Muscle Injury and Sequelae

Prevalence, Epidemiology, and Definitions

Injury of muscle, whether contusion, laceration, or strain, is one of the most frequently encountered injuries in athletes. Strain injury accounts for 90% of sports injuries of muscle and may result in significant disability.

Muscle injuries are not limited to athletes, however, and the interpreting radiologist must be comfortable with recognizing their radiologic presentation. Normal muscle activity relies on innervation and perfusion; thus, injuries that result in denervation or decreased circulation will impair function.

Anatomy

Normal skeletal muscle function is dependent on intact muscle fibers, normal innervation, and intact blood flow. Skeletal muscle is composed of myofibers supported by organized connective tissue. Individual myofibers are surrounded by a delicate sheath called the endomysium. Tens to hundreds of myofibers are then surrounded by the perimysium. The epimysium is the connective tissue that surrounds the entire muscle belly. This ordered connective tissue network mechanically supports the contraction of individual cells into an organized force that is converted into motion.

Individual muscles are composed of multiple motor units. Each motor unit contains either slow fibers (slow contraction but resistant to fatigue) or fast-twitch fibers (fast but less resistant to fatigue). The composition of fast and slow fibers in a given muscle is dependent on the muscle group and genetics.

Muscle is attached to bone by a tendon. The tendon is formed at each end of a muscle from a condensation of connective tissue fibers. The tendon is then attached to bone via Sharpey fibers. Injury to the muscle-tendon-bone unit can occur at any level.

Biomechanics

Skeletal muscle biomechanics can be separated into various actions: concentric contraction (shortening of a muscle), eccentric contraction (lengthening the muscle during contraction), and isometric contraction (contraction without change in length). All three actions are seen in the biceps brachii muscle during the different phases of a biceps curl: concentric contraction as the elbow is flexed, isometric contraction as the weight is held at the top of the curl, and eccentric contraction as the biceps lengthens while the weight is lowered. Eccentric exercise tends to result in less recruitment of motor units and is associated with less metabolic demand than concentric exercise.

Strain injury is one of the most common sport injuries and represents a tear of fibers from excess force or tension placed on the muscle. Any muscle can sustain a strain injury, but muscles that contain a higher percentage of fast-twitch fibers or that cross two joints, such as the hamstrings and rectus femoris, are at higher risk. An important additional risk factor is prior injury. Finally, muscles subjected to eccentric forces are more likely to sustain tears than those used in concentric exercise. Tears tend to occur at the weakest point of the bone-tendon-muscle unit, and this varies with age. In children, the apophysis represents a weak point and bone avulsions are relatively common, whereas in older patients, tendon tears are more common with the onset of tendon degeneration. Younger adults are more subject to muscle injuries. These tend to occur at the structurally weakest portion of the muscle, where muscle fibers join the tendon, also known as the myotendinous junction. The myotendinous junction is not a single point in a muscle but represents a continuum that stretches nearly the entire length of some muscles, such as the hamstring.

Pathology

Muscle injury can occur with ischemia, denervation, inflammation, infection, or physical force. In physical trauma, the healing process represents repair rather than regeneration; thus, a scar will be the end result of healing depending on the magnitude of the original injury. Healing of muscle is separated into three phases: destruction, repair, and remodeling.

The destruction phase of healing is initiated with rupture of the muscle, which is followed by necrosis, hematoma, and inflammation. Once the destruction phase subsides, the gap created in the injured muscle is bridged by two processes: the regeneration of muscle fibers and the formation of scar tissue. The muscle regenerative process, which decreases with age and is dependent on mechanical stress, directly competes with the formation of connective tissue scar. Connective tissue scarring is initially the mechanical weak link in the repair process, and reinjury from early mobilization or extensive scar formation can result in a true mechanical barrier preventing regeneration of myofibers across a gap. Most muscle injuries do not result in excessive scar formation. The repair process must also be accompanied by revascularization and reinnervation of the injured muscle. It can take weeks for an injured muscle to return to preinjury level, which is why professional athletes are subjected to conditioning regimens to reduce the likelihood of injury.

Imaging Findings

Muscle imaging is principally undertaken with MRI and ultrasound (US). Other modalities have limited uses in certain muscle pathology, as explained later in this chapter. MRI has long been considered the investigation of choice. The advantage MRI confers over US is that it can assess the entire muscle group in one examination. It is also better suited than US for the examination of deeper muscles. It is extremely sensitive to subtle abnormalities that persist in the muscle later in the healing process and would be missed on US. It is also non–operator dependent, allowing assessment between radiologists to be more accurately performed.

Developments in technology have led to US becoming increasingly important in assessment of muscle pathology. High-resolution probes are capable of excellent spatial resolution, and dynamic imaging can reveal abnormalities that would be missed on MRI. US visualizes the intrinsic muscle structure at fibrillar level, offering some advantages when distinguishing fiber disruption from muscle edema. It is a quick, cost effective, accessible test, and portable units allow athlete assessment on the field of play if necessary. Physiologic assessment of blood flow can also be performed. The US is dependent on the skill of the operator, and a scrupulous technique is required to ensure full examination and accurate diagnoses.

Muscle Trauma

The importance of imaging in muscle injury is highlighted by the key role played by the musculoskeletal radiologist in diagnosis, prognostication, appropriate management, and rehabilitation decisions, particularly in the athlete.

Muscle injuries can be classified into two main categories: intrinsic and extrinsic. Intrinsic injuries (or strains) are the result of the failure of muscle fibers during contraction whereas extrinsic injury results from trauma applied by an external force. The first commonly results in disruption of muscle fibers near the myotendinous junction (MTJ)—the weakest part of the muscle-tendon-bone unit. Extrinsic lesions result from laceration or compression of muscle against an underlying hard bony surface from a direct blow.

Intrinsic: Muscle Strain or Tear

Magnetic resonance imaging is considered the most sensitive modality to diagnose and characterize strain injuries of muscles. However, the US appearances of acute muscle injury have also been well described. Ultrasound has been shown to be sensitive for the detection of acute injuries but inferior to MRI for low-grade injury, along with nonacute and deep injury. Some deep muscles are particularly problematic or impossible to examine with ultrasound—for instance, the proximal psoas muscle, the obturator internus, and the piriformis. Whichever modality is used, the imaging findings of a muscle strain will depend on the severity and acuity of the injury.

Magnetic Resonance Imaging

Evaluation of muscle injury should be performed in the axial plane and a plane longitudinal to the muscle using both T1-weighted imaging and a fluid-sensitive sequence such as short tau inversion recovery (STIR). Because muscle tears can occur in multiple synchronous locations, the entire length of the muscle should generally be imaged, so coil selection will be predicated on the size of the muscle in question. In the lower extremity, imaging of both legs is recommended. Some injuries, particularly those that are chronic, may be more readily detected if compared with the unaffected extremity.

In the acute setting, fluid-sensitive sequences such as STIR or fat-suppressed T2-weighted sequences are the most important images for the detection of injury, with increased signal centered on the tear representing the pathology. Once the injury is visualized, the radiologist should attempt to characterize the extent of damage. The severity of the injury will dictate the extent of findings. In a grade 1 injury, increased signal is centered on the myotendinous junction, with fluid signal tracking into the adjacent muscle producing a feathered appearance ( Fig. 43-1 ).

FIGURE 43–1, A , Grade 1 myotendinous injury. Axial, fat-suppressed, fast spin-echo, T2-weighted MR image demonstrating faint increased signal (arrowhead) in muscle adjacent to thickened myotendinous junction. B , Grade 1 myotendinous tear. Coronal, fat-suppressed, fast spin-echo T2-weighted MR image showing faint increased signal in muscle (arrowheads) adjacent to thickened myotendinous junction.

Grade 2 injury represents a partial tear of the myotendinous junction and is distinguished from grade 1 injury by a wavy appearance of the myotendinous junction and the presence of a hematoma ( Fig. 43-2 ).

FIGURE 43–2, A, Axial, fat-suppressed, fast spin-echo, T2-weighted MR image of a grade 2 myotendinous tear of biceps femoris. Hematoma (black arrow) is demonstrated, and there is muscle fiber retraction from the myotendinous junction (arrowhead) . Further intramuscular hematoma is seen peripherally close to the epimysium (white arrow) . B , Sagittal, fast spin-echo, T2-weighted MR image in the same patient shows the irregular and thickened myotendinous junction (arrows) .

In grade 3 injury, the myotendinous junction is completely disrupted ( Fig. 43-3 ).

FIGURE 43–3, Coronal, fat-suppressed, fast spin-echo MR image of anterior thigh. Complete disruption of the myotendinous injection is seen (arrowhead) , indicating a grade 3 injury.

In grade 2 and grade 3 lesions, a hematoma may be confused with neoplasm. The presence of feathery edema extending away from the mass should favor a hematoma rather than tumor. Additional findings may include perifascial edema, which can be seen in up to 87% of acutely injured athletes. Although grading these lesions is important from a research perspective, in clinical practice it can be difficult to differentiate between grades of injury.

MRI has been used to predict the time to recovery from injury, which is particularly important in elite athletes. The literature on this topic is somewhat contradictory, but, in general, the more extensive the injury, the more time it takes to recover. The imaging features of the injury that have an effect on prognosis include the length of injured muscle, the percentage of cross-sectional area of injury compared with normal muscle, and the presence of a gap and/or hematoma in the involved muscle. On follow-up imaging, increased signal intensity in the damaged muscle may be seen on T2-weighted MRI even without clinical symptoms or pain at the time of resumption of athletic activity. It is not clear if this finding indicates an increased risk of reinjury.

Whereas most injuries occur in the central portion of a muscle, some tears may be centered at the epimysium between two muscles, such as the short head and long head of the biceps femoris. In the acute setting, increased signal intensity will still be seen but will be appreciated at the periphery of the involved muscle belly in cross section rather than in the center. In the chronic setting, a subtle area of focal low signal intensity will be visible in the periphery of the injured muscle ( Fig. 43-4 ).

FIGURE 43–4, A , Axial T1-weighted MR image of the thigh with low–signal-intensity thickening of the lateral aspect of the biceps femoris (arrowhead) representing chronic injury of the epimysium. B , Axial fat-suppressed, fast spin-echo, T2-weighted MR image of thigh. Low–signal-intensity thickening of the fascia (arrowhead) at the lateral aspect of the biceps femoris representing chronic injury of the epimysium.

Chronic injuries may be very hard to detect if the interpreting radiologist is not searching for their MRI manifestations. As stated previously, the affected extremity should be compared with the asymptomatic side. This comparison is most practical in the lower extremity. Chronic injuries present as subtle thickening of the myotendinous junction that is low in signal intensity on both T1- and T2-weighted images. The subtle thickening of the myotendinous junction may be accompanied by focal fatty atrophy in the injured muscle ( Fig. 43-5 ).

FIGURE 43–5, Axial T1-weighted MR image through the right thigh. There is a chronic injury to the rectus femoris. Note the thickened myotendinous junction (arrowhead) and focal high signal resulting from focal fat atrophy (arrow) .

Ultrasound

In order to adequately assess muscle injury, it is important to assess the size and depth of the muscle to be examined and to use an appropriately sized probe. For instance, superficial muscles such as those in the hand require small, high-frequency probes (7 to 15 Hz), whereas larger, deeper muscles such as those in the thigh will require a larger, lower frequency probe (3.5 to 10 Hz). Multiple focal zones should also be selected to improve resolution over the region of interest, and extended field-of-view imaging may be required to assess injuries to wide, long muscles or to accurately assess the size of an intramuscular lesion. Doppler imaging may also give information regarding regional blood flow and intramuscular lesion vascularity.

Normal muscle fibers are hypoechoic on US, but the connective tissue making up the endomysium, perimysium, and epimysium appears brightly reflective, being appreciated as linear bands in longitudinal or bright dots in transverse section. The transverse view of muscle on ultrasound has been likened to the appearance of a starry night sky ( Fig. 43-6A ). Tendon is seen on US as brightly reflective tissue with a fine linear architecture in longitudinal section, and the myotendinous junction is defined along the interface between the muscle and tendon (see Fig. 43-6 ).

FIGURE 43–6, A , Transverse US section through a normal biceps brachii muscle showing the normal “starry night” appearance resulting from highly reflective fascial tissue seen against the low reflective muscle fibers. The myotendinous junction is seen within the muscle (arrowheads) . H, Humerus. B , Extended field of view longitudinal US section through a normal biceps brachii muscle. In this plane the herringbone pattern of fascial bands merging on the myotendinous junction (arrowheads) can be appreciated. More distally the normal tendon emerges from the muscle as a continuation of the myotendinous junction (arrow) .

The reflectivity of tendon and the intramuscular connective tissue will depend on the alignment of the reflective fibers to the ultrasound beam, referred to as anisotropy. As the ultrasound beam moves from being perpendicular to the fibers, reflectivity will diminish. A scrupulous technique is required, as this phenomenon may mimic areas of pathologically altered reflectivity.

US appearances of muscle injury are dependent on the acuity and extent of the injury. The ideal time to evaluate a muscle tear is between 2 and 48 hours from the time of injury. Before 2 hours, a hematoma may not be visible, whereas after 48 hours, a hematoma may no longer be contained at the site of injury. If the perimysium is disrupted, a localized hematoma may not be detectable, thus potentially decreasing the diagnostic yield of the examination.

At the time of examination, the operator will be able to focus the evaluation on the point of maximal tenderness, although evaluation of the entire muscle including enthesis, MTJ, intramuscular septae, and epimysium should be performed. The neurovascular bundle should also be assessed, as swelling/hematoma can impinge on adjacent neural structures. Synergistic muscles should also be similarly examined. Comparison with the noninjured side can be performed if findings are equivocal.

As with MRI, injuries are graded according to severity. Several grading systems exist, but the most commonly used in the authors' practice follows.

Grade 1 injuries involve less than 5% of muscle fibers and include:

Normal findings
Subtle hypoechogenic areas representing small foci of muscle disruption; hyperechogenicity representing intramuscular edema/hemorrhage may also be seen

Grade 2 injuries involve more than 5% of muscle fibers ( Fig. 43-7 ) and show:

Altered echogenicity
Loss of perimysial striation at the MTJ
Foci of fiber disruption associated with hematoma formation
Increased Doppler signal adjacent to disrupted muscle fibers

FIGURE 43–7, Longitudinal extended field-of-view ultrasound showing a grade 2 tear of the distal biceps femoris myotendinous junction. Hypoechoic hematoma (asterisk) is seen separating the retracted muscle fibers (M) from the overlying myotendinous junction and tendon (arrows) . Retraction from the fascia separated the long head of biceps from the short head (SH) is also seen.

Grade 3 injuries are complete tears ( Fig. 43-8 ). US findings include:

Complete disruption of the muscle—retraction of the torn ends on dynamic testing
Bunched and retracted echogenic tendon surrounded by hematoma

FIGURE 43–8, Longitudinal extended field of view ultrasound showing a subacute grade 3 tear of the medial head of gastrocnemius. Organizing hematoma (asterisk) showing both solid and liquid elements is seen separating the markedly retracted muscle fibers (M) from the aponeurotic myotendinous junction.

Recovery is expected to take 1 to 2 weeks if the manifestations are limited to hematoma, but healing takes at least 4 to 6 weeks if a gap in muscle fibers is visible on examination. Scarring presents as hyperechoic zones in muscle, but chronic injury is more accurately assessed with MRI.

Extrinsic: Contusion

Muscle hematoma can occur in the setting of muscle strain but is also seen in direct muscle trauma, often referred to as muscle contusion. As previously described, it is important not to confuse this with neoplasm.

Muscle contusion is usually caused by a direct impact with a blunt object, typically seen in contact sports. It occurs due to microscopic muscle damage with disruption of fibers and capillaries leading to microhemorrhage and tissue necrosis. Most hematomas resorb spontaneously over the course of 6 to 8 weeks after trauma.

It can be difficult to determine whether the injury is due to muscle strain, direct trauma, or neoplasm based on imaging alone. A careful history should be taken, including anticoagulant use and recent trauma.

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