The reasons for<unk>  are * avascular necrosis (the blood supply was interrupted by the fracture) * the two ends are not apposed (that is, they are not next to each other) * infection (particularly osteomyelitis) * the fracture is not fixed (that is, the two ends are still mobile) * soft-tissue imposition (there is muscle or ligament covering the broken ends and preventing them from touching each other)
Nonunion
Pseudofracture
Retrolisthesis
Malunion
Joint dislocation
Intraarticular fracture
Bunion
Stress fracture