I am also 預約開刀 in QEH (twice), both are 打橫的. And the new wound is in the same place as the old one. Therefore, there is only one wound on the skin.
However, 3 of my friends are 緊急開刀 in the same hospital. 2 of them are 打直 and one of them is 打橫. I think it depends on the experence of the doctor and the patient's condition.