Re: why does an adult recover from a fracture much slower than a child?

What an interesting question! On my initial searches, I found a lot of references stating this fact, but not very many stating the reasons why. I included a review from the Merck manual on bone healing in general, general reasons why pediatric bone fractures heal faster than adult, and then another statement from STAT!Ref, an online medical resource. I think it�s clear from this outline that more research is needed to fully understand this phenomenon, as it is a complex process involving many variables. I hope this helps your understanding and further encourages your curiousity!

From the Merck Manual:
"When tissues, such as those of the skin, muscles, and internal organs, become injured, they tend to mend by having scar tissue take the place of healthy tissue. The scar tissue often compromises the tissue's appearance or function in some way. In contrast, bone is unique in that it heals with its own tissue�bone�rather than with scar tissue. This unusual capacity for regeneration enables a mending bone to heal itself after a fracture. Even shattered fragments of bone, with proper treatment, can often be restored to their normal function.

Fractures heal in three overlapping phases: inflammation, repair, and remodeling. Healing begins immediately with the inflammatory phase. In this phase, damaged soft tissue, bone fragments, and lost blood caused by the injury are removed by cells of the immune system. The region around the fracture becomes swollen and tender as cell activity and blood flow increase. The inflammatory phase reaches peak activity in a couple of days, but takes weeks to subside. This process accounts for most of the early pain people experience with fractures.

The repair phase begins within days of the injury and lasts for weeks to months. New repaired bone, called the external callus, is formed during this phase. When first produced, the callus has no calcium; it is soft and rubbery and cannot be seen on an x-ray. This new bone is neither strong nor stable, so that during this period the fractured bone can easily collapse and become displaced (that is, slip out of its proper place). At 3 to 6 weeks, the callus calcifies and becomes much stiffer and stronger and becomes visible on x-rays.

The remodeling phase (in which the bone is built back to its normal state) lasts many months. The bulky external callus is slowly resorbed and replaced by stronger bone; in this phase, the normal contours and architecture of the bone are restored. It is not likely that the bone will fracture again during this phase; however, people may experience mild pain with exertion."

Obviously, in children, bones are still growing. The cells (osteoblasts) and factors (calcium, vitamins, etc) required for this growth are also used in repairing fractures, so when children have a fracture, the repair machinery is already present and active at a high level. Additionally, there is more vasculature in pediatric bone, which allows faster delivery of nutrients and faster removal of waste products. To summarize, pediatric bones heal faster after fractures because those bones are growing and forming anyway.

From a medical reference (STAT!Ref):
The excellent osteogenic potential of pediatric periosteum permits rapid, aggressive fracture healing. Remodeling (gradual correction in alignment or size of a fractured bone back to normal) is generally far more rapid in children than in adults. Remodeling of angular deformities is particularly rapid when the deformity is in the same plane of motion as the nearest joint or when the deformity is near a rapidly growing physis.