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Ultrasound should be considered a complementary skill to MRI and is often applied as a first-line investigation.
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Ultrasound has a number of specific advantages over MRI including low cost, feasibility, dynamic capabilities, and ability to allow real-time interventions.
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Assessment of muscle pathology is considered at least an intermediate-level skill.
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Knowledge of anatomy and function of the muscle (e.g., point of origin and insertion and direction of muscle and fibers) is helpful when considering pathology.
Interest in the use of imaging for the investigation of muscle pathology has risen dramatically in response to technical improvements producing better image resolution, the growth of sports medicine (especially recreational), and an increased awareness by clinicians of the capabilities of imaging. Magnetic resonance imaging (MRI) and ultrasound are regarded as the investigations of choice for imaging muscle trauma and disease, with each modality having its own specific advantages.
Until recently, MRI was considered the superior choice. Its exquisite multiplanar tomographic images and ability to visualize muscle edema using a combination of T1- and T2-weighted images have enhanced its reputation. However, the image resolution of ultrasound has overtaken conventional MRI, and ultrasound’s superior accessibility and ability to obtain real-time dynamic images have made it a competitive alternative for the investigation of muscle damage. In clinical practice, ultrasound may be considered a first-line investigation or a supplementary technique to clarify the findings of other techniques such as MRI.
The major indications for performing ultrasound examinations vary according to the specialty. In sports medicine, traumatic muscle tears and strains form a significant proportion (30%) of injuries, because accurate assessment of muscle injury can be important in the diagnosis and in planning rehabilitation. In rheumatology, ultrasound has been used increasingly for inflammatory muscle conditions.
This chapter reviews the current role of ultrasound in the investigation of skeletal muscle disease. For optimal use of ultrasound, a thorough knowledge of muscle anatomy and physiology must dovetail with meticulous technique. According to the European League Against Rheumatism (EULAR) guidelines for rheumatologists, muscle sonography is designated an intermediate or advanced skill, which reflects some of the technical challenges of this discipline.
Anatomy and Physiology
Normal Muscle Anatomy
Skeletal muscles make up approximately 40% of the total human body weight; muscle mass is slightly higher in males than in females. Their primary function is to produce movement by contracting and relaxing in a coordinated manner. Muscle bellies are attached to bone by tendons at points known as the origin and insertion . In some muscles, the origin and insertion are of a similar size (e.g., biceps), but in others, there is a difference. For example, in the supraspinatus tendon, the origin (i.e., supraspinatus fossa) is much broader than the insertion (i.e., greater tuberosity of the humeral head). Muscle is composed of bundles of fibers (i.e., fascicles) that run parallel to each other, usually along the longitudinal axis of the muscle. Muscle fibers vary in length from a few centimeters in most muscles to up to 60 cm in, for example, the sartorius muscle.
Macroscopic Appearance
In different muscles, the fibers are orientated differently, and thus a knowledge of muscles and their orientation is useful in order to avoid misinterpretation of images when scanning. In some muscles, the fibers run along the line of the tendon, with proximal distal tapering to the tendon resulting in a fusiform configuration. In others, the fibers have a more oblique orientation in relation to the tendon; this is known as a pennate or feather-like distribution ( Fig. 12-1 ). Pennate orientations may be summarized as follows:
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Unipennate: The tendon lies on one side of the muscle and the fibers insert into it along the length of the muscle (e.g., extensor digitorum longus or flexor pollicis longus).
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Bipennate: The central tendon receives oblique fibers from both sides (e.g., rectus femoris).
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Circumpennate or multipennate: The central tendon receives fibers from all the way around (e.g., biceps brachii muscle, tibialis anterior).
Some muscles assume a spiral arrangement between the origin and the insertion (e.g., pectoralis major or supinator).
Microscopic Appearance
Microscopically, individual muscle fibers are surrounded by a thin fascial layer called the endomysium ( Fig. 12-2 ). The fibers are packaged together in bundles (i.e., fascicles), which are surrounded by the perimysium ; this is sometimes referred to as the fibro-adipose septum. It is thicker connective tissue than the endomysium and contains nerve endings and small blood vessels. These bundles are packaged together to form the muscles, which are surrounded by the thicker epimysium, where nerves and blood vessels are also located.
Normal Muscle Physiology
Physiologically, muscles are relatively heterogenous, consisting of two groups of muscle fibrils: T1 (i.e., red or slow twitch) and T2 fibers (i.e., white or fast twitch). Postural muscles consist mainly of T1 fibers, which are mitochondria rich and therefore can perform sustained, low-energy contractions. T2 fibers depend on the glycolytic pathway, and muscles rich in this fiber type produce much more forceful and rapid contractions. The arrangement of muscle fibers also determines muscle physiology; purely linear arrangements are optimal for distance movement (i.e., postural muscles), whereas pennate arrangements are better for producing maximal force.
Muscles that have a predominance of T2 fibers, have a pennate arrangement, and span more than two joints are subject to the greatest intrinsic forces and are therefore more susceptible to indirect muscle injury. The forces within a muscle depend on the way the muscle contracts. Isotonic contractions occur when a constant load is applied to a muscle and its length changes. The length can shorten, as with concentric contractions in which the muscle attachments move closer together, causing movement of the joint, or they can lengthen, as with eccentric contractions in which the muscle fibers stretch to slow movement. Eccentric contractions produce greater intrinsic forces than concentric contractions.
During exercise, blood flow through the muscle and connective tissues can increase 20-fold, with resultant muscle swelling and displacement of the overlying fascial planes (a volume increase of 10% to 15%). Edema has been identified on MRI in normal subjects after exercise, but no consistent change in echotexture has been described on ultrasound.
The function of muscle is to generate force through active contraction of the muscle fibrils within the muscle belly. This active force is transmitted to bone through the relatively inactive muscle tendons. In athletes and young adults, the main area of weakness in this muscle-tendon-bone unit is the myotendinous junction, where the transformation zone between the muscle fibrils and tendon is relatively inelastic. The junction is the area where there is greatest movement and force during muscle contraction. In the immature skeleton, the weakest area is the bone-tendon interface with the physis (leading to avulsion fracture), and in the older population, it is usually the degenerate tendon that tears. This explains why indirect muscle injuries are relatively uncommon in the latter two groups.
Ultrasound Examination Technique
A clinical history and physical examination are essential before undertaking an ultrasound investigation. The information gained allows the examination to be targeted toward the most relevant areas.
Fortunately, most skeletal muscles lie superficially within the body and are easily accessible by ultrasound. Linear transducers with multifrequency capability (with center frequency greater than 10 MHz) are preferred. However, lower-frequency linear (8.5 MHz) or curved-array (5 MHz) transducers may have to be used in obese or very muscular patients, especially in the gluteal region and proximal thigh. The optimal choice of transducer should be tailored to the individual muscle region and may have to be altered during the examination. In most muscle examinations, the use of thick coupling gel is adequate for assessment, but a stand-off pad is sometimes helpful for the investigation of muscle hernias because even minimal transducer pressure can maintain a hernia in reduction.
Dual-screen facility allows real-time comparison of two areas (usually the pathologic area and the asymptomatic opposite side), but it can also be used in a single area to double the transducer’s field of view. Many medium- to high-cost machines have panoramic or extended field of view capabilities, which can lengthen the field of view to 10 to 15 cm. This feature is useful for measuring muscle lesions that are larger than the transducer’s normal field of view and for demonstrating pathology (especially to nonsonographers).
Scanning should be undertaken in longitudinal and transverse planes of the symptomatic area. Pain resulting from muscle injury is usually well localized, although inflammatory conditions such as myositis cause more diffuse symptoms. This can be achieved by moving to the nearest anatomic area where the underlying muscular and tendinous anatomy can be defined and then scanning back to the area of abnormality by following the muscles and tendons in a continuous manner. The transverse plane is most useful in this respect.
After assessing the appearance of any pathology at rest, the abnormal area and surrounding tissues should be assessed dynamically with active or passive contraction, or both. This allows the consistency of the abnormality (i.e., solid or cystic), alteration in muscle function, and any movement of disrupted fibers (helping to differentiate grades of tears) to become more apparent. Additional maneuvers, especially in the case of muscle hernias, may be required because the hernia may become apparent only when the patient is standing (discussed later).
The angulation of the transducer can sometimes be important because the artifact produced can cause confusion. Anisotropy occurs if the region of interest is not perpendicular to the muscle, making it appear artificially hypoechogenic or the fibrous septa appear more echogenic. This may result in an overdiagnosis of muscle edema or tears. Care should be taken when applying pressure with the transducer; too much may make the muscle more echogenic and may obliterate any Doppler signals. The degree of transducer pressure should be considered when measuring muscle thickness.
Doppler investigation is usually not necessary in assessing muscle injury, except when there is clinical doubt regarding other underlying pathology (e.g., soft tissue sarcoma, inflammatory lesion, vascular abnormality). However, it may be of value for inflammatory muscle conditions. Three-dimensional imaging has little current practical application, although it has potential as a research tool ( Fig. 12-3 ).
Normal Ultrasound Appearance
Normal muscle bundles or fascicles are usually hypoechoic. In a longitudinal scanning plane, they are separated by multiple, long echogenic lines, which represent the connective tissue known as the perimysium or fibroadipose septa. The pennate appearance of some muscles is best appreciated with this view ( Fig. 12-4 A). The appearance of muscle varies according to the orientation of the muscle fibers within it. Smaller muscles (e.g., hand) tend to have a much finer echotexture than the larger muscles, such as those of the leg ( Fig. 12-5 ).
In a transverse plane, the lines are seen as dots or short linear shapes (see Fig. 12-4 B). Identification of the thicker echogenic outer fascial layer of the muscles (i.e., epimysium) allows differentiation of muscle groups. The endomysium cannot be seen on ultrasound. The perimysium and the epimysium contain blood vessels and nerves, which may be seen depending on size ( Fig. 12-6 ).
Within the muscle itself, the muscle fibers attach to a fibrous aponeurosis that eventually becomes the tendon. This is much denser and echogenic than the perimysium, and it can be seen in longitudinal and transverse planes ( Fig. 12-7 ).
During contraction (i.e., isometric or concentric) muscle echogenicity decreases as the muscle fibers thicken. Blood flow through the fascial layers may decrease during contraction but increase during and after exercise.
Pathologic Ultrasound Appearance
Most ultrasound investigations are done for trauma, although rheumatologists use ultrasound for the investigation of polymyositis and pyomyositis. Abnormalities in muscle may be described as being within the muscle belly itself or within the boundary area-muscle-fascia or muscle-tendon borders.
Muscle Injury
Muscle injury may be described as acute or chronic.
Acute Muscle Injury
Injuries can be classified as direct (e.g., contusion, laceration) or indirect (e.g., strain, tear).
Direct Muscle Injury
Muscle Contusion
Muscle contusion results from direct trauma that causes muscle fiber disruption and hematoma by compression of muscle against bone. Pathologically, the dominant process is hematoma, which begins to organize within 2 to 3 days. Healing occurs with muscle regeneration and fibrosis proportional to the extent of the injury.
Muscle contusion is commonly seen in contact sports (“dead leg”) or as part of polytrauma, usually occurring in the lower limbs. This is a clinical diagnosis obtained from patient history, but on examination, muscle function is relatively normal given the degree of pain. Clinically, the patient can be graded according to the restriction of joint movement nearest the site of impact. The grading system is divided into mild, moderate, or severe, with mild being joint movement greater than two thirds of full range, moderate a third to two thirds of full range, and severe less than a third of full range.
On ultrasound, an acute contusion (0 to 48 hours) appears ill-defined, with irregular margins and marked echogenic swelling of the fascicles and entire muscle. In severe clinical cases, dynamic imaging confirms that a complete tear is not present and documents the extent of muscle damage. At 48 to 72 hours, ultrasound appearances become better defined, with a clearer echogenic margin and the main area of the hematoma appearing hypoechoic ( Fig. 12-8 A). Subsequently, as the hematoma begins to organize, the echogenic periphery gradually fills in toward the center (see Fig. 12-8 B). In the following weeks, the contusion can be monitored for regeneration of muscle, scar tissue, or rarely, myositis ossificans (see “Complications”). However, in sporting injuries, most contusions heal with normal muscle regeneration, and chronic complications are relatively rare.
Muscle Laceration
Muscle laceration is a direct, penetrating injury that incises through the skin, subcutaneous tissues, and underlying muscle. Usually, the superficial injury heals well, but the underlying muscle injury has a high incidence of linear scar formation ( Fig. 12-9 ). Scar formation within the muscle decreases its ability to shorten and therefore decreases its ability to generate tension on contraction.
Lacerations are most commonly seen in trauma cases but can be associated with particular sports, such as ice hockey. Although in most cases, this decrease in function is not clinically relevant, if the muscles are required for a specific task or sporting activity, the limitations in developing maximal range of movement or power are more significant.
Ultrasound demonstrates the scar as a linear echogenic structure with relatively normal surrounding muscle architecture (see Fig. 12-9 ). Unlike the surrounding septa, the scar does not follow any normal anatomic plane, and it is thicker, longer, and more irregular than normal septa.
Indirect Muscle Injury
Indirect muscle injury is a common mechanism of sports injury. The incidence and muscle groups affected vary according to the sport. In soccer, the incidence is 30% to 38% of all injuries, and as in many other sports, the lower limb is most commonly affected.
Delayed-Onset Muscle Soreness
Delayed-onset muscle soreness (DOMS) develops when specific muscle groups undergo unaccustomed strenuous exercise. This usually occurs in recreational athletes who sporadically participate in sports. However, it can also occur in professional athletes with exercise of muscle groups not normally used in their own sport or when training is intensified after injury. Pathologically, it is thought to be a disruption of muscle fibrils, particularly at the myotendinous junction, where there are also large concentrations of pain receptors. Some studies have shown muscle enzymes to be elevated after 24 hours, and whether this results from direct fibril damage or from secondary lysosomal release is unknown.
Clinically, diffuse muscle pain develops 12 to 24 hours after activity, and it affects multiple limbs and is exacerbated by eccentric contractions. This helps to clinically differentiate DOMS from a muscle tear or strain, which usually causes immediate focal pain and is exacerbated by concentric contractions. DOMS usually resolves within 7 days without any specific treatment.
Because this is a clinical diagnosis, imaging is rarely necessary in most cases. However, in athletes when it can occur if training is intensified after injury, imaging can be useful in excluding other causes of severe pain if the clinical history is not clear. MRI can show edema in many muscles, but this is not a specific or sensitive finding; the abnormality can persist up to 82 days after clinical resolution. Ultrasound findings are usually normal, but its role lies in excluding a significant muscle strain or tear, which allows appropriate rehabilitation to continue.
Muscle Strain or Tear
Muscle strain or tear is an indirect injury caused by excessive force applied across the muscle rather than direct trauma. Muscles with increased T2 fibrils that span two joints and perform forceful eccentric contractions are more susceptible to this form of injury because they experience the greatest intrinsic forces. Some muscles are more commonly affected than others within a specific muscle group, especially the rectus femoris and biceps femoris.
The myotendinous portion of the muscle is the most likely area to be injured, but the junction of the muscle fibers and epimysium is also susceptible to injury. The myotendinous junction is histologically much more extensive than is apparent on imaging and can extend throughout 60% of the total muscle length. On exceeding the elastic limit of the muscle, the fibrils and fascicles are disrupted, with hemorrhage from the torn vascular fascia predominating in the first 24 hours. Subsequently, there is marked muscle edema with an inflammatory infiltrate. After 2 days, organization begins to occur along with early muscle regeneration. Muscle healing can take 3 to 16 weeks, depending on the extent of injury. The ability of myocytes to regenerate is good, but if the injury is extensive, there is always a potential for fibrous scar tissue to form.
Clinically, muscle strain or tear is characterized by immediate focal pain and decreased function that can be caused by muscle disruption or associated reactive spasm in adjoining muscles. Occasionally, a subcutaneous ecchymosis can occur, but it usually develops 12 to 24 hours later. A well-established clinical grading system has three components. Grade 1 injury is less than 5% loss of function. Grade 2 injury is more severe, but with some function preserved. Clinical grade 3 strains are complete muscle tears with no objective function and occasionally a palpable gap in the muscle belly. Differentiation of these clinical grades can be difficult, and imaging has an important role in these situations.
Described imaging grading systems have tried to correlate findings with the clinical grading system ( Figs.12-9 to 12-13 ). On ultrasound examination, grade 1 muscle injuries can show normal appearances or a small area of focal disruption (<5% of the muscle volume), and hematoma and perifascial fluid are relatively common.
