Abstract: BACKGROUND: Magnetic resonance imaging is the gold standard for the diagnosis of osteonecrosis of femoral head, and previous methods of predicting osteonecrosis of femoral head collapse based on magnetic resonance images mostly require the combined assessment of coronal and sagittal images. However, osteonecrosis of femoral head tends to occur bilaterally, most hospitals perform bilateral hip magnetic resonance imaging scans during clinical examinations, but the bilateral hip scans can only view coronal and cross-sectional images, and it is difficult to obtain sagittal images, which affects the assessment of the risk of collapse. Therefore, it is of clinical value to establish a method to assess the risk of early osteonecrosis of femoral head collapse by applying the images that can be obtained after bilateral hip magnetic resonance scanning. 
OBJECTIVE: To establish a method of applying coronal and cross-sectional images of bilateral hip magnetic resonance imaging to assess the risk of osteonecrosis of femoral head collapse. 
METHODS: The medical records of 111 patients (181 hips) with early-stage osteonecrosis of femoral head diagnosed at the outpatient clinic of Honghui Hospital Affiliated to Xi’an Jiaotong University from October 2017 to October 2019 were retrospectively analyzed. They were categorized into collapsed and non-collapsed groups according to the femoral head collapse at the final follow-up, with 69 hips in the collapsed group and 112 hips in the non-collapsed group. The angle of necrotic range on the images of median coronal plane, transverse plane or one level above and below it was measured on the magnetic resonance imaging system. The sum of the two angles of necrotic angle on the coronal and transverse planes was used as the combined necrotic angle. The average of the three combined necrotic angles of each hip was taken to get the average combined necrotic angle of each hip. Finally, the correlation between the three combined necrotic angles and the average combined necrotic angle with the collapse of osteonecrosis of femoral head was analyzed, and the specificity and sensitivity of the four combined necrotic angles in predicting collapse were evaluated by using receiver operating characteristic curves.
RESULTS AND CONCLUSION: (1) Totally 69 hips (38.1%) had femoral head collapse at the last follow-up and were included in the collapsed group; 112 hips (61.9%) did not have progression of collapse and were included in the non-collapsed group. (2) The difference between the collapsed group and the non-collapsed group in terms of Association Research Circulation Osseous (ARCO) stage was significant (P < 0.001). The difference in age, body mass index, follow-up time, gender distribution, side of onset, and causative factors was not significant (P > 0.05). (3) The results of independent samples t-test suggested that all four combined necrotic angles were significantly correlated with collapse (P < 0.000 1); and the differences in combined necrotic angles between the collapsed group and the non-collapsed group of ARCO stage I and the two groups of ARCO stage II were all significant (P < 0.000 1). (4) In the analysis of the receiver operating characteristic, the area under the curve of the average combined necrotic angle was greater than that of the combined necrotic angle on the lower level of the median, the middle level, and the upper level of the median. (5) The average combined necrotic angle had a higher accuracy in the prediction of collapse than the lower level of the median, the middle level, and the upper level of the combined necrotic angle. (6) It is concluded that the accuracy of the average combined necrotic angle in predicting the risk of osteonecrosis of femoral head collapse is higher, and the clinical practicability is stronger, so we can consider using this method to predict the risk of osteonecrosis of femoral head collapse.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

Key words: osteonecrosis of femoral head, collapse prediction, MRI, coronal plane, cross section, combined necrotic angle