Introduction
Immediately after a bone fracture and after the hemorrhage has resolved, adjacent cells known as fibroblasts start proliferating and creating granulation tissue. After this, the fibroblast is present in the fracture site, and the periosteum forms a type of flexible cartilage called hyaline cartilage (chondroblasts), and others make a type of weak bone called woven bone (osteoblasts). The mix of these different tissues forms a lump around the fracture site and acts as a bridge. This lump is called a callus. Over time, the soft tissue and cartilage in the callus turn into a solid bone. This process is called mineralization. Later on, the new bone is reshaped to match the original bone structure. Sometimes, in cases like cancer, infection, or certain diseases, this process will be altered, and the callus may not form correctly.
What Are the Stages of Fracture Healing?
Fracture healing is progressed under three stages:
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Inflammatory Stage: This stage may last from a few hours to days. When there is a bone fracture, blood vessels rupture and form a blot clot or hematoma. This will trigger an inflammatory response, where cytokines, prostaglandins, and growth factors are released as they are crucial factors in healing. The hematoma then organizes and is invaded by fibrovascular tissue, providing a framework for bone formation and the initial callus formation.
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Repair Stage: This stage may last from a few days to weeks. A thick callus will be developed around the fractured bone ends, originating from the hematoma. Bone-building cells are recruited from various sources to create new bone, which may be visible on X-rays within seven to ten days of injury. The soft callus then can transform gradually into a hard callus, and this process may take several weeks. While the soft callus is fragile and prone to bending if the fracture is not adequately supported. On the other hand, the hard callus is stronger, though still weaker than normal bone. At this stage, the fracture reaches "clinical union," which means that it is no longer painful to touch or move.
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Remodeling Stage: This stage may last from a few months to years. This can be the most lengthy stage, sometimes spanning over several years. Throughout the remodeling phase, the healed fracture and surrounding callus can react to movement, external pressures, functional requirements, and growth. Excess bone, known as the external callus, is gradually eliminated as it is no longer necessary, and the fracture site is refined and shaped until it appears closer to normal on X-rays. The epiphyses (ends of the bone) will align gradually in a proper manner, and any remaining misalignment will be slowly corrected, following the principles of modeling.
What Is the Significance of Bone Callus in Fracture Healing?
The endosteum and periosteum are the main sources of fibroblasts involved in healing fractures. These fibroblasts aid in healing by producing important materials like elastic fibers, collagen, and glycoproteins. Fibroblasts convert into osteoblasts through the regulation of various proteins and growth factors (bone morphogenic proteins (BMPs)) released by the body at the fracture site. This process results in an increase in total calcium levels, alkaline phosphatase levels, and the activation of genes related to bone formation.
The soft callus, initially made of cartilage, starts to undergo a process called endochondral ossification. Additionally, a medullary callus supports this soft bridging callus. The expression of RANK-L (apoptosis regulator gene) will stimulate further differentiation of various bone cells. Consequently, the cartilaginous callus will be broken down and become calcified. Woven bone will continue to be formed under the periosteum, and the new blood vessels will keep forming, thereby allowing more mesenchymal stem cells to migrate. Finally, this phase concludes with the formation of a hard, calcified callus that consists of immature bone. The formation of bone callus relies upon proper movement between the fragments of the fracture.
What Is the Significance of Bone Callus in Bone Grafting?
The significance of bone callus in bone grafting include:
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During surgical procedures for fractures, bone callus is often overlooked and discarded. This callus is visible at fracture sites.
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Surgeons commonly remove and discard the bone callus to ensure an accurate realignment and fixation during surgery.
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Lately, many orthopedic surgeons have volunteered to look upon the benefits of bone callus for bone grafting.
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Research is ongoing about the regenerative potential of bone callus.
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Bone callus has the ability to regenerate new bone, as evidenced by the formation of osteoblasts (bone-building cells), osteoinductive factors (factors that promote bone growth), and an osteoconductive structure (structure that encourages bone formation).
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Importantly, within three months, the bone callus has shown the potential to generate a new bone compared to a cancellous bone.
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Bone callus can offer several advantages as a potential graft material over traditional autologous bone grafts.
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Traditional graft harvesting can sometimes result in complications like bleeding, pain, infection, and soft tissue injury.
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Bone callus can be harvested from fracture sites, reducing the risk of donor site complications.
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Since bone callus is derived from the patient’s own tissues, it has excellent histocompatibility (compatible with tissue), thereby eliminating concerns about immunological rejection and the risks associated with allogeneic (donor-derived) bone grafts.
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Additionally, bone callus provides osteoinductive factors, osteoblasts, and an osteoconductive structure simultaneously, making it a highly effective osteogenic material.
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Moreover, its mechanical properties are superior to those of tissue-engineered bone, which makes bone callus a preferred option for grafting procedures.
Conclusion
Bone callus develops during the fracture healing process. A bone callus is a natural and temporary stage of the healing process. Fracture healing is an intricate process that involves a complex interaction of cells, growth factors, and the extracellular matrix. Specifically, osteoblasts and chondroblasts play an important role in callus formation. Eventually, the callus transforms into bone, gradually becoming rigid and solid over time. Bone callus is an important factor that promotes fracture healing. Orthopedic surgeons are keen to explore the potential of applying bone callus as autograft material.
