## 2 Materials and Methods
### Wettability Set-Up  
Wettability is usually defined as the ability of a liquid to spread over a surface [25]. The lubricant deposited on the solid surface under gravity tends to spread until the internal cohesion forces of the liquid are balanced with the forces rising from the surface tension [26]. To evaluate the wettability of the investigated ceramic femoral heads, the apparent contact angle was evaluated applying a static drop technique. The contact angle, \(\theta\), as depicted in Figure 1, is the angle at which the liquid-vapor interface meets the solid-liquid interface. The droplet on the solid surface under gravity spreads until the liquid internal cohesive forces, the surface tension forces and gravity are in balance. Once the equilibrium is reached (three phases minimum of the energy state) a contact angle \(\theta\) between the solid surface and the liquid droplet can be measured. Static contact angle \(\theta\) is greater than 90\({}^{\circ}\) when the surface energy of the solid-air interface assumes greater values than the liquid-air interface. With reference to water droplets (other liquids), we assumed a super-hydrophobic (super-amphiphobic) behavior for \(\theta\) greater than 150\({}^{\circ}\), hydrophobic (amphiphobic) behavior for \(\theta\) between 90\({}^{\circ}\) and 150\({}^{\circ}\), hydrophilic (amphiphilic) behavior for \(\theta\) between 10\({}^{\circ}\) and 90\({}^{\circ}\), and super-hydrophilic (super-amphiphilic) for \(\theta\) between 0\({}^{\circ}\) and 10\({}^{\circ}\)[27].  
The adopted experimental apparatus consisted of a horizontal stage used to place the specimen, which allowed to be adjusted in the z-directions, a micrometer syringe to form a liquid drop, a halogen and intensity adjustable light source to illuminate the simple, and a Nikon camera for the high-resolution images acquisition (Figure 2).  
Before measuring, the samples were cleaned in an ultrasonic bath for 15 min to remove any residues, dried with nitrogen gas, and cleaned with acetone. A liquid droplet of distilled water with a volume of 10 \(\mu\)L was used. The liquid droplet fell on the same dome surface of the ceramic femoral heads. After camera acquisitions, the contact angle between the drop and the surface of the samples was measured using a free and open-source graphics editor (GIMP 2.8, GNU Image Manipulation Program). Tests were conducted at room temperature (25 \({}^{\circ}\)C); each experiment was repeated four times and the experimental results were taken as the mean value to assure reproducibility. This technique represents a novel measurement approach, used nowadays in several fields of accurate dimensional measurements, but not yet consolidated in a standardized measurement protocol, and with a complex uncertainty determination; the possibility to obtain very high magnifications of the droplet pictures at high resolution, due to the high resolution camera adopted, lent satisfactory confidence in the obtained results.  
Statistical analyses were performed on measured data by using SPSS 14.0 (SPSS Inc, Chicago, IL, USA) software. Mann-Whitney U tests and median tests with independent samples were used to check differences in distribution and in median. Statistical significance was set at \(p<0.05\). The correlation between roughness measurements _vs_. follow-up and wettability of the heads were calculated using Pearson's r.  
Figure 1: Contact angle: (**A**) shows the vectorial representation indicating the direction of the force applied (F\({}_{\text{LA}}\) = liquid–air force; F\({}_{\text{SA}}\) = solid–air force; F\({}_{\text{LS}}\) = liquid–solid force. (**B**) is an example of a drop on a Biolox® Delta femoral head.  
Figure 2: Wettability set-up.