Upper Extremities
18 Forearm
Anatomy & Pathology
Routine Projections (ARRT Required)
Forearm – AP Projection
CR Location & Positioning
- SID: 40 inches (102 cm)
- Patient position: Seated at the table with the entire upper limb in one horizontal plane (shoulder, elbow, and wrist aligned).
- Adjustments: Hand fully supinated, elbow extended, dorsal surface of forearm on IR. Lean patient laterally so humeral epicondyles are equidistant from the IR for a true AP position.
- CR: Perpendicular to midpoint of the forearm; include both elbow and wrist joints.
- Pt. Instructions: Hold still and suspend breathing during exposure.
- Exposure: DR technique with moderate kVp (60–70 range); ensure both joints and soft tissues are visualized with adequate contrast and detail.
Evaluation Criteria
- Coverage: Entire forearm including wrist joint distally and distal humerus proximally; verify by seeing both styloid processes and olecranon process.
- Rotation checks: Humeral epicondyles in profile; radius and ulna parallel without crossover; open radioulnar joint space indicates true AP alignment.
- Motion checks: Cortical margins and trabecular bone sharply defined; any blurring indicates patient or part motion.
- Technique checks: Even exposure across proximal and distal forearm; bone and soft tissue visible without clipping or saturation; no artifacts.
- Clinical aim: Evaluate fractures, dislocations, or alignment abnormalities of radius and ulna; assess proximal and distal radioulnar articulation.
Forearm – Lateral Projection
CR Location & Positioning
- SID: 40 inches (102 cm)
- Patient position: Seated with shoulder, elbow, and wrist in the same horizontal plane.
- Adjustments: Elbow flexed 90°; medial aspect of forearm against IR; thumb side up. Ensure distal radius and ulna are superimposed; hand in true lateral with thumb relaxed along the side.
- CR: Perpendicular to midpoint of forearm; include both joints.
- Pt. Instructions: Hold still and suspend breathing during exposure.
- Exposure: DR technique with moderate kVp; adjust mAs for adequate penetration through both joints and soft tissue visualization.
Evaluation Criteria
- Coverage: Entire forearm from wrist to distal humerus; verify both joints included on image.
- Rotation checks:
- Distal radius and ulna superimposed
- Radial head superimposed over coronoid process
- Humeral epicondyles directly superimposed
- Radial tuberosity facing anteriorly
- Motion checks: Sharp trabecular pattern and crisp cortical outlines confirm no movement.
- Technique checks: Uniform brightness and contrast; visible soft tissue outline of both bones; no artifacts or grid cutoff.
- Clinical aim: Demonstrate displacement, angulation, or alignment in trauma; assess radial head relationship to ulna; evaluate for fractures or dislocations.
Hows & Whys of Forearm Radiography
Anatomy
- Which bone is lateral in anatomic position?
The radius is lateral; it lies on the thumb side of the forearm. - Which bone forms the primary articulation with the humerus?
The ulna articulates with the trochlea of the humerus at the elbow joint. - Which bone articulates with the carpal bones at the wrist?
The distal radius articulates directly with the scaphoid and lunate. - What type of joints are the proximal and distal radioulnar joints?
They are pivot joints that allow pronation and supination of the forearm. - What type of joint is the elbow?
A hinge joint, allowing flexion and extension.
Positioning
- Why must the hand be fully supinated for the AP projection?
Supination prevents the radius from crossing over the ulna and ensures both bones are parallel for a true AP view. - Why is the shoulder, elbow, and wrist kept in the same horizontal plane?
This alignment prevents distortion and allows both joints to be included without elongation or foreshortening. - Why is the elbow flexed 90° for the lateral projection?
Flexing the elbow 90° aligns the forearm in a true lateral position, ensuring correct superimposition of the radius and ulna. - Why is the thumb side placed upward for the lateral projection?
This maintains the forearm in a true lateral position and prevents internal rotation errors. - Why should both the wrist and elbow be included on the forearm image?
Because fractures often involve both the shaft and one or both joint articulations (e.g., Monteggia and Galeazzi injuries).
Technique & Image Evaluation
- How can you tell the hand was properly supinated on an AP image?
The radius and ulna are parallel, and there is no proximal crossover. - How can you tell the AP projection is properly aligned?
Humeral epicondyles appear in profile and the radioulnar space is open. - How can you tell the lateral projection is correctly positioned?
The radial head is superimposed over the coronoid process, distal radius and ulna are superimposed, and humeral epicondyles are directly over each other. - How can you tell there was motion during exposure?
Blurring of cortical margins or trabecular pattern indicates patient or part movement. - How can you tell exposure factors were appropriate?
Both bone cortices and soft tissue outlines are clearly visible from wrist through elbow without overexposure or noise.
Clinical Applications
- Which projections best demonstrate fractures of the radius or ulna?
AP and Lateral forearm projections—both required for assessment of alignment and displacement. - Which projection best shows anterior or posterior displacement of fracture fragments?
The Lateral projection, as it demonstrates the true depth of displacement. - Why is it important to include both joints?
Forearm injuries may involve a fracture at one end with dislocation at the other; omitting either joint risks missing significant pathology. - How can you differentiate a Colles’ from a Smith’s fracture radiographically?
A Colles’ fracture shows posterior (dorsal) displacement of the distal radius; a Smith’s fracture shows anterior (volar) displacement.
