Data are expressed as means ± standard deviation. A p-value < 0.05 was considered statistically significant. All statistical analyses were performed with the Statistical Package for Social Sciences (SPSS 10.0 for Windows, SPSS, Inc., Chicago, IL). The calculation of sample size was based on a power analysis. At a power of 80% using a significance level of p < 0.05, the sample size required was 20 subjects per study group.