Thumb AP is a radiography positioning projection of the Upper Extremity. Seated, hand internally rotated so the dorsal thumb surface lies flat on the IR. SID 40". The centering point is located first MCP joint of the thumb. The central ray is perpendicular to the first MCP joint. Image-quality criteria include thumb shown in true AP with no rotation or foreshortening, both thumb phalanges, the first metacarpal, and the trapezium included. A sample technique is 63 kVp with DR 0.6 or CR 1.6 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Distal and proximal phalanges (the thumb has only two).
- First metacarpal.
- IP, MCP, and first CMC joint spaces.
- Trapezium and CMC articulation.
Patient preparation
- WARNING: with possible spinal injury or severe trauma, do not attempt to move the patient; do not rotate a severely injured part or remove splints (Bontrager). Merrill's: do not attempt true AP or lateral positioning of a severely injured limb; take two projections 90 degrees apart, moving it as little as possible, and move the IR and tube if the patient cannot reach the position.
- Verify patient identity using two identifiers.
- Remove rings, bracelets, and radiopaque items from the hand.
- Seat the patient facing the table.
- Extend the arm and internally rotate the hand until the thumb's dorsal surface rests on the IR.
- Place the dorsal surface of the thumb flat against the IR.
Position & centering point
First MCP joint of the thumb.
Central ray
Perpendicular to the first MCP joint.
Exposure / technique
- kVp
- 63
- mAs
- DR 0.6 / CR 1.6
- SID
- 40" (102 cm)
- Notes
- Tabletop technique. No grid required. Sample technique: DR 0.6 / CR 1.6 mAs at 63 kVp, 40 in SID, for a 1.5 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Thumb shown in true AP with no rotation or foreshortening.
- Both thumb phalanges, the first metacarpal, and the trapezium included.
- IP and MCP joint spaces open.
- Soft tissue and bone trabeculae sharp with no motion blur.
- Collimation: 1 inch on all sides, 1 inch proximal to CMC joint.
Common errors
- 1 Thumb rotated or pronated off true AP projection.
- 2 Incomplete internal rotation lifts the dorsal thumb surface off the IR.
- 3 CR not at the first MCP joint.
- 4 Flexion of the thumb foreshortens phalanges.
Clinical indications
- Trauma evaluation for thumb fracture or dislocation. For skier's thumb (ulnar collateral ligament injury), Bontrager uses the PA stress view of both thumbs (Folio method).
- Pathologic processes such as osteoporosis.
- Assessment of thumb joint arthritis or degenerative changes.