Acta Orthopaedica 2011; 82 (5): 553–558 555 Figure 1 A. Relationship between implantation period and molecular weight. B. Relationship between implan- tation period and PDI. C. Relationship between molecular weight and bending strength. D. Relationship between molecular weight and bending modulus. no substantial difference in FTIR spectra between the CMW1 cements in fresh samples after they were cured and in samples retrieved 16 and 30 years after implantation (Figure 2). The spectra of methyl methacrylate homopolymer showed a dis- tinctive absorbance band around 1,730 cm–1, corresponding to the C=O stretch of the ester group (Hughes et al. 2003). This distinctive absorbance band did not differ between freshly prepared and retrieved CMW1 cements. These results indicate that no substantial hydrolysis of the ester group occurred in CMW1 cement in vivo, even after many years. The molecular weight was not related to the bending strength or the bending modulus (r = 0.040, p = 0.83; r = 0.046, p = 0.8, respectively) (Figure 1C and D). By contrast, the bending strength of each cement specimen was reduced with increas- ing time in vivo, but this was not statistically significant (r = –0.39, p = 0.03) (Figure 3A). In addition, there was no correla- tion between bending modulus and the length of the implanta- tion period (r = –0.038, p = 0.8) (Figure 3B). The density of the specimens was calculated from their dimension and weight. There was no correlation between the density of the cements and length of time in vivo (r = –0.20, p = 0.3) (Figure 4A). Density was found to be strongly cor- related to bending strength (r = 0.54, p = 0.002) but not to bending modulus (r = 0.38, p = 0.04) (Figure 4B and C). There was no correlation between the porosity of the specimens and the time in vivo (r = 0.27, p = 0.1) (Figure 5A). There was a strong correlation between the density and the porosity of the cement (r = –0.67, p < 0.001) (Figure 5B). The poros- A B C Figure 2. FTIR spectrum of control CMW1 cement (A) and CMW1 cement retrieved 16 years (B) and 30 years (C) after implantation. ity correlated with bending strength (r = –0.51, p = 0.004) (Figure 5C) but not with bending modulus (r = –0.18, p = 0.4) (Figure 5D). These results indicate that the porosity of the bone cement defined the in vivo mechanical properties of the