Background: Reimers’ bony migration percentage (bMP) is an established and widely applied indicator in the evaluation of the hip in children with cerebral palsy (CP). However, as bMP does not account for cartilaginous structures, it may not accurately represent true anatomic congruence in skeletally immature patients. Thus, we aimed to evaluate migration percentage, including cartilaginous structures (cMP), on hip arthrography to determine correlation with bMP and identify age-related predictive factors. Methods: This retrospective cross-sectional study included children with CP who underwent hip arthrography between April 2010 and March 2026. Both bMP and cMP were measured on the same single anteroposterior arthrographic radiograph to ensure direct comparability. For cMP, the lateral acetabular margin was defined as the outer edge of the labrum using the “rose-thorn” sign. A linear mixed model (LMM) evaluated the moderation of the bMP-cMP relationship by age. Results: The analysis included 173 hip examinations from 112 patients (mean age: 7.82 y). bMP and cMP demonstrated a strong positive correlation ( r =0.900, P <0.001) across all age groups. bMP consistently overestimated cMP, with the magnitude of this disparity increasing significantly as bMP values became larger ( r =0.712 for proportional bias). The LMM confirmed a significant bMP×age interaction (β=0.023, P =0.006). In younger children (5 years and younger), the overestimation by bMP was more pronounced than in older children (9 years and above). Conclusion: Due to its strong correlation with cMP, bMP remains a reliable indicator that reflects the relative magnitude of hip displacement. However, this discrepancy is amplified in cases with higher bMP and in younger children. As the skeleton matures with age, bMP more closely approximates the true anatomic hip morphology. Clinicians should consider this maturation-related bias when interpreting bMP. Level of Evidence/Clinical Relevance: Diagnostic Level III.
Tachibana et al. (Mon,) studied this question.