Summary of molecular weights for the retrieved cements Sample no. Implantation time (years) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 7 7 7 10 10 10 13 13 14 16 21 25 26 30 Mw (g/mol) 209,737 199,478 220,660 187,296 208,271 217,552 176,996 196,600 192,056 211,157 216,827 203,630 192,270 212,962 Mn (g/mol) 66,412 68,608 74,217 67,205 64,416 71,617 63,839 67,618 70,209 66,509 62,554 62,005 61,753 66,750 PDI 3.16 2.91 2.97 2.79 3.23 3.04 2.77 2.91 2.74 3.17 3.47 3.28 3.11 3.19 554 THA. We investigated various properties, including molecular weight, chemical structure, bending properties, density, and porosity in retrieved bone cements. Patients and methods Sample preparation CMW1 cements were retrieved from 14 patients who under- went acetabular revision because of aseptic loosening. The median time in vivo before revision was 15 (7–30) years. The retrieved samples were rinsed in saline solution and ethanol, and then stored at room temperature until they were exam- ined. Molecular weight analysis The average molecular weight and the molecular weight dis- tribution of the retrieved cements were assessed by gel per- meation chromatography (GPC). Molecular weight calibra- tion was established based on polystyrene standards. Briefly, samples were dissolved in tetrahydrofuran (THF) to a con- centration of 2 mg/mL at room temperature, and then filtered through 0.45-μm disk filters. Each sample was injected into a 30-cm long GPC gel column (Shodex, Tokyo, Japan) with an inner diameter of 8.0 mm, which was packed with THF with a pore size of 1,000 nm. The injection volume was 50 μL and the flow rate was 1 mL/min at 40°C. A differential refractive index detector (Hitachi L-2000; Hitachi, Tokyo, Japan) was used to monitor changes in the concentration of the sample. The molecular weight distributions of the samples relative to polystyrene were found in terms of the number-averaged molecular weight Mn, the weight-averaged molecular weight Mw, and the polydispersity index (PDI; Mw/Mn ratio). Fourier-transform infrared spectroscopy (FTIR) analysis Chemical analysis was performed using FTIR. The FTIR spectra were obtained using a Spectrum BX spectrometer (PerkinElmer, Waltham, MA). All transmission spectra were collected with a spectral resolution of 4 cm–1 and spectral range of 4000 to 600 cm–1 using KBr pellets. Unimplanted control CMW1 cement specimens were freshly prepared by mixing the powder and liquid components by hand accord- ing to the manufacturer’s instructions. They were then sent for FTIR analysis within 1 month of preparation. Bending properties The retrieved cement specimens were cut and scraped into rect- angular specimens (20 mm × 4 mm × 3 mm) using a rotational scraping machine (BUEHLER EcoMet 3000; BUEHLER Ltd., Lake Bluff, IL) with 1–9 specimens for each sample. The bending strength and bending modulus of each retrieved cement specimen and each freshly prepared CMW1 cement specimen were analyzed using an Instron 5500 instrument Acta Orthopaedica 2011; 82 (5): 553–558 (Instron, Norwood, MA) at 23 ± 1°C. The crosshead speed and the span were 0.5 mm/min and 15 mm, respectively, when using the 3-point bending method. The values for the bending modulus were derived from the stress-strain curves obtained from the bending tests, as described previously (Shinzato et al. 2002). Porosity analysis A microfocus X-ray computed tomography system (SMX-100CT-SV3; Shimadzu Co., Kyoto, Japan) was used to acquire microstructural information from the retrieved cements. The entire set of radiographs was deconvoluted by computer software to reconstruct a 3-D image of the micro- structure with a voxel size of 16 μm 3. The 3-D data were pro- cessed with commercially available 3-D image-processing software (VG Studio MAX 2.0; Volume Graphics, Heidelberg, Germany), and the porosity of the retrieved cements was cal- culated from the binary material images. The spatial bound- ary between the pores and the cement was established easily because of the large differences in density. Statistics Data from each test were compared by analysis of variance (ANOVA) to determine the overall significance of data trends. For all analyses, p < 0.01 was considered significant. Results The Mw of the samples ranged from 170,000 to 220,000 g/mol (Table). The molecular weight and PDI did not correlate with the time in vivo (r = 0.013, p = 1.0; r = 0.54, p = 0.05, respec- tively) (Figure 1A and B). Because the molecular weight of the polymer is proportional to the degree of polymerization of the monomer unit, this result suggested that scission of poly- mer chains did not occur in CMW1 cement in vivo. There was