The Femoral Head Edema Zone: A Novel Classification Scheme to Better Predict Osteonecrosis Progression

BACKGROUND: This study proposed a new classification, the Edema Zone classification, that uses magnetic resonance imaging images to grade the extent of edema in osteonecrosis of the femoral head. The purpose of the study was (1) to examine how the Edema Zone classification compared to the Japanese Investigation Committee (JIC) classification's prognostic ability for early conversion to total hip arthroplasty (THA), and (2) to determine how accurately and reliably the Edema Zone classification performed as a classification system. METHODS: This was a retrospective study of hips that converted to THA from a core decompression within 26 months compared to hips that received only core decompression. Preoperative magnetic resonance images were examined. Hips were graded by Edema Zone and JIC classifications. Edema Zone reliability was analyzed, and predictive accuracy of the classifications was compared. RESULTS: Inter-rater reliability for the edema zone classification was very high, with a κ of 0.87. Edema Zone grades were higher in hips that converted to THA (nine of 20 ≥ grade 3) compared to hips that did not (six of 74 ≥ grade 3, 8.1%, P < 0.001) and demonstrated a significant P-value for trend (P < 0.001). In contrast, JIC classification did not differ between patients who converted and those who did not (P = 0.83). Receiver operating characteristic curve analysis showed that the area under the curve for the Edema Zone classification was 0.71, substantially greater than the area under the curve for the JIC classification system at 0.52. CONCLUSIONS: Edema Zone classification showed exceptional inter-rater reliability and was a more accurate predictor of THA conversion compared to the JIC classification. Furthermore, the severity of the Edema Zone grade demonstrated a statistically significant impact on the likelihood of conversion to THA, with higher grades associated with increased conversion rates. This new classification system can be a powerful tool in guiding appropriate surgical intervention for patients who have osteonecrosis of the femoral head.