Stents and Orthopedics - Metallographic Preparation of Medical Implants

Plastic or metal tubes, inserted into the lumen of anatomic ducts or vessels to keep the passageway open, are called stents. Stents have a characteristic mesh design composed of biocompatible metal or polymers. They serve as scaffolds that push against the internal walls of anatomic vessels to open a blocked area guaranteeing natural flow and preventing the vessel from collapsing. These can be carotid, coronary, biliary, peripheral, or neurovascular and are made from 316L steel, titanium, platinum chromium, cobalt chromium alloys, titanium (Ti6Al4V ELI), and nitinol alloys. In addition, stents go through final finishing treatments so that the surface is passivated via a thermal or chemical treatment to reduce corrosion potential and improve biocompatibility. In order to characterize these coatings, metallographic cross-sections must be performed to validate their thicknesses and any interfacial interactions where surface modifications or coatings have been added. [1,2]

Metallic mesh-like stent designs are created through a wire knitting/braiding and laser tube machining process with the latter technique being the major method of design. Since laser machining is a high-energy process, it can cause microstructural changes to these alloys. Metallographic inspections of the stents are an important tool in qualifying the manufacturing process. For alloys susceptible to microstructural changes, a heat treatment process is usually suggested to relieve internal stresses and enhance fatigue properties. Nitinol stents are generally self-expanding and utilize the elastic properties of the alloy; therefore, a shape-setting process is needed to fix the final shape of the stent.

Orthopedics Medical Implants

People are living longer and wish to remain active into old age. Thanks to medical implants, patients are able to lead active lives and these implants put a higher emphasis on advancements in medical implants. These include artificial hip and knee joints, bone plates and screws, which are all made from titanium, and cobalt chrome alloys are attributed to their high durability.

In addition, these components can be coated with a hydroxyapatite (HA) coating in order to optimize the surface performance characteristics of the implant and at the same time maintain the mechanical properties of the underlying substrate. Normally, the ceramic coating has a rougher surface that helps in mechanical interlock of the component with body tissue [3]. Although several alloys are used for orthopedic implants, titanium and its medical grade alloys are preferred for their biocompatibility [4].

Since the composition of HA is similar to the inorganic component of the bone, it provides a biocompatible surface. HA serves as a scaffolding onto which bone cells can attach, migrate, and grow enabling improved bone in-growth. Moreover, the coating helps to reduce osteolysis around implants [3]. The thickness of the coating and assessment of its adhesion to the titanium substrate is the main analysis for HA coating. A thermal spray coating process is used to apply these coatings.

In a nutshell, the alloys used in these medical components present metallographic preparation challenges and require careful considerations because of their high probability of structural changes and damage caused during sectioning and grinding/polishing stages. Smearing, scratches, and mechanical deformation are common issues which can be difficult to remove and can prevent accurate measurement.

Sectioning

IsoMet High Speed Precision Saw

Before sectioning, samples coated with roughened ceramic top coats, for example, hydroxyapatite must be mounted using epoxy resins such as EpoKwick Fast Cure (FC) through a castable mounting technique. Mounting before sectioning offers additional strength to the coating with its inherent porosity. Furthermore, this enables the pores to be filled for void area fraction analysis. Dyes can also be included to improve contrast during optical analysis.

By using a larger abrasive cutter, uncoated rigid components such as hip titanium prostheses can be directly sectioned and precaution should be taken on how the samples are clamped to prevent damage. An appropriate diamond blade should be used to section titanium alloys, or alternatively recommended non-ferrous abrasive blade can be used. Following sectioning, the samples can be subsequently remounted using castable mounting for ceramic coated samples and hot compression mounting for uncoated ones.

Mounting

For coated samples, several low viscosity curing epoxies such as Epothin 2 and EpoKwick FC can be used, with the former having a longer cure time but normally preferred for their low peak exotherm temperatures. Due to their ability to penetrate into coatings filling up voids and other interconnected porous morphology, ow viscosity resins are usually preferred. If a high volume of samples is needed to be prepared over a short period of time, then the preferred resin would be EpoKwick FC. In order to reduce the high abrasion rates observed with cold mounted resins, samples can be mounted with the help of phenolic ring forms or circular metallic rings to ensure that flatness is achieved. By adding ceramic powder such as FlatEdge Filler to the Epoxy mount, the abrasion rate during grinding is lowered, supporting the edges of the specimen better and resulting in flatter samples. It is also possible to achieve flatness and uniformity in grinding for cold mounted samples by operating in central force during the grinding/polishing steps.

In order to ensure better impregnation of epoxy into pores or voids in coated samples, vacuum systems such as the Cast ‘n Vac 1000 can be used to help eliminate air pockets that may otherwise form at the resin/ implant interface as well as to ensure impregnation into the coating. Hot compression mounting equipment such as the Simplimet 4000 can be used to mount uncoated samples such as bone plates, artificial hips, and fasteners. A resin that ensures good edge retention with low abrasion such as EpoMet G or F is always recommended. For uncoated samples, acrylic-based resins such as Varidur 3003 and Varidur 200 can also be adopted that provide good abrasion and polishing rates.

Grinding and Polishing

Conventional methods of sample preparation entailed long tedious steps to expose the true microstructure of a component. Thorough understanding of the principles of material removal in grinding and polishing led to the development of advanced methods. These methods take into account the initial sectioning damage and how each step mitigates the damage level and its corresponding residual structural damage. This method has been named Z-axis thresholding (Figure 1) enabling shorter preparation procedures by guaranteeing efficient method selection.

Shown in Figure 1 are the z-axis curves for preparation procedure development and optimization based on stepwise deformation removal and its associated residual damage as a function of time. Z-axis curves correlate the total deformation after sectioning that needs to be eliminated, as illustrated in Figure 1 by levels W1 and W2, as a function of time and the stepwise z-axis removal. For a sectioning wheel with resultant damage level at W1, a 4-step procedure (A to D) is illustrated in Figure 1 that can be adopted to remove all the structural damage. The level of remnant deformation can be as low as shown in level W2 when sectioning is done correctly by considering feed rates, blade thickness, correct coolant direction to reduce heat build-up, and clamping. With damage at level W2, a 2 to 3 step procedure (C to D) is more than enough to expose the true microstructure. This has been the basis of the procedures tabulated below for various material type and combinations relevant to medical components.

Metallic Components with Ceramic Coating

Alumina, zirconia, or glass ceramics among others are the ceramic coatings on these alloys. While Hydroxyapatite (HA) coating is the most commonly used one, tricalcium phosphate and other calcium phosphates can also be adopted. Since these coatings are applied through a thermal spray technique, the key metallographic parameters that should be examined are their thicknesses, porosity, and coating/substrate interface. Figure 2 (A and B) shows an acetabular cup made of titanium and coated with hydroxyapatite and titanium coating through thermal spray technique. As shown in Figure 2 (C), the cup was cold mounted using epoxy resin for easy handling and to protect the coating before sectioning; the cup was again mounted in 50 mm moulds to facilitate semi-automatic preparation of sample.

Shown in Figure 3 (A) and (B) are polished samples of the cup, clearly demonstrating the substrate matrix, the interface with entrapped SiC particles seen from grit blasting action to roughen the substrate and enhance mechanical interlock with the thermal spray coating. It is obvious from the microstructures in Figure 3 that the HA coating is well adhered to the top layer and at times exhibits needle-like convoluted morphology around the fused titanium bond coat.

Table 1. Preparation procedure for Titanium/HA coatings on medical grade Titanium alloy Ti6Al4V

Surface Abrasive Lubricant/
Extender
Force
(Per Specimen)
Time (min:sec) Platen Speed Head Speed (rpm) Relative Rotation
1 CarbiMet P320 Water 30N Till planar 300 60 >>
2 Ultrapad 9 μm Metadi Supreme MetaDi Fluid 30N 07:00 150 60 ><
3 ChemoMet** 0.06 μm MasterMet 25N 05:00 150 60 ><

** last 15-20 second use water only >>Complimentary ><Contra

Note:

- For step 1, change the surfaces after 40-50 seconds of grinding
- For cold mounted samples, use phenolic ring or a metallic ring to aid grinding flat or use of central force mode

(A) and (B) illustrates an acetabular cup made of Titanium (substrate) and coated with HA and Titanium powder through flame spray technique. (C) shows encapsulated sample in epoxy for section and (D) illustrates remounted sectioned sample ready for grinding/polishing steps.

Figure 2. (A) and (B) illustrates an acetabular cup made of Titanium (substrate) and coated with HA and Titanium powder through flame spray technique. (C) shows encapsulated sample in epoxy for section and (D) illustrates remounted sectioned sample ready for grinding/polishing steps.

Showing (A) as polished surface bright field image showing the Ti6Al4V substrate and thermal sprayed titanium and hydroxyapatite (HA) top coat, (B) as-polished viewed with polarized light to reveal the alloy and sprayed titanium microstructures.

Figure 3. Showing (A) as polished surface bright field image showing the Ti6Al4V substrate and thermal sprayed titanium and hydroxyapatite (HA) top coat, (B) as-polished viewed with polarized light to reveal the alloy and sprayed titanium microstructures.

Table 2. Generic method for ceramic coatings on stainless steel substrates

Surface Abrasive Lubricant/
Extender
Force
(Per Specimen)
Time
(min:sec)
Platen Speed Head Speed
(rpm)
Relative Rotation
1 CarbiMet P320 Water 27N Till planar 300 60 >>
2 Ultrapad 9 μm Metadi Supreme MetaDi Fluid 27N 07:00 150 60 ><
3 Trident 3 μm Metadi Supreme MetaDi Fluid 27N 3:00 150 60 >>
4 ChemoMet** 0.06 μm MasterMet 27N 05:00 150 60 ><

** last 15-20 second use water only >>Complimentary ><Contra

The following procedure, Table 2, can be implemented in the case of coated or uncoated stainless steel substrates. This procedure includes a 3-μm diamond stage to help with damage removal when compared to the procedure given in Table 1.

Metallic Components

Titanium and its Alloys

It is difficult to prepare these alloys because of the development of a deformed surface layer. Sectioning damage is a general problem attributed to the alloys sensitivity to temperature and cold working. This deformation can lead to grain twinning and strain-induced transformation structures, while high temperature can result in phase distribution changes. With titanium, polishing with finer diamond can cause high deformation that cannot be removed easily. However, this remnant deformation is effectively removed through a chemo-mechanical polishing using oxide polishing suspension, for example MasterMet. Table 3 shows a typical procedure for titanium and its alloys. During the final polishing steps, attack polishes can also be implemented to remove the remnant deformation, which includes a suspension containing 1 part of 35% hydrogen peroxide combined with 5 parts of Mastermet, a colloidal silica suspension of 0.06 µm size. Shown in Figure 4 (A) is a polished titanium Ti6Al4V alloy substrate observed with polarized light exposing the microstructure and differential interference contrast microscopy (B) of the same region for topographic detail.

Table 3 is a generic procedure for Titanium and its alloys

Surface Abrasive Lubricant/
Extender
Force
(Per Specimen)
Time
(min:sec)
Platen Speed Head Speed
(rpm)
Relative Rotation
1 CarbiMet P320/P400 Water 27N Till planar 300 60 >>
2 Ultrapad 9 μm Metadi Supreme MetaDi Fluid 27N 10:00 150 60 ><
3 ChemoMet** 0.06 μm MasterMet 27N 10:00 150 60 ><

** last 15-20 second use water only >>Complimentary ><Contra

Nitinol is a nickel titanium alloy used in endovascular stents. The super elasticity nature of nitinol endears it to stent application because of its microstructure transformation and ability to retain some strain after deployment. Before implantation, the structure observed is austenitic in nature but this structure changes to martensitic when compressed and fitted on a catheter. Once the stent is fitted in the body, it expands and creates a reverse martensite to austenite transformation but since it is constrained within arterial walls, complete strain recovery doesn’t occur —a benefit often known as biased stiffness [5, 6]. An understanding of microstructural changes indeed helps in validating the observed microstructures after metallographic preparation. Table 4 represents a typical procedure, and Figure 5 shows the resultant microstructures.

Table 4 step procedure for Nitinol alloy

Surface Abrasive Lubricant/
Extender
Force
(Per Specimen)
Time
(min:sec)
Platen Speed Head Speed
(rpm)
Relative Rotation
1 CarbiMet P320 Water 27N Till planar 300 60 >>
2 Ultrapad 9 μm Metadi Supreme MetaDi Fluid 27N 05:00 150 60 ><
3 Trident 3 μm Metadi Supreme MetaDi Fluid 27N 05:00 150 60 >>
4 ChemoMet** 0.06 μm MasterMet 27N 05:00 150 60 ><

** last 15-20 second use water only >>Complimentary ><Contra

Showing (A) as polished Ti64Al surface with 5 parts Mastermet and 1 part hydrogen peroxide solution using polarized light, (B) shows corresponding differential interference contrast image (DIC) revealing topographic details of the polished surface.

Figure 4. Showing (A) as polished Ti64Al surface with 5 parts Mastermet and 1 part hydrogen peroxide solution using polarized light, (B) shows corresponding differential interference contrast image (DIC) revealing topographic details of the polished surface.

Showing (A) as polished Nitinol surface with 5 parts Mastermet and 1 part hydrogen peroxide (B) smaller sized sample polished and etched with 60 ml HNO3, 30 ml acetic acid and 20 ml HCl for 20-40 seconds to reveal martensitic microstructure and (C) surface etched with an etchant composed of HCl, Na2S2O5, K2S2O5 and NH4F viewed with polarized light to reveal a fine grain structure, (D) high magnification to illustrate individual grains and the characteristic martensitic microstructure.

Figure 5. Showing (A) as polished Nitinol surface with 5 parts Mastermet and 1 part hydrogen peroxide (B) smaller sized sample polished and etched with 60 ml HNO3, 30 ml acetic acid and 20 ml HCl for 20-40 seconds to reveal martensitic microstructure and (C) surface etched with an etchant composed of HCl, Na2S2O5, K2S2O5 and NH4F viewed with polarized light to reveal a fine grain structure, (D) high magnification to illustrate individual grains and the characteristic martensitic microstructure.

Stainless Steels

Traditionally, stainless steels, such as AISI 316L, have been used in orthopedics because they are low cost, offer good mechanical properties, and enable easy processing. Stainless steel alloys can cause other issues such as incompatibility with surrounding bone tissue because of their higher density and possibility to corrode the alloy in a body fluid environment over time. This is a major concern because the resultant effect of corrosion by-products can pose a severe health risk on human tissue. For example, nickel ions are known to act as allergens causing inflammations and potentially leading to carcinogenicity in the body, hence the development of low nickel or nickel-free steels. [7] However, stainless steels are still commonly used for surgical instruments and implants and can be found in fracture fixation plates and screws, stents, spinal implant devices, aneurysm clips etc. The prolonged use has been ascribed to surface modification to considerably improve surface passivity, and through an electropolishing process, to enhance surface Cr concentration for better passivity.

In this work, AISI 316LVM alloys were metallographically prepared with reference to Table 5 and corresponding microstructures shown in Figure 6 for a laser machined sheet used for making a stent. The goal was to investigate the machined channels and any microstructural changes caused by laser beam exposure.

A) show a sheet metal of AISI 316LVM steel as polished, (B) shows a rolled sheet and laser machined, (C) illustrates a high magnification image of a machined region to ascertain presence of heat affected zone and (D) shows corresponding austenitic microstructures for grain size measurement electroetched using 10% oxalic acid.

Figure 6. A) show a sheet metal of AISI 316LVM steel as polished, (B) shows a rolled sheet and laser machined, (C) illustrates a high magnification image of a machined region to ascertain presence of heat affected zone and (D) shows corresponding austenitic microstructures for grain size measurement electroetched using 10% oxalic acid.

Table 5 is a typical procedure for Austenitic stainless steels

Surface Abrasive Lubricant/
Extender
Force
(Per Specimen)
Time
(min:sec)
Platen Speed Head Speed
(rpm)
Relative Rotation
1 CarbiMet P220 to P320 Water 27N Till planar 300 60 >>
2 Ultrapad 9 μm Metadi Supreme MetaDi Fluid 27N 05:00 150 60 ><
3 Trident 3 μm Metadi Supreme MetaDi Fluid 27N 03:00 150 60 >>
4 Trident 1 μm Metadi Supreme Metadi Fluid 27N 03:00 150 60 >>
5 ChemoMet** 0.06 μm MasterMet 27N 02:30 150 60 ><

** last 15-20 second use water only >>Complimentary ><Contra

Cobalt Based Alloys

Cobalt alloys are structural tough materials with good biocompatibility and excellent corrosion and wear resistance. Compared to iron or titanium-based medical alloys, cobalt alloys are costly to manufacture through traditional machining processes and hence the latter two are preferred. With the development of additive manufacturing processes, Co-Cr alloys are increasingly being employed for medical implants attributed to improvement in near net shape manufacturing capability, requiring little to no machining. The Co-Cr alloys are best suited for implants that are designed to substitute zone and to be the chief load bearing part over time. These alloys are commonly used in knee condyles and artificial hips but also used in acetabular cups and as tibial trays. As shown in Figure 7, the key parameter of additive manufactured components is analysis of porosity levels within the bulk or near the surface layers. Porosity can happen owing to lack of fusion of powder feedstock during the build up of component layer.

Table 6 is a typical procedure for Cobalt alloys

Surface Abrasive Lubricant/
Extender
Force
(Per Specimen)
Time
(min:sec)
Platen Speed Head Speed
(rpm)
Relative Rotation
1 CarbiMet P220 to P320 Water 27N Till planar 300 60 >>
2 Ultrapad or TexMet P 9 μm Metadi Supreme MetaDi Fluid 27N 05:00 150 60 ><
3 TexMet C 3 μm Metadi Supreme MetaDi Fluid 27N 05:00 150 60 >>
4 TexMet C 1 μm Metadi Supreme Metadi Fluid 27N 03:00 150 60 >>
5 ChemoMet** 0.06 μm MasterMet 27N 03:30 150 60 ><

** last 15-20 second use water only >>Complimentary ><Contra

Showing (A) as polished surface of an additive manufactured Co-Cr acetabular cup viewed using differential interference contrast (DIC) through 10x objective, (B) showing porosity within the bulk alloy viewed using DIC through a 20X objective.

Figure 7. Showing (A) as polished surface of an additive manufactured Co-Cr acetabular cup viewed using differential interference contrast (DIC) through 10x objective, (B) showing porosity within the bulk alloy viewed using DIC through a 20X objective.

Summary

In the article, different coatings and alloys and used in medical implants were highlighted. Medical implants are increasingly being used because of their super elasticity, good mechanical strength, corrosion and wear resistant properties, and most significantly biocompatibility. For metallographic analysis of these components, a number of preparation procedures have been highlighted for stainless steel, cobalt alloys, and titanium and its alloys. Moreover, the article looked at hydroxyapatite (HA) coatings on titanium substrates and presented a common procedure for ceramic coatings on austenitic stainless steelsAISI316LVM. The article has also emphasized cobalt-based alloy preparation that is increasingly being used on load bearing medical components.

Reference

  1. Moravej, M., & Mantovani, D. (2011). Biodegradable Metals for Cardiovascular Stent Application: Interests and New Opportunities. International Journal of Molecular Sciences, 12(7)
  2. Brunette D.M., Tengvall P., Textor M., Thomsen P. (2013) Titanium in Medicine: Materials Science, Surface Science, Engineering, Biological Responses and Medical Applications. Springer.
  3. Coathup MJ, Blackburn J, Goodship AE, Cunningham JL, Smith T, Blunn GW. (2005) Role of hydroxyapatite coating in resisting wear particle migration and osteolysis around acetabular components. Biomaterials 26(19).
  4. Alexy, R. D., & Levi, D. S. (2013). Materials and Manufacturing Technologies Available for Production of a Pediatric Bioabsorbable Stent. BioMed Research International, 2013, 137985
  5. Duerig, T. W., Tolomeo, D. E., & Wholey, M. (2000). An overview of superelastic stent design. Minimally invasive therapy & allied technologies, 9(3-4), 235-246.
  6. Stoeckel, D. (2000). Nitinol medical devices and implants. Minimally invasive therapy & allied technologies, 9(2), 81-88.
  7. Yang, K., & Ren, Y. (2010). Nickel-free austenitic stainless steels for medical applications. Science and technology of advanced materials, 11(1), 014105.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

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