progressive bacterial infection of skin and subcutaneous tissue associated with extensive tissue loss, severe systemic toxicity, and sometimes, death [34]. It presents clinically as a localized cellulitis but with prominent systemic symptoms including fever, tachycardia, tachypnea, and profound malaise. The infection dissects along fascial layers causing extensive tissue necrosis that undermines surrounding structures. Early surgical excision of necrotic tissue is required to preserve tissue and prevent mortality.
  • Characteristic US findings are [34]:
    • - Irregular fascial thickening with fluid accumulation.
    • - Loculated abscess within a fascial plane.
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    • - Subcutaneous tissue swelling.
    • - Air bubbles are commonly present in soft tissue.
Figure 13.15 Abscess. Image from the same patient as shown in Figure 13.14 but in a different area shows a subcutaneous fluid collection (arrows). Ballottement with the transducer causes swirling of the echoes within fluid, confirming the liquid nature of the collection. US-guided aspiration yielded a small volume of gross pus.
Hemorrhage
Soft tissue trauma may cause discrete hematomas, intramuscular hemorrhage, subcutaneous fat necrosis, soft tissue inflammation, and myositis ossificans. Hemorrhage provides a fertile site for development of infection. US is used to detect fluid components and to guide aspiration to diagnose infection or to relieve pressure symptoms.
  • Hematoma are usually well marginated and confined to one compartment (Fig. 13.16). Acute clotted blood is homogeneously echogenic. With time and clot dissolution, the collection becomes more heterogeneous and eventually predominantly cystic. Subacute hemorrhage shows mixed areas of increased and decreased echogenicity. Liquefied hematomas or seromas are predominantly cystic with echogenic fibrinous strands and internal debris.
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  • Intramuscular hemorrhage appears infiltrative and mass-like, disrupting the normal uniform fascicular pattern of skeletal muscle. Comparison with adjacent muscles or with the same muscle on the opposite side is helpful in recognizing the hemorrhagic changes. Color Doppler is used to detect abnormal vascularity. Look carefully for a tumor nodule that might have hemorrhaged. If the history of trauma is equivocal or an intramuscular mass is suspected, follow-up US or MR examination is recommended.
Figure 13.16 Hematoma. A subcutaneous hematoma approximately 9 days old has a partially cystic, partially solid appearance. US-guided needle aspiration was performed to exclude infection. The needle tip (arrow) is well visualized within the hematoma. Only old blood was aspirated.
Figure 13.17 Tenosynovitis. A. Longitudinal view of the extensor hallucis longus tendon shows marked swelling of the tendon sheath (arrowheads) with diffuse thickening of the tendon (arrow). B. Transverse view confirms the irregular swollen echogenic tendon (arrow) and the large amount of fluid in the tendon sheath (arrowheads).
Tenosynovitis
Tenosynovitis refers to inflammation of the tendon or synovial tendon sheath. Causes include overuse, connective tissue disorders, and infection [35]. Infection usually results from puncture or penetrating injury. Septic tenosynovitis requires prompt drainage to avoid tendon necrosis or joint contamination [30].
  • US shows excessive fluid in the tendon sheath with or without swelling of the tendon (Fig. 13.17) [30,35].
  • Acute tendonitis appears as a swollen and hypoechoic tendon with ill-defined margins.
  • Doppler demonstrates increased blood flow surrounding the tendon sheath. More prominent blood flow is seen in patients with more pronounced symptoms [36].
  • Focal tendon sheath masses may be caused by rheumatoid nodules or pigmented villonodular synovitis.
  • Calcifications are seen within tendons in chronic tendonitis. They appear as echogenic foci with acoustic shadowing. Small calcifications may show comet tail artifacts.
Bursitis
Bursitis most often involves the subdeltoid, patellar, olecranon, or calcaneal bursa. Bursitis is caused by trauma, infection, or crystal-induced arthropathies.
  • The inflamed bursa is distended and fluid-filled (Fig. 13.18). Fluid may be anechoic but more commonly contains echogenic debris. Margins may be indistinct. Septations are often present. The nearby joint is often unaffected. Aspiration of the bursa is required if infection is suspected [30,31].
  • Chronic bursitis may show calcifications in the wall of the bursa.
Figure 13.18 Bursitis. The subdeltoid bursa (arrow) is distended with echogenic fluid in this patient with a rotator cuff tear.
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Muscle Injuries
Muscle injuries are caused by excessive pulling force (distraction) or by direct blunt trauma [35].
  • Distraction injury disrupts a variable number of muscle fibers. Small injuries produce small, linear, flame-shaped fluid collections between the muscle fibers. Partial or complete muscle rupture results in disrupted muscle fibers surrounded by larger fluid collections. Muscle fibers are seen to be discontinuous. These injuries are associated with acute functional impairment of the involved muscle [35].
  • Blunt force injuries result in intramuscular bleeding without extensive disruption of muscle fibers. Small hemorrhages are usually called contusions whereas larger hemorrhages are called intramuscular hematomas.
  • Rhabdomyolysis is necrosis of muscle fibers caused by ischemia that may be induced by crush injury, drug toxicity, or extreme overuse. Myoglobin and muscle enzymes are released into the blood stream. US shows areas of decreased echogenicity and nonhomogeneous muscle texture.
  • Fibrous muscle scars produce focal echogenic areas in the damaged muscle. Acoustic shadowing may be evident.
Tendon Rupture
Tendon injuries are usually classified as partial or complete tendon ruptures.
Figure 13.19 Partial Tendon Tear. The infraspinatus tendon shows a focal area of thinning (between cursors, +) with increased echogenicity of the tendon, indicating a chronic tendon injury.
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  • Complete tendon rupture is usually evident clinically. US shows a discontinuous tendon with the two fragments separated by hypoechoic blood or granulation tissue. Absence of visualization of a normally visualized tendon may be the only sign.
  • Partial tendon ruptures are seen as focal hypoechoic defects within the echogenic tendon or as focal thinning of the tendon (Fig. 13.19). Care must be taken to avoid calling tendon anisotropy a partial tendon tear. The tendons must always be imaged as near to perpendicular to the US beam as possible. Tendons routinely examined by US include the rotator cuff tendons, Achilles tendon, and the quadriceps and patellar tendons.
  • When a tendon sheath is present, partial ruptures appear as an anechoic cleft in the tendon with fluid distention of the tendon sheath [4].
  • Subluxation or dislocation of a tendon is documented by demonstrating absence of the tendon in its normal location [4]. This finding is often best demonstrated during dynamic imaging as the tendon and joint are moved through the normal range of motion.
Hernias
Hernias of the abdominal wall are common and are frequently a diagnostic challenge especially in obese patients. Hernias may present as a superficial soft tissue mass and must be differentiated from other musculoskeletal masses. US is very effective in the diagnosis of hernias. Abdominal wall hernias are classified as incisional, linea alba, umbilical, or Spigelian hernias. Groin hernias are classified as inguinal (indirect or direct) or femoral. Complications of hernia include pain, which may be positional, incarceration, and strangulation. Incarceration means that the hernia is not reducible but the blood supply is not compromised. Strangulation means that the blood supply to the hernia contents is reduced and that the tissue involved in the hernia is ischemic. Strangulation is an indication for immediate surgical repair [37].
  • The US diagnosis of a hernia is based upon visualization of abdominal contents protruding through tissue planes (Figs. 13.20, 13-21 and 13.22). For abdominal wall hernias the parietal peritoneum is identified by visualization of moving bowel beneath it. The presence of ascites makes identification of the parietal peritoneum easy. Hernia contents are then observed to move through the parietal peritoneum and into the abdominal wall. The Valsalva maneuver promotes motion of hernia contents and increases the size of the hernia. Some herniations occur only in the standing position. If a hernia is not present
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    in the area of concern with the patient supine, the US examination should be repeated with the patient standing [37,38].
  • The contents of hernias vary (Figs. 13.20, 13.21). Most hernias diagnosed by US contain fat and membranes (omentum or properitoneal fat) rather than bowel. Peritoneal fluid may also be present in the hernia sac.
  • The size of the hernia neck should be estimated. Small hernia necks increase the risk of incarceration and strangulation.
  • US signs of strangulation include edema, thickening of the wall of the hernia sac, thickening of bowel wall and absence of peristalsis in bowel contained within the hernia, and absent blood flow in hernia sac contents on Doppler examination.
  • Incisional hernias occur through surgical incisions in the anterior abdominal wall (Figs. 13.20, 13.21). Any abdominal wound may be a site of hernia development.
  • Linea alba hernias protrude through the fascia of the linea alba in the midline of the abdomen.
  • Umbilical hernia s are common and usually congenital caused by failure of complete closure of the abdominal wall around the umbilical cord. The hernia bulges at the umbilicus.
  • Spigelian hernias occur in characteristic location along the linea semilunaris (Fig. 13.22) [39]. Spigelian hernias are rare but difficult to diagnose and have a high risk of incarceration. The aponeuroses of the internal oblique, external oblique, and transversus abdominis muscles in the flank fuse along the medial edge of the three muscles forming the linea semilunaris. This fused aponeurosis divides medially to pass both anterior and posterior to the rectus muscle in the upper abdomen. In the lower abdomen, midway between the umbilicus and the symphysis pubis, the aponeurosis of the three flank muscles passes only anterior to the rectus muscle, leaving a weakened fascial
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    layer involving the lower one-fourth of the edge of the rectus muscle. Spigelian hernias form along this weak fascia plane.
  • Indirect inguinal hernias extend into the deep inguinal ring and a variable distance down the inguinal canal as far as the scrotum or labia. Indirect hernias lie medial to the inferior epigastric artery and anterior to the spermatic cord. Color Doppler aids in the identification of the inferior epigastric artery and in the differentiation of indirect and direct inguinal hernias [40].
  • Direct inguinal hernias develop as a result of weakening of the transversalis fascia. The fascia may tear or just stretch and bulge. Direct hernias lie lateral to the inferior epigastric artery and posterior and medial to the spermatic cord.
  • Femoral hernias are uncommon and difficult to diagnose because they lie deep within the femoral canal. Femoral hernias also occur because of weakening of the transversalis fascia often related to pregnancy. Thus, they are more common in women. The hernia sac protrudes through the femoral canal posterior to the inguinal ligament and medial to the common femoral vein.
Figure 13.20 Incisional Hernia. A hernia through a surgical wound created during cholecystectomy is well visualized in a patient with ascites (a). The ascites clearly defines the layer of parietal peritoneum (arrow) lining the peritoneal cavity. Omentum containing fluid between its layers herniates (arrowheads) into the abdominal wall. Ascites fluid (f) has also dissected into the hernia sac. The size of the hernia defect is measured by a cursor (+). Omentum is differentiated from bowel by absence of peristalsis and lack of continuity with bowel in the peritoneal cavity.
Figure 13.21 Incisional Hernia. A. The location of the peritoneal cavity (PC) is determined by real-time US examination looking for movement of abdominal organs with respiration and for bowel peristalsis. A hernia sac (H) containing omentum is seen protruding from the peritoneal cavity into the anterior abdominal wall through an abdominal wall defect marked by the arrowheads. B. In the same patient, an extended field-of-view image shows the hernia defect (arrowheads), the hernia sac (H), and the liver in the peritoneal cavity.
Figure 13.22 Spigelian Hernia. A. Transverse image of the anterior abdominal wall shows the fascia layers used as anatomic landmarks for identification of the weak fascial area where Spigelian hernias occur. The fascia of the external oblique (EO), internal oblique (IO), and transversus abdominis (TA) muscles fuse medially to join the fascia of the rectus abdominis (RA) muscle. The fascia is weakest lateral to the rectus and medial to the flank muscles (arrowhead) in the lower abdomen below the level of the umbilicus. The arrow indicates the location of the parietal peritoneum. B. A Spigelian hernia (H) protrudes through a defect (arrowheads) in the abdominal wall. The arrow indicates the location of the parietal peritoneum. PC, peritoneal cavity.
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