Ratio of the Medial Hip Joint Space as a Predictor for Collapse of the Femoral Head in Non-traumatic Osteonecrosis: A Retrospective Study and Finite Element Analysis

Abstract Background: There has been no indicators that can effectively predict femoral head collapse in ONFH so far. The aim of this study is to retrospectively analyze the first-visit medial space ratio of the hip joint to evaluate its efficacy in predicting ONFH-induced collapse and impacts on the mechanical environment of necrotic femoral head.Methods: In this retrospective analysis for traditional Chinese medicine (TCM), non-traumatic osteonecrosis of femoral head (NONFH) patients from January 2010 to December 2016 were selected. The medial space ratio at their first visit and and collapse of the femoral head during the follow-up were recorded. Patients were divided into group A, B, C, D, and E according to the grading of the medial space ratio at the first visit. A total of 14 hip joint models with distinct medial space ratios were established. The maximum stress intensities of the cartilage, cortical bone and the necrotic area (N-unit area) of the femoral head in the models were quantitated using a finite element analysis.Results: One hundred twenty-eight patients (142 hips) were included in this study. The average follow-up time was 5.4±1.5 years.The Kaplan-Meier survival analysis showed that survival rates of the first-visit medial space ratios in group C and D were significantly higher (exceeding 45.76%) than that in group E (22.73%). There was no significant difference in the survival rates between group C and D (P > 0.05). The finite element analysis showed that in either the necrosis or non-necrosis group, the maximum stress in cartilage, cortical bone and the N-unit area of the femoral head increased with the medial space ratio decreasing. Conclusion: The medial space ratio shows clinical implications of predicting the collapse of NONFH except for assessing the relationship between the femoral head and the acetabulum. The stress concentrations of cartilage, cortical bone, and the necrotic area of the femoral head are enhanced with the medial space radio decreasing. Once the medial space ratio falls below 4 and continues to decline, the risk of necrotic collapse will become higher.