Design of customized implants and 3D printing of symmetric and asymmetric cranial cavities.

A methodology has been developed in this work to design customized cranial implants from computed tomography (CT) scan images for symmetric as well as asymmetric defects. The two-dimensional CT scan images were converted into three-dimensional geometric models using software packages. Two cases of cranial cavities at different locations were considered for implant design using two different approaches. Case 1 is having a symmetric cranial cavity while Case 2 has an asymmetric frontal cranial cavity. The craniums with defects were 3D reconstructed. Customized cranial implants were made for the two cases. In Case 1, symmetry was used to design the cranial implant. Symmetry cannot be used in Case 2. In Case 2, the implant was designed by blending from the surface available adjacent to the missing portion of the cranium. 3D reconstructed bone models and customized implants were 3D printed in poly-lactic acid (PLA) using a fused deposition modeling process for form and fit evaluation. Finite element analysis was performed to compare the mechanical behavior of bone, and the two biomaterials - polyether ether ketone (PEEK), and Ti6Al4V. Static structural finite element analysis was performed to simulate the impact of falling off a bicycle with an impact on the cranial implants in the two cases. The load was modeled as a normal force acting on the surface of the implant. It was found that the stresses in the titanium alloy are comparable to those of PEEK for both the cases. However, the strains and deformation were found to be much smaller compared to those in PEEK. Therefore, the titanium alloy is the material of choice for both the cases among the materials under consideration. The designed implants are solid hence may face the challenge in bone ingrowth. In future studies, the implant can be made porous by incorporating a lattice structure to enhance osseointegration and promote bone ingrowth. Implants for both symmetric and asymmetric defect cases in cranium were successfully designed.

Applied Taguchi method for fatigue testing of customized hip implant.

PURPOSE: Human activities generate stresses, which vary with time and may result in fatigue failure of the customized hip implant. This study aims to investigate fatigue testing of customized hip implants using the minimum number of experiments by the Taguchi method, for 147 patients. This study was also useful to determine the influential geometrical parameters on the fatigue safety factor of customized hip implants. METHODS: Horizontal offset (HO), vertical offset (VO) and neck shaft angle (NSA) of the hip joint of 147 patients were measured on computed tomography (CT) scanned images. Stress and strain of hip implants were calculated by finite element analysis and validated by in vitro experimental tests. Fatigue safety factors were calculated by Goodman, Soderberg and Gerber's fatigue theories and maximum stresses. RESULTS: Analysis of variance results show that the highest impact on fatigue safety factors was equal to 54.38% for HO, 16.33% for VO, and was equal to 29.16% for NSA with reference to Goodman, Soderberg and Gerber's fatigue theories. The hip implant shape of experiment no. 8 has the highest safety factor value compared to all other hip implants. CONCLUSIONS: The results show that HO has the maximum influence on fatigue safety factors. The determination of influential geometric parameters may be useful to redesign customized hip implants in order to achieve the highest fatigue safety factor. The Taguchi method is suitable for fatigue testing of custom hip implant with a minimum number of experiments.