Imaging AVN, the ARCO Staging System and a First Case Study

Amie starts with a standard AP pelvic x-ray and advises checking the images as well as the report, since early signs are subtle and the radiologist may not have been asked about AVN. A normal x-ray in a suspicious patient should lead to MRI of the pelvis. Signs include subchondral cysts, which she explains to patients as a bruised apple, the crescent sign of a necrotic segment, sclerosis, the 50 pence sign of a femoral head losing its round shape, and finally flattening and collapse. The ARCO staging system, updated in 2021, has four stages: a normal x-ray with changes on MRI, sclerosis and cysts without collapse, femoral head flattening of less or more than 2 mm for 3A and 3B, and complete destruction of the femoral head. In the case study, a woman with groin pain labeled as lateral hip pain had diabetes, hypercholesterolemia and a BMI of 40, severe antalgic gait and very limited range. Her x-ray showed ARCO 3B changes, and she went on to a total hip replacement.