This patient had partial function
of their biceps motor branch
(above). To supplement this
function, an end-to-side nerve
transfer was performed using a
fascicle from the nearby ulnar
nerve (below)

This is a variation of the nerve transfer. If the injured nerve is partially functional, or has the potential to recover, you do not want to cut it so that a full nerve transfer can be performed. Instead you make a window in the outer nerve sheath and attach the nerve transfer to the side of the nerve.

In general, some partial injury to the recipient nerve occurs, and this allows for the end-to-side transfer to regenerate down the recipient nerve. More fascicular injury to the recipient nerve leads to a greater amount of regeneration from the transferred nerve.

This procedure is often used to supplement other nerve repair techniques.