Earlier blog posts have explained the processes of bone modeling and bone remodeling. Whether bone is modeling or remodeling depends on the balance of bone formation and bone resorption. This blog will describe these cellular processes in detail and provide background for a more detailed understanding of how bone formation and bone resorption impact bone graft materials.
Bone Remodeling vs. Bone Modeling
When using bone graft materials, it is important to understand how bone remodeling and bone modeling work, because these two processes impact how the body responds to bone graft materials. An ideal bone graft material’s primary function is to regenerate normal bone and to respond exactly like normal bone would respond.
To proceed, we must first learn the difference between bone remodeling and bone modeling.
- Bone Remodeling: Bone remodeling is the normal repair process that occurs throughout life to renew tiny defects in the bone resulting from normal wear and tear of our skeleton. Remodeling also maintains vascular homeostasis of certain elements that are important throughout the body. For example, calcium and phosphorus are essential elements that circulate in the blood but are stored in bone. Bone remodeling occurs when there is a balance between new bone formation by osteoblasts and old bone resorption by osteoclasts (Figure 1). When a balance exists, there is no net change in bone mass. There are, however, instances when either one of these processes overtakes the other. This is referred to as bone modeling.
- Bone Modeling: Bone modeling occurs when the bone formation and bone resorption phases are out of balance. An example of this is the decrease in bone mass due to excessive proliferation of osteoclasts, which can cause medical conditions such as osteoporosis. Conversely, increases in bone mass can occur in response to increased mechanical loads on the skeleton which stimulates osteoblasts and increases bone formation. This can occur through an increase in exercise, such as with weightlifting or high impact training.
Physiology of Bone Remodeling
Bone remodeling requires all three types of bone cells – osteocytes, osteoblasts, and osteoclasts – to work together. Like most cells, bone cells are stimulated or suppressed by a complex set of signaling factors, i.e., systemically or locally secreted proteins or peptides. There are five stages that bone goes through when it remodels. These stages are illustrated in Figure 1.