Bernice Aboud
Manager, Metallurgy Research
DePuy Orthopaedics Inc.,
a Johnson & Johnson Co.
Warsaw, Ind.

A CoCrMo Porocoat porous coated AML hip stem sports a CoCrMo femoral head, articulating against a Marathon cross-linked polyethylene liner in a Pinnacle acetabular porous coated cup.

A CoCrMo Porocoat porous coated AML hip stem sports a CoCrMo femoral head, articulating against a Marathon cross-linked polyethylene liner in a Pinnacle acetabular porous coated cup.


Microstructural comparison of high carbon standard ingot bar to the consolidated powder CCM Plus

Microstructural comparison of high carbon standard ingot bar to the consolidated powder CCM Plus.


Top micrograph is CCM Plus bar stock as-received. The bottom micrograph is CCM Plus that has been forged into a hip stem.


CAST SINTERED (100X)
CAST SINTERED (100X)
CCM PLUS FORGED & SINTERED (100X)

CCM PLUS FORGED & SINTERED (100X)

Micrographs show the dramatic difference in grain size between a casting which has been sintered and a forging which was sintered.

A CoCrMo Porocoat porous coated AML hip stem with a femoral head, articulating against the CoCrMo Ultamet metal-on-metal liner in a Pinnacle acetabular porous coated cup.


Over 300,000 total hip replacements (THR) take place in the U.S. every year. Unfortunately, the revision rate of total hip surgery is also increasing with the growing number of younger, more active patients. The demands placed on orthopedic devices by today's aging population require high-performance biomedical materials with better corrosion resistance, higher fatigue strength, and superior wear performance. The challenge is being met through partnerships between orthopedic manufacturers, researchers, and material suppliers. Their combined efforts have contributed greatly to long-term clinical performance of surgical implants.

Through retrieval evaluations and in-vitro studies of THR systems, useful information can be gleaned about the in-vivo performance of a component. The industry has learned that femoral heads, for example, must have ultralow surface roughness on the polished surface, near perfect sphericity, and scratch resistance to third-body particles. These properties will minimize the generation of polyethylene wear debris.

Polyethylene wear in an articulating joint is a major factor influencing fixation and durability of implants. The release of polyethylene wear debris into the joint space often leads to osteolysis then aseptic loosening, the main cause for long-term clinical failure of artificial joints. The wear mechanism may be responsible for about 9% of all total joint-replacement revisions each year.

The materials used to fabricate femoral heads can heavily affect the performance of modular components. It is consequently of great interest to improve, not only the wear performance of polyethylene, which has been the subject of intensive research over the past two decades in the orthopedic industry, but to also enhance the performance of the articulating metal components.

For prosthetic femoral heads, hips, knees, elbows, shoulders, and ankles a commonly used alloy is Co-28Cr-6Mo. This cobalt-based alloy meets ASTM Standard Specification in the F:04 Series for Medical Devices: ASTM F-75 for cast surgical implants, ASTM F-799 for forged components, and ASTM F-1537 for wrought material.

Although the carbon (C) content for these specifications can range to a maximum of 0.35% C, castings typically have a minimum 0.20% C, mainly to impart strength and hardness. Forgings are usually processed with forging stock containing a lower carbon content, around 0.05% and wrought alloys can range from 0.05 to 0.20% C.

Standard processing of low (0.05%) C wrought alloys are commonly cast as ingots and thermomechanically processed to boost mechanical properties to meet the demands of medical devices. Higher-carbon wrought alloys can also be processed using standard cast ingot practices. But these alloys can prove challenging to process because carbide segregation present in the as-cast ingot makes it difficult to hot work. These ingots can also suffer from other microstructural defects and porosity.

The desire for a more homogenous material with superior properties led to the development of a high-carbon wrought derivative using a proprietary powder-metal (P/M) process. The Micro-Melt process from Carpenter Technology Inc., Reading, Pa., uses a consolidated P/M billet (BioDur CCM Plus) with tightly controlled homogenous chemistry. Combinations of nitrogen and carbon improve CoCrMo alloy ductility, as well as yield and tensile strengths. Improvements in these properties, in turn, boost fatigue resistance, a property critical to the life cycle of implanted medical devices.

Micro-melt technology is based on gas atomization of the alloy, forming a prealloyed powder. The inert gas atomization yields a fine powder particle size (<150 m), with a homogenous composition as a result of the extremely rapid quench rate attained during the gasatomization process. The powder is then blended and put into a metal canister.

The powder-alloy charge is next hotisostatically pressed (HIPed) at a temperature and pressure selected to produce a fully dense compact. After consolidation, the compact can be thermomechanically processed to provide a desired production form. Properties of the HIPed alloy exceed the minimum requirements of ASTM F-1537 with improved strength, ductility, and toughness.

The high-carbon CCM Plus consolidated alloy features a homogeneous chemistry with a fine microstructure and uniformly distributed carbides of <10 m. The fine grain structure ( typically ASTM E-112 Grain Size of 12 and finer for hot rolled material, and GS 10 to 11 for material annealed at 2,050 to 2,100°F) and superior physical and mechanical properties lead to excellent clinical performance, high strength, and corrosion resistance when mating with polyethylene or when mating with itself in metal-to-metal designs. Recent tests have also shown that the high carbon in the alloy and good fatigue properties in consolidated CCM Plus forged hip stems offer an excellent alternative to traditional cast porous-coated hip stems. The material may also allow for the design of small, less-intrusive hip stems.

MICROSTRUCTURAL COMPARISON
A recent study of femoral heads is helpful in comparing bar stock made from CCM Plus consolidated powder and another high-carbon alloy (HC Ingot) processed using conventional ingot-metallurgy processes. Several lots of CCM Plus were metallurgically evaluated and compared to bars of the HC Ingot material. The carbon content of CCM Plus ranged from 0.24 to 0.25%. The carbon content for the HC Ingot materials ranged from 0.22 and 0.24%. Both materials were in the as-rolled condition and met the warm-worked properties of ASTM F-1537.

Transverse and longitudinal sections were taken, metallurgically prepared, and electrolytically etched using HCL to expose the carbide phases. The HC Ingot bar micrographs revealed fine chromium carbides preferentially segregated in the rolling direction of the bar. HC Ingot had an ASTM Grain Size of 10 with a hardness range from 43 to 46 HRc (Rockwell C-scale). The CCM Plus had a more homogenous distribution of carbides both transversely and longitudinally. The carbide particles were less than 10 m with an ASTM Grain Size of 12. Hardness ranged from 45 to 46 HRc.

Homogeneity and hardness uniformity throughout the bar, both longitudinally and transversely, are both important features, not only for ease of manufacturing, but also from a performance perspective. Machining difficulties arise when bars contain segregated areas of higher, nonuniform hardness resulting from microsegregation of carbides. Cutting tools can chip during turning if the lathe tools encounter a mass concentration of carbides. Femoral heads are 100% inspected using fluorescent penetrant inspection and will be rejected if surface defects are detected. During this evaluation, the frequency of rejects for the sample CCM Plus materials was zero.

SCRATCH RESISTANCE
Scratch tests are often performed as a bench test to determine the relative scratch resistance of a material. Harder and more homogenous microstructures have demonstrated an improvement in scratch resistance, maintaining a smooth polished articular surface necessary to minimize wear of the polyethylene counterface bearing

Scratch tests were performed on femoral heads manufactured from two different alloys: CCM Plus (0.25% C) and a conventionally processed low, 0.05%-C alloy (referred to as LC Ingot). Femoral heads for each alloy were cross sectioned and metallurgically prepared. The polished surface was scratched using a MST-CSEMEX microscratch tester (Neuchatel, Switzerland) with a 50- m radius diamond indenter and a constant force of 1.5 N.

There are several quantitative measurements than can be obtained from a scratch, however, built-up metallic material from a scratch, Rp, is likely the most detrimental factor affecting the polyethylene acetabular liner wear performance of an implant. Rp is a measurement of the highest profile peak to the centerline average of a scratch. The lower the Rp value, the less likely the material will abrade and wear against an articulating component.

Evaluations of scratches made along the core and perimeter of three femoral heads show that the CCM Plus averaged an Rp of 1.64 ± 0.05. The Rp data for the LC Ingot material was significantly higher in comparison, averaging Rp 1.80 ±0.07. There was no statistical difference between Rp values along the core of the CCM Plus heads to the outer perimeter, which speaks to the homogeneity of the material. The lower scratch peak of the CCM Plus heads was likely attributed to the higher hardness of the alloy. Macrohardness on the LC Ingot averaged 41 to 43 HRc and the CCM Plus was harder measuring 45 to 46 HRc.

The higher carbon composition of Co-28Cr-6Mo contributes to the hardness of the alloy that theoretically will resist scratches while in-vivo. This will, in turn, reduce the potential generation of polyethylene wear debris as the femoral head articulates against a polyethylene acetabular liner.

HIP STEMS
Cobalt-based investment castings have enjoyed a long successful clinical history in many orthopedic applications, such as hips, femoral-knee components, and shoulders. The carbon content in castings typically stays within a range of about 0.25% C. This makes them suitable for porous-coating applications whereby small beads are sintered onto the substrate creating a 3D network that promotes bone ingrowth.

The carbides and interstitial carbon in the casting help pin down the grain boundaries, thus helping to reduce grain growth, but there is still grain growth during the high-temperature sintering operations. Although properties can partially recover after HIPing and homogenization, there is continued interest to improve the fatigue performance of porous-coated applications, specifically for hip stems.

Forgings from CoCrMo alloys can offer excellent strength in orthopedic implants over castings; however forgings have been historically used for cemented applications only. The forgeability of these materials is greatly affected by carbon content, which is typically held under 0.07%. This makes forged CoCrMo products unsuitable for porous coated applications.

Researchers faced with the challenges of providing stronger and smaller porous-coated hip implants investigated the use of CCM Plus for forgings. The high carbon in the alloy allows for porous-coated applications. Some difficulties were encountered in forging the higher carbon alloy as expected. But the effort successfully produced hip stems to near-net shape through various changes to forging parameters.

The forging parameters of the forged CCM Plus hip stems are controlled in order to limit recrystallization and grain growth. The forged CCM Plus microstructures show no significant increase in grain size or carbide dissolution when compared to the as-received CCM Plus. The forged hips stems were subsequently subjected to the high sintering temperatures. Micrographs show the dramatic difference in grain size between a sintered investment casting and a high carbon forging that was subjected to the same sintering process.

High cycle fatigue performed on porous coated hip stems forged from CCM Plus brought a 60% increase in fatigue strength over cast-sintered hips stems.

METAL-ON-METAL THR
In the mid-1960s and 70s, metal-onmetal cast CoCrMo hip/liner components were used for THR, but were soon abandoned, primarily because metalonpolyethylene hip systems were so successful. Early incidents of equatorial seizing and loosening also plagued the first generation metal-metal bearings, which gave further incentive to implant a polyethylene liner and metal femoral head. Recent concerns, however, about aseptic loosening and polyethyleneinduced osteolysis have led to a renewed interest in alternate metal-metal bearings for THR in the U.S.

A wave of alternate hard-bearing systems have been launched in recent years in the U.S. using new cobalt-based materialsand improved designs and manufacturing methods. Design, contact stresses, and materials are better understood today through tribological wear tests and retrieval studies. There is an increasing awareness of the importance materials play in the performance of implant devices. Tribological wear studies have linked the performance of metalmetal bearings to the metallurgy of cobalt-based alloys. Wear simulation studies have shown significantly lower wear rates with all metal hip-bearing systems.

It has also been shown that alloys with a higher composition of carbon may prove advantageous in long-term wear resistance when compared with the femoral head and acetabular cup devices fabricated from the lower carbon alloy. Given recent simulation and material characterization studies, the higher carbon cobalt wrought alloys appear to offer improvements over the previous generation of cast metal-metal prosthesis.-Wear simulation and characterizationstudies continue in this effort using CCM Plus materials, both in wrought bar form and as a forged component.

Nominal composition (wt %) of CoCrMo wrought alloys

Alloy

CCoCr Mo N
ASTM F1537 - Wrought

0.35 max

Balance

26.0/30.0

5.0/7.0

0.25

BioDur CCM Plus

0.25

65.5

28

6

0.18

BioDur CCM

0.05

65.5

28

6

0.16

 

Nominal mechanical properties of
CoCrMo alloys versus metallurgical condition

Alloy

Condition

Condition

Yield strength
0.2% offset (kpsi)

Ultimate tensile
strength (kpsi)

Elongation
(% in 4D)

Hardness,
HR
c

ASTM F1537 Requirements

Annealed

 

75

130

20

25

Unannealed

Hot work

101

145

12

28

Unannealed

Warm work

120

170

12

35

BioDur CCM Plus

Annealed

2,200°F

104

190

32

37

Annealed

2,050°F

123

203

28

41

Unannealed

Hot work

135

198

22

43

BioDur CCM

Annealed

2,050°F

85

150

25

30

Unannealed

Hot work

110

160

25

33

Unannealed

Warm work

135

190

26

40

 

MAKE CONTACT:

Carpenter Technology Inc.,
(610) 208-2000,
cartech.com

DePuy, a Johnson&Johnson Co.,
(574)267-8143,
depuyorthopaedics.com