Upper Limb Radiography: Wrists and Scaphoids

Wrist Positioning and Projections

Dorsal-Palmar (DP) Wrist Positioning
- Centre Point (CP): Located in the midline, midway between the radial and ulnar styloid processes. Alternatively, it can be mid-carpal.
- Inclusion Criteria: The Field of View (FOV) must include the Metacarpophalangeal (MCP) joints to the distal $1/3$ of the radius and ulna, including lateral soft tissue margins.
- Angulation Assessment:
  - There should be minimal superimposition of the distal radius with the inferior aspect of the scaphoid.
  - The distal radius should exhibit a moon-curved or crescent-shaped line.
  - No foreshortening of the metacarpal (MC) shafts should be present, indicating the patient did not flex the fingers and kept the palmar aspect flat on the detector.
- Rotation Assessment:
  - Equal concavity of the metacarpal shafts.
  - Minimal superimposition of the metacarpal bases and the distal radio-ulnar joint.
Oblique Wrist Positioning
- Centre Point (CP): In the midline, midway between the radial and ulna styloid processes. It may appear slightly medial to the mid-carpal region to ensure the $1^{st}$ MC is included.
- Inclusion Criteria: MCP joints to the distal $1/3$ of the radius and ulna; soft tissue margins included.
- Angulation Assessment:
  - Little superimposition of the distal radius with the inferior scaphoid.
  - Smooth crescent shape of the distal radius.
  - No foreshortening of the MC shafts, ensuring the forearm is in the same vertical plane as the hand and detector.
- Rotation Assessment:
  - The hand is externally rotated $45^{\circ}$ .
  - The proximal $3^{rd}$ , $4^{th}$ , and $5^{th}$ MCs are partially superimposed.
  - The distal ulna partially superimposes the distal radius.
Lateral Wrist Positioning
- Centre Point (CP): Radial styloid process. On assessment, it often falls over the lunate bone, slightly distal to the styloid.
- Inclusion Criteria: MCP joints to the distal $1/3$ of the radius and ulna; soft tissue margins included.
- Angulation Assessment:
  - Distal radius appears as a crescent or saucer-like shape.
  - The lunate carpal bone (the "cup") should have a crescent-shaped inferior outline.
- Rotation Assessment:
  - True lateral involves the thumb in a PA position.
  - Distal radius and ulna are superimposed.
  - Alignment of the "Apple" (capitate), "Cup" (lunate), and "Saucer" (distal radius) should be identifiable.

Scaphoid Positioning and Projections

Collimation Standards
- Initial Presentation: Include mid-metacarpals to the distal radius and ulna.
- Follow-up: Tighter collimation targeting only the scaphoid and surrounding carpal bones.
PA Scaphoid Positioning
- Centre Point (CP): Mid-carpal area, specifically $2\,cm$ distal and lateral to the radial styloid process.
- Angulation and Movement: Requires ulnar deviation (lateral angling of the wrist). This opens the carpal bones around the scaphoid to better visualize the cortical outline.
- Rotation Assessment: Equal concavity of the metacarpal shafts (specifically the $4^{th}$ and $5^{th}$ ) and minor superimposition of the metacarpal bases. Distal radius and ulna articulation should have very little superimposition.
Oblique Scaphoid Positioning
- Centre Point (CP): Mid-carpal area, $2\,cm$ distal and lateral to the radial styloid process.
- Angulation Assessment: Smooth crescent shape of the radius head; minimal superimposition of the distal radius and inferior scaphoid.
- Rotation Assessment: Hand externally rotated $45^{\circ}$ . The ulnar head and distal radius are slightly superimposed; proximal MCs $3-5$ are partially superimposed.
Lateral Scaphoid Positioning
- Centre Point (CP): Anatomical Snuff Box (ASB), distal to the radial styloid process.
- Rotation/Structure: True lateral with distal radius and ulna superimposed. Carpal bones superimpose, but the "apple, cup, saucer" alignment remains the key assessment factor.
Axial Scaphoid Positioning
- Centre Point (CP): Anatomical Snuff Box (ASB).
- Angulation: A $25^{\circ}$ cranial tube angle is used to elongate the scaphoid bone.
- Evaluation: The scaphoid should be free of superimposition, though minor overlap from the trapezium, trapezoid, or capitate may occur due to ulnar deviation.
- Rotation: Base of the $1^{st}$ MC should not superimpose the base of the $2^{nd}$ MC.

Positioning Errors and Corrective Actions

Exposure and Grids in Casts
- Radiographs through fiberglass or Plaster of Paris must maintain diagnostic quality (clarity of trabeculae pattern).
- Low Density Example: An image with $55\,kVp$ and $2\,mAs$ resulted in an Exposure Index (EI) of $89$ , which was non-diagnostic (too white).
- Correction: Increasing to $60\,kVp$ and doubling the mAs to $4\,mAs$ improved the EI to $336$ , rendering the image diagnostic.
Lateral Rotation Errors
- Under-rotation: The radius and ulna are not superimposed; the radius is positioned more anteriorly relative to the ulna.
  - Correction: Externally rotate the wrist (moving it posteriorly) to bring the radius over the ulna.
- Over-rotation: The radius is more posterior relative to the ulna.
  - Correction: Internally rotate the wrist (moving it anteriorly) to bring the radius anterior over the distal ulna.
Ulnar Deviation Limitations
- In acute injuries, patients may be unable to perform ulnar deviation. In these cases, the axial (elongated) scaphoid view becomes the most valuable projection for visualizing the cortical outline.

Clinical Pathologies and Trauma

Scaphoid Fractures
- Most common carpal bone fracture ( $70-80\%$ of all carpal fractures).
- Diagnosis typically requires a 4-view scaphoid series.
- Presentation: Pain and tenderness in the Anatomical Snuff Box (ASB) following a fall on an outstretched hand (FOOSH).
- Fracture Locations:
  - Waist of the scaphoid: $70-80\%$
  - Distal pole: $20\%$
  - Proximal pole: $10\%$
- Risks: Fractures may be occult (not immediately visible) and carry a risk of avascular necrosis.
Distal Radius Fractures: Colles vs. Smith
- Colles Fracture:
  - Extra-articular fracture of the distal radial metaphysis resulting from FOOSH.
  - Displacement: Posterior/dorsal displacement and angulation of the distal fragment.
  - Deformity: Termed "dinner fork deformity" when dorsal angulation is severe.
  - Associated with advancing age and osteoporosis.
- Smith's Fracture:
  - Often referred to as a "reverse Colles."
  - Displacement: Volar (palmar) angulation/displacement of the distal fragment.
  - Mechanism: Fall onto a flexed wrist or a direct blow to the back of the wrist. Common in younger patients due to high-energy trauma.
Age-Specific FOOSH Injuries
- $4-10$ years: Torus fracture of the distal radial metaphysis.
- $11-16$ years: Salter-Harris II fracture involving the physeal plate.
- $17-40$ years: Scaphoid fracture.
- Over $40$ years: Colles'-type fracture.
Alignment Rules
- Congruency Rule: The articular surfaces of the radius, lunate, and capitate should form a straight, parallel line.
- Visual Aid: The "Apple" (capitate) sits in the "Cup" (lunate), which sits in the "Saucer" (distal radius). A break in this line suggests carpal dislocation, torus fractures, or cortical breaks.