Hello , this is a skeletal bone deposition disorder that is often caused by fluid and calcium deposits in the joints especially hock joint (between thigh and leg).
There is no known treatment for the condition.
I recommend you provide anticoccidiosis drugs since mostly this is an opportunistic disease in such cases, give ESB 3 30% and Vetoxy 20 for a period of 5 days.
Ensure you keep the affected birds near a source of heat.