Distance between the C2 vertical line and the femoral heads: a quasi-invariant global sagittal alignment parameter in both standing and sitting positions for asymptomatic adults

BACKGROUND: This study aimed to investigate whether the distance between the C2 vertical line and the femoral heads (C2-FH) is quasi-invariant for the sitting position. METHODS: A cross-sectional, prospective study was conducted. A cohort of 59 asymptomatic volunteers was prospectively recruited between February 1, 2020, and February 31, 2020, at Nanjing Drum Tower Hospital of Nanjing University Medical School. The following radiographical parameters were evaluated: T4–T12 thoracic kyphosis (TK), cervical lordosis (CL), lumbar lordosis (LL), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), sagittal vertical axis (SVA), T1-pelvic angle (TPA), and the C2-FH. The inclusion criteria were as follows: age between 18 and 35 years and no symptoms related to neck or back pain. The exclusion criteria were as follows: a history of any prior lower extremity or spine surgery; and a history of inflammatory arthritis, neuromuscular disorders, or congenital anomalies. Individuals were divided into the following 3 groups according to the percentile of ΔPT (PT difference between standing position and sitting position): group A (1–25% ΔPT, n=14), group B (25–75% ΔPT, n=30), and group C (75–100% ΔPT, n=15). Summary statistics calculated using analysis of variance (ANOVA) were used to provide 95% confidence intervals for measurement errors. The significance level of all statistical analyses was set as P < 0.05. RESULTS: There was no significant difference in any of the parameters among the 3 groups in the standing position. In the sitting position, for different degrees of pelvic retroversion, there were significant differences between the 3 groups in the other parameters except for C2-FH (P=0.80; 95% CI: −20.486 to −3.893), CL (P=0.47; 95% CI: −5.645 to 4.964), TK (P=0.54; 95% CI: 17.058–25.1), and PI (P=0.44; 95% CI: 40.747–49.087). Surprisingly, C2-FH changed slightly among the 3 groups in both the standing (P=0.87) and sitting (P=0.80) positions. As ΔPT gradually increased, ΔSS, ΔLL, ΔSVA, ΔL1-L4, ΔL4-S1, ΔTPA, ΔPT/PI, and ΔPI-LL showed corresponding gradual changes (all P values <0.05). However, there was no significant difference in PI (P=0.39), CL (P=0.46), C2-FH (P=0.51), or TK (P=0.51). CONCLUSION: C2-FH is a quasi-invariant parameter in both the standing and sitting positions for asymptomatic adults. The mean value of C2-FH was −11.95 mm in the standing position and −1.01 mm in the sitting position. Stable C2-FH could serve as a reference during the surgical decision-making process in adult patients with spinal deformity and sagittal malalignment.