The procedure you are referring to is called an exchange nailing. The nail that is already present is removed. The canal where the nail was placed is now reamed out to make a larger diameter. Putting a larger nail in place helps stabilize the fracture.
Reaming the intramedullary canal helps stimulate bone union from both a mechanical and biologic perspective. The larger, stiffer implant stabilizes the bone. It also increases the contact area for bone growth around the pin. The bone that is removed during the reaming process can then be used as a bone graft around the fracture and pin sites.
Exchange nailing allows for early weight-bearing. The patient can move the leg and all the joints without restrictions. This helps restore function early. The technique is also less invasive than open incision procedures.
There are cases where serial reaming doesn’t work as a treatment for femoral nonunion. Studies show that as many as half the patients require another surgical procedure to achieve bone union. Open incision bone grafting may be needed instead for bone healing to take place.