What can a doctor do about a broken rib? i've not heard of a "cast" for rib fractures.

Pain control: Mostly, the md can provide pain control thru meds. Otherwise, not many options. Strapping or wrapping of rib fractures has fallen out of favor secondary to potential breathing and/or pneumonia concerns. Tight wrapping about the rib cage does help pain, but the wrapping should not be left on 24 hrs. Sleeping almost sitting up with a few pillows helps with night time pain.

Give it time: There is no specific treatment required for a broken rib. These heal in approximately 4-6 weeks.

Depends: It depends on if the broken rib needs surgery or not. Certain rib fracture patterns (multiple consecutive rib fractures/etc) really benefit from surgery. There is good evidence in the scientific literature supporting this. Single rib fracture(s) are typically treated with pain medication and use of the incentive spirometry device.