There is a condition called perosis which is a deformity of the leg bones.
I have read that it is caused by a nutritional deficiency of manganese.
Adding a manganese supplement to the feed for 3 to 5 days will help to prevent any new cases developing.
Birds become crippled and usually die of starvation or need to be culled as there is no cure for affected birds.
My advice if you suspect that your poultry have perosis is to seek veterinary advice.