Research paper
Development and mechanical characterization of porous titanium bone substitutes

https://doi.org/10.1016/j.jmbbm.2012.01.008Get rights and content

Abstract

Commercially Pure Porous Titanium (CPPTi) can be used for surgical implants to avoid the stress shielding effect due to the mismatch between the mechanical properties of titanium and bone. Most researchers in this area deal with randomly distributed pores or simple architectures in titanium alloys. The control of porosity, pore size and distribution is necessary to obtain implants with mechanical properties close to those of bone and to ensure their osseointegration. The aim of the present work was therefore to develop and characterize such a specific porous structure. First of all, the properties of titanium made by Selective Laser Melting (SLM) were characterized through experimental testing on bulk specimens. An elementary pattern of the porous structure was then designed to mimic the orthotropic properties of the human bone following several mechanical and geometrical criteria. Finite Element Analysis (FEA) was used to optimize the pattern. A porosity of 53% and pore sizes in the range of 860 to 1500 μm were finally adopted. Tensile tests on porous samples were then carried out to validate the properties obtained numerically and identify the failure modes of the samples. Finally, FE elastoplastic analyses were performed on the porous samples in order to propose a failure criterion for the design of porous substitutes.

Highlights

► Determination of the mechanical properties of SLM-made CP Ti. ► Design of a porous elementary pattern with mechanical properties close to those of bone. ► Testing of porous samples. ► Failure criterion for wireframe porous bone substitutes.

Introduction

In most cases, clinical failure of prosthetic solutions is due to the stress-shielding generated by the mismatch of mechanical properties between bone and implant (Karachalios et al., 2004). The mechanical properties of bone depend on many factors such as anatomical part, sex and age. The global elastic properties of cortical bone are usually described by the orthotropic tensor of elastic moduli defined by nine constants. Its longitudinal compressive strength was estimated to be 200 MPa by Reilly and Burstein (1975) and Reilly et al. (1974). In long bones, the cortical tissue represents approximately 25% of the whole bone volume. Consequently, as a first order approximation an apparent compressive strength of 50 MPa is considered in this study.

The intrinsic properties of bulk titanium are significantly higher than those of cortical bone. To minimize the negative impact of this stiffness incompatibility, substitutes with a controlled porosity can be introduced. Oh et al. (2003) obtained the Young’s modulus of porous titanium close to that of bone by changing the voids volume fraction fV. They reduced this property from 110 GPa (bulk titanium) to 10 GPa for fV=0.35.

Moreover, as demonstrated by many authors (Otsuki et al., 2006, Heinl et al., 2008, Xue et al., 2007, Hollander et al., 2006), the existence of porosity is also propitious for the substitute osseointegration, the essential factor of the long-term reliability of the implant. The question arises: do the size, shape, topology and volume fraction of voids have any influence on the substitute colonization? Only a few studies have addressed this crucial problem. There is no consensus in the literature about the optimal pore size to enhance bone ingrowth. In their pioneering work, Otsuki et al. (2006) showed that the pores of the structure have to be interconnected in order to ensure the bone ingrowth. These authors also compared the type of tissue formed as a function of time and implant internal architecture. They tested two levels of porosity (50% and 70%) combined with two ranges of pore sizes (between 250 and 500 μm and between 500 and 1500 μm) and concluded that the size of pores should range from 500 to 1500 μm for both levels of porosity to get a tissue of good quality. Xue et al. (2007) demonstrated that the bone colonization is impossible for a pore size under 100 μm. Chen et al. (2009) tested the osteogenesis on structures with pores ranging from 100 to 400 μm but did not speak out against higher pore dimensions. Hollander et al. (2006) tested porous structure with pore dimensions of 500, 700 and 1000 μm and concluded that the growth of human osteoblast is possible for all these types of porosities. Those authors did not indicate a maximal pore size either. Generally speaking, no clearly identified rule exists to allow the optimal design of implants or bone substitutes in commercially pure porous titanium (CPPTi).

Several manufacturing processes of porous titanium are known among which freeze casting (Yook et al., 2009), space holder technique (Niu et al., 2009), rapid prototyping (Li et al., 2006) or laser processing (Parthasarathy et al., 2010, Heinl et al., 2008, Krishna et al., 2007). Freeze casting and space holder processes mostly lead to randomly distributed pores whereas rapid prototyping and laser processing are adopted when a controlled architecture is required.

The freeze casting technique enables a porous material with a compressive strength close to that of bone to be obtained, but its main drawback underlined by Yook et al. (2009) is the pore size that does not exceed 300 μm for a long manufacturing time of 7 days. Higher pore sizes can be reached with the space holder technique but the mechanical properties obtained are lower than those of human bone (Niu et al., 2009). In both cases, the control of the elastic properties anisotropy seems to be difficult.

In the literature, rapid prototyping and laser processing are often reported to generate bulk titanium or porous titanium with simple pore architecture (Parthasarathy et al., 2010, Heinl et al., 2008, Krishna et al., 2007, Li et al., 2006). These techniques are generally used in the case of titanium alloys.

In this paper, the Selective Laser Melting (SLM) process was chosen to produce porous titanium with a particular periodic internal architecture. In a first part, the properties of titanium samples processed by SLM are studied and compared to the ISO Standard Specifications. Crystallographic texture and residual stress measurements are also examined to estimate the anisotropy of samples obtained by this technology.

In a second part, an elementary pattern is designed to approach the mechanical properties of human bone and to enable the optimal implant osteointegration. The final section is dedicated to the question of elastoplastic global properties and simple failure assessment for such porous implants.

Section snippets

Criteria for elementary pattern definition

To design and optimize an elementary pattern for bone substitutes, several criteria had to be simultaneously taken into account. With biocompatibility, the criteria adopted in this study are as follows:

  • Elastic (global) properties as close as possible to those of cortical bone (including the anisotropy).

  • A maximal allowed apparent stress not exceeding 50 MPa.

  • A maximal stress exerted at any point of the pattern smaller than σad=σY/k, where σY is the yield point of the CP Ti and k=1.5 corresponds

Elastoplastic properties of CP Ti grade 2 processed by SLM

Three standard tensile test specimens of CP grade 2 Ti were tested for each direction of melting. The true stress–true strain curves were obtained using the actual specimen cross sections. As an example, Fig. 4 shows the three curves obtained for the tension direction (T). It clearly appears that SLM process ensures a good repeatability as the maximum stress deviation with respect to the average curve does not exceed 2%. A similar conclusion can be drawn for the two other directions, not

Conclusions

An elementary pattern of a CPPTi structure mimicking the mechanical properties of bone and favouring bone colonization was developed in this study. This pattern is characterized by a porosity of 53% due to the periodic oblong pores of relatively important dimensions 0.9×0.86×1.5 mm3.

Analyses performed on the bulk SLM processed samples seemed to reveal that this technology is well adapted to the manufacturing of implants. Indeed, the classical elastoplastic properties of CP Ti samples obtained

Acknowledgement

The authors wish to thank Dr J.-M. Hiver from Institut Jean Lamour, Ecole des Mines de Nancy for his participation in the computed tomography analysis of the porous samples.

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