I would check the temperatures under your heat lamp to make sure they fall within the recommended temp. range or are higher than that, with a colder spot in the brooder to get away from it (they need the colder spot anyway).
90-95 degrees for the first week of life and decrease by 5 degrees per...
Since Eggcessive mentions perosis here are some more links:
http://www.merckmanuals.com/vet/poultry/nutrition_and_management_poultry/mineral_deficiencies_in_poultry.html
manganese
perosis
Here is a pic of perosis...
http://www.merckmanuals.com/vet/poultry/nutrition_and_management_poultry/vitamin_deficiencies_in_poultry.html
look under riboflavin if desired
I hope this helps.
http://blog.mypetchicken.com/2012/02/29/crooked-toes/
http://poultrykeeper.com/skeletal-and-muscular-disorders/crooked-toes
I am not sure if this was incubation temps or vitamin deficiency...but it wouldn't hurt to make sure you are offering vitamins (esp. riboflavin).