Neurosurgeon Amgad Hanna knew that what he saw in the operating room did not match the diagrams depicted in anatomy textbooks.

So he tested his hunch in the University of Wisconsin School of Medicine and Public Health’s anatomy lab, the same place first-year medical students encounter the cadavers that begin to teach them about the human body. There, dissections confirmed Hanna’s observations and proved nearly 500 years of anatomy texts wrong.

Hanna’s findings, published in the Journal of Neurosurgery, will rewrite how anatomy books depict the brachial plexus. More importantly, they will improve care for trauma patients who suffer injuries that leave them unable to fully use their arms.

A neurosurgeon holding textbooks
Amgad Hanna shows the anatomy texts with incorrect diagrams of the brachial plexus and the book he wrote, in blue, with the correct alignment of nerves.

Hanna, an associate professor of neurosurgery, is a national expert in repairing injuries to the brachial plexus, a spot beneath the collarbone where the nerves controlling the shoulders and arms branch away from the upper trunk nerve.

His typical patient is a young man injured by crashing a motorcycle, snowmobile or skateboard or falling from a tree stand while hunting. Landing with force on the arm or shoulder can break the collarbone and sever or damage the nerves where they branch out.

Patients can be left with a non-working limb, known as “flail arm,” and severe chronic pain. After time, with no stimulation from the nerves, muscles in the arms and shoulder wither.

If caught early enough, the damage can be repaired surgically, but it helps to have a correct understanding of anatomy.

“You can imagine that if you didn’t know the anatomy, and it was a case of trauma, you are going to repair the nerves the wrong way,” Hanna says.

Early in his career, he repaired the brachial plexus of a patient to match the anatomy text illustrations, and the surgery failed. He re-operated, grafting the nerves in the order he has now shown to be correct, and the patient regained use of his arm.

The difference is the order of the branching of three nerves:

  • The suprascapular nerve (S), which controls the exterior rotation of the shoulder
  • The posterior division (P), which controls the deltoid muscle
  • The anterior division (A), which controls the biceps

Anatomy illustrations had traditionally shown the order as SAP, but Hanna’s research shows that the correct order is SPA.

It is a misunderstanding that goes back to the Middle Ages, and Andreas Vesalius, who founded modern human anatomy by writing the text De humani corporis fabrica. A 1555 Vesalius illustration from the Rare Books and Special Collections in the Ebling Library at the UW School of Medicine and Public Health shows the brachial plexus branching in the incorrect SAP order.

The mistake was carried through modern times; most of the anatomy books on Hanna’s shelf also have the incorrect order. But Hanna says that other neurosurgeons who specialize in the peripheral nervous system have noted that reality does not match the books.

Hanna is now on the lecture circuit, delivering talks on the clinical implications of how new understanding affects surgical planning and patient outcomes. He also teaches medical students at the UW School of Medicine and Public Health, and can share how his cadaver research helped change five centuries of medical belief.