Objective: To evaluate whether changes in innominate alignment on standing A-P full-spine radiographs follow the proportional relationship described by the Gonstead 5:2 rule, which predicts approximately 2 mm of femoral head height difference for every 5 mm of innominate misalignment.
Methods: This retrospective observational study examined 29 adult subjects with pre- and post-treatment standing full-spine radiographs. The primary variables were the change in combined right-side innominate alignment and the change in right femoral head height. Simple linear regression was used to estimate the association between changes in innominate measurements and changes in femur height. Additional analyses included outlier-removed regression and separating into positive versus negative innominate change subgroups.
Results: In the full sample (n = 29), the estimated slope was 0.44. The 95% confidence interval for the full-sample slope was wide (0.12 to 0.76). There was a widely variable relationship between innominate change and femur height change (R² = 0.23). Removal of three statistical outliers reduced the slope to 0.21 (95% CI: –0.21–0.62) and significantly decreased model fit (R² = 0.04). Regressions run within the subgroups showed similarly inconsistent slopes in both the positive and negative innominate change groups. Although the full sample numerical estimate lies near the 0.40 ratio proposed by the Gonstead 5:2 rule, the wide confidence interval and weak model fit indicate that the empirical relationship does not meaningfully support that proportional claim at the level of individual subjects.
Conclusion: There is not a fixed relationship applicable to every patient to support the Gonstead 5:2 proportional rule. The relationship between changes in innominate alignment and femur head height was wide, inconsistent, and variable. These results suggest that clinicians should be cautious in assuming predictable femur head height changes from innominate alignment alone.
Author keywords: Chiropractic; Ilium; Pelvis; Chiropractic Adjustment; Leg Length Inequality
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