Like muscle and the brain, bones grow and strengthen under pressure; they weaken when not used. Healed bones aren't stronger then they were before the injury!
Known as Wolff’s law, that concept is "the reason astronauts return with reduced bone density after floating in microgravity. And conversely, it explains why studies show that tennis players and fencers develop greater bone mass in their dominant arms and legs.
But this adaptive measure goes only so far. Despite one misconception, there is no evidence that a bone that breaks will heal to be stronger than it was before.
When a bone fractures, it begins the healing process by forming a callus at the fracture site, where calcium is deposited to aid rebuilding, said Dr. Terry D. Amaral, director of pediatric orthopedic surgery at Montefiore Medical Center in the Bronx. In most cases, a plaster cast is applied to protect the fracture site. But because there is no weight or pressure on the limb, it becomes weaker — except at the site of the callus, where calcium is being deposited." IE, healing involves rapid mineralization, but in disuse or no use bones demineralize.
"As a result, there may be a brief period in the healing process when the fracture site is stronger than the surrounding bone. But they later reach equal strength, and the fracture site is no more or less likely to break again. Dr. Amaral said that in studies in which healed bones were subjected to bending and twisting forces, the repaired bones were capable of breaking anywhere, including the fracture site. 'They never comment that it’s stronger or weaker than the rest of the bone.'"
ANAHAD O’CONNOR "The Claim: After Being Broken, Bones Can Become Even Stronger". NY Times, Pub 10/18/10