6.5 Fractures: Bone Repair

Fractures come in various types, from simple breaks to complex shatters. Each type affects healing differently. Understanding these differences helps in proper treatment and recovery. Bone repair is a remarkable process involving multiple stages and cell types.

The body's response to a fracture is a coordinated effort of inflammation, callus formation, and remodeling. This intricate process can take weeks to years, depending on the fracture's severity and location. Proper care during healing is crucial for optimal recovery.

Types and Characteristics of Fractures

Types of fractures

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• Simple (closed) fracture occurs when bone breaks without piercing the skin fragments remain aligned and stable
• Compound (open) fracture happens when bone breaks and pierces through the skin increasing risk of infection and complications
• Comminuted fracture involves bone shattering into three or more pieces often caused by high-impact trauma (car accidents, falls from height)
• Greenstick fracture is an incomplete fracture more common in children where bone bends and cracks but does not completely break
• Stress fracture involves tiny cracks in the bone caused by repetitive stress common in weight-bearing bones of athletes and dancers (metatarsals, tibiae)
• Pathologic fracture occurs when fracture is caused by weakened bone due to underlying disease (osteoporosis, bone cancer, osteogenesis imperfecta)

Bone Repair Process

Stages of bone repair

1. Inflammation stage (1-5 days post-fracture) involves hematoma formation at fracture site, inflammatory cells (macrophages, neutrophils) removing debris, and fibroblasts and mesenchymal cells migrating to the area
2. Soft callus formation (2 days to 2 weeks post-fracture) occurs when fibroblasts produce collagen fibers to bridge the fracture gap and chondroblasts produce cartilage matrix, providing initial stability
3. Hard callus formation (2-6 weeks post-fracture) happens as osteoblasts replace soft callus with woven bone, providing more stability and strength
   • Callus formation is crucial for stabilizing the fracture site and promoting ossification
4. Bone remodeling (6 weeks to several years post-fracture) involves osteoclasts removing excess woven bone and osteoblasts replacing it with mature lamellar bone, allowing bone to return to its original shape and strength
   • This process of bone remodeling continues long after the initial fracture healing is complete

Cellular roles in bone healing

• Hematoma provides a scaffold for cell migration and proliferation and contains growth factors and cytokines to stimulate healing
• Inflammatory cells (macrophages, neutrophils) remove debris and dead tissue and secrete growth factors to attract fibroblasts and mesenchymal cells
• Fibroblasts produce collagen fibers to form soft callus, providing initial stability to the fracture site
• Chondroblasts produce cartilage matrix in the soft callus, contributing to the initial stabilization of the fracture
• Mesenchymal stem cells differentiate into osteoblasts and chondroblasts, essential for the formation of new bone and cartilage
• Osteoblasts produce woven bone to replace soft callus and secrete osteoid which mineralizes to form hard callus
• Osteoclasts remove excess woven bone during remodeling phase, helping reshape the bone to its original form

Role of periosteum and endosteum in fracture healing

• The periosteum, a membrane covering the outer surface of bones, contains osteoprogenitor cells that contribute to callus formation and new bone growth
• The endosteum, lining the inner surface of bones, also contains osteoprogenitor cells that aid in fracture healing from the inside out