Statistical analysis
All results are expressed as mean ± standard deviation. Patient characteristics of the TNS and control groups were analysed using the Wilcoxon signed-rank test. JOA scores were compared using the one-way analysis of variance with the Tukey–Kramer post hoc test (JMP 15, SAS Institute Japan Ltd., Tokyo, Japan). Kappa coefficients were calculated for the reproducibility of scores between examiners in the grades of the Engh's classification and CH using SPSS version 21 (IBM, Armonk, NY, USA) to assess the reliability of the measurements. Differences in the incidence of SS and CH and the frequency of SS according to Gruen zone were determined using Pearson's Chi-squared test. Statistical significance was set at 