In the non-drainage group , the incidence of superficial infection , ecchymosis and tension vesicles was significantly higher than in the drainage group ( 10.6 % vs 2.4 % , P = 0.031 ; 12.9 % vs 3.6 % , P = 0.026 ; 16.5 % vs 4.8 % , P = 0.013 , respectively ) .