Mechanical Loading of Bone

Original Editor - Yasmina Hossman Top Contributors - Claire Knott, Lucinda hampton, Kim Jackson and Yasmina Hossam

Introduction

Bone mass is regulated by both mechanical and biochemical stimuli. Like muscles, bones add mass when they are loaded (physical activity) and will lose mass if they are not (immobilisation or time spent at low or zero gravity)

Smart Material: Bone is a remarkable living material that comes in two forms with different porosities and different macrostructure, but with the same highly organised microstructure and nanostructure. As bone accumulates damage, it is removed and replaced. When the mechanical demands on bone increase the bone mass increases, while reductions in the loading leads to the removal of bone, thus bone can be considered a ‘smart material’.

Remodelling: The ongoing replacement of old bone tissue by new bone tissue is called remodelling. Bone formation, repair and remodelling is controlled and produced by four types of cell, namely osteoblasts, osteoclasts, osteocytes and bone lining cells. Bone remodelling is regulated by signals to these cells generated by mechanical loading. Exactly how loads are transferred into bone, how the bone cells sense these loads and how the signals are translated into bone formation or removal is unknown.

Detecting Mechanical Load

The main sensor of mechanical forces is the osteocytes network, which transmits orders to effector cells ie osteoclasts, and osteoblasts, by the secretion of several cytokines which modulate the concentration of the bone microenvironment.

Osteocytes:

• The most abundant cells found in bone (90-95% of all bone cells) with long dendritic processes to communicate with surrounding cells.
• Detect the change of deformation by load and send signals to the osteoblasts and osteoclasts to repair and strengthen the bone.
• The biochemical reaction triggered by these mechanosensitive osteocytes leads to osteogenesis (new bone formation).

Key molecules in sensing and transducing:

• Calcium channels (an ion channel which shows selective permeability to calcium ions)
• MAP kinases (involved in cellular responses to various stimuli and regulate cell growth, cycling, differentiation, mitosis and apoptosis).

The response of bone to mechanical appeals will depend on the quality of these appeals, both in terms of intensity and of frequency. Because of desensitization of the system with continuous loading, intermittent activity is expected and has been shown to be more efficient. The response will also depend on the sensitivity of the sensor, which decreases with age and is modulated by the hormonal environment, particularly estrogens.A study in 2002 by Robling et. al. found that participants gained more bone mass by breaking up loading sessions into multiple smaller sessions separated by recovery periods compared with those completing all loading in single uninterrupted sessions .

Osteogenic Loading

When just standing upright, the simple act of resisting gravity puts a load on bone, but our body is well adapted to this load. To build stronger bones, a much higher load — that is, greater compression and bending — is needed to encourage our bodies to spend the necessary energy and resources.

The load put on bone can be measured in terms of multiples of body weight. The higher the load, the better able the activity is to stimulate bone growth.Most physical activity loads bone to a degree, but for strong osteogenic stimulation, the load needs to reach around 4 times body weight. Calculations of multiples of body weight look like this for common physical activities:

• Swimming: 0 (Your load is actually lessened in water.)
• Standing: 1
• Brisk walking : 1–2
• Running/jogging:  3–4
• Power jumping: 4+
• Resistance, strength training:  4 to 10 (Depending on impact.)

What Does This Mean for the Physiotherapist?

Further research is needed to better understand the whole process of bone formation regulation. Greater knowledge of the mechanisms that govern the adaptive responses of the bone cells may lead to the development of new strategies towards enhanced bone healing protocols and fracture prevention. The clinical implications for physiotherapists are difficulty in designing optimal bone loss prevention strategies and programs as well as directing the management of conservatively or surgically managed fractures in our patients.

References