Wrist and Forearm Radiographic Positioning Study Notes

Wrist and Forearm Anatomy

The anatomy of the wrist and forearm consists of several critical bony structures including the following:

• Carpal Bones (Distal Row): Scaphoid, Capitate, Trapezoid, Trapezium, Hamate, and Pisiform.

• Carpal Bones (Proximal Row): Triquetrum and Lunate.

• Forearm Bones:

  • Ulna: Features the Olecranon process, Trochlear notch, Coronoid process, Head, Body, and Ulnar styloid process.

  • Radius: Features the Head, Neck, Radial tuberosity, Body, and Radial styloid process.

General Wrist Radiography Principles

• Image Receptor (IR): $8 \times 10\,\text{inch}$ ($18 \times 24\,\text{cm}$) lengthwise or crosswise (to accommodate two or more images on one IR).

• Patient Position:

  • The patient is seated at the end of the radiographic table.

  • The patient should be seated low enough to place the axilla in contact with the table surface.

  • The shoulder, elbow, and wrist joints must be placed in the same plane to allow for a proper right-angle rotation of the ulna and radius during lateral positioning.

PA Projection of the Wrist

• Part Position:

  • Rest the forearm on the table and center the wrist to the IR.

  • The hand and forearm should lie parallel to the long axis of the IR.

  • Slightly arch the hand at the metacarpophalangeal (MCP) joints by flexing the digits. This places the wrist in close contact with the IR.

  • Special Considerations: If the wrist is swollen, ask the patient to flex the wrist slightly. If the wrist is in a cast or splint, positioning is determined by comparison with the opposite side.

• Central Ray (CR): Perpendicular to the midcarpal area.

  • Scientific Note: Daffner, Emmerling, and Buterbaugh recommend angling the CR when the patient is in the PA position for better demonstration of specific bones. A CR angle of $30^\circ$ toward the elbow elongates the scaphoid and capitate. An angle of $30^\circ$ toward the fingertips elongates only the capitate.

• Structures Shown: A PA projection of the carpals, distal radius and ulna, and proximal metacarpals. It provides a slightly oblique rotation to the ulna. If the ulna is the primary area of interest, an AP projection is preferred.

• Evaluation Criteria:

  • Demonstration of distal radius, ulna, carpals, and proximal half of metacarpals.

  • No rotation in carpals, metacarpals, or radius.

  • Clear soft tissue and bony trabeculation.

  • No excessive flexion that would obscure metacarpals with digits.

AP Projection of the Wrist

• Part Position:

  • Rest the forearm on the table with the arm and hand supinated.

  • Elevate the digits on a suitable support to ensure the wrist is in close contact with the IR.

  • The patient should lean laterally to prevent rotation of the wrist.

• Central Ray (CR): Perpendicular to the midcarpal area.

• Structures Shown: Carpal interspaces are better demonstrated in an AP image compared to a PA image. This is because the oblique direction of the interspaces is more closely parallel with the divergence of the X-ray beam.

• Evaluation Criteria: Similar to the PA projection; requires distal radius, ulna, carpals, and proximal half of metacarpals without rotation.

Lateral Projection (Lateromedial) of the Wrist

• Part Position:

  • The patient flexes the elbow $90^\circ$ to rotate the ulna into the lateral position.

  • Center the IR to the carpals and adjust the forearm/hand so the wrist is in a true lateral position.

• Central Ray (CR): Perpendicular to the wrist joint.

• Structures Shown: Lateral projection of proximal metacarpals, carpals, and distal radius and ulna. This position is useful for demonstrating anterior or posterior displacement in fractures.

• Special Notes:

  • Burman et al. suggested obtaining a lateral position of the scaphoid with the wrist in palmar flexion, which rotates the bone anteriorly into a dorsovolar position.

  • Fiolle described carpe bossu (carpal boss), a small bony growth on the dorsal surface of the third Carpometacarpal (CMC) joint, which is best demonstrated in a lateral position with palmar flexion.

• Evaluation Criteria:

  • Superimposed distal radius and ulna.

  • Superimposed metacarpals.

  • Increased exposure factors are required to compensate for greater part thickness compared to PA/AP projections.

PA Oblique Projection (Lateral Rotation)

• Part Position: From a pronated position, rotate the wrist laterally (externally) until it forms an angle of approximately $45^\circ$ with the IR plane. Extend the wrist slightly. For scaphoid examination, adjust the wrist into ulnar deviation.

• Central Ray (CR): Perpendicular to the midcarpal area, entering just distal to the radius.

• Structures Shown: Carpals on the lateral side, specifically the trapezium and scaphoid. The scaphoid is otherwise superimposed on itself in a direct PA projection.

• Evaluation Criteria: Well-demonstrated scaphoid and trapezium. Slight interosseous space between the 3rd-4th and 4th-5th metacarpal shafts. Slight overlap of the distal radius and ulna.

AP Oblique Projection (Medial Rotation)

• Part Position: From a supinated position, rotate the wrist medially (internally) until it forms a semi-supinated position of approximately $45^\circ$ to the IR.

• Central Ray (CR): Perpendicular to the midcarpal area, entering the anterior surface midway between medial and lateral borders.

• Structures Shown: Separates the pisiform from adjacent carpal bones. Provides a distinct view of the triquetrum and hamate.

• Evaluation Criteria: Triquetrum, hamate, and pisiform free of superimposition and in profile.

PA Projections with Deviations

Ulnar Deviation

• Part Position: PA position; move elbow away from the body and turn hand outward into extreme ulnar deviation.

• Central Ray (CR): Perpendicular to the scaphoid. Clear delineation may require CR angulation of $10$ to $15^\circ$ proximally or distally.

• Structures Shown: Corrects the foreshortening of the scaphoid and opens spaces between adjacent carpals.

Radial Deviation

• Part Position: PA position; turn hand medially (moving elbow toward the body) into extreme radial deviation.

• Central Ray (CR): Perpendicular to the midcarpal area.

• Structures Shown: Opens interspaces between carpals on the medial side.

Stecher Method (PA Axial Projection of Scaphoid)

• Method Description: Used to project the scaphoid without self-superimposition.

• Part Position: Finger end of IR is elevated $20^\circ$ on a support. Center wrist in PA projection. Bridgman suggested using ulnar deviation.

• Central Ray (CR): Perpendicular to the table, directed to the scaphoid.

• Variations:

  1. Wrist and IR horizontal; CR angled $20^\circ$ toward the elbow.

  2. CR angled toward the digits to demonstrate fracture lines that angle superoinferiorly.

  3. Stecher Clenched Fist Method: Have the patient clench their fist. This elevates the distal scaphoid to lie parallel with the IR and widens the fracture line (no CR angle used).

Rafert-Long Method

• Context: Scaphoid fractures account for $60\%$ of carpal bone injuries. This method is used when routine radiographs do not identify a fracture.

• Technique: A four-image multiple-angle series.

• Part Position: Wrist in PA projection and extreme ulnar deviation.

• Central Ray (CR): Four separate exposures at $0^\circ$, $10^\circ$, $20^\circ$, and $30^\circ$ cephalad.

Clements-Nakayama Method (PA Axial Oblique for Trapezium)

• Part Position: Wrist in lateral position on the ulnar surface. Place a $45^\circ$ sponge wedge against the anterior surface. Rotate hand to contact the sponge and ulnar deviate the wrist.

• Central Ray (CR): Angled $45^\circ$ distally to enter the anatomic snuffbox and pass through the trapezium.

Tangential Projection (Carpal Bridge)

• Part Position: Hand palm-up at a right angle to the forearm. Alternatively, elevate forearm on sandbags and place IR vertically against the flexed wrist.

• Central Ray (CR): Directed $1.5\,\text{inches}$ proximal to the wrist joint at a caudal angle of $45^\circ$.

• Structures Shown: Demonstrates carpal bridge. Useful for scaphoid fractures, lunate dislocations, calcifications, foreign bodies, and chip fractures of the dorsal aspect of carpal bones.

Gaynor-Hart Method (Tangential Projection of Carpal Canal)

• Clinical Significance: The carpal canal (carpal tunnel) contains finger flexor tendons and the median nerve. Compression of the median nerve causes pain. Used to identify fractures of the hook of hamate, pisiform, and trapezium (common in athletes).

Inferosuperior Approach

• Part Position: Hyperextend the wrist. Rotate the hand slightly toward the radial side (prevents superimposition of hamate and pisiform).

• Central Ray (CR): Directed to the palm at a point $1\,\text{inch}$ ($2.5\,\text{cm}$) distal to the base of the 3rd metacarpal at an angle of $25$ to $30^\circ$ to the long axis of the hand.

Superoinferior Approach

• Part Position: Patient dorsiflexes wrist and leans forward. Marshall suggested using a $45^\circ$ angle sponge if dorsiflexion is limited (note: increases OID and magnification).

• Central Ray (CR): Tangential to the carpal canal, angled toward the hand $20$ to $35^\circ$ from the long axis of the forearm.

• Evaluation Criteria: Carpals in an arch arrangement; pisiform in profile and free of superimposition.

Forearm Radiography

• Image Receptor (IR): $14 \times 17\,\text{inches}$, lengthwise (divided if taking both views on one IR).

• Requirements: IR must include the entire forearm from the olecranon process (ulna) to the styloid process (radius).

AP Projection of Forearm

• Part Position: Entire limb in the same plane. Supinate hand, extend elbow. Humeral epicondyles must be equidistant from the IR.

• Caution: Pronation of the hand crosses the radius over the ulna at the proximal third and rotates the humerus medially, causing an oblique projection.

• Central Ray (CR): Perpendicular to the midpoint of the arm.

• Evaluation: Open elbow joint (if shoulder is in same plane). Slight superimposition of radial head, neck, and tuberosity over proximal ulna.

Lateral Projection (Lateromedial) of Forearm

• Part Position: Elbow flexed $90^\circ$. True lateral position (thumb side up).

• Central Ray (CR): Perpendicular to the midpoint of the arm.

• Evaluation Criteria: Superimposition of radius and ulna at the distal end. Radial head superimposed over the coronoid process. Radial tuberosity facing anteriorly. Superimposed humeral epicondyles.