Musculoskeletal (MSK) disorders frequently present to the emergency department (ED), spanning minor soft tissue injury, tendon rupture and limb-threatening infection. Radiography remains the initial imaging modality for many MSK presentations but lacks sensitivity and specificity for soft tissue abnormalities. High-frequency ultrasound (US) offers high-resolution evaluation of tendons, muscles and subcutaneous tissues and is non-invasive, radiation-free and widely available.
Core Techniques That Improve Diagnostic Confidence
An effective MSK US examination relies on probe selection, scanning orientation and dynamic manoeuvres. High-frequency linear transducers, typically 7–15 MHz or higher, are used for superficial structures, while lower-frequency curvilinear transducers may be used for deeper tissues or in obese patients. Structures are assessed in longitudinal and transverse planes and comparison with the contralateral side supports differentiation between normal and abnormal appearances.
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Dynamic assessment can improve acute evaluation, particularly when oedema or haemorrhage reduces conspicuity on static imaging. Gentle graded compression supports distinction between diffuse interstitial fluid and a discrete collection and can demonstrate mobile internal debris. Colour Doppler adds vascularity assessment, supporting differentiation between hyperaemic inflammatory change and avascular collection.
Trauma-Focused Findings Across Tendons and Muscles
Tendon rupture is a high-yield ED indication because US can show fibre discontinuity, retraction and associated haematoma. Achilles tendon rupture may occur at the musculotendinous junction, within the mid-substance or at the calcaneal insertion, with the mid-tendon region described around 4–6 cm proximal to the calcaneal insertion. Acute haematoma and oedema can fill a rupture gap and appear isoechoic, so systematic two-plane scanning combined with dynamic manoeuvres supports confident detection.
Quadriceps tendon rupture is described as uncommon and typically affecting men over 40. Partial rupture may appear as focal hypoechoic defects, while complete rupture can show disruption separated by hypoechoic or anechoic haematoma. In the shoulder, acute rotator cuff rupture most often follows trauma, with the supraspinatus frequently involved and full-thickness tears commonly described. For full-thickness supraspinatus tears, reported US sensitivity is 94% to 97% and reported specificity is 92.5% to 100%, aligning closely with magnetic resonance imaging. Full-thickness rupture appears as complete discontinuity with fluid in the gap, while partial-thickness injury can appear as focal hypoechoic defects or fibre disruption.
Muscle injuries are commonly located at the myotendinous junction. US can show echogenic oedema or haemorrhage and hypoechoic or anechoic fibre disruption with haematoma. Grading is described from Grade 1 injuries involving under 5% tissue damage and typical healing within 1–2 weeks to Grade 3 complete rupture at the myotendinous junction with haematoma and a recovery period of 5–8 weeks to reduce recurrence risk.
Infection, Collections and Occult Fracture Detection
US assessment of soft tissue infection helps distinguish cellulitis, abscess and necrotising fasciitis. In cellulitis, US typically reveals a cobblestoning pattern with hypoechoic reticulated subcutaneous tissue reflecting interstitial fluid, and Doppler demonstrates hyperaemia. Abscesses appear as well-defined hypoechoic or anechoic collections, often with posterior acoustic enhancement and internal debris, and Doppler can show a hypervascular capsule around an avascular core.
Necrotising fasciitis can show fascial fluid and subcutaneous gas as echogenic foci with shadowing, and the presence of gas is described as a key feature supporting differentiation from cellulitis and soft tissue oedema. Reported US sensitivity for necrotising fasciitis ranges from 85% to 100% and reported specificity ranges from 44% to 98%.
Septic arthritis is described as a medical emergency with risk of rapid joint damage. US supports detection of joint effusion and image-guided arthrocentesis. Computed tomography (CT) is recommended when US findings are inconclusive, when deeper or adjacent bone involvement such as osteomyelitis is suspected or when the clinical picture is severe or not improving. Immediate empiric antibiotic therapy is described when there is strong clinical suspicion of sepsis.
Soft tissue haematoma is described as a localised collection of blood related to trauma, procedures or anticoagulant therapy, with spontaneous haematomas described as increasingly common with growing anticoagulant use. Sonographic appearance evolves from hyperacute hyperechoic change to later anechoic fluid as clot liquefies, and then complex appearances with septations and fluid-fluid levels. Computed tomography angiography (CTA) is described as the primary imaging modality for evaluating haemorrhage extent and identifying active bleeding, guiding decisions including arterial embolisation with high technical and clinical success rates. Conventional US is limited for identifying active bleeding, while contrast-enhanced US improves accuracy by visualising microbubble extravasation as a direct marker of ongoing haemorrhage.
Occult fractures are described as fractures not visible on initial radiographs but later confirmed by other imaging or follow-up. Systematic reviews and cohort studies demonstrate high sensitivity and specificity of US for radiographically occult fractures, particularly of the scaphoid, ankle, foot and in paediatric populations. In children, US can depict cortical discontinuity and indirect signs such as joint effusion, with direct visualisation reported in over 90% of cases. In the knee, US-detected lipohaemarthrosis is described as highly sensitive and specific and more sensitive and specific than radiographs for early occult intra-articular fracture detection. US is operator-dependent, so CT or MRI is recommended when findings are inconclusive or suspicion remains high.
High-frequency MSK US supports ED decision-making across tendon rupture, muscle injury, infection, haematoma and selected occult fractures, particularly where radiography has limited performance for soft tissue abnormalities. Diagnostic confidence improves with appropriate transducer selection, two-plane scanning, contralateral comparison, dynamic manoeuvres, graded compression and targeted Doppler assessment. Defined escalation to CT, MRI, CTA or contrast-enhanced US supports management when findings are inconclusive or risk is high.
Source: Emergency Radiology
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