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You are here: Home / Basics and Biomechanics / Fracture Healing: Stages, Types, Influencing Factors, and Timeline

Fracture Healing: Stages, Types, Influencing Factors, and Timeline

Dr Arun Pal Singh ·

Last Updated on May 2, 2025

Fracture healing is a highly coordinated biological and mechanical process that restores the structural and functional integrity of bone after injury. It involves a sequence of cellular responses influenced by multiple intrinsic and extrinsic factors. Understanding these stages and mechanisms is essential for evaluating clinical progress, recognizing complications, and optimizing treatment outcomes.

In this article, we will discuss the steps of fracture healing in a broken bone, the types of fracture healing, and the factors that affect this healing process.

Contents hide
1 What Is Fracture Healing?
2 Types of Fracture Healing
2.1 Primary (Direct) Bone Healing
2.2 Secondary Fracture Healing
3 Relevant Anatomy and Physiology
4 Stages of Fracture Healing
4.1 Inflammatory Phase (0–7 days)
4.2 Reparative Phase (1-3 weeks)
4.3 Remodeling Phase (weeks to months)
5 Factors Affecting Fracture Healing
6 Complications in Fracture Healing
6.1 Delayed Union
6.2 Non-Union
6.3 Malunion
7 Timeline of Bone Healing
8 Key Takeaways
9 References

What Is Fracture Healing?

Fracture healing is the physiological restoration of bone continuity and strength after a break or disruption. It occurs through a natural process of inflammation, repair, and remodeling, and can proceed via two distinct biological pathways—primary (direct) and secondary (indirect) healing.

Fracture healing begins with hematoma formation, a collection of inflammatory agents that enable the bone-forming cells to form bone tissue to bridge the defect created by the fracture.

Types of Fracture Healing

 Primary (Direct) Bone Healing

  • Occurs with rigid internal fixation and absolute stability
  • No visible callus formation
  • Involves direct osteonal remodeling across the fracture site
  • Histologically similar to normal bone remodeling

Primary bone fracture healing can be divided into

  • Gap healing: If internal fixation leaves a gap between fragments, the fracture heals by gap healing.
    • Firstly, the width of the gap is filled by direct bone formation.
    • An initial scaffold of woven bone is laid with an orientation transverse to that of the original. In the next stage, bone remodeling occurs to replace the woven bone to form normal bone.
  • Contact Healing: Contact healing occurs where fragments are in direct apposition with [< 0.1 mm distance] and no interfragmentary strain exists. This allows the osteons (the basic unit of structure of compact bone) to grow across the fracture site, parallel to the long axis of the bone.
    • The process is initiated by osteoclasts forming cutting cones that traverse the fracture line at 50-100 µm/day.
    • The advancing osteoclasts cut the bone, and trailing osteoblasts lay down the new bone, followed by capillary penetration for blood supply to ultimately create regeneration of the normal bone architecture.

Secondary Fracture Healing

Secondary bone fracture healing occurs when there is no rigid fixation of the fractured bone ends, which leads to the development of a fracture callus. It includes an inflammatory phase, a reparative phase, and a remodeling phase as described below.

Secondary fracture healing occurs with non-rigid fixation, as fracture braces, external fixation, bridge plating, intramedullary nailing, etc.

Bone healing can occur as a combination of the above two processes, depending on the stability throughout the construct.

FeaturePrimary Bone HealingSecondary Bone Healing
Stability RequirementAbsoluteRelative
Callus FormationAbsentPresent (soft and hard callus)
Fixation TypeRigid internal fixation (e.g., plate)Casting, intramedullary nail, or external fixator
Remodeling ProcessCutting cones (direct remodeling)Callus remodeling over time
Healing TimeTypically slowerTypically faster
Histological PatternDirect bone formation (intramembranous ossification)Endochondral ossification (via cartilage intermediary)

Relevant Anatomy and Physiology

From without inwards, the bone is composed of periosteum, cortex, and medullary cavity.

Parts of long bone
Structure of a long bone

The periosteum is a fibrous membrane covering the surface of the bone, which consists of an outer fibrous layer, and an osteogenic inner cellular layer which is the primary source of precursor cells that develop into chondroblasts ( cartilage cells) and osteoblasts (bone cells) during the repair.

The cortex is made up of compact bone, which gives it the desired strength to withstand all possible mechanical stains.

Inside the cortical cylinder is the hollow called the medullary cavity, which is filled with red or yellow bone marrow.

Stages of Fracture Healing

fracture healing process image
A cut section through a bone where a fracture is healing. Normally the bone is like a cylinder. Imagine cylinder cut into halves along its longitudinal axis and you would get a similar picture.

Actual fracture healing is a continuous process, and the events of different phases may overlap in their occurrence.

stages of fracture healing
Stages of fracture healing

Inflammatory Phase (0–7 days)

The bone breaks due to injury, along with its soft tissue envelope [periosteum and surrounding muscles]. The bleeding occurs due to the rupture of blood vessels crossing the fracture line.

A hematoma is formed within the medullary canal, between the fracture ends, and beneath any elevated periosteum.

Due to loss of blood supply, the immediate ends of fracture fragments undergo necrosis, which leads to an immediate and intense acute inflammatory response.

The blood vessels dilate, and there is an exudation of plasma to the injured site. This brings acute inflammatory cells – macrophages, neutrophils, and platelets, which release several factors such as

  • Plasma-derived growth factor [PDGF]
  • Tumor necrosis factor-alpha**
  • Transforming growth factor-beta
  • Interleukins IL-1, IL-6, TNF-alpha, IL-10, IL-12

** Absence of this factor [as in HIV infection] delays ossification

These factors are detected as early as 24 hours after injury.

Fibroblasts and mesenchymal cells migrate to the fracture site, and granulation tissue forms around the fracture ends, followed by the proliferation of osteoblasts and fibroblasts.

Reparative Phase (1-3 weeks)

This phase is characterized by the differentiation of mesenchymal cells into chondroblasts and osteoblasts.

High oxygen concentration and mechanical stability favor bone formation, whereas low oxygen and instability lead to the formation of cartilage.

Soft callus composed of fibrous tissue and cartilage is formed first followed by gradual mineralization into hard (woven bone) callus. T

Enchondral ossification converts soft callus to hard or bony callus, a type of woven bone. The medullary callus also supplements the bridging soft callus.

fracture_callus-copy.jpg
Callus formation

As this phase of repair takes place, the bone ends gradually become enveloped in a fusiform mass of callus as noted in the picture above.

Remodeling Phase (weeks to months)

Remodeling leads to replacement of woven bone by lamellar bone, restoration of cortical structure and medullary canal.

Gradual improvement in strength and alignment occurs over months to years depending on load-bearing forces.

Remodeling in accordance with Wolff’s Law. Wolff’s law suggests that bones remodel based on mechanical loading. The bones will remodel and strengthen where they experience the most stress and weaken where the stress is the least.

In remodeling, osteoclasts resorb the woven bone trabeculae, and new struts of bone are laid down that correspond to lines of force over a prolonged period of time.

Remodeling is thought to be modulated by electrical signals.

The cellular module that controls remodeling is the resorption unit, consisting of osteoclasts, which first resorb bone, followed by osteoblasts, which lay down new Haversian systems.

Factors Affecting Fracture Healing

Both local and systemic variables influence the rate and degree of fracture healing. These are

  • Age- Young patients heal rapidly and have a remarkable ability to remodel and correct angulation deformities. These abilities decrease once skeletal maturity is reached.
  • Nutrition- A substantial amount of energy is needed for fracture healing. An adequate metabolic stage with sufficient carbohydrates and proteins is necessary.
  • Systemic Diseases- Osteoporosis, diabetes, and those compromising immunity can delay healing
  • Head Injury- Fractures heal in shorter times because the head injury may increase osteogenic response.
  • Type of Bone- Cancellous bone heals faster than cortical bone.
  • Degree of Trauma- Extensive the injury may lead to a poorer outcome.
  • Open Fractures: These let the hematoma escape and may be associated with bone healing problems.
  • Blood Supply- Inadequate blood supply to the injured bony regions impairs healing.
  • Immobilization- Inadequate reduction, excessive traction, or interposition of soft tissue will prevent healing.
  • Infection- Infections cause necrosis and edema and take energy away from the healing process and may increase the mobility of the fracture site.
  • Local Pathologies- Local lesions of the bone may cause poor fracture healing. Examples are malignancy, Paget disease, fibrous dysplasia, irradiation and necrosis

Complications in Fracture Healing

Delayed Union

  • Slower than expected healing timeline
  • Usually resolves with conservative extension of immobilization

Non-Union

  • It is the failure of fracture to unite beyond 6–9 months
  • May require surgical intervention, bone grafting, or stimulators

Malunion

  • The fracture heals in an improper position
  • Malunion can lead to functional or cosmetic deformity

Timeline of Bone Healing

Bone TypeAverage Healing Time
Phalanges3–6 weeks
Metacarpals4–6 weeks
Distal radius6–8 weeks
Tibia/femur12–20 weeks
Spine12+ weeks (variable)

Note: Healing time varies based on age, bone type, treatment method, and complications.

Key Takeaways

  • Fracture healing is a staged, biologically active process involving inflammation, repair, and remodeling.
  • Healing occurs via primary or secondary mechanisms, depending on the fixation technique.
  • Numerous systemic and local factors affect the pace and quality of bone union.
  • Monitoring clinical signs and imaging findings helps detect healing delays or failure early.

References

  • Giannoudis PV, Einhorn TA, Marsh D. Fracture healing: the diamond concept. Injury. 2007 Sep;38 Suppl 4:S3-6. [Link].
  • Jagodzinski M, & Krettek C. Effect of mechanical stability on fracture healing—An update. Injury. 2007;38(Suppl 1):S3-10. [Link]
  • Claes L, Recknagel S, Ignatius A. Fracture healing under healthy and inflammatory conditions. Nat Rev Rheumatol. 2012;8(3):133–143. [Link]
  • Bowers KM, Anderson DE. Delayed Union and Nonunion: Current Concepts, Prevention, and Correction: A Review. Bioengineering (Basel). 2024 May 22;11(6):525. [Link]

Basics and Biomechanics This article has been medically reviewed by Dr. Arun Pal Singh, MBBS, MS (Orthopedics)

About Dr Arun Pal Singh

Dr. Arun Pal Singh is a practicing orthopedic surgeon with over 20 years of clinical experience in orthopedic surgery, specializing in trauma care, fracture management, and spine disorders.

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Dr. Arun Pal Singh is an orthopedic surgeon with over 20 years of experience in trauma and spine care. He founded Bone & Spine to simplify medical knowledge for patients and professionals alike. Read More…

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