Effect of vacuum mixing on the mechanical properties of antibiotic-impregnated polymethylmethacrylate bone cement' Michael J. Askew, Mark F. Kufel, Paul R. Fleissner, Jr., Ivan A. Gradisar, Jr., Sara-Jane Salstrom,+ and James S. Tan* Musculoskeletal Research Laboratory, Department of Orthopaedic Surgery, and *Infectious Disease Research Laboratory, Department of Medicine, Akron City Hospital, Akron, Ohio Polymethylmethacrylate bone cement, containing either no added antibiotic, 0 . 5 g of Vancomycin, 1 . 0 g of Vancomycin, or 1.0 g of Tobramycin, was mixed either in air or a vacuum chamber. Following storage in a water bath at 37°C for 48 h, the specimens were tested in four-point bending. The porosity of the specimens was assessed radiographically, and their antibacterial activity was monitored for 21 days. The bending strength of the vacuum mixed specimens containing no antibiotic was 40% greater than that of similar air-mixed specimens. However, there were no significant differences in the bending strength of either the air-

or vacuum-mixed specimens when any of the antibiotic dosages were added. The bending modulus of the vacuum-mixed specimens, containing no antibiotic, was significantly greater than the moduli of all the other specimen groups which did not differ from each other. Vacuum mixing reduced the apparent porosity of the specimens fivefold, and while the addition of antibiotic did not effect porosity of the airmixed specimens, that of the vacuummixed specimens was doubled. Although initial rapid decreases were seen, leaching of antibiotic from the cement and antibacterial activity continued through the 21day monitoring period.


Antibiotic-impregnated bone cement has been studied pharmacokinetically and clinically, and has been shown to be effective prophylactically against infections following joint replacement surgery.'-5 However, mechanical tests have shown that the antibiotic reduces the tensile, shear, and flexural strengths of the Recently, it has been shown that centrifugation can be used to reduce the porosity of the leading to increased strength and possibly longer fatigue life.12-14~16 The effect of centrifugation on antibiotic-impregnated bone cement has been in~estigated.'~ Similarly, vacuum mixing of the cement has been shown to reduce its poro~ity'~-~' and to increase its and fatigue life.2' In this study, the flexural mechanical properties of vacuum-mixed and air (ambient)mixed samples of antibiotic-impregnated bone cement, aged and tested in water at 3TC, have been investigated. Journal of Biomedical Materials Research, Vol. 24, 573-580 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0021-9304/90/050573-08$04.00



Flexural testing was conducted according to the IS0 Standard IS05833/1 (proposed revision 1986).’l Four-point bending tests of bone cement samples (Zimmer Regular, Zimmer, Warsaw, IN) were carried out in a water bath at 37°C on a materials testing machine (model 812, MTS Systems Inc., Minneapolis, MN) at a support displacement rate of 15 mm/min. The bending load, measured by the load cell of the materials testing system, and the midspan deflection, measured by a linear variable displacement transducer (LVDT, model 7307-X2-A0, Pickering and Co.), were recorded on an X-Y plotter. The bending modulus and the bending strength were calculated according to the methods of IS0 standard 583311.’’ For air mixing, the cement was mixed for 40 s in an open disposable plastic mixing bowl with a plastic spatula performing a clockwise-counterclockwise mixing cycle at a rate of approximately 30 cycles/min. The cement was then poured into a mold. For vacuum mixing, the Zimmer Vacuum Mixing System in its mixing bowl mode was used. The system was used as per the manufacturers instructions and developed 566 +- 3 mm Hg (562-570 mm Hg) of vacuum in 20 s and achieved a near constant vacuum of 628 k 2 mm Hg (621-629 mm Hg) after 60 s. Mixing of the liquid monomer and the powder component was begun 20 s after initiation of the vacuum and proceeded for 40 s using a pattern of alternating clockwise and counterclockwise mixing cycles at a rate of approximately 25 cycles/min. Following mixing the cement was allowed to sit in the vacuum for an additional 30 s, and was then poured into a mold. The mold consisted of a rectangular hole, 90 mm x 75 mm, in a 3.5-mmthick polytetrafluoroethylene (Teflon) plate. The size of the mold was such that it was filled by one standard commercial package of bone cement consisting of 40 g of the powder component and 20 mL of the liquid monomer. The mold was placed on a glass plate with a 0.2 mm thick sheet of polyester film between the glass plate and the mold. After the mixed cement had been poured into the mold, a second polyester film sheet and glass plate were placed on top of the mold and secured by clamps. After 1 h, the cement plate was removed from the mold and cut into specimen strips, 75 mm long, 10 mm wide and 3.5 mm thick, using a water-cooled saw. Each prepared cement plate yielded six test samples. The sides and one face (compression surface in flexural testing) of each sample were wet-ground on 400-grit carborundum paper. The final dimensions of each specimen were measured with a micrometer and recorded. The specimens were then stored in a water bath at 37°C for 48 h. For cement samples impregnated with antibiotic, 1.0 g or 0.5 g of either Tobramycin or Vancomycin (Nebcin and Vancocin, respectively, E. Lilly and Co., Indianapolis, IN), in powder form, was thoroughly mixed with 40 g of the powder component of the bone cement prior to the addition of 20 mL of the liquid monomer. The antibiotic dosages used, the method of mixing, and the numbers of specimens prepared are shown in Table I.



TABLE I Bending Strength and Modulus Results for Control and Antibiotic Bone Cement Samples Antibiotic

Mixing Technique

No. of Samples

Bending Strength (MW

Controls Controls Tobramycin” (1.0 9) Tobramycin”

Air Vacuum Air

12 17 12

47.7 5.9 66.7 k 7.3 45.1 t 7.4

2326 2842 2305




* 6.0


* 170














* 140




* 4.9


* 165

(1.0 €9

Vancomycinb (0.5 9) Vancomy cinb (1.0 g)

Vancomycinb (1.0 a)


Modulus (MW k


* 308 * 250 271

“Nebcin, E. Lilly and Co., Indianapolis, IN. bVancocin HCL, E. Lilly and Co., Indianapolis, IN.

Differences in the mechanical property results obtained from specimens made from separate commercial cement packages and from the different mixing techniques and the addition of the antibiotics were assessed by analysis of variance (ANOVA, F test) followed by Tukey tests ( p < 0.05). Student’s t -tests were also conducted on selected groups of specimens. Following flexural testing, the samples were radiographed to assess their overall porosity and the presence of any large defects, particularly at the fracture site. In addition, 5-mm cubes of antibiotic-impregnated cement were incubated at 37°C either in a dry test tube or in one filled with 2 mL of phosphate buffered saline, pH = 7.4. Antibacterial activity in the cubes and in the fluid were determined by the agar well diffusion technique22on days 0, 1, 2, 3, 4, 5, 7, 14, and 21. RESULTS

Bending strength and bending modulus results for specimens prepared from different commercial cement packages were not significantly different ( p < 0.0005). Thus, data from all specimens in a particular mixing technique and antibiotic group were pooled for analysis and discussion. The bending strengths of the cement specimens, as calculated according to the IS0 standard,*l are shown in Table I. On average, a 40% increase in specimen strength resulted from vacuum mixing of the control, non-impregnated, specimens. This difference, significant at the p < 0.0005 level by t -test, was the only significant difference between specimen groups ( F = 19.7, df = 6/90; Tukey, p < 0.05). Strength results for all groups involving antibiotics or air mixing showed no significant differences by the Tukey test. For air-mixed specimens, the addition of 1 g of Tobramycin re-



sulted in a strength reduction of only 6%, while the addition of 1 g of Vancomycin to air-mixed cement resulted in less than a 1%reduction in strength. However, for the vacuum-mixed specimens, a 21% reduction in strength was seen following addition of 1 g of Tobramycin, a 23%reduction following addition of 0.5 g of Vancomycin, and a 22% reduction following addition of 1 g of Vancomycin. These reductions in the strength of antibiotic-imprepated vacuum-mixed bone cement relative to the non-impregnated vacuum-mixed specimens were significant at p < 0.0005 (Student’s t-test). The bending moduli for the cement specimens, as calculated according to the IS0 standard,” are shown in Table I. The bending modulus of the non-impregnated vacuum mixed specimens was significantly greater than that of all the other specimen groups ( F = 10.14, df = 6/90; Tukey p < 0.05). The bending moduli of the antibiotic-impregnated specimens, vacuumor air-mixed, did not differ significantly from that of the non-impregnated air-mixed specimens. However, the modulus of the vacuum-mixed nonimpregnated specimens was 27% greater than that of the air-mixed non-impregnated specimens ( p < 0.001, Student’s t-test), and was significantly greater than that of all the antibiotic impregnated vacuum-mixed specimen groups ( p < 0.001, Student’s t-test). An increase in the antibiotic level of Vancomycin from 0.5 g to 1.0 g resulted in a 7% reduction in the bending modulus ( p < 0.02, Student’s t-test). The porosity of the specimens was assessed from the radiographs by counting the number of voids of greater than 1 mm in diameter, Table 11. There were at least 5 times as many voids visible in the air-mixed specimens than in the vacuum-mixed specimens. The number of voids in the air-mixed specimens was not changed by the addition of the antibiotic, but the number of voids was doubled in the vacuum-mixed specimens. Inspection of the fracture lines indicated that in only 11 of the 97 specimens had the fracture occurred through one of these 1-mm or more diameter voids. Smaller voids were evident in the specimens and their number appeared to be reduced by vacuum mixing and increased by the addition of antibiotic, but these smaller voids were not quantified. Test cubes impregnated with Tobramycin and incubated dry showed minimal deterioration of antibacterial activity throughout the test period. When incubated wet, these specimens lost 90% of their activity by day 3, but Tobramycin activity was detected in the incubation fluid throughout the

TABLE I1 Specimen Porosity: Average Number of Radiographically Evident Voids per Specimen, Greater than 1 mm in Diameter ~~

Control Air mixed (12 specimens) 5.2 Vacuum-mixed (18 specimens) 0.6 “Vancocin HCL, E. Lilly and Co., Indianapolis, IN.

Vancomycina (1.0 9) 5.1 1.3



test period. Similar results were found for the Vancomycin cubes; which, when incubated in the saline, lost 80% of their activity by day 4 and over 95% by day 14. DISCUSSION

Preparation of the specimens used in this study followed the cement manufacturers directions except that the liquid monomer, powder polymer, and the mixing apparatus were stabilized at ambient room temperature at the time of mixing. Chilling before mixing has been recommended, but its The effect on mechanical properties is uncertain and maybe preparation and testing of the specimens followed the protocol of the IS0 standard,’l except that the displacement rate of the materials testing machine was 15 mm/min rather than 5 mm/min suggested by the IS0 standard.” Rates used by other investigators have varied from 1.3 mm/min up to 25 mm/min.8~’s-2”*23 The values of the bending strength of the bone cement specimens found in this study (Table I) were similar to those reported in the literature which Vacuum mixing of the antibiotic-free specirange from 42 to 82 MPa.8T’9r24-29 mens resulted in a statistically significant increase in their strength as reported by Alkire et al.17 Arroyo,18 Lidgren et al.,” and Wixson et a1.20 Reduction of the number of voids present in these specimens also occurred as the result of vacuum mixing as has been observed by these other author~.’~-~’ Arroyo,” who applied 500 mm Hg of vacuum for 60 s, reported an order of magnitude decrease in porosity, similar to that seen in the present data (Table 11). Lidgren et a1.” observed a 15-30% reduction in porosity due to a vacuum of 600 mm Hg, while Alkire et al.,I7testing at 300 mm Hg to 700 mm Hg, reported that at least 400 mm Hg of vacuum was needed to affect the porosity. A statistically significant reduction of the bending strength of the vacuum-mixed specimens and a doubling of the number of observed voids, 1 mm or more in diameter, resulted from the addition of antibiotic. No reduction in bending strength due to the addition of the antibiotics tested was seen in the air-mixed specimens, and there was no apparent change in the number of observed voids. However, the number of such voids in the antibiotic-impregnated, vacuum-mixed specimens remained 5 times lower than that of the air-mixed specimens. It may be that the fracture process involves voids of smaller size (only 11% of the fractures occurred through the 1 mm or larger voids) which were not quantified. The addition of the antibiotic, and its leaching in the water bath, may increase the number of such smaller voids resulting in the observed reduced bending strength. The presence of voids of all sizes that are to be found in air mixed cementm may dominate the fracture behavior of the specimens and conceal any effect additional porosity due that the addition and subsequent leaching of the antibiotic may have. The bending moduli calculated in this study are similar to values in The bending modthe literature which range from 2000 to 3000 MPa.’8,’9,24-2y



ulus of the vacuum-mixed but antibiotic-free specimens was found to be significantly greater than that of the air-mixed control specimens, as had been reported by Lidgren et al.I9 Arroyo” reported an increase in modulus of the cement with vacuum mixing but the difference was not statistically significant. The level of antibacterial activity in the cement specimens, when incubated in saline at 37”C, decreased rapidly initially,2,4,5 but antibacterial activity, at levels adequate to inhibit most bacteria associated with prosthetic joint infections, was detected in the specimens and in the incubation solution for both of the tested antibiotics throughout the 21-day test period. When incubated dry, the cement specimens demonstrated little loss of antibacterial activity. This demonstrates that the tested antibiotics are stable through the polymerization process of the cement, and are able to leach out of the cement in a wet saline environment. Thus high levels of antibiotic can be provided to the tissue around the cement in the critical period immediately after surgery, and inhibitory levels are maintained for at least 21 days. CONCLUSIONS

1. Vacuum mixing of polymethylmethacrylate bone cement significantly increased its bending strength and modulus and reduced the number of voids present that were 1 mm or larger in size. 2. The addition of antibiotics (Vancomycin and Tobramycin) to vacuummixed bone cement, incubated in a water bath at 3 7 T , reduced its bending strength and modulus to that of air-mixed bone cement that was similarly incubated. The addition of antibiotic to air-mixed cement did not change its bending strength or modulus. 3. The antibacterial activity of both Tobramycin and Vancomycin survived the mixing and polymerization process of the cement. Adequate antibacterial activity continued through 21 days following polymerization. This work was supported, in part, by The Akron City Hospital Foundation, the Infectious Disease Research Laboratory of Akron City Hospital, the Northeastern Ohio Universities College of Medicine, and the Women’s Auxiliary Board of Akron City Hospital. The authors wish to thank Zimmer, Inc. for providing the polymethylmethacrylate bone cement and the vacuum mixing equipment.

References 1. H. W. Buchholz, R. A. Elson, and K. Heinert, “Antibiotic-loaded acrylic cement,” Clin. Orthop., 190, 96-108 (1984). 2. K. E. Marks, C. L. Nelson, and E. P. Lautenschlager, ”Antibiotic impregnated acrylic bone cement,” J. Bone J . Surg., 58A, 358-364 (1976). 3. D. J. Schurman, C. Trindade, H. I? Hirshman, K. Moser, G. Kjiyama, and P. Stevens, “Antibiotic-acrylicbone cement composites,” J . Bone It. Surg., 60A, 978-984 (1978). 4. S. B. Trippel, ”Current concepts review: Antibiotic impregnated cement in total joint arthroplasty,” I . Bone It. Surg., 68A, 1297-1302 (1986).



7. 8.

9. 10.


12. 13. 14.

15. 16.

17. 18. 19.

20. 21. 22. 23.


H. Wahlig and E. Dingeldein, ”Antibiotics and bone cements,”Acta Orthop. Scand., 51, 49-56 (1980). D. Dall, E. Ger, T. Miles, and A. Forder, “A preliminary report on the material strength and antibacterial activity of methylmethacrylateantibiotic mixtures,” 1. Bone Jt. Surg., 58B, 390 (1976). A. Hofman and W. Krause, ”Mechanical strength of antibiotic impregnated acrylic bone cement,” Trans. 28th Orthopaedic Research Society Meeting, 1982, p. 246. E. P. Lautenschlager, J. J. Jacobs, G. W. Marshall, and P. R. Meyer, ”Mechanical properties of bone cements containing large doses of antibiotic powders,” J, Biomed. Muter. Res., 10, 929-938 (1976). J.W. Moran, A. S. Greenwald, and M. Matejczyk, ”Effect of gentamicin on shear interface strengths of bone cement,” Clin. Orthop., 141, 96-101 (1979). R. C. Nelson, ”Effect of antibiotic type and concentrations on the mechanical properties of acrylic cement as a function of aging environment and time,” Trans. 4th Annual Meeting Society for Biomaterials, 1978. A.M. Weinstein, D.N. Bingham, B. W. Sauer, and E.M. Lunceford, “The effect of high pressure insertion and antibiotic inclusions upon the mechanical properties of polymethylmethacrylate,” Clin. Ovthop., 121, 67-73 (1976). D. W. Burke, E. I. Gates, and W. H. Harris, ”Centrifugation as a method of improving tensile and fatigue properties of acrylic bone cement,” J. Bone It. Surg., 66A, 1265-1273 (1984). J. P. Davies, D. W. Burke, D. 0. O‘Connor, and W. H. Harris, “Comparison of the fatigue characteristics of centrifuged and uncentrifuged Simplex P bone cement,” J. Orthop. Res., 5(3), 366-371 (1987). J. I? Davies, D. 0. O’Connor, D. W. Burke, and W. H. Harris, ”Influence of antibiotic impregnation on the fatigue life of Simplex P and Palacos acrylic bone cements, with and without centrifugation,” J. Biomed. Muter. Res., 23, 379-397 (1989). M. Jasty, N. F. Gensen, and W. H. Harris, “Porosity measurements in centrifuged and uncentrifuged commercial bone cement preparations,” Trans. 10th Annual Meeting, Society for Biomaterials, 1984, p. 46. C. M. Rimnac, T. M. Wright, and D. L. McGill, ”The effect of centrifugation on the fracture properties of acrylic bone cements,” I . Bone Jt. Surg., 68A, 281-287 (1986). M. J. Alkire, E. J. Dabezies, and P. R. Hastings, ”High vacuum as a method of reducing porosity of polymethylmethacrylate,” Orthopaedics, 10, 1533-1539 (1987). N. A. Arroyo, ”Physical and mechanical properties of vacuum mixed bone cement,” Trans. 12th Annual Meeting, Society for Biomaterials, 9, 187 (1986). L. Lidgren, H. b a r , and J. Moller, “Strength of polymethylmethacrylate increased by vacuum mixing,” Acta Orthop. Scand., 55, 536-541 (1984). R. L. Wixson, E. P. Lautenschlager, and M. Novak, ”Vacuum mixing of methylmethacrylate bone cement,” Trans. 31st O.R.S., 10 327 (1985). Implants for Surge ry -Acrylic Resin Cements, Part 1: Vrthopaedic Appiications, (Revised version of I S0 58330-1979) International Standards Organization, Geneva, Switzerland, 1986. J. V. Bennett, J. L. Brodie, E. J. Benner, and W. M. M. Kirby, “Simplified accurate method for antibiotic assay of clinical specimens,” Appl. Microbiol., 14, 170-177 (1966). E. P. Lautenschlager, G. W. Marshall, K. E. Marks, J. Schwartz, and C. L. Nelson, ”Mechanical strength of acrylic bone cements impregnated with antibiotics,“ 1. Biomed. Muter. Res., 10, 837-845 (1976).



24. J. R. deWijn, T. J. J. H. Sloff, and F. C. M. Driessens, ”Characterization of bone cement,” Acfu Orfhop. Scund., 46, 38-51 (1975). 25. N. S. Eftakhar and C. W. Thurston, “Effect of irradiation of acrylic cement with special reference to fixation of pathological fractures,” J. Biomechanics, 8, 53-56 (1975). 26. S. S. Haas, G. M. Brauer, and M. A. Dickson, “A characterization of polymethylmethacrylate bone cement,” J. Bone Jf. Surg., 57A, 380-391 (1975). 27. N.J. ’Holm, ”The modulus of elasticity and flexural strength of some acrylic bone cements,” Actu Orfhop. Scund., 48, 436-442 (1977). 28. A. Knoell, H. Manwell, and C. Bechtol, ”Graphite fiber reinforced bone cement,” Ann. Biorned. Eng., 3, 225-229 (1975). 29. S. Saha and S. Pal, ”Mechanical properties of bone cement: A review,” 1. Biomed. Mafer. Res., 18, 435-462 (1984). 30. U. Linden, ”Porosity of manually mixed bone cement,” Clin. Orfhop., 231, 110-112 (1988).

Received March 2, 1988 Accepted October 24, 1989

Effect of vacuum mixing on the mechanical properties of antibiotic-impregnated polymethylmethacrylate bone cement.

Polymethylmethacrylate bone cement, containing either no added antibiotic, 0.5 g of Vancomycin, 1.0 g of Vancomycin, or 1.0 g of Tobramycin, was mixed...
495KB Sizes 0 Downloads 0 Views