# Roughness Digital Characterization and Influence on Wear of Retrieved Knee Components
## 2 Materials and Methods
### Roughness Measurements and 3D Image Generation  
The wear analysis was performed by means of surface roughness and topographic measurements. All the retrieved specimens were analyzed on three different areas, corresponding to the position of the knee during walk (30\({}^{\circ}\) and 60\({}^{\circ}\)) or at rest (0\({}^{\circ}\)). An area of interest of 1.5 mm \(\times\) 1.5 mm (2.25 mm\({}^{2}\)) was taken into account for each angle and on medial and lateral compartments. Figure 1 shows the measured areas on each prosthesis. Three main roughness indices were considered to characterize the roughness of the specimens: Ra, Rt, and Rsk. The most significant indicator to characterize the surface conditions is the mean roughness, Ra, that represents the amplitude of the arithmetical mean value of the absolute values of the deviations of the real profile compared to the mean profile trace. The peak-peak height Rt is defined as the vertical distance between the highest peak and lowest valley along the measured trace for the whole analyzed surface. The Rsk represents the degree of symmetry of the surface peaks with respect to the mean plane. The values of Rsk may be:  
* Equal to zero for a normal distribution;
* Lesser than zero for a negative distribution and a plain profile;
* Greater than zero for a positive distribution and a profile with many peaks. Roughness measurements were performed by means of the contact profilometer. Hommel Tester T8000 (Hommel Werke, Luedinghausen, Germany), following an internal and consolidated protocol [17, 18, 19, 20] and the following operating parameters:  
* A diamond stylus tip with radius 0.020 mm;
* A measuring length of 1.5 mm;
* A cut-off of 0.25 mm;
* A travel speed of 0.15 mm/s. \begin{table}
\begin{tabular}{l c c c c c c c} \hline
**Pz** & **Gender** & **Weight (kg)** & **Height (cm)** & **Side** & **Diagnosis** & **Reason for Revision** & **F.U. (Years)** & **Size** \\ \hline \#1 & F & 80 & 160 & Right & Fracture & Septic loosening & 1.5 & 3 \\ \hline \#2 & M & 94 & 170 & Left & Unknown & Septic loosening & 0.7 & 3 \\ \hline \#3 & M & 71 & 170 & Right & Other & Aseptic loosening & 2.5 & 3 \\ \hline \#4 & F & 74 & 166 & Left & Aseptic loosening & Aseptic loosening & 2.1 & 3 \\ \hline \#5 & F & 78 & 164 & Right & Arthritis & Septic loosening & 1.0 & 3 \\ \hline \#6 & M & 106 & 180 & Left & Arthritis & Aseptic loosening & 1.6 & 4 \\ \hline \#7 & M & 50 & NA & Left & Post-traumatic arthrosis & Septic loosening & 0.2 & 2 \\ \hline \#8 & F & 75 & 160 & Left & Arthritis & Septic loosening & 2.5 & 3 \\ \hline \#9 & M & 60 & 170 & Right & Fracture & Septic loosening & 1.7 & 3 \\ \hline \#10 & F & 52 & 150 & Left & Valgus deformity & Aseptic loosening & 1.3 & 2 \\ \hline \#11 & M & NA & NA & Left & Arthritis & Septic loosening & 1.3 & 3 \\ \hline \#12 & F & 74 & 153 & Right & Other & Septic loosening & 0.4 & 2 \\ \hline \#13 & M & 95 & 180 & Right & Arthritis & Septic loosening & 0.5 & 3 \\ \hline \#14 & F & 107 & 165 & Left & Arthritis & Pain without loosening & 2.1 & 4 \\ \hline \#15 & F & 59 & 155 & Right & Condyle necrosis & Septic loosening & 0.1 & 3 \\ \hline \#16 & M & 70 & 163 & Right & Rigidity & Rigidity & 0.6 & 3 \\ \hline \#17 & M & 108 & 182 & Right & Other & Aseptic loosening & 4.1 & 4 \\ \hline \#18 & M & 75 & 160 & Right & Fracture & Septic loosening & 7.6 & 3 \\ \hline \end{tabular} \(\text{Pz}=\text{patient}\); NA = not available; FU = follow up.  
\end{table} 
Table 1: Specimen selection, patient characteristics, follow up, and prosthesis size-number. All the specimens were prepared following the same procedure: before the roughness measurement, the surfaces were cleaned with acetone and then dried at room temperature in a controlled environment. Roughness was detected in A/P (antero/posterior) direction, as it was the main orientation of the scratches. After the roughness measurements, the topographic surface acquisitions were performed on the retrievals.