Thumb PA Oblique is a radiography positioning projection of the Upper Extremity. Seated, hand pronated, thumb obliqued 45 degrees. SID 40". The centering point is located first MCP joint of the thumb. The central ray is perpendicular to the MCP joint. Image-quality criteria include thumb shown in 45-degree oblique with minimal overlap of digits, 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 articulations.
- Trapezium and CMC joint region.
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 at the end of the table.
- Extend the arm with hand pronated (palm down).
- Abduct the thumb slightly; natural pronation places it in about a 45-degree oblique.
Position & centering point
First MCP joint of the thumb.
Central ray
Perpendicular to the MCP joint.
Exposure / technique
- kVp
- 63
- mAs
- DR 0.6 / CR 1.6
- SID
- 40" (102 cm)
- Notes
- Tabletop technique. Palm facing down (pronated hand). 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 45-degree oblique with minimal overlap of digits.
- Both thumb phalanges, the first metacarpal, and the trapezium included.
- Joint spaces open and clearly visible.
- Bone trabeculae and soft tissue sharp, no motion blur.
- Collimation: 1 inch on all sides, 1 inch proximal to CMC joint.
Common errors
- 1 Palm lifted off the IR: palmar contact with slight thumb abduction gives the 45-degree oblique.
- 2 Supinated hand negates the oblique positioning.
- 3 CR not at the first MCP joint.
- 4 Excessive flexion foreshortens the thumb.
Clinical indications
- Fractures and dislocations of the phalanges, distal metacarpal, and associated joints.
- Suspected thumb joint arthrosis or degenerative disease.