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Musculoskeletal

Musculoskeletal Anatomy (Upper Extremity)

The upper extremity comprises the shoulder girdle, elbow, wrist, and hand with their articulations, tendons, ligaments, and neurovascular bundles. MRI and MR arthrography assess internal derangement, while CT and radiographs define bony injury and ultrasound evaluates tendons dynamically. Knowledge of the rotator cuff, elbow ligaments, carpal alignment, and nerve courses guides musculoskeletal interpretation.

Key Structures

  • Shoulder: glenohumeral joint with labrum, rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis), biceps long head, and the acromioclavicular joint
  • Elbow: humeroradial, humeroulnar, and proximal radioulnar joints; ulnar collateral ligament, radial collateral complex, and the common flexor/extensor origins
  • Wrist: proximal (scaphoid, lunate, triquetrum, pisiform) and distal (trapezium, trapezoid, capitate, hamate) carpal rows, the TFCC, and scapholunate/lunotriquetral ligaments
  • Hand: metacarpals and phalanges with collateral ligaments, volar plates, and flexor/extensor tendon apparatus with annular pulleys
  • Tendons: rotator cuff, biceps, common flexor/extensor, and the digital flexor/extensor tendons
  • Neurovascular bundles: brachial plexus terminal branches (median, ulnar, radial, axillary) and the axillary/brachial/radial/ulnar arteries
  • Carpal tunnel and Guyon canal transmitting the median and ulnar nerves respectively

Imaging Planes & Modalities

  • MRI in three planes: oblique coronal and oblique sagittal for the rotator cuff, axial for the labrum, biceps, and subscapularis
  • MR arthrography improves labral and ligament assessment (shoulder SLAP/Bankart, wrist intrinsic ligaments, elbow UCL)
  • Radiographs first-line for fracture, alignment, and arthritis; specialized views (scaphoid, axillary) as needed
  • CT for complex/occult fractures, nonunion, and preoperative bony detail
  • Ultrasound for dynamic tendon evaluation, effusions, and guidance; excellent for superficial structures
  • Wrist imaging in neutral and stress positions to assess carpal instability and TFCC

Landmarks & Normal Values

  • Acromiohumeral interval normally ~7-14 mm; narrowing suggests cuff tear/superior migration
  • Scapholunate interval normally <3 mm; widening (Terry Thomas sign) indicates SL ligament tear
  • Normal scapholunate angle ~30-60 degrees; capitolunate alignment assesses DISI/VISI instability
  • Supraspinatus tendon best profiled on oblique coronal images near its footprint insertion
  • Carpal tunnel contains the median nerve and nine flexor tendons; nerve cross-sectional area helps assess entrapment
  • Radial inclination (~22 degrees) and volar tilt (~11 degrees) of the distal radius on radiographs

Anatomical Variants

  • Os acromiale (unfused acromial apophysis) predisposing to impingement
  • Sublabral foramen and Buford complex (cord-like middle glenohumeral ligament with absent anterosuperior labrum) mimicking tears
  • Bipartite/accessory carpal bones and a bifid median nerve/persistent median artery
  • Ulnar variance (positive/negative) affecting TFCC and ulnar impaction
  • Accessory ossification centers and sesamoids in the hand mimicking fractures
  • Anconeus epitrochlearis muscle and variant anatomy around the cubital tunnel

Pathology Correlation

  • Rotator cuff footprint anatomy defines partial versus full-thickness tears and retraction/atrophy grading
  • Labral anatomy underlies SLAP and Bankart lesions after dislocation
  • Carpal alignment predicts scapholunate/lunotriquetral instability and SLAC/SNAC wrist arthritis after scaphoid pathology
  • Nerve courses explain entrapments: median at the carpal tunnel, ulnar at the cubital tunnel/Guyon canal, radial at the spiral groove
  • Elbow ligament anatomy relates to UCL injury (overhead athletes) and instability
  • Scaphoid vascular supply (retrograde, distal to proximal) explains proximal pole avascular necrosis after fracture

Pitfalls & Mimics

  • Magic-angle artifact in curved tendons (supraspinatus, peroneals analog) mimicking tendinosis at ~55 degrees
  • Normal labral variants (sublabral foramen, Buford complex) misread as labral tears
  • Partial-volume and chemical-shift artifacts at bone-cartilage interfaces
  • Accessory ossicles and unfused apophyses mistaken for avulsion fractures
  • Occult scaphoid fractures negative on initial radiographs; MRI/CT or follow-up needed
  • Intrasubstance tendon signal and myotendinous striation misinterpreted as tears

References

  • Stoller. Magnetic Resonance Imaging in Orthopaedics and Sports Medicine
  • Helms, Major, Anderson. Musculoskeletal MRI
  • Manaster et al. Diagnostic Imaging: Musculoskeletal
  • Netter. Atlas of Human Anatomy

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.