Journal of the Mechanical Behavior of Biomedical Materials
Research paperCharacterization of cellular solids in Ti6Al4V for orthopaedic implant applications: Trabecular titanium
Introduction
In the past ten years Electron Beam Melting (EBM) was mainly used for rapid prototyping due to the long time required in pre-production and production and to the high costs generated for single component manufacturing (Chua, 2009, Kalpakjian and Schmid, 2009). Nowadays, EBM techniques can be successfully used in a wide range of industrial applications: from purification of semi-conductors (Pires et al., 2003) to welding processes (Hershcovitch, 2005), for the production of amorphous alloys (Guan et al., 2005) and for producing porous materials (Cansizoglu et al., 2008).
In a chamber, under vacuum conditions, the electron ray melts the alloy at approximately 2000 °C where 1650 °C is the melting temperature of Titanium Grade 5 Alloy (Niinomi, 1998). Some parameters influence mainly the structure of the cellular solids: the surface tension of the melt alloy (1.1±0.1 N/m, Schneider et al., 2002); the EBM ray diameter (μm); the diameter of the Ti6Al4V powder particles; and, the influence of gravity on the liquid melt pool during melting. The latter depends on liquid that, when standing on powder and not on solid metal, may penetrate into the substrate. This usually occurs during the production of metallic arms of trabecular structures. The tolerance capability of this technology is about ±300 μm.
In the last few years different studies were carried out on the biomedical applications of EBM: Li et al. (2009) investigated the mechanical properties of a Ti6Al4V honeycomb-like structure with approximately 66% porosity; Heinl et al. (2008) investigated the microstructure and bioactivity of two different trabecular solids (80% and 60% porosity) before and after chemical etching; Harrysson et al. (2008) and Cansizoglu et al. (2008) worked together and investigated the relationships between the geometrical parameters (single cell dimension, orientation, angle) and the mechanical properties of the solids.
Moreover, in a recent study of Thomsen et al. (2009) it has been demonstrated that the surface properties of EBM Ti6Al4V display biological short-term behaviour in bone equal to that of conventional wrought titanium alloy.
As proved by Christensen et al. (2007) the potential advantages of the revolutionary EBM technology, in terms of manufacturing method, is its conformity with the ASTM standards for surgical implant applications; quick processing and design complexity and the related benefits for obtaining a metallic biomaterial used in rapid bone ingrowth with good mechanical qualities. All of which promote this technology in the orthopaedic implants industry.
The success of bone response and prosthetic implant depends on the surface characteristics of the materials applied. As reported in the literature, the surface characteristics of materials influence the osteoblast adhesion on biomaterials; therefore, porosity and pore size of biomaterial play a critical role in bone formation in vitro and in vivo (Anselme, 2000). Specifically the minimum pore size required for improving osseointegration is 300 μm (Frosh et al., 2004). On the other hand, an upper limit in porosity and pore size must be defined by constraints which refer to the mechanical properties and the anatomical dimensions of the pores of the specific bone-tissue repaired. Furthermore the growth of human osteoblasts in drill channels, having a diameter of 600 μm, was significantly quicker than in all other channels with diameters ranging from 300, 400, 500, and 1000 μm (Frosh et al., 2004).
According to the experimental findings reported in the literature regarding the improvement of vascularisation and of oxygenation of tissue–that is to maximize osseointegration of the implantable materials–the EBM manufacturing process allows to obtain innovative prosthetic implants as shown in Fig. 1 by modulating the cellular solids structure in Ti6Al4V. The continuity between the solid part of the acetabular cup cavity and the external multi-planar hexagonal cell structure imitating the morphology of the trabecular bone has been designed, the name of this microstructure is Trabecular Titanium. In fact, it is not a coating consequently the porous part cannot detach nor are any galvanic effects produced between the implant areas, composed of different materials, as common in coated implants. Traditional application of porous coatings can be detrimental to the material properties of the implant and may lead to problems at the interface such as shedding of the porous surface (Li et al., 2009). By having the porous structure completely integrated at the time of implant manufacturing this issue is avoided.
The advantage of acetabular cups in Trabecular Titanium is mainly in the outer surface. It is an extremely rough surface that, together with a 1 mm press fit it, ensures strong fixation making the acetabular cup ideal for primary surgery (Fig. 2). It is also a good option for acetabular fixation in hip arthroplasty.
The ability of natural bone to re-grow in narrow porosities cellular solids and in metal foams are under intensive study for innovative metallic coatings in prosthetic implants (Anselme, 2000, Baleani et al., 2000, Christensen et al., 2007, Frosh et al., 2004).
The aim of this study is to characterize the morphology and the microstructure of Ti6Al4V cellular solid samples manufactured by Electron Beam Melting (EBM) which evaluates the main mechanical properties and the possibility of applying it in prosthetic implants.
Section snippets
Experimental details
A tri-dimensional structure has been built using a CAD 3D program. The basic element of the CAD 3D matrix is a hexagon developed on three planes. The matrix built on this element is then cut using boolean operators on planes and curves to obtain the desired shape (Fig. 3).
Two different samples in Ti6Al4V alloy were tested in this project: a smaller structure (sample A) and a larger structure (sample B). The name of both of these structures is Trabecular Titanium™ (Dalla Pria et al., 2008).
Results and discussion
Powder chemical composition was analyzed by the producer obtaining the results in Table 1.
The composition is completely in line with the composition standards of Ti6Al4V alloy. The distribution of the particles dimension is around 45–100 μm in diameter.
Fig. 5 shows the structure of sample (A) as obtained with the EBM process and Ti6Al4V alloy powders, followed by blasting, to remove any residual metallic not melted powder. The metallic surfaces appear pleated and irregular, the single porosity
Conclusions
The experimental results reported in this study illustrated that with the application of EBM technology it is possible to obtain components having the following characteristics:
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the porosity of both samples (63% for sample A, 72% for sample B) is comparable with the literature values for human spongy bone;
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the porosity dimension, especially pore size of the sample (A), is comparable with the values that can be found in the literature for structures that improve osseointegration; it remains to be
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