If there is doubt about the interpretation of the x-ray, stress x-rays, tomograms or CT scan may be used for confirmation. Cause
The reasons for non-union 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)

Risk factors
Related to the person:
Age: Common in old age
Nutritional status : poor
Habits : Nicotine and alcohol consumption
Metabolic disturbance : Hyperparathyroidism
Can be found in those with NF1
Genetic predisposition
Causes related to fracture:
Related to the fracture site
Soft tissue interposition
Bone loss at the fracture
Infection
Loss of blood supply
Damage of surrounding muscles
Related to treatment
Inadequate reduction
Insufficient immobilization
Improperly applied fixation devices. Types of Nonunion
There are typically three types of nonunion described. Hypertrophic nonunion
In a hypertrophic nonunion, the fracture site contains adequate blood supply but the fracture ends fail to heal together. X-rays show abundant callus formation. This type of nonunion is thought to occur when the body has adequate biology, such as stem cells and blood supply, but inadequate stability, meaning the bone ends are moving too much. Typically, the treatment consists of increasing stability of the fracture site with surgical implants. Atrophic nonunion
In an atrophic nonunion, x-rays show little to no callus formation. This is usually due to impaired bony healing, for example due to vascular causes (e.g. impaired blood supply to the bone fragments) or metabolic causes (e.g. diabetes or smoking).
Failure of initial union, as when bone fragments are separated by soft tissue, may also lead to an atrophic non-union. Atrophic non-unions can be treated by stimulating blood flow and encouraging healing. This is often done surgically by removing the end layer of bone to provide raw ends for healing and the use of bone grafts. Oligotrophic nonunion
As the name implies, an oligotrophic nonunion demonstrates some attempt by the body to heal the fracture. These are thought to arise from adequate biology but displacement at the fracture site. Diagnosis
A diagnosis of nonunion is made when the clinician feels there will be no further bone healing without intervention. The FDA defines it as a fracture at least 9 months old that has not shown any signs of radiographic healing within the last 3 months. CT scans offer a closer look at the fracture and may also be used to evaluate how much of the fracture has healed. Blood tests can evaluate if the patient has adequate levels of nutrients such as calcium and vitamin D.  Blood tests can also look for markers of infection such as ESR and CRP. Treatment
Surgery
Currently, there are different strategies to augment the bone-regeneration process, however, there is no standardised clinical treatment guideline yet. Surgical treatment options include:
Debridement: radical surgical removal of necrotic or infected soft tissue and bone tissue is deemed essential for the healing process. Immobilization of the fracture with internal or external fixation.