The periosteum, endosteum, and Haversian canals are the resources of pluripotent mesenchymal stem cells that initiate the development of your healing tissues. The bridging callus found on radiographs predominantly occurs through the periosteum. For that reason, even if it could have healed with no surgery, it would just take longer. https://www.theorthopaedicandpainpractice.com/what-is-a-long-bone-fracture/