Basic Pathology Concepts

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Chapter 2 Basic Pathology Concepts

imageAdditional videos for this topic are available online at www.expertconsult.com.

The full text of this chapter can be accessed online at www.expertconsult.com.

Muscle and Tendon Injury

Muscle and tendon injuries may be categorized as acute and chronic. Acute injuries tend to take the form of direct impact injury, stretch injury during contraction (strain), or penetrating injury. Acute muscle injury can be clinically categorized as grade 1 (no appreciable fiber disruption), grade 2 (partial tear or moderate fiber disruption with compromised strength), and grade 3 (complete fiber disruption).1 At sonography, muscle contusion and hemorrhage acutely appear hyperechoic (Fig. 2-1).2,3 Excessive and intense muscle activity may produce diffuse muscle hyperechogenicity if imaged acutely from transient muscle edema (Fig. 2-2).4 Partial fiber disruption indicates partial-thickness tear, whereas complete fiber disruption indicates full-thickness tear. One hallmark of full-thickness tear is muscle or tendon retraction, which is made more obvious with passive movement or active muscle contraction. Hemorrhage will later appear more hypoechoic (Fig. 2-3), although a heterogeneous appearance with mixed echogenicity is common (Fig. 2-4). As soft tissue hemorrhage resorbs, a hematoma will become smaller and more echogenic, beginning at the periphery (Fig. 2-5). A residual anechoic fluid collection or seroma may remain (Fig. 2-6). Hemorrhage located between the subcutaneous fat and the adjacent hip musculature can occur with trauma as a degloving-type injury, called the Morel-Lavallée lesion (Fig. 2-7).5 Residual scar formation appears hyperechoic (Fig. 2-8). Heterotopic ossification may remain and is hyperechoic with posterior acoustic shadowing (Fig. 2-9). An area of damaged muscle may ossify, termed myositis ossificans (Fig. 2-10), and ultrasound can show early mineralization before visualization on radiography.6 Often, computed tomography (CT) is needed to demonstrate the peripheral rim of mineralization characteristic of myositis ossificans, given shadowing seen on ultrasound. Prior trauma to muscle or its nerve supply can result in muscle atrophy, which causes increased echogenicity and decreased size of the muscle (Fig. 2-11).

With direct impact injury, the belly of a muscle is typically involved with hematoma and variable fiber disruption (Fig. 2-12). In contrast, stretching of a contracting muscle typically results in injury at the musculotendinous junction and is more common with muscles that span two joints, such as the hamstring muscles of the thigh (Fig. 2-13) and the medial head of the gastrocnemius (Fig. 2-14). It is important to consider the architecture of the muscle imaged in evaluation for myotendinous injury. For example, a muscle with a unipennate architecture (e.g., the medial head of the gastrocnemius) shows injury at the myotendinous junction located at its periphery (see Fig. 2-14).7 A muscle with circumpennate or bipennate architecture (e.g., the indirect head of the rectus femoris) may show injury at its distal musculotendinous junction or within the muscle belly as a central aponeurosis tear (see Fig. 6-59A in Chapter 6).8 Musculotendinous injury also demonstrates variable echogenicity from hemorrhage and fluid, depending on the age of the injury and the degree of fiber disruption. Passive joint movement or active muscle contraction can demonstrate retraction at the site of the injury that indicates full-thickness tear. Particularly in children, these types of acute tendon injuries may be associated with bone fragment avulsion at the tendon attachments, which appear hyperechoic with possible shadowing.

With penetrating injury or laceration, acute muscle and tendon injury may occur at any site (see Fig. 7-47 in Chapter 7). The obvious physical examination findings usually guide the ultrasound evaluation. Muscle and tendon injuries are again classified as partial-thickness or full-thickness tears. Dynamic imaging is helpful in this distinction because it makes retraction related to full-thickness tears more conspicuous. Gas introduced during the penetrating injury can make evaluation extremely difficult; air appears hyperechoic with heterogeneous posterior shadowing. It is also important to recognize adjacent soft tissue or osseous injuries outside the muscle because lacerations may cause peripheral nerve injury as well (see Fig. 6-80B in Chapter 6).

Chronic muscle and tendon injuries are usually the result of overuse, with tendon degeneration and possible tear. It has been shown that such involved tendons show eosinophilic, fibrillar, and mucoid degeneration but do not contain acute inflammatory cells; therefore, the term tendinosis is used rather than tendinitis.911 At sonography, tendinosis appears as hypoechoic swelling of the involved tendon, but without tendon fiber disruption (see later chapters). Several tendons may commonly show increased blood flow on color or power Doppler imaging in the setting of tendinosis, such as the patellar tendon, Achilles tendon, and common extensor tendon of the elbow. This increase in blood flow is not due to inflammation but rather represents neovascularity. Tendinosis may progress to partial-thickness and full-thickness tendon tear. Chronic muscle and tendon injuries that result in tear can be associated with atrophy of the muscle, which appears hyperechoic and decreased in size. After surgery, misplaced hardware or screw-tip penetration beyond the bone cortex may cause excessive wear of an adjacent tendon (Fig. 2-15). Ultrasound is helpful in this diagnosis because artifact from metal hardware does not obscure overlying soft tissues. In addition, dynamic imaging with joint movement or muscle contraction can determine whether a tendon is in contact with metal hardware with specific positions (Video 2-1).image

Bone Injury

The normal osseous surfaces are smooth and echogenic with posterior shadowing and possibly reverberation when imaged perpendicular to the sound beam. The hallmark of an acute fracture is discontinuity of the bone cortex with possible step-off deformity (Fig. 2-16).12 Adjacent mixed echogenicity hemorrhage may also be present. A stress fracture, for example involving a metatarsal, may initially appear as a focal hypoechoic area adjacent to bone, which may progress to fracture step-off deformity or hyperechoic callus formation (see Fig. 8-146 in Chapter 8).13 This is typically associated with point tenderness induced by pressure from the transducer. A patient also commonly indicates focal pain in the area. It is important at the completion of any ultrasound examination to ask the patient about such focal symptoms because they are often clues to underlying pathology that may not be otherwise evaluated.

Other types of bone injuries involve avulsion at tendon and ligament attachments. In these situations, a small fragment of bone with variable shadowing is seen attached to the involved tendon or ligament (see Fig. 8-143 in Chapter 8). Asymmetrical widening and irregularity of an open growth plate with point tenderness can indicate a physeal injury (Fig. 2-17).14 It is important to differentiate the findings of bone injury at ultrasound from bone irregularity resulting from osteophytes. This differentiation is possible because osteophytes occur at margins of synovial joints usually without point tenderness, whereas a fracture shows a cortical step-off deformity. Correlation with radiography may also be considered to assist with this differentiation.

In many situations in which a fracture is identified at ultrasound, the fracture is unsuspected. The indication for the examination is often to evaluate a soft tissue or joint abnormality after a “negative” radiograph. This is not uncommon in the foot and ankle, where multiple overlapping osseous structures may make radiographic diagnosis of fracture difficult. The other situation is the greater tuberosity fracture of the proximal humerus, which can be overlooked at radiography because of suboptimal patient positioning or suboptimal radiographic technique (see Fig. 3-103 in Chapter 3).15 It has been shown that ultrasound is more effective than radiography in the diagnosis of rib fracture (see Fig. 2-16A).16,17 As a fracture begins to heal, early hypoechoic callus becomes hyperechoic hard callus, which can eventually bridge the fracture gap or step-off deformity.12 This can also be applied to limb-lengthening procedures, in which ultrasound can detect new bone before radiography. Ultrasound has also been shown to be effective in diagnosis of tibial fracture nonunion with static interlocked nail placement; ultrasound can detect healing before radiography, whereas visualization of the hyperechoic nail indicates no overlying callus formation (Fig. 2-18).12 Another advantage of ultrasound is in the evaluation of nonossified structures, such as the distal humeral epiphyses in children and the anterior costocartilage.18,19

Infection

The imaging appearances of soft tissue infection are largely predicted by the route of infection spread. For example, in adults, infection commonly occurs through a puncture wound or skin ulcer. This produces infection of the soft tissues or cellulitis, which may have several appearances (Fig. 2-19). Acutely, cellulitis appears as hyperechoic and thickened subcutaneous tissue.20,21 Later, hypoechoic or anechoic branching channels are visualized, with distortion of the soft tissues and possibly increased flow on color or power Doppler imaging.20 Such branching channels can coalesce as purulent fluid and can progress to frank abscess, where ultrasound-guided aspiration may be of benefit.20 However, ultrasound-guided aspiration may be less effective in the setting of methicillin-resistant Staphylococcus aureus infection.22 When evaluating for cellulitis, the findings of anechoic perifascial fluid and gas (appearing as hyperechoic foci with comet-tail artifact or dirty shadowing) at the deep fascia can indicate necrotizing fasciitis.23 The differential diagnosis for ultrasound findings of hyperechoic subcutaneous fat, as seen with acute cellulitis, includes fat necrosis (Fig. 2-20); however, the latter condition is usually more focal, may be multiple, and is without physical examination findings of infection.24

image

FIGURE 2-20  Fat necrosis.

Ultrasound images from two different patients show (A) hyperechoic subcutaneous area (arrows) and (B) focal hyperechoic nodule (arrows) representing fat necrosis.

(From Walsh M, Jacobson JA, Kim SM, et al: Sonography of fat necrosis involving the extremity and torso with magnetic resonance imaging and histologic correlation. J Ultrasound Med 27:1751–1757, 2008. Reproduced with permission from the American Institute of Ultrasound in Medicine.)

The ultrasound appearance of abscess is variable but predominantly appears as well-defined hypoechoic heterogeneous fluid collection with posterior through-transmission and hyperemia on color or power Doppler imaging (Fig. 2-21).25 A thick hyperechoic and hyperemic wall may also be seen, as may soft tissue gas.26 Uncommonly, an abscess may be isoechoic or hyperechoic relative to the adjacent soft tissues (see Fig. 2-21D). In this situation, in which it may be difficult to identify an abscess, increased through-transmission and swirling of echoes within the abscess with transducer pressure are helpful features to indicate the presence of a fluid component (Videos 2-2 and 2-3).27 image Increasing the depth and field of view around a possible abscess often causes the increased through-transmission to become more conspicuous relative to the surrounding tissues.

Some infections occur after surgery and may be located immediately adjacent to metal hardware (see Fig. 2-21E). Ultrasound is ideal for evaluation in this situation because the reverberation artifact from the hardware occurs deep to the metal and does not obscure the overlying soft tissues.28 Soft tissue infection may also involve a bursa, which can produce complex fluid and synovitis, and possibly gas, which appears hyperechoic with comet-tail artifact (Fig. 2-22) (Video 2-4).image Unlike a nonspecific abscess, a bursal fluid collection tends to be more defined and, more importantly, occurs in an area of a known bursa. If an area of soft tissue infection is identified adjacent to bone, then osteomyelitis should be considered (Fig. 2-23). In the presence of cortical irregularity resulting from erosions or destruction, osteomyelitis is likely, although confirmation with magnetic resonance imaging (MRI) is often needed to assess the extent of infection fully.

Another route of infection is hematogenous, which may manifest as a muscle abscess, as septic arthritis, or as osteomyelitis. This mode of infection is more common in children, intravenous drug abusers, or patients with sepsis. In the correct clinical scenario, septic arthritis is suspected when there is fluid distention of a joint recess, which may range from anechoic to hyperechoic, with possible hypoechoic or isoechoic synovial hypertrophy (see later). The echogenicity of fluid or the presence of flow on color or power Doppler imaging cannot predict the presence of infection, and therefore ultrasound-guided percutaneous fluid aspiration should be considered. When distention of a joint recess is not anechoic, the possibility of complex fluid versus synovial hypertrophy must be considered. To help in this distinction, compressibility of the recess, redistribution of the contents with joint positions, and lack of internal flow on color Doppler imaging suggest complex fluid rather than synovial hypertrophy. When synovial hypertrophy related to a septic joint is present, discontinuity or irregularity of the adjacent bone cortex suggests erosions and possible osteomyelitis (Fig. 2-24). Joint inflammation and synovitis from infection are indistinguishable from other inflammatory conditions, such as rheumatoid arthritis. In children, hematogenous spread of infection may also directly infect the bone. In this situation, a subperiosteal abscess may be identified because the periosteum is loosely adherent in children when compared with adults (Fig. 2-25).

image

FIGURE 2-24  Septic sternoclavicular joint.

Ultrasound image shows heterogeneous distention of the sternoclavicular joint capsule (arrows). Note erosions (arrowheads) of the sternum (S) and clavicle (C).

(From Johnson M, Jacobson JA, Fessell DP, et al: The sternoclavicular joint: can imaging differentiate infection from degenerative change? Skeletal Radiol 39:551–558, 2010.)

Arthritis

The foregoing descriptions relate to infection of soft tissues and bone. However, inflammation may have noninfective causes. Other inflammatory conditions, such as rheumatoid arthritis, can produce joint findings (effusion, synovial hypertrophy, and erosions), which can resemble infection.29 Often, the distribution of the abnormalities and the clinical history assist with the differential diagnosis. Infection more commonly causes abnormalities at one site, and this diagnosis must be excluded before considering single-site involvement of a systemic inflammatory arthritis. The following represents general concepts of some inflammatory conditions with additional examples and text found in later chapters.

Rheumatoid Arthritis

The characteristic features of rheumatoid arthritis include synovial hypertrophy and erosions. Ultrasound can be used from early diagnosis to assessment of response to therapy and can guide injections or aspirations. Synovial hypertrophy appears as hypoechoic (Fig. 2-26) or, less commonly, isoechoic (Fig. 2-27) or hyperechoic relative to subdermal fat, poorly compressible tissue within a joint or a joint recess.30 Synovial hypertrophy may also involve other synovial spaces, such as a bursa or tendon sheath (Fig. 2-28). Flow may be seen on color or power Doppler imaging, depending on the inflammatory activity of the synovitis. When assessing for hyperemia of synovial hypertrophy, it is important to minimize transducer pressure to avoid occluding or dampening flow (Fig. 2-29) (Video 2-5).image Joint synovial hypertrophy may be seen in the dorsal recesses of the wrist, the volar and dorsal recesses of the metacarpophalangeal and interphalangeal joints of the hand, and the metatarsophalangeal joints of the feet. 31,32 Erosions appear as discontinuity of the bone cortex seen in two orthogonal planes (Fig. 2-30). Such erosions begin in the marginal regions of a joint, where the bone cortex is not covered with hyaline cartilage and is directly exposed to joint inflammation. Ultrasound is sensitive to bone cortex abnormalities but is not specific for erosions, with a reported false-positive rate of 29% for diagnosis of erosions.33 The finding of synovial hypertrophy directly over a cortical irregularity also increases the likelihood that an erosion is present. Correlation with radiographic and clinical findings remains important, in addition to the distribution of imaging findings. For example, rheumatoid arthritis commonly involves the metacarpophalangeal joints of the hands (especially the second), the metatarsophalangeal joints of the feet (usually at least the fifth), and the wrist joints. A rheumatoid nodule typically appears as a hypoechoic nodule at ultrasound (Fig. 2-31).34

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