The study has been designed to find out whether heat loss increases significantly in laminar air flow operating rooms. Furthermore the investigators will find out whether active warming minimizes this heat loss. Increased heat loss increases the amount of perioperative hypothermia. It has been shown that perioperative hypothermia is associated with severe complications such as an increased incidence of surgical wound infections. On the other hand laminar air flow is explicitly used to decrease air-borne infections. Our study results will help to optimize thermal management of orthopedic patients.