New injectable and radiopaque antibiotic loaded acrylic bone cements

New Injectable and Radiopaque Antibiotic Loaded AcrylicBone Cements

Lidia Hernandez, M. Eugenia Munoz, Isabel Goni, Marilo Gurruchaga

Universidad del Paıs Vasco 20009-San Sebastian, Spain

Received 26 December 2006; revised 22 January 2008; accepted 25 January 2008Published online 7 May 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.31105

Abstract: The use of antibiotic loaded bone cements (ALBCs) has become a common clinical

practice in the prevention and treatment of prosthesis-related infections. However, due to

antibiotic resistance, there is a general interest in broadening the antibacterial spectrum of

currently used drugs. The aim of this work is to formulate ALBCs for specific use in

vertebroplasty and kyphoplasty, and to study the effect of the addition of ciprofloxacin alone

and in combination with vancomycin on some properties of the cement. The cements were

formulated using bismuth salicylate as the radiopacifier. The setting properties, residual

monomer content, release of antibiotics, rheological behavior, injectability, and mechanical

properties of these formulations were studied. They showed long setting times and low curing

temperatures. From the release studies, antibacterial properties are assumed because the

concentration of released antibiotic was higher than the minimum effective. Although the

experimental cements had slightly reduced mechanical properties, the other alterations shown

were negligible. ' 2008 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 87B: 312–320,

2008

Keywords: bone cement-PMMA; antimicrobial; vertebral; viscosity, mechanical properties

INTRODUCTION

One of the first modifications of acrylic bone cements con-

cerns one of the more serious complications of orthopedic

surgery: bacterial bone infections (osteomyelitis).1 In the

early 70s, Buchholtz and Engelbrecht2 already included

gentamicin sulphate (GS) in a standard formulation to treat

an infection in a total hip arthroplasty.2 Thanks to their

success, the use of antibiotic loaded bone cements

(ALBCs) has become common clinical practice in the pre-

vention and the treatment of prosthesis-related infections3

with the aim of attaining high local drug levels, while

maintaining low systemic levels.4 Apart from GS, other

antibiotics such as erythromycin, clindamycin or tobramy-

cin5 are also being used as additives for bone cements.

However, due to antibiotic resistance, there is a growing in-

terest in the addition of other antibiotics or mixtures of

antibiotics to bone cements, which broadens the antibacte-

rial spectrum of currently used drugs.3

In recent years, new surgical techniques have emerged,

such as vertebroplasty (VP) or kyphoplasty (KP), which

mainly consist of the percutaneous injection of acrylic bone

cement into a damaged vertebra to reduce the pain by sta-

bilizing the vertebral body.6 KP, moreover, entails the

inflation of a balloon tamp prior to the injection in an

attempt to restore vertebral body height and reduce the

kyphotic deformity.7 Up to now, physicians have adapted

classical acrylic formulations to these techniques,8 on the

one hand by increasing radiological visibility by means of

adding high atomic number metals in powder form, such as

barium, tantalum or tungsten.9,10,11 In our case to prevent

the need for this intervention by the physician, our group

has worked on this by adding bismuth salicylate (BS) as a

radiopacifier to a cement formulation and have obtained

good results.12,13 On the other hand, in an attempt to inject

the cement through the needle easily and also to increase

the working time, some physicians reduce the viscosity by

adding more monomer to the cement mixture14 or by chill-

ing the constituents of the cement prior to the mixing.15 In

a previous study, we found that the use of polymethyl

methacrylate (PMMA) beads of bigger size and low molec-

ular weight is a better way to decrease maximum curing

temperature and increase the working time.16 Moreover, de-

spite the low incidence of infection in these procedures,17

apart from preoperative intravenous antibiotics,18 inciden-

ces of the addition of powdered antibiotics to the cement

formulation have been reported in the operating theater.11

In light of the reality described above, the aim of this

work is to formulate ALBCs for specific use in VP and

KP, and to study the effect of the addition of two antibiot-

Correspondence to: M. Gurruchaga (e-mail: [email protected])

' 2008 Wiley Periodicals, Inc.

312

ics on some properties of the cement, such as setting prop-

erties, residual monomer content, release of antibiotics,

rheological behavior, injectability and mechanical proper-

ties. Ciprofloxacin (CFX) alone and in combination with

vancomycin (VM) were chosen for this study. The high

penetration depth of CFX into bone and the success of the

combined therapy for the elimination or prevention of

implants infection in hip surgery were the reasons for

including these antibiotics in the new cements.19

MATERIALS AND METHODS

Materials and Specimen Preparation

Table I summarizes the formulations studied: two commer-

cially available (I and II) and three experimental (III to V)

loaded with BS as the radiopacifier and CFX and VM, as

antibiotics.

The experimental formulations were prepared using a

solid: liquid ratio of 2:1. The liquid component consisted

of methylmethacrylate (MMA, Merck) monomer and N,N-

dimethyl-p-toluidine (Merck, 1 wt % with respect to the

liquid phase), in all cases. The solid component consisted

of benzoyl peroxide (Merck, 1.25 wt % with respect to the

solid phase), 10% of BS (Fluka), the corresponding antibi-

otic (pure CFX was purchased from Fluka and the VM

hydrochloride from Normon. The PMMA beads employed

in the solid phase were a mixture of Colacryl DP300

(Lucite) and Plexigum (Rohm and Haas) in a ratio of

80/2016 (wt).

The preparation of commercial cement samples was car-

ried out according to the manufacturer’s instructions. The

components of bone cement were hand-mixed and before

the dough state was reached the mass was placed in the

corresponding Teflon mould and allowed to cure for 1 h at

378C.Either for the experimental formulation specimen’s prep-

aration or their characterization, the methods described in

the ISO 5833 standard for bone cements used in arthro-

plasty20 were followed. However, we must take into

account that there are not specific rules for VP cements.

Curing Parameters

Exotherms of the polymerisation were registered in compli-

ance with ISO 5833.20 The mixture was introduced in a

Teflon mould connected to a high sensitivity thermotester

and the changes in temperature were recorded as a function

of the time from the moment of mixing. The maximum

temperature (Tmax) attained by the bulk was recorded and

the setting time (tset) was determined as the time taken to

reach a temperature midway between room temperature

and Tmax. The given values are the mean of at least two

determinations, as required by the ISO standard.

Residual Monomer Content

Proton nuclear magnetic resonance (1H NMR) spectroscopy

was selected to determine the percentage of residual mono-

mer. 20 mg of cement cured for 1 h at 378C were dissolved

in deuterated chloroform containing tetramethylsilane as

the internal standard to obtain the 1H NMR spectra using a

FT-NMR Bruker spectrophotometer operating at 300 MHz.

To assign the peaks and estimate the residual MMA con-

tent we took into account the references given by other

authors.21 Measurements were performed in triplicate.

Mechanical Properties

Compressive properties were determined according to the

ISO 5833 standard.20 Specimens testing was carried out af-

ter 24 h of storage in air at room temperature. At least six

12-mm high and 6-mm wide cylinders were tested for each

formulation on the 4301 INSTRON testing machine

equipped with a cell load of 100 kN, with a cross-head dis-

placement of 22 mm/min. Additional tests were performed

on the specimens after 1 month of immersion in saline so-

lution at 378C.The bending tests were performed in three-point bending

mode, and following the ISO 5833 standard specifica-

tions.20 At least six specimens (75 3 10 3 3.3 mm3) were

tested on a 4301 INSTRON machine after conditioning

the samples in water at 378C for 50 h. The displacement

rate for the bending tests was 5 mm/min. After tests, frac-

ture surfaces were examined by scanning electron micros-

copy (SEM) in a Hitachi S-2700 with an accelerating

voltage of 15 kV. Previously, samples were gold coated

(Fine Coat Ion Sputter JFC-1100).

Izod impact testing was carried out according to the

ASTM D256 standard.22 The specimens measured 63.5 312.7 3 6.35 mm3 and were machined to obtain a notch of

458, with a radius of 0.25 mm and a depth under the notch

of 10.16 mm. No fewer than 10 specimens were tested in a

CEAST 6548/000 pendulum equipped with a 1 J hammer.

Rheological Behavior and Injectability

Rheological oscillatory measurements were selected to

characterize the viscoelastic phase of the cement between

TABLE I. Brief Description of the Acrylic BoneCements Studied

Formulation Main Characteristics

I (Simplex P) Radiopaque Cement (10% BaSO4)

II (Osteopal G) Radiopaque Cement (10% ZrO2), loaded

with antibiotic (GS)

III Radiopaque Cement (10% BS)

IV Radiopaque cement (10% BS), loaded with

antibiotic (5% CFX)

V Radiopaque Cement (10% BS), loaded with

a mixture of antibiotics (3% CFX 13%VM)

313INJECTABLE RADIOPAQUE ANTIBIOTIC LOADED CEMENT

Journal of Biomedical Materials Research Part B: Applied Biomaterials

the mixing and the complete setting by means of complex

(or true) viscosity (g*), since this parameter contain both

the real (elastic) and loss (viscous) components.23 Evolu-

tion of g* as a function of time was measured in a Rheo-

metric Ares rheometer in dynamic oscillation mode using

plate-plate configuration, at a frequency of 1 Hz. The ra-

dius of the plates was 25 mm and the gap between the

plates was 2 mm. The rheometer was used in a constant

strain mode with strain amplitude of 1% and measurements

were conducted at room temperature (258C). A significant

benefit of this technique is the ability to extract viscoelastic

parameters such as the storage (G0) and loss moduli (G00)and their ratio, called loss tangent (tan d 5 G00/G0).24

For the Injectability tests, we followed the method pro-

posed by other authors.25 Injectability (I%) was defined as

the weight percent of cement injected into a Teflon recipient,

expressed as a percentage of the total amount of cement

charged into the syringe. Time of mixing was considered as

the time needed to reach a homogeneous mass ready to be

injected. In order to ascertain the time during which the

cement had the appropriate consistency and fluidity to be

injected (the injection time), tests were performed slowly until

the solidity of the mass impeded the injection.

Water Uptake Capacity

Taking into account the high presence of water in the

human body, water absorption of any polymeric material is

of importance for surgical applications since it influences

the mechanical properties of the cement and monomer

release. Moreover, in this case in which the salicylic salt is

used as radiopacifier, water absorption can to the hydrolysis

of the salt and the subsequent release of salicylic acid.12,13

Thus, specimens were immersed in phosphate buffered sa-

line (PBS, pH 5 7.4) at 378C for 1 month. Water absorp-

tion (%A) and the percentage of elution (%E) of the

cements were calculated using the following expressions

[Eqs. (1) and (2)]26:

%A ¼ Mw �Mf

M0

3 100 ð1Þ

%E ¼ M0 �Mf

M0

3 100 ð2Þ

where M0 is the dry sample weight, Mf the dry weight after

testing and Mw the weight at the equilibrium absorption

point.

Release of Antibiotics

The release experiments were performed using cement

sheets measuring 10 3 25 3 1 mm3, by two different

methods: In the first experiment, the determination of the

GS released from Osteopal G was performed by immersing

the cement sheet in 100 mL of PBS. The measurements

were taken from an aliquot of 10 mL, which was immedi-

ately replaced with fresh PBS. In the second case, the

quantities of GS released were measured at 332 nm by

means of the UV-spectrophotometry (UV CECIL CE2041)

following the derivatizing method described by Zhang

et al.27 The relationship between UV absorption and con-

centration was determined by means of solutions of differ-

ent known concentrations, giving a linear plot (C 561.91A, r 5 0.9971).

Because of the simultaneous release of salicylic acid,

CFX and VM from formulations IV and V, high perform-

ance liquid chromatography (HPLC) was employed to sepa-

rate the substances released before their quantification.

Specimens’ sheets were immersed onto 15 mL of PBS and

incubated without stirring. Samples of 0.500 mL were

taken at different times, and immediately replaced with

fresh solution. The HPLC equipment consisted of a Perking

Elmer LC-250 pump, a UV-VIS detector Perking Elmer

LC-95, and a Waters lBoundapack 3.9 3 300 mm C-18

column. The wavelengths used were 272 nm for CFX and

211 nm for VM. The separation of CFX required the addi-

tion of a counterion (PIC A, Waters), so the mobile phase

for the determination of CFX was a methanol/ aqueous so-

lution of PIC-A (60:40). For the determination of VM a

methanol/water (80:20) solution was used as the mobile

phase at the same flow rate, which was 1 mL/min. The reten-

tion time of the CFX and VM peaks relative to the standard

were 5.7 and 6.8 min. The calibration curves were obtained

for the complete set of measurements, displaying correlation

coefficients of 0.999 for CFX and 0.994 for VM. Samples

were assayed in duplicate.

Radiopacity

The X-ray photograph of the cement specimens was taken

using a standard clinical General Electric X-ray instrument

(set at 25 KV and 11 MAS). The specimens were rectangu-

lar sheets measuring 25 3 10 3 1 mm3.

Statistical Analysis

Results were evaluated using the one-way Anova statistical

analysis with respect to the reference formulations. The

error protection method used in this research was the Fisher

PLSD method and the confidence interval used was 95%.

Because of the obvious differences between the com-

mercial cements and experimental formulations, the statisti-

cal analysis was focused on the radiopaque cement with

10% of BS (Formulation III). Taking this cement as a ref-

erence, comparisons were made between commercial

cements and the new formulations.

RESULTS

Setting Parameters

As we can see from the setting diagrams (Figure 1), experi-

mental formulations (III to V) exhibit significantly higher

314 HERNANDEZ ET AL.


setting times than commercial cements, and significantly

lower maximum temperatures. As we have demonstrated in

a previous study, this is due to a careful selection of the

PMMA beads employed in the solid phase of the experi-

mental formulations.16 The characteristics of these beads

(particle size overall) make possible to obtain optimum

cement setting at long polymerization times and not very

high polymerization temperatures. In this case, it is also

important to point out that the addition of these quantities

of powdered CFX and VM did not affect the polymeriza-

tion kinetics, since no significant differences were found

between the curing parameters (Tmax and tset) of experimen-

tal cements.

Residual Monomer Content

Results of the determination of the residual monomer con-

tent are shown on Table II. The differences observed ini-

tially in the setting kinetics did not significantly affect the

residual monomer content, which remained at similar val-

ues for all cement samples after 1 h of setting at 378C.

Mechanical Properties

Compressive Properties. The results of the compres-

sion tests are given in Table III and indicate that, in dry

specimens, the addition of antibiotics does not significantly

affect the compressive strength (rc) or Young’s modulus

(Ec) of the experimental formulations. Immersion in saline

solution for 1 month decreases both parameters due to the

plastizicing effect of the absorbed water and the porosity

induced by the release of antibiotics and salicylic acid.

This is more noticeable in formulation V due to its higher

weight loss and release, giving a significant decrease of the

compression parameters with respect to Formulation III.

The compressive parameters of experimental cements are

comparable with commercial ones and all specimens

showed a higher rc than the minimum value required by

the ISO 5833 standard (70 MPa). On the other hand, it has

to be clearly stated that Table III shows results at two dif-

ferent time points and two different test conditions and,

consequently, the presented values are not comparable.

Bending Tests. The bending strength (B) and modulus

(E) of all formulations are also presented on Table III. All

experimental formulations show significantly lower values

of B than commercial bone cements, and the values are

below the minimum value required by the ISO standard

(50 MPa). This is probably due to the poorer dispersion of

the ingredients in these cements compared with that of the

commercial ones, as can be seen in the fracture surfaces

examined by SEM (Figure 2). Since there are no significant

differences between experimental formulations, we can

deduce that the addition of antibiotics does not further

deteriorate the bending strength of the experimental

cements. Regarding the modulus, except for the significant

increase found in Simplex P, the rest of the formulations

do not show differences in this parameter, which complied

with the ISO standard (E[ 1800 MPa).

Impact Strength. Determining the impact strength is im-

portant for acrylic implants since a strong positive relation

was found between this parameter and fracture toughness.28

Regarding impact strength results (Table III), once again

Simplex P showed significant superiority, whereas Osteopal

G and Formulation III did not show statistical differences.

In this test, the addition of antibiotics weakened the

cements, since a significant decrease impact resistance was

observed in Formulations IV and V with respect to Formu-

lation III.

Figure 1. Exotherms of polymerization of all formulations studied.

TABLE II. Residual Monomer Content After 1 h of Curing at 378C (% Mr), Results of the InjectabilityTests and Water Uptake Behavior of the Various Formulations

Formulation % Mr (mol)

Injectability Tests Water Uptake

Time of Mixing (min) Injection Time (min) I (%) A (%) E (%)

I (Simplex P) 2.60 (0.25) – – – 1.29a (0.01) 0.00a (0.00)

II (Osteopal G) 2.03 (0.38) 1a 2.0 45.94a (5.73) 2.89a (0.09) 0.50a (0.13)

III 2.34 (0.45) 3 5.8 80.11 (4.68) 2.16 (0.10) 0.72 (0.10)

IV 1.92 (0.11) 3 6.4 74.55a (3.05) 2.01 (0.09) 0.74 (0.14)

V 2.50 (0.23) 3 6.2 75.19a (5.56) 2.62a (0.04) 1.22a (0.10)

Results are given as mean and standard deviation (SD) in parentheses.a Values with significant differences with respect to Formulation III (p\ 0.05).



Rheological Behavior and Injectability

Figure 3 (upper) shows performance during the curing of

complex viscosity (g*) versus elapsed time from mixing

for all formulations, which reflects the different characteris-

tics of each type of cement and the different ways in which

they are intended to be used.24 Simplex P is a hand-dough

usage cement, which quickly reaches quite high values of

g* that remain stable during working time. After this, due

to its early curing, its g* increases very quickly. Osteopal

G is sold as ‘‘low viscosity cement.’’ We can see that it

really does exhibit low values of g*. However, we can

observe that due to the early setting of this cement there is

a very rapid increase of g* at about 5–6 min after the mix-

ing. The three experimental cements, which are formulated

for injectable use, at the very outset of the experiment,

showed initial values of g* similar to Osteopal G, but due

to their delayed setting, the g* remain almost constant at

these low values for longer times.

Figure 4 shows the plots of G0, G00 (upper), and tan d(G00/G0) (down) for all the cements. Although initially G00

tends to be greater than G0, from the moment that we can

see in the figure, G0 is greater than G00 and both compo-

nents increase over time. However, G0 increases more rap-

idly since the material changes from being predominantly

viscous to being predominantly elastic. This becomes evi-

dent when we observe the decrease of tan d over time. As

in the previous tests, the addition of antibiotics did not

affect, for a long period, the behavior of the experimental

formulations.

Results of the injectability tests are shown in Table II

except for formulation I, which is formulated for hand-

dough usage. These values corroborate the rheological

behavior observed before, since all experimental formula-

tions show similar results, that is, higher injectability val-

ues and longer injection times than Osteopal G, due to

their longer setting times. Differences between injectability

parameters of the experimental formulations did not reach

statistical significance (p[ 0.05) and the addition of antibi-

otics did not change the injection behavior of the cement

formulations, as expected from rheological results.

Water Uptake Capacity

Results for absorption (A) and elution (E) are also shown

in Table II. Simplex P has a moderate water uptake and a

negligible weight loss after 1 month of immersion in saline

solution. The rest of the formulations showed a significant

increase in these parameters due to the higher polarity of

the ingredients of these formulations. Regarding the elution

results, which give us an idea of the total release of sub-

stances from the cements, all experimental formulations

experienced a lost of weight due to the release of salicylic

acid. Formulations III and IV did not show statistical differen-

ces between them despite the incorporation of CFX to the for-

mulation, which could be attributable to the low solubility of

CFX or even to the affinity for the polymer.29 However, the

addition of a more soluble antibiotic as in the case of VM

obviously enhanced the weight loss of the cement.

Antibiotic Release

Release kinetic plots obtained for all the antibiotic-loaded

formulations are shown in Figure 5. In many cases, release

was fast at the beginning of the experiment due to the dis-

solution of the particles located at or near the surface of

the specimen, which is called ‘‘burst effect.’’ In fact, we

have observed this ‘‘burst effect’’ for GS in Osteopal G

cement and for VM in formulation V. However, for CFX,

we did not observe the burst effect. After 1 month, we can

consider that release is almost finished. If we observe the

kinetics, CFX is released at significantly lower percentages

than GS and VM. There is a trend showing more gradual

release of CFX in formulation V due to the incorporation

of VM. However, the difference did not reach statistical

significance.

Radiopacity

Minimally invasive techniques for spinal surgery, such as

VP or KP, require injectable materials with high radiopac-

ity to accurately follow the evolution of the cement within

the vertebral body. Figure 6 shows the X-Ray image of the

TABLE III. Mechanical Properties of the Cements Studied: Compressive Strength (rc) and Modulus (Ec),Bending Strength (B) and Modulus (E), and the Impact Resistance (IR)

Formulation

Compression Tests

Bending Tests Impact TestsDry Specimens Wet Specimens

rc (MPa) Ec (MPa) rc (MPa) Ec (MPa) B (MPa) E (MPa) I R (KJ/m2)

I (Simplex P) 112a (3) 1590a (60) 105a (4) 1545a (60) 81a (5) 2600a (80) 1.06a (0.03)

II (Osteopal G) a104 (4) a1740 (40) 1a93 (1) a1340 (80) a67 (3) a2310 (220) a0.91 (0.07)

III a104 (2) a1780 (70) 1a94 (3) a1695 (50) a41 (3) a2500 (70) a0.85 (0.06)

IV a105 (4) a1825 (65) 1a93 (2) a1680 (20) a39 (1) a2480 (90) 0.75a (0.05)

V a101 (4) a1840 (60) 185a (4) a1740 (60) a41 (1) a2470 (70) 0.71a (0.06)

For the compression tests, results of dry and wet specimens (soaked in saline solution at 378C for 1 month) are presented. Results are given as mean and standard deviation

(SD) in parentheses.a Values with significant differences with respect to Formulation III (p\ 0.05).



cements studied in this study. Experimental formulations

show higher radiopacity than traditional commercial

cements used in arthroplasty (Simplex P and Osteopal G),

due to the presence of the bismuth atom in the BS, which

has been proven to give excellent X-ray visibility,12,13

reducing the need of adding extra radiopacifiers.

DISCUSSION

Although the role of ALBC in the prevention of infection

remains controversial because of issues regarding drug re-

sistance, efficacy and cost, a recent review30 concluded that

its use for prophylaxis should be restricted to high-risk

groups and it should be considered as a defense against

direct contamination at the time of surgery, or during the

Figure 2. Fracture Surfaces Examined by SEM: (a) Simplex P, (b)

Osteopal G, (c–e) Formulations III, IV, and V, respectively. Bar scales

represent 75 lm.

Figure 3. Evolution of complex viscosity vs. time from mixing for all

formulations.

Figure 4. Rheological values of loss and storage modulus (upper)

and evolution of Tan delta as a function of time (down).



postoperative period as the wound seals. Thus the research

of new formulation of ALBC must go on.

The new highly radiopaque and injectable cements

loaded with antibiotics formulated in this research for spe-

cific use in the field of vertebroplasty or kyphoplasty,

showed long setting times, which is positive when consid-

ering their injectablility, as their viscosities remained at

low values for longer times than conventional cements.

Moreover, their low curing temperatures minimize the ther-

mal damage to the surrounding tissues. In the cases in

which low polymerization temperature are attained, the

measurement of monomer presence is particularly impor-

tant to demonstrate the high polymerization yield. This pa-

rameter remained at similar values for all cement samples

after 1 h of curing at body temperature.

Because of the variety of methods reported in bibliogra-

phy, it is difficult to establish proper comparisons between

the different studies realized about the effect of the addi-

tion of antibiotic on the mechanical performance of the

cements.31 In general, a weakening effect is observed as a

result of the presence of differentiated particles,5 since anti-

biotic particles act as voids or defects. The greater the anti-

biotic concentration the bigger the antibiotic domains from

which rupture initiates.32–35 In general, the compression

strength is not affected by small additions of antibiot-

ics.31,32,35,36 However, at higher percentages, this parameter

can decrease below the minimum required by the ISO

standard (70 MPa).35,36 The effect is more noticeable in

flexural tests34,36 and, above all, in fatigue tests, since in

these assays there is a significant decrease of the fatigue

life with the addition of small quantities of antibiotics to

the cement formulation.32,33,37 With the aim of minimizing

the detrimental effects of the addition of antibiotics, some

authors recommend the use of commercial formulations

with antibiotics included. These formulations are more con-

sistent and homogeneous than hand-mixed ones, which can

show lower strengths.2 Other options to improve the

cements properties are the vacuum mixing or the centrifu-

gation of the formulation, two processes which result in a

lower porosity and in an enhancement of the fatigue life.

However, the lower porosity will hinder the release of anti-

biotics.

In our study, as expected from the studies reported

above, the addition of antibiotics did not affect the com-

pression performance of the new cements, which complied

with the standard even after 1 month of immersion in

saline medium. However, the flexural test for experimental

cements showed low values and impact strength, which has

been correlated previously with the fatigue performance of

the cements, decreased with the addition of antibiotics. It

has been shown from fracture surfaces (Figure 2) that

whereas commercial cements showed smooth surfaces

where any of the ingredients could be distinguished, the ex-

perimental cements show differentiated particles, which

corresponds to BS, CFX, or VM. So, either dissolution of

the radiopacifier12 or the mechanical mixing of the constit-

uents is considered to be able to produce a more consistent

and reproducible mixture, thus enhancing the mechanical

properties of these formulations.28 This affirmation is based

on the studies of other authors.31,32 In one of them,32 they

found that bone cements in which tobramycin had been

hand-blended was weaker than their prepackaged counter-

part.

The results from the rheological study and the injectabil-

ity tests emphasize again the differences between the com-

mercial and our new formulations specifically designed to

maintain low viscosity for use as cements in vertebroplasty.

As we have demonstrated in the previous studies,12,16 the

slower the decrease of tan d, the easier the cement is to

inject. Thus, the fast increase of the viscosity and the ear-

lier decrease of tan d showed by the commercial cements

indicate the lack of suitability for use as formulations in

VP and KP. These observations are in accordance with the

results of the injectability tests, that is, higher injectability

values and longer injection times for the experimental

cements compared with the commercial cements.

The release of active substances from the cement matrix

is affected by a number of factors, from which we can

highlight: (a) the kind and number of antibiotics38; (b) their

concentration—higher concentrations give as a result an

increase of the total amount of antibiotic released. This is

due to the fact that the ‘‘extra’’ antibiotic acts as a soluble

additive that leaves a network of void or holes, thus pro-

moting a higher release. Taking this into account, the addi-

Figure 6. X-Ray Photograph of the bone cements studied.

Figure 5. Release of GS from Osteopal G, CFX from formulations

IV and V, and VM from formulation V.



tion of soluble additives, such as lactose39 or polivinilpirro-

lidone40 has been reported in a bid to increase the total

amount of antibiotic released; (c) the cement itself: its po-

rosity, chemical composition, its surface characteristics (to-

pography, surface area), the technique of mixing the

ingredients of the formulation (vacuum mixing decreases

porosity), or the addition of antibiotic with or without a

mixer; (d) the characteristics of the release medium (stir-

ring, temperature, volume, or composition) also affect to

the results of the release experiment. All these factors and

the lack of standardized release experiments give as a con-

sequence conflicting results in no end of reported studies.

Moreover, In vivo new factors can be considered, for

instance, the composition of the surrounding medium,

which is obviously different from the physiological

mediums used in the release studies.

Nevertheless, most studies are in keeping with the

releasing mechanism. The antibiotics release in two steps:

first, there is a rapid dissolution of the antibiotic located at

or near the surface, which is essentially a surface phenom-

enon. Usually, an immediate dissolution of antibiotic can

be measured; this is called ‘‘burst effect.’’ After this, there

is a long and gradual release of the antibiotic located at

inner zones of the specimen, which can continue during

some days or months. It has been proven41 that at this

stage of the experiment, the drug is released as the water

absorption process is completed and the release pathways

are created.

In our experiments, we can consider that release is

almost finished after 1 month. If we observe the kinetics,

we can affirm that solubility of the drug is essential for the

release process, since CFX (low solubility, pure) is released

at significantly lower percentages than GS and VM. There

is a trend showing more gradual release of CFX in formu-

lation V due to the incorporation of VM. This leads us to

believe that the porosity induced by the release of VM

would enhance the release of CFX. However, the difference

did not reach statistical significance. It seems that low solu-

bility and the influence of the PMMA are of greater impor-

tance than the diffusion. We must add that even if the

release of CFX was very slow, the quantities released are

far greater than the minimum inhibitory concentration,

which has been reported to be 0.125 lg/l42.The greater X-ray visibility showed by experimental

cements makes them very suitable for use in vertebroplasty

and kyphoplasty. We are confident that the use of powder

mixing techniques or the dissolution of the radiopaci-

fier12,13 would improve the homogeneity and the visibility

even more.

CONCLUSIONS

The new highly radiopaque and injectable cements loaded

with antibiotics formulated for specific use in the field of

VP or KP showed long setting times, which is positive

when considering their injectability, as their viscosities

remained at low values for longer times than conventional

cements. Moreover, their low curing temperatures minimize

the thermal damage to the surrounding tissues.

The release of the antibiotics tested in this research

seem to offer antibacterial properties to the cements since

the concentration attained was higher than the effective

minimum.

In spite of experimental cements did not show better me-

chanical properties than standard cement, we observed that

the addition of antibiotics did not affect the compression per-

formance of the new cements, which complied with the stand-

ard even after 1 month of immersion in saline medium.

Moreover, we are confident about the improvement of the

flexural and impact tests results by using industrial mixing

methods or using the salicylic salt dissolved in the monomer.

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New injectable and radiopaque antibiotic loaded acrylic bone cements

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