Preparation of animal model. (A) Preparation of skull defect. For each rabbit, two 3.5 mm holes were drilled on each side of skull, respectively. The two holes on one side of skull were used as control group, and the two holes on the other side of skull were used as experimental group in each rabbit. (B) Postoperative outline of negative vacuum tube. The vacuum tube was placed near two experimental holes. For each rabbit, −50 kPa of negative pressure was applied to two experimental holes for 15 min, which was performed 4 times per day.