Ankle AP is a radiography positioning projection of the Lower Extremity. Supine, leg extended and not rotated, CR perpendicular to the ankle joint. SID 40". The centering point is located midway between the malleoli. The central ray is perpendicular to the ankle joint between the medial and lateral malleoli. Image-quality criteria include medial and superior joint space open; lateral mortise not fully open, some superimposition of the distal fibula by the distal tibia and talus. A sample technique is 70 kVp with DR 1.8 or CR 3.2 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Distal tibia and fibula clearly demonstrated.
- Medial and lateral malleoli (the lateral one lies about 15 degrees more posterior).
- Medial mortise open; the lateral mortise stays closed on a true AP.
- Distal tibiofibular joint, overlapped on a true AP.
- Talus and the proximal half of the metatarsals.
Patient preparation
- Verify patient identity using two identifiers.
- Remove footwear, socks, and any radiopaque foot jewelry.
- Position the patient supine with the leg extended.
- Leave the foot in its natural position; do not force dorsiflexion, which can hurt and add injury.
- Keep the whole lower leg unrotated; the intermalleolar line is not parallel to the IR.
Position & centering point
Midway between the malleoli.
Central ray
Perpendicular to the ankle joint between the medial and lateral malleoli.
Exposure / technique
- kVp
- 70
- mAs
- DR 1.8 / CR 3.2
- SID
- 40" (102 cm)
- Notes
- Tabletop technique. Leave the foot in its natural position; forced dorsiflexion is painful. Sample technique: DR 1.8 / CR 3.2 mAs at 70 kVp, 40 in SID, for an 11 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Medial and superior joint space open; lateral mortise not fully open.
- Some superimposition of the distal fibula by the distal tibia and talus.
- Distal third of the tibia and fibula, malleoli, talus and proximal half of the metatarsals.
- Distal tibiofibular joint partially overlapped, as expected on a true AP.
- Bone trabeculae sharp, no motion blur.
- Collimation: lateral skin margins (Bontrager); 1 inch on the sides, 8 inches lengthwise (Merrill's).
Common errors
- 1 Rotating the leg medially opens the lateral mortise, turning the AP into a mortise view.
- 2 Forcing dorsiflexion is painful and can add injury; leave the foot as it lies.
- 3 CR not directed midway between the malleoli.
- 4 Any leg rotation changes the expected overlap of the distal fibula by the tibia and talus.
Clinical indications
- Bony lesions or diseases of the ankle joint, distal tibia and fibula, and proximal talus.
- Bony lesions or diseases of the proximal fifth metatarsal.
- An open lateral mortise may suggest spread from ruptured ligaments; the AP mortise evaluates it.