# Cranial Implant Design Applying Shape-Based Interpolation Method via Open-Source Software
## 2 Methods
### Design of cranial Implants Using MITK Software  
As an alternative to commercial 3-matic software, MITK was used to design the patient-specific cranial implant using a shape-based interpolation method. After the CT data of defected skull were loaded, a new \"Segmentation\" was created to design the cranial implant. Next, the paint size 14 in the 2D Tools box (red rectangle) was used to draw the line along the defective skull, as shown in Figure 2.  
The line was drawn while the left side of the mouse was held in a clicked position; the mouse was moved along the perimeter of the defect skull following the curvature of the healthy skull. The axial slice was enlarged for segmentation editing. If the line was over drawn, the \"2D Tools\" (red rectangle in Figure 3) were used to edit the line, either by using the \"Erase\" tool to delete the whole line or the \"Wipe\" tool to delete part of the line. The \"Wipe\" tool has different sizes of circles, similar to the size of the \"Paint\" tool. The \"Add\" and \"Subtract\" tools were also used to add or remove certain parts of the line. Figure 1: Fitting evaluation of patient-specific cranial implant on the defect skull. The size of \"Paint\" tool was used as a thickness indicator before drawing the cranial implant by choosing the right thickness of the implant before drawing, based on the healthy part of the skull. The \"Paint\" size in millimetres was 5.95 mm, 5.10 mm, 4.25 mm, 3.40 mm, and 2.55 for the size of 14, 12, 10, 8, and 6, respectively. A line was drawn to represent the thickness of the implant to cover the defect. The first line to be drawn may be selected from the most top or most bottom of the CT data axial slices, following which the same line is drawn on every tenth or fifth slice. In this example, the total axial slices within the defective area are 132 slices (slice no 91 to 223). Starting with slice no 91, every tenth slice was selected to draw the line for implant design, one by one, until slice 221, which were 91, 101, 111, 121, 131, 141, 151, 161, 171, 181, 191, 201, 211, and 221. The total number of slices involved for shape-based interpolation was 14 slices. Figure 4 shows the example of a line drawn in axial slice numbers 151, 161, and 171. Figure 3: The axial slice of the segmentation was enlarged to view the line drawn and for editing purposes. Figure 2: The “Paint” tool was used to draw the line along defective skull. Once the line drawing of every tenth axial slice was performed, the 2D interpolation was executed applying the shape-based interpolation method, and the 3D cranial implant was successfully created, but not all surfaces of implants were reconstructed due to under-interpolation as shown in Figure 5.  
As the selected axial slice was performed at every tenth slice, some areas were not interpolated; thus, holes could be seen on the implant. However, it can still be edited on the axial slice to cover the hole or defect of the interpolation, and the redo process can be carried out, as shown in Figure 6.  
Figure 4: Slices (**A**) 151, (**B**) 161, and (**C**) 171 of every tenth slice. Figure 5: Green circles indicated unreconstructed surfaces due to under-interpolation. Alternatively, another method of selecting every fifth slice for interpolation was tested, as shown in Figure 7.  
When every fifth slice interpolation method was performed, the slices involved were 91, 96, 101, 106, 111, 116, 121, 126, 131, 136, 141, 146, 151, 156, 161, 166, 171, 176, 181, 186, 191, 196, 201, 206, 211, 216, and 221. The total number of slices was 27. The result of the patient-specific cranial implant using the shape-based interpolation method on every tenth slice and fifth slice were saved in STL format and exported to STL format, as shown in Figure 8.  
Figure 1:0 The patient-specific cranial implant with every tenth slice interpolation (**left**) and every fifth slice interpolation (**right**). Figure 1:1 The final cranial implant before editing (**left**) and after editing (**right**).