Mechanostat
With Mechanostat refers to a model, which the bone remodeling describes (modeling and remodeling). It was established in 1960 by Harold Frost in the Utah Paradigm of Skeletal Physiology and is a supplement to Wolff's Law .
According to this, bone growth and bone resorption is determined by the maximum elastic deformation of the bone. The reason for the deformation of the bone is the short-term maximum forces that occur (can be measured in vivo using mechanography and quantitative computer tomography, for example ). This process ( control loop ) takes place for a lifetime. The bone thus adapts its mechanical function, i.e. its geometry and thus the bone strength, to the daily requirements for a lifetime. Accordingly, in the healthy muscle-bone control cycle, there is a linear relationship between muscle cross-sectional area (as a surrogate for the typical maximum muscle strength) and bone cross-sectional area (as a surrogate for bone density ).
This fact also has consequences for bone loss ( osteoporosis ), as this can be counteracted by suitable training that generates the necessary peak forces to stimulate bone growth, for example vibration training .
Modeling and remodeling
Frost speaks of four areas of elastic bone deformation (strain - see definition below) that lead to different consequences:
-
Disuse:
Strain <approx. 800 μStrain: Remodeling (bone remodeling and bone repair ) takes place, bone mass and bone strength is reduced -
Adapted State:
strain between approx. 800 μStrain and approx. 1500 μStrain: Remodeling (bone remodeling and bone repair) takes place, bone mass and bone strength remain unchanged -
Overload:
Strain> approx. 1500 μStrain: Modeling (bone building) takes place, bone mass and bone strength are increased -
Fracture:
Strain> approx.15,000 μStrain: breaking point, the bone breaks.
Thus, the typical bone, for example the tibia , has a safety factor of around 5 to 7 between maximum typical deformation (maximum 2000 to 3000 μStrain) and its breaking point (around 15,000 μStrain)
Unit: strain E.
The deformation of the bones is measured in μStrain, where 1000 μStrain = 0.1% change in length.
- Strain E for length l and change in length Δ l :
It must be taken into account here that the strength of the bone is heavily dependent on the geometry and the direction in which the force is applied. The tibia, for example, has a breaking point in the axial direction of about 50 to 60 times the body weight. However, perpendicular to this axis, the breaking limit is lower by a factor of 10 or more.
Different bones can have different modeling and remodeling thresholds. For the tibia, for example, the modeling threshold is around 1500 μStrain (= 0.15% change in length), while the threshold on the cranial bone is around a factor of 6 to 8 lower. Since the pure material properties, such as density and strength, of these two bones do not differ, this means that the cranial bone has a significantly higher safety factor compared to the tibia (i.e. breaking limit compared to the typical load), because with a low modeling threshold, lead clearly smaller daily forces to "thicker" bones.
Examples
Typical examples of the influence of maximum forces and the resulting deformations on the muscle-bone control loop are long - term space travelers and patients with spinal cord injury after an accident ( paraplegics ). In the case of a wheelchair user, muscles and bones in the unused leg area are drastically reduced, while muscles and bones are maintained or even built up in the heavily used arm area. The same effect also occurs with long-term astronauts, because due to the lack of gravity in space, they cannot exert sufficient maximum forces on the bones, especially the leg area.
If the bone mass were decisive for the condition of the bone, every long-term astronaut and every wheelchair user would suffer from osteoporosis. In fact, in both cases it is not a question of a diseased bone, but merely the lack of stimulus for the stimulation of bone maintenance or bone growth through maximum forces or the resulting deformation of the bones. This is also proven by the fact that muscle and bone losses in long-term astronauts are fully compensated for after returning to earth with sufficient training duration.
Web links
- ISMNI - International Society of Musculoskeletal and Neuronal Interactions
literature
- ↑ Frost HM: Defining Osteopenias and Osteoporoses: Another View (With Insights From a New Paradigm) , Bone 1997, Vol. 20, No. 5, 385-391, PMID 9145234
- ^ A b Frost HM: The Utah Paradigm of Skeletal Physiology Vol. 1 , ISMNI, 1960
- ^ A b Frost HM: The Utah Paradigm of Skeletal Physiology Vol. 2 , ISMNI, 1960
- ↑ Frost HM: The Utah paradigm of skeletal physiology: an overview of its insights for bone, cartilage and collagenous tissue organs , J Bone Miner Metab. 2000; 18: 305-316, PMID 11052462
- ↑ Frost HM, Schoenau E .: The muscle-bone unit in children and adolescents: an overview , 2000, J. Pediatr Endorcrinol Metab 13: 571-590, PMID 10905381
- ↑ Schoenau E., Neu CM, Beck B., Manz F., Rauch F .: Bone Mineral content per Muscle Cross-Sectional Area as an Index of the Functional Muscle-Bone Unit , J Bone Mineral Res, Vol. 17, p 1095-1101, 2002, PMID 12054165
- ↑ Schießl H., Frost HM, Jee WSS: Estrogen and BoneMuscle Strength and Mass Relationships , Bone, Vol. 22, pp. 1-6, 1998, PMID 9437507
- ↑ Eser P. et al .: Relationship between duration of paralysis and bone structure: a pQCT Study of spinal cord injured individuals , Bone, Vol. 34, pp. 869-880, 2004, PMID 15121019
- ↑ Eser P. et al .: Bone Loss and Steady State after Spinal Cord Injury: A Cross Sectional Study Using pQCT , J Muskuloskel Neuron Interact, Vol. 4, pp. 197-198, 2004, PMID 15121019
- ↑ Blottner D., Salanova M., Püttmann B., Schiffl G., Felsenberg D., Buehring B., Rittweger J .: Human skeletal muscle structure and function preserved by vibration muscle exercise following 55 days of bed rest , Eur J. Appl Physiol, 2006, Vol. 97, pp. 261-271, PMID 16568340
