Image Interpretation Course
by Heidi Nunn DCR(R) PgCert
The Foot
Normal anatomy | Accessory ossicles | Phalanges
5th metatarsal | Stress fractures | Tarsal avulsions | Lisfranc | Midtarsal fracture-dislocation
(click on images to enlarge)
Normal anatomy
- DP film - The medial border of the base of the 2nd metatarsal should be in line with the medial border of the middle cuneiform. There is overlap of the lateral metatarsal bases and tarsal bones (cuboid and lateral cuneiform).
- Oblique film - The medial border of the base of the 3rd metatarsal should be in line with the medial border of the lateral cuneiform. Now the 1st/2nd mt bases, and the medial and middle cuneiforms overlap:
Accessory ossicles – Normal variants
- Unfused secondary ossification centres. These are well corticated osseous fragments that are located in characteristic places. Should not be misinterpreted as fractures; refer to Keats Atlas of Normal Variants if unsure. Some of the more common ossicles are:
Os tibiale externum – postero-medial to the navicular.
Os peroneum – lateral to the cuboid.
Os calcaneus secundarius – adjacent to the anterior process of the calcaneum.
Phalanges
- Fractures and dislocations are common. Magnification and correct windowing of the image is often necessary for detection.
Fifth metatarsal fractures
- Avulsion fractures are common at the tuberosity at the base of the 5th metatarsal. This is at the insertion of the peroneus brevis tendon, and avulsions occur due to inversion injuries:
- Jones fracture – refers to a transverse fracture of the proximal shaft of the 5th metatarsal. Has a poorer prognosis and different treatment to avulsion fractures (requires non-weight bearing).
- Beware in children the normal unfused apophysis. This should not be mistaken for a fracture, by observing that the apophysis lies longitudinal to the long axis of the metatarsal. A fracture line will run transversely:
- The normal apophysis may appear displaced and/or fragmented. However this is usually normal:
- An apophysis is located at a site of ligament or tendon attachment. The apophysis does not contribute to the longitudinal growth of bone. This differs from an epiphysis which is located adjacent to a joint and does contribute to longitudinal growth.
Stress fractures of the metatarsals
- Radiographic findings not evident initially, however approx.10 days later a transverse radiolucency indicates the site of fracture followed by periosteal reaction and fluffy callus formation indicating healing. Profuse callus formation often subsequently develops. Usually 2nd/ 3rd distal metatarsal shaft affected:
Avulsion injuries of the tarsal bones
- Check the lateral cortex of the cuboid for subtle avulsion fractures:
- Avulsion fractures of the dorsal surface of the navicular and head of talus are common and often missed. These can only be identified by a lateral view:
- Avulsion of the anterior process of the calcaneum by the bifurcate ligament is relatively common but can be subtle, resulting from an inversion injury. This is usually seen on the oblique view of the foot:
- A 2nd common site is seen lateral to the calcaneum (on the DP foot) at the insertion of the extensor digitorum brevis muscle:
Injuries to the anterolateral aspect of the calcaneum may be very subtle but, if missed, can be debilitating:
Lisfranc fracture-dislocation (tarso-metatarsal)
- Refers to fractures at the base of the metatarsals (usually the 2nd) accompanied by lateral subluxation at the tarso-metatarsal joints. The base of the 2nd metatarsal is held in a mortise by the three cuneiform bones. When it is fractured distal to this, the base often remains held in the mortise by the ligaments, with lateral subluxation of the metatarsals:
- May be subtle, however, by observing the alignment at the base of the 2nd and 3rd metatarsals with the adjacent cuneiforms, this significant injury should not be missed. May have associated cuboid fracture.
Midtarsal fracture-dislocation
- Disruption of the talo-navicular and calcaneo-cuboid joints, with medial or lateral dislocation of the foot. Occurs due to high velocity trauma; these injuries are usually easy to detect as the foot is obviously deformed:
