Bone is a complex, highly organized andspecialized connective tissue. It is characterized physically by the fact thatit is a tissue that is hard, rigid and strong, and microscopically by thepresence of relatively few cells and much intercellular substance formed ofcollagen fibers and stiffening substances 1,2.Bone consists of 65% mineral, 35% organicmatrix, cells, and water.
The minerals in bone is in the form of small needles,plates, and rods shape of crystals located within and between collagen fibers.This mineral is widely impure hydroxyapatite, Ca10 (PO4)6(OH)2, containing constituents such as citrate, magnesium, carbonate fluoride, and strontium incorporatedinto the crystal lattice or absorbed onto the crystal surface. Foreignsubstances such as tetracyclines, polyphosphates, bisphosphonates, andbone-seeking radionuclides can also be incorporated with high affinity. Theorganic matrix consists of 90% collagen and about 10% of variousnon-collagenous proteins4,8.During skeletal growth, removal andreplacement of bone proceeds at a rapid pace.
The rate of turnover of theskeleton approaches 100% per year in the first year of life, declining to about10% per year in late childhood, and then usually continues at approximatelythis rate or more slowly throughout life. Much of the turnover of bone duringgrowth results from bone-modeling, but presumably at least some remodeling alsooccurs. After the completion of skeletal growth, the turnover of bone resultsprimarily fromremodeling. Modeling and remodeling result from coordinatedresorption and formation of bone over extensive regions of the tissue, overprolonged periods of time.
The discipline of mechanics is the physicalscience that deals with the effects of forces on objects. The concern here iswith the mechanics of deformable objects, in particular, bone. Bones arephysical objects that obey the laws of mechanics. The primary laws of mechanicsthat concern deformable objects like bone are the three laws of motion by SirIsaac Newton in 1687 and the law of elasticity of solid materials described byRobert Hooke in 1678. The following three Newtonian laws are the basis ofclassical mechanics:1- A body remains at rest or moves at aconstant speed in a straight line unless acted on by a force. This is astatement of the principle of inertia.2- The total force acting on a body is equalto the mass of the body times its acceleration; that is, f = ma, where f and aare vectors oriented in the same direction.
3- If a body exerts a force on a second body,the second body exerts a force on the first body that is equal in magnitude andopposite in direction to the first force. This is the law of action andreaction.Hooke’s law states that there is a linearrelation between the force and deformation of a solid object. The laws ofNewton and Hooke form the foundation of the mechanics of elastic objects. Themechanical behavior of bone in normal physiological situations is quite similarto the mechanical behavior of an elastic object4.It is obvious that mechanical forces have amajor influence on the bone modelingand remodeling processes in both corticaland trabecular bone, since their effects on bonemorphology are obvious (Wolff,1892). The pathways by which mechanical forces areexpressed in osteoclast andosteoblast activity is currently one of the main unresolvedissues in bonemechanobiology.
The current concept is that the bonearchitecture iscontrolled by a local regulatory mechanism. This idea originatesfrom Roux (1881), whoproposed that bone remodeling is a self-organizingprocess. Frost captured these conceptsin his ‘mechanostat’ theory (Frost, 1964,1987). It assumes that local strains regulatebone mass.
If strain levels exceeda so-called mechanical ‘set-point’, new bone is formed. If strain levels arebelow this set-point, bone is removed. It is a qualitative theory, but itformsthe theoretical basis for several mathematical and computational theories thatweredeveloped to study bone adaptation.Themechanostat does not specify the cellular level mechanisms behindthe(re)modeling process.
In other words, it does not describe how localmechanical signalsare detected, nor how they are translated to bone formationand resorption. Osteocytesmay play an important role here. Several studiesrevealed that these cells respond tomechanical stimulation.
Together withthelining cells they form a system that seems well equipped for signaltransduction. It could be that mechanically induced osteocyte signals aretransferredthrough the canaliculi to the bone surface where they controlosteoclast and osteoblastactivity. Whether this is true remains to be proven9.