The shin bone is the larger of the two bones found in the lower leg.The shinbone is also referred to as tibia in medical parlance.if you are a woman then it could be because of wearing heels.Wearing heels does not allow you to have a balanced walk and this can alter the position of the shin bone leading to depression in the shin bone.Other reasons could be if you have overexercised putting too much pressure on your legs by doing excessive cycling or running. You need to see a doctor at once to ascertain the reason behind the depression in your shinbone.A simple x-ray will tell the truth.