What Causes The Knuckles On The Toes To Turn Brown And Calloused?

The knuckles on both big toes are dark brown and calloused. It started out light brown, and lightly calloused last summer, but now has really darkened and hardened. I ve used a pedifile on it...it softens it but stays dark brown and after a few days is hard again. Is this something to worry about, or simply accept? I am a 50 year old female; about 20 pounds overweight.