How is finnys second broken leg from the first?

Brinker, who is far more obsessed with Finny's accident than either Gene or Finny are, is holding some kind of inquiry into the accident, with a few other boys there to act as an examining jury. Brinker brings Leper, who was at the scene of the accident, out to speak; when Leper says that Gene did indeed push Finny, Finny freaks out and runs from the room. He falls on the marble steps outside, and re-breaks his leg. Obviously his first injury made his leg very weak and prone to further injury.