Adhesive bone cement containing hydroxyapatite particle as bone compatible filler Kazuhiko Ishihara,* Hiroki Arai, and Nobuo Nakabayashi Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Kanda-surugadai, Chiyoda-ku, Tokyo 101, Japan Sadao Morita and Kotaro Furuya Department of Orthopedic Surgery, Faculty of Medicine, Tokyo Medical and Dental University, Yushima, Bunkyo-ku, Tokyo 107, Japan Acrylic bone cement containing hydroxyapatite (HA) as a filler was developed using 4-methacryloyloxyethyl trimellitate anhydride (4-META) to promote adhesion both to bone a n d HA. The mechanical strengths of the cement did not decrease significantly with increasing HA in the cement by 4-META. However, strengths decreased with increasing HA content in the absence of 4-META. Scanning electron
micrographic examination of fractured surfaces of the cement clearly showed that the HA particles adhered to the matrix resin when 4-META was added. Thus, it was important to maintain the original mechanical strengths for 4-META. The HA particles along the surface increased with increased HA content in the cement. The cement adhered to bone with a tensile bond strength was higher than 10 MPa.
INTRODUCTION
In orthopedic and dental surgery, the fixation or attachment of artificial implants to hard tissues such as bone and tooth continues to be a major area of interest. The standard method of implant fixation is to use an acrylic cement primarily composed of poly(methy1 methacrylate) (PMMA).’ Though immediate fixation of the implants is an advantage in this method, shrinkage of the cement during polymerization can inf hence loosening of the implants. It is very important to minimize gaps between the hard tissues and the implants. Adhesion may be effective in solving this p r ~ b l e m . ”In~our previous article, we presented a new cement composed of 4-methacryloyloxyethyl trimellitate anhydride (4-META) and methyl methacrylate (MMA) as monomers, tri-nbutyl borane (TBB) as an initiator and PMMA powder (4-META/MMA-TBB cement) which adhered both to bone and metals: The tensile strength between bone and metals adhered with the modified cement was above 7 MPa. We also found that the 4-META/MMA-TBB cement could adhere to ceramics, *To whom correspondence should be addressed. Journal of Biomedical Materials Research, Vol. 26, 937-945 (1992) ccc 0021-9304/92/070937-09$4.00 0 1992 John Wiley & Sons, Inc.
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hydroxyapatite (HA), and a composite of HA and fluoroapatite (FAP) with a tensile bond strength of 15 MPa.’ Recently, several types of bioactive ceramic coatings on prostheses designed to enhance attachment of the prostheses to bone have been This type of coating has the potential advantage of producing an intermediate region between bone and the implant to enhance the transition of stress between them. Moreover, the bone active ceramic coating has shown new bone growth on the surface and direct attachment of the implant to the bone. However, bone growth requires a longer period to fix the prostheses. If an acrylic bone cement contained a bone active component, fast fixation of the prostheses and direct bone attachment on the surface might be expected. An adhesive bone cement containing HA particles as a bone compatible filler within a 4-META/MMA-TBB cement is reported in this communication. The mechanical properties, surface characteristics of the cement, and the adhesion to bone were investigated with attention on the effect of 4-META and HA compositions in the cement.
EXPERIMENTAL
Materials
4-META was prepared as described previously and recrystallized with benzene.’ HA particles of 5 and 15 pm, average diameters, were supplied from Pentax Co. Ltd., Tokyo, Japan and used without further treatment. The MMA, PMMA, and TBB (San-Medical Co., Kyoto, Japan) were of standard commercial origin.
Preparation of bone cement containing HA
In 0.4 mL of monomer, 5 mg of TBB (three drops from a syringe with a 22gauge needle) were added and stirred quickly to oxidize the TBB in air to introduce the initiation phase. One gram of the powder component of the bone cement (PMMA and a mixture of PMMA and the desired amount of HA) was mixed with the monomer solution and stirred until the mixture became paste like with a high viscosity like a conventional bone cement. This viscous mixture was injected and cured in a Teflon mold to make specimens for testing. Measurement of mechanical strength
The compressive, tensile, and bending strengths of the cements were measured using an autograph testing machine (Shimadzu DDS-500, Kyoto, Japan), with a crosshead speed of 2 mm/min. The specimen size was 6 mm X 12 mm for compressive and 2 mm X 2 mm X 30 mm for tensile and bending strength measurements. The cement was cured in a Teflon mold and kept for 60 min at
ADHESIVE BONE CEMENT CONTAINING HA
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room temperature. The specimens were removed from the mold and dried in zlucuo for 15 h to determine the dry weight. One group of specimens was immersed in water at 37°C for 7 days. Fractured surfaces were examined with a scanning electron microscope (SEM, Jeol JSM-5400, Tokyo, Japan). The mean of each mechanical test group was compared by analysis of variance and Student’s t test, with the value of statistical significance set at the p < 0.05 level. The number of samples used for each measurement was 5.
Measurement of tensile bond strength Tensile bond strength was measured by the same procedure as described previously using human femur^.^ The mean of each bond strength test group was compared by analysis of variance and the Student’s t test, with the value of statistical significance set at the p < 0.01 level. The number of samples used was 4. In the same way, the cement was placed on a stainless-steel (SUS-304) rod with a smooth surface 4 mm in diameter and another rod pasted onto it. The tensile bond strength was measured after the specimen was stored in water for 24 h at 37°C.
Other measurement X-ray photoelectron spectroscopic (XI’S) analysis was carried out on the 5 mm in diameter specimens of cured bone cement using a Shimadzu ESCA750 (Kyoto, Japan). Weight gain due to water adsorption in the cured specimen and the tensile test was measured using a Mettlar H-20 type balance (min = 0.01 mg) after immersion in water for 1 month at 37°C.
RESULTS A N D DISCUSSION
Mechanical properties of bone cement containing HA Figures 1 through 3 show dependence of the mechanical properties of bone cement on the HA composition with or without 4-META. In the absence of 4-META, the values of the compressive proportional limit did not depend on the HA composition in the powder component; whereas, the values of the tensile strength and bending proportional limit decreased with increased HA composition. These mechanical properties were improved by the addition of 4-META in the MMA-TBB cement. Moreover, a combination of HA in the powder did not have an adverse effect on the mechanical strength even with a higher HA ingredient composition. These findings clearly showed that 4-META effectively promoted adhesion of HA to the matrix resin5 and the bonding prevented weakening due to HA. The SEM examination of frac-
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Figure 1. Compressive proportional limit of 4-META/MMA-TBB and MMA-TBB cements containing HA particles. Average diameter of HA; (0,o) 5 Fm, (A,.) 15 pm. Open plot, 5% 4-META-MMA/TBB cement, closed plot, MMA-TBB cement.
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20 _ I 0 20 40 60 80 HA composition in PMMA powder (%)
Figure 2. Tensile strength of 4-META/MMA-TBB cement and MMA-TBB cement containing HA particles 15 p m in average diameter. Open plot, 5% 4-META/MMA-TBB cement, closed plot, MMA-TBB cement.
tured surfaces shown in Figure 4 clearly supported this consideration. Adhesive failure was visible between the HA and PMMA matrix in the absence of 4-META, whereas cohesive failure in the PMMA matrix was observed with 4-META cement [Fig. 4(C) and 4(D)]. A similar state was observed in 40 wt% HA-containing 4-META/MMA-TBB cement. Therefore, the tensile strength of 4-META/MMA-TBB cement containing HA particles depended on the strength of the original cement without HA. Priming of fillers introduced defects when they were not bonded to the matrix. However, in the case of 4-META/MMA-TBB cement, HA particles did not have an adverse effect on the cement since 4-META created bonding to the HA.
ADHESIVE BONE CEMENT CONTAINING HA
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HAcompositionin PMMA powder (wt%)
Figure 3. Bending proportional limit of 4-META/MMA-TBB cement and MMA-TBB cement containing HA particles. Average diameter of HA; (0,o) 5 p m , ( A , A ) 15 pm. Open plot, 5% 4-META-MMA/TBB cement, closed plot, MMA-TBB cement.
The diameter of the HA particles was not a dominant factor in the mechanical strength of 4-META/MMA-TBB cement with HA particles; however, as shown in Figure 3, the bending strength of MMA-TBB cement with HA particles of 5 pm diameter was lower than that with 15-pm HA particles. This result correlated with the surface area of the H A particles which was thought to be the mechanical defect of the cement. Figure 5 shows the bending strength of 4-META/MMA-TBB or MMA-TBB cements after immersion in water. Though the values were decreased compared with the dry state (Fig. 2), 4-META/MMB-TBB cement maintained 30 MPa above the MMA-TBB cements at all ranges of HA composition. The amounts of water absorbed in the MMA-TBB cement (40% HA) and 4-META/ MMA-TBB cement (40% HA) were 3.1 wt% and 0.8 wt%, respectively. Thus, HA and cement with 4-META composition suppressed water absorption effectively, and maintained the mechanical properties of the cement even in a wet condition. This is considered a very important point because bone cement is in fact used in the body. Surface characteristics of HA cement
The surface of 4-META/MMA-TBB cement containing HA particles was analyzed by XI'S to estimate the distribution of HA on the surface. The relationship between the value of the calcium (Ca)/carbon (C) ratio and the HA composition in the powder component is shown in Figure 6. The Ca/C value was slightly increased with HA composition in the powder component and then drastically increased at 60% HA composition when 5-pm HA particles were used. The Ca/C value corresponded to the surface distribution of HA particles. These XPS data suggest that the HA particles could contact bone directly.
ISHIHARA ET AL.
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C
D
Figure 4. SEM view of fractured surface of 4-META/MMA-TBB cement and MMA-TBB cement containing HA particles 15 pm in average diameter after tensile strength measurements. (A, B); MMA-TBB cement, (C,D); 5% 4METAJMMA-TBB cement. Composition of HA in PMMA powder; (A,C) 20 wt%, (B, D) 60 wt%.
104 0
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HA composition in PMMA powder (wt%)
Figure 5. Bending proportional limit of 4-META/MMAITBB cement and MMA-TBB cement containing HA particles 15 pm in average diameter after immersion in water for 7 days at 37°C. Open plot, 5% 4-METAMMA/TBB cement, closed plot, MMA-TBB cement.
Boone and coworkers investigated bone attachment to HA-coated polymem8HA particles molded into the surface of thermoplastic implants or cast into the subsurface of thermoset implants significantly improved the bond strength between the polymers and bone by allowing direct bone apposition and some mechanical interlocking with the bone. However, the SEM picture
ADHESIVE BONE CEMENT CONTAINING HA
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HA composition in PMMA powder (%)
Figure 6. Surface characteristics of 4-META/MMA-TBB cement containing H A particles. Average diameter of HA particle, (0)5 pm, (A) 15 p m .
of the fractured surface after a push-out test from the bony site was very similar to Figure 4(A) and 4(B). It revealed the existence of gaps between HA particles and polymer implants and the fractures occurred at the interface. This was attributed to a lack of adhesion of the HA particles to the polymer. That is, it was suggested that the adhesion of HA particles to the polymer improved the bond strength between bone and cement. The 4-META/MMATBB cement could be useful for adhering HA particles to the surface of the bone cement. Effect of 4-META on adhesion of HA cement to bone
Figure 7 shows the effect of the 4-META concentration in MMA of 4META/MMA-TBB cement containing 40% HA on the tensile bond strength to bone. Though the tensile bond strength of bone cement without 4-META was about 5 MPa, the value was above 10 MPa in the case of 3 or 5 wt% 4-META. Cohesive fracture of the bone occurred in the 4-META/MMA-TBBcement; on the other hand, adhesive failure between bone and MMA-TBB cement was found by SEM examination of the fractured surface. Thus, 4-META could play an important role in improving the bond strength of cement to bone. However, when the 4-META concentration in MMA reached 10 wt%, the bond strength decreased. Figure 8 shows the HA composition dependence of the tensile bond strength of 5% 4-META/MMA-TBB cement. There was no significant difference even when the HA composition was changed. The 4-META/MMA-TBB cement also adhered to stainless steel with a tensile strength of 10 MPa, even if the HA particle concentration was 60 wt% in the powder. This result indicated that the 4-META/MMA-TBB cement containing HA particles could be suitable for the intermediate zone between bone and orthopedic prostheses such as artificial bone since it can adhere to both components.
ISHIHARA ET AL.
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0
2 4 6 8 4-META concentration in MMA (wt%)
10
Figure 7. 4-META concentration dependence of tensile bond strength of 4-META/MMA-TBB cement containing HA particle 15 p m in average diameter to bone. HA composition in PMMA powder is 40 wt$.
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HA composition in PMMA powder (wt%)
Figure 8. Tensile bond strength of 4-META/MMA-TBB cement containing HA particles to bone. 4-META concentration in MMA is 5 wt%. Average diameter of HA particle, (0)5 p m , (A) 15 p m . Closed plot represents the value of MMA-TBB cement.
In conclusion, HA particles can be contained in 4-META/MMA-TBB cement with no adverse effect on several mechanical properties. Moreover, the surface of the cement possesses an adhesion ability to bone and direct bonding of HA particle in the cement to bone will be expected. The evaluation of cement properties by implantation into animals is under way and the results will be reported in the near future. Since a part of this study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan (02205034 and 03205031), one of the authors (N. N.) would like to express his appreciation for this support.
ADHESIVE BONE CEMENT CONTAINING HA
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References 1. J. Charnley, Acrylic Cement in Orthopedic Surgery, E&S Livingstone,
Edinburgh, 1970. S. Raab, A.M. Ahmed, and J. W. Provan, “Thin film PMMA precoating for improved implant bone-cement fixation,” 1.Biomed. Mater. Res., 16, 679-740 (1982). 3. N. Nakabayashi, E. Masuhara, E. Mochida, and I. Ohmori, ”Development of adhesive pit and fissure sealants using a MMA resin initiated by a tri-n-butyl borane derivatives,“ J Biomed. Mater. Res., 12, 149-165 (1978). K. Ishihara and N. Nakabayashi, ‘Adhesive bone cement both to bone and metals: 4-METAinMMAinitiated with tri-n-butyl borane,”]. Biomed. Mater. Res., 23, 1475-1482 (1989). Y. Abe, M. Kiyomura, K. Nagata, and N. Nakabayashi, ’Adhesion of 4-META/MMA-TBB resin to hydroxyapatite, fluoroapatite and their mixture,” J. 1.Dent. Mater., 5, 310-316 (1986). K. de Groot, R. Geesink, C. P. A.T. Klein, and P. Serekian, “Plasma sprayed coatings of hydroxyapatite,” 1.Biomed. Mater. Res., 21, 1375-1381 (1987). K. A. Thomas, J. F. Kay, S. D. Cook, and M. Jarcho, ”The effect of surface macrotexture and hydroxyapatite coating on the mechanical strengths and histologic profiles of titanium implant materials,” 1.Biomed. Muter. Res., 21, 1395-1414 (1987). 8. P. S. Boone, M.C. Zimmerman, E. Gutteling, C. K. Lee, J. R. Parsons, and N. Lamgrana, “Bone attachment to hydroxyapatite coated polymers,” 1.Biomed. Mater. Res., Appl. Biomater., 23(A2), 183-199 (1989). 9. M. Takeyama, N. Kashibuchi, N. Nakabayashi, and E. Masuhara, “Studies on dental self-curing resin (17)-Adhesion of PMMA with bovine enamel or dental alloys,” 1.Jpn. Soc. Dent. Apparatus Mater., 19, 179-185 (19 78). 2.
Received April 10, 1991 Accepted December 11, 1991
micrographic examination of fractured surfaces of the cement clearly showed that the HA particles adhered to the matrix resin when 4-META was added. Thus, it was important to maintain the original mechanical strengths for 4-META. The HA particles along the surface increased with increased HA content in the cement. The cement adhered to bone with a tensile bond strength was higher than 10 MPa.
INTRODUCTION
In orthopedic and dental surgery, the fixation or attachment of artificial implants to hard tissues such as bone and tooth continues to be a major area of interest. The standard method of implant fixation is to use an acrylic cement primarily composed of poly(methy1 methacrylate) (PMMA).’ Though immediate fixation of the implants is an advantage in this method, shrinkage of the cement during polymerization can inf hence loosening of the implants. It is very important to minimize gaps between the hard tissues and the implants. Adhesion may be effective in solving this p r ~ b l e m . ”In~our previous article, we presented a new cement composed of 4-methacryloyloxyethyl trimellitate anhydride (4-META) and methyl methacrylate (MMA) as monomers, tri-nbutyl borane (TBB) as an initiator and PMMA powder (4-META/MMA-TBB cement) which adhered both to bone and metals: The tensile strength between bone and metals adhered with the modified cement was above 7 MPa. We also found that the 4-META/MMA-TBB cement could adhere to ceramics, *To whom correspondence should be addressed. Journal of Biomedical Materials Research, Vol. 26, 937-945 (1992) ccc 0021-9304/92/070937-09$4.00 0 1992 John Wiley & Sons, Inc.
ISI-IIHARA ET AL.
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hydroxyapatite (HA), and a composite of HA and fluoroapatite (FAP) with a tensile bond strength of 15 MPa.’ Recently, several types of bioactive ceramic coatings on prostheses designed to enhance attachment of the prostheses to bone have been This type of coating has the potential advantage of producing an intermediate region between bone and the implant to enhance the transition of stress between them. Moreover, the bone active ceramic coating has shown new bone growth on the surface and direct attachment of the implant to the bone. However, bone growth requires a longer period to fix the prostheses. If an acrylic bone cement contained a bone active component, fast fixation of the prostheses and direct bone attachment on the surface might be expected. An adhesive bone cement containing HA particles as a bone compatible filler within a 4-META/MMA-TBB cement is reported in this communication. The mechanical properties, surface characteristics of the cement, and the adhesion to bone were investigated with attention on the effect of 4-META and HA compositions in the cement.
EXPERIMENTAL
Materials
4-META was prepared as described previously and recrystallized with benzene.’ HA particles of 5 and 15 pm, average diameters, were supplied from Pentax Co. Ltd., Tokyo, Japan and used without further treatment. The MMA, PMMA, and TBB (San-Medical Co., Kyoto, Japan) were of standard commercial origin.
Preparation of bone cement containing HA
In 0.4 mL of monomer, 5 mg of TBB (three drops from a syringe with a 22gauge needle) were added and stirred quickly to oxidize the TBB in air to introduce the initiation phase. One gram of the powder component of the bone cement (PMMA and a mixture of PMMA and the desired amount of HA) was mixed with the monomer solution and stirred until the mixture became paste like with a high viscosity like a conventional bone cement. This viscous mixture was injected and cured in a Teflon mold to make specimens for testing. Measurement of mechanical strength
The compressive, tensile, and bending strengths of the cements were measured using an autograph testing machine (Shimadzu DDS-500, Kyoto, Japan), with a crosshead speed of 2 mm/min. The specimen size was 6 mm X 12 mm for compressive and 2 mm X 2 mm X 30 mm for tensile and bending strength measurements. The cement was cured in a Teflon mold and kept for 60 min at
ADHESIVE BONE CEMENT CONTAINING HA
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room temperature. The specimens were removed from the mold and dried in zlucuo for 15 h to determine the dry weight. One group of specimens was immersed in water at 37°C for 7 days. Fractured surfaces were examined with a scanning electron microscope (SEM, Jeol JSM-5400, Tokyo, Japan). The mean of each mechanical test group was compared by analysis of variance and Student’s t test, with the value of statistical significance set at the p < 0.05 level. The number of samples used for each measurement was 5.
Measurement of tensile bond strength Tensile bond strength was measured by the same procedure as described previously using human femur^.^ The mean of each bond strength test group was compared by analysis of variance and the Student’s t test, with the value of statistical significance set at the p < 0.01 level. The number of samples used was 4. In the same way, the cement was placed on a stainless-steel (SUS-304) rod with a smooth surface 4 mm in diameter and another rod pasted onto it. The tensile bond strength was measured after the specimen was stored in water for 24 h at 37°C.
Other measurement X-ray photoelectron spectroscopic (XI’S) analysis was carried out on the 5 mm in diameter specimens of cured bone cement using a Shimadzu ESCA750 (Kyoto, Japan). Weight gain due to water adsorption in the cured specimen and the tensile test was measured using a Mettlar H-20 type balance (min = 0.01 mg) after immersion in water for 1 month at 37°C.
RESULTS A N D DISCUSSION
Mechanical properties of bone cement containing HA Figures 1 through 3 show dependence of the mechanical properties of bone cement on the HA composition with or without 4-META. In the absence of 4-META, the values of the compressive proportional limit did not depend on the HA composition in the powder component; whereas, the values of the tensile strength and bending proportional limit decreased with increased HA composition. These mechanical properties were improved by the addition of 4-META in the MMA-TBB cement. Moreover, a combination of HA in the powder did not have an adverse effect on the mechanical strength even with a higher HA ingredient composition. These findings clearly showed that 4-META effectively promoted adhesion of HA to the matrix resin5 and the bonding prevented weakening due to HA. The SEM examination of frac-
ISHIHARA ET AL.
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E!
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100
HA composition in PMMA powder (wt%)
Figure 1. Compressive proportional limit of 4-META/MMA-TBB and MMA-TBB cements containing HA particles. Average diameter of HA; (0,o) 5 Fm, (A,.) 15 pm. Open plot, 5% 4-META-MMA/TBB cement, closed plot, MMA-TBB cement.
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20 _ I 0 20 40 60 80 HA composition in PMMA powder (%)
Figure 2. Tensile strength of 4-META/MMA-TBB cement and MMA-TBB cement containing HA particles 15 p m in average diameter. Open plot, 5% 4-META/MMA-TBB cement, closed plot, MMA-TBB cement.
tured surfaces shown in Figure 4 clearly supported this consideration. Adhesive failure was visible between the HA and PMMA matrix in the absence of 4-META, whereas cohesive failure in the PMMA matrix was observed with 4-META cement [Fig. 4(C) and 4(D)]. A similar state was observed in 40 wt% HA-containing 4-META/MMA-TBB cement. Therefore, the tensile strength of 4-META/MMA-TBB cement containing HA particles depended on the strength of the original cement without HA. Priming of fillers introduced defects when they were not bonded to the matrix. However, in the case of 4-META/MMA-TBB cement, HA particles did not have an adverse effect on the cement since 4-META created bonding to the HA.
ADHESIVE BONE CEMENT CONTAINING HA
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ao
HAcompositionin PMMA powder (wt%)
Figure 3. Bending proportional limit of 4-META/MMA-TBB cement and MMA-TBB cement containing HA particles. Average diameter of HA; (0,o) 5 p m , ( A , A ) 15 pm. Open plot, 5% 4-META-MMA/TBB cement, closed plot, MMA-TBB cement.
The diameter of the HA particles was not a dominant factor in the mechanical strength of 4-META/MMA-TBB cement with HA particles; however, as shown in Figure 3, the bending strength of MMA-TBB cement with HA particles of 5 pm diameter was lower than that with 15-pm HA particles. This result correlated with the surface area of the H A particles which was thought to be the mechanical defect of the cement. Figure 5 shows the bending strength of 4-META/MMA-TBB or MMA-TBB cements after immersion in water. Though the values were decreased compared with the dry state (Fig. 2), 4-META/MMB-TBB cement maintained 30 MPa above the MMA-TBB cements at all ranges of HA composition. The amounts of water absorbed in the MMA-TBB cement (40% HA) and 4-META/ MMA-TBB cement (40% HA) were 3.1 wt% and 0.8 wt%, respectively. Thus, HA and cement with 4-META composition suppressed water absorption effectively, and maintained the mechanical properties of the cement even in a wet condition. This is considered a very important point because bone cement is in fact used in the body. Surface characteristics of HA cement
The surface of 4-META/MMA-TBB cement containing HA particles was analyzed by XI'S to estimate the distribution of HA on the surface. The relationship between the value of the calcium (Ca)/carbon (C) ratio and the HA composition in the powder component is shown in Figure 6. The Ca/C value was slightly increased with HA composition in the powder component and then drastically increased at 60% HA composition when 5-pm HA particles were used. The Ca/C value corresponded to the surface distribution of HA particles. These XPS data suggest that the HA particles could contact bone directly.
ISHIHARA ET AL.
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C
D
Figure 4. SEM view of fractured surface of 4-META/MMA-TBB cement and MMA-TBB cement containing HA particles 15 pm in average diameter after tensile strength measurements. (A, B); MMA-TBB cement, (C,D); 5% 4METAJMMA-TBB cement. Composition of HA in PMMA powder; (A,C) 20 wt%, (B, D) 60 wt%.
104 0
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HA composition in PMMA powder (wt%)
Figure 5. Bending proportional limit of 4-META/MMAITBB cement and MMA-TBB cement containing HA particles 15 pm in average diameter after immersion in water for 7 days at 37°C. Open plot, 5% 4-METAMMA/TBB cement, closed plot, MMA-TBB cement.
Boone and coworkers investigated bone attachment to HA-coated polymem8HA particles molded into the surface of thermoplastic implants or cast into the subsurface of thermoset implants significantly improved the bond strength between the polymers and bone by allowing direct bone apposition and some mechanical interlocking with the bone. However, the SEM picture
ADHESIVE BONE CEMENT CONTAINING HA
943
HA composition in PMMA powder (%)
Figure 6. Surface characteristics of 4-META/MMA-TBB cement containing H A particles. Average diameter of HA particle, (0)5 pm, (A) 15 p m .
of the fractured surface after a push-out test from the bony site was very similar to Figure 4(A) and 4(B). It revealed the existence of gaps between HA particles and polymer implants and the fractures occurred at the interface. This was attributed to a lack of adhesion of the HA particles to the polymer. That is, it was suggested that the adhesion of HA particles to the polymer improved the bond strength between bone and cement. The 4-META/MMATBB cement could be useful for adhering HA particles to the surface of the bone cement. Effect of 4-META on adhesion of HA cement to bone
Figure 7 shows the effect of the 4-META concentration in MMA of 4META/MMA-TBB cement containing 40% HA on the tensile bond strength to bone. Though the tensile bond strength of bone cement without 4-META was about 5 MPa, the value was above 10 MPa in the case of 3 or 5 wt% 4-META. Cohesive fracture of the bone occurred in the 4-META/MMA-TBBcement; on the other hand, adhesive failure between bone and MMA-TBB cement was found by SEM examination of the fractured surface. Thus, 4-META could play an important role in improving the bond strength of cement to bone. However, when the 4-META concentration in MMA reached 10 wt%, the bond strength decreased. Figure 8 shows the HA composition dependence of the tensile bond strength of 5% 4-META/MMA-TBB cement. There was no significant difference even when the HA composition was changed. The 4-META/MMA-TBB cement also adhered to stainless steel with a tensile strength of 10 MPa, even if the HA particle concentration was 60 wt% in the powder. This result indicated that the 4-META/MMA-TBB cement containing HA particles could be suitable for the intermediate zone between bone and orthopedic prostheses such as artificial bone since it can adhere to both components.
ISHIHARA ET AL.
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0
2 4 6 8 4-META concentration in MMA (wt%)
10
Figure 7. 4-META concentration dependence of tensile bond strength of 4-META/MMA-TBB cement containing HA particle 15 p m in average diameter to bone. HA composition in PMMA powder is 40 wt$.
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HA composition in PMMA powder (wt%)
Figure 8. Tensile bond strength of 4-META/MMA-TBB cement containing HA particles to bone. 4-META concentration in MMA is 5 wt%. Average diameter of HA particle, (0)5 p m , (A) 15 p m . Closed plot represents the value of MMA-TBB cement.
In conclusion, HA particles can be contained in 4-META/MMA-TBB cement with no adverse effect on several mechanical properties. Moreover, the surface of the cement possesses an adhesion ability to bone and direct bonding of HA particle in the cement to bone will be expected. The evaluation of cement properties by implantation into animals is under way and the results will be reported in the near future. Since a part of this study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan (02205034 and 03205031), one of the authors (N. N.) would like to express his appreciation for this support.
ADHESIVE BONE CEMENT CONTAINING HA
945
References 1. J. Charnley, Acrylic Cement in Orthopedic Surgery, E&S Livingstone,
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Received April 10, 1991 Accepted December 11, 1991
