Metal-on-Metal Articulation and Wear

FREQUENTLY ASKED QUESTIONS

Irina Timmerman and Harlan Amstutz, MD

About  Wright's new CONSERVE® PLUS Total Hip Resurfacing Implant

Wright's new CONSERVE® PLUS Total Hip Resurfacing Implant has many advantages:

• Preserves bone

• More accurately reproduces a patient's anatomy

• Increases range of motion (ROM)

• Contains no plastic parts

• Allows patients to maintain an active lifestyle

• Allows easy revision (if necessary) since it does not violate the femoral canal

 The CONSERVE® PLUS system is a metal-on-metal system.
In the past, many of the total resurfacing devices were
manufactured from plastics which wore out quickly.
Today, the CONSERVE® PLUS implant is manufactured with metal
only. This results in an articulation of metal on metal instead
of metal on plastic.

Plastic is a great material, but it is soft. When metal and plastic
rub against each other, the plastic begins to break down.
Outside of the body, this is not a huge issue. But, inside the
hip joint, that plastic debris may cause osteolysis. Osteolysis is
a disease in which the body attacks the plastic debris, as well
as the human tissues around that plastic. This loss of tissue
may cause the implant to loosen, which in turn can lead to

pain and possible failure of the implant. Metal-on-metal
resurfacing devices eliminate the soft plastic that can cause
increased chances of osteolysis.

Each CONSERVE® PLUS device ranges in size from 36-56mm on
the femoral side. These sizes compare very favorably in
regards to range of motion with traditional 22, 28, and
32mm femoral heads. The maximum range of motion with
the CONSERVE® PLUS system is 167° compared to 130° with
traditional 28mm heads. This range of motion decreases
the likelihood of postoperative dislocation which is the
second leading cause of hip replacement failures.

No matter what your age, total resurfacing can help you
return to your active lifestyle. The CONSERVE® PLUS
resurfacing implant – a superior quality implant that not
only conserves bone, but has an excellent range of motion,
as well as long-term durability – is a great treatment
alternative for an active individual facing hip surgery.

HOW LONG HAS METAL-ON-METAL ARTICULATION BEEN IN USE?

George McKee of Norwich, England was the first to use metal-on-metal

with modified Thompson stems and a one-piece cobalt chrome socket

combination in THR in 1953. The design was primitive but many

lasted for more than 7 years. Although metal wear was detected in devices

that were revised, McKee did not observe any undesirable effects of that

debris on the soft tissues or the bone1. The early history of M/M devices,

including the Dr. Amstutz’ experience with the McKee device in New York,

has been previously published

WHAT IS THE OPTIMUM MATERIAL FOR METAL-ON-METAL ARTICULATION?

Metal-on-metal articulation is typically associated with the cobalt

chromium molybdenum alloy. Typically these alloys are divided into two

categories: high carbon, where the C content is above 0.20%; and low

carbon, where the C content is less than 0.05%. Several studies comparing

both groups have been conducted. Earlier studies presented

inconclusive results. By comparison, later studies isolated the contribution

of factors such as surface finish, clearance, sphericity and carbon content.

There is now general consensus in the industry that the high carbon

alloy has much better wear resistance than the low carbon type.

CoCr alloy preferred for self-bearing applications because of high hardness

and self-healing capacity.

At least three types of CoCr alloys are candidates for m-m implants.

Description of ASTM Grades of CoCr

F1537-94

o  Low Carbon (<0.05%C)

o  Forged

o  Grain Size <10 µm

F1537-94

o  High Carbon ( >0.20%C)

o  Forged

o  Grain Size <10 µm

F75-92

o  High Carbon

o  Cast

o  30 µm< Grain Size <1000µm

In addition, there are two types of processes used in manufacturing the

cobalt chrome molybdenum components. One method is casting the

components (used by Wright for the CONSERVE® Plus and CONSERVE®

Total implants) and the other is forging the material. Although the

chemical composition can be exactly the same between the two materials,

there is a structural difference. The grain size of the forged alloy is typically

less than 10 microns, whereas the grain size for the cast material ranges

from 30 to 1000 microns. There is also a marked difference in the

appearance of the carbides, in that the carbide regions tend to be smaller in

the forged material. Metal liners and femoral heads have been produced at

Wright with both types of material. A limited number of couples were

tested in a hip wear simulator. The test showed less wear with cast high

carbon alloy than the forged alloy. Due to the limited number of samples,

the difference had low statistical reliability.

DOES THE CLEARANCE BETWEEN ARTICULATING COMPONENTS PLAY A ROLE

IN WEAR DEBRIS GENERATION?

Absolutely! This is probably the most influential factor in wear behavior.

The proper clearance is essential for entrapping the synovial fluid between

the articulating surfaces. This fluid is largely responsible for separating the

surfaces while the joint is in motion and, thereby, reducing wear. If the gap

between components is too small or too large you will see a sharp increase

in wear rates.

A study conducted by Isaac, Dowson and others (DePuy International, Leeds,

UK) compared wrought and as-cast components with various clearances

between those two groups. The results of the hip simulator study strongly

indicated that clearance plays a major role in wear rates, and that wear

appears to be relatively insensitive to changes in materials that have

similar chemical compositions but different microstructures.

DOES THE CONSERVE® PLUS ACETABULAR SHELL WITH THE BIG FEMORAL

HEAD USE THE SAME CLEARANCE FOR ALL SIZES?

No. The clearance between components is size-dependent. The larger the

diameter, the larger the gap between the components. The range for the

entire family of sizes is from 90 to 200 microns of diametral clearance,

each bearing size having an optimized gap for maximum fluid film

thickness

I’VE HEARD A LOT ABOUT HEAT-TREATED COBALT CHROME COMPONENTS VERSUS AS

CAST COMPONENTS. WHAT ARE THEY TALKING ABOUT AND IS THERE A DIFFERENCE?

Cobalt Chrome Molybdenum components that are cast usually go through

the hot isostatic pressing (HIP) and solution annealing processes to

remove microporosities often found in castings, and to improve the

ductility and homogeneity of the material. The microstructure of this type

of heat-treated material looks different from that of the original casting.

It is important to note that even though heat treated material looks

different it doesn’t affect wear.

Two global metal-on-metal resurfacing manufacturers use the heat-treated

process for the castings (Corin, LTD. and WMT, Inc.). Midland Medical, the

producer of the Birmingham Hip Resurfacing (BHR) implant, leaves the

castings untreated.

The BHR product champion, Derek McMinn, MD

claims that heat treatment can lead to carbide depletion and, in turn, it can

adversely affect wear rates. One pin on disk type test suggests that as

cast material wears slightly less than HIP cast material, however, the

data shows so much scatter that the results are inconclusive. In addition,

the linear tracking motion of the type of pin-on-disk used in that study is

very different from the actual hip motion. A linear tracking pin-on-disk test

is conducted by sliding the cylinder on the flat surface back and forth along

one axis. The actual movement of the femoral head inside the socket produces

crossing path motion. Studies in hip simulators are more relevant

since they more closely resemble the actual hip function by reproducing

this crossing path motion. It has been shown that a linear tracking pin-ondisk

test under-estimates UHMWPE wear rates by 10 to 100 times, and

over-estimates metal-on-metal wear rates as compared to hip simulators

and retrieval studies14. Midland Medical has not published any data from a

hip simulator to support their claim. Also, zero clinical studies have been

conducted which suggest BHR components create less wear than heat

treated components.

Bowsher, et al conducted a hip simulator wear study in which 40mm diameter

metal-on-metal bearings, either  as cast  or heat treated, were compared

side by- side. Wear rates were compared for the running-in state ( first 1

million cycles), steady state, and also fast jogging. In all three

conditions, there was no difference between wear rates of the two forms of

the alloy. The authors concluded that HIPing and solution annealing do not

adversely affect the wear rates of large diameter metal-on-metal articulations

Furthermore, one additional study was presented at the recent June,

2003 Conference on Metal-on-Metal Devices in Montreal that corroborate

the Bowsher study

WHAT IS THE STEADY-STATE WEAR?

Typically, metal-on-metal couples in the hip simulators go through the run-in

or wear-in  period where the weight loss due to wear increases linearly.

At some point, usually between 500,000 and 1 million cycles, the wear

increase drops dramatically or stops altogether. It is then said that the

metal-on-metal couple reached the steady-state  of wear. Both wear-in

and steady-state  are demonstrated in

DOES THE SURFACE FINISH AFFECT WEAR RATES?

Surface finish has a definite effect on wear rates. The rougher the surface

finish, the higher the peaks of material that eventually will be removed.

Typical surface finish for the CONSERVE® resurfacing components is 0.008

microns (micrometers). This is an order-of-magnitude smoother than the

finish on typical metal femoral heads articulating with polyethylene

inserts used for THR.

DO LARGER HEADS WEAR LESS THAN SMALLER HEADS?

Theoretically, if the metal couple is dry, larger heads should wear more than

smaller heads due to their longer sliding distance per step. However, in

the presence of the fluid the opposite is true, larger diameter heads should

wear less because of their greater sliding velocity. Calculations show that

larger diameter wear couples can form a thicker synovial fluid film

between components

Hmin = 1.64D(_Ú/ED)0.65(W/ED2)-0.21

WHERE: Hmin is the minimum film thickness

D is the head diameter

Ú is the entraining velocity

According to the formula above, the larger the articulating diameter, the

larger the Hmin value. A thicker fluid film means less contact between

hard surfaces during motion and, presumably, less wear. Does this theory

prove itself ? The study cited above compared 22mm, 26mm, and 35mm

diameter metal-on-metal articulations and found no difference between

the three. Isaac compared 16mm, 22mm, 28mm, 36mm, and 54.5mm

diameter couples15 and, for diameters 28mm and larger, it was determined

that wear decreases with increasing head diameter.

In the study of the 54mm articulating couple (the largest size currently

available in the BFH product line) conducted at WMT, the wear rates

were found to be very similar to the wear rates for the 44mm CONSERVE®

Plus articulating couple performed at another institution.

 WHAT DO THEY MEAN WHEN THEY SAY THAT COBALT CHROME IS SELF-HEALING

 A cobalt chrome articulation has the ability to polish out the scratches from

 abrasive damage such as third-body wear. In retrieval studies, the deep

 scratches have often been partially or entirely polished out of the main

contact zones.

WHAT IS THE AVERAGE PARTICLE SIZE FOR METAL WEAR DEBRIS?

In one study, the cobalt chrome particles from a McKee-Farrar metal-on-metal

articulation were in the range of 6 to 744 nm (nanometers), with an average

size of 42 nm11. By comparison, polyethylene particles range from 0.05 to 5

micrometers (50 to 5000 nm).

CAN A METAL-ON-METAL ARTICULATION PREVENT OSTEOLYSIS?

Since a metal-on-metal articulation does not eliminate wear entirely, there is

always the potential for an osteolytic reaction. There are reports of isolated

cases of osteolysis with metal-on-metal joints. However, these are mostly

limited to the first-generation metal-on-metal components. Those were

implanted with acrylic cement, which can fragment and generate thirdbody

abrasive particles. It is believed that the metal debris is too small, in

comparison to the polyethylene particles, to initiate an osteolytic reaction.

A study of several metal-on-metal components (MetasulTM total hip

replacements and McMinn surface replacements) investigated the bone

and tissue reactions to the metal debris. It was noted that metallosis (a

grey-black appearance of the soft tissue) was present with the surface

replacements and the total hip replacements. Macrophages filled with

metallic particles were found in all tissues, but in larger amounts in those

with metallosis. Giant cells and small areas of histiocytic granulomas were

also present. The authors noted that there were fewer macrophages and

giant cells than typically seen in tissues around metal-polyethylene joints,

and although an inflammatory response to the metal particles was present,

this was not as severe as the response to the cement particles. The authors

concluded that the long-term response to these very small CoCr particles

should be monitored. There has been no observed occurrence of metallosis

in connection with CONSERVE® Plus or CONSERVE® Total implants.

WHAT ABOUT METAL ION RELEASE?

Metal ions find their way into the tissues through wear particles or through

corrosion mechanisms. These ions then travel into the blood stream and

eventually expel in the urine. The topic of metal ion release will be discussed

in greater detail in a separate technical monograph.

CONCLUSIONS:

Many factors affect metal-on-metal wear behavior. Some of them are more

significant than others. Surface finish, appropriate radial clearance and

high carbon content have been shown to play the greatest role in reducing

wear rates.

The microstructure of the alloy does not play a key role in wear behavior.

While  as cast  and heat treated alloys were directly compared in hip simulators

by the scientists from Corin, DePuy, Centerpulse, and in some

indepen-dent laboratories, proponents of  no heat treatment  regimes

have not provided us with laboratory or clinical data to date. McMinn’s

claim of better metallurgy with the  as cast  components is based primarily

on  pin-on-disk  type testing. The  pin-on-plate  or  pin-on-disk  type

experiment can compare the wear of different materials as a flat surface,

but the mechanism of these tests has nothing in common with the

motion of the hip joint.

Hip simulators offer the most reliable way to assess wear in the laboratory,

but keep in mind that the outcome greatly depends on the method, testing

equipment, and measuring equipment. Since we are dealing with tiny

amounts of debris, test results may vary greatly from one hip simulator

study to another. Take that into account when comparing data between

two tests conducted by different people and with different equipment.

And finally, the best proof of a good design is in the clinical outcome.

To date, there have been no published reports of the clinical performance

of the BHR device. The CONSERVE® Plus metal-on-metal articulation has

a good clinical history with over 6 years and over 600 patients. The paper

presenting the clinical results of the first 400 CONSERVE® Plus hip

resurfacing cases performed at the JRI has been accepted for publication

by the Journal of Bone and Joint Surgery.

REFERENCES

1. McKee, G. K., and Watson-Farrar, J.: Replacement of arthritic hips by the McKee-Farrar prosthesis. J. Bone Joint

Surg.,48B:245-259, 1966.

2. Wilson, P. D.; Amstutz, H. C.; Czerniecki, A.; Salvati, E. A.; and Mendes, D. G.: Total hip replacement with fixation by

acrylic cement. A preliminary study of 100 consecutive McKee-Farrar prosthetic replacements. J. Bone Joint Surg.

Am.,54A:207-236, 1972.

3. Amstutz, H. C., and Grigoris, P.: Metal-on-metal bearings in hip arthroplasty. Clin Orthop,329:S11-34, 1996.

4. McKellop, H.; Park, S.-H.; Chiesa, R.; Doorn, P.; Lu, B.; Normand, P., Grigoris, P.; and Amstutz, H.: In vivo wear of three

types of metal-on-metal hip prostheses during two decades of use. Clin.Orthop.,329 Suppl:S128-140, 1996.

5. Chan FW, Bobyn JD, Medley JB, Krygier JJ: Comparison of Alloys and designs in a hip simulator study of metal-on-metal

implants. CORR, Vol.329 148-159,1996S

6. Chan FW, Bobyn JD, Medley JB, Krygier JJ, Tanzer M: Wear and lubrication of metal-on-metal hip implants. CORR, No.

369, pp 10-24, 1996.

7. Nolan FJ, Farrar R, Schmidt MB, Phillips H, Tucker JK: Effect of head size and diametrical clearance on wear production

of a new metal-on-metal hip prosthesis. 42th Annual Meeting of the Orthopaedic Research Society, Poster #71, 1997.

8. Bowsher JG, Nevelos J, Pickard J, Shelton JC: Do heat treatments influence the wear of large diameter metal-on-metal hip

joints? An in-vitro study under normal and adverse gait conditions. 49th Annual Meeting of the Orthopaedic Research

Society, Poster #1398, 2003.

9. Schmalzried TP: Metal-on-metal: historical perspectives and lessons learned through retrieval studies. Seminars in

Arthroplasty, Vol.9, No.2 pp 135-142, 1998.

10. Varano R, Bobyn JD, Medley JB, Yue S: Does alloy heat treatment influence metal-on-metal wear? 49th Annual Meeting

of the Orthopaedic Research Society Poster #1399, 2003.

11. Doorn PF, Campbell PA, Worrall J, Benya PD, McKellop HA, Amstutz HC: Metal wear particle characterization from

metal on metal total hip replacements: transmission electron microscopy study of periprosthetic tissues and isolated

particles. Journal of Biomedical Material Research 1998 Oct;42(1):103-11.

12. Klapperich C, Graham J, Pruitt L, Ries MD: Failure of a metal-on-metal total hip arthroplasty from progressive osteolysis.

Journal of Arthroplasty Vol.14 No.7, 1999, pp 877-881.

13. Campbell P, McKellop H, Alim R, Mirra J, Nutt S, Dorr L, Amstutz HC: Metal-on-metal hip replacements-wear performance

and cellular response to wear particles. ASTM STP1365,1999, pp 193-209.

14. McKellop HA: Testing Issues in Metal-Metal Tribology. Second International Conference on Metal-Metal Hip Prostheses,

Jun22 2003, Montreal.

15. Isaac G, Hardaker C, Flett M, Dowson D: Factors Affecting Wear in Metal-Metal Hips, An Overview of Simulator Testing.

Second International Conference on Metal-Metal Hip Prostheses, Jun22 2003, Montreal

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