15.1 Foot, Toes & Ankle

Osteology of the Foot and Ankle

The foot is built from 26 bones: 14 phalanges (two in the great toe, three in each lesser toe), 5 metatarsals, and 7 tarsals. The tarsal group is the exam's favorite recognition target: the calcaneus (largest, forms the heel), the talus (articulates with the tibia and fibula to form the ankle mortise), the navicular (medial midfoot), the cuboid (lateral midfoot), and the three cuneiforms (medial, intermediate, lateral). Two small sesamoid bones sit beneath the head of the first metatarsal. The longitudinal arch runs front to back and is best evaluated under load. Distally, the tibia contributes the medial malleolus and the fibula contributes the lateral malleolus; together with the talus they form the ankle mortise, a three-part joint whose even, open appearance is the primary evaluation criterion for a well-positioned mortise image.

Toes

Routine toe imaging uses three projections. The AP (dorsoplantar) projection may be taken with the central ray (CR) perpendicular or, more often, 10 to 15 degrees toward the heel (posteriorly) to open the interphalangeal and metatarsophalangeal joints. The AP oblique rotates the foot 30 to 45 degrees medially to demonstrate the phalanges and distal metatarsals free of superimposition. The lateral (mediolateral or lateromedial) isolates the individual toe of interest, using tape or gauze to hold neighboring toes clear. Because the toes are thin, extremity technique is used: small focal spot, tabletop (no grid), and low kVp (around 50 to 60) for high recorded detail.

Foot

The routine foot series is AP axial, AP oblique (medial rotation), and lateral. On the AP axial foot, the CR is angled 10 degrees toward the heel and enters the base of the third metatarsal; this opens the tarsometatarsal (Lisfranc) region and the intertarsal joints. The AP oblique with 30 degrees medial rotation throws open the third through fifth metatarsal bases, the cuboid, and the sinus tarsi, and demonstrates the tuberosity of the fifth metatarsal in profile (a common avulsion-fracture site). The lateral (mediolateral) places the plantar surface perpendicular to the receptor and superimposes the metatarsals; it shows the longitudinal arch and the navicular. When the clinician needs the arch under weight, a weight-bearing lateral foot with a horizontal beam is performed so the arch is evaluated while the patient stands and loads the foot.

Calcaneus and Ankle

The plantodorsal (axial) calcaneus angles the CR 40 degrees cephalad to the long axis of the foot, entering at the base of the third metatarsal; the dorsiflexed foot with the plantar surface vertical yields an elongated calcaneus free of superimposition and demonstrates the calcaneal tuberosity. A lateral (mediolateral) calcaneus places the CR perpendicular about 1 inch inferior to the medial malleolus. For the ankle, the AP uses a perpendicular CR midway between the malleoli with no rotation; note that a true AP does not fully open the mortise because the distal fibula overlaps the talus laterally. The AP mortise corrects this by internally rotating the leg and foot 15 to 20 degrees, placing the intermalleolar line parallel to the receptor and opening the entire mortise evenly. A 45-degree medial (internal) AP oblique demonstrates the distal tibiofibular articulation, and the lateral (mediolateral) superimposes the malleoli with the CR at the medial malleolus.

Projection, CR, and Evaluation Reference

Technical Factors and Evaluation Criteria

Lower-extremity distal work is done at the tabletop without a grid, using a small focal spot and moderate kVp (about 55 to 70) to preserve recorded detail. Collimate closely to the part and place a lead marker in the primary field at the time of exposure. Acceptance criteria for the AP oblique foot include open third through fifth intermetatarsal spaces, the cuboid free of superimposition, and the base of the fifth metatarsal in profile, a frequent avulsion site (the pseudo-Jones or dancer fracture). For the AP mortise, the tibiotalar joint and both the medial and lateral portions of the mortise must appear uniformly open; overlap of the distal fibula and talus signals too little internal rotation. For the plantodorsal calcaneus, the tuberosity and sustentaculum tali should be demonstrated without the malleoli obscuring the body.

Worked Scenario

A patient who inverted the ankle and reports lateral midfoot pain warrants an AP, AP oblique, and lateral foot to catch a fifth-metatarsal base avulsion, whereas heel-strike pain after a fall from height points to a calcaneus series (axial plus lateral). Matching the mechanism of injury to the projection set is a recurring ARRT reasoning pattern.

Common Traps

The most-missed distinction on the exam is AP ankle versus AP mortise: a plain AP is perpendicular with no rotation, while the mortise requires 15 to 20 degrees internal rotation. Do not confuse this with the 45-degree oblique. Another trap is the calcaneus angle: 40 degrees cephalad for the plantodorsal, not 40 caudad. Finally, remember the AP axial foot angle points toward the heel (posteriorly), and that the sesamoids of the great toe are demonstrated tangentially rather than on a routine AP. Anchoring each answer to the mortise's even appearance and the third-metatarsal centering point keeps positioning questions manageable under time pressure.