Review Question - QID 215618

This patient has tarsal tunnel syndrome, which causes compression of the tibial nerve. The peroneus brevis (E) is innervated by the superficial peroneal nerve, which would remain unaffected in this case.

Given the number of nerves that coexist in the foot and ankle, innervation can be complex. The tibial nerve plays a large role in this innervation scheme, and is primarily responsible for actions related to ankle plantarflexion, flexion of the toes, and plantar sensation. The tibial nerve has two primary branches, the medial and lateral plantar nerves, which are responsible for innervating all intrinsic foot muscles aside from the extensor hallucis brevis and extensor digitorum brevis. The medial calcaneal branch is the other branch of the tibial nerve which plays a lesser role, only providing sensation to the plantar/medial heel. Tarsal tunnel syndrome is the most common cause of tibial nerve compression and has multiple etiologies with a common theme of nerve impingement as it passes through its fibro-osseous tunnel. With no treatment and a long period of compression, symptoms may worsen and muscle deinnervation may be noted on MRI.

Pomeroy et al. reviewed entrapment neuropathies of the foot and ankle and their respective effects. They note that understanding the anatomy and physical examination of the area is important in prompt and accurate diagnosis. Treatment can then be aimed at reducing compression and decreasing causes of intrinsic impingement nonoperatively or, at times, through surgery.

Donovan et al. discussed MRI findings of entrapment neuropathies about the foot and ankle. The noted that most causes were either positional or related to mechanical compression in the nerve's fibrous tunnel. They argue that MRI is useful in tracking the course of the peripheral nerve as well as accurately identifying associated changes in muscular/soft tissue integrity.

Figure A demonstrates a coronal T1 MRI sequence which demonstrates normal muscular anatomy labeled as follows: (1) Abductor hallucis (2) Quadratus plantae (3) Flexor digitorum brevis (4) Abductor digiti minimi (5) Peroneus brevis.

Incorrect Answers:
Answer 1: A depicts the abductor hallucis muscle which is innervated by the medial plantar branch of the tibial nerve.
Answer 2: B depicts the quadratus plantae, which helps to flex the 2nd-5th toes and is innervated by the lateral plantar branch of the tibial nerve.
Answer 3: C depicts the flexor digitorum brevis which is innervated by the medial plantar branch of the tibial nerve.
Answer 4: D depicts the abductor digiti minimi which is innervated by the lateral plantar branch of the tibial nerve.