Types of non-union

• Hypertrophic non-union: A mechanical failure of union – where there is good vascularity and healing potential, but excessive instability leading to movement at the fracture site. Most common in midshaft long bone fractures where small movements are amplified by the lever arm of the diaphysis. May also be seen in cases with unstable fixation, such as loose plates. XR/CT shows an elephant foot appearance with abundant callus, but disorganised, non-bridging trabeculae. Once stability is restored (often by surgical fixation), union can be reliably expected.
• Atrophic non-union: A biological failure of healing, when there is poor vascular supply to the fracture site. This may occur in well-established vascular watershed areas (e.g. proximal scaphoid, 5th metatarsal metaphysis). Alternatively, injuries from high-energy trauma with comminution and periosteal stripping can also have impaired local blood supply. Imaging shows tapered bone ends with no callus formation. Treated with surgical intervention—debridement of non-viable bone, rigid fixation and bone grafting.
• Pseudoarthrosis: Pseudoarthrosis is the creation of a false joint due to a chronically unstable fibrous non-union subjected to continuous motion while healing. XR demonstrates a clear fracture gap, fibrous pseudocapsule and the fragments remain mobile. With evolving fibrosis, pseudoarthroses can become nonpainful. In low-demand individuals, there are times where allowing a pseudoarthrosis to form may be the therapeutic goal. For example, ‘bag of bones’ non-operative treatment for proximal or distal humerus fractures, where a non-operative candidate is expected to achieve painless fibrous union. Other examples include radial head excision and non-operative treatment of Charcot joints, where painless function is the goal.
• Infected non-union: Patients with co-existing fractures and bone infection are high risk for non-union. This might include patients with open fractures, pre-existing osteomyelitis or those who develop surgical site infection post-operatively. Examination can show signs of deep/superficial infection, while imaging demonstrates periosteal reaction and lytic, moth-eaten areas of bone. Infected metalwork is often surrounded by a lytic perimeter. Management is typically dictated by a tertiary bone infection unit, and frequently involves a combination of surgical debridement, antibiotic suppression, external fixators and and staged/delayed revision fixations.
• Gap non-union: In patients with segmental fractures and high-energy injuries, there is a risk of bone loss and devitalised bone fragments. Where this is anticipated intra-operatively, large defects may be replaced by a cement spacer (Masquelet technique) in the acute phase, followed by delayed reconstructive surgery. Defects with >50mm gap are likely to require multiple surgeries and prolonged recovery times.

Patient factors increasing risk of non-union

Smoking: Reduces oxygenation, impairs osteoblast function, and decreases vascular ingrowth (2–3x higher non-union rate).

Diabetes and/or peripheral vascular disease: Restricted blood supply and microvascular damage reduces blood flow to healing fracture site.

Endocrine Disorders: Hypothyroidism, hyperparathyroidism, chronic kidney disease and osteoporosis impair bone turnover and healing.

NSAIDs: NSAIDs inhibit prostaglandins, which are a key component of early fracture healing.

Corticosteroids: Impair osteoblast function and reduce bone formation.

Advanced Age: In addition to increasing frailty and comorbidities, bone turnover and periosteal response are both reduced with increasing age.

Malnutrition: Deficiencies in vitamin D, calcium or protein can slow osteogenesis.

Obesity: Increased mechanical loads and higher risk of micromovement across a fracture site. May also be associated with metabolic disturbances (e.g., insulin resistance).