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How Bone Heals¹

Bone repair follows a bone formation cascade during which time new bone is generated to replace or repair the damaged bone. The steps in the bone formation cascade are:

1. Reactive Phase:

   With a typical traumatic bone injury, temporary bleeding around the fracture site creates a hematoma which, along with blood cells and adjacent periosteum ultimately supports the healing process. However in a bone fusion procedure this bleeding is not present and therefore its beneficial effects are not at work. In these cases, a bioresorbable implant or scaffold may be placed between the adjacent bone segments to facilitate the regenerative process. Osteoprogenitor cells attach and bind to proteins on the scaffolding implant and fibroblasts begin to replicate to form a loose aggregate of cells, interspersed with newly developing small blood vessels. This process begins the revascularization of the bone and surrounding tissue to facilitate healing.

2. Reparative Phase:

   During this phase, the infrastructure for bone formation is created. The osteoprogenitor cells differentiate into osteoblasts which are the cells responsible for bone formation. During this process, other stem cells differentiate into vascular and endothelial cells to further propagate growth and healing. The fibroblasts, from the reactive phase, are responsible for creating and modifying the extracellular matrix which provides the structural support found in all connective tissue. Collagenous proteins (Type I collagen) form a fibrous network at the bone-healing site. As well, other proteins are secreted to further promote bone growth. The vascular network continues to develop through this process as well.

   As the extracellular matrix matures, it becomes mineralized with calcium phosphate. The osteoblasts begin to form woven bone, which is a haphazard organization of the collagen fibers and hence is inherently weak. It forms quickly and provides some structure, but is eventually replaced by lamellar bone. The fibroblasts, through several steps, develop into hyaline cartilage, which in combination with the woven bone, bridge the gap between the native bone segments thus creating the initial bridge which adds strength, integrates revascularization and facilitates additional bone regeneration.

3. Remodeling Phase:

   The woven bone is the first to be replaced by lamellar bone, followed by the hyaline cartilage. This happens as soon as the collagen matrix becomes mineralized. At this point, the developing vascular channels penetrate the matrix accompanied by osteoblasts. The osteoblasts continue to form lamellar bone. This lamellar bone is replaced by trabecular or cancellous bone, also sometimes referred to as ‘spongy bone’. It is transition bone and is less dense making it softer than and thus not as strong as the final bone structure referred to as 'compact bone.' Osteoclasts, a type of bone cell that removes bone tissue, resorbs the trabecular bone and during the remodeling process, it is replaced with compact bone which is the strongest and most dense bone structure.