Mark W. McFarland, DO
Bone fractures are a common form of skeletal injury that affect individuals of all ages which undergo a complex and coordinated process of healing. This article provides a comprehensive review of the stages of human bone healing and discusses the intricate biological mechanisms involved, from the initial inflammatory response to the final remodeling phase. The stages of bone healing involve a complex interplay of cellular and molecular sequelae, ultimately leading to the restoration of bone structure and function. Understanding the stages of bone healing is essential for me to properly diagnose and treat fractures of all kinds.
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- Hematoma Formation and Inflammatory Phase:
The initial response to a fracture is the inflammatory stage. Inflammation is a vital protective mechanism that initiates the healing process. It involves the release of various chemical inflammatory mediators, such as cytokines and growth factors, which recruit inflammatory cells to the fracture site and attract mesenchymal stem cells (MSCs) to initiate the next stage of healing.
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Upon fracture, a hematoma (mass of clotted blood) forms at the site of the bone break due to the disruption of blood vessels. The hematoma serves as a scaffold for subsequent stages of bone healing. Neutrophils and macrophages play crucial roles in removing debris and initiating tissue repair. The inflammatory stage typically lasts for a few days.
- Soft Callus Formation and Granulation Tissue Formation:
During the soft callus formation stage, MSCs differentiate into chondroblasts (cells that form cartilage) and osteoblasts (cells that form bone), forming a soft callus composed of fibrocartilage and collagen. This matrix bridges the fracture gap and simultaneously, the formation of granulation tissue, rich in blood vessels and fibroblasts, occurs. These blood vessels supply oxygen and nutrients necessary for bone healing. The soft callus acts as a temporary scaffold, stabilizing the fracture and allowing for the subsequent deposition of bone tissue. The soft callus stage typically lasts for a few weeks.
- Hard Callus Formation and Endochondral Ossification:
Osteoblasts, which are responsible for bone formation, invade the soft callus and deposit new bone tissue. The soft callus gradually undergoes mineralization and transforms into a hard callus, composed of woven, immature bone. This process, known as endochondral ossification, involves the transformation of cartilage into bone. The hard callus provides structural stability and gradually bridges the fracture gap. This stage can last several months, depending on the severity and location of the fracture.
- Remodeling Phase:
The remodeling phase is the final stage of bone healing, where the newly formed woven bone is reshaped into its mature and mechanically sound structure. Osteoclasts (another type of specialized bone cell) resorb excess bone. Simultaneously, osteoblasts deposit new bone tissue in a process called bone modeling. The remodeling process can take several years, during which the bone gradually adapts to its mechanical demands.
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Factors Affecting Bone Healing:
Many factors can negatively influence the stages of bone healing, including:
- bone fracture type (of which there are many)
- location of the fracture on the body
- patient age
- comorbidities
- smoking
- poor nutrition
- impaired blood supply/non-union
- surgical reduction of the fracture
- infection
- diabetes
As an orthopaedic physician, it is my responsibility to treat each patient according to their specific type of fracture, and their own general physical condition, taking into account other considerations, like age, diabetes or if they smoke. There are many treatment options and tools that I may utilize.
I may need to prescribe a bone growth stimulator, which is a device that generates a gentle electric current, known to encourage bone growth. The patient wears the device for 4-6 hours a day over the area of the fracture to stimulate bone remodeling. Perhaps the patient isn’t getting proper nutrition and needs to work with a dietician to learn how to eat healthier, mineral-rich foods to stimulate the growth of stronger bones. Thankfully, most fractures won’t require my intervention and will heal just fine on their own, given time and care.
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