What is the association between MRI and radiography in measuring femoral head migration?

Background: Conventional pelvic radiographs are traditionally used for assessing femoral head migration in residual acetabular dysplasia (RAD). Knowledge of the importance of cartilaginous structures in this condition has led to increased use of magnetic resonance imaging (MRI) in assessing both osseous and cartilaginous structures of the pediatric hip.
Aim: Therefore, we assessed the relationship between migration percentages (MP) found in MRI and conventional radiographs. Secondly, we analyzed the reliability of MP in MRI and radiographs.
Materials and Methods: We retrospectively identified a consecutive series of 16 patients (2 male, mean age 5 years (2 to 8 years)), examined for RAD during a period of 2½ years. Four raters performed blinded repeated measurements of osseous migration percentage (MP), cartilaginous migration percentage (CMP), in MRI and radiographs. Pelvic rotation and tilt indices were measured in radiographs to account for influence of pelvic positioning. Bland Altman (BA) plots and intraclass correlation coefficients (ICC) were calculated for agreement and reliability.
Results: BA plots for MP(R) and MP(MRI) produced a mean difference of 6.4 Limits of agreement (-11 to 24) with higher disagreements at low average MP values. Mean MP(R) differed from mean MP(MRI) (17% versus 23%, P<0.001) MP(R) had the best interrater reliability with an ICC of 0.92 (0.86-0.96), compared to MP(MRI) and CMP with ICC values of 0.61 (0.45-0.70) and 0.52 (0.26-0.69), respectively. Intrarater reliability for MP(R), MP(MRI) and CMP all had ICC values above 0.75 and did not differ statistically significantly. Differences in MP(MRI) and MP(R) showed no correlation to pelvic rotation index, pelvic tilt index or interval between radiograph and MRI exams.
Interpretation / Conclusion: Pelvic radiographs underestimated MP when compared to pelvic MRI. These results should prompt the clinician to consider which image modality to use when assessing RAD cases. We propose CMP as a new imaging measurement, and conclude that it has good intrarater reliability but moderate interrater reliability. Measurement of MP in radiographs and MRI had mediocre to excellent reliability.