A shank is:
1. The narrow part of the sole of a shoe under the instep.
2. A piece of material, such as metal, that is used to reinforce or shape this part of a shoe.
Not having any other ready sources of metal, prisoners would remove this part of the shoe and sharpen an edge or point on it.
So the article is proceeding from the definition of shank as “a piece of shoe”? Doubtful. More likely is the loaded definition so they can continue to feed biased perceptions on this issue.