The lesion is located in the cortical and medullary cavities of the long bone epiphysis or the bone stem and is round, oval or cylindrical in shape. It develops mainly along the longitudinal axis of the long bones, with high translucency, mildly distended growth, smooth cyst wall and clear borders, showing irregular round or oval hypodense shadow with clear margins, no punctate calcification points inside, fluctuating expansion of the cyst wall, and rarely new bone growth is seen. Unicompartmental cysts show round or oval hypointense shadow with clear border and uniform density, no bone septum, a few cysts may show pseudo multicompartmental cyst hypointense shadow due to bone crest, large partition shadow is seen, bone septum is mostly perpendicular to the longitudinal axis of long bone, mildly inflated, peripheral expansion is rarely seen, local bone cortex is irregularly thinned but not ruptured, smooth margin, no periosteal reaction and periosteal hyperplasia. Bone cysts often cause pathological fractures, and some of them may disappear by themselves after fracture of small cysts, while large cysts may shrink or increase in size, combined with pathological fracture bone fragments into the cyst called fragment trapping sign, which helps to identify.