## 3 Results
#### Case Study: The Skin Assessment
An FFF printer (MakerBot Replicator+, Banbury, UK) was used to manufacture out of plastic the skull and the devices and the molds to cast silicone and reproduce the skin. FFF parameters used in the process were layer height of 0.2 mm, extruder of 0.4 mm, and printing temperature of 215 \({}^{\circ}\)C. Non-soluble PLA supports were used in all prints and later removed by hand. Fabricated models were, in all cases, manually cleaned and refined. The silicone used in this application was the Smooth-On Ecoflex 00-50 (Smooth-On-Macungie, PA, USA). We chose this material after having qualitatively testing various specimens following the indications described in [16]. The skull and the skin molds were printed in polylactide (PLA). The skin was designed to fit over the plastic model of the skull, as shown in Figure 10a. As with the skull, prototypes of the implant can also be made from plastic using a 3D printer. First, the physical simulation confirmed that the conditions were borderline. The same suction action that stretched the skin in the most sunken region had, however, worsened the condition of the skin, particularly thin and fragile along the edges of the defect (white regions visible in Figure 10a). The Frontal region, in particular, did not allow further incisions without encountering certain complications after surgery short of introducing expanders. Consequently, the assessment suggested devising a minimal incision that would avoid the frontal, posterior and lateral regions as far as possible and designing a device that would not increase skin tension in the same areas. Figure 10b shows in green the eligible region for the skin incision. Red indicates the areas that must be avoided according to the skin assessment. The only solution to avoid skin expanders was then to plan a minimally invasive surgery and, consequently, to design a cranial prosthesis able to be correctly positioned despite the small incision. In addition, the device should have avoided increasing skin tension in the lateral, posterior and frontal sides. Many attempts with various surgical strategies and different implants and supplementary devices for positioning the implant were simulated on the produced models to find the best combination of surgery pathways and device features able to fulfill the constraints defined through the skin assessment without using skin expanders. Different iterations of the devices have been tested with physical simulations (e.g., see Figure 11), with the surgeon performing surgery on the phantom. Figure 10: (**a**) Skin model correctly applied over the plastic model of the defective skull. (**b**) Eligible region for surgical cuts. All these attempts resulted in an innovative minimally invasive procedure with a surgical cut that was 1/3 smaller than a traditional craniplasty. The pathways were maintained within the established limits, planned to be the smallest possible to allow the prosthesis to pass. The patient-specific device had to slide inside the dissection to its resting position. Physical references properly designed on the implant ensured its right positioning. However, the small size of the incision had essentially made the surgery a blind operation. The prosthesis was 0.4 mm thin; the implant did not exceed the frontal boundary of the defect, while it leaned 15 mm on the other 3 sides, allowing the fixation through screws on the bone. Because of the minimal incision located in the upper part of the skull, screws were inserted transcutaneously in the lateral side. To make this possible, an additional device was designed to provide physical references that allowed effortless identification of the holes through the skin. The additional device could be mounted on the plate through four pins (Figure 12b) and then easily removed once the fixing is realized. Simulations demonstrated that the developed procedure and devices would have reduced surgical incisions and the skull surface exposed during surgery. Furthermore, the stresses imposed on the skin flap were significantly lowered.