Chemism of Networking and Biological Functions of Different Types of Phosphate Cements

This article provides an overview of calcium phosphate systems used in different formulations of cement mixtures. Chemism of reactions that occur in contact of these systems with water medium shows diversity and specificity of each of selected types of mixtures. As it can be clearly seen from reactions that occur between components of cement mixtures, highly active calcium deficient hydroxyapatite is always an end product in all reactions. This makes these systems extremely biocompatible and suitable for use in dentistry from the standpoint of chemical and biological degradability.


INTRODUCTION
Hydroxyapatite is an important biomaterial due to its resemblance to apatite mineral contained in natural teeth and bones [1].Calcium phosphate cements (CPC) have been developed because of their ability to adjust to a given model and form hydroxyapatite during the process of biological maturation in body [2].So far, there have been developed a few different types of phosphate cements [3].CPC powder usually consists of tetrakalcium phosphate (TTCP), Ca 4 (PO 4 ) 2 O, or tricalcium phosphate, β-Ca 3 (PO 4 ) 2 and anhydrous dicalcium phosphate (DCPA), CaHPO 4 .In order to obtain corresponding cement paste, component powders are mixed with water leading to the formation of hydroxyapatite [4].CPC exhibit an excellent biocompatibility and osteoconductivity.Moreover they are easily absorbed creating new bone [4].First cements based on phosphate components were applied in human clinical practice in 1996 for the repair of craniofacial defects [4].Since then they have been proved useful for a variety of dental indications, such as a root canal filling etc. Composites that contain CPC and various polymers (resin) may be applied for pulp capping and restoration of damaged cavity walls.They are suitable for the initiation of dentine remineralization process in in vitro conditions [5].However, in combination with biopolymers, such as chitosan, they build strong and flexible composites that can be used for the repair of periodontal bone [6].
The only drawback of CPC is the preparation just before use where powder and liquid must be mixed to form the cement paste which hardens very quickly and may be problematic during its incorporation into defect.The preparation of mixture immediately before use has other serious drawbacks [7] such as increasing time of surgery.However, if mixing and preparation are not satisfactory, some additional problems related to instruments and materials sterilization and maintaining sterile environment may appear.To operate in such conditions, mixing components is usually improvised giving the paste that does not have satisfactory rheological characteristics, or the amount of paste at a given moment is not sufficient to fill the present craniofacial defects.These difficulties have triggered the development of finished CPC, where the CPC powder is mixed with non-aqueous medium, miscible with water (alcohol) under well-controlled conditions.The advantage of this waterless paste is that it does not harden in a tool that is used for application; it only hardens by hydration in contact with aqueous environment.When the paste in non aqueous solution comes in contact with saline, it changes non-aqueous solution with water causing the paste hardening.At the same time, the setting time becomes longer with inadequate strength of hardened cement paste [7].Longer setting time causes some clinical problems due to instability of cement paste to maintain a desired shape during its networking and provide satisfactory mechanical properties.Therefore, it is necessary to find more favorable formulations of cement mixtures, and use various additives to accelerate setting and increase strength.It is important to note that some formulations of these mixtures show high level of cytotoxicity and can cause cell death in in vitro conditions.Consequently, ideal cements should be non-toxic, have good mechanical properties and harden quickly.Such cements in addition to basic components usually contain different types of additives and gel agents with diverse biological properties that have significant effects on reducing setting time of cements and viability of osteoblastic cells, both in vitro and in vivo.
For better review of all the features related to formulations of cement mixtures based on phosphate systems, this article gives formulations of various cement mixtures, specific reactions that occur in the process of their hydration and networking, as well as their chemism that results in the formation of calcium deficient hydroxyapatite important for biological compatibility of these systems [8].

PHOSPHATE CEMENTS AND CHEMICAL REACTIONS CHARACTERISTIC FOR THEIR NETWORKING
Chemical reactions that occur during networking of calcium orthophosphate cements depend on their composition; however, there are two main types of reactions.The first type occurs by classical rules of acid-base reactions, where relatively acidic calcium orthophosphates react with relatively alkaline orthophosphate producing almost neutral compounds.The first cement that was designed by Brown and Chow is typical example of such cement, where TTCP (alkaline phosphate) reacts with anhydride DCP (weakly acidic phosphate) in aqueous suspension, precipitating poorly crystalline hydroxyapatite (low alkaline phosphate) [8][9][10]: Earlier view was that this reaction gives stoichiometric hydroxyapatite as product.Following studies have found that first formed nuclei were stoichiometric hydroxyapatite that further formed calcium deficient hydroxyapatite.These studies have been confirmed by other researchers.Hydroxyapatite formation by reaction (1) leads to release of acidic or alkaline by-products.pH of cement liquid phase is about 7.5 for TTCP and dicalcium phosphate dihydrate, while pH for TTCP and dicalcium phosphate anhydrous is about 8.The influence of CaO/P ionic relation of TTCP on the properties of TTCP -DCP cement was also the subject of various studies.
In addition to these types of cements, cements based on β-TCP (almost neutral phosphate) and monocalcium phosphate monohydrate (acidic phosphate) that form dicalcium phosphate dihydrate (low alkaline phosphate) have been studied [8,11]: If orthophosphoric acid or anhydrous monocalcium phosphate (instead of monocalcium phosphate monohydrate) and α-TCP (instead of β-TCP or calcium deficient hydroxyapatite) are used, different types of reactions are possible such as [8,11]: Suitable formulations for cement pastes are mixtures of amorphous calcium phosphate with α-TCP or dicalcium phosphate dihydrate, anhydrous dicalcium phosphate and α-TCP, oktakalcijum phosphate and TTCP and partially crystallized calcium orthophosphate and anhydrous dicalcium phosphate.
Another type of networking reaction is hydrolysis reaction of metastable calcium orthophosphate in aqueous medium.In such reaction Ca/P ratio is the same at the beginning and at the end of the reaction.Such components are used as mono phase (mono components) of cement mixture.These types of cements consist of amorphous calcium phosphate, or α-TCP or β-TCP or nanocrystalline or γ-irradiated TTCP and aqueous medium as a second phase, where recrystallization and formation of calcium deficient hydroxyapatite happens in all cases, according to the following reactions [8,[11][12][13]: In all reactions, addition of small amounts of low crystallized hydroxyapatite (about 2%) accelerates kinetics of a given reaction.Monophasic cements containing K + and Na + within the structure of calcium deficient hydroxyapatite (with Ca/P ratio of about 1.64) harden and network after mixing with sodium citrate and sodium orthophosphate.After networking, this mixture provides weak cement (compressive strength of about 15 MPa) composed of ion-substituised calcium-deficient hydroxyapatite, similar to bone mineral.Self-networking cement can be formed from thermally disassembled hydroxyapatite.Hydration process of calcium orthophosphate cements is slightly exothermic and runs in 5 stages: initiation period, induction period, accelerating period, deceleration period and termination period.Heat release during solidification process of calcium orthophosphate is low.This is very good because there is no danger of damaging surrounding tissue.The process of cement solidification is initially controlled by dissolution of reactants (during the first 4 hours) and then by diffusion through the layer of reaction product of calcium deficient hydroxyapatite [8,11,12,13].
Networking of calcium orthophosphate cements occurs most frequently during first 6 hours through the conversion of 80% of reactants into the final product.The volume of cement during networking is almost constant.After hardening the cement is transformed into brittle ceramics with tensile strength that is 5 to 20 times lower than compressive strength.Therefore, these cements are commonly used with metal implants or in places that are not exposed to heavy loading (craniofacial region).All these observations are valuable only for in vitro networking of cements.In in vivo conditions, formation of carbonate hydroxyapatite is favored as compared to noncarbonated calcium deficient hydroxyapatite.

APATITE CEMENTS
Apatite cements precipitate in poorly crystallized hydroxyapatite and/or calcium deficient hydroxyapatite as the end product of networking reaction.Due to the presence of carbonates, apatite cements such as Norian SRS® and Biocement D® form non-stoichiometric carbonate apatites or dalit as end products, the chemical formula [Ca 8.8 (HPO4) 0.7 (PO 4 ) 4.5 (CO 3 ) 0.7 (OH) 1.3 ].Calcium deficient hydroxyapatite and carbonate hydroxyapatite formed in aqueous medium have low crystallinity and are similar to biological apatite of bones and teeth.These qualities are essential for their good absorption in vivo [8,14].
Conventional apatite cements contain TCP and/or TTCP, while monocomponent calcium deficient hydroxyapatite cements contain sodium.Reactivity of apatite cements on the basis of TCP varies depending on the fraction of crystalline tricalcium phosphate phase, its crystallinity and particle size.Generally, high reactivity was observed in less stable phases (increasing from β-TCP, α-TCP to amorphous calcium phosphate).It increases with decreasing particle size.One characteristic of cements is the weak link among crystals (calcium deficient hydroxyapatite or carbonated hydroxyapatite), therefore crystals can be easily separated from cement volume, especially after their partial dissolution.When this happens, osteoclasts and other cells can readily digest apatite crystals.
Immediately after implantation, cement is exposed to blood and other body fluids, which delays its setting time.Internal setting time for apatite cement is between 15 and 20 minutes (for the cement composition by Brown and Chow).This may lead to procedural complications.During that time, the amount of liquid should be reduced to a minimum.Therefore, apatite cements are tractable pastes that are difficult to inject.Networking time can be reduced by adding additives to the fluid phase.Such additives are phosphoric acid, monocalcium phosphate monohydrate and other soluble orthophosphates.These additives facilitate dissolution of solid phase by reducing the pH of the solution.In such cases, the setting time is 10-15 min [8,[14][15][16].
Influence of soluble orthophosphates (such as Na 2 HPO 4 or NaH 2 PO 4 ) on the setting time of apatite cements is explained by dissolution of dicalcium phosphate anhydrate and formation of calcium deficient hydroxyapatite during networking, avoiding the formation of calcium deficient hydroxyapatite in early stage (Figure 1).
Particle size, temperature of liquid phase and initial presence of hydroxyapatite germs in solid phase affect the time of linking.Reducing the particle size strongly affects reduction of initial and terminal binding, accelerates solidification and hydration kinetics during cement hardening.Specific surface area has similar effects (with increasing area the speed of binding increase and vice versa).
Relationship between the particle size of starting calcium orthophosphates and mechanical properties of hardened cements have not been precisely defined, although in some formulations a significant increase in compressive strength was observed.However, results are controversial and depend on many factors; therefore, they vary from author to author.The only certain thing is that all these cements are brittle and cannot be used in areas exposed to loading.Networking process of most apatite cements occurs in accordance with equations (1)(2)(3)(4)(5)(6) in conditions close to physiological pH.For the precipitation of Norian SRS® and Cementek® dominant reactions are reactions of calcium deficient hydroxyapatite or carbonated hydroxyapatite precipitation, through the following steps [8,9]: 5.2CaHPO Initial chemical reaction is fast and causes the formation of dicalcium phosphate dihydrate whereas networking of the cement paste is secondary reaction.The second step in which dicalcium phosphate dihydrate reacts is slower and takes place over several hours with a lag of unreacted α-Ca 3 (PO 4 ) 2 and CaCO 3 according to the equation (8).This step is conditional for the cement hardening.Similar two-step mechanism of hardening is also present in cements that contain monocalcium phosphate monohydrate and CaO.
In the first step, monocalcium phosphate monohydrate reacts with CaO giving dicalcium phosphate dihydrate that in the second step reacts slowly giving calcium deficient hydroxyapatite with the remnants of CaO.Networking mechanism was investigated on three component mixtures TTCP, β-TCP and monocalcium phosphate monohydrate in appropriate proportions to satisfy the ratio of Ca/P = 1.67.At the beginning of the process there were chemical reactions [8,10]: First reaction of formation of dicalcium phosphate dihydrate corresponds to crosslinking stage.Thereafter TTCP reacts with previously created dicalcium phosphate dihydrate and β-TCP giving calcium deficient hydroxyapatite, according to the reactions [8]: and Formation of calcium deficient hydroxyapatite is very slow and corresponds to solidification stage.Although octa-calcium phosphate is not registered, its formation as an intermediate phase has been observed in some studies.According to some studies octa-calcium phosphate, as an intermediate phase, is formed faster than calcium deficient hydroxyapatite, being its precursor.

BRUSHITE CEMENTS
Brushite cements have dicalcium phosphate dihydrate as main product, resulting from crosslinking reactions given in equations (1 and 2).Several formulations are known (β-TCP and monocalcium phosphate monohydrate, β-TCP and H 3 PO 4 and TTCP, monocalcium phosphate monohydrate and CaO).All brushite cements are based on acid-base reactions.Dicalcium phosphate dihydrate precipitates at pH ≤ 6, meaning that brushite crosslinking is happening in acidic conditions.Formulation H 3 PO 4 and β-TCP have advantages over the formulation β-TCP and monocalcium phosphate monohydrate due to: i) easier and faster preparation, ii) better control of chemical composition and reactivity, iii) improved physical and chemical properties, such as longer networking and greater tensile strength due to better homogeneity of cement mixture [8,19,20].
As solubility of calcium phosphate decreases with increasing alkalinity, the time for networking of brushite cements depends on solubility of basic phase.If this phase is more soluble, crosslinking is faster.The time needed for crosslinking of a combination of monocalcium phosphate monohydrate and alkyl calcium phosphate is shorter starting from hydroxyapatite (a few minutes), via β-TCP (30-60 s) to α-TCP (a few seconds).However, hardening takes much longer (a day).Additives that inhibit crystal growth of dicalcium phosphate dihydrate are successfully used to increase the time needed for networking of mixtures of monocalcium phosphate monohydrate and β-TCP.
Unlike apatite cements, brushite cements may be in the liquid form, however, they are crosslinked very fast.They are also biocompatible and bioresorptive.Due to better solubility of dicalcium phosphate dihydrate as compared to calcium deficient hydroxyapatite and metastability of dicalcium phosphate dihydrate under physiological conditions, brushite cements degrade faster than apatite cements.They are quickly resorbed in vivo and have low strength.Short time for networking and low mechanical strength are main reasons why this cement is rarely clinically applied.The use of sodium citrate and citric acid as networking inhibitors provide workable and less viscous pastes of brushite cements.Similar effect can be achieved by adding hondrotoin 4-sulfate and glycolic acid.Cements containing orthophosphoric acid as starting reactant also show better workability.Crosslinking reaction in such cases can be described by chemical reaction [8,20,21]: Some studies have shown that brushite cements can cause tissue inflammation in the first week after in vivo implantation.Such tissue reaction is related to the process of partial transformation of dicalcium phosphate dihydrate into calcium deficient hydroxyapatite, which leads to release of phosphoric acid, according to the reaction [8,21]: Transformation of dicalcium phosphate dihydrate in calcium deficient hydroxyapatite occurs through two successive processes: dissolution process and precipitation process.It can be slowed with the addition of magnesium ions to the cement paste reducing the possibility of inflammation.Reaction (13) indicates that an excess of acid released in the reaction (14), can be compensated by reaction of (13) using β-TCP.Implantation of previously crosslinked brushite material is third option, because solid material is more tolerant as an implant than a paste.
According to some studies, linear velocity of brushite cements degradation is about 0.25 mm per week.This rapid degradation can lead to formation of immature bone.Addition of β-TCP granules to cement paste solves this problem because β-TCP acts as a bone anchor and enhances the formation of mature bone [19,20,21].

RHEOLOGICAL PROPERTIES OF PHOSPHATE CEMENTS
It is well known how to control viscosity of calcium phosphate pastes.They belong to the group of so-called non-Newtonian fluids, in terms of viscosity of fluid shear.Cement pastes possess transient properties, which mean that their viscosity is function of shear stress and time.Calcium phosphate pastes are thixotropic.In addition to particle size (which increasing size reduces viscosity), additives added to cement composites, such as citrate ions and polyacrylic acid, reduce viscosity and cohesion by reducing interaction between the particles.
Networking time is the time for reaching mechanical stability of the cement paste.This time can be experimentally determined using the method of Gillmor's pin or Vikat's test.It is now an open topic for numerous studies.Besides networking time of cement pastes, a special significance has cement solidification rate and its dependence on particle size of the system, addition of nucleation phase or dissolving an appropriate additive (accelerator or retarder) in cement mixture etc. [22].

BIOLOGICAL PROPERTIES OF PHOSPHATE CEMENTS
The current strategy to repair bone defects is based on using bone substituent, which is quickly absorbed creating a new, mature bone (Figure 2).This problem is not related just to chemistry but also to geometry of bone substituent which should be optimized.Especially important are bone substituents in which cells and blood vessels could be easily introduced.Therefore, they must have pores larger than 50 μm.To solve this problem, cement pastes are combined with very soluble solids, hydrophobic liquids or gas bubbles.Unfortunately, gas created macropores are not linked, which limits the success of this strategy.In other approaches pastes are combined with hydrogels such as sodium alginate, dextran, sodium hyaluronate, hydroxypropylmethyl cellulose, etc.The content of hydrogel as solid matter is very low (a few percent), therefore, cells can easily penetrate hydrogel parts between the granules.Another important parameter is particle size of ceramic bone substitute in the paste [23].
Biological response to a paste that contains nano and micrometer particles is different from response to a paste with millimeter particles.Particles less than 5 μm can cause damage.Therefore, caution is needed when using such particles.In the last decade, there has been a great research on high-resorptive bone substituents.Some of these materials, such as β-TCP are resorbed differently from gypsum and brushite that are resorbed by simple dissolution.Under conditions of thermodynamic equi-librium, the solution obtained by dissolution of gypsum has about 10 times more calcium ions than serum bodily fluid.On the other hand, serum does not contain sulfate ions.The result of gypsum dissolution is rapid bone growth which is complicated by the presence of fibrous tissue in defective centers.
Brushite is less soluble than plaster.In physiological fluids it is poorly soluble.By dissolution brushite loses its mechanical strength very quickly, transforming its center in apatite.Also, there is a resulting fibrous part between bone and resorbed cement.However, this part disappears when apatite remains in the block of cement paste.As apatite is alkaline compound, its precipitation makes surrounding medium acidic.Such cement (Brown and Chow cement) provokes in vivo reaction characteristic for brushite cement when implanted in large quantity.The use of fast resorbing cements leads to rapid transformation of bone defects in mature bone but has risks of negative biological response and/or very rapid disappearance from implanted defects [8,24].

CONCLUSION
Phosphate cements show great diversity in terms of chemism of reactions that lead to hardening through their hydration.They are very resorbable, therefore, they can be used as bone replacement materials.Their disadvantages are primarily related to sometimes too rapid resorption of material and lack of adequate mechanical support to the system in which they are used as implants.Their application in tooth fillings, repairing damaged teeth walls, dentin reparations even in in vivo conditions is very interesting and valuable.Combination with different types of polymers and additives can significantly improve some of their characteristics, such as setting time and mechanical strength, which makes them very interesting for applications in various fields of dentistry.

UVOD
Hi drok si a pa tit (HAp) je ve o ma va žan bi o ma te ri jal zbog svo je izu zet ne slič no sti s apa tit nim mi ne ra lom sa dr ža nim u pri rod nim zu bi ma i ko sti ma [1].Kal ci jum-fos fat ni ti po vi ce men ta (CPC) su raz vi je ni zbog spo sob no sti da se pri la go de da tom mo de lu i da to kom pro ce sa bi o lo škog sa zre va nja u or ga ni zmu for mi ra ju HAp [2].Do sad je raz vi je no ne ko li ko raz li či tih vr sta fos fat nog ce men ta [3].CPC prah naj če šće se sa sto ji od te tra kal ci jumfos fa ta (TTCP) [Ca 4 (PO 4 ) 2 O], od no sno tri kal ci jum-fos fa ta [β-Ca 3 (PO 4 ) 2 ] i bez vod nog di kal ci jum-fos fa ta (DCPA) [CaH-PO 4 ], pri če mu se kom po nent ni pra ho vi, da bi se do bi la od gova ra ju ća ce ment na pa sta, me ša ju s vo dom, na kon če ga do la zi do stva ra nja HAp [4].Vr ste CPC po ka zu ju od lič nu bi o kom pa ti bilnost i oste o kon duk tiv nost.Uz to oni se la ko re sor bu ju stva ra ju ći pri to me no vu kost [4].Pr ve re cep tu re ce men ta za sno va nog na fos fat nim kom po nen ta ma pri me nje ne su u hu ma noj kli nič koj prak si 1996.go di ne za po prav ku kra ni o fa ci jal nih ošte će nja [4].Ota da do da nas CPC su se po ka za li ve o ma po god nim za raz liči te den tal ne in di ka ci je, kao što je pu nje nje ka na la ko re na zu ba i sl.Kom po zi ti ko ji uklju ču ju u svo ju re cep tu ru CPC i raz li či te vr ste po li me ra (smo la) mo gu se pri me ni ti za pre kri va nje zubne pul pe i po pu nu ošte će nih zi do va zu ba.Ve o ma su po god ni i za sti mu li sa nje pro ce sa re mi ne ra li za ci je den ti na u uslo vi ma in vi tro [5], dok u kom bi na ci ji sa bi o po li me ri ma, kao što je hi tozan, gra de ve o ma čvr ste i flek si bil ne kom po zi te ko ji mo gu da se pri me ne za pa ro don tal nu re pa ra ci ju ko sti [6].
Je di ni ne do sta tak CPC je što je neo p hod no da se pre upo trebe (na li cu me sta) prah i teč na kom po nen ta sa vr še no iz me ša ju, da bi se for mi ra la ce ment na pa sta ko ja ve o ma br zo stvrd nja va, što mo že da uzro ku je po te ško će pri nje noj pri me ni.Pri pre ma sme se ne po sred no pre upo tre be ima i svo je ozbilj ne ne do stat ke [7], jer po ve ća va vre me hi rur ške in ter ven ci je, a ako se to me još do da da ni sa mo me ša nje i pri pre ma po ne kad ni su za do vo lja vaju ći, on da do la zi do do dat nih pro ble ma u ve zi sa ste ri li za ci jom ma te ri ja la i in stru me na ta, te odr ža va nja neo p hod nog ste ril nog okru že nja.Za rad u ta kvim uslo vi ma me ša nje kom po ne na ta obič no se im pro vi zu je, što mo že da do ve de do to ga da do bi je na pa sta ne ma za do vo lja va ju će re o lo ške oso bi ne, kao i da ko li či na pa ste u da tom tre nut ku ni je za do vo lja va ju ća za po pu nu kra ni ofa ci jal nih ošte će nja.Ove po te ško će pod sta kle su raz voj go to vih vr sta CPC, kod ko jih se CPC prah me ša s ne vo de nim me di ju-mom, ko ji je me šljiv sa vo dom (al ko hol), u do bro kon tro li sa nim uslo vi ma.Pred nost ova kve bez vod ne pa ste je u to me što ona ne očvr šća va u ala tu ko jim se na no si, s ob zi rom na to da CPC očvr šća va hi dra ta ci jom je di no u kon tak tu s vo de nim okru ženjem.Pa sta u ne vo de nom ras tvo ru do vo di se po tom u kon takt s fi zi o lo škim ras tvo rom, unu tar ko jeg se iz me nju je ne vo de ni ras tvor s vo dom, usled če ga pa sta očvr šća va.Pri tom do la zi do po ve ća nja vre me na ve zi va nja, uz ne za do vo lja va ju ću čvr sto ću očvr sle ce ment ne pa ste [7].Dugo vre me ve zi va nja uslo vlja va i od go va ra ju će kli nič ke pro ble me zbog ne sta bil no sti ce ment ne pa ste da to kom svog umre ža va nja odr ži že lje ni ob lik i obez be di me ha nič ke oso bi ne sa mom uzor ku.Zbog to ga su neo p hod ni iz na la že nje po god ni jih for mu la ci ja ce ment nih me ša vi na, uz prime nu raz li či tih do da ta ka, ko ji ubr za va ju ve zi va nje i po ve ća va ju čvr sto ću.Va žno je is ta ći da ne ke for mu la ci je ta kvih me ša vi na po ka zu ju vi sok ni vo ci to tok sič no sti i mo gu iza zva ti će lij sku smrt u uslo vi ma in vi tro.Zbog sve ga na ve de nog, ide al no bi bi lo da se raz vi ju ti po vi ce men ta ko ji ima ju do bre me ha nič ke oso bi ne i očvr šća va ju br zo, a da pri tom ni su tok sič ni.Ta kve vr ste ce men ta, po red osnov nih kom po ne na ta, naj če šće sa dr že raz li či te ti po ve adi ti va i ge li ra ju ćih agen sa, ko ji po ka zu ju najra zli či ti ja bi o lo ška svoj stva i bit no uti ču na sma nje nje vre me na ve zi va nja ce men ta i vi ja bil nost oste o blast nih će li ja ka da se na đu u od go va ra ju ćem in vi vo ili in vi tro okru že nju.
Ra di bo ljeg pre gle da svih mo guć no sti u ve zi s for mu li sa njem osnov nog sa sta va ce ment nih me ša vi na na osno va ma fos fat nih si ste ma, u ra du su, po red ra zno vr snih po la znih for mu la ci ja cement nih me ša vi na, da te i spe ci fič ne re ak ci je ko je se do ga đa ju u pro ce su nji ho ve hi dra ta ci je i umre ža va nja i nji ho vi he mi zmi, či ji je re zul tat na sta nak hi drok si a pa ti ta s ne do stat kom kal ci juma (engl.cal ci um-de fi ci ent hydroxyapa ti te -CDHAp), što je iz u zet no va žno sa sta no vi šta bi o lo ške kom pa ti bil no sti ta kvih si ste ma [8].

FOSFATNI CEMENTI I HEMIJSKE REAKCIJE TIPIČNE ZA NJIHOVO UMREŽAVANJE
He mij ske re ak ci je ko je se do ga đa ju to kom umre ža va nja kalci jum-or to fos fat nih vr sta ce men ta za vi se od nji ho vog sa sta va, a sve se u osno vi svo de na dva glav na ti pa re ak ci ja.Pr vi tip se de ša va pre ma kla sič nim pra vi li ma ki se lo-ba znih re ak ci ja, u ko-ji ma re la tiv no ki se li kal ci jum-or to fos fa ti re a gu ju s re la tiv no baznim or to fos fa ti ma pro iz vo de ći pri bli žno ne u tral na je di nje nja.Pr vi ce ment ko ji su di zaj ni ra li Braun (Brown) i Čou (Chow) ti pi čan je pri mer ta kvog ce men ta, u ko jem TTCP (ba zni fosfat) re a gu je s an hi dri dom DCP (sla bo ki se li fos fat) u vo de noj su spen zi ji, pre ci pi ti ra ju ći u sla bo kri stal ni HAp (sla bo ba zni fos fat) [8,9,10]: Ra ni je se sma tra lo da ta kva re ak ci ja da je kao pro iz vod ste hi ome trij ski HAp.Is tra ži va nja ko ja su po tom sle di la po ka za la su da pr vo na sta ju nu kle u si go to vo ste hi o me trij skog HAp, da bi pri daljem ra stu ovih nu kle u sa na stao ob lik CDHAp.Ta kvi na la zi do bi li su po tvr du i u is tra ži va nji ma mno gih dru gih auto ra.For mi ra nje HAp pre ma re ak ci ji (1) vo di ka ot pu šta nju ili ba znih ili ki se lih nus pro i zvo da.Vred nost pH teč ne fa ze ce men ta je oko 7.5 za TTCP i di kal ci jum fos fat-di hi drat, dok je pH oko 8 za TTCP i an hi dro va ni di kal ci jum fos fat.Uti caj Ca/P jon skog od no sa TTCP na oso bi ne TTCP-DCP ce men ta bio je ta ko đe pred met ne kih is tra ži va nja.
Po red ta kvih vr sta ce men ta, is tra ži va ni su i ti po vi ce men ta na osno vi β-TCP (go to vo ne u tra lan fos fat) i mo no kal ci jum fosfat-mo no hi dra ta (ki se li fos fat), da bi na stao di kal ci jum fos fatdi hi drat (sla bo ba zni fos fat) [8,11]: Ako se kao re a gu ju će kom po nen te ko ri ste or to fos for na kise li na ili an hi dro va ni mo no kal ci jum-fos fat (ume sto mo no kalci jum fos fat-mo no hi dra ta) i α-TCP (ume sto β-TCP ili CDHAp), ta da su mo gu ći raz li či ti ti po vi re ak ci ja, kao što je i ova [8,11]: Kao po god ne for mu la ci je za ce ment ne pa ste ko ri ste se i sme se amorf nog kal ci jum-fos fa ta sa α-TCP ili di kal ci jum fosfat-di hi dra tom, an hi dro va nog di kal ci jum fos fa ta i α-TCP, ok takal ci jum-fos fa ta i TTCP i de li mič no is kri sta li sa nog kal ci jumor to fos fa ta i an hi dro va nog di kal ci jum-fos fa ta.
Dru gi tip re ak ci ja umre ža va nja či ne re ak ci je hi dro li ze meta sta bil nih kal ci jum-or to fos fa ta u vo de noj sre di ni.Kod ta kvih re ak ci ja Ca/P od nos isti je i na po čet ku i na kra ju re ak ci je.Takve kom po nen te ko ri ste se kao mo no fa ze (mo no kom po nen te) ce ment ne sme se.Ta kve vr ste ce men ta se sa sto je od amorf nog kal ci jum fos fa ta, ili α-TCP ili β-TCP, ili na no kri stal nog ili γ-ozrače nog TTCP i vo de nog me di juma kao dru ge fa ze, u ko jem dola zi do re kri sta li za ci je i na sta ja nja CDHAp, u svim na ve de nim slu ča je vi ma, sle de ći re ak ci je da te jed na či na ma [8,11,12,13]: U svim re ak ci ja ma do da tak ma le ko li či ne sla bo is kri sti li sanog HAp (oko 2%) ve o ma ubr za va ki ne ti ku da te re ak ci je.Mo-no fa zni ce men ti ko ji sa dr že K + i Na + unu tar struk tu re CDHAp (sa Ca/P od no som oko 1,64) umre ža va ju i otvrd nja va ju pri me ša nju s ras tvo rom na tri jum-ci tra ta i na tri jum-or to fos fa ta.Po sle umre ža va nja ova sme sa da je slab ce ment (pri ti sne čvrsto će oko 15 MPa) ko ji je sa sta vljen od jon ski sup sti tu i sa nog CDHAp, ko ji je sli čan mi ne ra lu ko sti.Sa mo u mre ža va ju ći ce ment mo že se na pra vi ti i iz ter mič ki raz lo že nog HAp.Hi dra ta ci o ni pro ces kal ci jum-or to fos fat nih vr sta ce men ta je bla go eg zo terman i pro ti če u pet sta di ju ma: ini ci ja ci o ni pe riod, in duk ci o ni pe riod, ak ce le ra ci o ni pe riod, pe riod uspo re nja i ter mi na ci o ni pe riod.Br zi na oslo ba đa nja to plo te to kom pro ce sa očvr šća va nja kal ci jum-or to fos fa ta je ni ska.To je ve o ma do bro jer ne ma opasno sti za ošte će nje okol nih tki va.Pro ces očvr šća va nja ce men ta je u po čet ku kon tro li san ras tva ra njem re ak ta na ta (to kom pr va če ti ri sa ta) i po tom di fu zi jom kroz sloj na sta log pro iz vo da reak ci je CDHAp [8,11,12,13].
Umre ža va nje kal ci jum-or to fos fat nih vrsta ce men ta de ša va se naj če šće to kom pr vih šest sa ti kroz kon ver zi ju 80% re ak ta na ta u fi nal ni pro iz vod.Za pre mi na ce men ta to kom umre ža va nja je sko ro stal na.Po sle očvr šća va nja ce ment se tran sfor mi še u kr tu ke ra mi ku sa za te znom čvr sto ćom ko ja je od pet do dva de set puta ma nja od pri ti sne čvr sto će.Zbog to ga se ta kve vr ste ce men ta naj če šće ko ri ste za jed no s me tal nim im plan ta ti ma ili na me stima ko ja ni su iz lo že na op te re će nju (kra nio fa ci jal na re gi ja).Sva ova za pa ža nja su vred na sa mo u uslo vi ma umre ža va nja ce men ta in vi tro.U uslo vi ma in vi vo naj če šće je na sta ja nje kar bo nat nih HAp u od no su na ne kar bo nat ni CDHAp.

VRSTE APATITNOG CEMENTA
Ti po vi apa tit nog ce men ta pre ci pi ti ra ju u sla bo kri sta li sa ni HAp i/ili CDHAp kao kraj nji pro iz vod re ak ci je umre ža va nja.Zbog za stu plje no sti kar bo na ta, vr ste apa tit nog ce men ta kao što su No rian SRS® i Bi o ce ment D® kao kraj nje pro iz vo de for mi ra ju ne ste hi o me trij ske kar bo nat ne apa ti te ili da lit, he mij ske for mu le Ca 8.8 (HPO4) 0.7 (PO 4 ) 4.5 (CO 3 ) 0.7 (OH) 1.3 .CDHAp i kar bo nat ni HAp na sta li u vo de noj sre di ni ima ju slab kri sta li ni tet i slič ni su bi olo škom apa ti tu ko sti ju i zu ba.Ove oso bi ne su bit ne za nji ho vu ve o ma do bru re sorp ci ju u uslo vi ma in vi vo [8,14].
Kon ven ci o nal ne vr ste apa tit nog ce men ta sa dr že TCP i/ili TTCP, dok mo no kom po nent ne vr ste ce men ta CDHAp sa dr že na tri jum.Re ak tiv nost apa tit nog ce men ta na osno vi TCP va ri ra kao funk ci ja ude la kri stal ne tri kal ci jum fos fat ne fa ze, nje nog krista li ni te ta i ve li či ne če sti ca.Ge ne ral no, vi so ka re ak tiv nost uočena je kod ma nje sta bil nih fa za (ra ste od β-TCP, pre ko α-TCP, ka amorf nom kal ci jum-fos fa tu).Ona ra ste i sa sma nje njem ve li či ne če sti ca.Od li ka ce men ta je sla ba ve za me đu kri sta li ma (CDHAp ili kar bo nat nog HAp), ta ko da se kri sta li mo gu la ko odvo ji ti iz ce ment ne za pre mi ne, po seb no na kon de li mič nog nji ho vog rastva ra nja.Kad se to do go di, oste o kla sti i dru ge će li je mo gu la ko da sva re apa tit ne kri sta le.
Ne po sred no na kon im plan ta ci je sva ka vr sta ce men ta je iz lože na de lo va nju kr vi i dru gih te le snih teč no sti, što od la že vre me nje go vog ve zi va nja.Unu tra šnje vre me ve zi va nja za ti po ve apa titnog ce men ta je iz me đu 15 i 20 mi nu ta (za vr ste ce men ta sa sta va pre ma Bra u nu i Čou), što mo že da do ve de do pro ce du ral nih kom pli ka ci ja.Za to vre me ko li či na teč no sti tre ba da je sve de na na mi ni mum.Zbog to ga, apa tit ni ce ment je la ko ob ra di va pa sta ko ju je te ško ubri zga ti.Vre me umre ža va nja mo že se sma nji ti i adi ti vi ma u teč noj fa zi.To su fos for na ki se li na, mo no kal ci jumfos fat-mo no hi drat i dru gi ras tvor ni or to fos fa ti.Oni po spe šu ju ras tva ra nje čvr ste fa ze sma nje njem pH ras tvo ra.U ta kvim sluča je vi ma vre me ve zi va nja je 10-15 mi nu ta [8,14,15,16].
Uti caj ras tvor nih or to fos fa ta (kao što su Na 2 HPO 4 ili NaH-2 PO 4 ) na vre me ve zi va nja svih vr sta apa tit nog ce men ta ob jašnja va se ras tva ra njem an hi drit nog di kal ci jum-fos fa ta i for mira njem CDHAp to kom umre ža va nja, uz iz be ga va nje for mi ra nja CDHAp u ra noj fa zi (Sli ka 1).Ve li či na če sti ca, tem pe ra tu ra teč ne fa ze i ini ci jal no pri su stvo HAp kli ca u čvr stoj fa zi ta ko đe uti ču na vre me umre ža va nja.Sma nje nje ve li či ne če sti ca ve o ma uti če na sma nje nje vre me na po la znog i kraj njeg ve zi va nja, ubr za nje očvr šća va nja i hi dra taci o nu ki ne ti ku pri očvr šća va nju ce men ta.Na sli čan na čin uti če i spe ci fič na po vr ši na (s nje nim po ve ća njem ra ste i br zi na vezi va nja, i obr nu to).
Ve za iz me đu ve li či ne če sti ca po la znih kal ci jum-or to fos fa ta i me ha nič kih oso bi na očvr slog ce men ta ni je pre ci zno utvr đena, ma da je kod ne kih for mu la ci ja pri me će no znat no po ve ća nje kom pre siv ne čvr sto će.Ipak, po da ci su opreč ni i za vi se od mno gih fak to ra, ta ko da va ri ra ju od auto ra do auto ra.Je di no što je iz ve sno je ste da su sve vr ste apa tit nog ce men ta kr te i ne mo gu se ko ri sti ti na me sti ma ko ja su iz lo že na op te re će nju.Pro ces umre ža va nja ve ći ne ti po va apa tit nog ce men ta de ša va se u skla du s jed na či nama (1-6) u uslo vi ma bli skim fi zi o lo škom pH.Kod pre ci pi ta ci je No rian SRS® i Ce men tek® do mi nant ne su re ak ci je pre ci pi ta ci je u CDHAp ili u kar bo nat ni HAp, kroz sledeće ko ra ke [8,9]: Po la zna he mij ska re ak ci ja je vr lo br za i uslo vlja va na sta ja nje di kal ci jum fos fat-di hi dra ta i ve o ma br zo umre ža va nje ce ment ne pa ste.Dru gi ko rak u ko jem re a gu je di kal ci jum fos fat-di hi drat je spo ri ji i od vi ja se to kom ne ko li ko sa ti uz za o sta ja nje de la ne iz rea go va nog α-Ca 3 (PO 4 ) 2 i Ca CO 3 sa gla sno jed na či ni (8).Taj ko rak uslo vlja va očvr šća va nje ce men ta.Sli čan dvo ste pe ni me ha ni zam očvr šća va nja za pa ža se i kod ti po va ce men ta ko ji sa dr ži mo nokal ci jum fos fat-mo no hi drat i CaO.
U pr vom ko ra ku mo no kal ci jum fos fat-mo no hi drat re a gu je sa CaO, da ju ći di kal ci jum fos fat-di hi drat, ko ji u dru gom ko ra ku re a gu je mno go spo ri je da ju ći CDHAp sa osta ci ma CaO.Me ha nizam umre ža va nja is tra ži van je i na tri kom po nent nim sme sa ma TTCP, β-TCP i mo no kal ci jum fos fat-mo no hi dra ta u od go va ra jućoj pro por ci ji, da bi se za do vo ljio uslov Ca/P=1,67.Od mah na po čet ku pro ce sa do šlo je do he mij skih re ak ci ja [8,10]: Sta di ju mu umre ža va nja od go va ra pr va re ak ci ja na sta ja nja di kal ci jum fos fat-di hi dra ta.Po sle to ga TTCP re a gu je s prethod no na sta lim di kal ci jum fos fat-di hi dra tom i β-TCP da ju ći CDHAp, sa gla sno re ak ci ja ma [8] (12) Re ak ci ja na sta ja nja CDHAp je vr lo spo ra i od go va ra sta diju mu očvr šća va nja.Iako ok ta kal ci jum-fos fat ni je re gi stro van, nje go vo for mi ra nje kao sred nje fa ze uoče no je u ne kim is tra živa nji ma.On se, na i me, br že for mi ra ne go CDHAp, pret ho de ći mu ta ko.

VRSTE BRUŠITNOG CEMENTA
Ti po vi bru šit nog ce men ta su oni u ko ji ma je glav ni pro iz vod di kal ci jum fos fat-di hi drat, ko ji na sta je re ak ci ja ma umre ža va nja da tim jed na či na ma (1 i 2).Ne ko li ko for mu la ci ja ta kvih vr sta ce men ta je po zna to (β-TCP i mo no kal ci jum fos fat-mo no hi drat, β-TCP i H 3 PO 4 i TTCP, mo no kal ci jum fos fat-mo no hi drat i CaO).Svi ti po vi bru šit nog ce men ta za sni va ju se na ki se lo-ba znim reak ci ja ma.Di kal ci jum fos fat-di hi drat pre ci pi tu je pri vred no sti pH od naj vi še 6, ta ko da bru šit umre ža va u ki se lim uslo vi ma.For mu la ci ja H 3 PO 4 i β-TCP-a ima pred no sti u od no su na for mula ci ju β-TCP i mo no kal ci jum fos fat-mo no hi dra ta zbog: 1) lak še i br že pri pre me; 2) bo lje kon tro le he mij skog sa sta va i re ak tivno sti; 3) po bolj ša nih fi zič ko-he mij skih oso bi na, kao što su du že umre ža va nje i ve ća za te zna čvr sto ća usled bo lje ho mo ge no sti ce ment ne sme se [8,19,20].
Ka ko se ras tvor lji vost kal ci jum-or to fos fa ta sma nju je s po veća njem ba zi ci te ta, vre me umre ža va nja bru šit nog ce men ta za vi si od ras tvor lji vo sti osnov ne fa ze, ta ko da ve ćoj ras tvor lji vo sti te fa ze od go va ra br že umre ža va nje ce men ta.Vre me umre ža va nja za kom bi na ci ju mo no kal ci jum fos fat-mo no hi dra ta i ba znih kalci jum-fos fa ta po ve ća va se u ni zu od HAp (ne ko li ko mi nu ta), pre ko β-TCP-a (30-60 se kun di), do α-TCP (ne ko li ko se kundi).Sa mo očvr šća va nje i po red ova ko br zog umre ža va nja tra je mno go du že (dan).Adi ti vi ko ji spre ča va ju rast kri sta la di kal cijum fos fat-di hi dra ta uspe šno se ko ri ste da bi se po ve ća lo vre me umre ža va nja sme sa mo no kal ci jum fos fat-mo no hi dra ta i β-TCP.
Za raz li ku od apa tit nog ce men ta, ti po vi bru šit nog ce men ta mo gu bi ti u po la zu teč ni, da bi se po tom br zo umre ži li.Iz ra zi to su bi o kom pa ti bil ni i bi o re sor bil ni.Za hva lju ju ći bo ljoj ras tvor ljivo sti di kal ci jum fos fat-di hi dra ta u po re đe nju sa CDHAp i me tasta bil no sti di kal ci jum fos fat-di hi dra ta pod fi zi o lo škim uslo vi ma, bru šit ni ce ment br že de gra di ra od apa tit nog.Br zo se re sor bu je u uslo vi ma in vi vo i ima ve o ma sla bu čvr sto ću.Krat ko vre me umre ža va nja i ni ska me ha nič ka čvr sto ća su glav ni raz lo zi što se ovaj ce ment ši re kli nič ki ne pri me njuje.Pri me nom na tri jumci tra ta i li mun ske ki se li ne kao uspo ri va ča umre ža va nja do bi ja ju se vi še ob ra di ve i ma nje vi sko zne pa ste bru šit nog ce men ta.Sličan efe kat po sti že se do dat kom hon dro toin-4-sul fa ta i gli kol ne ki se li ne.Vr ste ce men ta ko je sa dr že or to fos for nu ki se li nu kao po la zni re ak tant ta ko đe po ka zu ju bo lju ob ra di vost.Re ak ci ja umre ža va nja u ta kvim slu ča je vi ma mo že se opi sa ti he mij skom re ak ci jom [8,20,21]: Ne ka is tra ži va nja su po ka za la da bru šit ni ce ment mo že da iza zo ve za pa lje nje tki va u pr voj ne de lji na kon im plan ta ci je in vi vo.Ta kva re ak ci ja tki va ve za na je za pro ces de li mič ne transfor ma ci je di kal ci jum fos fat-di hi dra ta u CDHAp, pri če mu do la zi do oslo ba đa nja fos for ne ki se li ne, sa gla sno re ak ci ji [8,21]: Tran sfor ma ci ja di kal ci jum fos fat-di hi dra ta u CDHAp od vi ja se kroz dva suk ce siv na pro ce sa: pro ces ras tva ra nja i pro ces preci pi ta ci je.Ona mo že bi ti uspo re na sa do dat kom mag ne zi ju mo vih jo na ce ment noj pa sti, ko ji sma nju ju mo guć nost upa le.Re ak ci ja (13) po ka zu je ka ko je mo gu će vi šak ki se li ne oslo bo đen u re ak ci ji (14) kom pen zo va ti re ak ci jom (13) po mo ću β-TCP.Im plan ta ci ja pret hod no umre že nog bru šit nog ma te ri ja la je tre ća op ci ja, zato što je čvr sti ma te ri jal to le rant ni ji kao im plan tat ne go pa sta.
Pre ma ne kim is tra ži va nji ma, za bru šit ni ce ment li ne ar na brzi na raz grad nje je oko 0,25 mm ne delj no.Ova br za de gra da ci ja mo že da vo di for mi ra nju ne zre le ko sti.Do da tak β-TCP gra nu la ce ment noj pa sti re ša va ovaj pro blem, jer β-TCP de lu je kao košta no si dro (an ker) i po spe šu je stva ra nje zre le ko sti [19,20,21].

REOLOŠKE OSOBINE FOSFATNOG CEMENTA
Re la tiv no je po zna to ka ko se kon tro li še vi sko znost kal ci jumfos fat nih pa sti.One pri pa da ju gru pi tzv.ne njut nov skih flu i da, sa sta no vi šta za vi sno sti vi sko zno sti od si le smi ca nja.Ce ment ne pa ste ima ju tzv.tran zi jent ne oso bi ne, što zna či da je nji ho va visko znost funk ci ja na po na smi ca nja i vre me na.Kal ci jum-fos fatne pa ste su tik so trop ne.Po red ve li či ne če sti ca (či ji rast sma nju je vi sko znost) i adi ti vi ko ji su do da ti ce ment nim kom po zi ti ma, kao što su ci trat ni jo ni i po li a kril na ki se li na, ta ko đe uti ču na vi sko znost i ko he zi ju, sma nju ju ći ih usled sma nje nja in ter ak ci je me đu če sti ca ma.
Vre me umre ža va nja je vre me po sti za nja me ha nič ke sta bilno sti ce ment ne pa ste.To vre me se mo že eks pe ri men tal no od redi ti ko ri šće njem me to de Gil mo ro ve (Gil lmor) igle ili Vi ka to vog (Vi kat) te sta.To je i sa da otvo re na te ma broj nih is tra ži va nja.Po red vre me na umre ža va nja za ce ment ne pa ste, po se ban zna čaj ima br zi na očvr šća va nja ce men ta i nje na za vi snost od ve li či ne če sti ca si ste ma, do dat ka nu kle a ci o ne fa ze ili ras tva ra nja od go vara ju ćeg adi ti va (ubr zi va ča ili uspo ri va ča ve zi va nja) u ce ment noj sme si itd.[22].

BIOLOŠKE OSOBINE FOSFATNOG CEMENTA
Sa da šnja stra te gi ja po prav ke ko šta nih ošte će nja u osno vi je veza na za pri me nu ko šta nih sup sti tu en ta, ko ji se br zo re sor bu ju for mi ra ju ći no vu, zre lu kost (Sli ka 2).Ovaj pro blem ni je sa mo pro blem he mi je, već i ge o me tri je ko šta nog sup sti tu en ta ko ju tre ba op ti mi zo va ti.Po seb no su va žni ko šta ni sup sti tu en ti u ko je je la ko uve sti će li je i krv ne su do ve.Zbog to ga oni tre ba da pose du ju po re ko je su ve će od 50 μm.Da bi se re šio taj pro blem, ce ment ne pa ste su kom bi no va ne s ve o ma ras tvor nim čvr stim sup stan ca ma, hi dro fob nim teč no sti ma ili me hu ro vi ma ga sa.Na-ža lost, ma kro po re stvo re ne ga som ni su po ve za ne, što ogra ni ča va uspeh ta kve stra te gi je.U dru gim pri stu pi ma pa ste se kom bi nu ju s hi dro ge lo vi ma, kao što su na tri jum-al gi nat, dek stran, na trijum-hi ja lu ro nat, hi drok si pro pil me til ce lu lo za itd.Sa dr žaj hidro ge la kao čvr ste ma te ri je je vr lo ni zak (ne ko li ko pro ce na ta), ta ko da će li je mo gu la ko da pro dru u hi dro gel ne de lo ve iz me đu gra nu la.Dru gi va žan pa ra me tar je ve li či na ke ra mič kih če sti ca ko šta nog sup sti tu en ta u pa sti [23].
Bi o lo ški od go vor na pa ste ko je sa dr že na no me tar ske i mikro me tar ske če sti ce je dru ga či ji od od go vo ra kad po sto je ve će mi li me tar ske če sti ce.Če sti ce ma nje od 5 μm mo gu da pro uzro ku ju ošte će nja.Za to tre ba bi ti opre zan ka da se ko ri ste ta kve če sti ce.Po sled nje de ce ni je ak tu el na su is tra ži va nja vi so ko resor bil nih ko šta nih sup sti tu e na ta.Ne ki od ovih ma te ri ja la, kao što je β-TCP, re sor bu ju se dru ga či je od gip sa i bru ši ta, ko ji se re sor bu ju pro stim ras tva ra njem.U uslo vi ma ter mo di na mič ke rav no te že, ras tvor do bi jen ras tva ra njem gip sa ima oko de set pu ta vi še kal ci ju ma ne go se rum te le sne teč no sti.Se rum uz to ne sa dr ži sul fat ne jo ne.Kao re zul tat ras tva ra nja gip sa do la zi do ve o ma br zog ra sta ko sti, ko ji je kom pli ko van fi bro znim tki vom u ošte će nim cen tri ma.
U po re đe nju sa gip som, bru šit je za je dan red ve li či ne ma nje ras tvo ran.U fi zi o lo škim teč no sti ma on je sla bo ras tvo ran.Pri ras tva ra nju bru šit ve o ma br zo gu bi svo ju me ha nič ku čvr sto ću, tran sfor mi šu ći se u cen tru u apa tit.Ta ko đe, na sta je fi bro zni deo iz me đu ko sti ju i re sor bu ju ćeg ce men ta.Ipak, taj deo iš če za va ka da apa tit osta ne u blo ku ce ment ne pa ste.Ka ko je apa tit ba zno je di nje nje, nje go va pre ci pi ta ci ja či ni ki se li jim okol ni me di jum.Ta kav ce ment iza zi va re ak ci ju u uslo vi ma in vi vo ko ja je ti pič na za bru šit ni ce ment ka da se im plan ti ra u ve li koj ko li či ni.Ko rišće nje br zo re sor bu ju ćih vr sta ce men ta vo di br zoj tran sfor ma ci ji ko šta nih ošte će nja u zre lu kost, ali no si i ri zi ke ne ga tiv nog bi olo škog od go vo ra i/ili ve o ma br zog iš če za va nja iz im plan ti ra ne za pre mi ne ošte će nja [8,24].

ZAKLJUČAK
Fos fat ni ce ment po ka zu je iz u zet nu ra zno vr snost sa sta no vi šta he mi za ma re ak ci ja ko je vo de njego vom očvr šća va nju kroz proce se hi dra ta ci je.On je i vr lo re sor bi lan, pa se zbog to ga mo že ko ri sti ti kao za me na za kost.Nje go ve lo še oso bi ne ve za ne su pr ven stve no za po ne kad isu vi še br zu re sorp ci ju ma te ri ja la i za ne do volj no ade kvat nu me ha nič ku po dr šku si ste mu kod ko jeg se ko ri sti kao im plan tat.Nje go va pri me na kod is pu na zu ba, re para ci je ošte će nih zi do va zu ba, re pa ra ci je den ti na čak i u uslo vi ma in vi vo ve o ma je dra go ce na.Kom bi na ci jom s raz li či tim vr sta ma po li me ra i adi ti va ne ke nje go ve oso bi ne, kao što su br zi na vezi va nja i me ha nič ka čvr sto ća, mo gu da se bit no una pre de, što ga či ni ve o ma za ni mlji vim za pri me nu u raz li či tim obla sti ma sto ma to lo gi je.

ZAHVALNICA
Rad je po dr žan sred stvi ma Mi ni star stva pro sve te, na u ke i tehno lo škog raz vo ja u okvi ru pro jek ta 172026 i sa stav ni je deo mono gra fi je "Na no me di ci na, naj ve ći iza zov 21. ve ka".