highly significantP < 0.001
Statistical analysis The data were analyzed with SPSS for Windows (version 18, SPSS Inc, Chicago, IL, USA) and expressed as the means±standard deviations (SD).The significant thresholds were: P < 0.05 was considered significant, P < 0.01 was considered highly significant, P < 0.001 was considered very highly significant, while P < 0.0001 was considered extremely significant.