Fracture Healing: Types, Mechanisms, and Challenges

Fracture Healing

A bone fracture is a break in the continuity of bone, leading to mechanical instability. It often involves injury to the surrounding soft tissues, such as blood vessels, and can result in compromised function of the locomotor system. Fractures can occur due to falls, accidents, sports injuries, or penetration injuries on the battlefield. They are a public health issue with high healthcare costs, with an estimated 15 million fractures occurring annually in the United States alone.

Types of Fractures

Fractures can be classified into several types based on the characteristics of the forces resulting in the fracture:

• Simple and comminuted fractures: These occur due to a single injury or a high-velocity impact, such as a fall from a height or a car accident. Simple fractures involve bones being broken into two pieces, while comminuted fractures result in bones being broken into several pieces.
• Stress fractures: These are overuse injuries resulting from repetitive loading, such as running or cycling. They involve microdamages accumulating over time, healing via normal bone remodeling.

Healing Mechanisms

Fracture healing is a complex process involving a series of biological events. It can be divided into primary and secondary healing mechanisms:

• Primary healing: This involves the cortex attempting to reestablish continuity between the fracture fragments. It occurs when the alignment stability and decrease in interfragmentary motion of the fracture are sufficient. During this process, osteoblasts derived from mesenchymal stem cells (MSCs) lay down osteoid on the exposed bone surfaces, eventually leading to the formation of new bone.
• Secondary healing: This occurs in non-rigid fixation modalities, such as braces, external fixation, plates in bridging mode, and intramedullary nailing, which achieve a mechanical strain between 2 and 10%. During secondary healing, the fracture fragments are maintained in an aligned position, allowing the formation of granulation tissue and bony callus. Eventually, the callus is remodeled into new bone through the coordinated activity of osteoclasts and osteoblasts, which remove and replace the bone tissue, respectively.

Fracture Healing and Strain

The type of fracture healing is determined by the mechanical strain at the fracture site. Primary healing occurs with mechanical strain below 2%, while secondary healing occurs with mechanical strain between 2 and 10%. A strain greater than 10% can lead to non-union or delayed union.

Complications and Challenges

Failed or delayed healing can affect up to 10% of all fractures and can result from factors such as comminution, infection, tumors, and disrupted vascular supply.

In recent years, there has been a growing interest in understanding the healing process at the molecular level and developing new therapeutic strategies to enhance fracture healing. One approach is the use of electrical stimulation (ES) to generate endogenous electrical fields, which may facilitate tissue formation at the bony ends.

In conclusion, fracture healing is a complex and intricate process that aims to restore the damaged bone to its pre-injury state. It involves a series of biological events, primary and secondary healing mechanisms, and is influenced by the mechanical strain at the fracture site. Continued research in this field will help to improve our understanding of fracture healing and develop more effective treatment strategies.