An analysis of the interactions between the spine, pelvis, and lower limbs in asymptomatic adults with limited pelvic compensation

Background The interactions between the spine, pelvis, and lower limbs are dynamic based on the "cone of economy" concept; thus, different global radiographic parameters could be regarded as reflections of different centers of gravity. We conducted this retrospective study to evaluate the offsets of different centers of gravity in asymptomatic populations and to investigate how the global sagittal alignment is supported. Methods The following parameters were measured: cervical lordosis, thoracic kyphosis (TK), lumbar lordosis (LL), pelvic incidence (PI), pelvic tilt (PT), the ratio between PT and PI (PT/PI), sacral slope, PI minus LL (PI-LL), the sagittal vertical axis (SVA), cranial SVA to ankle center (Cr-A), CrSVA to the femoral head center (Cr-FH), C2SVA to the femoral head center (C2-FH), pelvic translation (P. Shift), and knee angle (KA). Participants were divided into subgroups based on the PT/PI ratio. Mean values were compared using the t-test, and correlations were assessed using Pearson's coefficient. Results A total of 82 asymptomatic adults were enrolled. The average PT/PI in subgroup 1 was the smallest, showing that individuals in this group may have limited pelvic retroversion. No significant differences in Cr-FH, Cr-A, or C2-FH were found between subgroups (all P>0.1), implying that global alignment was well supported in each group. Specifically, C2-FH showed minor changes between subgroups (P=0.998), showing that C2-FH may be a target for sagittal compensation. There were positive correlations between PT/PI and both P. Shift and SVA (r=0.930 and r=0.606, respectively). However, Cr-FH, Cr-A, and C2-FH were not significantly correlated with P. Shift or PT/PI (all P>0.05). Weak correlations existed between Cr-A, Cr-FH, and age (all P>0.2). Conclusions This study revealed that the Cr-FH and C2-FH offsets are stable across the population and could be maintained by regulating only the sagittal spinal curvature when pelvic compensation is limited. Cr-FH is not affected by age in the asymptomatic population. Thus, the stable Cr-FH and C2-FH could provide references for surgeons during the surgical decision-making process in patients with adult spinal deformity with sagittal malalignment.