Thumb Lateral is a radiography positioning projection of the Upper Extremity. Seated, hand turned slightly medially so the lateral surface of the thumb is on the IR. 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 true lateral 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 joints.
- Trapezium, superimposed, 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 at the end of the table.
- Start with the hand pronated, the thumb abducted, and the fingers and hand slightly arched.
- Turn the hand slightly medially until the thumb is in true lateral, its whole lateral surface on the IR; support the hand if needed.
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. Lateral surface of the thumb against the IR, thumbnail perpendicular to it. 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 lateral with no rotation or foreshortening.
- Both thumb phalanges, the first metacarpal, and the trapezium included.
- Joint spaces open and clearly visible.
- Soft tissue and bone trabeculae sharp, no motion blur.
- Collimation: 1 inch on all sides, 1 inch proximal to CMC joint.
Common errors
- 1 Hand supinated or pronated rotates thumb off true lateral.
- 2 Thumb not true lateral: the anterior proximal phalanx and first metacarpal should look concave.
- 3 CR not at the first MCP joint.
- 4 Thumb flexion foreshortens phalanges.
Clinical indications
- Fractures and dislocations of the phalanges, distal metacarpal, and associated joints.
- Suspected thumb joint instability or dislocation.
- Pathologic processes such as osteoporosis and osteoarthritis.