The possible causes of subcutaneous effusion after skull repair include meningeal defect, local inflammatory reaction, allergic reaction and so on. 1. Meningeal defects: If there is any imperfect repair of meningeal damage during skull repair surgery, localized subcutaneous effusion may occur after the surgery, which needs to be extracted in time, or the meninges should be closed surgically to prevent infection. 2. Local inflammatory reaction: after cranial bone repair, due to the stimulation of foreign body friction, while the wound is in the edema period, local subcutaneous effusion may occur. 3. Allergic reaction: Currently, most of the cranial bone repair materials used in the clinic are biocompatible materials with good biocompatibility, and allergic reaction is extremely rare, but it has been reported in the clinic. In the case of subacute or delayed allergic reaction, tissue edema and liquefaction of subcutaneous effusion will occur. The presence of subcutaneous effusion after cranial repair suggests a prompt visit to the hospital for further testing.