BACKGROUND:The gluteus medius not only abducts the hip joint,but also plays an important role in limiting the external movement of the femoral head.At present,there is a lack of research on the correlation between gluteus medius status and non-traumatic femoral head necrosis. OBJECTIVE:To investigate the relationship between the gluteus medius width ratio and the medial space ratio of the hip joint and the progression of non-traumatic femoral head necrosis,and to explore the effect of gluteus medius atrophy on the surface and necrotic zone stress of the femoral head necrosis through finite element analysis. METHODS:Retrospective analysis of unilateral non-traumatic femoral head necrosis patients admitted to Third Affiliated Hospital of Guangzhou University of Chinese Medicine was performed.All patients were followed up for an average of more than 2 years.They were divided into a collapsed group and a non-collapsed group based on whether there was collapse of the femoral head during the follow-up.Medial space ratio,gluteus medius width ratio,Sharp angle,gluteus medius length ratio,and gluteus medius activation angle were measured and calculated.The differences in these indicators were compared between the two groups.At the first visit and follow-up at 3,6,12,and 24 months,the medial space ratio and gluteus medius width ratio were measured and calculated to explore the changes of these two indicators in the course of non-traumatic femoral head necrosis.In addition,using three-dimensional finite element analysis,a Japanese Investigation Committee classification C1 type femoral head necrosis model was constructed based on CT data.At the same time,based on MRI data,a model of the gluteus medius muscle was constructed and divided into a gluteus medius muscle atrophy group(gluteus medius width ratio:74%-76%)and a gluteus medius muscle normal group(gluteus medius width ratio:94%-96%).Each group constructed 10 models,with 6 degrees of freedom of the distal femur constrained to zero.600 N pressures were applied along the Z-axis to the upper surface of the sacrum.The stress distribution,maximum stress values on the surface and necrotic area of the femoral head,and the maximum displacement of the necrotic area were compared between two groups of models. RESULTS AND CONCLUSION:(1)A total of 153 patients(67 males and 86 females)with 153 hips were included in this study.(2)At the 24-hour follow-up,the medial space ratio of the collapsed group was significantly higher than that of the non-collapsed group(P<0.05).The gluteus medius width ratio of the collapsed group was significantly lower than that of the non-collapsed group(P<0.05).There was no statistically significant difference in Sharp angle,gluteus medius activation angle,and gluteus medius length ratio between the two groups(P>0.05).(3)Since the follow-up time exceeded 3 months,the gluteus medius width ratio of the collapsed group was lower than that of the non-collapsed group(P<0.05).Since the follow-up time exceeded 12 months,the medial space ratio of the collapsed group was higher than that of the non-collapsed group(P<0.05).(4)Pearson correlation analysis showed a significant positive correlation between follow-up time and medial space ratio in the collapsed group(P<0.05),and a significant negative correlation between follow-up time and gluteus medius width ratio(P<0.05).The regression coefficient of gluteus medius width ratio was larger than that of medial space ratio.(5)The group with middle gluteal muscle atrophy showed significant stress concentration on the surface of the femoral head,and the stress zone was significantly located on the outside.The maximum stress on the surface of the femoral head in the group with middle gluteal muscle atrophy was significantly greater than that in the group with normal middle gluteal muscle(P<0.05).There was significant stress concentration in the necrotic area of the middle gluteal muscle atrophy group,and the maximum stress was located at the edge of the necrotic area.The maximum stress and maximum displacement in the necrotic area of the middle gluteal muscle atrophy group were significantly greater than those of the normal group(P<0.05).(6)It is indicated that gluteus medius width ratio is an effective indicator for evaluating changes in gluteal muscle atrophy.In the progression of non-traumatic femoral head necrosis,atrophy of the gluteus medius muscle first occurs,followed by widening of the medial hip joint space.The mechanical mechanism may be that the atrophy of the gluteus medius muscle affects the stability of the hip joint,leading to external displacement of the femoral head,and increasing stress and displacement on the surface and necrotic area of the femoral head.