Wear Reduction Technology In Total Knee Arthroplasty 71281763

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Wear-Reduction Technology in Total Knee Arthroplasty

Steven Haas, MD, MPH 1, Ramprasad Papannagari, MS 2, Mark Morrison, PhD 2, Shilesh Jani, MS 2

1 Hospital for Special Surgery, New York, NY, USA

2 Smith & Nephew, Inc., Orthopaedics, Memphis, TN, USA

Summary

Due to the increasing burden of revision in total knee arthro-

plasty (TKA), sustainable improvements in implant longevity

may require the continued development of advanced bear-

ing materials. The LEGION™ Primary Knee System featuring

VERILAST™ technology is the first device to combine an OX-

INIUM™ Oxidized Zirconium femoral component with a highly

crosslinked ultra-high molecular weight polyethylene (UHM-

WPE) tibial insert to form an advanced TKA bearing. Follow-

ing the review of published volumetric wear rates, this bear-

ing coupling was found to provide the lowest observed wear

of any contemporary TKA device, potentially supporting the

equivalent of 30 years of normal use in vivo. This evidence

supports the assertion that both tibial and femoral bearing

surfaces can significantly affect TKA wear. Moreover, the use

of VERILAST™ technology may reduce long-term revision risk

and support device longevity in younger, more active pa-

tients.

‡Based on in-vitro wear simulation testing, the LEGION Primary Knee System with VERILAST technology is expected to provide

wear performance sufficient for 30 years of actual use under typical conditions. The results of in-vitro wear simulation testing

have not been proven to quantitatively predict clinical wear performance. Also, a reduction in total polyethylene wear volume or

wear rate alone may not result in an improved clinical outcome as wear particle size and morphology are also critical factors in

the evaluation of the potential for wear mediated osteolysis and associated aseptic implant loosening. Particle size and

morphology were not evaluated as part of the testing.

Vol 01, No 01 - December 2010

Bone&JointScience

 Our Innovation in Focus

Page – 2

Bone&JointScience Vol 01, No 01 - December 2010

Wear-Reduction Technology in Total Knee Arthroplasty

‡Based on in-vitro wear simulation testing, the LEGION Primary Knee System with VERILAST technology is expected to provide

wear performance sufficient for 30 years of actual use under typical conditions. The results of in-vitro wear simulation testing

have not been proven to quantitatively predict clinical wear performance. Also, a reduction in total polyethylene wear volume or

wear rate alone may not result in an improved clinical outcome as wear particle size and morphology are also critical factors in

the evaluation of the potential for wear mediated osteolysis and associated aseptic implant loosening. Particle size and

morphology were not evaluated as part of the testing.

The Importance of Wear Resistance in TKA

Tibial component wear, attendant osteolysis and loosening

have been identified as the primary causes of long-term fail-

ure in TKA [1–3]. In 1999 alone, 22,000 TKA revision proce-

dures occurred in the United States at an estimated cost of

over $260 million [4]. In 2005, there were 38,300 revisions in

the United States. This number is expected to grow to over

268,000 by 2030 [5]. In order to support optimal patient care

and reduce accelerating healthcare costs, technologies must

be introduced that support improved device longevity. Specifi-

cally, improved tibial and femoral bearing technologies could

limit long-term revision risk in TKA, especially in relatively

young and active patients.

Polyethylene and Wear Performance

The History of UHMWPE

UHMWPE was first utilized in TKA in 1968, setting a standard

for knee replacement that continues today (Figure 1) [6]. More

than 40 years later, every TKA in the world still utilizes a UHM-

WPE tibial bearing. However, polyethylene wear remains a pri-

mary cause of long-term failure [3]. During normal articulation,

millions of microscopic polyethylene wear particles are re-

leased into the tissues surrounding the knee joint. These par-

ticles can cause a cascade of biological responses leading to

osteolysis, aseptic loosening, and eventual revision [7]. In or-

der to address these risks many attempts have been made to

improve polyethylene wear performance, including the unsuc-

cessful introductions of Poly II in 1977 and Hylamer in 1991. In

Figure 1: TKA Milestone Timelines

Page – 3

Bone&JointScience Vol 01, No 01 - December 2010

Wear-Reduction Technology in Total Knee Arthroplasty

‡Based on in-vitro wear simulation testing, the LEGION Primary Knee System with VERILAST technology is expected to provide

wear performance sufficient for 30 years of actual use under typical conditions. The results of in-vitro wear simulation testing

have not been proven to quantitatively predict clinical wear performance. Also, a reduction in total polyethylene wear volume or

wear rate alone may not result in an improved clinical outcome as wear particle size and morphology are also critical factors in

the evaluation of the potential for wear mediated osteolysis and associated aseptic implant loosening. Particle size and

morphology were not evaluated as part of the testing.

mal balance between wear performance and mechanical

properties for TKA [16].

OXINIUM™ Oxidized Zirconium in TKA

Cobalt chrome (CoCr) alloy has served as the standard mate-

rial for femoral components in TKA for more than 40 years

(Figure 1). However, the surfaces of retrieved CoCr femoral

components have been shown to exhibit roughening that can

significantly increase polyethylene wear [17–19]. This evidence

suggests that a femoral bearing surface with improved wear

performance could improve implant longevity.

In contrast to the UHMWPE milestones shown in Figure 1, the

introduction of OXINIUM™ (Smith & Nephew, Inc., Memphis,

TN, USA) Oxidized Zirconium femoral components in 1998 was

the first major TKA bearing advancement on the femoral side

in 40 years. This material was developed to combine the ob-

served wear benefits of ceramics with the toughness of met-

als. The resulting bearing surface is resistant to in-vivo rough-

ening, is less abrasive than CoCr, and has enhanced

biocompatibility, without any risk of catastrophic fracture [20–

25]. Retrieval studies have shown that Oxidized Zirconium

femoral components exhibit minimal scratching. A matched

pair analysis performed at The Hospital for Special Surgery

showed that in vivo femoral scratching was 12 times greater in

the CoCr components compared to Oxidized Zirconium (Fig-

ure 2) [25, 26].

Wear Performance of CoCr and OXINIUM

TKA Bearings

With a CoCr bearing, the only way to significantly improve wear

performance is by increasing the irradiation dose of the poly-

ethylene. However, as previously described, this improved

wear performance must be balanced against unfavorable

changes in mechanical properties. OXINIUM femoral compo-

nents effectively alter the dynamic between irradiation dose,

wear resistance and mechanical properties. Compared to

CoCr, OXINIUM results in less UHMWPE wear at any given ir-

radiation dose, without any sacrifice in mechanical properties

contrast, crosslinked polyethylene has been used since 2001

and has been shown to be highly successful clinically [8].

The Development of Crosslinked UHMWPE

It is well established that the wear resistance of UHMWPE

quickly improves with increased irradiation dose. However,

this gain in wear resistance is attained at the expense of me-

chanical properties. If greater wear resistance is desired, the

UHMWPE can be exposed to a higher radiation dose, but the

mechanical properties will be further decreased. This balance

is particularly important in TKA, where contact stresses are

higher than in total hip arthroplasty (THA). Based on material

and device testing, a highly cross-linked UHMWPE with a ra-

diation dose of 7.5 Mrad (75 kGy) appears to result in an opti-

0.0

1.3

0.4

5.0

2.5

3.8

5.0

OxZr CrCoMo

Significantly less scratching OxZr

p=0.005

Results—Femoral component

Femoral Scratching Pitting Delamination Striations Total

component

OxZr 0.4±0.7 0.3±0.6 0.1±0.3 0.5±0.7 1.6±1.3

CrCoMo 5.0±0 0.1±0.3 1.6±2.3 0±0 9.8±0.5

p 0.44 0.05 0.05 0.005

1.6

9.8

OxZr CoCr

Results—Femoral component

Femoral

component

OxZr 1.6±1.3

CoCr 9.8±0.5

p 0.005

p=0.005

The total average score was significantly lower for the OxZr components (1.6±1.3

vs. 9.8±0.5, p=0.005) [26].

Figure 2: Comparison of wear grades (scratching,

pitting, delaminations, striations) by visual score

for OxZr and CoCr bearing surfaces.

Page – 4

Bone&JointScience Vol 01, No 01 - December 2010

Wear-Reduction Technology in Total Knee Arthroplasty

‡Based on in-vitro wear simulation testing, the LEGION Primary Knee System with VERILAST technology is expected to provide

wear performance sufficient for 30 years of actual use under typical conditions. The results of in-vitro wear simulation testing

have not been proven to quantitatively predict clinical wear performance. Also, a reduction in total polyethylene wear volume or

wear rate alone may not result in an improved clinical outcome as wear particle size and morphology are also critical factors in

the evaluation of the potential for wear mediated osteolysis and associated aseptic implant loosening. Particle size and

morphology were not evaluated as part of the testing.

(Figure 3). For example, the wear rate of OXINIUM against a 7.5

Mrad crosslinked UHMWPE is approximately equivalent to that

of CoCr against a 10 Mrad crosslinked UHMWPE with pristine,

new components.

Utilizing an OXINIUM femoral component instead of CoCr pro-

vides a reduction in wear equivalent to an additional 3 Mrad

irradiation dose. In the end, mechanical properties are im-

proved because about 25% less radiation exposure is neces-

sary to achieve the same wear resistance.

The previously described testing conditions represent an ide-

al situation with pristine, new components, featuring highly

polished surfaces. However, the presence of third-body de-

bris such as bone cement, bone chips, or debris shed from

ingrowth surfaces can significantly diminish the gains in wear

resistance provided by crosslinked UHMWPE [27]. Using an in-

vitro tumbling protocol designed to simulate roughening from

third-body debris [28], the polyethylene wear against tumbled

OXINIUM components was compared to the wear produced

by tumbled CoCr femoral components. Results indicated that

the abrasion resistance of OXINIUM appears to prevent

scratching by third-body debris, enabling improved wear re-

sistance (Figure 3).

Muratoglu et al [29] examined the wear of conventional UHM-

WPE and highly crosslinked polyethylene on new and retrieved

CoCr femoral components. Their data indicated that femoral

scratching increases wear in both crosslinked and conven-

tional polyethelene. The increase was over 800% for the

crosslinked polyethylene, but only 266% for conventional

UHMWPE [29]. Based on this data, the scratch-resistant prop-

erties of OXINIUM appear to be especially important in main-

taining the wear resistance of crosslinked polyethylene.

Wear Performance of VERILAST™

The LEGION™ Primary Knee System featuring VERILAST™ tech-

nology (Smith & Nephew, Inc., Memphis, TN USA; Figure 4) is

the first TKA device to combine the advanced wear properties

of 7.5 Mrad highly crosslinked ultra-high molecular weight poly-

ethylene (XLPE) tibial inserts with the superior abrasion resis-

LEGION™ Primary Knee System featuring

VERILAST™ technology (Smith & Nephew, Inc.,

Memphis, TN USA).

Figure 4: LEGION™ Primary Knee

System

Plot of the mean wear rates (± standard deviations) in a knee simulator for

UHMWPE crosslinked to various doses against either CoCr or OXINIUM femoral

components in pristine (solid symbols and lines) and tumbled (open symbols and

dashed lines) conditions.

0 10 30 40

50

20

0 2 6 8 10 124

Wear Rate (mm3 / Mc

ycles)

Radiation Dose (Mrad)

Tumbled

Tumbled CoCr

Tumbled OXINIUM

Pristine CoCr

Pristine OXINIUM

Pristine

Figure 3: Plot of the mean wear rates

Page – 5

Bone&JointScience Vol 01, No 01 - December 2010

Wear-Reduction Technology in Total Knee Arthroplasty

‡Based on in-vitro wear simulation testing, the LEGION Primary Knee System with VERILAST technology is expected to provide

wear performance sufficient for 30 years of actual use under typical conditions. The results of in-vitro wear simulation testing

have not been proven to quantitatively predict clinical wear performance. Also, a reduction in total polyethylene wear volume or

wear rate alone may not result in an improved clinical outcome as wear particle size and morphology are also critical factors in

the evaluation of the potential for wear mediated osteolysis and associated aseptic implant loosening. Particle size and

morphology were not evaluated as part of the testing.

tance of OXINIUM femoral components. This advanced bear-

ing couple could provide improved implant longevity in TKA.

In order to evaluate bearing performance, wear rates from in-

dependent, published studies were compared to wear results

for VERILAST (Figure 5). Volumetric wear rates for CoCr and

conventional UHMWPE (CoCr/CPE) range from 20–43 mm3/

Mcycles. The wear rates for CoCr and crosslinked UHMWPE

(CoCr/XLPE) is significantly less, ranging from 4–13 mm3/Mcy-

cles. In contrast, a wear rate of 0.58 mm3/Mcycles was ob-

served for VERILAST in the 45 Mcycles test.

These results are especially impressive considering the test-

ing protocols that were utilized. The VERILAST bearing was

tested with the kinematically aggressive Leeds protocol [35].

Moreover, the bearing was tested for 45 Mcycles. Simulator

tests reported in the literature are typically conducted for only

5 to 20 Mcycles [30–43].

Some specific examples of reported wear cycles include the

following:

– Crosslinked polyethylene (Prolong) using NexGen CR

(Zimmer, Warsaw, IN) TKR – 20 Mcycles (Popoola et al [39]).

Mean volumetric wear rates (+/- std. dev.) of CoCr against conventional polyethylene (CPE), CoCr against crosslinked polyethylene (XLPE) and OXINIUM against XLPE

(VERILAST) [30-36].

Figure 5: Comparison of mean volumetric wear rates

Volumetric wear rate (mm3 / Mcycles

24.40

4.10

23.00

13.00

43.40

6.10

20.20

6.50

34.60

7.30

0 10 30 40 5020

NexGen PFC Sigma Scorpio Triathlon Vanguard

CoCr/CPE CoCr/XLPEVERILAST

Genesis II

23.45

6.41

0.58

LEGION

VERILAST

Page – 6

Bone&JointScience Vol 01, No 01 - December 2010

Wear-Reduction Technology in Total Knee Arthroplasty

‡Based on in-vitro wear simulation testing, the LEGION Primary Knee System with VERILAST technology is expected to provide

wear performance sufficient for 30 years of actual use under typical conditions. The results of in-vitro wear simulation testing

have not been proven to quantitatively predict clinical wear performance. Also, a reduction in total polyethylene wear volume or

wear rate alone may not result in an improved clinical outcome as wear particle size and morphology are also critical factors in

the evaluation of the potential for wear mediated osteolysis and associated aseptic implant loosening. Particle size and

morphology were not evaluated as part of the testing.

– Crosslinked polyethylene (Durasul) using Natural Knee II

(Zimmer, Warsaw, IN) TKR – 10 Mcycles (Muratoglu

et al [40]).

– Insall-Burstein I (Zimmer, Warsaw, IN) and Kinematic

(Howmedica, Rutherford, NJ)

TKR – up to 11 million cycles (Walker et al [41]).

– Insall-Burstein II (Zimmer, Warsaw, IN) TKR – approx.

11 million cycles (Beaule et al [42])

– JOURNEY (Smith & Nephew, Memphis, TN) TKR – up to

10 million cycles (Ries et al [43])

Conclusion

The amount of volumetric wear observed per million cycles of

testing suggests that the LEGION primary knee coupled with

VERILAST bearing technology may remain viable in vivo for the

equivalent of 30 years of normal use. Moreover, when tested un-

der relatively extreme simulation conditions, this system demon-

strated the lowest wear rate of any contemporary TKA device.

These wear results may be particularly relevant for younger pa-

tient populations. While the longevity of contemporary TKA im-

plants has remained relatively unchanged, the typical patient

has not. Initially, TKA was primarily performed in patients over

the age of 65. However, today an ever increasing number of

patients are having surgery in their 40’s and 50’s [44, 45]. An

estimated device longevity of approximately 15 years may be

sufficient for older populations, but the increased demands of

younger patient groups require an additional 5–15 years of in-

vivo use prior to revision TKA. This demographic shift suggests

that advanced bearing technologies should be adopted to re-

solve an inevitable increase in long-term revision TKA rates.

Page – 7

Bone&JointScience Vol 01, No 01 - December 2010

Wear-Reduction Technology in Total Knee Arthroplasty

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Page – 8

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