What can a doctor do to fix a broken knee cap?

ORIF. O.R.I.F. Or open reduction and internal fixation is when an orthopedic surgeon identifies the fracture fragments (with open surgery), then places the pieces back in their anatomic positions (reduction) and then holds the pieces in place with various devices (screws, plates, wires) - - - (internal fixation). This is often the preferred treatment for displaced patella or kneecap fractures.
Surgery. If the knee cap is broken, it can often be repaired with surgery that includes using screws and wires to keep it together while it heels.