Page 91 - Clinical relevance of current materials for cranial implants

Flexural strength
The fractured implant and the CMW-3 specimens were sawed with a 0.3-mm-wide
diamond saw (Ukam Industrial, Valencia, CA, USA) into x-10-x 13-mm rectangular
specimens (10 (n=10 per group) and wet grinded with standard metallographic grinding
paper (P500, P1000, and P1200). The specimens from the fractured cranioplasty were
harvested from the center of the implant. Before testing the specimens were immersed
in a water bath at 37.0 ± 1.0 °C for 50 ± 2 h. The flexural strength was determined at 37.0
± 2.0 °C, using a three-point-bending test with a crosshead speed of 1.0 mm/min and
a distance between the supports of 10.0 mm. Each specimen was tested until fracture. 4 The ultimate flexural strength (σ) was calculated using the following equation:
2
where F is the maximum load exerted [in newtons], l is the distance between the supports [in millimeters], b is the width and h is the height of the specimen [in millimeters].
Finite element analysis
The fractured cranioplasty (Figure 3) was visually inspected and a simplified 3D model was created to predict the stresses in the expected point of failure (Figure 4). More precisely, this point showed typical characteristics of an initial point of fracture, the so-called mirror-hackle zone 19,20, a groove originating near the edge of the implant, and an exposed pore located at the thinnest portion of this groove. Finite Element modeling was carried out using FEMAP software (FEMAP 11.1.0, Siemens PLM Software, Plano, Texas, USA); the analyses were performed with Nastran software (NX Nastran; Siemens PLM Software, Plano, Texas, USA).
In vivo fractured PMMA cranioplasty
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