Fracture healing

Fracture healing occurs naturally after traumatic bony disruption. This process begins begins with hemorrhage and progresses through three stages:

• inflammatory
• reparative
• remodeling

This process can be supported by various treatment options with immobilisation a mainstay, and inappropriate treatment may may result in a variety of complications. Depending on the fracture site, normal healing healing may take from 3-12 weeks. 

Normal fracture healing

For normal fracture healing to occur a number of requirements must be met:

• viabilty of fragments (i.e. intact blood supply)
• mechanical rest: this can be achieved by not moving and external immobilisation, e.g. cast or internal fixation
• absence of infection

The process of healing is different depending on the configuration of the fracture fragments and can be divided into three main categories:

1. spontaneous (indirect/secondary) healing
2. contact (angiogenic/primary) healing
3. gap healing

Spontaneous (indirect/secondary) healing

This is the most common 'natural' healing process, whereby the fracture ends are placed placed close to each other (but not "apposed"), with intervening haematoma and variable displacement and/or angulation. 

Though the healing process of a fracture can be divided into various phases, it should rather be understood as a a biological continuum. The periosteum, endosteum, and Haversian canals are the sources of pluripotent mesenchymal cells that that initiate the formation of the healing tissues (three blastema theory). The bridging callus seen on radiographs mainly arises from the periosteum.

The process of fracture healing

The fracture haematoma initiates the healing response. Within 48 hours, chemotaxic signaling mechanisms attract the inflammation cells necessary to promote the healing process.

Within 7-14 days, granulation tissue is formed between the fragments, leading to vascularisation of the haematoma. On radiographs, there may be increased translucency of the fracture during this stage, due to bone resorption.

Cells within the granulation tissue proliferate and begin to differentiate into fibroblasts and chondroblasts. They produce an extracellular organic matrix of fibrous tissue and cartilage, wherein woven bone is deposited by osteoblasts. This stage usually lasts 4-16 weeks. The newly formed callus is still damageable by shear forces, whereas axial traction and pressure promote the matrix formation.

The mesh of woven bone is then replaced by lamellar bone, which is organized parallel to the axis of the bone.

Eventually, remodeling of the bone takes place, restoring its normal cortical structure depending on load distribution. This is an ongoing process that may last for several years. In children, remodeling occurs faster than in older people and may compensate malunion to some degree.

Average healing times of common fractures

• phalanges: 3 weeks
• metacarpals: 4-6 weeks
• distal radius: 4-6 weeks
• lower arm: 8-10 weeks
• humerus: 6-8 weeks
• femoral neck: 12 weeks
• femoral shaft: 12 weeks
• tibia: 10 weeks

Contact (angiogenic/primary) healing

Contact healing occurs between adapted fragments when there is less than 0.1 mm distance and neutralisation of interfragmentary strain. This is the aim of stable internal fixation. The process is initiated by osteoclasts forming cutting cones, that traverse the fracture line at 50-100 µm/d. They are accompanied by capillaries and osteoblasts, that form lamellar bone primarily oriented in the right direction (Haversian remodeling). In stable osteosynthesis, there should be no formation of periosteal callus. Otherwise this must be regarded as a sign of instability (irritation callus).

Gap healing

If internal fixation leaves a gap, even of less than 1 mm, between fragments, lamellar bone is at first deposited perpendicular to the long axis. Haversian remodeling does not start until the gap has been filled by this process.

Disturbed fracture healing

Normal fracture healing can be disrupted in numerous ways:

• delayed union
  • fracture healing takes about twice as long as expected for a specific location
• non-union (pseudoarthrosis)
  • fracture healing does not occur within 6-9 months
  • common sites: scaphoid bone, femoral neck, tibial shaft
• malunion
  • healing in the wrong position
  • can be partially compensated for by remodeling of the bone (except for rotational malunion)
• refracturing
  • as a rule of the thumb, fixation material should be removed within about 18 months after consolidation of the fracture
  • leaving it in place may lead to cortical atrophy and a higher risk of refracturing

Permanent damage to the healing tissue (blastema), typically in the chondral phase. Risk factors include:

• local: insufficient immobilisation, distraction, soft tissue interposition, bone loss, impaired blood supply, soft tissue damage, improper fixation method, infection
• general/systemic: old age, poor nutritional status, drugs and medications, metabolic diseases

Types of pseudarthroses

• hypertrophicpseudoarthrosis
  • rich in callus(elephant foot)
  • poor in callus (horse hoof)
• oligotrophicpseudoarthrosis
  • no callus on radiographs, rounded fracture ends
• atrophic pseudoarthrosis pseudoarthrosis
  • bone resorption due to infection or impaired blood supply
• hypotrophic (non-vital) pseudoarthrosis
  • necrotic fragments (increasing fragment density without callus formation)
• defect pseudarthrosis
  • missing fragments
• infected pseudoarthrosis: see: osteomyelitis

Treatment options for disturbed healing

• sufficient immobilisation
• electrical stimulation
• bone grafting
• revision of internal fixation