Femoral head necrosis is a disease that scares patients and causes headaches for doctors. The former is mainly caused by femoral neck fracture, acetabular fracture and hip dislocation, while the latter is mainly caused by the application of corticosteroids and heavy alcohol consumption in China. Non-traumatic femoral head necrosis mainly affects young adults between 20 and 50 years old, and about 80% of patients have bilateral onset. Studies of the natural history of the disease show that without effective treatment about 80% of cases of osteonecrosis of the femoral head will develop femoral head collapse within 0.5 to 3 years. How to diagnose femoral head necrosis? 1.X-ray examination The X-ray examination of early femoral head necrosis generally has no obvious abnormality, but in the high-quality X-ray film sometimes can see the femoral head within the bone trabeculae streak unclear, there are spots of high-density and/or low-density intermingled lesions, slightly later lesions will appear by the high-density demarcation line wrapped low-density lesions. In more advanced lesions, subchondral bone resorption and CrescentSign may be seen. The CrescentSign indicates the loss of subchondral bone resorption, loss of cartilage support, and, without effective treatment, rapid head collapse and flattening. The crescent sign can therefore be regarded as the dividing line between early and late stage femoral head necrosis. 2.Radionuclide bone scan and single photon emission computed tomography (SPECT) Bone scan is an important screening test with high sensitivity and low specificity, because it reflects the changes in the level of cellular metabolism at the lesion site, so it can show abnormal signals at the early stage of the disease (the first few days after the appearance of symptoms), while the X-ray examination is often negative at this time. Therefore, it is important for the early diagnosis and screening of osteonecrosis. In the early stage of osteonecrosis, the focal area is ischemic and the bone scan shows a low resorptive signal, then, due to the inflammatory response of the surrounding normal tissue caused by necrotic tissue and capillary proliferation, a ring of high resorptive signal appears at the edge of the low resorptive signal lesion. This has been described as “Cold in Hot”. This is the characteristic bone scan performance of osteonecrosis, that is, the presence of this bone scan signal can be diagnosed femoral head necrosis. MRI is based on the water content of cells and tissues, so in the early stage of the disease, ischemia causes cellular metabolic disorders, resulting in edema and degeneration of cells and tissues, MRI can show the lesion area, therefore, its sensitivity is no less than that of bone scan. MRI can also show the lesion by using different temporal images, different levels and different angles, so its accuracy is also high. In early stage femoral head necrosis, a low density line is visible on the T1 image of MRI film, which is actually the demarcation line between normal bone and necrotic bone. This high-density line is actually a band of hyperplastic granulation tissue, which is low signal on T1 image and high signal on T2 image because of its high water content. MRI can also show the location and extent of the lesion, which can provide a basis for selecting a treatment method and determining the treatment plan. It can also be used to evaluate the effectiveness of a particular treatment method because it can show the subtle changes in the bone.