You can break a rib without obvious injury; it often happens to drunks who wake up the next morning with pain, & don’t remember having fallen. But it can also happen from bone cancer, usually one that’s spread to the ribs from elsewhere. Then it’s called a “pathologic fracture,” and can be caused by injury as mild as rolling over in bed or coughing.
We suspect this in a patient with chest pain, if we find tenderness on just one point on a rib. If we pound on the rib from a point further away, & it hurts at the point of tenderness (like pound on the back, & it hurts by the armpit), we’re more suspicious.
To diagnose this, we order rib x-rays, not chest x-rays. If the x-ray is normal, we diagnose a muscle strain. But if the pain keeps on, in just that one point, we may order a bone scan or MRI.