Presentation
Left ankle sprain - a few days prior to the presentation. Initial radiograph negative for a fracture. Immobilization for 1 week followed by ultrasound check for the lateral ankle ligaments.
Patient Data
Left ankle lateral ligaments were intact and normal(images not uploaded).
An extra structure with muscle echopattern is present in the tarsal tunnel lying deep to the flexor retinaculum. It is located between the tibial nerve and the flexor hallucis longus tendon.
A similar structure is present in the asymptomatic right ankle tarsal tunnel.
- medial malleolus
- tibialis posterior tendon
- flexor digitorum longus tendon
- posterior tibial artery
- posterior tibial paired veins
- tibial nerve
- an extra-muscle - accessory flexor digitorum longus muscle
- flexor hallucis longus tendon
- flexor retinaculum
An accessory muscle is present in bilateral tarsal tunnels.
Case Discussion
An accessory flexor digitorum longus muscle is present bilaterally. There is no associated tarsal tunnel syndrome or flexor hallucis longus muscle tenosynovitis.
An accessory muscle can be easily missed by ultrasound because it does not generate the abnormal echopattern of a pathology you are looking for during a scan. What we see is a normal echopattern in MSK ultrasound with an abnormal location. If you count the contents of a tunnel during a scan, you can not miss it.