Did Saint Peter have leprosy, or perhaps some other cause of injury to the ulnar nerve? It would seem so, according to Dr. Bennett Futterman, professor of anatomy in New York. In a recent book he points out that the traditional blessing of the Pope—ring and little finger bent inward as in the clawhand deformity produced by ulnar nerve damage—is also displayed in some of the earlier paintings of St. Peter by Medieval and Renaissance masters.

Reference

Futterman, B. Analysis of the Papal Benediction Sign, Clinical Anatomy, June 28, 2015.