State of the art
The replacement of parts of the skeletal system by means of prostheses is still not completely optimized for those districts that need both functional and aesthetic requirements; the replacement of head or face bones are a typical example of this need. As regards the morphological reconstruction, the mirroring technique is the only method used to virtually build the missing part on a 3D model of the head obtained starting from CT images. On the other hand, the material choice is still controversial; in literature different examples can be found: prostheses made up of acrylic material poured into a mould obtained using the stereolitography (SL) technique [1-2]; Ti prostheses manufactured by machining using CAD/CAM techniques [3-4]; composite material prostheses obtained through SL technique [5]; porous Hydroxiapatite prostheses obtained through rapid prototyping technique [6-7]. The above mentioned solutions still have some drawbacks: insufficient mechanical characteristics [8-9], too long manufacturing time [10], high weight or presence of defects related to the manufacturing processes. All these limits can be overcome by using the biocompatible Ti [11] (which allows to reduce the weight and to enhance the mechanical performances of the prosthesis) and any innovative metal forming processes (e.g. the SPF Process, in which the Ti, subjected to relatively low strain rates, exhibits elongations higher than 1000% thanks to the activation of specific deformation mechanisms [12-14]). Even though manufacturing cycle time is still high, SPF allows to obtain complex geometries (typical of patient-specific maxillo-facial or cranial reconstructions [15-18]). In order to define SPF process parameters (especially trough numerical simulations [18,19]), it is essential to correctly model and characterize the material [20-23]. On the other hand, the problem of the processing time increase which characterizes the SPF process, can be overcome by using SPIF, which also allows to obtain complex patient-specific geometries even if characterized by a lower precision than the SPF. Some examples of prostheses obtained by SPIF technique are present in literature: reverse engineering techniques and SPIF have been used for the production of customized orthopedic braces [24]; similar approaches have been used for maxillar prostheses, manufactured both in Ti and polymeric material [25], and cranial prostheses, starting from commercial pure Ti sheets [26-27]. However, all these studies are only engineering based and do not take into account either clinical aspects or problems related to the patient.