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Radiopacity of tantalum-loaded acrylic bone cement C Persson1*, L Guandalini1, F Baruffaldi1, L Pierotti2, and M Baleani1 1Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Bologna, Italy 2Servizio di Fisica Sanitaria, Policlinico S. Orsola Malpighi, Bologna, Italy The manuscript was received on 15 July 2005 and was accepted after revision for publication on 19 June 2006. DOI: 10.1243/09544119JEIM88

Abstract: Radiopacifying agents are commonly added to bone cements to enhance the visibility of the cement in radiography. The radiopacifiers usually employed may, however, have undesired effects on the mechanical properties of the cement. A potentially new radiopacifier is tantalum, which in the present work was evaluated in terms of radiopacity. Bone cements containing different percentages of tantalum were compared with plain bone cement as well as with formulations containing different percentages of the commonly used radiopacifier barium sulphate. The radiopacity was assessed quantitatively and qualitatively, by measuring with a digital densitometer the optical density of the cement on X-ray films, and consulting the expertise of ten orthopaedic surgeons. It was found that tantalum does present radiopacity, but not as high as barium sulphate under the specific conditions applied to this study. Keywords: bone cement, radiopacity, tantalum, barium sulphate

1 INTRODUCTION Cementing with polymethyl methacrylate (PMMA) is a method extensively used for the fixation of prostheses in joint replacements. To distinguish readily the bone cement from body tissues during surgery and to detect and forecast possible failure by radiography, a radiopacifier is added to the rather transparent cement. The radiopacifier, usually a metal or a metallic compound, enhances the visibility of the PMMA since it is of higher atomic number and attenuates the X-rays. Bone cements generally contain radiopacifiers such as barium sulphate or zirconium dioxide. Unfortunately, some detrimental effects of these radiopaque agents on the mechanical behaviour of PMMA have been observed [1–3]. A potential alternative radiopacifier is tantalum, which is already being used in other medical applications, such as stents [4], and in the embolization of cerebral arteriovenous malformations [5]. It is also proposed as a contrast agent in the form of an oxide in dental filling materials [6]. The radiopacifying effect of a material is, however, a complex property that is best evaluated in its specific situation of use. It depends * Corresponding author: Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Via di Barbiano 1/10, Bologna 40136, Italy. email: [email protected]

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not only on the atomic number of the material but also on the density and the type of interaction of the X-rays with the material. For diagnostic radiology the most important interaction is the photoelectric effect. Other possible interactions are the classic diffusion and the Compton effect, which are both noise effects from a diagnostic point of view. The aim of this work was to evaluate the radiopacity effect of tantalum in bone cement for orthopaedic applications.

2 MATERIALS AND METHODS To evaluate fully the radiopacity effect of the alternative material, various formulations with different percentages of tantalum were compared with plain PMMA cement and formulations with different percentages of the commonly used barium sulphate. Both radiopacifiers were in powder form with a mean particle size of the order of 1 mm, although the highest frequency in both distributions was in the range 0.1–0.3 mm. A commercial bone cement without radiopacifier (CemexA; Tecres, Verona, Italy) was selected as control cement and as the base for the radiopaque formulations. The cements with different percentages of radiopacifier were prepared by mixing Proc. IMechE Vol. 220 Part H: J. Engineering in Medicine

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C Persson, L Guandalini, F Baruffaldi, L Pierotti, and M Baleani

the barium sulphate or the tantalum with the PMMA powder. The obtained powder was then mixed with the appropriate amount of liquid to maintain the correct PMMA/MMA ratio of the bone cement. The mixing was done following supplier and ISO5833 recommendations. The components were mixed manually in a thermostatic chamber at 23±1 °C and at a humidity between 40 and 60 per cent. The following formulations were investigated (the weight percentage refers to the amount of PMMA powder, not to the amount of cured bone cement): (a) control, i.e. the acrylic bone cement PMMA without radiopacifier; (b) 10%BS–PMMA with 10 wt % barium sulphate (standard configuration); (c) 20%BS–PMMA with 20 wt % barium sulphate; (d) 30%BS–PMMA with 30 wt % barium sulphate; (e) 5%Ta–PMMA with 5 wt % tantalum; (f) 10%Ta–PMMA with 10 wt % tantalum; (g) 20%Ta–PMMA with 20 wt % tantalum; (h) 40%Ta–PMMA with 40 wt % tantalum. A simplified protocol was defined to simulate the clinical scenario of a cemented hip prosthetic stem. Steel stems were machined with a middle crosssectional dimension that was fixed to replicate an average dimension of a prosthetic stem. The cement mantle was obtained by injecting the mixed cement into a polyethylene mould and inserting the steel stem immediately after. After curing, the simulated cemented prosthesis was extracted from the mould. A model of the moulded specimen is shown in Fig. 1. The final thickness of the cement mantle was 1.5 mm. This value corresponds to a thin cement layer expected in vivo [7] and it represents a critical condition in terms of visibility. The eight specimens underwent simultaneously X-ray exposure. Two different conditions were considered: (a) in air, to investigate the radiopacifying effect without noise in the image; (b) in 15 cm of water with the cemented stem inserted into a piece of diaphyseal bone of

human femur, to simulate surrounding soft tissue as well as the bone, as for a typical hip prosthesis. Several X-ray exposures were made to find the optimal conditions for obtaining the film with the best contrast. Regarding the radiological parameters, a suitable tension peak (expressed in kilovolt peak, kVp) is normally chosen first, and then the optimum intensity of exposure (expressed in milliampereseconds, mA s) is found. In diagnostic radiology for hip implants, a value between 60 and 75 kVp is normally used [8], and in this case a value of 64 kVp was chosen by the radiologist. Then, depending on the environment of the specimens, different values of the intensity were applied. The best image for the specimens surrounded by air only was obtained with 20 mA s. When the stem was inserted into the bone and water was added, the selected intensity value was 63 mA s. To obtain the best image contrast, it is also important to optimize the printing process. When obtaining a digital X-ray image, the number of printed grey levels can be adjusted. If the visualization is limited to the active grey levels, the best contrast is achieved during printing. Therefore, this criterion was applied in printing the films. To evaluate the radiopacity of the materials, the optical density (OD) of the bone cement was measured on the X-ray films, in accordance with the standard test method ASTM F640-79(00). The optical density is defined as OD=log

(incident light/transmitted light) 10 A digital densitometer (Model 07-440; Victoreen Inc., NY, USA) was used to measure the OD of the different materials on the X-ray films. It has a detectable density range of 0–4.50 with an accuracy of ±0.02 at a density of 3.00 with a 1 mm aperture. For each specimen, measurements of a diameter of 1 mm were made, and mean and standard deviation values were calculated. For the specimens in air only, 12 measurements were taken at six different levels of the cement mantle of each specimen, two measurements for

Fig. 1 Moulding of the cement mantle Proc. IMechE Vol. 220 Part H: J. Engineering in Medicine

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each level (one for each side). For the specimens inserted into bone and immersed in water, the same 12 measurements were taken on each specimen. In this case, however, the measurement session was repeated eight times, changing the cemented stem– bone combination, to delete measurement drift due to bone inhomogeneities. Finally, the contrast index (CI) was calculated as the difference in OD between the specific measure taken on the material under investigation and the background. The data were analysed by analysis of variance (ANOVA) and post-hoc analysis (Scheffe’s test). To determine the radiopacifying effect of Ta compared with BS, the average of the percentage difference in CI values was calculated for the two comparable formulations (i.e. 10%Ta versus 10%BS and 20%Ta versus 20%BS). Additionally, an alternative investigation was made to verify qualitatively the effectiveness of the two radiopacifying materials. The X-ray films were cut in a manner that made it possible to show the stems in pairs. Then, ten orthopaedic surgeons were asked to indicate the stem with the highest radiopacity of each pair. The data were analysed by contingency analysis. In both analyses a significance level of 5 per cent was selected.

3 RESULTS The CI values for the film with the specimens surrounded by air only are shown in Fig. 2(a). The results of statistical analysis showed that the addition of the radiopacifier affects the radiopacity of the material (ANOVA, p