Patients who will undergo Oral and Maxillofacial or Plastic surgery that require reconstruction with microvascular flap at Hospital Ramón y Cajal, Madrid. A group of 67 patients treated according to a protocol for prevention of postoperative deep venous thrombosis with Bemiparina that will be compared with data collected from a group of 134 patients previously treated with a similar protocol but enoxaparin as antithrombotic agent.