## 2 Engineering Principles for Clinicians
### How to Evaluate the Requested CMF Implant? A Guide for the Surgeon  
#### 2.2.1 Principles of Mechanical Defects Formation  
Before learning how to perform an effective visual inspection, it is necessary to understand some main defects connected with the 3D PBF printing. They can be divided into macroscopic and microscopic, and they usually depend on the wrong setting of the parameters of the machine, but they are even related with some criticalities of the geometry to be printed and its positioning inside the melting chamber. It is of prominent importance to underline that almost every defect can be avoided or corrected by conducting a systematic analysis and optimization of the parameters involved in the 3D printing process (Table 1). Porosity: the presence of pores inside the metallic structure could be critical for the fatigue resistance of the component [19]. Notably, porosities usually represent the trigger points for cracks propagation. These pores usually measure from 1 to 20 \(\upmu\)m and can extend up to the surface. Balling: melt ball formation occurs when the molten material solidifies into spheres instead of solid layers. The result is a rough and bead-shaped surface that produces an irregular layer deposition with detrimental effects on the density and quality of the part [20]. Balling increases the surface roughness and contributes to the formation of a large number of pores. Surface defects: the presence of a rough and non-homogeneous surface represents a critical issue for the final component. In PBF processes, surface roughness has two main contributors: the stair-stepping effect due to the layer-wise production, and the actual roughness of the metal surface. The surface finishing depends on the surface orientation with respect to the growth direction [21]. In particular, downward and upward surfaces are known to have considerably different roughness properties. The former present much lower surface quality. It is important to highlight the dependence of the fatigue resistance on the surface roughness of the stressed surface: the higher the roughness the lower the fatigue performance of the component. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**Critical Feature Description** & **Recommendation for Prosthesis Design** \\ \hline Vertical thin walled structures with thickness from 1 to 0.1 mm & \\ \hline \hline Pipe-like, hollow cylinders with different external diameters and thicknesses & Avoid thin walled parts having thickness smaller than 0.2 mm \\ \hline Small quarters of spherical shells presenting undercut surfaces & \\ \hline \hline Abrupt transition from semi-void section to full section & Avoid abrupt section variations whenever possible \\ \hline Small full cross section parallelepipeds and cylinders having variable fillets/radii at their bases & It is possible to print small features with small or no fillets/radii at their conjunctions with other surfaces \\ \hline Horizontal cylindrical holes with diameters the range 2–8 mm, without internal supports & Horizontal holes are feasible having maximum diameter of about 8 mm, but they can be inaccurate; better results can be achieved by assuming a drop-like cross section shape \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Criticalities that might occur during the manufacturing process and design recommendations to avoid them. * Geometric defects: the PBF produced parts may exhibit different kinds of dimensional and geometric deviations from the nominal model: shrinkage and oversizing are the most common ones. Other sources of inaccuracy are represented by warping (a curling phenomenon that yields a curved profile of down facing surfaces intended to be flat) and by the formation of super elevated edges. These phenomena deteriorate the surface topology and the dimensional accuracy while interfering with the efficiency of the recoating system as well as damaging the adjacent pieces. Other distortions affect critical features like thin walls, overhanging surfaces, and acute corners [22].