Suggest Remedy For Darkening Of Knuckles

hello dr, i have a doubt.i used to bend my fingers(&make the tik tik sound) in a day(40-50 time).its my habit.i cant avoid this.& now my fingers becoming black in color(3 bends of all fingers).my skin color is white.so..is it due to my this habit?&..is this habit make any problem to me?pls reply me.....