Mechanostat

With Mechanostat, we describe a model that describes bone growth and bone resorption ( modeling and remodeling ). It was erected by Harold Frost in Utah Paradigm of Skeletal Physiology in 1960 and is a supplement of Wolff 's law.

Thus, bone growth and bone degradation by the maximum elastic deformation of the bone is determined. Reason for the deformation of the bone are the maximum forces occurring short-term (in-vivo measured for example by means mechanography and quantitative computed tomography). This process ( loop) is a life rather long. Thus, the bone adapts its mechanical function, ie its geometry and bone strength, a lifetime to meet the daily requirements. Accordingly, in the healthy control loop muscle - bone is a linear relationship between muscle cross -sectional area ( as a surrogate for the typical maximum force of the muscle ) and bone cross-sectional area ( as a surrogate for bone strength).

This fact has also just for bone loss (osteoporosis ) consequences, as this can be counteracted by suitable training, which generates the required peak powers for the stimulation of bone growth, such as the vibration training.

Modeling and Remodeling

Frost speaks of four areas of bone elastic deformation ( strain - see definition below ) that lead to different consequences:

• Disuse: Strain < about 800 μStrain: Remodeling ( bone remodeling and bone repair ) takes place, bone mass and bone strength is reduced
• Adapted State: strain between about 800 and about 1500 μStrain μStrain: Remodeling ( bone remodeling and bone repair ) takes place, bone mass and bone strength remains unchanged
• Overload: Strain > circa 1500 μStrain: Modeling ( bone formation ) takes place, bone mass and bone strength is increased
• Fracture: Strain > about 15000 μStrain: breaking point, breaking bones.

Thus has the typical bone, such as the tibia, a safety factor of about 5-7 between maximum typical deformation (maximum 2000 to 3000 μStrain ) and its breaking limit (about 15000 μStrain )

Unit: Strain E

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

• Strain E with length l and length change DELTA.l:

To be considered here is that the strength of the bone is strongly dependent on the geometry and on the direction of force application. The tibia, for example, has in the axial direction as a breaking point from the 50 - to 60 - times body weight. Perpendicular to this axis, however, the breakdown limit is lower by a factor of 10 or more.

Different bone can have entirely different modeling and remodeling thresholds. For the tibia, the modeling threshold is for example about 1500 μStrain ( = 0.15% change in length ), the cranial bone, however, the threshold is about a factor of 6-8 lower. Since the pure material properties such as density and strength of these two bones do not differ, it means that the skull bones compared to the tibia significantly higher safety factor (ie breaking point compared to the typical load ) has, since at low Modeling threshold already perform significantly smaller daily forces to " thicker " bone.

Examples

Typical examples of the influence of the maximum forces and resulting deformations on the control circuit muscle bones are long-term space travelers and patients with a spinal cord injury after an accident ( Paraplegic ). Thus, in a wheelchair muscles and bones are reduced drastically in the unused leg area, while in the much-used arm region maintain muscle and bone or even built. The same effect also occurs in long-term astronauts, as these can also have insufficient maximum forces on the bones of the leg portion in particular due to the lack of gravity in space.

If purely for the condition of the bone, the bone mass are decisive so any long-term astronaut and each wheelchairs would suffer from osteoporosis. In fact, however, is in both cases not a diseased bone, but merely the lack of stimulus for the excitation of bone preservation and bone growth through maximum forces and the resulting deformation of the bones. This is also the fact that muscle and bone losses are fully compensated for long term astronauts after returning to Earth with sufficient training time again.