Musculoskeletal Ultrasound

Musculoskeletal ultrasound uses a high frequency (7 to 18 MHz) linear probe to image muscles, tendons, ligaments, joints, bursae, peripheral nerves, bone surfaces, fascia, soft tissue masses and the skin and subcutaneous tissue. It is done in real time, with dynamic manoeuvres (movement of a joint or tendon, probe compression, stress testing) and comparison with the opposite side.

Colour and power Doppler show hyperaemia in inflammation and tumours. Ultrasound is used to direct aspiration, injection, biopsy, lavage of calcific tendinopathy and hydrodissection of nerves. The examination takes 10 to 30 minutes for a joint region, with no radiation and no contrast, and is cheaper and more available than MR.

Musculoskeletal pain is among the commonest reasons for consultation. Rotator cuff tears are found in about 20% of the general population and more than 50% of those over 70 years (many asymptomatic).

Ultrasound has a sensitivity and specificity of 90 to 95% for full thickness rotator cuff tears and 80 to 90% for partial tears, and is highly sensitive for effusion, synovitis, erosions in rheumatoid arthritis, crystal deposition, tendon rupture, muscle tears and foreign bodies. It is less useful for deep structures, bone marrow, intra-articular cartilage and the hip labrum.

Indications. Shoulder pain (rotator cuff, subacromial bursitis, biceps tendon, calcific tendinopathy, acromioclavicular joint). Tendon disorders (Achilles, patellar, tibialis posterior, extensor and flexor tendons of the hand and wrist, tennis and golfer elbow, plantar fascia). Muscle injury (strain, tear, haematoma, myositis ossificans).

Joint effusion and synovitis (rheumatoid arthritis, spondyloarthritis, gout, septic arthritis, haemophilic arthropathy). Joint aspiration and injection under ultrasound control. Soft tissue masses (lipoma, ganglion, Baker cyst, nerve sheath tumour, sarcoma screening), lymph nodes, and foreign bodies. Nerve entrapment (carpal tunnel, ulnar neuropathy at the elbow, peroneal neuropathy).

Bone cortex lesions (fracture, periosteal reaction, osteomyelitis). Giant cell arteritis (temporal artery halo sign). Paediatric musculoskeletal infection and fractures in selected settings. Tendon and ligament instability on dynamic scanning.

Contraindications and precautions. None absolute. Open wounds need sterile gel and covers. Injection requires allergy check, coagulation review in anticoagulated patients, and exclusion of infection at the site. Septic arthritis suspected clinically needs aspiration without delay. Patients should be examined with the joint in a relaxed position, and with the opposite side for comparison.

Preparation and technique. No preparation is needed. Each structure is scanned in two planes (long and short axis), keeping the probe perpendicular to the tendon to avoid anisotropy (artefactual hypoechogenicity from an oblique angle). Dynamic scanning (shoulder abduction, ankle dorsiflexion, finger flexion) shows impingement, subluxation, snapping and tendon rupture.

Doppler settings are set to detect slow flow, with minimal probe pressure. Compression assesses cysts and collections.

Normal appearances. Normal tendons show a fibrillar pattern: parallel echogenic lines in the long axis and multiple echogenic dots in the short axis, with a smooth outline and no Doppler signal. Ligaments are compact, echogenic and fibrillar. Muscle is hypoechoic with echogenic septa (feathery pattern in long axis, "starry night" in the short axis).

Nerves show a honeycomb pattern in the short axis (hypoechoic fascicles within echogenic perineurium) and are fibrillar in the long axis. Bone cortex is a smooth continuous echogenic line with posterior shadowing. Hyaline cartilage is a thin anechoic band. Joints contain a small amount of anechoic fluid. Fat is hypoechoic with echogenic strands, bursae are collapsed potential spaces.

Interpretation.

• Tendinosis: thickened, hypoechoic tendon with loss of the fibrillar pattern and possible neovascularity on Doppler; partial tear: focal anechoic defect involving part of the thickness; full thickness tear: complete defect with a gap, retraction of ends and a fluid filled gap, with the "naked tuberosity" sign in the rotator cuff and non-visible tendon in the cuff cable. Dynamic scanning confirms continuity.
• Tenosynovitis: fluid and synovial thickening around the tendon, with hyperaemia on Doppler.
• Calcific tendinopathy: echogenic deposit in the tendon with or without shadowing (hyperechoic arc with posterior shadow in formative phase, fragmented or fluid like in resorptive phase).
• Bursitis: fluid filled bursa (subacromial subdeltoid, olecranon, prepatellar, retrocalcaneal, trochanteric), with thickened synovium and hyperaemia in inflammatory or septic bursitis.
• Muscle injury: grade 1 (feathery pattern disturbance with oedema), grade 2 (partial tear with fibre disruption and haematoma), grade 3 (complete rupture with retraction); haematoma evolves from echogenic to anechoic; myositis ossificans shows peripheral calcification; chronic scarring shows echogenic, shortened fibres.
• Joint disease: effusion (anechoic or echogenic if complex), synovial hypertrophy (hypoechoic, non-compressible tissue), hyperaemia on Doppler (active synovitis), bone erosions (cortical breaks seen in two planes), osteophytes, and cartilage thinning; gout shows the "double contour" sign (hyperechoic band over cartilage), tophi (heterogeneous hyperechoic nodules with halo) and "snowstorm" effusion; pseudogout shows cartilage calcification (hyperechoic spots within cartilage).
• Septic arthritis: complex effusion with debris and synovitis, but findings are not specific, and aspiration is required.
• Baker cyst: fluid collection between the medial gastrocnemius and semimembranosus tendons communicating with the knee joint; rupture gives fluid tracking into the calf.
• Ganglion cysts: well defined anechoic or low echo lobulated cyst, often with a neck connected to a joint or tendon sheath, avascular.
• Soft tissue tumours: lipoma (elongated, isoechoic or hyperechoic to fat, compressible, avascular), nerve sheath tumour (fusiform, hypoechoic, connected to nerve, "target" sign), haemangioma and vascular malformation (compressible, with flow), and sarcoma (large, heterogeneous, vascular, deep or fast growing mass over 5 cm); any suspicious mass needs MR and biopsy.
• Nerve entrapment: carpal tunnel syndrome (median nerve cross-sectional area at the tunnel inlet over 10 mm2, flattening within the tunnel, loss of fascicular pattern, increased vascularity, flexor retinaculum bowing); ulnar nerve enlargement at the elbow; hypoechoic swelling proximal to the entrapment site.
• Foreign body: echogenic focus with shadowing (glass, metal, stone) or reverberation, with a hypoechoic halo of granulation tissue (wood and thorn may be isoechoic or hypoechoic and need careful scanning).
• Bone: cortical break or step in fracture (rib, scaphoid, children's torus fractures), periosteal elevation and subperiosteal collection in osteomyelitis (in children), and cortical erosions.
• Plantar fasciitis: plantar fascia thickness over 4 mm, hypoechoic, with perifascial fluid.
• Giant cell arteritis: dark halo around the temporal artery wall, non-compressible artery, and halo seen on two planes.

Critical values and findings needing immediate action.

• Septic arthritis (complex effusion with fever and a hot, swollen joint): urgent aspiration for microscopy, Gram stain and culture, followed by antibiotics and drainage or washout.
• Necrotising soft tissue infection (gas in fascial planes, thickened fascia with fluid): emergency surgical exploration and debridement.
• Abscess or deep collection: drainage with antibiotics.
• Acute complete rupture of the Achilles, quadriceps or patellar tendon: surgical referral within days.
• Compartment syndrome (tense compartment, swollen muscle, pain out of proportion): urgent fasciotomy decision; pressure measurement; ultrasound does not exclude it.
• Deep vein thrombosis found as an incidental finding in a swollen limb: anticoagulation.
• Suspected sarcoma (large, deep, vascular, rapidly growing mass): urgent referral to a sarcoma service, with MR before biopsy.
• Positive halo sign in suspected giant cell arteritis: start high dose steroid and arrange urgent biopsy or imaging.
• Open fracture or joint penetration by a foreign body: surgical care.

Limitations and artefacts.

• Anisotropy mimics tendinosis or tear when the probe is not perpendicular to the fibres; rocking or heel toe movement of the probe resolves it.
• Operator dependence is considerable, and the learning curve is long; findings should be correlated with the clinical picture.
• Deep structures (hip labrum, cruciate ligaments, bone marrow, intra-articular cartilage, and structures behind bone) cannot be assessed; MR is used.
• Asymptomatic abnormalities are common (rotator cuff tears, tendinosis, effusions, small erosions), so ultrasound findings must match symptoms before treatment decisions.
• Calcifications cast shadows that hide deeper structures, and gas, scar and post-surgical changes make interpretation difficult.
• Doppler is affected by probe pressure, wall filter and temperature; cold limbs show less flow.
• Muscle tears may not be seen in the first hours before haematoma formation.
• Bone lesions, stress fractures and occult fractures are better shown by radiograph, CT or MR.
• Measurement of nerve cross-sectional area has overlap between normal and abnormal; electrophysiology is needed.
• Foreign body detection is less sensitive for small or organic objects.

Special groups.

• Children: ultrasound avoids sedation and radiation, and is useful for fractures, infection, hip disease, soft tissue masses and joint effusions; growth plates and cartilage appear as hypoechoic structures that should not be mistaken for fracture.
• Athletes: dynamic scanning of tendon and muscle injuries and return-to-play assessment.
• Patients with inflammatory arthritis: serial scans for synovitis and erosions, and ultrasound control of injection.
• Patients with haemophilia: detection of joint bleeding and synovitis.
• Anticoagulated patients: injection and aspiration are possible with local rules for the risk of bleeding.
• Pregnancy: safe imaging of soft tissue complaints without radiation.
• Older adults: tendon degeneration is universal, and treatment should follow symptoms.
• Regions: pyomyositis, tropical muscle abscess, septic arthritis (including sickle cell osteomyelitis and salmonella), tuberculosis of joints and tendon sheaths, filarial and parasitic cysts (cysticercosis, hydatid), and neglected trauma are common; ultrasound is often the only imaging available for aspirating abscesses and for guiding drainage.

Follow up tests. Plain radiographs for fracture, arthritis and calcification, MR for deep joint structures, cartilage, marrow, labrum, ligaments, and for tumours, CT for complex fractures, joint aspiration with cell count, Gram stain, culture, crystals and polymerase chain reaction, inflammatory markers (C-reactive protein, erythrocyte sedimentation rate), serum uric acid,

rheumatoid factor and anti-cyclic citrullinated peptide antibodies, human leucocyte antigen B27, nerve conduction studies and electromyography, biopsy of masses after imaging, temporal artery biopsy, and referral to orthopaedics, rheumatology or sports medicine.