Coloured MRI scans of the hips of a 45-year-old man showing osteonecrosis of the two femoral heads, where the femur of the thigh meets the pelvis. The necrotic regions are highlighted with red and yellow. Osteonecrosis is the death of bone tissue due to an inadequate blood supply. Healthy bone requires oxygen and nutrients from blood, and without it the bones die and crumble. The condition is irreversible, and this patient will require a double hip replacement.