Abstract Objective:Osteonecrosis of the femoral head (ONFH) is a common and difficult disease. The effect of cystic area on femoral head mechanics in patients with ONFH is unknown. The purpose of this study was to investigate the effect of cystic areas of osteonecrosis of the femoral head on stress distribution and disease progression in the femoral head.Methods:A total of 85 patients (106 hips) diagnosed with ARCO stage II non-traumatic and non-surgical treatment of ONFH from February 2017 to November 2018 were retrospectively analyzed. All patients were followed up for an average of more than 2 years. According to whether the femoral head collapsed during the follow-up, they were divided into collapse group and non-collapse group. The age, gender, etiology, height, weight, BMI, JIC classification, presence of cystic areas and diameter of cystic areas were compared between the two groups. In addition, five spherical cystic areas of different diameters of 0, 5, 10, 15, and 20 mm were constructed to simulate the hip joint load of a person during standing by finite element methods, and the maximum stress, mean stress, and maximum stress values in the necrotic area of the femoral head without cystic areas and in the area 1 mm around the cystic areas with different diameters were observed and analyzed.Results:All 85 patients (106 hips) completed the follow-up, with an average age of 37.4 ± 8.12 years. The mean follow-up time was 2.8 ± 0.6 years. Forty-five patients (57 hips) with ONFH who had femoral head collapse were included in the collapse group, and the remaining 40 patients (49 hips) were included in the non-collapse group. There was significant difference in JIC classification between the two groups (P < 0.05), most of which were C2 type in the collapse group and B type in the non-collapse group. There was a difference between the two groups in whether cystic areas appeared in the femoral head, with 49.1% in the collapse group showing cystic areas, which was significantly higher than that in the non-collapse group (18.4%) (P < 0.05). In addition, the diameter of the cystic areas was significantly larger in the collapsed group than in the non-collapsed group (P < 0.05). The maximum von Mises stress value and mean von Mises stress value around the cortical bone, necrotic area and around the cystic area of the femoral head increased with the increase of the cystic diameter. Furthermore, linear regression analysis showed a linear positive correlation between the maximum stress, mean stress in the necrotic area in the femoral head, and the maximum stress value in the 1 mm area around the cystic area and the diameter of the cystic area.Conclusion:The maximum stress and average stress of necrotic area can be increased in cystic area, and the increase of stress in cystic area is more obvious. Stress concentration areas can be generated around the cystic areas. The presence and increased diameter of the cystic areas accelerates the collapse of the ONFH femoral head.