I fell on my arm and it started hurting immediately and there was swelling. But the doctor didn’t find any fracture when he took an xray. He put a cast anyway. About 2 weeks later, it was still hurting a little so I got another xray and it showed a small fracture. How come the fracture didn’t show on the first one?

Although modern X-rays have excellent resolution, not all fractures are visible especially they are hairline fractures or minor compression fractures. In these injuries, the bone’s structure often has minimal disruption. The likely reason the fracture was visible 2 weeks later is because the healing process takes time and involves formation of a dense callous across the fracture that is visible by X-ray. The general rule of thumb is that minor fractures not visible by X-ray at the time of injury are often visible on repeat X-ray 7-14 days later.