# Biomechanical Performance of the Cemented Hip Stem with Different Surface Finish
## 2 Experimental Work
### Interface Strengths  
The bonding strengths of the cement-metal specimens obtained from the mechanical tests are presented as the mean value \(\pm\) the standard deviation and are summarized in Table 1. The tensile and shear strengths of the interface between the cement and the roughened metal specimen were 1.73 (\(\pm\)0.61) MPa and 3.80 (\(\pm\)0.48) MPa, respectively. For specimens bonded with cement and polished metal specimens, the tensile strength was measured as 2.81 (\(\pm\)1.15) MPa and the shear strength as 1.39 (\(\pm\)0.55) MPa. The normal and tangential stiffness of the interface formed with the sand-blasted surfaces were 15.48 (\(\pm\)5.05) MPa/mm and 13.95 (\(\pm\)3.807) MPa/mm, respectively. However, for polished surfaces, the normal and tangential stiffness of the interface were 19.17 (\(\pm\)8.62) MPa/mm and 8.21 (\(\pm\)2.28) MPa/mm, respectively. One-way ANOVA indicates that there is a significant difference between the interfacial strengths generated by metals with smooth surfaces and with rough surfaces, with a \(p\)-value of 0.009 for both tensile and shearing strengths. Compared to roughened surfaces, polished surfaces showed superior bonding strength and normal stiffness in resisting interface separation under tensile loading (\(p\) = 0.009). Conversely, the roughened metals showed higher interfacial shear strength and tangential stiffness than the polished metal in resisting shearing deformation. Several experimental studies with various configurations of the specimen showed different results [9; 10; 11; 12; 26; 27; 28], which makes direct comparisons of interfacial bonding strength among studies difficult. For example, [PERSON] and [PERSON] [9] reported that the tensile bonding strength between grit-blasted CoCr rods (Ra =1-2 \(\upmu\)m) and bone cement was about 5.0 MPa. In contrast to the positive tendency in the variation of tensile strength observed with surface roughness, [PERSON] et al. [10] reported that polished metal rods had an inferior tensile bonding strength with cement when compared to roughened metal rods. They found that the tensile strength of the bonded interface was about \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Surface Roughness of Metal Specim**} & \multicolumn{2}{c}{**Under Tensile Loading**} & \multicolumn{2}{c}{**Under Shearing Loading**} \\ \cline{2-5}  & **Strength (MPa)** & **Stiffness (MPa/mm)** & **Strength (MPa)** & **Stiffness (MPa/mm)** \\ \hline Polished surface & 2.81 \(\pm\) 1.15 & 19.17 \(\pm\) 8.62 & 1.39 \(\pm\) 0.55 & 8.71 \(\pm\) 3.91 \\ Ra = 0.6 \(\upmu\)m & (\(n\) = 22) \({}^{*}\) & (\(n\) = 22) \({}^{*}\) & (\(n\) = 15) \({}^{*}\) & (\(n\) = 15) \({}^{*}\) \\ \hline Sand-blasted surface & 1.73 \(\pm\) 0.61 & 15.48 \(\pm\) 5.05 & 3.80 \(\pm\) 0.48 & 13.95 \(\pm\) 3.81 \\ Ra = 6 \(\upmu\)m & (\(n\) = 18) \({}^{*}\) & (\(n\) = 18) \({}^{*}\) & (\(n\) = 18) \({}^{*}\) & (\(n\) = 18) \({}^{*}\) \\ \hline \hline \end{tabular} \({}^{*}\) Value (\(n\)) represents the number of specimens for tests.  
\end{table} 
Table 1: Interface tensile strength and bonding stiffness for cement/metal specimens. Figure 3: Typical relationship between the surface traction and displacement of the bonded interface under tensile and shear loading, respectively. 7.3-9.9 MPa for metallic rods with coarse grit-blasted surfaces (Ra = 6.67 \(\upmu\)m), but was 2.7-4.9 MPa for metal samples with a blasted ceramic coating (Ra = 0.97 \(\upmu\)m). [PERSON] et al. [26] conducted pull-off tests of cement-metal bonded specimens and found that polished metal (Ra = 0.1 \(\upmu\)m) yielded higher tensile bonding strength than roughened metal (Ra = 15 \(\upmu\)m). In a study conducted to investigate interfacial shear strength, [PERSON] et al. [7] showed that the shear strength of the cement-metal interface ranged from 0.56 to 1.89 MPa, and that the values were positively related to the surface roughness of the metal specimen. [PERSON] et al.