The purpose of this clinical study is to compare the safety and effectiveness of shockwave treatment combined with standard-of-care treatment, to standard-of-care treatment alone to induce healing of a chronic plantar foot ulcer in subjects with diabetes mellitus. For the purpose of this study, the definition of plantar foot ulcer is a wound or open sore that involves the plantar(bottom) aspect of the foot, and the definition of chronic is a duration of 6 weeks or greater with a lack of response to treatment.