gms | German Medical Science

Trauma, calvarian tumors and defects following decompressive surgery are the main reasons for large cranial defects. Indications for reconstruction of these defects are cosmetic and/or protection against mechanical impact. The intraoperative free-hand modelling of skull implants, in particular with large defects, turns out to be technically difficult and operating times increase. We report a technique of Rapid Prototyping (RP) for custom reconstruction of cranial defects using Fused Deposition Modeling (FDM) which is based on routine radiographic diagnostics.

In a cadaver specimen, a craniectomy was performed and a model of the defect was built using Fused Deposition Modeling. Implant design was based on the preoperative imaging data using a mirror image of the contralateral side modified by “free hand” CAD. The CAD data of the mold were transferred to a .STL-file format to generate rapid prototyping models. Different acrylic implants were built using the mold. In addition alternative prefabricated implants were built, based on a wax matrix free-hand formed on the FDM model. The wax matrix was then imprinted in a silicone form and the wax was removed. Subsequently a conventional free-hand formed implant was modelled directly into the cranial defect. The different implants were radiographically imaged and the reconstructions were compared to the preoperative images.

Free-hand intraoperative direct modeling of PMMA implants into the cranial defect showed the highest inter-individual variation of the implant fit and cosmetic result. The intraoperative modeling times were more than four times longer than implantation of prefabricated implants and ranged from 26-44 minutes. Both prefabricated implants based on free-hand wax models or CAD designed Fused Deposition Modeling produced implants showed excellent cosmetic results and very low inter-individual variation. However, wax modeling into an artificially produced defect was very labor intensive and required a three day processing time.

Our results demonstrate that FDM generated anatomical models and negative molds of the defects provide a rapid and highly accurate method of custom implants of skull defect reconstructions, which is based on routine CT data and commercially available hardware and CAD software components. It is a cost-effective alternative to industrially produced implants. The implant fabrication can be performed preoperatively by the operating surgeon.