Soong Classification Using Radiographs Only Moderately Correlates With Distal Radius Plate Position on Computed Tomography

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Background: The Soong classification grades the prominence of volar locking plates used to treat distal radius fractures in relation to the volar rim. The basis of the classification scheme is that increasing plate prominence over the volar rim and distal radius watershed zone is associated with increased likelihood of flexor tendon irritation and need for plate removal. However, recent studies report mixed results on the predictive value of the Soong classification for these outcomes. We hypothesized that the decreased predictive accuracy of Soong classification is due to misclassification between Soong grades secondary to a suboptimal correlation between the Soong classification on radiographs (XRs) and computed tomography (CT). Methods: Fifty volarly displaced distal radius fractures treated with a volar locking plate in the international and publicly available ICUC database were reviewed. All cases with a postoperative XR and CT were included. Soong classification of the volar locking plate in relation to the volar ulnar rim was determined on both XR and CT by 2 independent, fellowship-trained hand surgeons using CT imaging as the gold standard. The distribution of Soong grades on XR and CT was compared using Pearson’s χ2 test, and correlation was calculated using the Matthews correlation coefficient (MCC). A multi-class confusion matrix was used to calculate each grade’s positive predictive value (PPV). Results: We found an MCC of 0.65, indicating only moderate correlation between the 2 modalities. Per individual Soong grade, the PPV was the highest for grade 2 (0.96), with lower PPVs for grade 0 (0.63) and grade 1 (0.60). Conclusions: The distribution of Soong grades was significantly different when using XR versus CT ( P < .001). Level of Evidence: Oxford Centre for Evidence-Based Medicine, diagnostic, level 2b