Mechanostat

The Mechanostat is a model describing bone growth and bone loss. It was promoted by Harold Frost and described extensively in the Utah Paradigm of Skeletal Physiology in the 1960s. The Mechanostat is a refinement of Wolff's law described by Julius Wolff (1836–1902).

According to the Mechanostat, bone growth and bone loss is stimulated by the local mechanical elastic deformation of bone. The reason for the elastic deformation of bone is the peak forces caused by muscles (e.g. measurable using mechanography). The Adaptation (feed-back control loop) of bone according to the maximum forces is considered to be a lifelong process. Hence bone adapts its mechanical properties according to the needed mechanical function – bone mass, bone geometry and hence bone strength (see also Stress-strain index, SSI) is adapted according to the everyday usage / needs.

Due to this control loop, there is a linear relationship in the healthy body between muscle cross sectional area (as a surrogate for typical maximum forces the muscle is able to produce under physiological conditions) and the bone cross sectional area (as a surrogate for bone strength).

These relations are of immense importance especially for bone loss situations like in osteoporosis, since an adapted training utilizing the needed maximum forces on the bone can be used to stimulate bone growth and hence prevent or help to minimize bone loss. An example for such an efficient training is vibration training or whole body vibration.

Frost defined four regions of elastic bone deformation which result in different consequences on the control loop:

According to this a typical bone, e.g. the tibia has a security margin of about 5 to 7 between typical load (2000 to 3000 μStrain) and fracture load (about 15000μStrain).

The elastic deformation of bone is measured in μStrain. 1000μStrain = 0.1% change of length of the bone.

It has to be considered that bone strength is highly dependent on geometry and direction of the acting forces in relation to this geometry. The fracture load for axial forces of the tibia for example is about 50 to 60 times the body weight. The fracture load for forces perpendicular to the axial direction is about 10 times lower.

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