All of the following factors facilitate nonunion except -
A hematoma or blood clot occurs at the fracture site.
This hematoma provides two important factors for fracture healing:-


First, the hematoma provides a small amount of mechanical stability.
Second, and perhaps more importantly, the hematoma brings osteoblast and chondrocytes precursors to the fracture site in large numbers that can begin to differentiate into osteoblast and chondrocytes to begin producing matrix.


Other three options are causes of nonunion.
Haematoma formation
Periosteal injuries
Absence of nerve supply
Chronic infection