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 ^{[1][2][3][4][5]} in the 1960’s. 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 elastically deformation of bone. The reason for the elastically deformation of bone are the peek 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 life-long 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 every-day 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)^[6][7].

These relations are of immense importance especially for bone loss situations like in Osteoporosis, since a 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.

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Modeling and Remodeling

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

• Disuse:
  Strain < circa 800μStrain: Remodeling (bone adaptation and bone repair) Bone mass and bone strength is reduced.
• Adapted State:
  strain between ca. 800μStrain and ca. 1500μStrain: Remodeling (bone repair) Bone mass and bone strength stays constant
• Overload:
  Strain > circa 1500μStrain: Modeling (bone growth) bone mass and bone strength his increased
• Fracture:
  Strain > circa 15000μStrain: maximum elastically deformation excceded - bone fracture.

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

Unit: Strain E

The elastically deformation of bone is measured in μStrain^[2][3]. 1000μStrain = 0,1% change of length of the bone.

• Strain E at length l and change of length Δl:
  

It has to be considered that bone strength is highly dependant 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.

Different type of bones can have different modelling and remodelling thresholds. The modelling threshold of the tibia is about 1500μStrain (= 0,15% change of length), the modelling threshold for bone of the skull is lowered by the factor of 6 to 8. Since the physical material properties of bone (as a material) are not altered in the different bone types of the body, this difference in modelling threshold results in a increased bone mass and bone strength and hence in an increase safety factor (relation between fracture load and typical loads) for the scull compared to the tibia. A lower modelling threshold means that the same typical daily forces result in a ‘thicker’ and hence stronger bone at the scull.

Examples

Typical exampüles of the influence of maximum forces and the resulting elastical deformations on bone growth or bone loss are extended flights of Astronauts and Cosmonauts as well as patient with paraplegia due to an accident. For example a patient in a wheel chair who is using his arms but due to his paraplegia not his legs will suffer of massive muscle and bone loss only in his legs due to the lag of usage of the legs. However the muscles and bones of the arms which are used every day will stay the same or might even be increased depending on the usage^[8][9].

The same effect can be observed for long flight Astronauts or Cosmonauts^[10]. While they still use there arms in an almost normal manner due to the lag of gravity in space there are no maximum forces induced on the bones of the legs.

If only bone mass or loss of bone mass would be the indicator for Osteoporosis every Astronaut or Wheel-Chair driver would be considered to be considered to be suffering from Osteoporosis. However, in fact both examples are not the result of a disease but of the lag of usage and hence the lag of adequate mechanical stimulation of bone. This lag of maximum forces on the bone and hence reduced maximum elastic deformations of the bone result in a downward adaptation of bone to optimize the used amount material to the every day usage. This control-loop is proven by the fact, that Astronauts gain muscle and bone again once they are back on earth and bone receives again the typical every day loading of before the space flight.

Literature