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Acta Biomaterialia 4 (2008) 1465–1471

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Mechanisms underlying the limited injectability of hydrauliccalcium phosphate paste

Mohamed Habib a, Gamal Baroud a,*, Francois Gitzhofer b, Marc Bohner c

a Laboratoire de Biomecanique, Departement de Genie, Universite de Sherbrooke, Sherbrooke, QC, Canada J1K 2R1b Centre de Recherche en Energie, Plasma et Electrochimie (CREPE), Universite de Sherbrooke, Sherbrooke, QC, Canada J1K 2R1

c Dr. Robert Mathys Foundation, Bettlach, Switzerland

Received 20 October 2007; received in revised form 10 February 2008; accepted 20 March 2008Available online 1 April 2008

Abstract

Calcium phosphate (CaP) cements are being increasingly used for minimally invasive hard tissue implantation. Possible approaches toimprove the bad injectability of hydraulic calcium phosphate pastes have been discussed and investigated in a number of recent publi-cations. However, the liquid-phase separation mechanism leading to the limited injectability has not yet been addressed. Liquid-phaseseparation means that the liquid-to-powder ratio (LPR) of the extruded paste is higher than the LPR of the paste left in the syringe. Thegoal of this paper was to remedy this situation by looking at the liquid-phase migration occurring during the injection of a paste from asyringe through a cannula. Experimentally, it was seen that the liquid content of both the syringe paste and the extrudate decreased dur-ing the paste injection. Moreover, a high extrusion velocity, small syringe size, short cannula and high LPR favored a good injectability.These results could be partly explained in light of rheological measurements performed with the investigated paste.� 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Injectable bone substitute; Calcium-phosphate applications; Limited injectability; Experiments

1. Introduction

CaP cements are being increasingly used in minimallyinvasive interventions to treat fragility fractures [1]. Thisis due to these cements possessing a number of interestingproperties. Namely, their structure and composition resem-bles human bones, resulting in good biocompatibility andosteoconductivity [2]. In addition, they set practically iso-thermally compared to acrylic cements [3]. However, theyhave some critical drawbacks. One of them is their poorability to be injected through a thin long cannula attachedto a syringe, such as in minimally invasive clinical applica-tions [4–6].

The injectability of these cements has been discussedrecently in a number of publications (Table 1). There seems

1742-7061/$ - see front matter � 2008 Acta Materialia Inc. Published by Else

doi:10.1016/j.actbio.2008.03.004

* Corresponding author. Tel.: +1 819 821 8000x61344; fax: +1 819 8217163.

E-mail address: Gamal.Baroud@USherbrooke.ca (G. Baroud).

to be agreement on the difficulty of injecting cements.However, there is presently no common understanding ofthe meaning of ‘‘injectability”. In most research works,injectability has been related to the viscosity of the CaPcement, or in other words to the injection force requiredto deliver the cement paste, regardless of its quality orhomogeneity [5–8]. A recent study conducted by Bohnerand Baroud [4] introduced the concept of filter pressing,in which the pressure applied to the cement paste provokesa phase separation after a certain injection time: the liquidcomes out without the particles. However, neither the loca-tion nor the mechanism of the filter pressing phenomenonwas examined. In this study, we focused on the mechanismunderlying the limited injectability of CaP cements. First,the uniformity of the extruded cement was studied overthe course of the injection to gain a better understandingof the separation forces. In addition, several processparameters, such as liquid-to-powder ratio (LPR), plasticlimit (PL), delivery rate and geometry, were investigated.

vier Ltd. All rights reserved.

Table 1Articles with focus on injectability

Reference How injectability wasstudied

Parameters studied Comments

LPR Rheology PSD Additives Process parameter

[4] IWPp p

Polymeric Ionic Cannula diam. and extrusionspeed

Analytical model wasdeveloped

[5] Injection pressure Polymeric Ionic[8] Injection force Polymeric[6] IWP

pIonic Time after mixing

[9] _p

[10] IWP Ionic[7] IVP

pPolymeric Ionic

[11] IWPp

Ionic[12] IWP Polymeric[13] IWP Ionic[14] IWP Ionic[3] IVP

p pAnalytical model wasexamined

IWP, injected weight percentage; IVP, injected volume percentage.

1466 M. Habib et al. / Acta Biomaterialia 4 (2008) 1465–1471

These examinations were done in the light of the filterpressing phenomenon. These results were then comparedto force and rheological measurements to understand bet-ter the interactions between injection forces, rheologicalproperties and filter pressing.

Instead of using a typical CaP cement suspension, weuse a b-tricalcium phosphate (b-TCP) suspension in thisstudy. The main difference between the two is that theCaP suspension sets or cures, whereas the b-TCP suspen-sion does not.

Due to the setting process, the rheological properties ofCaP cements are transient and constantly changing. Com-bined with this is the complication that the injection pro-cess may destroy the evolving structure and thereby affectthe measurements. Consequently, the results obtained aredifficult to interpret.

In contrast, the advantage of the b-TCP suspension isthat it allows us to remove the complex curing processand to focus on the properties that are important for injec-tability. This is especially true since the ideal CaP cementshould not react during the injection period. Anotheradvantage is that since b-TCP does not cure, it has a muchlonger handling time in which experiments can be con-ducted without time constraints. Additionally, the resultsobtained are more reproducible.

2. Materials and methods

2.1. Powder characterization

b-Tricalcium phosphate (b-TCP; Ca3 (PO4)2; Fluka No.21218) was used as a model powder to investigate the injec-tability of CaP cements. The b-TCP powder selected herehas similar physical properties compared to the b-TCPand a-TCP powders commonly used for CaP cements. Asa result, the hydraulic pastes obtained by mixing this spe-cific b-TCP powder with an aqueous solution have similarrheological properties to those of most CaP cement formu-

lations except that the paste does not harden over time.Powders were characterized by various advanced tech-niques. The particle morphology was examined by scan-ning electron microscopy (SEM; JSM-840-A JEOL).Particle size distribution (PSD) was determined by lasergranulometry (Matersizer 2000, Malvern). For that pur-pose, powders were dispersed in isopropanol with 0.1%sodium pyrophosphate. Specific surface area was measuredwith surface area analyzers (Autosorb-1, Quantachrome)following the Brunauer, Emett and Teller theory (BET).The crystalline composition of the powders was determinedby X-ray diffraction. The measurements were done on anX’pert Pro MRD (Panalytical) powder diffractometer usinga monochromatic source (Cu Ka1, k = 1.5405980 A,45 kV, 40 mA) in the following conditions: scan range:2h = 5–70�, scan speed: 0.020� s�1, scan step: 0.020�). Thedifferent phases present in the powders were checked bymeans of JCPDS (Joint Committee on Powder DiffractionStandards) references patterns (b-TCP: JCPDF 9-169; a-TCP: JCPDF 29-359; HAP: JCPDF 9-432; TTCP: JCPDF25-1137; CaO: JCPDF 37-1497).

The PL was determined according to the proceduredescribed by Bohner and Baroud [4], in which the PL isobtained by measuring the minimum amount of liquid thathas to be added to a powder to form a paste. PL isexpressed in milliliters per gram.

The rheological tests were conducted by using a TA rhe-ometer (TA Instrument). Ionized water was used as anaqueous media to make the suspension. Three differentLPR values were used (40, 50 and 65 wt.%). After 1 minof mixing, the suspension was transferred immediately intothe concentric geometry rheometer. Each sample was sub-jected to a flow procedure with a shear rate peak hold.Shear rate was held at different values starting from 0.002[1/s] to 100 [1/s]. At each hold value of the shear rate, bothviscosity and shear stress values were collected. Curveswere plotted of the viscosity and the shear stress vs. theshear rate.

•1

•2

•3

•4

•5 X, F

•1

•2

•3

•4

•5

Syringe

Cannula

•1

•2

•3

•4

•5

Fig. 1. Injectability experiment setup with injection force (F) for adisplacement X (numbers specify sampling location of the remainedsamples inside the syringe as will be shown in Fig. 8).

Table 2Injectability experiments parameters

Parameters Experimental

LPR (%) 37, 40, 50,60Flow rate (cm min�1) 1.27, 2.54, 25.4Syringe size (ml) 5, 10, 20

M. Habib et al. / Acta Biomaterialia 4 (2008) 1465–1471 1467

2.2. Injectability test

The powder–aqueous solution mixtures were mixed byhand for 1 min to form a paste (the liquid was added tothe powder in a plastic beaker and stirred by hand approx-imately once per second until the mixture became homoge-neous), and then transferred into a syringe connected to acannula. A hydraulic press was used to inject the pastethrough both the syringe and the cannula. The forcerequired to inject the paste and the extrudate percentagewere measured at fixed feed rates. The injectability repre-sented by the extrudate volume fraction was assessed interms of particle size distribution, particle shape, LPR,injection rate, syringe size (5, 10 and 20 ml) and with orwithout cannula. Extruded paste samples were taken every30 s and the LPRs were measured and compared with theLPR of the initial paste to examine the extrudate homoge-neity. The paste remaining within the syringe was carefullycut with a sharp knife across the axis into a number of sec-tions, each 5 mm long. The resultant samples were thenweighed and dried at 150 �C. The samples were kept inthe oven until a constant weight was reached. The weightsbefore and after drying allowed the determination of themoisture content of the samples. The experimental setupused to determine the paste injectability is represented inFig. 1. The experimental parameters are summarized inTable 2.

3. Results

Fig. 2 presents the X-ray diffraction diagrams and theSEM image of the CaP powder. The powder consists ofcrystalline b-TCP with randomly distributed shapes. Theparticles sizes were monomodally distributed with meandiameters at 50% (6.3 lm) as shown in Fig. 3. The PLwas 0.327 ± 0.006 ml g�1.

We report the rheology results as they enhanced ourunderstanding of the injectability. CaP paste showed shear

4.5 4.0 3.5 3.0

d (Å

(1)

(2)

Fig. 2. XRD diagrams and SE

thinning behavior with a yield stress. Both the shear viscos-ity and the yield stress depended strongly on the LPR. Spe-cifically, the yield stresses were around 66 ± 2 Pa,19 ± 2 Pa, and 8 ± 0 Pa for 40%, 50% and 65% LPR sus-

ββ

2.5 2.0 1.5

)

β-TCP

M photograph of b-TCP.

0.1 10 100 10000

2

4

6

8

10

12

14

16

18

20

0

1

2

3

4

5

6

7

8

9

10

Nu

mb

er (

%)

Particle size (µm)

Vo

lum

e (%)

1

Fig. 3. Particle size distribution in number (left scale) and in volume (rightscale) of b-TCP.

140

160

180

200

[N]

No

t ame

1468 M. Habib et al. / Acta Biomaterialia 4 (2008) 1465–1471

pensions, respectively (Fig. 4a, arrows). For the three stud-ied LPRs, the viscosity rapidly dropped with an increase inshear rate to a level below 10 Pa s (Fig. 4b).

The injectability of the hydraulic paste seemed poor. Forthe 40% LPR paste, only 62 ± 3% of the paste initially

0 20 40 60 80 1001

10

100

Sh

ear

stre

ss [

Pa]

Shear rate [1/s]

LPR 40% LPR 50% LPR 65%

0 20 40 60 80 1000.1

1

10

100

1000

10000

Vis

cosi

ty [

Pa.

s]

Shear rate [1/s]

LPR 40% LPR 50% LPR 65%

a

b

Fig. 4. Rheological results showing (a) zero shear stress (arrows) decreaseswith increasing LPR; (b) shear thinning behavior of the CaP pastes.

present in the 10 ml syringe could be injected with a reason-able force. Thereafter, the remaining paste in the syringewas not amendable to injection (Fig. 5).

Examining both the extruded paste and the pasteremaining in the syringe provided a deeper understandingon the mechanism underlying the drop in injectability. Bystudying the LPR of the extrudate samples taken every30 s, it was found that the extruded paste had a signifi-cantly higher LPR than that of the original paste(Fig. 6). Specifically, this effect was in the range of a fewLPR percent and was most visible for a low LPR ratio suchas a 37% LPR. Furthermore, by studying the LPR of thepaste remaining in the syringe, which was not amendableto injection (Fig. 7), a shortage of water content was mea-sured. The LPRs of the slices taken perpendicular to theprincipal direction of extrusion were smaller than theLPR of the initial paste. For instance, the LPRs of the

00

20

20

40

40

60

60503010

80

8070

100

120

Ext

rusi

on

fo

rce

Extruded percentage [%]

nd

able to

injectio

n

Fig. 5. Variation of the extrusion force in function of the extrudatepercentage for 40% LPR paste (arrow points at the syringe blocking area).

20 40 60 80 100 120 140 160 18035

40

45

50

55

60

Ext

rud

ate

LP

R [

%]

Time [s]

E-LPR 37% E-LPR 40% E-LPR 50% S-LPR 37% S-LPR 40% S-LPR 50%

Fig. 6. Homogeneity of the CaP powder in function of time for differentliquid-to-powder ratio (LPR%). E and S are for the extruded paste and theinitial paste, respectively.

0.5 1.0 1.5 2.0 2.520

22

24

26

28

30

32

34

36

38(5)

(4)(3)

(2)

LP

R [

%]

Distance [cm]

LPR 37% LPR 40%

(1)

Fig. 7. LPR as a function of the distance of plunger movement for twodifferent starting LPRs in percentage (numbers to specify samplinglocation of the remaining samples inside the syringe).

0 50 100 150 200 250 300 350 40020

40

60

80

100

Ext

rud

ed f

ract

ion

[%

]

f (PL,LPR)

Fig. 9. The fraction percent can be extruded as a function of (PL, LPR)model obtained from Ref. [4].

M. Habib et al. / Acta Biomaterialia 4 (2008) 1465–1471 1469

non-injectable fraction of the 40% LPR paste ranged from32% to 34% (Fig. 7). It is noteworthy that this small changein the LPR inside the syringe (from 40% to 32%) can con-vert the paste from a paste to a wet powder (PL = 32.7% or0.327 ml g�1).

The LPR greatly affected the paste injectability, or inother words the extrudate volume fraction. A fully inject-able paste was obtained using a 60% LPR. A significantdrop in injectability was observed for LPRs lower than45% (Fig. 8). Conversely, the extrusion force was greatlyincreased by a decrease in the paste LPR (Fig. 8).

Besides composition, process parameters also affectedinjectability: increasing the extrusion rate (high velocity)led to a better injectability. On using a high feed rate(25.4 cm min�1) and a 10 ml syringe attached to a cannula,the extrudate volume fraction reached 79 ± 4%, while itreached 62 ± 1% with a smaller feed rate of 1.27 cm min�1

for the 40% LPR paste (Table 3). The same trend was

35 40 45 50 55 60 6540

50

60

70

80

90

100

4

6

8

10

12

14

16

Ext

rud

at [

%]

LPR [%]

Fo

rce [N]

Fig. 8. Force required at 1 cm displacement in Newton and the extrudatevolume fractions (%) measured at the chosen extrusion force 20 pounds(�90 N) in percentage as a function of LPR percentage of the CaPpowder.

observed for the 50% LPR paste. The volume fraction thatcan be extruded was 91 ± 2% using a feed rate of1.27 cm min�1. This ratio increased to 94 ± 1% when usinga feed rate of 2.54 cm min�1.

The syringe size and the presence of a cannula slightlyaffected the extruded volume fraction. A small 5 ml syringeenhanced injectability by about 5%. Surprisingly, injectingthe paste without a cannula only improved the injectedfraction by 2.5% (Table 3). The analysis of variance wasdone for each parameter as a single factor experiment anal-ysis according to Montgomery [21].

4. Discussion

This article has focussed on the injectability of hydraulicCaP pastes. Attention has been paid to the mechanismsunderlying the poor injectability of such pastes. In additionto examining the force and volume fractions of theextruded paste, several process and powder parameters ofinterest for minimally invasive clinical applications wereinvestigated.

The findings of the article underlined the shift in waterconcentration of the paste over the course of the injection.Namely, excess water was found in the extruded paste(Fig. 6) and reduced water content was found in the pasteremaining in the syringe (Fig. 7), suggesting that the liquid-phase was partly filtered through the paste particles duringinjection. This phenomenon is known as ‘‘filter pressing” or‘‘phase migration”. Although the extent of the composi-tional changes occurring during filter pressing is small,the rheological properties of the paste are very significantlyinfluenced by it.

This study showed further that the forces required forthe paste delivery (Fig. 8) are very reasonable and can beapplied manually by physicians. Only when filter pressingoccurs does the injection force increase too much for man-ual injection. In fact, no matter how big the forces appliedto the plunger beyond that point, the paste is no longeramendable to injection (Fig. 5). Accordingly, the force

Table 3Extruded volume of cement made with CaP powder (b-TCP)

LPR (%) b-TCP powder(%)

Plastic limit(ml g�1)

Flow rate(cm min�1)

Syringe size(m)

Extrudedvolume (%)

Statistical analysis factor

37 100 0.32 1.27 10 42.2 ± 2 a = 0.05, 1 � b = 0.99, P < 7.24E � 1340 100 0.32 1.27 10 62 ± 2.945 100 0.32 1.27 10 79 ± 2.550 100 0.32 1.27 10 89.5 ± 1.560 100 0.32 1.27 10 10065 100 0.32 1.27 10 100

50 100 0.32 1.27 10 89.5 ± 1.550 100 0.32 2.54 10 94 ± 0.96

50 100 0.32 1.27 10a 89.5 ± 1.5 a = 0.1, 1 � b = 0.98, P < 0.0650 100 0.32 1.27 10b 91 ± 1.150 100 0.32 1.27 10c 92 ± 0.96

50 100 0.32 1.27 5 95 ± 1.01 a = 0.05, 1 � b = 0.93, P < 0.00750 100 0.32 1.27 10 89.5 ± 1.550 100 0.32 1.27 20 90 ± 1.1

40 100 0.32 1.27 10 62 ± 2.9 a = 0.05, 1 � b = 0.99, P < 0.00140 100 0.32 2.45 10 67 ± 1.0940 100 0.32 24.5 10 78.8 ± 3.8

a Syringe with cannula.b Syringe with no distal tip but with cannula.c Syringe only.

1470 M. Habib et al. / Acta Biomaterialia 4 (2008) 1465–1471

itself can be used to predict the point in time to stop theinjection but is not itself a measure of the injectability ashas been previously used. Although the mechanistic findingand results are new, this phenomenon has already beenanticipated by Bohner and Baroud [4]. Further, this phe-nomenon has been partially studied in, and is relevant to,some non-medical applications [15–18]. Interestingly, waterleakage was observed around the gasket of the plunger ofthe syringe as well. It should be emphasized that anincrease in the LPR was observed to increase the injectabil-ity. This result has been observed in the past and thus is notsurprising [19]. However, this approach is not a practicalsolution because not only does it result in highly porouscements, which are mechanically weak, but also in poorlycohesive pastes [20]. Further, an increase of feed rate(2.54 cm min�1) enhanced the injectability by 5%, whichcan be anticipated from the shear thinning behavior ofthe paste. Also, since filter pressing is time dependent, ahigher injection velocity does not provide the liquid withenough time to migrate, and as a result the cake thicknessis reduced [4,15–17]. Also, shorter injection time implieshigher flow rate and the lower viscosity, which againenhances the injectability. We refer the readers here tothe work conducted by Bohner and Baroud [4] for addi-tional reading. However, a higher flow rate results in alower window of observation for physicians during mini-mally invasive applications, which may compromise thesafety of the patient; thus there is a limit how quickly apaste can be delivered. Further, the finding of this studyis consistent with the function (Eq. (19)) of Bohner andBaroud [4], in which the injectability was defined as a func-tion of both the PL and the LPR. The data of this studyseem to strengthen the validity of this function (Fig. 9).

Both the syringe size and the cannula impacted injecta-bility. Smaller syringes increased the injectability. This isbelieved to be due to the change in the geometry of theconnection between syringe and cannula. Further, shortercannulas reduced the filter pressing and thus a higher vol-ume fraction of the paste could be injected. This could bedue to the low pressure drop attributed to the shortlength of the cannula, hence reducing the pressurerequired to inject the paste according to Hagen–Poiseu-ille’s law. Surprisingly, full injectability could not bereached even when no cannula was used. This impliesthat filter pressing occurred even for very small forces.Accordingly, a solution might be obtained by reducingthe ability of the mixing liquid to pass through the pow-der. Thus, injectability of cement could be improved byincreasing the viscosity of the mixing liquid or reducingthe permeability of the powder. The former strategy hasbeen applied extensively in the past, for example by add-ing a soluble polymer (e.g. polysaccharide) to the mixingsolution [4–7,12]. The latter strategy has been less welldocumented. Gbureck et al. [13] added different fine pow-ders to an a-TCP powder, resulting in a large improve-ment in paste injectability. Adding a few wt.% ofprecipitated hydroxyapatite particles (specific surface areaclose to 50 m2 g�1) to a b-TCP-water paste increased theextruded fraction by a few percent despite a simultaneousincrease in the injection force (Marc Bohner, unpublisheddata). However, as the use of bimodal size distribution isknown to reduce not only the paste permeability but alsothe PL, it is not clear to what extent the increase in injec-tability is due to a decrease in permeability or a decreasein PL (in both cases, the change in PL was notdocumented).

M. Habib et al. / Acta Biomaterialia 4 (2008) 1465–1471 1471

5. Conclusion

Liquid-phase migration seems to be the mechanismunderlying the limited injectability of CaP biomaterialsused in minimally invasive clinical applications tostrengthen osteoporotic bones. That is, the forces requiredto deliver the mineral paste are reasonable until the injec-tion comes to a halt. The older paradigm of the force beinginsufficient to deliver the CaP hydraulic paste seems nowoutdated and the new paradigm is that there is excess liquidin the extrude while the water content of the paste remain-ing in the syringe is reduced. With the new paradigm, newavenues to enhance injectability, such as the powderdesign, will be explored in future research activities.

Acknowledgments

The kind support of Jake Barralet, Canada ResearchChair in Osteoinductive Biomaterials, for the zeta potentialmeasurements is acknowledged. Further, the financial sup-port of the Robert Mathys Foundation (Switzerland) isacknowledged.

References

[1] Brown WE, Chow LC. A new calcium phosphate, water-settingcement. In: Brown PW, editor. Cements research progress. Wester-ville, OH: American Ceramic Society; 1986. p. 351–79.

[2] Tofighi A, Mounic S, Chakravarthy P, Rey C, Lee D. Settingreactions involved in injectable cements based on amorphos calciumphosphate. Key Eng Mater 2001;192–195:769–72.

[3] Baroud G, Bohner M, Heini P, Steffen T. Injection biomechanics ofbone cements used in vertebroplasty. Biomed Mater Eng2004;14:487–504.

[4] Bohner M, Baroud G. Injectability of calcium phosphate pastes.Biomaterials 2005;26:1553–63.

[5] Leroux L, Hatim Z, Freche M, Lacout JL. Effects of variousadjuvants (lactic acid, glycerol, and chitosan) on the injectability of acalcium phosphate cement. Bone 1999;25(2):S31–4.

[6] Khairoun I, Boltong MG, Driessens FCM, Planell JA. Some factorscontrolling the injectability of calcium phosphate bone cements. JMater Sci Mater Med 1998;9:425–8.

[7] Wang X, Ye J, Wang H. Effects of additives on the rheologicalproperties and injectability of a calcium phosphate bone substitutematerial. J Biomed Mater Res B Appl Biomater 2006;78(2):259–64.

[8] Ratier A, Freche M, Locout JL, Rodriguez F. Behaviour of aninjectable calcium phosphate cement with added tetracycline. Int JPharm 2004;274:261–8.

[9] Liu C, Shao H, Chen F, Zheng H. Rheological properties ofconcentrated aqueous injectable calcium phosphate cement slurry.Biomaterials 2006;27:5003–13.

[10] Gbureck U, Barralet JE, Spatz K, Grover LM, Thull R. Ionicmodification of calcium phosphate cement viscosity. Part I: hypo-dermic injection and strength improvement of apatite cement.Biomaterials 2004;25(11):2187–95.

[11] Barralet JE, Grover LM, Gbureck U. Ionic modification of calciumphosphate cement viscosity. Part II: hypodermic injection andstrength improvement of brushite cement. Biomaterials2004;25(11):2197–203.

[12] Ginebra MP, Rilliard A, Fernandez E, Elvira C, San Roman J,Planell JA. Mechanical and rheological improvement of a calciumphosphate cement by the addition of a polymeric drug. J BiomedMater Res 2001;57(1):113–8.

[13] Gbureck U, Spatz K, Thull R, Barralet JE. Rheological enhancementof mechanically activated a-tricalcium phosphate cements. J BiomedMater Res B 2005;73B:1–6.

[14] Burguera EF, Xu HH, Weir MD. Injectable and rapid-setting calciumphosphate bone cement with dicalcium phosphate dehydrate. JBiomed Mater Res B Appl Biomater 2006;77(1):126–34.

[15] Yu AB, Bridgwater J, Burbidge AS, Saracevic Z. Liquid maldistri-bution in particulate paste extrusion. Powder Tech 1999;103:103–9.

[16] Bayfield M, Haggett JA, Williamsin MJ, Wilson DI, Zargar A.Liquid phase migration in the extrusion of icing sugar pastes. TransIChemE 1998;76(B):39–46 [March].

[17] Rough SL, Wilson DI, Bridgwater J. A model describing liquid phasemigration within an extruding microcrystalline cellulose paste. TransIChemE 2002;80(A):701–14 [October].

[18] Bradley J, Yuan W, Draper O, Blackburn S. A study of phasemigration during extrusion. Key Eng Mater 2004;264–268:57–60.

[19] Baroud G, Cayer E, Bohner M. Rheological characterization ofconcentrated aqueous beta-tricalcium phosphate suspensions: theeffect of liquid-to-powder ratio, milling time, and additives. ActaBiomater 2005;1(3):357–63.

[20] Bohner M, Doebelin N, Baroud G. Theoretical and experimentalapproach to test cohesion of calcium phosphate pastes. EuropeanCells Mater 2006;12:26–35.

[21] Montgomery DC. Design and analysis of experiments. 6th ed. NewYork: John Wiley; 2005, pp. 60–118.