Interesting. I wish they said what breed were the 4000 or so hens in the study. I would guess leghorns, because they are very small for the size egg they lay, as opposed to brown egg layers, which tend to be somewhat larger.
The article and research didn't address it, but I wonder if another factor in the keel bone fractures is the tremendous amount of calcium needed for that many eggs per year. While it is a matter of course to provide oyster shell and layer feed with calcium, that is still a huge amount of calcium cycling through a hen's body for 300+ eggs per year, as opposed to RJF's couple dozen. I have not seen anyone on BYC address whether this is a problem for bone health long term. Or maybe I just don't know enough about chickens and this has been addressed and is one of the reasons for production breeds' reproductive issues?