A convertible patient-specific guide for use in arthroplasty has a body having a patient-specific three-dimensional undersurface closely mateable engageable in only one position with a corresponding joint surface of a specific patient. The convertible guide includes patient-specific guiding formations extending from the body and configured for guiding both a total arthroplasty of the joint and a partial arthroplasty of the joint, such that the arthroplasty to be performed is intraoperatively changeable.