Fracture Healing Process: Repair of Fractured Bone

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In this Video you will learn about the process of fracture healing.

Fracture healing: is an intricate and fluent regenerative process that aims at restoring the damaged bone to its pre-injury state and cellular composition.
A fracture is a break or a crack in a bone. A fracture occurs when force exerted against a bone is stronger than the bone can structurally withstand.
The type of fracture healing is governed by the achieved mechanical stability at the fracture site and, consequently, the strain. An appropriate mechanical stimulation, such as strain, facilitates tissue formation at the bony ends. The amount of the involved strain dictates the biological behavior of the cells involved in the healing process and, consequently, the type of bone healing. Primary bone healing ensues with mechanical strain below 2%, whereas secondary bone healing ensues when the mechanical strain is between 2 and 10%.
There are two main modes of bone healing; primary bone healing is dictated by absolute stability constructs that achieve a mechanical strain below 2%. It is an intramembranous bone healing that occurs through Haversian remodeling. The other type is secondary bone healing which occurs in non-rigid fixation modalities such as braces, external fixation, plates in bridging mode, intramedullary nailing, ..etc. These fixation modalities achieve a mechanical strain between 2-10%. And it occurs via endochondral bone healing. Bone healing can involve a combination of
primary and secondary processes based on the stability throughout the construct.
Primary Bone Healing vs. Secondary Bone Healing:
Primary bone healing, also known as direct or lamellar bone healing, occurs when there is rigid fixation of the fractured bone fragments without significant movement.
Secondary Bone Healing:
Secondary bone healing, also known as indirect or callus-mediated healing, is the more common form of bone healing where there is some degree of movement or instability at the fracture site.

Following the fracture, secondary healing begins, which consists of four steps:

Hematoma formation
Granulation tissue formation
Bony callus formation
Bone remodeling

Initiation of Hematoma: Upon the occurrence of a bone fracture, blood vessels within the affected area are disrupted, causing bleeding into the surrounding tissues. This accumulation of blood forms a hematoma, essentially a localized blood clot, at the site of the fracture.

Inflammatory Response: The hematoma triggers an inflammatory response, attracting immune cells such as macrophages to the fracture site. These cells initiate the removal of debris and damaged tissue, setting the stage for the regenerative phase.

Granulation tissue Formation:
Following hematoma formation, the next pivotal stage involves the generation of granulation tissue.
Granulation tissue is primarily composed of a dynamic interplay of cells, including fibroblasts, endothelial cells, and immune cells. Fibroblasts contribute to the synthesis of collagen, a structural protein essential for tissue strength, while endothelial cells aid in establishing new blood vessels within the developing tissue
Soft Callus: The soft callus is a subsequent stage in bone healing, emerging from the granulation tissue. It represents a preliminary structure that bridges the fracture gap and provides initial stability to the fractured bone.
Bony Callus Formation:
Transformation to Hard Tissue: As bone healing progresses, the soft callus undergoes a transformative process. Chondrocytes within the soft callus gradually differentiate into osteoblasts, cells responsible for bone formation.

Remodeling and Finalization:
Bone Remodeling: Following the formation of the bony callus, the bone undergoes a remodeling phase. Osteoclasts, specialized cells, work to reshape and refine the newly formed bone tissue. This process ensures the optimization of bone structure in response to mechanical demands.
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