A middle aged man was sanding when he felt a sharp pain at the base of his right middle finger. The finger was held partially flexed. A thin foreign body (presumed splinter) could be felt at the distal palm (at the level of the junction of the palm and finger). Flexing and extending the finger was very painful.
An echogenic linear mass can be seen running transverse from just below the skin. It appears to pass through the radial slip of the flexor digitorum superficialis tendon. (Note that the FDP tendon appears black due to inosotropy)
Ultrasound if very useful for locating foreign bodies, particularly those that are radiolucent.
Angling the probe back and forth can help distinguish between FDS and FDP as the slightly different directions of the fibres cause anisotropy at different angles of insonation. In this view FDP is black and the slips of FDS (moving around FDP to insert more proximally) lie on either side.