Knee Extension Is Related to the Posteriorly Deviated Gravity Line to the Pelvis in Young Adults: Radiographic Analysis Using Low-Dose Biplanar X-ray

Abstract

Purpose: We sought to compare the radiographic parameters concerning the sagittal alignment of the standing whole-body skeletons between the knee extension group and control group using the low-dose biplanar X-ray system in a young adult population without knee pain, and to investigate the associated variables for the sagittal knee angle (sagKA) among the radiographic parameters of global sagittal alignment. Materials and Methods: We reviewed whole-body standing sagittal radiographs of 124 young adults taken from December 2018 to May 2020 in a single institution. We compared the radiographic parameters concerning the lower extremity sagittal alignment and global sagittal alignment between the knee extension group and control group. The factors correlated with sagKA were evaluated using multiple linear regression analysis. Results: The sagittal vertical axis (SVA), the horizontal offset between the gravity line (GL) and the posterior edge of S1 endplate (GL-S), and the horizontal offset between the GL and the hip center (GL-H) were-11.6±21.3 mm, 5.1±23.8 mm, and-25.1±27.1 mm in the knee extension group, respectively, which were significantly smaller than those in the control group. The C7 plumb line (C7PL) and GL were deviated posterior to the sacrum and the hip center in the knee extension group, with the mean sagKA of-5.6° in young adults. Conclusion: The GL-H using GL, not the SVA using C7PL, was a significant radiographic factor associated with the sagKA.