This feature is a unique combination of text (voice) and video that more clearly presents and explains procedures in musculoskeletal medicine. These videos will be available on the journal’s Website. We hope that this feature will change and enhance the learning experience.
Walter R. Frontera, MD, PhD
Musculoskeletal ultrasound (US) identifies a broad range of pathologies, that is, traumatic, inflammatory, and neoplastic. Similar to other imaging modalities, for example, x-ray, computed tomography, and magnetic resonance imaging; typical sonographic images of certain pathological/abnormal conditions can be rendered and “highlighted” for the daily practice/language of musculoskeletal sonographers. Of note, metaphoric “signs” promote learning and retention of the characteristic appearances (via the limbic system, affecting memory consolidation and retrieval) and allow the examiners to easily recall the specific condition.1,2 In this regard, the following text and accompanying figures/videos represent a collection of findings pertaining to commonplace pathological conditions. This article is the second part of a series—after the characteristic/metaphoric descriptions of normal musculoskeletal structures.3
Most of the supraspinatus tendon tears ensue in the distal insertion (critical) zone. Whereas short-axis view (“tire appearance”) is quite demonstrative for a detailed description of the rupture,3 long-axis view of the tendon distal to the acromion would also be guiding in the initial step. In other words, the convex “bird’s beak”3 might no more be present or, even worse, be replaced by an irregular and “scary” crocodile mouth (Fig. 1).4
Supraspinatus tendon tears often appear as anechoic or hypoechoic on US. During short-axis imaging, a healthy tendon is convex (looking like a “tire”)3 and cannot be compressed under sono-palpation. In case of a full-thickness tear, the appearance will turn into a flat tire whereby deltoid muscle and the subdeltoid bursa would be filling the space.5 Needless to say, if the image of a flat tire is not straightforward, compression with the probe would be noteworthy to make it apparent (Video 1, https://links.lww.com/PHM/B859).
Soft tissue calcifications can be caused by a broad range of pathologies, that is, tendinopathy, myositis ossificans, rheumatic conditions, and malignancy.6 Notably, whereas dense (hard) and large calcific deposits produce a typical posterior acoustic shadowing,6,7 less dense (soft) or small calcifications may not always be accompanied by shadowing. Likewise, the phase of calcification may also impact whether the lesion will produce this artifact.6 Although calcifications vary in density, location, size, and shape, the sonographic appearance of superficial ones can resemble a “single-cistern” bridge (Fig. 2) and the river represents the acoustic shadowing artifact.
HALO/EYE OF THE TIGER
Synovial (parietal and visceral) sheaths are structures that facilitate the sliding of tendons.8 In the presence of inflammation within tendon sheaths (long head of the biceps tendon, wrist extensor tendons etc.), the fluid appears as a dark halo on US examination. Of note, because the synovial sheath can sometimes be associated with the joint space (e.g., shoulder), it is necessary to differentiate primary tendon problems from other articular pathologies. Furthermore, as comparison is an important advantage (as well as a prerequisite) of US examination, bilateral halos may be reminiscent of tiger eyes (Fig. 3) (Video 2, https://links.lww.com/PHM/B860).
In gout(y arthritis), the main sonographic findings suggestive for monosodium urate crystal deposition are tophi, the double contour sign, and the snowstorm. The characteristic image of the latter consists of multiple hyperechoic spots floating within the synovial fluid and surrounded by the synovium (Fig. 4). These microtophi correspond to aggregates of monosodium urate monohydrate crystals. Concerning the snowstorm appearance, the sensitivity and specificity in detecting monosodium urate deposition have been reported as 30.3% and 90.9%, respectively.9
When a peripheral nerve becomes entrapped, the deformation results in pressure gradients, which redistributes the tissue to areas of lower pressure. In addition, compression inhibits normal intraneuronal axoplasmic transport and various substances; for example, transmitter substance vesicles and cytoskeletal elements accumulate.10 Eventually, mechanical/ischemic factors can cause nerve swelling usually just proximal to the entrapment site. In certain cases (e.g., after carpal tunnel surgery), nerve edema can also be present distal to the entrapment site and appear as an hourglass.11 In daily clinical practice, peripheral nerves are usually/easily tracked in short axis, and if an abnormality (e.g., increased nerve caliper or hypoechogenicity) is detected, the probe is rotated 90 degrees and the typical appearance of bottle neck can be observed in the long-axis view. Needless to say, the neck represents the point of nerve compression (Fig. 5).
In the universe, a family of asteroids is usually identified for their brilliant tails. Likewise, in the musculoskeletal system, metallic foreign objects (screw, implant, etc.) give this reverberation artifact (long hyperechoic bands) owing to their feature of randomly reflecting the US beams. In daily clinical practice, it is also not uncommon to detect certain (otherwise unknown) foreign objects after recognizing these comet tails (Fig. 6) (Video 3, https://links.lww.com/PHM/B861).
RAT TAIL, TARGET, AND BAG OF WORMS
High-resolution US is considered as the mainstay for detecting/diagnosing peripheral nerve sheath tumors. The most prevalent types of solitary (benign) tumors in adults are schwannoma and neurofibroma. The former is derived from neoplastic schwann cells and the latter originates from schwann cells as well as from other tissues of the nerve sheath. Although the definitive diagnosis is histopathological, both display some characteristic findings on US examination. For instance, a round shape strongly suggests a schwannoma, a fusiform shape is relatively typical for a neurofibroma, and an oval shape is common for both. In contrast to neurofibromas, schwannomas appear in an eccentric position with regard to the peripheral nerve trunk.12 Accordingly, the characteristic rat tail (a thin hyperechoic line entering and exiting the nerve) appearance (Fig. 7) is present in approximately half of the schwannoma patients.13 Another US feature—which is more common in neurofibromas—is the target sign.14 It is characterized by a hyperechoic fibrous center with a hypoechoic periphery (Fig. 8) that contains predominantly myxomatous material.15 A subtype of neurofibromas that frequently develops in individuals with neurofibromatosis type 1 is plexiform neurofibroma. In contrast to localized neurofibromas that arise from a single peripheral nerve, plexiform neurofibromas originate from multiple nerves and involve perineural components as well as other soft tissue components. As such, a bulging mass that comprises tortuous bundles of enlarged, disorganized nerves and connective tissues appears as bag of worms on macroscopic inspection and US alike.16 The US appearance is that of multiple hypoechoic nodules, poor margins, and several feeding vessels within hyperechoic connective tissue (Fig. 9). Notably, these tumors are at significant risk for eventual malignant transformation.
Sono-tracking of a skeletal muscle in short-axis view can show sudden disappearance of the normal “starry sky” appearance (alternation of hypoechoic fascicles and hyperechoic perimysium)—being replaced by a large black hole. Similar to the absence of light in black holes of the universe, no acoustic interfaces can be identified inside an acute/subacute muscle injury due to hemorrhage instead of connective tissue and muscle fibers. Under prompt sono-palpation, compressibility/displacement of the black hole can confirm the lesion as well as the possibility of aspiration5 (Video 4, https://links.lww.com/PHM/B862).
Sono-tracking of a denervated skeletal muscle in short-axis view can clearly show fibroadipose involution. In other words, unlike the “starry sky,” a coarse pattern with low visibility of the hypoechoic background (muscle fibers) and increased hyperechoic connective tissues (perimysium) can be observed, that is, a misty muscle (Fig. 10). Of note, the blurred muscle tissue would also reduce the sonographic visibility of deeper anatomical layers—owing to the presence of excessive/pathological acoustic interfaces that attenuate the ultrasonic beam.
Myositis ossificans is a rare complication that mostly occurs after traumatic large muscle injury. US can significantly be contributory in the early diagnosis, even at stages when radiographs are negative.17 Multiple/irregular hypoechoic and hyperechoic layers accompanied by posterior acoustic shadowing (as discussed above) is the main scenario mimicking clouds or cloudy weather18 (Video 5, https://links.lww.com/PHM/B863).
PISTOL GRIP DEFORMITY
Femoroacetabular impingement syndrome is a clinical entity that refers to the disrupted relationship between the two bones due to morphological abnormalities from either side. The two basic types (i.e., cam and pincer) can be evaluated by x-ray, magnetic resonance imaging, and computed tomography,19 but US examination might be of additional values if performed dynamically.20 During long-axis imaging for a cam lesion, the shape of the proximal femur resembles a pistol grip (Video 6, https://links.lww.com/PHM/B864).
BOOMERANG AND SPEECH BUBBLE
The bursa between semimembranosus and medial head of the gastrocnemius tendons is the place of origin for popliteal/Baker’s cyst. While its normal or mildly swollen shape resembles a boomerang, the appearance looks more like a speech bubble in case of further fluid collection (Fig. 11). The latter ensues mainly because of the dilatation of the superficial arm of the bursa. Of note, although the accumulation often contains homogeneous/anechoic fluid, different sonographic findings—ranging from fibrous septa to hypertrophic/floating synovial villi floating inside the cavity—might not be uncommon. Accordingly, in addition to the shape of a Baker’s cyst, the appearance of its content would also be important before an onward intervention (Video 7, https://links.lww.com/PHM/B865).
In several joints of the musculoskeletal system, the presence of an intra-articular triangular fibrocartilage can be related to a peculiar pathological finding that looks like a volcano (eruption). For instance, longitudinal scan over the acromioclavicular joint or the medial aspect of the knee joint shows—in some patients—the bulging of the aforementioned intra-articular fibrocartilage also bulging the overlying capsular tissue. In this sense, the joint line can be considered as the mouth of the volcano and the ejected/extruded meniscocapsular tissue as the boiling magma. Needless to say, clear recognition of these pathologies might easily navigate the clinician for prompt interventional planning to “cool down” the natural phenomenon (Figs. 12 and 13) (Video 8, https://links.lww.com/PHM/B866).
SHARK HEAD AND LIPS
Tennis leg refers to strain lesions of the myotendinous junction of the medial head of the gastrocnemius muscle. Intramuscular tear, deteriorated pennation pattern, fluid in the fascial planes, and hematoma can be visualized under US examination.5,21 Especially, the presence of fluid between the medial head of the gastrocnemius and soleus muscles/aponeuroses might appear as a shark head with open mouth during long-axis view. On the other hand, in short-axis imaging, the same vista might sometimes look like lips (Fig. 14) (Video 9, https://links.lww.com/PHM/B867).
In patients with clinical suspicion of Morton’s neuroma, (short-axis view) dynamic assessment can be performed over the metatarsal heads using the Mulder maneuver. A hypoechoic mass protruding outward or a typical artifact when it is located inward can be quite pathognomonic.5 The outward “jumping” neuroma can appear as the activation of an atomic bomb, that is, exploding and generating an atomic mushroom (Video 10, https://links.lww.com/PHM/B868).
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