Fracture Healing and Its Stages 

Fracture Healing

Bone is very peculiar in the body, which is repaired and replaced by bone itself after an injury, unlike other tissues.

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Types of Fracture Healing

Primary Healing

The cortex of one side of the fracture unites with the cortex of the other side without callus formation. This occurs only with rigid compression plating. Cutting cones are formed initially, which consist of leading osteoclasts followed by a capillary bud surrounded by osteoblasts

Primary healing is of two types: 

1. Contact healing: Active haversian remodeling occurs in about 4 weeks. No Callus formation 
2. Gap Healing: In Gap <1 mm, the gap is filled with woven bone followed by haversian remodeling without going through secondary healing stages

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Secondary Healing 

It involves the classical stages of fracture healing. The fracture heals with a callus. Callus initially formed, which is later converted to lamellar bone. Healing of cortical bone differs from cancellous bone

Stages of Secondary Fracture Healing

First observed by “Sir John Hunter.” Later staged by McKibbin, Heppenstall, and Frost into 8 stages

Stage of impaction

Impact occurs at the moment of fracture & lasts until there is complete dissipation of energy

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Stage of Induction

This phase involves the formation of hematoma. When the bone is fractured, the blood seeps out & forms a hematoma that is contained by the periosteum or by the soft tissue envelope formed by muscles, fascia, & the skin. The osteoprogenitor cells are induced to form bone cells. This is a very short phase that ends as inflammatory cells begin to appear at the fracture site.

Stage of Inflammation

This begins within 48 hours of impact & lasts until cartilage & bone start appearing. Inflammatory cells appear first & clear the hematoma. Fibroblast, Mesenchymal cell & Osteoprogenitor cell appears later. Clinically, this Stage is associated with

• Pain
• Swelling
• Warmth
• This stage may last for about 7 days

Stage of Granulation Tissue

The sensitized precursor cells form the soft granulation tissue between the two fractured fragments. The granulation tissue here is made up of blood vessels, fibroblasts & osteoblasts. This soft tissue holds the fractured fragments together without having any rigidity. Simultaneously, the proliferating cells appear from the medullary canal & they also start growing toward each other. The blood clot has little or no role in the ultimate repair & is absorbed by the macrophages & giant cells in the proliferating granulation tissue. This stage may last for about 2 to 3 weeks.

Stage of Soft Callus

This phase begins with the disappearance of pain & swelling and lasts until the bony fragments are united by cartilaginous tissue & are no longer freely mobile. By the end of this stage, some stability is gained, which prevents shortening, but angulation can still occur. If at all non-unions happens, It happens in this stage.

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Soft Callus response 

It will occur in all types of fracture regardless of type of fixation. It begins within 2 weeks of fracture. It is tolerant of movement. Cells present in the periosteum are responsible for this response

External Bridging Callus  

This occurs when bone ends are not in continuity. Cells involved are derived from surrounding soft tissues & neovasculature. It forms rapidly & has the ability to bridge gaps. It is tolerant of movement

Late Medullary Callus

It occurs after plate fixation with imperfect reduction of fracture ends. Cells responsible for this are derived from intramedullary vasculature. It is not tolerant of movements 

Callus is visible radiologically by 3 Weeks. This stage may last for 4 to 12 weeks

Stage of Hard Callus/Consolidation

At this stage, the intracellular matrix is deposited with calcium salts, which form the woven bone (immature bone). Woven bone is produced mainly by periosteum & to a lesser extent by endosteum. It provides the platform on which lamellar bone is laid. This process of conversion of immature woven bone to mature lamellar bone is called consolidation. Here, clinical union occurs. This is a slow process & may take 3 to 6 months

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Stage of Modeling

At this stage, woven bone is converted to lamellar bone & the medullary canal is reconstituted. Modeling is done by BMU (Bone Modeling Unit). BMU first produces osteoclasts that remove a pocket of preexisting hard tissue and then produces osteoblasts that replace it with newly formed bone. This will take a period of 3 to 4 months. Complete replacement of all the callus with the lamellar bone takes 1 to 4 years

Rapid Acceleratory Phenomenon (RAP)

The initial phase of rapid remodeling is called RAP. It increases the healing rate by 2 to 10 times. RAP occurs after 

• Fractures
• Osteotomies
• Bone grafting
• Arthrodesis

It begins a few days after fracture, peaks at 1 to 2 months, and subsides by 6 to 24 months. Inadequate RAP contributes to delayed unions & non-unions.

Stage of remodeling

This is the process of reshaping of bone over a period of time. According to Wolff's Law, remodeling can occur completely in children & adolescents but never in adults. This stage may last for years 

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Distraction Histiogenesis

Distraction histiogenesis is a biological phenomenon that can be utilized to induce the formation of new bone and soft tissue. This technique has been used after corticotomy or osteotomy of bone.

A distraction force is applied with an external fixator like the Ilizarov circular fixator or a uniplanar fixator.

Latency Phase

The period from bone division to the onset of distraction. Inflammation and soft tissue callus formation of the fractured bone

• Soft callus formation
  • Starts 3 to 7 days
  • Lasts up to 2 to 3 weeks
• Latency period: 5 to 7 days

Distraction phase

Characterized by the application of traction forces to osteotomized segments

• Rate: 1 mm/day
• Rhythm: 0.25 mm every 6 hours OR 0.5 mm twice daily
• Duration: 1 to 3 weeks

Consolidation Phase

• Cessation of traction forces to the removal of the distractor.
• Newly formed bone
  • Mineralizes
  • Increases in bone density
• Duration: 3 to 4 months

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Factors Affecting Fracture Healing

1. Injury variables
2. Patient variables
3. Tissue variables
4. Treatment variables

Signs of Fracture Union 

Can be made by clinical signs

• Should be confirmed by radiological examination

Clinical Signs	Radiological Signs
Absence of free movement between the fractured fragments. Absence of tenderness at the fracture site. Absence of pain on angulatory stress applied to the fracture site	Visible callus bridging the fractured site and connecting the fractured fragments. Continuity of bone trabeculae across the fracture: Bridging callus

Important One-liners 

• Primary healing is when the cortex of one side of the fracture unites with the cortex of the other side without callus formation.
• Secondary healing is when a fracture is healed with callus formation.
• Stage of induction involves the formation of hematoma
• The process of conversion of immature woven bone to mature lamellar bone is called consolidation
• Distraction histiogenesis is a biological phenomenon that can be utilized to induce the formation of new bone and soft tissue. 
• Continuity of bone trabeculae across the fracture: Bridging callus

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