Bone remodeling

The idea that hard bone changes a little every day may sound unfamiliar. Yet repair work is constantly taking place inside bones. Two types of cells do this work.

• Osteoclasts — Break down and remove old or damaged bone.
• Osteoblasts — Fill the excavated area with a framework for new bone and allow minerals to accumulate on it.

These two types of cells work together. When we are young and healthy, new bone replaces as much as is removed, maintaining bone mass. With aging or hormonal changes, however, removal can outpace replacement. A typical example is the decline in estrogen after menopause. If this imbalance persists over time, bone mass decreases, bringing osteopenia or osteoporosis into the picture.

Remodeling proceeds in sequence through small teams of cells at multiple sites on the bone surface. First, osteoclasts excavate old bone in a relatively short phase. This is followed by a longer phase in which osteoblasts fill the area with new bone and harden it. A single cycle at one site takes several months to complete, with the duration varying by location and age.

Because of this timetable, bones adapt to change more slowly than muscles do. Muscles may change within a few weeks of starting exercise, but bones take longer. Exercise for bone health is therefore better suited to a gradual increase in load rather than a rushed approach. This is explored in more detail in Load-capacity relationship and Load management.

Within the bone matrix, bone cells called osteocytes form a network. Osteocytes act as sensors, detecting the tiny bending of bone and the flow of fluid within it when we walk or run. These signals regulate how much work osteoclasts and osteoblasts do. Bone shape and density gradually change: areas that are frequently loaded are built stronger, while areas that are rarely used are reduced.

This pattern has long been recognized. In the 19th century, German physician Julius Wolff described how the internal architecture of bone aligns with the direction of loading, an idea commonly called “Wolff’s law.” In the late 20th century, Harold Frost proposed the “mechanostat” model, in which bone acts like a regulatory system that seeks to maintain strain within a certain range.

There are also examples in the opposite direction. Prolonged bed rest or time in a weightless environment removes the load on bones, leading to a relatively rapid decrease in bone mass. This clearly illustrates that bone is a tissue that responds to use.

However, more use does not always make bone stronger. Its response depends on the magnitude, rate, and novelty of loading, as well as recovery time. If the same area is loaded excessively and repeatedly over a short period, remodeling may not keep up, allowing damage such as a stress fracture to develop faster than repair.

• Try weight-bearing activities. Exercises in which your feet bear weight against the ground, such as walking, climbing stairs, and light jogging, provide a beneficial stimulus to the bones of the legs and pelvis.
• Include strength training. The pull of muscles on bones is also an important source of loading. Start with light weights and increase them gradually.
• Reduce prolonged periods of lying down or sitting. The longer bones go without loading, the more they respond toward reduced use.
• Pay attention to nutrition and sleep, too. Calcium, protein, and vitamin D—the building materials for bone—along with sufficient sleep, provide a foundation for remodeling.
• Increase exercise gradually. A sudden, large increase in exercise volume leaves too little time for bones to adapt. Rather than increasing it all at once, build up little by little over several weeks.

Bone mineral density is a measurement of how densely minerals are packed within bone. Bone remodeling is the process that explains why that measurement changes. Rather than viewing a single number as a still image, it is easier to understand it as the result of a balance between removal and replacement that changes over time.

The Korean transliteration of “bone remodeling” is another name for the same process as the Korean term for bone renewal. Because it is taken directly from the English term “bone remodeling,” this name is also frequently used in hospitals and health information.

A few similar-sounding expressions can be confusing. Heterotopic ossification refers to bone tissue forming somewhere other than bone, such as within muscle. Its location differs from remodeling, which occurs within existing bone. The phrase “bone regenerates” is also used broadly in everyday language. The replacement of bone tissue is a cellular process that unfolds over a long period, so it happens on a different timescale from changes felt over a day or two.