Pre-operative patient-specific alloplastic implant design and manufacturing: cranioplasty application

Cranial defects are often result of decompressive craniotomies performed after brain injuries, tumor extraction and trauma, leaving patients with severe functional and aesthetic problems. The partially or total reconstruction (cranioplasty) is expensive, especially for low-income patients. The standard protocol is to use the pre-fabricated implants made of titanium or Peek using complex CNC machines. In this work we present a method for design and build a low cost alloplastic implants using the commercial bone cement based on Polymethyl-Methacrylate (PMMA), free software and rapid prototyping. The design process starts with data image acquisition from high resolution CT scans of a patient defect, the image is segmented carefully using 3DSlicer and then exported as a cloud point format. Subsequently, the reconstruction of the defect the design of the implant was carried out using the software Blender. To avoid interference with the surrounding tissue of the skull, it is necessary to create an internal offset between the implant and cranial bony contour, ranging from 0.8 to 1 mm. The mold consisted of two parts, considering the injection and vent holes in the design, that were printed using a 3D printer machine with rigid and flexible filaments. A manual syringe was used to prepare and inject the bone cement into the molds that were covered with release agent. The design of the implant and molds took 2.6 hours on average for each subject, the printing cost of the molds, the skull phantom and the syringe mixer was approximately $86, the bone cement was $ 315.0 per case without including the CT and the 3D designer expert costs. The method presented in this work has been applied to ten patients who had skull defect from 36 to 100 cm3. The implants were secured into position with metal fixation system. The surgical procedure took on average about 2 hours. Therefore, by using this procedure, surgeons have a flexible option for preoperatively implant preparing and building with excellent anatomic geometry, optimal fit and a stable reconstruction, reducing significantly the cost, surgical time and risk.