There are other procedures not performed in the surgery room (e.g.: in the padded box or in the ultrasound-cardiology room), on a standing horse because they do not require a general anaesthesia of the animal.
These are less heavy surgeries, less at risk because not invasive or of a short duration (e.g.: trepanning, dental surgery, laparoscopy, castration, etc.). Sterile equipment is required for laparoscopy. For trepanning, dental extraction or field-like castration, the surgeon will only wear sterile gloves.
In such cases, the sterile procedure is applied, and the room door must remain closed during the whole surgery.