Advances and utility of diagnostic ultrasound in musculoskeletal medicine
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Musculoskeletal ultrasound (US) can serve as an excellent imaging modality for the musculoskeletal clinician. Although MRI is more commonly ordered in the United States for musculoskeletal problems, both of these imaging modalities have advantages and disadvantages and can be viewed as complementary rather than adversarial. For diagnostic US, relative recent advances in technology have improved ultrasound’s ability to diagnose a myriad of musculoskeletal problems with enhanced resolution. The structures most commonly imaged with diagnostic musculoskeletal US, include tendon, muscle, nerve, joint, and some osseous pathology. This brief review article will discuss the role of US in imaging various common musculoskeletal disorders and will highlight, where appropriate, how recent technological advances have improved this imaging modality in musculoskeletal medicine. Additionally, clinicians practicing musculoskeletal medicine should be aware of the ability as well as limitations of this unique imaging modality and become familiar with conditions where US may be more advantageous than MRI.
KeywordsUltrasound Musculoskeletal Sports injuries
Advantages and disadvantages of diagnostic US
US also has the advantage of being a dynamic study. For example, the affected part can be imaged in real time, observing for pathologic movement in tendon, bursa, muscles, or joints. Unfortunately, MRI does not offer this luxury, as there would be movement artifact distorting image quality. With diagnostic US, the patient simultaneously provides feedback and vital information to the examiner during the dynamic examination that may reveal tendon subluxation, joint subluxation, or ligamentous incompetence. Since the diagnostic US exam is real time, the patient and even the referring physician can receive results immediately and then can outline a treatment strategy within the same visit.
Unlike MRI, the traditional form of musculoskeletal imaging, clinicians are often unfamiliar with the images produced by US. Despite this, many specialties such as rheumatology, orthopedics, physiatry, and family practice are integrating this imaging modality into their daily practices. The portability of US machines makes this feasible. Portable machines allow clinicians to examine patients in their offices, in the training room, and even on the field. Outside of a mobile scanner, this is not feasible with MRI.
One limitation that diagnostic ultrasound has is its dependence on body habitus. Ultrasound wave penetrance into tissue is inversely proportional to the wave frequency. For instance, a 12 MHz linear array transducer can visualize very superficial structures with high resolution, but imaging of a hip joint or rotator cuff in an obese or extremely muscular individual can be extremely limited. Although recent advances have improved high frequency linear array transducers, a lower frequency curvilinear transducer (3–5 MHz) may be needed to provide adequate penetration for deeper structures. With greater depth of penetrance, though, resolution can be sacrificed, making musculoskeletal US limited as a modality in obese or muscular patients. However, recent advances in tissue harmonics have improved visualization and resolution of deeper structures even in these challenging cases [3, 9].
The evaluation of tendon pathology is probably the most common clinical indication to obtain a diagnostic musculoskeletal ultrasound. On US, normal tendon, which is composed of fascicles of collagen fibers running in parallel, appear as fibrillar hyperechoic (brighter) bands. In normal conditions, there will also be a flat hypoechoic structure surrounding the tendon, representing a synovial sheath containing a small amount of fluid . An exception to this is the Achilles tendon, which has a closely adherent paratenon and is often normally imperceptible. In cases of tenosynovitis, there will be an increase in synovial sheath fluid indicative of underlying inflammation. Exceptions do exist. A significant amount of fluid surrounding the bicep may indicate primary bicipital tendonitis. However, fluid around the biceps tendon may be a secondary sign of a complete rotator cuff tear as fluid communicates with the glenohumeral joint through the subacromial bursa .
Danielson et al. has recognized that tendonopathy may be due to abnormal penetrating neovascularization carrying nociceptive fibers . Power or Color Doppler enables the examiner to identify these abnormal penetrating vessels in cases of Achilles tendonopathy [23, 24]. An eccentric calf-strengthening program is recommended to decrease this neovascularization . This program theoretically decreases these infiltrating vessels via repetitive constriction. If this strengthening program does not resolve the patients’ complaints, Alfredson has reported that injecting a sclerosing chemical into these aberrant vessels under US guidance results in normalization of the Achilles tendon and significant reduction of clinical symptoms .
Another advantage of ultrasonographic examination of tendons includes the ability to perform dynamic imaging. Small tendon tears on initial examination may go undetected. With sonopalpation or motion, further tendon separation may become apparent . Owing to hematoma formation and associated debris, a complete Achilles tendon tear may be poorly demarcated. Yet with ankle dorsiflexion, a discontinuity will be more easily demonstrated . Real time dynamic subluxation of tendons cannot be visualized with current MRI technology. With US however, biceps, peroneal, or posterior tibial subluxation or dislocation can be visualized with dynamic maneuvers. For the biceps, this may involve elbow flexion combined with forearm supination and glenohumeral external rotation. In the ankle, peroneal tendon subluxation over the lateral malleolus can be demonstrated with combined active ankle dorsiflexion and eversion .
Another common athletic ligamentous injury involving the ankle joint is a tear of the anterior talofibular ligament and if more severe, the calcaneal fibular ligament. Although ultrasound cannot detect underlying bone edema, it can aid in grading the severity of the tear, which may be helpful for prognosis and return to play. Like the elbow, applying additional stress may also aid in determining severity of ankle ligamentous injury .
Although it cannot detect intra-articular knee pathology such as meniscal or cruciate tears adequately, ultrasound can easily visualize the medial, lateral, and patellar ligaments quite readily. Particularly when combined with dynamic stress testing, US has been proven to be a sensitive test for detecting medial collateral ligament (MCL) tears . The normal MCL is composed of a hyperechoic superficial and deep band separated by a hypoechoic layer representing loose areolar tissue . In partial MCL injury or sprain, thickening of the ligament will occur and the superficial and more commonly, the deep band will appear with decreased echogenicity. Complete rupture of the ligament will appear as an interruption of the hyperechoic bands within the ligament and there can be an associated fluid collection .
US can also aid in detecting the cause of the underlying effusion. Although MRI is superior at visualizing intra-articular pathology, Color or Power Doppler can detect concomitant increased blood flow detected in the synovium of inflammatory or infectious arthritis. The synovium in infectious or inflammatory arthritis is thickened, hypertrophic, and edematous, appearing as a hypoechoic band between muscle or fat [35, 36]. Most infectious effusions also have some component of echogenicity but may also give the appearance of a compressible hypoechoic mass . Evaluation of large synovial joints is most easily performed at the suprapatellar recess of the knee, anterior synovial recess of the hip, and posterior synovial recess of the shoulder . Measuring the thickness of the synovium in inflammatory arthritis has been shown to be a reliable means of following the effectiveness of therapy [37, 38].
Articular hyaline cartilage appears as a thin, hypoechoic line juxtaposed to the subchondral cortical bone. Early ultrasonographic findings compatible with cartilage pathology, in particular inflammatory and osteoarthritis, include edema. With edema there will be an increased thickness of the articular cartilage with inhomogenity and an ill-defined cartilage margin. Comparison with the opposite side may be helpful to obtain a baseline, however arthritic conditions are often symmetric. Chondral and osteochondral defects, which can occur through trauma, infarction, or osteonecrosis, can also be detected as loose bodies. If calcified, these loose bodies will appear with acoustic shadowing on musculoskeletal US .
Fibrocartilage, such as that found in the knee menisci, is composed of densely packed collagen fibers with interposed chondrocytes. This infrastructure is responsible for the homogenous hyperechoic appearance seen on US. Though US cannot penetrate into the joint proper to directly visualize cartilaginous injuries, there are secondary signs that may indicate underlying cartilage injury. Meniscal cysts in particular are most commonly located on the lateral joint line and often communicate with horizontal meniscal tears. They are frequently seen as hypoechoic, and occasional anechoic (without echogenicity) structures adjacent to the meniscus and often require surgical intervention for treatment .
Labral tears of the hip or shoulder can be identified using diagnostic musculoskeletal US, particularly if the defect extends to the peripheral joint margin where the cartilage can be examined . Though MR arthrography remains the gold standard, it is an invasive and expensive procedure. With US, a non-invasive and relatively inexpensive test, there are a number of findings that indicate the presence of underlying labral pathology. Paralabral cysts of the hip or suprascapular ganglia are associated with concomitant labral pathology of the hip and knee joints, respectively. Van Holsbeeck has reported that US is particularly useful if used immediately after a dislocation. The intra-articular hemorrhage serves as a natural contrast medium and improves direct imaging of a labral tear . However, until further research is performed utilizing US, MRI arthrography would still be considered the gold standard for imaging labral injuries.
It has been estimated that 30% of sports injuries affect muscles. The portability, ease of use, and superior spatial resolution make ultrasonography an excellent imaging modality for detecting and classifying these injuries. Additionally, ultrasound can also identify non-traumatic or primary muscle pathology such as myositis [24, 40]. Occasionally patients are unable to completely localize the area involved especially when it involves a large muscle group such as the biceps femoris. However, extended FOV technology has made capturing large areas of muscle tissue feasible. US has an advantage over MRI when imaging obliquely running muscles. With MRI, the clinician has to follow obliquely running muscles like the sartorius on various cuts and sequences. However, the musculoskeletal ultrasonographer can follow this muscle from its origin to its insertion during one scan. Also if a muscle and its tendon is torn and retracted, MRI may not identify the location of the entire tendon. For example, in a complete quadriceps tear, standard knee MRI protocol may not include the distal torn portion. However, US offers the ability to track the torn portion proximally and is helpful in measuring the degree of retraction .
There has been recent interest in imaging of the peripheral nervous system with US. One of the most common studied peripheral nerve entrapments is Carpal Tunnel Syndrome (CTS). In this condition, typically the proximal portion of the nerve becomes swollen while the portion coursing through the tunnel is compressed . In one study, cross-sectional area of greater than 10.5 mm2 was compatible with electrophysiological abnormalities seen on nerve conduction studies . Additional abnormal US findings seen in CTS include a decrease in median nerve echogenicity and loss of the normal fascicular pattern. With more severe cases, there may also be an increase in blood flow within the nerve on Color Doppler . Though US can be useful to help guide a therapeutic steroid injection near a neural structure for pain relief, a randomized study comparing it to blind injections has not been performed. However, by directly visualizing peripheral nerves, inadvertent injury can be avoided during injections. This complication has been described when performing blind carpal tunnel injections .
Morton’s neuroma is an abnormal fibrous condition of the digital nerve most commonly located between the 3rd and 4th and second and third web spaces of the feet and may produce pain and paresthesias of the respective toes. When a neuroma is present, US examination of the plantar surface between the metatarsal heads will reveal an ill-defined, poorly reflective ovoid or fusiform mass measuring 5–7 mm in diameter [19, 45]. US can be additionally helpful in this painful condition since it can help guide a local steroid injection for pain relief .
Bursae are sac-like structures that facilitate movement of musculotendonous structures and are optimally visualized with diagnostic US . Inflammation of these structures, commonly due to increased friction or trauma, can become a source of pain and dysfunction. The more common clinical conditions associated with these structures include subacromial, greater trochanteric, pes anserine, and olecranon bursitis. Normally, these structures on US appear as a thin hypoechoic line no more than 1–2 mm in height with hyperechoic boundaries reflective of a fluid tissue interface (Fig. 2) . When enlarged, these bursae may be mistaken for soft tissue tumors yet they are fluid filled and therefore often compressible. Comparison with the opposite and hopefully asymptomatic side will provide a “normal” control for that patient. In chronic bursitis, the synovial walls of the bursa may become thickened with proliferative tissue and may have associated calcifications and internal hyperechoic debris . The differentiation between infectious, metabolic, or inflammatory bursitis in these cases may be difficult, but US guided aspiration for fluid analysis helps decipher this clinical conundrum.
Diagnostic US can serve as an excellent imaging modality for most musculoskeletal problems. Recent improvements in technology allow one to image various structures including tendon, muscle, joints, and even nerve with excellent resolution. Portability allows examination not only in the office but also in the training room and playing field. Low cost, real time imaging, and its ability to be used as a guide for interventional procedures make this imaging modality ideal for most musculoskeletal clinicians.
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