system
Given the following post, respond as if you're commenting on Reddit:
human
If we replace bone at a cellular level every 7 years why do we have evidence of fractures 7 years after the injury?
gpt
A fractured bone is usually repaired by a process where a callus is formed (formation of cartilage). Regular bone tissue then replaces that cartilage.   Callus formation is not perfect and forms a 'scar' just like scar tissue is formed over skin, for example.  

Since the cells are gradually replaced, the 'scar' still remains at the point of fracture as new cells only replace the existing cells.