DESIGN, ANALYSIS AND FABRICATION OF POROUS TITANIUM IMPLANTS USING ELECTRON BEAM MELTING FOR CRANIOFACIAL APPLICATIONS

Abstract

Patient specific implants for the reconstruction of craniofacial defects have gained importance due to better performance over their generic counterparts. This is due to, the precise adaptation to the region of implantation, reduced surgical times, and better cosmesis. Titanium implants built using traditional manufacturing processes are often heavy compared to the parts they replace and can cause discomfort to the patients. The variation in mechanical properties as elastic modulus between the implant and bone reduces the longevity of the implant. In mandible reconstruction, post implant dental reconstruction poses additional problems. Recent introduction of direct digital manufacturing technologies as electron beam melting (EBM) and Selective Laser Melting for processing of titanium has led to a one step fabrication of near net shape porous custom titanium implants with controlled porosity to meet the requirements of the anatomy and functions at the region of implantation.

The first part of this research is directed towards development of a design strategy using representative volume element based technique, in which precisely defined porous implants with customized stiffness values are designed to match the stiffness and weight characteristics of surrounding healthy bone tissue. Dental abutment structures have been incorporated into the mandibular implant. Finite element analysis is used to assess the performance of the implant under masticatory loads. This design strategy lends itself very well to rapid manufacturing technologies such as Selective Laser Sintering (SLS) and Electron Beam Melting (EBM) processes.

The second part of the research consists of an image based micro-structural analysis and mechanical characterization of porous Ti6Al4V structures fabricated using the EBM rapid manufacturing process. SEM studies have indicated complete melting of the powder material with no evidence of poor inter-layer bonding. Micro-CT scan analysis of the samples indicate well formed titanium struts and fully interconnected pores with porosities varying from 49.75 - 70.32%. Compression tests of the samples showed effective stiffness values ranging from 0.57 (+0.05) - 2.92(+0.17) GPa and compressive strength values of 7.28(+0.93) - 163.02(+11.98) MPa. For nearly the same porosity values of 49.75% and 50.75%, with a variation in only the strut thickness in the sample sets, the compressive stiffness and strength decreased significantly from 2.92GPa to 0.57GPa (80.5% reduction) and 163.02MPa to 7.28MPa (93.54 % reduction) respectively. Grain density of the fabricated Ti6Al4V structures was found to be 4.423g/cm3 equivalent to that of dense Ti6Al4V parts fabricated using conventional methods.

In conclusion, a methodology for fabrication of craniofacial implants that would have better aesthetics, and improved masticatory functions, enhancing patient comfort and compliance, has been developed. From a mechanical strength viewpoint, we have found that the porous structures produced by the electron beam melting process presents a promising rapid manufacturing process for the direct fabrication of customized titanium implants for enabling personalized medicine with reduced lead time and cost.