Correlations Analysis of Different Pelvic Tilt Definitions: A Preliminary Study.

Background: Pelvic tilt (PT) is described as the pelvic orientation along the transverse axis, yet 4 PT definitions were established based on radiographic landmarks: anterior pelvic plane (PT_a), the center of femoral heads to sacral plate (PT_m), pelvic outlet (PT_h), and sacral slope (SS). These landmarks quantify a similar concept, yet understanding of their relationships is lacking, and their differences are sometimes ignored. Purpose: This study aimed to examine the correlations and differences of PT definitions for education and research purposes. Methods: This study reviewed 105 sagittal pelvic radiographs of patients (68 men and 37 women) awaiting hip surgery at a single clinic. Hip hardware and spine pathologies were examined for subgroup analysis. Two observers annotated 4 PTs in a gender-dependent manner and repeated it after 6 months. The linear regression model and intraclass correlation coefficient (ICC) were applied with a 95% confidence interval. Results: The SS showed no correlation to the other 3 PT definitions, except for females in the hip hardware subgroup (n = 17). PT_m demonstrated very strong linear correlation to PT_h (r > 0.9) under the linear model PT_m = 0.951 × PT_h - 68.284. Conclusion: The PT_m and PT_h can be calculated from each other under a simple linear regression equation, which enables comparisons between them. SS presented poor correlations to the other PT parameters, except for the female subgroup with hip implant that required further analysis; PT_a-related comparisons showed high anatomical variations between patients.