A bone fracture is when there is damage in the continuity of the bone. Bone fractures can occur due to a high force of impact that the bone cannot withstand or due to a medical condition that weakens the bone such as osteoporosis or osteogenesis imperfecta. Common types of bone fractures include transverse, linear, oblique, spiral, greenstick, and comminuted. Comminuted can be the most severe as the bone has broken into many small pieces.
Bone metabolism
Bones contain crystals of minerals (calcium and phosphate) called hydroxyapatite that are bound to a matrix made up of collagen. The mineral part of bone provides support while the collagen part of bone provides the framework. The cells within bone consist of osteocytes, osteoblasts, and osteoclasts. Each type of cell is necessary for the build-up and break down of bone. Osteoblasts deposit new bone tissue while osteoclasts resorb bone tissue. Osteocytes are the most abundant bone cell and are derived from osteoblasts. Osteocytes reside in lacuna within the bone matrix and communicate through channels called canaliculi. Through these channels, they can secrete substances that can inhibit or recruit osteoblasts and osteoclasts. This process can be influenced by mechanical stressors, hormones, and the amounts of calcium and phosphorous in the bloodstream.
Bone remodeling process in fracture healing
The bone remodeling process of a fracture occurs in four stages. The early stages of a fracture lack vascularization thus bone formation cannot occur immediately. The first stage occurs instantly following the fracture. A hematoma is formed around and within the fractured area. The hematoma lays the foundation for callus formation which helps in the stabilization of the bone. This period lasts about seven days. Mesenchymal stem cells (MSCs) are recruited from the nearby soft tissues to the area where the hematoma formed. The MSCs are necessary for the generation of the callus tissues, the second stage in the bone remodeling process. Once the MSCs are recruited, the callus formation occurs by the matrix production of type I and type II collagen which forms a cartilaginous tissue that makes up the callus. This stage occurs 2-3 weeks following the fracture. This soft callus is replaced by a harder calcified callus. In the third stage chondrocytes within the callus continue to proliferate leading to a calcified extracellular matrix. This calcified callus is better for biomechanical support but does not fully restore the properties of normal bone and does not occur until 4-6 weeks after the initial fracture. This leads to the fourth and final stage, a second resorption period. In this resorption phase, osteoclasts are recruited to resorb the calcified callus while osteoblasts deposit new bone tissue. Although this process is started about 6-8 weeks after the fracture, it can take years to fully regenerate the bone structure. It’s important for the success of fracture healing that the area contains an adequate blood supply and mechanical stability. Since this process is very long, it’s common for fractures to heal in adverse anatomical positions and lead to non-unions.
