Metallographic Preparation of Orthopedic Medical Devices

D. J. Medlin Zimmer Incorporated, Warsaw, Indiana, USA

G. M. Lucas and G. F. Vander Voort Buehler Ltd, Lake Bluff, Illinois, USA

Abstract

Metallographic sample preparation methods for porous coated

implant devices can be difficult due to inadequate fill of the

mounting materials into the porous metallic structures.

Inadequate fill of the mounting material during sample

preparation leads to problems such as edge rounding, uneven

etching, and metal smearing during polishing. These

problems make proper microstructural identification and

analysis difficult and/or inaccuate.

Two porous coated implant components were

metallographically prepared by five different sample

preparation methods to determine which method would

develop the best metallographic specimens. Edge retention

was best when the specimens were electroless nickel-plated

and mounted in Epomet®-F thermosetting resin. This

mounting material had the best fill in the porous coating areas

and resulted in superior microstructural clarity. Three other

preparation methods, including vacuum impregating with two

epoxy resins and mounting in phenolic resin, resulted in

adequate metallographic images. The Sample-Kwick® cast

acrylic resin resulted in more edge rounding and uneven

etching than the other mounting materials when evaulating

porous coated metallographic specimens.

Introduction

Metals have a diverse application in the medical field as

implantable, internal (in-vivo), structural, load-bearing

replacement components and surgical instruments. A few

examples of metallic components include hip and knee

replacements, fracture fixation plates, screws, cables, surgical

blades and tools, etc. The field of metallography plays a

significant role in the quality control of metals used to

manufacture medical implants. Metallographic techniques are

used to examine raw materials (metals) prior to fabrication of

the devices and systematic examinations during and after

specific processing steps to insure the final product will be

safe and effective when used in patients.

Some implant designs have porous metallic coatings on the

surface to improve the adhesion at the bone/metal interface by

bone in-growth (or on-growth) of bone tissues into the

metallic coatings. Traditional metallographic techniques can

be insufficient in properly preparing porous metallographic

specimens and revealing microstructures due to problems such

as: edge rounding, incomplete fill of mounting material,

porous metal smearing, bimetallic polishing and etching

problems, color metallographic etching issues, etc.

The purpose of this investigation is to find more efficient and

thorough methods to prepare porous coated metallic

specimens by resolving typical porous metal preparation

issues and allowing improved and more complete

examinations of the microstructures.

Specimens Evaluated

Two porous coated metallic samples were evaluated in this

evaluation. Sample 1 was an acetabular cup made from Ti-

6Al-4V-ELI alloy (ASTM F-136) with a commercially pure

(CP) titanium (ASTM F-67) fiber metal wire coating (mesh)

on the surface1. Sample 2 was a femoral hip stem made from

a Co-Cr-Mo alloy forging (ASTM F-799) with Co-Cr-Mo

beads (ASTM F-75) sintered to the surface1.

Specimen Mounting Procedure

The scope of this study was to find an improved method of

metallogrphically preparing porous coated specimens by

obtaining more complete impregnation and fill of the

metallographic mounting materials into the voids of porous

metal coatings. Five different combinations of mounting

compounds, specimen coating materials, and mechanical

impregnation procedures were evaluated in an effort to reduce

edge rounding and incomplete mounting material fill. Table 1

list the five different mounting procedure combinations.

Table 1: The five different mounting procedures used in this

analysis.

Test

Number

Mounting Combination

1

Phenocure™ thermosetting phenolic resin

2

Electroless Ni-plating and Epomet®-F

thermosetting resin

3

Vacuum impregnation with low-viscosity Epo-

Thin® epoxy resin

4

Vacuum impregnation with Epo-Heat™

epoxy resin

5

Sample-Kwick® cast acrylic resin

Titanium Alloy Preparation Procedure

Sample 1 was a Ti-6Al-4V acetabular cup with a

commercially pure (CP) titanium wire mesh diffusion bonded

to the surface. After mounting the specimens, refer to Table 1,

the specimens were ground with a 320-grit abrasive silicon-

carbide Carbimet® paper. A force of 18 N (4 lbs) at 250 rpm

was used with the specimen holder and platen rotating in

opposite directions (contra rotation). This process was water

cooled and was ground until the specimen was planar. The

polishing process initiated with a 9-μm Metadi Supreme®

polycrystalline diamond suspension on a Ultra-Pol™ silk

cloth. A force of 18 N (4 lbs) at 200 rpm was used with a

contra rotation between the specimen holder and the platen for

approximately 4 minutes. Next, the specimens were polished

with 3-μm Metadi Supreme® polycrystalline diamond

suspension on a Texmet® 1000 pad with 18N (4 lbs) of force

at 200 rpm. Contra rotation was used for approximately 4

minutes. The final polishing procedure was Mastermet-2®

slurry on a Chemomet® pad with 31N (7 lbs) at 150 rpm. The

Mastermet-2® slurry is a 0.02-μm colloidal silica attack polish

made by mixing 1 part H2O2 (30% concentration) to 6 parts

Mastermet-2. The specimen holder and the platen were

rotated in the same direction (comp rotation) for 7 about

minutes.

Cobalt Alloy Preparation Procedure

Sample 2 was a Co/Cr/Mo femoral hip stem with Co/Cr/Mo

beads sintered to the surface. After mounting the specimens,

see Table 1, the specimens were ground planar with a 125-μm

diamond Apex™ DGD disk with 18N (4 lbs) of force at 250

rpm. The specimens polished with contra rotation between the

specimen holder and the platen and ground until the specimen

was planar. The specimens were then ground with 320-grit

silicon-carbide Carbimet paper with 18N (4 lbs) of force at

250 rpm and contra rotation for approximately 2 minutes.

Polishing was done with 9-μm Metadi Supreme®

polycrystalline diamond suspension on a Ultra-Pol™ silk

cloth. A force of 18 N (4 lbs) at 200 rpm was used with a

contra rotation between the specimen holder and the platen for

approximately 4 minutes. Next, the specimens were polished

with 3-μm Metadi Supreme® polycrystalline diamond

suspension on a Texmet® 1000 pad with 18N (4 lbs) of force

at 200 rpm. Contra rotation was used for approximately 3

minutes. The next polishing procedure was a two part 4

minute cycle. First, a Mastermet-2® slurry on a Chemomet®

pad with 36N (8 lbs) at 150 rpm was used and then at mid-

cycle (after about 2 minutes) a Mastermet alumina suspension

was used. The specimen holder and the platen were rotated in

the same direction (comp rotation). The final procedure was a

1 hour vibratory polish using Masterprep™ alumina

suspension on a Microcloth® pad.

Etching Procedure

The titanium alloys were etched with Kroll’s Reagent and

modified Weck’s Reagent, as shown in Table-2. The Weck’s

Reagent was used for color metallographic imaging. The Co-

Cr-Mo alloys were etched with an HCl and H2O2 (3%

concentration) mixed in a 5 to 1 ratio, also shown in Table 2.

Table 2. The etchants used for the titanium and Co/Cr/Mo

alloys. Weck’s Reagent is a color etchant2-4

.

Etchant

Name

Procedure Composition

Kroll’s

Reagent

(titanium)

Immerse 5-

30 seconds

10 ml HF

5 ml HNO3

85 ml H2O

Weck’s

Reagent

(titanium)

Immerse

for 15-30

seconds

5 g ammonium bifluoride

4 ml HCl

100 ml H2O

Colbalt

Etchant

Immerse 2-

4 minutes

100 ml HCl

20 ml H2O2 (3% conc.)

Metallographic Results – Acetabular Cup

Sample 1 (Ti-6Al-4V acetabular cup with CP titanium wire

mesh) was initially mounted in a Phenocure™ thermosetting

phenolic resin and polished with the titanium alloy

preparation method. Figure 1a shows the microstructure

etched with Kroll’s Reagent and Figure 1b shows the color

tinted microstructure etched with Weck’s Reagent. These

acetabualr cup components were diffusion bonded to

metallurgically attach the CP-titanium wire mesh to the Ti-

6Al-4V substrate. The metallurgical bond between the CP-

titanium wires and between the wires and the Ti-6Al-4V

substrates can be seen. Minimal edge rounding and excess

edge etching is apparent in these images.

Figure 1:Titanium fiber metal diffusion bonded to Ti-6Al-4V

substrate and mounted in phenolic mounting material. Figure

1a (top) is etched with Kroll’s Reagent and Figure 1b (above)

is color etched with Weck’s Reagent.

Additional specimens from Sample 1 were electroless nickel-

plated and then mounted in an Epomet®-F thermosetting

resin. Coating the specimen with a layer of nickel helps

maintain the integrity of the specimen edges during polishing

and keeps the entire surface of the metal within the same focal

plane when examining the specimen in a metallograph. Figure

2a shows the microstructure etched with Kroll’s Reagent and

Figure 2b shows the microstructure colored etched with

Weck’s Reagent. Both micrographs reveal the nickel-plating

layer around the surface of the wires and substrate and show

very little evidence of edge rounding or uneven etching at the

interface between the metal and mounting material. Figure 3

shows the microstructure at a higher magnification and the

nickel-plating layer completely coats the exposed metal

surfaces. The metallic bond between the round wires and the

substrtate are very clear at this magnification.

Figure 2:Titanium fiber metal diffusion bonded to Ti-6Al-4V

substrate and electroless nickel plated and then mounted in

and Epomet®-F thermosetting resin.. Figure 2a (top) is etched

with Kroll’s Reagent and Figure 2b (above) is color etched

with Weck’s Reagent.

Another set of specimens from sample 1 were vacuum

impregnated with low-viscosity Epo-Thin® epoxy resin.

After etching, this mounting method and mounting material

exhibited some evidence of edge rounding and uneven etching

at the interface between the Epo-Thin epoxy and the metal.

The uneven etching is due to the retention of acids during

etching usually due to interface cracks forming between the

mounting material and the metal. The entrapped acid slowly

leaks out of the interface crack and etches the immediate area

more than the rest of the metal surface. Figure 4a shows the

microstructure etched with Kroll’s Reagent and Figure 4b

shows the microstructure after color etching with Weck’s

Reagent. The arrows indiates the areas with edge rounding

and uneven etching. Figure 5 shows the uneven etching at the

interface between the metal and the mounting material.

200 µm a

200 µm 200 µm

a

200 µm

200 µm b 200 µm 200 µm b

Figure 3:Titanium fiber metal diffusion bonded to Ti-6Al-4V

substrate and electroless nickel plated and then mounted in

and Epomet®-F thermosetting resin. This specimen was

etched with Kroll’s Reagent.

The fourth mounting material used on specimens from Sample

1 were prepared by vacuum impregnating with Epo-Heat™

epoxy resin. This mounting material showed similar edge

retention and uneven etching when compared to the Epo-Thin

resin. Figures 6a, 6b and 7 show the microstructures.

The last mounting material evaluated on specimens from

Sample 1 was the Sample-Kwick® cast acrylic resin. Figures

8a and 8b show substantial amounts of edge rounding and

uneven etching at the interfaces between the mounting

material and the metal. These edge retention problems would

make interface microstructural analysis more difficult when

compared with the other mounting materials.

Metallography Results – Femoral Hip Stem

The same five specimen preparation methods shown in Table

1 were used for Sample 2, the bead coated femoral hip stem

made from a Co/Cr/Mo alloy. In general, metallographically

preparing and etching the Co/Cr/Mo alloy is more difficult

than the titanium based alloys. Etching of the Co/Cr/Mo

alloys must be performed within a few minutes of final

polishing to obtain optimum results. Waiting several hours

between the final polishing procedure and etching may make

proper etching difficult.

The results from Sample 2 were similar to the results from

Sample 1. Electroless nickel-plating and mounting with

Epomet® thermosetting resin produced the best

metallographic images with minimal amounts of edge

rounding and very uniform etching. The Phenocure™

thermosetting phenolic resin and the two vacuum impregnated

epoxy resin preparation methods revealed adequate results

with minimal edge retention issues. Figure 9 shows the bead

coated layer mounted with the phenolic resin and Figure 10

Figure 4:Titanium fiber metal diffusion bonded to Ti-6Al-4V

substrate vacuum impregnated with low-viscosity Epo-Thin®

epoxy resin. Figure 4a (top) is etched with Kroll’s Reagent

and Figure 4b (above) is etched with Weck’s Reagent.

shows the excellent results using the nickel-plated Epomet®-F

resin. Comparing these two micrographs with the results in

Figure 11, it is apparent that the Sample-Kwick acrylic resin

does not retain the sample edges like the previous two

preparation methods. The Co/Cr/Mo beads in Figure 11 are

not as clear and defined as they are in Figures 9 and 10 and

this could result in misleading or inaccurate metallographic

interpretation and analysis.

The two vacuum impregnation sample preparation methods

resulted in satisfactory results with minimal edge retention

problems.

.

50 µm 200 µm

a

200 µm b 200 µm

Figure 5:Titanium fiber metal cup sample vacuum

impregnated with low-viscosity Epo-Thin® epoxy resin.

.

Figure 6:Titanium fiber metal cup sample vacuum

impregnating with Epo-Heat™ epoxy resin. Etched with

Kroll’s Reagent (top) and Weck’s Reagent (above).

Figure 7:Titanium fiber metal cup sample vacuum

impregnating with Epo-Heat™ epoxy resin and etched with

Kroll’s Reagent.

Figure 8:Titanium fiber metal cup sample was mounted in

Sample-Kwick® cast acrylic resin and etched in Kroll’s

Reagent. Figure 8a (top) and Figure 8b (above).

50 µm

200 µm a

200 µm

200 µm b

50 µm

200 µm

a 200 µm

b 50 µm

Figure 9. Bead coated Co/Cr/Mo hip stem in Phenolic

mounting material etched in HCl-H2O2.

Figure 10. Bead coated Co/Cr/Mo hip stem electroless nickel-

plated, mounted in Epomet®-F thermosetting resin, and

etched in HCl-H2O2.

Summary

Metallographic specimen preparation procedures have been

developed to adequately prepare porous implant devices with

minimal problems such as edge rounding, uneven etching,

incomplete fill of the mounting material, metal smearing, and

color etching problems. Edge retention and uniform etching is

best with the electroless nickel-plating and Epomet®-F

thermosetting resin preparation procedure. Adequate results

were obtained with the Phenocure™ thermosetting phenolic

resin and the two vacuum impregnated epoxy resins. The

poorest edge retention resulted from the Sample-Kwick®

acrylic resin for this type of application.

Figure 11. Bead coated Co/Cr/Mo hip stem mounted in

Sample-Kwick acrylic resin and etched in HCl-H2O2.

References

1. American Society for Testing Materials, Annual Book of

Standards, Medical Devices and Services, Volume 13.01,

2004.

2. G. Vander Voort, Metallography: Principles and Practice,

ASM International, 1984.

3. D.J. Medlin and R. Compton, Metallography of

Biomedical Orthopedic Alloys, ASM Handbook, Volume

9, Metallography and Microstructures, 10th

Edition, 2004.

4. L.E. Samuels, Metallographic Polishing by Mechanical

Methods, Third Edition, ASM International, 1985.

200 µm

200 µm

200 µm