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your dog's pads are made to absorb the shock impact when walking running etc. They can crack or get cut and continuous walking on the area can keep spreading the cut/injury open causing the pad inability to heal. Some of the pad may come off in some situations. This leads to pain which is causing the limp. if it looks serious you should bring him in for an exam, he may need a splint to keep him from walking on it.
I am directing you to sites regarding the purpose of the foot pads as well as what conditions/injuries are cause for concern.
Foot pads- dogs
Foot pad injuries