They occur when the edge of a toenail starts growing into the surrounding skin, causing pain, swelling, and discoloration, ...

When the nail grows into the skin, it causes pain ... A pincer toenail occurs when the sides of the toenail grow inward toward the skin, causing pain, redness, and inflammation.

But just because it's under the skin, the follicle does not stop ... and when that happens the nail can actually grow into the flesh of the toe. As you can imagine, it's pretty painful, and ...