I wish there was a non biased study! There isn't, though.
Eggshells are calcium...you can grind them easily in a coffee grinder. But you DON'T want to add calcium to a commercial diet; overfeeding calcium can cause ofrworsen joint problems of all kinds especially in growing dogs...(there HAVE been a number of studies on this.)
People do use eggshells & various other kinds of calcium (usually bonemeal, a much better source) in cooked diets for dogs b/c those just use boneless meat.
Eggs are a great additive to any diet, though.