## 4 Discussion  
From a biological point of view, a high surface wettability is desirable because it promotes the material-tissue interactions [28]. From the corrosion point of view, more hydrophobic surfaces are desirable because the interaction with the external environment is minimized. Wettability can be seen as a characteristic to be correlated to the material tendency to corrode inside the human body [16; 29]. In this study we investigated changes in the roughness surface in different in vitro and in vivo ceramic femoral heads, considering also MT and surface wettability. We found an average roughness measured on explanted specimens greater than the new ones and the in vitro specimens, as shown in the results section. Measurements of the observed wettability angle showed hydrophilic surface with quite high wettability for all femoral heads considered in this study, with a variation of the contact angle \(\theta\) between values of \(37.20^{\circ}\) to \(55.63^{\circ}\) in the case of retrieved femoral heads, \(33^{\circ}\) and \(60^{\circ}\) in the case of new Biolox(r) Delta femoral heads and \(34^{\circ}\) to \(61^{\circ}\)in the cases of in vitro simulated Biolox(r) Forte Femoral Heads. No statistical significance (\(p>0.05\)) was observed between the wettability vs. the roughness measurements of all the whole femoral heads studied in this work. No statistical significance (\(p>0.05\)) was observed between the follow-up (fu) vs. the roughness measurements of all the whole femoral heads. A statistical significance (Pearson correlation, \(p=0.472\)) was observed between the fu vs. the Rt roughness measurements for the explanted femoral heads. Statistical significance (Pearson correlation, \(p=0.706\)) was also found on the whole femoral heads with regard to the wettability vs. the Rt roughness measurements. A very strong correlation was found between follow-up and roughness measurements for all the explanted femoral heads (\(p<0.0001\)). No differences were found by comparing the percentage values of the MT relative to the Biolox(r) Forte (5.38%) and Delta (5.29%) femoral heads. It is known that roughness should act on the wettability apparent contact angle, which is not the same as the local one (the contact angle on an ideal mirror-polished surface) since different local equilibrium states on surface topographic asperities are allowed [26]. A particular importance was attributed to these results since friction and wear processes are known to be affected by the surface wettability of the prosthetic materials [16], even if definite correlations between wettability and tribological behavior are still to be established [17]. Biolox(r) Delta femoral heads can be coupled with a ceramic or a polyethylene counterbody. As is known from literature, better lubrication--reduced friction and wear--is realized when the tribosystem is composed by hydrophilic materials [30; 31]. The prosthetic joint lubrication can be referred to the three classical lubrication modes (boundary, mixed, full film/elastohydrodynamic) depending on the loading and the motion conditions [32]. When the specific film thickness \(\lambda\), depending on the synovial film thickness and on the contact surface roughness, is large (\(\lambda>3\)), the lubricating gap could ensure a complete separation between the surface asperities allowing the hydrodynamic lubrication mode [14]. When \(\lambda<1\), boundary lubrication is expected with contact between the surface asperities [15]. The transitional mixed lubrication mode occurs for \(1<\lambda<3\). The hydrodynamic lubrication modelling is commonly  
Figure 5: Wettability angles behavior for the different conditions analyzed. approached by the Reynolds equation under its classical hypothesis. Of course the Reynolds model should be improved considering the non-Newtonian behavior of the synovial lubricant [33; 34], allowing fluidodynamic lubrication models closer to the tribosystem's real behavior.