The amputation risk is high when diabetic foot ulcers, complicated by osteomyelitis, fail to heal after non-surgical standard-of-care treatment. A new treatment regimen has been developed recently and has been proven feasible. This treatment regimen consists of surgical debridement, followed by bone void filling with gentamicin-loaded calcium sulfate-hydroxyapatite biocomposite and closure of soft tissues and skin, in combination with antibiotic therapy and offloading. This treatment regimen has not been investigated prospectively. Therefore, this multicenter prospective cohort study was designed, with the primary objective of investigating postoperative wound healing. Patients with diabetic forefoot ulcers, complicated by osteomyelitis, will be included. The most relevant exclusion criteria are: Severe diabetic foot infection, severe limb ischemia, and foot deformity causing high pressure and friction on the diabetic foot ulcer. After inclusion, subjects will undergo study phase 1, which is observation of the standard-of-care non-surgical treatment. When standard-of-care non-surgical treatment is unsuccessful, subjects will be included in study phase 2, which consists of treatment by surgical debridement of the diabetic foot ulcer and underlying osteomyelitis, followed by bone void filling with gentamicin-loaded calcium sulfate-hydroxyapatite biocomposite and closure of soft tissues and skin, followed by a postoperatieve treatment regimen which involves wound care, 10 days of antibiotic therapy and offloading. The primary outcome measure of this study is the proportion of subjects with post-operative wound healing, which will be investigated clinically and will be objectified by a review panel of blinded, independent experts based on digital photographs. Follow-up will be performed until wound healing or for a maximum of 20 weeks. The primary outcome measure is the proportion of subjects with postoperative wound healing during 20 weeks of follow-up. Secondary outcome measures are: days until postoperative wound healing, proportion of subjects with persistent osteomyelitis post-operatively, proportion of subjects undergoing amputations during follow-up, foot function index scores at inclusion and after 20 weeks follow-up.