Sesamoids Tangential, Lewis Method is a radiography positioning projection of the Lower Extremity. Patient prone, toe flexed back on IR. CR perpendicular and tangential. SID 40". The centering point is located posterior aspect of the first MTP joint. The central ray is perpendicular to the IR, tangential to the posterior first MTP joint; a slight angle may be needed. Image-quality criteria include medial and lateral sesamoids of the first metatarsal head visible, posterior margins of the 1st to 3rd distal metatarsals in profile, showing correct dorsiflexion. The SID is 40 inches (102 cm).
Anatomy demonstrated
- Medial sesamoid of the first metatarsal head.
- Lateral sesamoid of the first metatarsal head.
- Plantar surface of the first metatarsal head.
- Open space between the sesamoids and the first metatarsal.
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 footwear, socks, and any radiopaque foot jewelry.
- Position the patient prone.
- Dorsiflex the great toe and rest it on the IR to hold the position.
- Dorsiflex the foot: sole about 15-20 degrees from vertical (Bontrager); ball of the foot vertical (Merrill's).
- This position is uncomfortable and often painful: do not keep the patient in it longer than necessary. If the patient cannot tolerate prone, Bontrager reverses it, supine, with a long gauze strip holding the toes.
Position & centering point
Posterior aspect of the first MTP joint.
Central ray
Perpendicular to the IR, tangential to the posterior first MTP joint; a slight angle may be needed.
Exposure / technique
- kVp
- N/A
- mAs
- N/A
- SID
- 40" (102 cm)
- Notes
- Great toe dorsiflexed on the IR. Bontrager: plantar surface about 15-20 degrees from vertical, digital 60 +/- 5 kV, no printed mAs. Merrill's: ball of the foot perpendicular to the horizontal plane. In the 2022 ARRT outline; not in the 2027 outline (from March 1, 2027).
Image-quality criteria
- Medial and lateral sesamoids of the first metatarsal head visible.
- Posterior margins of the 1st to 3rd distal metatarsals in profile, showing correct dorsiflexion.
- Plantar surface of the first metatarsal head clearly shown.
- Open space between the sesamoids and the 1st metatarsal.
- Bone trabeculae sharp, no motion artifact.
- Collimation: at least the first three distal metatarsals (Bontrager); 3 x 3 inches (Merrill's).
Common errors
- 1 Great toe not dorsiflexed and resting on the IR.
- 2 Foot not dorsiflexed to about 15-20 degrees from vertical superimposes the sesamoids.
- 3 CR not directed to the first MTP joint.
- 4 Rotating the long axis of the foot superimposes bone on the sesamoids.
Clinical indications
- Suspected sesamoid fracture or osteonecrosis.