Interesting "little" Lincoln stories

[RJNorton]

Mr. Lincoln saw me glance at his foot. It was much swollen. Both of his feet, in fact, were in bad condition. I said nothing, but he commenced talking about them.

Abraham Lincoln knew a chiropodist named Isachar Zacharie.

On September 22, 1862, Lincoln wrote of Dr. Zacharie:

"Dr. ← Zacharie → has operated on my feet with great success, and considerable addition to my comfort."

However, I have never read exactly what the nature of the operation was - does anyone know the nature of Dr. Zacherie's operation on Lincoln's feet?