A 17 year old male patient presented after a motorcycle accident where he was not wearing a helmet. Upon his arrival at the hospital with a diagnosis of TBI (traumatic brain injury), he was evaluated by the Department of Neurosurgery who decided to treat the patient with a decompressive craniotomy. A CT was done to complete the diagnosis and plan the surgery. The patient had a bifrontal 20 cm cranial defect from the temporal fossa of one side to the other and 10 cm vertical distance (Fig. 4). The thickness of the implant in this case was of 6 mm. It is 1 mm thicker than that of the previous case due to patient anatomic differences in diploe thickness. After surgery, he remained hospitalized for 2 weeks before discharging him. He was referred to us after 3 months of physical therapy once the Neurosurgical Department considered cranial reconstruction feasible. The patient was evaluated and was found to have no neurologic damage and an intact scalp. The patient was deemed an excellent candidate for the placement of a 3D PMMA pre-operative manufactured cranial prosthesis, 2 days after which the patient had a good evolution and was discharged. After 2 months, the patient returned to his normal school activities.
In Mexico these procedures have to be payed in full by the patient, limiting their reach and practice in the general population since they are costly. A customized titanium implant costs around US$5000, and those made from PEEK around US$7000 or more depending on their size. The customized prostheses proposed by the authors have a cost of about US$600, including the digital design, printing of a 3D prototype and the PMMA prosthesis itself. Both titanium and PMMA are the most commonly used alloplastic materials.
The CT scan data is stored in the standard format DICOM (Digital Images and Communications in Medicine) which allows generating an interface between the medical equipment and any other device to visualize the images. Through the DICOM viewer, Osirix  generated a three-dimensional reconstruction of all the CT cross-sectional images. A bone filter is applied in order to only observe the bone structure, achieved by taking as reference its attenuation degree.
The implant is generated using the software of the computer-aided design (CAD) ZBrush 4R5  since Osirix  is only a viewer.
The implant piece, Vimplant, is generated from the CT skeletal reconstruction of the patient's cranium. With volumetric reconstruction, Vtotal, and considering its symmetry; an imaginary division is made on the sagittal plane, obtaining two volumes, Vleft and Vright. A boolean operation is applied to the volumes Vleft and Vright: Vimplant = Vleft - Vright (Fig. 5).
The implant design must have a precise shape and volume according to each patient's cranial anatomy. Finally, the data is exported in a stereolithography extension file (STL) and fed to the printer.
A CUBE 3D (from 3D System) printer is used to print out a PLA prosthesis from the STL file using a fused deposition model by means of a 1,75 mm filament at a 260  C extruder temperature (Fig. 6). Once the printing process is finished, which takes about 20 h, the scaffolds are removed with a low-speed motor and a carbide bur and the adjustment is verified (Fig. 7). The prosthesis is then taken to the dental laboratory.
The prosthesis is placed in metal containers to obtain two plaster impressions (an internal and an external surface impression). A transparent PMMA OPTI-CRYLR is poured into the space created by the internal plaster impression and pressed with the external mold. The casts are opened and the prosthesis is revealed after a 25-min polymerization time (Fig. 8). The prosthesis is then cut, perforated, and polished with a low-speed motor. Precise anatomical compliance to the model is verified and approved by the multi-disciplinary team (including the neurosurgeons), and the prosthesis is then placed on a mannequin for academic and illustrative purposes (Fig. 9). After obtaining the prosthesis in the laboratory, it is washed with normal saline solution and then submerged in a chlorine based antiseptic solution (Microdacyn 60 by Oculus lnnovative Sciences in Petaluma, CA.) for 15 min before taking it to the hospital where it is sterilized with ethylene oxide gas before its implantation (Fig. 10).
Cranioplasties were carried out 3 months after craniotomy in conjunction with the neurosurgeons. The prostheses were then placed over each of the defects to adjust them in vivo with minimum adjustments with the same low-speed electric motor, and fixed with 3 long titanium bridge plates, each held down with 2 screws (Fig. 11). The procedure ended without any unexpected events. The patients were hospitalized for 2 days and discharged after a single drain is removed from each patient to continue their care as outpatients (Fig. 12).
The patients were evaluated every week for 6 months to register PMMA behaviour, biological sefety and any eventualities (Fig. 13).