Growth factors of Bone

Osteoconductive

This term describes factors which help form bone by providing a scaffold on which cells, such as osteoblasts, lay down vascularized new bone. Perhaps the most simple example is freeze dried bone graft, which is devoid of living tissue but contains the inherent framework to build new bone. Ceramics also possess this ability and are frequently utilized. Ceramics are usually fundamentally composed of calcium phoshate in the form of hydroxyapatite and/or tricalcium phosphate. Both of these substances provide a framework for bone formation; additionally, they are reabsorbed and remodeled at varying rates. Recently, combinations have been produced that provide structural stability for several months and have been useful in fracture care. Synthetic polymers such as polylactic acid (PLA) and polyglycolic acid (PGA) possess osteoconductive properties and are used frequently as plates and screws for stabilizing fractures and graft placement.

Osteoinductive

Osteoinductive is a term used to describe substances that are capable of inducing bone to grow. Unlike osteoconductive substances, these factors are capable of causing extraskeletal tissue to form bone. This class of compounds include Bone Morphogenic Proteins, TGF-Beta family, Insulin-Like growth factors, fibroblastic growth factors (FGFs), and Platelet Derived Growth Factors (PDGFs)

TGF-Beta family

TGF-Beta family of proteins induces the production of type II collagen and proteoglycans that are instrumental in fracture healing. This protein has dramatic effects on callus formation and regulation and can be found throughout the early fracture hematoma within 24 hours from fracture. It is deposited in the hematoma by platelets; however, the protein is found throughout mesenchymal cells, chondrocytes, and osteoblasts. TGF-Beta has been used clinically as a coating for porous implants and has demonstrated the ability to enhance bone ingrowth.

Bone Morphogenic Protein (BMP)

A cytokine that induces undifferentiated perivascular mesenchymal cell to transform into osteoblasts. This low molecular weight hydrophobic glycoprotein is highly “osteoinductive” meaning that the substance can induce bone growth. This induction is powerful enough that it can cause bone to form in extraskeletal sites. BMPs were first discovered in demineralized bone matrix and several different types have been found. BMP-7 (also known as OP-1) is often mixed with a collagen carrier and utilized frequently as an adjuct in spinal fusion and non-union surgeries. BMP-2 and BMP-3 (also called osteogenin) have similar effects with variable potency. In addition, BMPs help regulate bone matrix formation and BMP-1 has an important function in the processing of collagen from its procollagen precursors.

Platelet Derived Growth Factor (PDGF)

A protein found within platelets that has many functions. Most importantly, it has a chemotactic function that attracts inflammatory cells to the fracture hematoma. In vitro, it appears to stimulate replication of osteoblasts and increase production of type I collagen. While it can induce bone formation, no experiment has shown it to increase callus strength.

Fibroblast Growth Factor and Insulin Like Growth Factor both have varying effects on fracture callus and appear to enhance fracture healing.