Influence of porosity on mechanical properties and in vivo response of Ti6Al4V implants

Abstract

Metallic biomaterials are widely used to restore the lost structure and functions of human bone. Due to the large number of joint replacements, there is a growing demand for new and improved orthopedic implants. More specifically, there is a need for novel load-bearing metallic implants with low effective modulus matching that of bone in order to reduce stress shielding and consequently increase the in vivo lifespan of the implant. In this study, we have fabricated porous Ti6Al4V alloy structures, using laser engineered net shaping (LENS), to demonstrate that advanced manufacturing techniques such as LENS can be used to fabricate low-modulus, tailored porosity implants with a wide variety of metals/alloys, where the porosity can be designed in areas based on the patient's need to enhance biological fixation and achieve long-term in vivo stability. The effective modulus of Ti6Al4V alloy structures has been tailored between 7 and 60 GPa and porous Ti alloy structures containing 23-32 vol.% porosity showed modulus equivalent to human cortical bone. In vivo behavior of porous Ti6Al4V alloy samples in male Sprague-Dawley rats for 16 weeks demonstrated a significant increase in calcium within the implants, indicating excellent biological tissue ingrowth through interconnected porosity. In vivo results also showed that total amount of porosity plays an important role in tissue ingrowth.

Fig. 1

(a) Schematic depiction of LENS™ process (b) typical porous Ti6Al4V samples fabricated using LENS™.

Fig. 1

(a) Schematic depiction of LENS™ process (b) typical porous Ti6Al4V samples fabricated using LENS™.

Fig. 2

Influence of specific energy input on the relative density of laser processed Ti6Al4V alloy samples.

Fig. 3

Micrographs showing pore connectivity (a) Part build section, 180W, 15g/min, 10 mm/s, 0.762mm, relative density 80% (b) Same as (a) Perpendicular section, (c) Part build section, 200 W, 30g/min, 25mm/s, 1.27mm, relative density 70% (d) Same as (c) Perpendicular section.

Fig. 3

Micrographs showing pore connectivity (a) Part build section, 180W, 15g/min, 10 mm/s, 0.762mm, relative density 80% (b) Same as (a) Perpendicular section, (c) Part build section, 200 W, 30g/min, 25mm/s, 1.27mm, relative density 70% (d) Same as (c) Perpendicular section.

Fig. 3

Micrographs showing pore connectivity (a) Part build section, 180W, 15g/min, 10 mm/s, 0.762mm, relative density 80% (b) Same as (a) Perpendicular section, (c) Part build section, 200 W, 30g/min, 25mm/s, 1.27mm, relative density 70% (d) Same as (c) Perpendicular section.

Fig. 3

Micrographs showing pore connectivity (a) Part build section, 180W, 15g/min, 10 mm/s, 0.762mm, relative density 80% (b) Same as (a) Perpendicular section, (c) Part build section, 200 W, 30g/min, 25mm/s, 1.27mm, relative density 70% (d) Same as (c) Perpendicular section.

Fig. 4

Typical microstructure of (a) as-received powder (b) laser processed samples.

Fig. 4

Typical microstructure of (a) as-received powder (b) laser processed samples.

Fig. 5

X-ray diffraction pattern of laser processed structures and as-received Ti6Al4V alloy powder.

Fig. 6

0.2% proof strength of laser processed porous Ti6Al4V samples.

Fig. 7

Young's modulus of laser processed porous Ti6Al4V samples.

Fig. 8

Ca⁺⁺ concentration in porous Ti6Al4V alloy implants. Each bar indicates the mean concentration of Ca⁺⁺ and each vertical line indicated the S.E.M. of 5 rats per group. *** P<0.001 vs. III, §§§ P<0.001 vs. II

Fig. 9

LENS processed porous Ti6Al4V implants after 16 weeks implantation in rat intramedullary defects.