These are most likely calluses caused from lying on those joints and most often occur on the elbows. There are many ways to treat them but the most effective is to keep them from lying on hard surfaces repeatedly that press on that particular area. Some vets recommend having them lay on a dog bed instead of lying on concrete or hard surfaces for instance. Other vets recommend topical treatment such as cod liver oil or other emolients that penetrate the calluses and soften them. Still others recommend excision if they have become too large and that will reduce the size of them. If using something topical, I would check with a vet, however, to make sure that the substance (when licked off as they will surely do it) is not toxic or can cause problems for the dog.