Ultrasound in Musculoskeletal Practice

Sound Wave Principles

Ultrasound uses sound waves with frequencies above the audible range (more than 20kHz). In MSK imaging, frequencies of 5-18MHz are standard. These high-frequency waves are generated by piezoelectric crystals in the transducer, transmitted into tissue, reflected at tissue interfaces, and received by the same transducer to construct the image.

Key relationships:

• Frequency and resolution: Higher frequency gives better axial resolution (ability to distinguish structures along the beam axis). At 15MHz, axial resolution is approximately 0.1mm.
• Frequency and penetration: Higher frequency waves are absorbed more rapidly by tissue, limiting depth of penetration. At 15MHz, useful penetration is approximately 3-4cm. At 5MHz, penetration reaches 15-20cm.
• This trade-off is the fundamental physics concept: you cannot have both maximum resolution and maximum depth simultaneously.

Acoustic impedance: Each tissue has a characteristic acoustic impedance (Z = density × speed of sound). Ultrasound waves are reflected at interfaces between tissues with different impedances. The greater the impedance mismatch, the stronger the reflection — this is why cortical bone (very high impedance) produces an extremely bright surface reflection.

Coupling gel: Air has very different acoustic impedance from tissue. Without gel, nearly all ultrasound energy is reflected at the skin-air interface. Coupling gel eliminates this interface, allowing sound to enter the tissue efficiently.

Common MSK Ultrasound Artefacts

Anisotropy (most important in MSK): When the beam strikes a tendon at any angle other than perpendicular, the reflected sound is directed away from the transducer, creating a falsely hypoechoic (dark) appearance. This is the most common cause of false-positive tendon pathology. Always confirm findings on two axes and resolve with gentle transducer tilt.

Acoustic shadowing: Dense structures (bone cortex, calcification) reflect or absorb nearly all sound, creating a dark shadow behind them. Useful for identifying calcification but obscures deeper structures.

Posterior acoustic enhancement: Fluid-filled structures (cysts, effusions) transmit sound more readily than surrounding tissue, producing increased brightness behind the fluid. This helps confirm the fluid-filled nature of anechoic structures.

Reverberation: Multiple reflections between two highly reflective parallel surfaces create repeated echo artefacts at regular intervals deeper in the image. Common near metal surfaces and at air-tissue interfaces.

Edge artefact (refraction): Hypoechoic lines at the curved edges of round structures (cysts, tendons in cross-section). Can mimic tears at the margin of a tendon.

Understanding artefacts is essential for avoiding misdiagnosis.

Rotator Cuff Ultrasound

Ultrasound is the primary imaging modality for rotator cuff assessment in many centres, with sensitivity comparable to MRI for full-thickness tears (89-95%) in experienced hands.

Standard shoulder ultrasound protocol:

1. Biceps tendon (long head): Transverse and longitudinal views in the bicipital groove. Assess for tenosynovitis (fluid around the tendon), subluxation, dislocation, and tears.
2. Subscapularis: Internal rotation brings the tendon to the anterior scanning window. Assess for partial and full-thickness tears. Dynamic assessment with external rotation demonstrates the tendon rolling over the lesser tuberosity.
3. Supraspinatus: Modified Crass position (hand on back pocket). Long axis and short axis assessment. Full-thickness tears show a defect through the entire tendon. Partial-thickness tears appear as focal hypoechoic or anechoic areas.
4. Infraspinatus/Teres minor: External rotation with the arm adducted. Less commonly torn but assessed as part of the complete protocol.
5. AC joint: Superior assessment for osteophytes, effusion, and instability.

Dynamic assessment: Impingement testing under real-time ultrasound allows direct visualisation of subacromial bursal thickening and cuff compression during abduction — this is unique to ultrasound and impossible with MRI.

Ultrasound-Guided Procedures

Ultrasound guidance dramatically improves injection accuracy and safety by providing real-time needle visualisation:

• Shoulder subacromial injection: Accuracy improves from approximately 70% (blind) to over 95% (guided). The needle is visualised entering the subacromial space in real time.
• Glenohumeral joint injection: Posterior approach under ultrasound guidance — the needle enters between infraspinatus and the posterior labrum. Accuracy improves from approximately 50% to over 95%.
• Hip joint injection: Anterior approach targeting the femoral head-neck junction. Essential for obese patients where blind injection accuracy drops significantly.
• Knee joint aspiration: Suprapatellar pouch approach. Ultrasound confirms the needle is within the effusion before aspiration.
• Tendon sheath injection: De Quervain stenosing tenosynovitis, trigger finger — precise delivery into the sheath without tendon injury.
• Barbotage (calcific tendinitis): Ultrasound-guided needling and aspiration of calcific deposits in the rotator cuff.

Key principles: (1) In-plane technique allows full-length needle visualisation. (2) Always confirm the needle tip position before injecting. (3) Observe the injectate spreading to confirm correct placement.

DDH Screening

Ultrasound is the gold standard for assessment of the neonatal hip (developmental dysplasia of the hip, DDH) because the femoral head is predominantly cartilaginous and invisible on plain radiography before ossification begins (3-6 months of age).

Graf classification: The standard method uses a coronal ultrasound image of the hip in the standard section (the iliac bone, labrum, and femoral head are all visible). From this image:

• Alpha angle: Angle between the iliac bone and the bony acetabular rim. Normal: more than 60 degrees. Less than 50 degrees = dysplastic.
• Beta angle: Angle between the iliac bone and the cartilaginous rim (labrum). Normal: less than 55 degrees.

Graf types:

• Type I (Normal): Alpha more than 60 degrees, well-formed bony roof
• Type II (Immature/Dysplastic): Alpha 50-60 degrees, various subtypes based on age
• Type III (Subluxated): Alpha less than 43 degrees, cartilaginous roof displaced
• Type IV (Dislocated): Alpha less than 43 degrees, labrum pushed inferiorly

Dynamic assessment (Barlow and Ortolani manoeuvres under real-time ultrasound) can demonstrate hip instability that may not be evident on static imaging.

Strong evidence supports ultrasound for rotator cuff assessment and guided interventions.

Broader applications of ultrasound continue to expand in orthopaedic practice.