Characterization of a Reticulated, Randomized Porous Structure Produced by Additive Manufacturing
Mark L. Morrison1, Marc E. Taylor1, Carolyn Weaver1
Smith & Nephew Advanced Surgical Devices, Memphis, TN
1
Disclosures: All authors (3A, Smith & Nephew. 4; Smith & Nephew.)
INTRODUCTION: Additive manufacturing (AM), commonly referred to as 3D printing, is a relatively
new manufacturing method that involves the use of a laser or electron beam, for example, to sinter
polymer or metal powders into a solid part that is built layer-by-layer. This unique fabrication method
provides greater design flexibility compared to standard, subtractive manufacturing (i.e, machining).
Although standard porous coatings such as beads or fiber mesh have demonstrated clinical success [1,
2], advanced porous structures can provide enhanced friction and ingrowth that could be beneficial in
more challenging cases such as young/more active patients or the compromised bone encountered in
revision surgeries. The purpose of this study was to characterize some of the key metrics of a customdesigned, randomized porous structure produced by additive manufacturing and to compare it to
cancellous bone.
METHODS: A custom, reticulated, randomized porous structure was fabricated layer-by-layer from Ti6Al-4V powder with an EOS laser sintering system (Krailling, Germany). Various coupons (Figure 1)
were built to characterize the chemical composition, the solid tensile properties, the microstructure, and
the compressive, tensile and shear properties of the porous structure. The chemical compositions and
tensile properties of solid, sintered materials (n=3 lots) were analyzed per ISO 6892-1:2009. Coupons
with both solid and porous features were sectioned, metallographically mounted, polished and etched to
examine the microstructure. Compressive properties of fully porous cylinders (Ø10.2×11.7 mm) were
evaluated through monotonic/static testing (n=6) at a displacement rate of 25.4 mm/min. The yield
strength was determined by a 2% offset method, and the peak strength was determined by calculating
the slope between two consecutive data points and identifying the stress prior to the first negative slope.
Tensile properties of samples with solid ends and porous gage sections (Ø10×57.15 mm) were
evaluated through monotonic/static testing (n=8) at a displacement rate of 2.54 mm/min. The yield
strength was determined by a 0.2% offset method. Shear properties of samples with solid ends and
porous gage sections (Ø19.05×50.8 mm) were evaluated through monotonic/static testing (n=6) at a
displacement rate of 2.54 mm/min. The shear yield strength was determined by a 0.2% offset method.
All of the samples for mechanical testing were fabricated in the worst-case orientation and build
locations on the build plate. As a result, these strengths represent the minimums for this porous structure
in this AM system.
Figure 1: Example image of a fully porous
coupon fabricated through additive
manufacturing.
Figure 2: SEM image of the reticulated,
randomized, porous structure.
RESULTS: Both the chemical composition and the monotonic/static tensile properties of the AM solid material met the requirements for wrought Ti-6Al-4V
in ASTM F1472-14 and ISO 5832-3:2012 (Table 1). A low magnification, SEM image of the porous structure (Figure 2) illustrated the randomness and
interconnected porosity of the structure and the similar appearance to cancellous bone. Both solid and porous features were composed of the Widmanstätten
or “basket-weave” microstructure, which is typical of the standard Ti-6Al-4V alloy when heated above the β transus. The moduli for this porous structure
were similar to those reported for cancellous bone, and the strengths were significantly higher (Table 1).
DISCUSSION: This study has shown that laser sintering of Ti-6Al-4V powder is capable of producing solid components that meet the requirements for
standard, wrought Ti-6Al-4V that has been used in medical devices for decades. A custom, randomized porous structure was designed, fabricated through
powder bed laser sintering and characterized. Analysis of the porous structure demonstrated that it was an interconnected network of pores that resembled
cancellous bone in appearance. The combination of similar moduli and superior strength compared to cancellous bone suggested that medical devices
fabricated from this porous structure could be flexible enough to reduce stress shielding of the surrounding bone while withstanding the significant stresses
encountered in vivo.
SIGNIFICANCE: Advanced, reticulated porous
structures could provide enhanced osseointegration
and improve survivorship in challenging applications
such as revision surgeries with compromised bone
where worse outcomes are well documented. This
study has shown that a randomized, reticulated
porous structure can be designed and fabricated
through additive manufacturing with properties that
meet current standards for wrought materials but with
similar moduli and significantly higher strengths
compared to cancellous bone.
REFERENCES: [1] Corten, J Arthrop, 2011;26(8):
1350; [2] Tan, ORS, 2014, 0914; [3] ASTM F147208; [4] Morgan, J Biomech, 2001;34(5):569; [5] FDA
Guidance Document for Testing Orthopedic Implants
With Modified Metallic Surfaces Apposing Bone or
Bone Cement, 1994; [6] Rohl, J Biomech, 1991;24
(12):1143; [7] Keaveny, J Biomech, 1994;27(9):
1137.
Table 1: Mean (±standard deviation) results from testing of solid and porous additivemanufactured coupons. For comparison, the specified requirements and values reported in the
literature for cancellous bone are also summarized.
Output
Required
Cancellous
Property
Metrics
Minimums
Bone
Solid 0.2% Tensile Yield Strength (MPa)
1044±12
>860 [3]
-Solid Ultimate Tensile Strength (MPa)
1090±14
>930 [3]
-Solid Elongation at Break (%)
13.5±0.9
>10 [3]
-Porous Compressive Modulus (GPa)
4.3±0.1
-0.6-3.2 [4]
Porous 0.2% Compressive Yield
76.5±2.0
-3-17 [4]
Strength (MPa)
Porous 2% Compressive Yield Strength (MPa)
101.2±4.3
--Porous Compressive Peak Strength (MPa)
106.3±3.5
--Porous Tensile Modulus (GPa)
2.80±0.22
-0.6-2.7 [4]
Porous Tensile Yield Strength (MPa)
27.3±5.2
-2-11 [4]
Porous Ultimate Tensile Strength (MPa)
100.1±2.6
20 [5]
0.9-23.1 [6-7]
Porous Shear Yield Strength (MPa)
39.0±2.5
--Porous Ultimate Shear Strength (MPa)
63.3±2.5
20 [5]
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ORS 2016 Annual Meeting Poster No. 2004