Page 92 - Clinical relevance of current materials for cranial implants

Chapter 4
The implant and surrounding bone were modeled into a sphere-like shape with an outside diameter of 140 mm. The thickness of the implant model was chosen as 4.5 mm, the average thickness of the fractured implant. The dimensions of the groove and exposed pore in the broken implant were measured using digital dial calipers (Mitutoyo, Kawasaki, Japan). The models were composed of 17,345 parabolic tetrahedron solid elements. The implant was fixed to the patients’ cranial bone during surgery and held into place by surrounding tissues; therefore the interface between the implant and the surrounding bone was designed as fixed, allowing no movement in any direction. Material characteristics of the models were: PMMA: Young’s modulus = 2158 MPa (experimental result of the fractured cranioplasty), Poisson ratio = 0.38; bone: Young’s modulus = 15 GPa, Poisson ratio = 0.3. Two analyses were performed using different loads at the outside of the implant and perpendicular to the surface. One with a load of 100 N on the central node and one with a load of 100 N on the node opposite to the weak spot. The material properties used in the models are shown in Table 1.
Table 1: The maximum tensile stress (σ), in MPa, and the Young’s modulus (E), in GPa, of the materials used in the models.
Material
Bone Implant
σ E
- 10 70 2.2
The maximum tensile stress (solid maximum principle stress) and the displacements in the implant and surrounding bone layer were calculated. In post-processing, the contour option ‘average elemental’, without use of the ‘corner data’, was used to visualize the results.
The amount of absorbed energy in the implant under load was calculated as follows:
Where E is the absorbed energy [in joules], F is the applied load [in newtons], and tf is the displacement in the direction of the applied load [meters].
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