Background  In order to evaluate the sagittal balance of the spine and pelvis, spinal-pelvic imaging parameters have become a research hotspot in recent years, and sacrum morphologic parameters have attracted much attention as a new parameter system. The study of related factors is of great significance to improve the clinical application value of sacrum morphological parameters.

  Objective  To investigate the influence of age, body position, and sex on sacrum morphological parameters.

  Methods  Whole spine radiographs and spine MRI data were collected from the individuals who underwent physical examination in our hospital from January 2013 to December 2020, and sacrum morphological parameters (S1-2T, SK, α, β, and θ) and spinal-pelvic parameters (lumbar lordosis LL and pelvic incidence PI) were measured for 120 juveniles (in the standing position), 131 adults (in the standing position), and 88 adults (in the supine position). Scatter plots of PI, S1-2T, SK, α, β, θ, and age were generated. Sacrum morphological parameters in the standing position were compared between the juveniles and the adults, and Pearson correlation analysis was performed for S1-2T, SK, α, β, θ, PI, and LL in both groups; sacrum morphological parameters in adults were compared between the standing position and the supine position, correlation analysis was performed for S1-2T, SK, α, β, θ, and LL, and the linear regression equations of θ and LL were obtained. After control for the variable of body position, sacrum morphological parameters were compared between adult male and female individuals.

  Results  SK, α, and PI increased with age and remained stable after an individual became an adult, with SK and α increasing by about 10° and PI increasing by about 20°, while there were no significant changes in S1-2T, θ, and β. There was no significant difference in sacrum morphological parameters between the juveniles and the adults (P＞0.05). There was no correlation between PI and LL in the juveniles (P＞0.05), but there was a correlation between SK, α, θ and LL (P＜0.05). There were significant differences in S1-2T, SK, and LL in the adults in different body positions (P＜0.05), while no significant differences were found in α, β, and θ (P＞0.05). The correlation between θ and LL was the strongest in the standing position and the supine position, with correlation coefficient of -0.520 and -0.530, respectively. In the supine position, LL=-0.3398× θ + 63.12(r=-0.530), and in the standing position, LL=-0.4509× θ + 79.45(r=-0.520); There were no significant differences in sacrum morphological parameters between the male and female adults in the same body position (P＞0.05). In the male adults, there was a significant change in S1-2T from the standing position to the supine position, while in the female adults, there were significant changes in S1-2T, SK, and α from the standing position to the supine position (P＜0.05).

  Conclusion  Age has little influence on sacrum morphological parameters, and the correlation between age and LL is stronger than that between PI and LL in juveniles. α, β, and θ are not affected by the changes in body position, and θ can be used to predict LL in different body positions. There are no significant differences in sacrum parameters between male and female adults in the same body position, and sacrum morphology in female individuals is more easily affected by the change in body position than that in male individuals.