Struthers' ligament is a ligament that extends between the shaft of the humerus

Humerus

The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....

 and the medial epicondyle

Medial epicondyle

Medial epicondyle can refer to:* Medial epicondyle of the humerus * Medial epicondyle of the femur...

 of the humerus. It is not a constant ligament, and can be acquired or congenital. Its clinical significance arises form the fact that the median nerve

Median nerve

The median nerve is a nerve in humans and other animals. It is in the upper limb. It is one of the five main nerves originating from the brachial plexus....

, passes in the space between the ligament and the humerus, and in this space the nerve may be compressed leading to supracondylar process syndrome.

The ligament is usually ignored and not mentioned because its not always found.
It was described by John Struthers

John Struthers (anatomist)

Sir John Struthers, LRCSE, MD, LLD, FRCSE, FRSE was Professor of Anatomy at the University of Aberdeen....

 who wrote about it in 1848 and 1854.