Ultrasonography is gaining an increasing role within the specialty of emergency medicine, and this role continues to grow for orthopedic conditions. Soft tissue and musculoskeletal ultrasound is now recognized as one of the eleven core emergency ultrasound applications.1 This modality offers several advantages over traditional imaging methods including the ability to perform dynamic imaging of the affected body part, the ability to easily compare findings on the affected side with those on the unaffected side, and the lack of exposure to the harmful effects of radiation. The last advantage mentioned is particularly important when evaluating the pediatric population who are more susceptible to ionizing radiation, which is delivered in large doses with imaging techniques such as CT.2,3
Common musculoskeletal ultrasound applications in the emergency department (ED) include tendon evaluation, muscle evaluation, joint evaluation for effusion, foreign body identification, and procedural guidance.4–6 There are several studies documenting the usefulness of ultrasound in trauma, especially with regard to the evaluation of bony trauma. It may be used in conjunction with plain radiographs to evaluate for fractures and may even be superior to plain radiographs in certain types of fractures, including rib and scaphoid fractures (Fig. 5–1).7–9 In addition, recent research has also suggested that this modality is useful in diagnosing extremity fractures in military or sideline settings where other imaging capabilities are not readily available.10–11 This role of ultrasound in the acute setting is also expanding to include evaluation of musculoskeletal infections. The localization of soft-tissue collections by ultrasound helps narrow the differential diagnosis based on the finding of fluid in the dermis, joint, bursa, or muscle. For this reason, ultrasound can be used to detect simple abscesses, pyomyositis, septic bursitis and tenosynovitis, joint effusions, and subperiosteal fluid associated with osteomyelitis.12
Comparison of plain radiography and ultrasound in rib fractures. A. Normal chest radiograph in a patient with left-sided pleuritic chest pain after trauma (arrow notes the location of an occult rib fracture). B. Ultrasound showing cortical disruption of the rib (arrows).
Musculoskeletal Ultrasound Techniques
The following is an overview of the basic musculoskeletal imaging that may be useful and can be easily performed in the ED. There is a focus on how to obtain the images for evaluation, normal imaging findings, and how to identify deviations from the normal patterns. If your findings deviate from the normal appearance of the structure, pathology should be expected, and further investigation should be pursued with appropriate imaging techniques, lab studies, or expert consultation.
Most EDs utilize ultrasound imaging systems with a high-frequency 7 to 12 MHz linear transducer. This transducer is ideal for superficial (<3–4 cm deep) musculoskeletal imaging. It provides good resolution in the near field with less penetration into deeper structures (Fig. 5–2A,B). For imaging that requires deeper penetration (>3–4 cm deep), such as hip ultrasound, a lower-frequency 2 to 5 MHz curvilinear transducer should be used (Fig. 5–2C,D). The typical width of an ultrasound beam is 0.2 to 1 mm thick, therefore careful interrogation of musculoskeletal structures is imperative not to overlook a small abnormality. Ultrasound imaging, regardless of the structure being evaluated, should be performed in both the longitudinal and transverse axes of the structure. When evaluating small parts or parts with abnormal contours, a water bath or a stand-off pad will help increase through transmission of the ultrasound waves being transmitted and improve image quality (Fig. 5–3).13
Transducer selection. A, B. Linear transducer for superficial imaging. C, D. Curvilinear transducer for deeper imaging.
Adjuncts for musculoskeletal imaging. A. Water bath and associated ultrasound image showing the metacarpal phalangeal joint. B. Stand-off pad using a liter of normal saline.
Ultrasonographic evaluation of tendons is used to identify traumatic tendon rupture and infections of the tendon and tendon sheath. Tendons are best evaluated with the linear transducer in both the longitudinal and transverse axes. On ultrasound, tendons exhibit an echogenic fibrillar pattern that is linear in nature without disruption.14 This finding is more notable when imaging in the long axis of the tendon (Fig. 5–4A). Disruption of the normal linear pattern should prompt further evaluation for an acute tear (Fig. 5–4B). Dynamic evaluation is helpful when imaging tendons, as this may enlarge an area of hematoma or rupture not previously visualized on static imaging.
Tendon evaluation for trauma. A. Normal patellar tendon in long axis. B. Patellar tendon with a large (50%–60%) tear in long axis. C. Patellar tendon exhibiting anisotropy when imaged in full extension.
Of note, it is important to maintain a perpendicular angle between the ultrasound beam and the tendon being imaged, as tendons demonstrate anisotropy. Anisotropy is an artifact that occurs when the ultrasound beam and the tendon are not perpendicular to each other. It may create a dark area within the tendon that can be easily be mistaken for pathology (Fig. 5–4C).15
When there is concern for an infectious process involving the tendon, any amount of fluid within a tendon sheath (>2 mm) should be considered abnormal and may suggest a tenosynovitis (Fig. 5–5).
Tendon evaluation for infection. A. Wrist extensor tenosynovitis with anechoic areas of fluid surrounding the individual extensor tendons. B. Normal contralateral tendon for comparison.
Ultrasound is used to detect muscle tears and infections such as myositis or abscess. Muscles may be evaluated with either the linear or the curvilinear transducer, depending on how deeply situated the muscle in question lies within the body. Again, the muscular tissue should be evaluated in the long and transverse axes. On ultrasound, muscle appearance ranges from hypoechoic to echoic in nature and is encased in a hyperechoic fascial sheath (Fig. 5–6).16 Dynamic imaging will provide important information regarding muscle structure and function. Disruption of the normal fiber organization or the inability to contract the muscle under ultrasound is suggestive of a tear (Fig. 5–7).5 Enlargement of the muscle belly as a whole, loss of the normal architecture, and diffuse hypoechogenicity when compared to the contralateral muscle is suggestive of myositis (Fig. 5–8A).12 A well-circumscribed anechoic area or an area of hypoechogenicity within the muscle belly should be concerning for an abscess (Fig. 5–8B).17
Muscle. A. Normal muscle tissue in long axis. B. Normal muscle tissue in short axis.
Muscle evaluation in trauma. A. Physical examination findings suggestive of muscular tear. B. Chronic muscle tear with muscle bellies located laterally and hematoma located medially (M, muscle; arrows, hematoma).
Muscle evaluation for infection. A. Biceps muscle in short axis showing blurring of the margins and generalized hypoechogenicity, suggestive of myositis. B. Biceps myositis with a focal area of abscess (arrows).
Ultrasonic evaluation of the joint is useful to identify joint effusions. For most joints, the linear transducer can be used, but evaluation of the hip joint will likely require the curvilinear transducer. Every joint contains a small amount of joint fluid for normal function, but any joint fluid in excess of normal should prompt the provider to pursue joint aspiration for fluid analysis if acute pathology is suspected (Fig. 5–9). The suggested norms for joint fluid, measured in millimeters, are provided in Table 5–1. Ultrasound is not only useful for identifying joint effusions but is also useful for distinguishing their presence from other soft-tissue abnormalities.18
Joint. A. Normal wrist ultrasound (S, scaphoid; R, radius). B. Wrist effusion with anechoic distention of the joint space (arrows).
TABLE 5–1The Normal Joint Space on Ultrasound. Higher Measurements Suggest an Effusion ||Download (.pdf) TABLE 5–1 The Normal Joint Space on Ultrasound. Higher Measurements Suggest an Effusion
|Joint ||Normal Joint Space (mm) |
|Upper Extremity || |
|Shoulder || |
|Posterior Joint Recess ||2–5 |
|Elbow || |
|Anterior or Posterior ||1–2 |
|Joint Recess || |
|Wrist || |
|Volar or Dorsal ||<1 |
|Joint Recess || |
|Lower Extremity || |
|Hip || |
|Anterior Joint Recess ||5 (or <2 mm difference than the contralateral side) |
|Knee || |
|Suprapatellar Joint Recess ||1–2 |
|Ankle || |
|Anterior Joint Recess ||1.8–3.5 |
Ultrasound is used to detect fractures or the secondary signs of fractures. The high-frequency linear transducer should be used for bony evaluation and the bone should be evaluated in both the long and transverse axes. The cortex of the bone should appear as a hyperechoic linear structure without disruption (Fig. 5–10). When there is any disruption or buckling of the cortex a fracture should be suspected. Ultrasound is also useful for identifying secondary signs of fracture that are not readily visualized with plain radiography. These include soft-tissue edema overlying the bone and hematoma formation adjacent to the fracture site (Fig 5–11).
Bone. A. Normal cortical bone in long axis (R, radius). B. Normal cortical bone in short axis (R, radius; U, ulna).
Bone evaluation in trauma. A. Fibula fracture with cortical disruption and soft-tissue edema (FX, fracture). B. Fibular fracture with cortical disruption and soft-tissue edema.
Foreign Body Identification
Ultrasound may be employed to identify foreign bodies within soft tissues. Using a high-frequency probe, ultrasound is better equipped to detect radiolucent foreign bodies (plastic and wood) than conventional radiography and fluoroscopy.19 In one experimental model, ultrasound identified wood and plastic foreign bodies with a sensitivity of 83% and a specificity of 59%.20 Emergency physicians trained in this technique exhibit a similar rate of detection as ultrasound technologists and radiologists.21
There is an increasingly large role for ultrasound when performing procedures of the musculoskeletal system. The procedures in which ultrasound may be useful include fracture reduction, joint aspiration, joint injection, hematoma blocks, and peripheral nerve blocks. A few recent studies have shown ultrasound to be useful in forearm fracture reduction in the pediatric population.22–24 Ultrasound guidance of hematoma blocks has also been shown to be superior to a landmark-based approach, which should help facilitate adequate fracture reduction.25 When ultrasound guidance has been studied for joint aspiration at the level of the knee, it was shown to cause less procedural pain, improve physician’s confidence with the procedure, and provide greater synovial fluid for analysis.26,27 Peripheral nerve blocks under ultrasound guidance for acute bony trauma and joint dislocation are becoming more widely used as they have been shown to decrease ED length of stay when compared with procedural sedation for upper-extremity injuries.28–32