Characterization of the structure and permeability of titanium foams for spinal fusion devices

Titanium foams produced via the space-holder method are used for spinal fusion devices since their combination of an open-cell structure and bone-like mechanical properties promises potentially excellent bone ingrowth. Earlier studies have indicated that the size of the pores and interconnects must be greater than 100microm for effective bone ingrowth and vascularization. Hence, the quantification of the pore and interconnect size is required for efficient scaffold design. In this study, microcomputed tomography (microCT) was used to obtain the three-dimensional (3D) structure of Ti foams with three levels of porosity (51%, 65% and 78%). Novel algorithms were then applied to quantify both the pore and interconnect size of Ti foams as a function of porosity. All foams possessed a modal pore and interconnect size in excess of 300microm, satisfying the requirement of being greater than 100microm. The pore and interconnect size also dominates the flow properties or permeability of open-cell structures. Therefore, the microCT data was also used to generate a mesh for computational fluid dynamics analysis to predict the permeability. The calculated permeability (117-163x10(-12)m(2) depending on direction) for the Ti foams with 65% porosity was first validated against experimental measurements (98-163x10(-12)m(2)) and then compared to prior authors' measurements in healthy cancellous bovine bone (233-465x10(-12)m(2)). The close match among all the permeability values proves the suitability of the material for biomedical skeletal-implant applications.