What happens to bones that do not heal?
Fracture healing occurrs in several steps:


haemorrhage: blood and surrounding cells fill the space created by the fracture.
fibrous callus: chondrocytes colonize the fracture space, with neovascularization.
bony callus: osteocytes colonize the fracture space and rearrange in woven bone
remodelling: in long bones, woven bone is remodelled into lamellar bone and creates a new haversian system


If the fracture is not immobilized enough (not enough rest), several things can happen that are mutually exclusive:


normal healing: luckily, the bone is not too much mobilized and is able to heal normally
fracture displacement: the bone heals, but in an incorrect alignment. This gets partially corrected over time, but the correction mechanism is limited, especially in adults.
pseudarthrosis: the worst case. instead of normal healing, cartilage grows to protect the bony extremities that are mobilized and prevents further healing. This causes pain, and will require surgery in most cases.