Musculoskeletal Ultrasonography

MSK US Basics - Probing the Pixels

• Transducers: High-frequency linear array (>7.5 MHz) for superficial structures; curvilinear for deeper structures.
• Echogenicity:
  • Hyperechoic (bright): Bone cortex, tendons, ligaments, nerves (fibrillar pattern).
  • Hypoechoic (dark): Muscle, hyaline cartilage, fluid.
  • Anechoic (black): Cysts, bursae (uncomplicated fluid).
• Anisotropy: Tendon/ligament echogenicity changes with probe angle; perpendicular for max brightness. 📌 Heel-toe maneuver to overcome.
• Artifacts: Reverberation, acoustic shadowing (e.g., gallstones, bone), posterior acoustic enhancement (e.g., cysts).

⭐ Anisotropy is a key property of tendons and ligaments, where their echogenicity changes depending on the angle of insonation. Always scan perpendicular to the structure to avoid misinterpreting it as pathology (e.g., a tear).

Shoulder US - Cuff & Beyond

• Rotator Cuff (RC) Tears:
  • Supraspinatus (SST): Most common. Signs: defect, cortical irregularity, non-visualization.
    • Full-Thickness (FTT) vs. Partial-Thickness (PTT). Articular PTT (rim rent).
    • Cartilage interface sign (FTT).
  • Infraspinatus (IST), Subscapularis (SSC), Teres Minor (Tm).
• Biceps Long Head (LHB) Tendon:
  • Tenosynovitis (halo sign), tendinosis, tear, subluxation (empty bicipital groove).
• Subacromial-Subdeltoid (SASD) Bursa: Fluid >2 mm indicates bursitis.
• Acromioclavicular (AC) Joint: Osteoarthritis (OA), synovitis, distal clavicle osteolysis.
• Other: Calcific tendinitis (echogenic foci ± shadow); dynamic impingement tests.

⭐ The "critical zone" of the supraspinatus tendon, a common site for tears, is located approximately 1 cm proximal to its insertion near the biceps tendon (LHB).

Upper Limb Distal US - Joint Ventures

• Elbow Pathologies:
  • Lateral Epicondylitis (Tennis Elbow):
    • Common Extensor Tendon (CET): Thickened (>4-5mm), hypoechoic, tears, calcification.
    • Hyperemia on Power Doppler.
  • Medial Epicondylitis (Golfer's Elbow):
    • Common Flexor Tendon (CFT): Similar findings to CET. Ulnar nerve may show changes.
• Wrist & Hand Pathologies:
  • Carpal Tunnel Syndrome (CTS):
    • Median nerve: Swelling proximal to tunnel; CSA >10-12 mm² at pisiform level (inlet). (📌 Measure Nerve Carefully Soon)
    • Notching, ↓echogenicity, hypervascularity.

    ⭐ Median nerve cross-sectional area (CSA) >10 mm² at the carpal tunnel inlet is a primary US criterion for CTS.

  • De Quervain's Tenosynovitis:
    • Thickened 1st extensor compartment retinaculum.
    • Tenosynovitis/fluid around Abductor Pollicis Longus (APL) & Extensor Pollicis Brevis (EPB) tendons.
  • Ganglion Cysts:
    • Anechoic/hypoechoic, well-defined, compressible cyst.
    • Posterior acoustic enhancement; stalk may be visible.

Lower Limb Proximal US - Limb Lowdown

• Hip Joint:
  • Anterior approach: Detects effusion (anechoic/hypoechoic fluid), synovitis.
  • Femoral head contour, iliopsoas tendon/bursa.
  • Lateral approach: Gluteal tendons (medius, minimus) for tendinopathy, tears.
• Knee Joint:
  • Suprapatellar pouch: Effusion, synovitis (most sensitive site).
  • Patellar tendon: Tendinopathy (thickening, hypoechogenicity), tears.
  • Baker's (popliteal) cyst: Medial gastrocnemius & semimembranosus tendons.
  • Menisci, collateral ligaments (less common by US).

⭐ Baker's cyst, a distension of the gastrocnemio-semimembranosus bursa, is a common finding in knee US, often associated with intra-articular pathology like osteoarthritis or meniscal tears.

Lower Limb Distal & Interventions - Ankle Action & Needles

• Ankle Sprains (Dynamic Scan)
  • ATFL: Most common; plantarflexion/inversion stress.
  • CFL: Dorsiflexion stress. Deltoid (medial): Eversion stress.
• Achilles Tendon
  • Normal: <6 mm thick, fibrillar pattern.
  • Tendinopathy: Thickened, hypoechoic, neovascularization (Doppler +ve).
  • Tears: Partial/complete; assess with Kager's fat pad.
• Plantar Fasciitis
  • Thickening >4 mm at calcaneal origin, hypoechoic.
• US-Guided Interventions
  • Needle guidance: In-plane (longitudinal to beam, preferred) vs. out-of-plane.
  • Common: Aspirations (cysts, effusions), injections (steroid, PRP).

⭐ ATFL (Anterior Talofibular Ligament) is the most frequently injured ankle ligament, typically appearing thickened and hypoechoic on ultrasound after injury.

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High‑Yield Points - ⚡ Biggest Takeaways

• MSK USG excels for superficial soft tissues: tendons, ligaments, nerves.
• Dynamic assessment is a key advantage for real-time joint/tendon motion.
• Anisotropy artifact: tendons appear hypoechoic if not perpendicular to beam.
• Diagnoses tendinopathies, tears (rotator cuff, Achilles), bursitis, nerve entrapments.
• Doppler USG assesses inflammation and vascularity (e.g., synovitis).
• Accurately guides interventions like aspirations and injections.
• Limited for deep joint cartilage; MRI is superior for this assessment.