Injectable and Rapid-Setting Calcium Phosphate Bone Cementwith Dicalcium Phosphate Dihydrate

Elena F. Burguera,1 Hockin H. K. Xu,2 Michael D. Weir2

1 Instituto de Ceramica de Galicia, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain

2 Paffenbarger Research Center, American Dental Association Foundation, National Institute of Standards and Technology,Gaithersburg, Maryland 20899-8546

Received 31 March 2005; revised 3 June 2005; accepted 8 June 2005Published online 23 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30403

Abstract: Calcium phosphate cement (CPC) sets in situ with intimate adaptation to thecontours of defect surfaces, and forms an implant having a structure and composition similarto hydroxyapatite, the putative mineral in teeth and bones. The objective of the present studywas to develop an injectable CPC using dicalcium phosphate dihydrate (DCPD) with a highsolubility for rapid setting. Two agents were incorporated to impart injectability and fast-hardening to the cement: a hardening accelerator (sodium phosphate) and a gelling agent(hydroxypropyl methylcellulose, HPMC). The cement with DCPD was designated as CPCD,and the conventional cement was referred to as CPCA. Using water without sodium phosphate,CPCA had a setting time of 82 � 6 min. In contrast, CPCD exhibited rapid setting with a timeof 17 � 1 min. At 0.2 mol/L sodium phosphate, setting time for CPCD was 15 � 1 min,significantly faster than 40 � 2 min for CPCA (Tukey’s at 0.95). Sodium phosphate decreasedthe paste injectability (measured as the paste mass extruded from the syringe divided by theoriginal paste mass inside the syringe). However, the addition of HPMC dramatically in-creased the paste injectability. For CPCD, the injectability was increased from 65% � 12%without HPMC to 98% � 1% with 1% HPMC. Injectability of CPCA was also doubled to 99%� 1%. The injectable and rapid-setting CPCD possessed flexural strength and elastic modulusvalues overlapping the reported values for sintered porous hydroxyapatite implants andcancellous bone. In summary, the rapid setting and relatively high strength and elasticmodulus of CPCD should help the graft to quickly attain strength and geometrical integritywithin a short period of time postoperatively. Furthermore, the injectability of CPCD mayhave potential for procedures involving defects with limited accessibility or narrow cavities,when there is a need for precise placement of the paste, and when using minimally invasivesurgical techniques. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 77B:126–134, 2006

Keywords: calcium phosphate cement; hydroxyapatite; injectability; rapid setting; bonerepair

INTRODUCTION

Approximately one million bone grafts are performed eachyear in the United States to treat osseous defects.1,2 This needis increasing dramatically as the world population ages.3

Autograft, the gold standard in bone repair, is restricted bybone availability, donor site morbidity, and contouring diffi-culty. Allografts and xenografts raise concerns of immunore-

jection and disease transmission. These factors are providingmuch of the driving force for the development of syntheticbiomaterials. Calcium phosphate biomaterials have gainedclinical acceptance for bone substitution and augmenta-tion.4–11 Porous calcium phosphate scaffolds have been de-veloped to facilitate tissue ingrowth12–14 by using controlledpore architectures15 and three-dimensional fabrication tech-niques.16

Calcium phosphate cements can be easily manipulated andshaped, provide intimate adaptation to the contours of defectsurfaces, and set in situ in the bone cavity to form a solidrestoration.17 The concept of calcium phosphates as possiblecement materials was first introduced in 1982.18 The firstcalcium phosphate cement (CPC) was reported in 1987.17

Since then, many compositions have been formulated.17,19–22

Correspondence to: H. Xu, (e-mail: hockin.xu@nist.gov)E. F. Burguera is now with Materials Science and Engineering Department, Vir-

ginia Polytechnic Institute and State University, Blacksburg, VA.Contract grant sponsor: USPHS NIH; contract grant number: R01 DE14190Contract grant sponsor: NISTContract grant sponsor: ADAF

© 2005 Wiley Periodicals, Inc.

126

CPC is comprised of a mixture of tetracalcium phosphate[TTCP: Ca4(PO4)2O] and dicalcium phosphate anhydrous(DCPA: CaHPO4).17 The CPC powder can be mixed with anaqueous liquid to form a paste23–26 where the water providesa vehicle for the dissolution of the reactants and the precip-itation of the product. The set cement has a structure andcomposition similar to hydroxyapatite, the putative mineralin teeth and bones: 2Ca4(PO4)2O � 2CaHPO4 3Ca10(PO4)6(OH)2.17,26 Due to its excellent osteoconductivityand bone replacement capability, CPC is highly promising fora wide variety of clinical applications.17,23–26

In other in vitro studies, dicalcium phosphate dihydrate(DCPD, CaHPO4 � 2H2O), a compound with a relatively highsolubility, was used to replace the DCPA component in CPCto form hydroxyapatite 27,28: 2Ca4(PO4)2O � 2CaHPO4 �2H2O 3 Ca10(PO4)6(OH)2 � 4H2O. The conventional ce-ment from DCPA–TTCP was referred to as CPCA, and thecement from DCPD–TTCP was termed CPCD.28 The highsolubility of DCPD resulted in faster setting and higher earlystrength for CPCD.28 Thirty minutes after powder and liquidmixing, CPCD developed a flexural strength of 4.2 MPa,while CPCA was not set and consequently had no strength.28

Fast setting is desirable because a long setting time can resultin the crumbling of the inserted paste when it comes in earlycontact with physiological fluids, or when bleeding occursdue to the difficulty to achieve complete hemostasis in somecases.21,29,30 High early strength is needed to prevent early-stage implant failure or disintegration.28

In addition to rapid setting and early strength, the inject-ability of a CPC paste is also an important property. This isespecially true in procedures involving defects with limitedaccessibility or narrow cavities, when there is a need forprecise placement of the paste to conform to a defect area,and when using minimally invasive surgical techniques.31–37

Several factors affecting the injectability of a number ofcalcium phosphate cements have been examined.31–37 How-ever, the injectability of the high early-strength CPCD

27,28

remains to be investigated.The objective of the present study was to investigate the

injectability of CPCD in comparison with the conventionalCPCA, and to examine the effects of incorporating a harden-ing accelerator and a gelling agent. The tested hypotheseswere: (1) incorporating sodium phosphate into the cementliquid would impart rapid setting to the cement, but woulddecrease the paste’s injectability; (2) incorporating hy-droxypropyl methylcellulose would improve the paste’s in-jectability, without compromising the mechanical strength;and (3) the use of DCPD with its high solubility for rapidsetting would not reduce the paste’s injectability compared toDCPA.

MATERIALS AND METHODS

Synthesis of Cement Powders

TTCP was synthesized from a solid-state reaction betweenCaHPO4 and CaCO3 (J. T. Baker Chemical, Phillipsburg,

NJ), which were mixed and heated at 1500°C for 6 h in afurnace (Model 51333, Lindberg, Watertown, WI).17 Theheated mixture was quenched to room temperature andground dry in a ball mill (Retsch PM4, Brinkman, NY). TheTTCP powder was then sieved using a standard testing sieve(W. S. Tyler Inc., Mentor, OH) with openings of 38 �m toremove the large particles. The TTCP powder that wentthrough the sieve was collected and analyzed. The particlesize distribution was measured by a sedimentation methodwith the use of a centrifugal particle analyzer (SA-CP3,Shimazu, Kyoto, Japan), which yielded a TTCP particle sizerange of about 1 �m to 60 �m and a median particle size of20 �m. The commercial DCPA powder was ground and thenanalyzed using the same particle size analyzer, which re-ported a particle size range of 0.4 �m to 6 �m and a medianparticle size of 1.2 �m. The TTCP and DCPA powders weremixed in a micromill (Bel-Alert Products, Pequannock, NJ)in equimolar amounts to form the powder for the cementdesignated as CPCA.

Commercial DCPD powders were first used in our pilotstudies,27 but the resulting TTCP–DCPD pastes exhibitedundesirable long setting times of � 1 h. This was likely dueto unidentified impurities in the commercial powders.27

Hence DCPD in the present study was prepared in our labo-ratory. To synthesize DCPD, the pH of a DCPD–monocal-cium phosphate monohydrate singular point solution (pH �1.9, 4°C) was slowly raised via the addition of CaCO3.27,28

DCPD that precipitated before the pH reached 3.5, which issignificantly below the hydroxyapatite–DCPD singular pointof 4.2, was collected to avoid possible contamination of theDCPD by hydroxyapatite.27,28 The DCPD was ground andthen analyzed using the same particle size analyzer thatshowed a particle size range of 0.5 �m to 4 �m and a medianparticle size of 1.3 �m. The DCPD powder was then mixedwith the TTCP powder at a molar ratio of 1:1 to form thepowder for the cement referred to as CPCD.

Cement Liquids

Two types of liquid were used: (1) aqueous sodium phosphatesolution and (2) aqueous sodium phosphate–hydroxypropylmethylcellulose solution. The sodium phosphate solutionswere prepared by diluting a 3 mol/L phosphate solution(Na2HPO4 and NaH2PO4, Abbott Laboratories, North Chi-cago, IL) with distilled water at the following sodium phos-phate concentrations: 0 mol/L (distilled water without sodiumphosphate), 0.1 mol/L, 0.2 mol/L, 0.3 mol/L, 0.4 mol/L, and0.5 mol/L. In a previous study,30 sodium phosphate impartedfast setting to CPCA. But the very fast-setting property mightexert adverse effects on the paste injectability because ofpaste thickening and stiffening caused by the rapid setting.Hence, the purpose here was to examine the effect of sodiumphosphate on setting time as well as injectability for bothCPCA and CPCD.

For the second type of liquid, hydroxypropyl methylcel-lulose (HPMC, Sigma, St. Louis, MO) was added to theliquid containing 0.2 mol/L sodium phosphate because this

127INJECTABLE AND RAPID-SETTING CALCIUM PHOSPHATE CEMENT

concentration imparted rapid setting while only moderatelyreducing the paste’s injectability. HPMC mass fractions inthe liquid were: 0 % (without HPMC), 0.5 %, 1 %, 2 %, and3 %. HPMC was used here as a gelling agent because previ-ous studies showed that it improved paste cohesive-ness.30,38,39

Setting Time Measurement

Two groups of specimens were tested. The first group had a2 � 6 full factorial design, with two cements (CPCD andCPCA) and the sodium phosphate solution having the sixconcentrations described above.

The second group had a 2 � 5 full factorial design, withtwo cements (CPCD and CPCA) and five liquids having thefive HPMC mass fractions described above.

Each powder and liquid were manually mixed with aspatula at a powder:liquid mass ratio of 2:1, slightly lowerthan the 2.5:1 used in a previous study on injection.32 Thepurpose of the lower ratio was to form a flowable paste whileproviding room for additions of macropore forming parti-cles40 and reinforcing fibers41,42 to the paste in future studies.The paste was filled into a stainless-steel mold of 6 mmdiameter and 3 mm depth.39 Each specimen was incubated ina humidor with 100% relative humidity at 37°C. Followingthe method used in previous studies,30,38,39 when the powdercomponent of the specimen did not come off when scrubbedgently with fingers, the setting reaction had occurred enoughto hold the specimen together. The time measured from thepowder and liquid mixing to this point was used as the settingtime.30,38,39

Injectability Testing

The same factorial designs as in the Setting Time Measure-ment were subjected to an injectability test. A 10-mL syringe(Free-Flo, Kerr, Romulus, MI) with a diameter of 10 mm andan opening of 2.8 mm was used. The syringe was similar tothose used in previous studies on calcium phosphate injectionwhile the opening was slightly larger than the 2 mm usedearlier.31,32 This was because we planned to add strengthen-ing fibers28,41,42 of possibly 3-mm length to the injectablepaste in future studies. Each cement powder of 2 g was mixedwith 1 g of the corresponding liquid and placed into thesyringe. The syringe was placed between the compressionplates of a computer-controlled Universal Testing Machine(5500R, MTS Systems Corp., Cary, NC). At 1.5 min from themixing, compression was started and the cement was ex-truded from the syringe at a crosshead speed of 15 mm/minuntil a maximum force of 100N was achieved, following themethods reported in previous studies.31,32 Injectability wasdetermined as the mass of the paste extruded from the syringedivided by the original mass of the paste inside the sy-ringe.31,32,36 The force of 100N was selected because it couldbe applied in clinical applications.31,32

Specimen Fabrication

To compare mechanical properties of injectable CPCD andCPCA and to investigate the effect of incorporating HPMC, a

2 � 5 full factorial design was tested with two materials(CPCA and CPCD) and five liquids with the five HPMCfractions described in Cement Liquids. Steel molds of 3mm � 4 mm � 25 mm were used to make flexural speci-mens.28,41,42 Each specimen was allowed to set in the humi-dor for 4 h, then demolded and immersed in distilled water at37°C for 20 h.30,40 The CPC conversion to hydroxyapatitewas shown to be largely complete during the 24 h incuba-tion.17,26,40

Mechanical Testing

Specimens were fractured using a three-point flexural testwith 20-mm span at a crosshead speed of 1 mm/min on thesame Universal Testing Machine.40,41 Four-point flexure is apreferred test because it samples a volume of the speci-men,43,44 while three-point flexure samples a thin plane in thespecimen. When the loading was started in four-point flexure,usually one of the two upper pins of the four-point fixturetouched the specimen first. After preloading, the two loadingpins then both touched the specimen and shared the appliedload. Some of the relatively weak CPC specimens brokeunder the first touching pin, instead of breaking somewherebetween the two loading pins. A flexural strength of 3 MPa to5 MPa (Figure 3) is much lower than, for example, 60 MPaof a dental composite43 and 200 MPa of a dental glass–ceramic.44 Therefore, three-point flexure was used in thepresent study. The flexural strength was calculated by S �3PmaxL/(2bh2), where Pmax is the maximum load on theload-displacement (P-d) curve, L is the span, b is specimenwidth, and h is specimen thickness. The elastic modulus E �(P/d)(L3/[4bh3]), where P divided by the corresponding d isthe slope of the P-d curve in the linear elastic region. Thework-of-fracture (toughness) was calculated by WOF �A/(bh), where A is the area under the P-d curve, which is thework done to deform and fracture the specimen.40,41

Microscopy and Statistics

The fracture surfaces of selected specimens were sputtercoated with gold and examined with a scanning electronmicroscope (SEM, JEOL 5300, Peabody, MA). Two-way andone-way analyses of variance (ANOVA) were performed todetect significant effects of sodium phosphate concentrationand HMPC mass fraction on setting time, injectability, andmechanical properties. Tukey’s multiple comparison wasused to compare the data at a family confidence coefficient of0.95. Linear and nonlinear regressions were used to analyzethe data. One standard deviation was used as the estimatedstandard uncertainty of the measurements. These valuesshould not be compared with data obtained in other labora-tories under different conditions.

RESULTS

Effects of Sodium Phosphate

Figure 1 plots setting time T and injectability I versus sodiumphosphate concentration, C. Two-way ANOVA identified

128 BURGUERA ET AL.

significant effects (p � 0.001) of C and cement composition(CPCD vs. CPCA), with a significant interaction between thetwo factors (p � 0.001). Adding sodium phosphate to theliquid resulted in faster setting. The curves in Figure 1(A)were best fits using a three-parameter exponential decaymodel.45 This yielded T � 35 � 47e�10C (correlation coef-ficient, R2 � 0.99) for CPCA, and T � 11 � 7e�3C (R2 �0.92) for CPCD. They show that T decreased at a higherpower of 10 for CPCA than the 3 for CPCD. As C increased,T approached 35 min for CPCA and 11 min for CPCD. Ateach sodium phosphate concentration, CPCD had a shortersetting time than CPCA (Tukey’s multiple comparison atfamily confidence coefficient of 0.95).

Increasing sodium phosphate concentration significantlydecreased the paste injectability, I [Figure 1(B)]. The lines in

Figure 1(B) are linear best fits to the experimental data,yielding I � 99–173C for CPCD with R2 � 0.92, and I �90–155C for CPCA with R2 � 0.95. The two materialsexhibited a similar trend of rapidly decreasing injectability,with I at 0.5 mol/L sodium phosphate being about 1/5 of theinjectability at 0 mol/L sodium phosphate.

Effects of Hydroxypropyl Methylcellulose (HPMC)

The above experiment showed that the addition of sodiumphosphate imparted rapid setting, but also decreased the pasteinjectability. Hence, HPMC was added to the liquid contain-ing an intermediate concentration of 0.2 mol/L sodium phos-phate to improve the paste’s injectability. Figure 2(A) showsthat HPMC did not adversely prolong the setting time. Ac-tually, both CPCD and CPCA exhibited shorter setting timesat intermediate HPMC concentrations. For CPCA, setting

Figure 2. (A) Setting time and (B) injectability versus HPMC. Thecement liquid contained 0.2 mol/L sodium phosphate. Adding HPMCto the liquid did not prolong the setting time; in fact, the setting timewas reduced at HPMC fractions of 0.5% and 1%. The addition ofHPMC dramatically increased the paste injectability. Each value ismean � SD; n � 4.

Figure 1. (A) Cement setting time and (B) injectability versus sodiumphosphate concentration. The cement liquid contained no hy-droxypropyl methylcellulose (HPMC). Each value is the mean of fourmeasurements with the error bar showing one standard deviation(mean � SD; n � 4). The conventional CPCA had a long setting timewith water, while adding sodium phosphate significantly decreasedthe setting time. The new CPCD exhibited much faster setting thanCPCA. Increasing the sodium phosphate concentration decreased theinjectability.

129INJECTABLE AND RAPID-SETTING CALCIUM PHOSPHATE CEMENT

time (mean � SD; n � 4) was 32 � 1 min at 1% HPMC,significantly faster than 40 � 2 min at 0% HPMC and 39 �1 min at 3% HPMC (p � 0.05). Setting time of CPCD wasabout three times faster than CPCA, and reached 11 � 1 minat 1% HPMC.

HPMC addition dramatically increased the paste inject-ability [Figure 2(B)]. For CPCD, injectability was increasedfrom 65% � 12% without HPMC to the maximum measur-able 98% � 1% with 0.5% HPMC. For CPCA, injectabilityachieved the maximum measurable 99% � 1% with 1%HPMC. Both CPCs remained at the maximum injectability asthe HPMC concentration was increased to 2% and 3%.

Mechanical Properties

HPMC improved paste injectability. To examine whetherHPMC adversely affects mechanical properties, the flexuralstrength, elastic modulus, and work-of-fracture were mea-sured (Figure 3). Both CPCD and CPCA showed no decreasein strength with increasing HPMC fraction. CPCD had sig-nificantly higher strengths than CPCA at 0%, 1%, and 2% ofHPMC (p � 0.05). CPCD had significantly (p � 0.05) higherwork-of-fracture than CPCA at all HPMC fractions except3%. For elastic modulus, two-way ANOVA showed no sig-nificant effect for HPMC (p � 0.073) or cement composition(CPCD vs. CPCA) (p � 0.756).

Fracture Surfaces

SEM micrographs of fracture surfaces are shown in Figure 4for CPCA. CPCA had small crystals that appeared to beneedle-shaped (arrow), with a thickness of approximately 100nm and a length of 300 nm. No significant differences werefound between fracture surfaces with or without HPMC.

In Figure 5(A), CPCD consisted of a mixture of needle-shaped nanocrystals and platelets. Occasionally, relativelylarger platelet crystals were also observed in the fracturesurfaces of CPCD [Figure 5(B)]. The features in Figure5(A,B) were observed in CPCD without HPMC or withHPMC. The relatively large platelets were not observed inCPCA. As reported in previous studies, crystals of suchshapes and sizes were not present in the starting powder ofthe cement, and X-ray diffraction analysis showed that the setcement had converted to hydroxyapatite.28

DISCUSSION

Injectable calcium phosphate cements can be placed easily insurgery and have potential for minimally invasive techniques.This may widen the applications of CPC to surgical sites thatare not freely accessible by open surgery and areas thatinvolve narrow defects and in situ fracture fixation. It hasbeen reported by surgeons that calcium phosphate cementsare poorly injectable.35,37 This is because when a calciumphosphate powder and water are mixed into a paste anddelivered through a wide bore needle or cannula, a filter-pressing phenomenon can occur in which the liquid is pushed

out but a major portion of the powder remains inside thesyringe, leading to the phase separation of liquid and solid.36

Previous studies on calcium phosphate cements have inves-tigated the dependence of injectability on powder-to-liquid

Figure 3. (A) Flexural strength, (B) work-of-fracture (toughness), and(C) elastic modulus versus hydroxypropyl methylcellulose (HPMC)mass fraction. The liquid contained 0.2 mol/L sodium phosphate.Each value is the mean � SD; n � 6.

130 BURGUERA ET AL.

ratio and time after mixing,31 particle shape,35 and particlesize.37 The effects of adding a polymeric drug,32 incorpora-tion of citric acid,33 use of oscillatory mixing,46 and ionmodification36 have also been shown to influence the inject-ability.

Injectability and rapid setting can be opposing propertiesbecause a rapid setting paste may start setting in the syringethus increasing the paste thickness and rigidity, and reducingits injectability. Improvements in injectability via powder orliquid modifications may inadvertently result in slow setting.Therefore, any enhancement in paste injectability should notcompromise other critical properties, such as setting time. Forexample, the addition of glycerol improved the injectabilityof a calcium phosphate cement, but greatly increased itssetting time.47 A long setting time could cause problemsbecause of the cement’s inability to support stresses duringthis time period.21,29,30 For example, a severe inflammatory

response was observed when CPC failed to set and disinte-grated, likely due to low initial mechanical strength.21,29

In the present study, using dicalcium phosphate dihydrateprepared in our laboratory, a gelling agent, and a hardeningaccelerator, a unique cement was developed that was not onlyinjectable but also rapid setting. CPCD with 0.2 mol/L sodiumphosphate and 1% HPMC achieved a setting time of 11 mintogether with 99% of paste injectability under the specifiedexperimental conditions. In comparison, the conventionalCPCA was injectable but had a long setting time of 32 mmwith 0.2 mol/L sodium phosphate and 1% HPMC. The rapidsetting of the injectable CPCD should yield a graft that canattain early strength and geometrical integrity.

Sodium phosphate concentration had a significant effecton setting time [Figure 1(A)]. TTCP [Ca4(PO4)2O], DCPD(CaHPO4 � 2H2O), and DCPA (CaHPO4) dissolved in water

Figure 5. SEM of fracture surfaces of CPCD showing in situ formationof hydroxyapatite crystals at body temperature (37°C). (A) CPCD

consisted of nanosized crystals and platelets. Arrow indicates a plate-let. (B) Larger platelet hydroxyapatite crystals were occasionally ob-served. No significant differences were observed with or withoutHPMC.

Figure 4. SEM of specimen fracture surfaces showing the in situformation of nanohydroxyapatite crystals at body temperature (37°C).CPCA had nanosized hydroxyapatite crystals that were elongated(arrow). The cement liquid contained 0.2 mol/L sodium phosphate. Nosignificant differences were observed in fracture surfaces of speci-mens with or without HPMC.

131INJECTABLE AND RAPID-SETTING CALCIUM PHOSPHATE CEMENT

as Ca2�, PO43� and OH� ions, which then reprecipitated to

form hydroxyapatite, Ca10(PO4)6(OH)2. The Na2HPO4 in theliquid increased the phosphate concentration, which in turnaccelerated the setting reaction to form hydroxyapatite. How-ever, such rapid setting adversely and dramatically decreasedthe paste’s injectability [Figure 1(B)]. This drawback wasovercome via the addition of HPMC [Figure 2(B)].

HPMC is a derivative of cellulose, which is one of themost commonly occurring polysaccharides, and it is noncy-totoxic.30,38,39 It was used in this study as a gelling agentbecause of its ability to hydrogen bond to water keeping it inthe solution, thereby forming a viscous solution that improvesCPC’s washout resistance.30,38,39 The mechanism via whichHPMC improved the injectability was likely that HPMCimproved the paste cohesiveness and made it more difficultfor the solid phase and the liquid phase to separate. Thepresence of HPMC probably also provided lubrication thatallowed the solid particles in the paste to slide over eachother, making the paste easier to move during injection. InFigure 2(B), the pastes with HPMC concentrations of 1% to3% had all passed the injectability test. The injectability testof the present study did not further identify the differencesbetween all the pastes above the “pass” level. It should benoted that the injectability of a paste is not an absolute value;it depends on the test conditions including the orifice size andthe injection force. When these parameters change (which isto be experimented in further studies), the paste injectabilityvalues may vary.

The mechanism via which HPMC reduced the setting timeat intermediate fractions [Figure 2(A)] was likely that HPMCimproved the distribution of the particles in the paste. TTCPparticles had a median size of 20 �m. DCPD and DCPAparticles are much smaller, with median sizes of 1.3 �m and1.2 �m, respectively. Due to their high surface areas, thesmaller particles tend to stick together and form agglomerates(Figure 2 in Ref. 27). It is possible that a thin HPMC filmcoated the individual particles, thus separating the particlesfrom each other and thus preventing the particles from form-ing agglomerates, resulting in a more homogeneous particledistribution in the paste. In the case of CPCD, instead ofDCPD particles contacting other DCPD particles in an ag-glomerate, a more homogeneous distribution may enhancethe reaction between the acidic DCPD particles and the basicTTCP. The same applies to CPCA. The setting time wasshortened for both CPCD and CPCA at 1% of HPMC (Figure3). However, when the HPMC content was increased to 3%,the setting time increased. The liquid with 3% HPMC becamequite viscous, which likely retarded the movement of Ca2�,PO4

3� and OH� ions to form hydroxyapatite. Indeed a previ-ous study showed that the CPC conversion to hydroxyapatitewas slowed when the content of another gelling agent chi-tosan was increased in the cement liquid.48 Further studies areneeded to examine these mechanisms, the distribution ofparticles in the paste, and the effects of particle size37 andshape35 on injectability and cement setting.

Improvement in calcium phosphate injectability viaHPMC did not adversely affect the mechanical properties

(Figure 3). Flexural strength of sintered porous hydroxyapa-tite implants was reported to range from 2 MPa to 11 MPa.10

Cancellous bone had a tensile strength of about 3.5 MPa.49

The rapid-setting and injectable CPCD, developed in thepresent study, possessed a flexural strength of approximately5 MPa. While the measurement methods may differ and thestrengths may not be directly comparable, the strength ofCPCD appeared to overlap the strengths of sintered poroushydroxyapatite and cancellous bone. Sintered hydroxyapatiteimplants require machining to fit a prepared bony defect, andare usually not resorbable or replaceable by new bone.50,51

Histologic analyses showed that nonresorbable hydroxyapa-tite implants had induced little new bone fill, and very lim-ited, if any, periodontal bone regeneration.51 In comparison,the injectable CPCD of the present study can be placed andshaped easily with intimate contacts to neighboring bone, andharden in situ to form hydroxyapatite. While animal studiesare needed to examine the bioresorbability of the injectableCPCD, hydroxyapatite from CPC is in general biocompatibleand resorbable.23–26 This is because the latter is formed in anaqueous environment at body temperature, hence is moresimilar to biological apatites than sintered hydroxyapatiteformed at high temperatures.52 The elastic modulus of theinjectable CPCD was approximately 2.5 GPa. This comparesto an elastic modulus measured in flexure of 12.8 GPa forcortical bone53 and 0.3 GPa for cancellous bone.54

SUMMARY

A rapid-setting and fully injectable calcium phosphate ce-ment was developed via the use of tetracalcium phosphate,dicalcium phosphate dihydrate, and the combined incorpora-tion of a hardening accelerator and a gelling agent. Thehardened cement possessed strength and elastic modulus val-ues overlapping those of sintered porous hydroxyapatite im-plants and cancellous bone. The rapid setting ability and arelatively high strength should help protect the implant fromcatastrophic fracture or disintegration under stresses. SEMrevealed the formation of nanosized rodlike hydroxyapatitecrystals and slightly larger platelet crystals in the cement.Compared to sintered hydroxyapatite, the new calcium phos-phate cement has advantages including complete injectabil-ity, in situ hardening, formation of hydroxyapatite, and closecontact with neighboring bone. It has the potential to bedelivered through needles and applicators for practices thatinvolve minimally invasive methods, narrow defects, andsites of limited accessibility for open surgeries, while stillproviding mechanical strength and elastic modulus matchingthose for sintered porous hydroxyapatite implants and can-cellous bone.

We thank Drs. S. Takagi, L. C. Chow, F. C. Eichmiller, G. E.Schumacher, and S. H. Dickens for discussions, and A. A. Giusep-petti and L. E. Carey for assistance. E. F. Burguera acknowledgesthe Great-West Life Annuity for financial support through theirscientist-in-training program. Certain commercial materials andequipment are identified in this paper to specify experimental pro-

132 BURGUERA ET AL.

cedures. In no instance does such identification imply recommen-dation by NIST or the ADA Foundation or that the material identi-fied is necessarily the best available for the purpose.

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