Statistical analysis

Normal distribution of the data was tested by univariate Kolmogorov Smirnov test and a histogram graphic was prepared. Normality was obtained by logarithmic transformation of age (log base 10).

Data were presented as mean, standard deviation, minimum, maximum, frequency, and percent value based on various characteristics. Intergroup comparisons were made by using independent samples t-test or univariate analysis of variance. Post hoc comparisons after ANOVA were made by Tukey HSD test.

Comparison of nominal variables were made by chi-square test. The effects of atrophy and hernia at L4-L5 and L5-S1 levels were tested by two-way analysis of variance.

Two tailed significance level was adjusted to P < 0.05. All statistical analyses were performed using the NCSS10 software (NCSS, LLC).