The Journal ofArthroplasty Vol. 10 No. 4 1995

Effec t of Vary ing Sur face P a t t e r n s o n A n t i b i o t i c E l u t i o n F r o m A n t i b i o t i c - l o a d e d B o n e C e m e n t

B a s s a m A. M a s r i , M D , F R C S C , C l i v e P. D u n c a n , M D , F R C S C ,

C h r i s t o p h e r R B e a u c h a m p , M D , F R C S C , N a n c y J. P a r i s , B A S c ,

a n d J e n n i f e r A r n t o r p , B A S c

Abstract: In an effort to improve the antibiotic elution characteristics of the pros- thesis of antibiotic-loaded acrylic cement, an in vitro study was conducted. Tobramycin-loaded bone cement blocks of three different surface patterns with dif- ferent surface area-to-volume ratios were used. The elution of tobramycin over a 2- month period was investigated. There was a gradual decline in the tobramycin elution rate over time. The surface pattern with the increased surface area-to-vol- ume ratio showed a significant increase in the tobramycin elution rate over the first week of the study. The surface pattern with ridges but no change in the surface area- to-volume ratio did not result in a statistically significant increase in the tobramycin elution rate. Key words: antibiotic, elution, cement, surface, arthroplasty, infection.

The in t roduc t ion of an t ib io t ic - loaded bone cement by Buchholz and Engelbrecht ~ significantly changed the m a n a g e m e n t and out look of infected total joint ar throplas ty prostheses. Other antibiotic depots have been investigated2-~; however , bone cement continues to be the mos t c o m m o n l y used carrier today. The elution characteristics of antibiotics f rom bone cement have been well described. 7-'°

The prosthesis of antibiotic-loaded acrylic cement (PROSTALAC) system ~-14 has been the t r ea tment of choice for infected total hip and knee prostheses at our insti tution for the past 3 years (Fig. 1). We have already repor ted an infection recurrence rate of only 6% in 64 such t reated infected total hip and knee arthroplasties, x4 This system consists of t empo- rary hip and l~nee prostheses that are made pr imar- ily of an t ib io t ic - loaded bone c e m e n t wi th a low-friction articulating surface. This prosthesis acts

From the Department of Orthopaedics, Vancouver Hospital and Health Sciences Centre, and University of British Columbia, Vancouver, British Columbia, Canada.

Reprint requests: Bassam A. Masri, MD, Department of Orthopaedics, Vancouver Hospital and Health Sdences Centre, Third Floor, Laurel Pavilion, 910 W. Tenth Avenue, Vancouver, British Columbia, Canada V5Z 4E3.

as a functional spacer that permits joint mobil i ty and stability, while the pat ient awaits re implanta- t ion of a definitive prosthesis. While in situ, this prosthesis ehites high concentrat ions of antibiotics, wi th levels well main ta ined up to 3 mon ths follow- ing implantat ion. 15

Schurman et al., 16 as well as others, ~7.~8 suggested that the in vitro antibiotic elution f rom acrylic cement is propor t iona l to the surface area of exposed cement in a liquid medium. The purpose of this study was to test this hypothesis by means of an in vitro experiment, in which the effect of increasing the surface area of bone cement by applying a prac- tical, easily manufac tured pat tern to the surface was investigated, to determine if the elution characteris- tics of the PROSTALAC system could be improved by a simple alteration of the surface pattern.

Materials and Methods

Machinable polyethylene molds were used to manufac ture bone cement blocks. Three different surface pat terns were used to produce ridged sur- face p a t t e r n s on fou r of the six sur faces of the cemen t blocks. Simplex P bone cement

453

454 The Journal of Arthroplasty Vol. 10 No. 4 August 1995

Fig. 1. Hip prosthesis of antibiotic-loaded acrylic cement (PROSTALAC).

(Howmedica, Rutherford, NJ) containing 2.4 g t o b r a m y d n per 40 g bone cement was used. The three surface patterns were as follows. Group 1 spec- imens had a smooth surface and acted as the control pattern. The dimensions of each block were 0.5 x 0.5 x 1 inches. These blocks had a surface area of 2.5 square inches and a vo lume of 0.25 cubic inches. Group 2 specimens had a surface pat tern that con- sisted of four rows of hemispheric ridges 1/8-inch diameter on each of the four rectangular sides of the block, giving a surface area of 3.04 square inches and a volume of 0.31 cubic inches. Group 3 speci- mens had a surface pat tern of eight rows of hemi-

spheric ridges 1/16-inch diameter on each of the four rectangular sides of the block, giving a surface area of 3.04 square inches and a vo lume of 0.28 cubic inches. Both groups 2 and 3 gave a surface area increase; however, there was a proport ional increase in volume in the group 2 blocks, but not in the group 3 blocks. There was no significant increase in the surface a rea- to-volume ratio be tween groups 1 and 2. The surface a rea- to-volume ratio of group 3 was 9% greater than that of group 1 (Table 1).

The surface pat terns were chosen in consultat ion wi th our PROSTALAC mold manufac tu re r , to ensure that the chosen pat terns could be easily manufac tu red and incorporated into the system, if the results of the s tudy war ran ted such a change. The group 2 pa t te rn was chosen to show that any significant change in the elut ion rate of tobramycin f rom bone cement observed in this s tudy was due to the relative increase in surface area, and not to the addition of ridges to the surface.

Six sets of t o b r a m y c i n - l o a d e d bone c e m e n t blocks were made using the three groups of molds. The cement blocks were r emoved f rom the molds and stored overnight at 4°C. The blocks were then immersed in 18 mL of no rma l saline at 37°C. The beakers were covered and incubated at 37°C for the durat ion of the exper iment . Samples of 2 mL each were obtained at 1, 6, 24, and 48 hours, t hen weekly for 1 month ; one final spec imen was obtained 1 m o n t h later. Prior to sampling, the blocks were r emoved and the solution was stirred vigorously l0 t imes clockwise and I0 t imes coun- terclockwise to ensure un i fo rm sampling. The beakers and blocks were rinsed and the blocks were re immersed in a fresh 18 mE aliquot of normal saline. These samples were frozen at -20°C until they were analyzed.

Tobramycin assays on the eluant fluid was per- fo rmed using a fluorescence polarizat ion i m m u n o - assay. The tob ramyc in concentra t ions ob ta ined were use to calculate the tobrarnycin elut ion rates for the three different groups.

A one -way analysis of var iance (ANOVA) was used to detect a statistically significant difference in

Table 1. Surface Areas and Volumes of Different Patterns

Surface Area Vo lume A (SA/Volume) Group Pattern ( square inches) (cubic inches) SA/Vo lume (%)

l Smooth 2.5 0.25 10 0 2 Four rows, 1 /8- inch diameter 3.04 0.31 9.8 -2 3 Eight rows, 1 /16- inch diameter 3.04 0.28 10.9 9

The change in surface a rea - to -vo lume ratios wi th respect to the control pa t te rn (group 1) is s h o w n as A(SA/Volume). Note tha t group 2 blocks represent the effect of adding ridges w i thou t increasing the surface area- to -vo lume ratio. In group 3, the addit ion of ridges caused a 9% increase in the surface a rea - to -vo lume ratio.

Surface Pattern and Tobramycin Elution • Masri et al. 455

mean elution rates be tween groups 1, 2, and 3 (P < .05). A separate analysis was performed for each elution period. For those periods that showed a statistically significant difference in the means (P < .05), Duncan's mult iple-range test was used to test be tween the three subgroups for statistical sig- nificance. The change in mean elution rate over time was also investigated using ANOVA to test whe the r the change in elution rates with time was of statistical significance (P < .05).

Results

groups. Using Duncan's multiple-range test, we were able to demonstra te a statistically significant difference in the m ean elution rates of groups 1 and 3 only for the first week. The significance level was .01 up to 48 hours and .05 at 1 week. There was no statistically significant difference in m ean elution rates at later t ime intervals, because of the signifi- cant decline in elution. Using the previously men- t ioned significance levels, Duncan's multiple-range test also showed that there was no statistically sig- nificant difference in the m ean elution rates of groups 1 and 2 at any time during this study.

Successful assays were obtained for five of six blocks in each of the three groups. Based on the known volume of saline in which each block was immersed, the amoun t of eluted antibiotics was easily calculated. Because the saline bath was dis- carded with each assay, it was possible to calculate the total amoun t of eluted antibiotics at any one time in the study. This allowed us to calculate the tobramycin e lut ion rates for each group. The tobramycin concentrat ions and elution rates for the durat ion of the study are shown in Table 2.

Tobramycin concentra t ions and elut ion rates deteriorated with time (Figs. 2, 3). Using ANOVA, this deterioration was statistically significant (F ratio = 355.32, tl 2 = .9576). Using Duncan's multi- ple-range test with a significance level of .05, we determined that this statistically significant decline cont inued up to and including the 48-hour assay. Further assays, however, failed to show a signifi- cant change in the elution rate, suggesting a plateau in the elution rate.

When the mean elution rates of blocks from groups 1, 2, and 3 were compared using ANOVA, there was a statistically difference be tween the means of the three groups up to and including 1 week into the study (P < .05) (Table 3). Beyond the 1-week mark, there was no statistically significant difference in the mean elution rates of the three

Discussion

The elution of numerous antibiotics, such as oxacillin, cloxacillin, methici l l in, l incomycin, cephalosporins; colistin, fuddin, neomycin, kan- amycin, ampicillin, gentamicin, tobramycin, ery- thromycin, and penicillin/-1° from bone cement has been well demonstrated. This elution has been shown to be maintained for prolonged periods in in vivo and in vitro studies. 7-~°,1~,~9-21 The mechanism of elution of antibiotics from bone cement has been debated in the literature. Flynn described two types of mechanisms for release of biologically active agents from solid depots. 22 The first mechanism is that of dissolution of the drug part ide within the liq- uid milieu. This allows for the formation of sol- vent-solute capillaries within the matrix of the solid depot. The other mechanism involves diffusion of the drug through the solid matrix. Bayston and Milner demonstrated that antibiotics continue to elute from bone cement for a finite period; following that, they failed to demonstrate any bactericidal effect of antibiotic-loaded bone cement blocks? 3 When the bone cement blocks ceased to elute any more antibiotics, they extracted the residual antibi- otics. Ninety-four percent of the gentamicin used in their bone cement blocks was extracted following cessation of elution of antibiotics. Other antibiotics

Table 2. Mean Tobramycin Concentrations and Elution Rates

Tobramycin Concentrat ion (rag/L) Tobramycin Elut ion Rate (mg/h)

Time Group 1 Group 2 Group 3 Group 1 Group 2 Group 3

I hour 87.I7 98.8 I09.17 1.39 1.78 2.01 6 hours 58 73 82.5 0.4 0.52 0.59 24 hours 65.5 87.17 100.67 0.15 0.2 0.23 48 hours 50.33 46.83 60.6 0.09 0.12 0.14 1 week 93.33 112.33 130 0.04 0.05 0.05 2 weeks 66 76.83 89.5 0.02 0.03 0.03 3 weeks 58 46.5 56.33 0.01 0.02 0.02 4 weeks 52.67 37.17 45 0.01 0.02 0.02 8 weeks 132 103.33 105.83 0.01 0.01 0.01

456 The Journal of Arthroplasty Vol. 10 No. 4 August 1995

Fig. 2. Change in average tobramycin concentration in the saline bath with time.

140

~, 100 , , / / X \ \ , , , / , , ' , , . . " , / . / ,oo ,/"X;, //" '~ 6o \" , '/

4 0

20

~ " . . ' /

I I I I I :

6 24 48 1 6 8 336 504 672 l ime (h~

1344

seemed to elute a larger proportion of the drug. Only 60.6% of impregnated sodium fusidate was extracted from the Palacos R bone cement (Smith & Nephew Richards, Memphis, TN), and only 56.8% was extracted from CMW bone cement (Wright, Arlington, TN). In another experiment, they failed to observe any elution of antibiotics from antibiotic- loaded bone cement when plain bone cement was attached to the surface of the loaded cement. Based on these results, the authors concluded that antibi- otics diffuse through the matrix of bone cement. This diffusion is slow, and the available analytic methods were unable to measure such a phenomenon. A crit- ical review of the data presented does not support such a conclusion, and the diffusion theory is not supported by their work. Elson et al. showed that antibiotics may diffuse from loaded bone cement across necrotic bone into the surrounding fluid milieu. ~9 This important observation supports the use of antibiotic-loaded bone cement in the t reatment of osteomyelitis; however, it does not support the dif- fusion theory because of the porous nature of bone.

Other authors argued against the theory of diffu- sion of drugs f rom bone cement . Wroblewski demonstrated that at most 11.87% of sodium chlo- ride eluted from CMW bone cement, and that this

elution was proportional to the surface area of the tested bone cement disks. ~s He concluded that salt did not diffuse through the matr ix of bone cement and that elution is a surface p h e n o m e n o n related to the exposed surface of antibiotic-loaded bone cement. Levin demonstrated effective elntion of d indamycin and cephalothin from Simplex P bone cement. '4 In an effort to elucidate the mechanism of elution, he immersed bone cement blocks in water for a 13-month period. He noted a steady gain in mass of about 5% per year, most of which occurred during the first year. I-Ie concluded that the elution of antibiotics is due to the dissolution of surface antibiotic particles within water that are absorbed through the pores of the bone cement. Increasing the surface area available is thus expected to improve the rate of elution. Marks et al. demon- strated that in vitro Palacos R elutes larger amounts of oxacillin, gentamicin, and cefazolin and main- tains this elution for a longer period than Simplex P bone cement. 9 Based on scanning electron micro- scopy studies that showed a larger pore size in Palacos R bone cement compared with Simplex P bone cement, Marks et al. concluded that their observation of improved elution with Palacos R

Fig. 3. Change in average tobramycin elution rate with time.

.= \ ' , ~1~ \',,

[] V,

\'i) 0 . 5

0 '

48 168 336 "[im, e (hcs)

: "" =' I 504 672 1344

- - Group 1

- - - - Group 2

. . . . Group 3

Surface Pattern and Tobramycin Elution • Masri et al. 457

Table 3. Statistical Analysis of the Mean Elution Rates

One-way Analysis of Variance (ANOVA)

Time F Ratio F Probability (P value)

1 hour 11.69 .001 6 hours 11.15 .002 24 hours 8.27 .006 48 hours 6.89 .01 1 week 4.41 .04 2 weeks 3.55 .06 3 weeks 2.35 .14 4 weeks 1.59 .25 8 weeks 0.54 .6

The ANOVA showed that there was a significant difference in elution rates between the three groups up to 2 weeks into the study. Duncan's multiple-range test showed that this difference was due to a statistically significant difference between groups 1 and 3 only.

bone cement is related to the increased pore size with the subsequent increase in the microscopic overall surface area of exposed bone cement. 9 This finding fur ther supports the hypothesis that elution of antibiotics f rom bone cement is a passive surface p h e n o m e n o n that is not dependent on diffusion th rough the matr ix of bone cement . In a series of exper iments designed to study the mechan i sm of elution of antibiotics f rom bone cement, Baker and Greenham demons t ra ted that antibiotic molecules escape the confines of bone cement through pores and cracks wi thin the cement mat r ixY In one experiment , they demonst ra ted that nei ther gen- tamicin nor me thy lene blue was able to diffuse th rough a 0 .8-ram disk of plain Palacos R bone cement w h e n the disk was glued to the end of a test tube containing a gentamicin and methy lene blue solution. W h e n this plain cement disk was sec- tioned, no tide m a r k was seen, suggesting that there was no diffusion of me thy lene blue across the bone cement. This finding argues against the diffusion theory. Baker and Greenham also studied the elu- t ion characteristics of bone cement disks of various thicknesses. The elution rate was independent of the thickness of the disk w h e n the disk was thicker than 10 gin. This also suggests that elution of antibi- otics is a surface p h e n o m e n o n that is dependent on the surface area of exposed bone cement . With disks th inner than 10 gin, the core of the disk is so super- ficial that core antibiotics are able to escape. Ten micrometers might represent the surface layer that was capable of eluting antibiotics in that experi- ment . The same authors also demonst ra ted empty pores and voids wi thin bone cement using scanning electron microscopy following elution of gentam-

icin. These voids represented the empty pores that are left behind following elution of antibiotic parti- cles.

On the basis of the above studies, there is no doubt that elution of antibiotics is a surface phe- n o m e n o n that is related to pores and cracks wi thin the bone cement matrix. It is then no surprise that the major i ty of impregnated antibiotics remain en t rapped within the cement core, and that elut ion is improved with increasing surface area.

The purpose of this s tudy was not to demons t ra te that increasing the surface area of bone cement does indeed improve the elution characteristics. Our goal was to investigate the efficacy of a clini- cally applicable surface pa t te rn that is easily applied to the surface of our t empora ry PROSTALACs in significantly improving antibiotic elution. The two pat terns tested were the pat terns suggested by the manufac tu re r of our prostheses. The na r row ridge pattern, which gave a 9% increase in the surface a rea - to -vo lume ratio, is easily applied to our pros- theses and will significantly increase early antibiotic elution rates, as was shown in this study. This increase in elution was no ted only for the first week of the study. Our inability to demons t ra te a significant difference in the elution rates after the 1-week m a r k is mos t likely related to the diminu- t ion of the elut ion rate to such a low level that any compar ison with any degree of accuracy is impossi- ble. The presence of surface ridges that do not al low an increase in the surface a rea - to -vo lume ratio, as in group 2 in this study, did not al low for a statisti- cally significant increase in tobramycin elution. This seems to suggest that it is indeed the relative increase in surface area, and not the mere presence of ridges on the surface, that al lowed an increase in antibiotic elution.

Despi te the stat ist ical s ignif icance of these resul ts , the clinical s igni f icance of such an increase in antibiot ic e lu t ion is no t known . The increase in e lu t ion seen in this s tudy occurred over the first w e e k only. This is a t ime w h e n antibiotic e lu t ion is max ima l , and local antibiot ic levels o f ten exceed the m i n i m a l inh ib i tory con- cen t ra t ion for mos t pa thogens . In per ipros the t ic infect ions , g lycoca lyx f o r m a t i o n m a y g rea t ly increase the m i n i m u m inh ib i to ry concen t ra t ion of o therwise sensit ive organisms, and an increase in the ear ly antibiot ic concen t ra t ions m a y be of benefi t . The ex ten t of this change in m i n i m u m inh ib i to ry concen t ra t ion is variable, depends on the individual organism, and is difficult to predic t wi th accuracy. Moreover , w h e n re la t ively resis- t an t organisms are be ing t rea ted wi th ant ibiot ic- loaded bone cement , it m a y be advan t ageous to

458 The Journal of Arthroplasty Vol. 10 No. 4 August 1995

maximize early local antibiotic concentra t ions in order to maximize early bacterial kill and mini- mize the risk of infection persistence or recur- rence. Fur ther in vitro and in vivo work is needed before m a n y of these questions m ay be answered.

The findings of this s tudy m a y also be applied in other ways. If ant ibiot ic- loaded cement beads are used in the t r ea tment of infected fractures 2~ or prostheses, 26,27 a donu t instead of a bead may improve the elut ion by increasing the surface area. By appl icat ion of the same principles, smaller beads are preferable to larger beads because of the relatively increased surface area while using the same vo lume of bone cement . If cement blocks are used in the t r ea tmen t of infected total knee arthroplasties, 2s the addit ion of a textured pa t te rn may improve the elut ion rate as well.

In summary, we were able to describe a surface pa t te rn that w h e n applied to the surface of PROSTALACs, might be capable of increasing t o b r a m y c i n e lu t ion rates for the first week. Whe the r this increase in elut ion is of clinical sig- nificance remains to be seen. This s tudy also pro- vides a model by which antibiotics that are less well released f rom bone cement, such as van- comycin, may be studied to a t tempt to maximize their elut ion potential .

Acknowledgment

The authors thank Dr. John Keating for his help with the statistical analysis.

References

1. Buchholz EIW, Engelbrecht I-I: Uber die Depotwirkung einiger Antibiotica bei Vermischung dem Kunstharz Palacos. Chirurg 41:511, 1970

2. Duncan CE Masri B: Antibiotic depots. J Bone Joint Surg 75B:349, 1993

3. Gerhart TN, Roux RD, Horowitz Get al: Antibiotic release from an experimental biodegradable bone cement. J Orthop Res 6:585, 1988

4. Lawson IB, Duncan CE Beauchamp CP et al: The elution of tobramycin from Silastic: a potential vehi- de for local antibiotic delivery. Poster presentation at Canadian Orthopaedic Association Annual Meeting, Calgary, Canada, 1990

5. Perry CR, Ritterbusch JK, Rice SH et al: Antibiotics delivered by an implantable drug pump: a new application for treating osteomyelitis. Am J Med 80(suppl):222, 1986

6. Shinto Y, Uchida A, Korkusuz F et al: Calcium hydroxyapatite ceramic used as a delivery system for antibiotics. J Bone Joint Surg 74B:600, 1992

7. Goodell JA, Flick AB, Herbert JC, Howe JG: Preparation and release characteristics of tobramycin-impregnated polyrnethylmethacrylate beads. Am J Hosp Pharm 43:I454, 1986

8. Hoff SE Fitzgerald RH, Kelly P J: The depot adminis- tration of penicillin G and gentamicin in acrylic bone cement. J Bone Joint Surg 63A:798, 1981

9. Marks KE, Nelson CL, Lautenschlager EP: Antibiotic- impregnated acrylic bone cement. J Bone Joint Surg 58A:358, 1976

10. Murray WR: Use of antibiotic-containing bone cement. Clin Orthop 190:89, 1984

l l. Duncan CP, Beauchamp CP: The antibiotic-loaded hip replacement: a valuable tool in the management of the complex infected total hip arthroplasty. J Bone Joint Surg 73B (suppl II): 115, 1991

12. Duncan CP, Beauchamp CP: Total hip replacement for the management of chronic hip infection. J Bone Joint Surg 74B(suppl III):275, 1992

13. Duncan CP, Beauchamp CP, Masri Bet al: The antibi- otic loaded joint replacement system: a novel approach to the management of the infected knee replacement. J Bone Joint Surg 74B(suppl III):296, I992

14. Duncan CP, Beauchamp CP, Breault M e t aI: Antibiotic loaded joint replacement system: a novel approach to the management of the infected knee or hip replacement. Presented at the 60th Annual Meeting of the American Academy of Orthopaedic Surgeons, San Francisco, CA, 1993

15. Masri BA, Duncan CP, Beauchamp CP et al: Antibiotic elution from bone cement, an in vivo and in vitro study. Presented at the 48th Annual Meeting of the Canadian Orthopaedic Association, Montreal, Canada, June I993

16. Schurman DJ, Trindade C, Hirshman HP et al: Antibiotic-acrylic bone cement composites: studies of gentamicin and Palacos. J Bone Joint Surg 60:978, 1978

17. Baker AS, Greenham LW: Release of gentamicin from acrylic bone cement: elution and diffusion studies. J Bone Joint Surg 70A:1551, 1988

18. Wroblewski BM: Leaching out from acrylic bone cement: experimental evaluation. Clin Orthop 124:31 I, 1977

19. Elson RA, Jephcott AE, McGechie DB, Verettas D: Antibiotic-loaded acrylic cement. J Bone Joint Surg 59B:200, 1977

20. Hill J, Klinerman L, Trustey S, Blowers R: Diffusion of antibiotics for acrylic bone-cement in vitro. J Bone Joint Surg 59B:197, 1977

21. Seligson D, Popham GJ, Voos K et al: Antibiotic- leaching from polymethylmethacrylate beads. J Bone Joint Surg 75A:714, 1993

22. Flynn GL: Influence of physico-chemical properties of drugs and system of release of drugs from inert

Surface Pattern and Tobramycin Elution • Masri et al. 459

matrices, p. 73. In Tanquary C, Lacey RE (eds): Controlled release of biologically active agents: advances in experimental biology and medicine. Vol. 47. Plenum, New York, 1974

23, Bayston R, Milner RDG: The sustained release of antimicrobial drugs from bone cement. J Bone Joint Surg 64B:460, 1982

24. Levin PD: The effectiveness of various antibiotics in methylmethacrylate. J Bone Joint Surg 57B:234, 1975

25. McNally MA, Small JO, Tofighf HG, Mollan RAB: Two-stage management of chronic osteomyelitis of

the long bones: the Belfast technique. J Bone Joint Surg 75B:375, 1993

26. Borden LS, Gearen PF: Infected total knee arthro- plasty: a protocol for management. J Arthroplasty 2:27, 1987

27. Hovelius L, Josefsson G: An alternative method for exchange operation of infected arthroplasty. Acta Orthop Scand 50:93, 1979

28. Wilde AN, Ruth JT: Two-stage reimplantation in infected total knee arthroplasty. Clin Orthop 236:23, 1988