Bone Healing

Bone is a Living Tissue. It is formed of living cells in a matrix designed to withstand load and to constantly replace and repair itself. When injured, bone immediately begins the repair process, with dedicated cells immediately taking over and beginning a complex signaling event that ultimately results in complete restoration.  Because healing is phase oriented, certain systems must in place at the right time to effect the healing cascade. This begins with inflammation, which is immune system mediated. A hematoma forms, which is filled with platelets, cytokines and growth factors. There are important meditators at work- PDGF (platelet derived growth factor), VEGF (vascular endothelial growth factor), BMPs (bone morphologic proteins), interleukins, TGFa (transforming growth factor), and more. These signaling molecules recruit osteo-progenitor cells and ignite angiogenesis. The system must maintain balance between normal inflammatory response and excessive or prolonged inflammation which can impair the process.  Angiogenesis means "new blood vessel formation," a process critical to repair. The biology of bone healing thus begins with restoration of blood flow.  There are two main types of bone healing - primary and secondary. Primary healing is direct laying down of matrix between two fracture ends. This occurs when there is adequate blood flow, close approximation of bone ends, stability at the fracture site, and low tissue strain. Secondary bone healing occurs when there is relative stability, not absolute rigidity. This involves a cartilage intermediary, a tissue type visible in the healing process.  This concept of strain is critical to understanding how bone behaves. Strain is basically the change in gap length divided by the original gap length. Different tissue types tolerate different amounts of strain. Granulation tissue tolerates high strain, bone tolerates very low strain. This means that the tissue type that will form at a fracture site is heavily dependent on the mechanical environment. Bone healing does not like extremes. Too much motion leads to delayed healing and persistent fibrous tissue. Too little will suppress callous formation and stress shield the bone leading to poor biologic recruitment.  Healing likes the environment to be stable enough but not necessarily absolutely rigid. This is the art in surgical practice - determining how and when bone needs stability, how much stability, and when to progress to load and stress again.  Of course, mechanics alone are not a sure bet that the bone will heal. Biologic constitution also is a factor. Low vitamin D, low calcium levels, high sugar levels, presence of carbon monoxide, and more can lead to poor healing of bone tissue.   The content of this podcast is for educational and informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.