Clinical Medicine

Doctor of med. sci., professor A.L. Isakadze, professor G. G. Eliava, doctor of medicine T.R. Svanishvili, professor T.G. Tsintsadze, assoc. professor L.S. Topuria

Composition structure of bone tissue and its change during physical load

and formation of pathological state

Tbilisi State Medical University

Georgian Technical University

Taking into account functional status of bone tissue is very important both for determination of pathogenesis of different diseases and for determination of size of physical load in sport and drawing up a right rehabilitation scheme. Ongoing inflammation and degenerative-dystrophic changes in bone, cartilaginous and connective tissues form the basis of various types of rheumatic diseases, which cover roughly 25 percent of population.

Collagen, elastin and connective substance are a basis of biological tissues. After natural and artificial mechanical effect takes place formation of mechanical motions in biological tissues, organs and systems, transmission of waves and origination of deformations and strains.

Biological tissues have complicated anisotropic structure, which is depended of functions executed by them.

Bone is a solid body, basic properties of which are strength and elasticity.

Bone tissue strength should be high since it is basic material of musculo-skeletal (locomotion) system. Strength is depended on chemical composition, general structure and inner structure of armature, strength and number of components, orientation of basic components towards longitudinal axis of bone, as well as on age, individual conditions of body growth, bone sections, from where sample is taken etc.

Analysis of bone tissue structure and mechanical behavior shows that it can be represented as environment with five structural levels (I. Knets, 1979).

First level is represented by tropocollagen molecules and inorganic crystals. Tropocollagen molecule consists of three left spiral polypeptide chains, which are formed by right spiral strengthened by hydrogen bonds.

Second level is represented by collagen microfibrilles and inorganic crystals. Microfibrille consists of five spirulate tropocollagen molecules. In longitudinal direction these molecules are disposed with stair-step shift, which is equal to 650Å. Their diameter is 35Å.

Third structural level is represented by fibres, which consist of a lot of collagenic microfibrilles and mineral crystals tightly connected to them with stereochemical bonds. Mineral crystals are disposed both at the surface of microfibrilles, and inside them and are predominantly oriented along longitudinal axis. This unity of organic and inorganic substances is considered as reinforcing components of bone tissue.

Fourth structural level consists of bone plates – the smallest independent constructional elements, which form compact bone substance. Collagenic-mineral substances, which are connected with bonding agent, contain the material of bone plates.

Fifth structural level is constructional element osteone. It consists of concentric bone plates. Their amount in osteone may vary from 5 to 20. Volumetric weight of bone tissue is approximately equal to 2,4 g/cm³.

First and second structural levels are considered as structural components at molecular level. Fourth and fifth structural levels are complied with already determined independent constructional elements. That’s why when considering bone tissue as continuous medium, the third structural level is accepted as basic one.

According to bone form, position and function there are complicated load diagram is composed from trajectories of deformation direction or strain action. Deformation trajectories represent trajectories of compression and tension. Energetically these trajectories correspond with compression and tension strain trajectories.

As is known from technical statistics maximally simplified structure should be represented by grid structure, where rods follow trajectories of strain action. For example, bridge, radio tower constructions are built from crossbars disposed cross-wise. Even in 1866 engineer K. Kulman expressed an opinion on similarity of bone structure and technical constructions. It is known that trajectories of compression and tension strains are represented by nebulous structure, i.e. by disposition of many cross-wise bone crossbars.

In places of maximum loads bone crossbars are disposed arc-like (arcwise). Arc systems along with pipe-like systems are belonged to the most strength ones. Arc-like principle of construction of crossbars of spongy substance is characteristic for proximal epiphysus of thigh bone etc.

In experiments carried out in vivo using ultrasound method it is established that bone structure, form and chemical composition are changed during different loads that has an impact on bone adaptation at all. Abovementioned changes are manifested in bone system of athletes, since their skeleton experiences intensive and long-term load.

Bone is getting adapted to the environmental conditions via remodeling, which includes its development, strengthening and resorption. Remodeling is a balance between bone adsorption (osteoclasts) and its formation (osteoblasts) and it is permanently changed due to such factors as physical activity, age and disease (Alexander, 1984a; Canyon, Rubin, 1984).

Shumski, Merten, Dzenis (1978) have observed more considerable deposits of macroelements and increased density of shin bone in athletes compared with control tested persons. Dalen and Ollson (1974) have found that macroelement content in bones of across country runners (25 years of appearance) was 20 percent higher in appendicular sections (distal radius (radial bone), elbow bone, calcaneal (heel) bone) and less than 10 percent higher in axial sections (lumbar vertebra and head of thigh bone), than in control test persons.

Even in weight-lifters (6 years of experience) has been observed increased content of macroelements in basic support sections (lumbar part of vertebral column, trochanter, neck of femur) compared with other body areas (medial part of radial bone) (Colleti, Edwards, Cordon et al, 1989).

Bone tissue resistance to breaking is basically manifested in determination of its strength during compression and tension. Both in vivo and in vitro researches clearly show that tension strain is the most dangerous. It is caused by the fact that bone tissue tension strength is equal to 60 percent of compression strength in average.

In case of bone ageing biological activity decreases, mineralization quality changes, as well as sequence of mineral crystals and osteons disposition, amount of bonding agent is getting smaller, some part of tissue disappears and takes place formation of pores.

Basic components of bone tissue are distributed as follows: fibers of organic substance – collagen contain 40-45 percents of total volume, crystals of inorganic crystalline substance – hydroxyapatite contain 40-45 percents and bone liquid – 15%, insignificant part of total volume – 1 percent is represented by organic and mineral substances.

Bone tissue tension strength to breaking varies from 150 to 177 MgPa and is depended on the cross-section zone, from where samples are taken, in particular, on strength of separate components: hydroxyapatite – from 600 to 700 MgPa, collagen – from 50 to 100 MgPa.

It is accepted that bone tissue fibers are characterized by predominantly elastic deformation, while matrix (the rest of tissue) is featured by plastic deformation and tissue experiences fragile breaking. Alongside with increase of volumetric part of fibers need to be reinforced and tangent modulus of matrix elasticity takes places increase of elasticity modulus of composition. In case of increase of volumetric part of fibers the effect of matrix elasticity modulus on elasticity modulus of composition lessens.

As researches show, optimum picture of human skeleton formation is largely depended on inner structure of bones. For instance, compact structure of longitudinal bones close to joints is replaced by porous structure that causes smooth change of bone strength and uniform distribution-delivery of load.

Bone resistance to breaking is high. Bearing ability of thigh in longitudinal direction exceeds 45000 N in men, while in women it is more than 39000 N. Limited strains of various samples of compacted bone to compression are from 120 to 170 MgPa.

Usually, breaking process is preceded by plastic deformations and micro fissures, while breaking process itself is basically considered as macroscopic phenomenon, during which bone breaks into two or three parts and neither molecular nor other forces act between them. Pafford et al. (1975) on the basis of experimental data considers compacted bone tissue as orthotropic medium. It is established that composition of substance located between fibrilles alongside with mineral components and collagenic fibers has significant impact on mechanical characteristics of bone tissue. Thus compacted bone tissue represents not two-phase, but three-phase composite material in relation to mechanical loads.

As research results show, the process of bone remodeling runs more effectively under periodical but not continuous action of physical loads (Lanyon, Rubin, 1984). More over, the values of articular forces to a greater extent influence on bone mass, than number of load cycles (Whalen, Carter, Steele, 1988). The possibility of osteoporosis development arises, which is characterized by increase in bone porosity, leading to decrease of their density and strength, and therefore, to rise of probability of their breaking. Artificial bones and implants, basically made from titanium are used for replacement and strengthening of damaged bones of human’s skeleton.

Titanium is well assimilated by human organism. But, as far as bone tissue is gentler material, in places of articulation with titanium parts it gradually breaks that sometimes leads to necessity of surgical interference with the purpose of implant replacement. But this state of things may change thanks to creation of foamy structure of titanium.

Peter Quadback from the Fraunhofer Institute for Manufacturing Technology and Advances Materials Research has elaborated with colleagues in Dresden (Germany) bone implants from titanium, which have foamy structure, which is practically copied from spongy structure of common bone tissue. This foamy structure of titanium gives implants mechanical properties similar to properties of human bones.

Titanium foam is manufactured as follows: polyurethane foam, saturated with titanium powder and other additional substances, is molded (formed) in accordance with necessary form. Afterwards the received conglomerate exposes to high temperature, which evaporates polyurethane and other substances, at the same time effect of high temperature forces titanium particles to fuse to each other, and these particles after this treatment are fusing into foamy structure. These developments are prospective and need further researches for their use in humans.

Synthetic polymeric materials with mechanical properties similar to properties of human bones are widely used in surgery in case of resorbable materials of temporary action and for other goals (R. Kasarava et al., 2009; Vasiljev A., 1971; Takemoto K., Inaku Y., Ottendrite R.M., Marcel Dekker Inc., 1987; Langer R., Tirrel D.A., 2004). Many degradable polymers such as poly (L-lactic acis) (PLLA), polycaprolactone and poly (propylene fumarate), amino acid-based poly(ester urea)s (PEu) and other degradable polymers are used in orthopedic applications (Wang S., Lu L., Yaszemski, 2006; Athanasiou K.A. et al., 2004; Ma PX, Choi JW, 2001; Tsuju, Iskada Y., 1995; Liao SS. et al., 2004; Wei CB, Ma Px., 2004; Navarro M. et al., 2008; Kimberly Sloan Stakleff et al., 2012).

Without any doubt when analyzing mechanical properties of tissue such as strength and elasticity or when using their substitutes we always must proceed from composite structure of bone and mechanical properties of its component parts.

Proceeding from composite structure of bones based on its changes during physical loads and pathologies it may be noted that it is necessary to carry out assessment of musculo-skeletal (locomotion) system, namely bone structure and form with the purpose of prevention and using ultrasound and other methods before exercise, during exercise period and after it, as well as in rehabilitation period. Examination of bone structure and form is significant in latent period and at prodromal stage, until formation of complete clinical picture of disease occurs; use of synthetic polymeric materials for creation of resorbable materials of temporary action, when maximal approach to natural mechanical properties of bones, peculiarities of their structure and form; as well as maximum getting rid of inflammatory and immune reaction origination is necessary; in case of use of permanent implants is necessary to make their mechanical properties as far as possible similar to properties of human bones in order to get rid of surgical reintervention; estimation and analysis of bone structure and form along with other functional samples will assist improvement of athletic training, getting rid of traumatism, while with the purpose of population prevention during screening studies will reduce the risk of manifestation of degenerative-dystrophic changes and respectively will prevent progression of rheumatic diseases.