Effect of oxidation on delamination of ultrahigh-molecular-weight polyethylene tibial components

The Journal of Arthroplasty Vol. 13 No. 3 1998

Effect of Oxidation on D e l a m i n a t i o n of Ultrahigh-molecular-weight P o l y e t h y l e n e Tibial C o m p o n e n t s C a r o l J. B e l l , B E n g , M S c , P e t e r Melanie

R. A b e y s u n d e r a ,

BSc, MBBS,

Polly M. King, BSc, MBBS,

S. W a l k e r ,

Jonathan

and

Gordon

BSc, PhD,

M . H. S i m m o n s , W. B l u n n ,

BSc, MBBS,

BSc, PhD

Abstract: Whether oxidation of ultrahigh-molecular-weight polyethylene (UHMWPE) causes delamination of the plastic in total knee arthroplasties (TKAs) was investigated. Examination of retrieved TKAs has shown that oxidation of UHMWPE can be caused by postirradiation damage leading to a subsurface band or, to a lesser extent, by mechanical forces during use leading to surface oxidation. Delamination cracks propagated through the subsurface oxidized band. In wear tests, delamination occurred in artificially aged UHMWPE where only subsurface oxidized bands had formed. Increased surface wear predominated where oxidation was associated with the surface of the plastic. Similarly, in tensile and fatigue tests of oxidized UHMWPE, there was a significant reduction in the ultimate tensile strength and in the fatigue resistance of specimens that had developed a subsurface band. Oxidation increased fatigue crack growth rate. It was observed that UHMWPE from different manufacturers varied in its resistance to oxidation. This study demonstrates that the effect of oxidation, which results in the development of a subsurface white band, combined with high subsurface shear forces observed in TKAs, is to enhance delamination wear. Key words: ultrahigh-molecular-weight polyethylene, total knee arthroplast}; delamination, oxidation.

D e l a m i n a t i o n is the m o s t catastrophic f o r m of w e a r in u l t r a h i g h - m o l e c u l a r - w e i g h t p o l y e t h y l e n e (UHMWPE) tibial c o m p o n e n t s . This is believed to be due to a fatigue failure of the material as a result of repetitive cyclic loading during e v e r y d a y activity. This failure is initiated by the d e v e l o p m e n t and p r o p a g a t i o n of subsurface cracks that eventually connect w i t h the surface of the tibial plastic c o m p o nent, releasing large flakes of w e a r debris [1]. Various factors affect the onset and extent of delami-

nation. The kinematics at the articulating surface are important, with subsurface crack initiation and p r o p a g a t i o n occuring during cyclic sliding rather t h a n rolling [2]. Design of the i m p l a n t is important, with d e l a m i n a t i o n a n d catastrophic fracture related to high-contact stresses a n d associated m o r e with bearing surfaces of low congruity [3,4]. Misalignm e n t of c o m p o n e n t s also appears to be important, with d e l a m i n a t i o n and cracking occurring as a result of increased stresses, particularly t o w a r d the edges of flat c o m p o n e n t s [5]. In addition to the g e o m e t r y of the knee, a n u m b e r of other factors a p p e a r to affect d e l a m i n a t i o n of the plastic. Walker et al. suggested that for p o l y e t h y l e n e with fusion defects, the time t a k e n for crack initiation is short and cyclic stress leads to crack initiation f r o m the defects, particularly at depths of a p p r o x i m a t e l y 1

From the Centrefor Biomedical Engineering, Royal National Orthopaedic Hospital, University CollegeLondon, United Kingdom. Reprint requests: Gordon W. Blunn, BSc, PhD, Centre for Biomedical Engineering, Royal National Orthopaedic Hospital Trust, Brockley HilI, Stanmore, HA7 4LP, UK. Copyright © 1998 by Churchill Livingstone ®. 0883-5403/1303-000653.00/0

280

Effect of Oxidation on Delamination of Polyethylene

m m below the surface [6], the area of highest von Mises stresses [7]. Stresses in the plastic will be higher in thinner components, and this effect will be increased if screw holes or ridges in the metal baseplate occur u n d e r the bearing area. A n o t h e r factor that m a y well affect delamination of tibial plastic is the material changes initiated by irradiation during sterilization. Nusbaum and Rose investigated the effect of g a m m a sterilization on UHMWPE and concluded that in-air irradiation caused oxidative degradation as well as crosslinkage of polyethylene molecules and probably increased the material's susceptibility to damage caused by low-level cyclic deformation [8]. Gamma irradiation initiates chain scission of polyethylene molecules, resulting in the generation of free radicals which can react with oxygen, causing oxidation leading to shorter molecular chains and altering abrasion resistance and material properties [9]. Increases in crystallinity and hardness t h e n result. This has recently b e e n s h o w n to result in the t i m e - d e p e n d e n t d e v e l o p m e n t of a subsurface band of more highly oxidized polyethylene. This process occurs in both shelf-stored and implanted components. A n u m b e r of studies have investigated the effect of increased oxidation on the wear of UHMWPE. Fisher has shown, using a pin-on-disk test, that polyethyelene from acetabular cups oxidized by shelf storage has a higher wear rate compared with material from cups obtained directly from the m a n u facturer [10]. Sutula et al. indicated that the subsurface band correlated with cracking and delamination in retrieved acetabular cups [ I I ]; however, Li et

•

Bell et al.

281

al. found no correlation of the subsurface band with wear in acetabular cups [12]. Our study investigated the relationship b e t w e e n the subsurface band of oxidized UHMWPE and the residual strain in retrieved tibial components, with delamination. Using a technique developed by Sanford and Saum [13], we also investigated the effect of artificially induced subsurface oxidation on the wear of UHMWPE in a test that simulates the sliding motion of total knee arthroplasties (TKAs). In addition, we investigated the effect of induced oxidation on the material properties of UHMWPE and related these to delamination in TKAs.

Materials and Methods Retrieved Specimens Thirty-four tibial polyethylene components were retrieved and analyzed for the presence of wear damage, specifically delamination. The majority were St. Georg Sledge (Waldemar Link, Hamburg, Germany) and Kinematic (Howmedica, Rutherford, N J) designs that had been implanted longer than 6 years. The study was expanded to introduce specimens that had been retrieved after shorter implantation times and of several different designs (Table 1). The p o l y e t h y l e n e of the retrieved c o m p o n e n t s showed delamination wear. The reasons for removal varied and are listed in Table 1. Loosening was the main reason for removal. To assess the location and a m o u n t of oxidation, subsurface failure, and position of stresses, the retrievals were cut

Table 1. Prosthesis Type, Number, Duration, and Reason for Removal Duration of I m p l a n t a t i o n (y) Prosthesis Type

No.

Range

Attenborough* Freeman~Kinematics

3 1 14

3-12 15.5 2-10

Minns Meniscal§ Porous Coated AnatomicH Rota Glide# St. Georg Sledge~

1 3 1 10

7 4-5 6 6-14

Tota] Condylar**

I

9

*Zimmer UK, Swindon, United Kingdom. t H o w m e d i c a International, Limerick, Ireland. SHowmedica, Rutherford, New Jersey. §Zimmer UK. ][Howmedica. #Corin Medical Ltd., Circencester, United Kingdom. q[Waldemar Link, Hamburg, Germany. **Howmedica.

Average

Reason for Removal (No. of Prostheses)

8 -6

Tibial loosening (2), infection (1) Loosening ( 1) Fractured tibial tray (3), loosening (8), infection (2), polyethylene wear (1) Subluxation ( 1 ) Loosening ( 1 ), pain ( 1 ), polyethylene wear ( 1 ) Loosening ( 1)

-4.5 -10

Loosening (2), infection (2), ligament instability (6) Pseudoarthritis ( 1 )

282

The Journal ofArthroplasty Vol. 13 No. 3April 1998

transversely through the middle of the wear track. Fifty-micrometer-thick sections were cut on a microtome perpendicular to the wear track. Sections were viewed in transmitted and polarized light to qualitatively examine the position of residual strains in polyethylene caused by stresses in the polyethylene. Sections were treated with chlorosulfonic acid (C SA). This acid stains oxidized polyethylene brown. To verify the effect of this acid, CSA-treated sections were compared with sections from the same components that were analyzed by Fourier transform infrared spectroscopy (FTIR) (this technique is described later in this section).

2.3 kN CONSTANT LOAD FROM SERVO HYDRAULIC ACTUATOR

ius SERIJ LUBRICI

~/PE C

BEARINGS Allowing sliding

Materials All specimens for the wear and material tests were m a c h i n e d to the required dimensions from ram e x t r u d e d bar stock of Poly Hi (GUR 415; Solldur, Fort Wayne, IN) UHMWPE. Compact tension specimens for crack growth rate m e a s u r e m e n t s were additionally m a c h i n e d from compressionm o l d e d RCH 1000 (GUR 412). All m a c h i n e d specimens were sterilized using g a m m a irradiation (cobalt-60 source), using standard implant dose levels (25 kGy).

Artificial Aging The effect of artifidal oxidation on the wear characteristics and mechanical properties of UHMWPE was examined. All aging was achieved by a method developed from that of Sanford and Saum [ 13]. The polyethylene specimens were placed in a pressurized container in oxygen at a pressure of 70 psi. This vessel was placed in an oven at 80°C for a set period. Specimens were removed from the vessel at various periods and the degree of oxidation was assessed by FTIR and by staining sections of polyethylene with CSA.

Wear Testing The effect of oxidation on wear was investigated by artificially aging polyethylene and comparing aged and u n t r e a t e d UHMWPE in a pin-on-plate wear test that simulated the sliding conditions occurring at the knee joint. Specimens were artificially aged (oxidized) for periods b e t w e e n 2 and 8 days. Plain irradiated specimens were also tested. The test configuration is s h o w n in Figure 1. The test machine consists of 6 hydraulically loaded cobaltc h r o m e cylinders that articulate with UHMWPE flat disks [I4]. A compressive load of 2.3 kN was applied, giving an initial contact pressure (ignoring plastic deformation) of 71 MPa. The plastic specimens are reciprocated at a frequency of 1 Hz along a

SLIDING DISTANCE 10 mm

1 Hz Fig. I. Schematic diagram of the wear test apparatus. UHMWPE, ultrahigh-molecular-weight polyethylene.

track length of 10 mm. A serum lubricant was used. The tests were stopped every 200,000 cycles; pins and disks were t h e n washed and dried in an oven for 24 hours. Weight loss was measured, and change in the wear track profile was d e t e r m i n e d using Talysurf (Rank Taylor, Hobson, Leicester, UK). To assess the effect of oxidation on the type and extent of wear, the wear track m o r p h o l o g y was also examined using a scanning electron microscope.

Mechanical Testing Specimens were m a c h i n e d from the same bar stock to the appropriate British Standards. Tensile tests were performed according to British Standard D 638-89. Specimens were tested in the as-irradiated state and after 4, 6, and 8 days of simulated oxidation in the pressure vessel. Tests were performed at r o o m t e m p e r a t u r e using a Hounsfield biaxial testing machine, with an elongation rate of 25 m m / m i n . Ultimate tensile strength and ultimate strain of specimens were calculated. Specimens were retained after testing, and fracture surfaces were e x a m i n e d in a scanning electron microscope. Rotating b e a m fatigue tests were p e r f o r m e d according to British Standard B S 3518, using a W h o l e > type apparatus (G.T.G. Engineering, Loughborough, UK). Specimens were tested in the as-irradiated condition and after 4 and 8 days of simulated oxidation in the pressure vessel. A constant bending m o m e n t of 0.66 Nm was applied to all specimens, and the n u m b e r of cycles to fracture was deter-

Effect of Oxidation on Delamination of Polyethylene

mined. A f r e q u e n c y of 2 Hz was used, in accordance with previous w o r k that s h o w e d t h e r m a l fatigue was not implicated as a m o d e of failure. Specimens w e r e retained after testing, and fracture surfaces w e r e e x a m i n e d in a scanning electron microscope. Fatigue crack p r o p a g a t i o n tests w e r e p e r f o r m e d on RCH i000 and Poly Hi according to British Standard E 647-93. Specimens w e r e m a c h i n e d into standard c o m p a c t tension specimens, the n o t c h e d radius being s h a r p e n e d w i t h a razor blade prior to testing. Testing was p e r f o r m e d on p o l y e t h y l e n e in the asirradiated condition and after 4 and 8 days of simulated oxidation in the pressure vessel. Specim e n s w e r e tested on a Hounsfield m a c h i n e using a cyclic displacement of 1 m m at a f r e q u e n c y of 0.93 Hz. P a r a m e t e r s w e r e controlled by computer and chosen to allow comparison between materials in various states of oxidation. Measurements of crack length were made with a 10-power traveling microscope as a function of n u m b e r of cydes.

1

2

•

Bell et al.

Oxidation Ratio 3 4

283

5

6

Cryst allinit y (%) 48

50

52

54

56

58

60

I

I

I

I

i

I

B 1000 Depth (~m) 2000

Fourier Transform Infrared Spectroscopy

Fourier transform infrared spectroscopy was performed to determine oxidation and crystallinity with depth from the articulating surface. This was performed using a Nicolet 740 FTIR spectrophotometer (Madison, WI) combined with a light microscope. The level of oxidation was determined according to the method of Li et al. [15] by comparing absorbance bands at 1,720 and 1,740 cm 1 with the polyethylene absorbance band at 3,600 a n ~. Crystallinity was determined according to the method of Gueugnaut et al. [16] by comparing absorbance peaks for amorphous polyethylene (1,303 cm -I) and crystalline polyethylene (1,894 a n ~). Measurements were taken every 250 ~ m from the surface. Results Observations of Retrieved Tibial Components

N u m b e r s of each type of design with m i n i m u m and m a x i m u m i m p l a n t a t i o n times are given in Table 1. I m p l a n t a t i o n times varied f r o m 2 to 15 years, with a variety of revision causes including infection, wear, and instability. Light microscope e x a m i n a t i o n of retrievals revealed the widely variable m o r p h o l o g y of the polyethylene, including fusion defects, i m p u r i t y partides, a n d cracking. These p h e n o m e n a w e r e highlighted b y the CSA t r e a t m e n t , w h i c h indicated, in addition, highly variable oxidation within the c o m p o n e n t s e x a m i n e d . T r e a t m e n t with CSA highlighted the subsurface bands, w h i c h w e r e observed

3000 g

a-•

Crystallinity(%) Oxidation Ratio

Fig. 2. (A) Thin section through a retrieved tibial component {Kinematic, duration, 5 years) showing subsurface oxidized band stained using chlorosulfonic acid. (B) This band corresponds to the peaks in oxidized ultrahighmolecular-weight polyethylene measured using Fourier transform infrared spectroscopy (Howmedica, Rutherford, NJ).

on sectioning prior to staining and, in some instances, oxidized subsurface bands that w e r e not evident before CSA treatment. Regions of UHMWPE that stained with CSA w e r e also areas w h e r e high concentrations of carbonyl and k e t o n e groups were m e a s u r e d using FTIR {Fig. 2). F r o m observation of CSA-treated thin sections and residual strains using polarized light microscopy, the regions of m a x i m a l strain and oxidation were coincident in several cases, being positioned u n d e r the surface adjacent to the w o r n region of the tibial plastic {Fig. 3). In 1 case, this was also related to d e l a m i n a t i o n of the c o m p o n e n t (Fig. 4); h o w ever, in the majority of d e l a m i n a t e d tibial components, cracking was initiated within the subsurface b a n d of high oxidation (Fig. 5). Almost all the retrieved c o m p o n e n t s e x a m i n e d s h o w e d that delamination was associated with oxidation rather t h a n fusion defects alone.

284

The Journal of Arthroplasty Vol. 13 No. 3 April 1998

A

Fig. 5. Thin sections through a retrieved tibial component stained with chlorosulfonic acid showing (A) delamination with cracks propagating along and (B) through the subsurface oxidized band.

Fig. 3. (A) Thin section through a retrieved component (St. Georg Sledge [Waldemar Link, Hamburg, Germany], duration, i 1 years) under polarized light demonstrating residual strain in the plastic. (B) The highest residual strain demonstrated by the colored fringes are coincident with the oxidized regions stained using chlorosulfonic acid. Also note that the boundaries of fusion defects stain when treated with chlorosulfonic acid.

the UHMWPE samples are d e p e n d e n t on incubation time. Samples incubated for 2 a n d 4 days s h o w m a x i m u m oxidation at the surface of the specimen. A subsurface oxidation b a n d develops only in specim e n s incubated b e t w e e n 6 and 8 days. Generally, there is an increase in the degree of oxidation w i t h increasing incubation time.

Wear Testing Artificial Aging Results f r o m FTIR analysis of samples of UHMWPE artificially aged b e t w e e n 2 and 8 days are s h o w n in Figure 6. The degree of oxidation and the position of m a x i m u m oxidation with reference to depth within

Fig. 4. Thin section through a retrieved tibial component (St. Georg Sledge [Waldemar Link, Hamburg, Germany], duration, 7 years) treated with chlorosulfonic acid showing that polyethylene is more oxidized under the wear track and how the plastic is delaminating with cracks propagating along the boundary of the oxidized and nonoxidized ultrahigh-molecular-weight polyethylene.

Several w e a r tests were carried out on specimens that w e r e aged for different periods. The type and m e c h a n i s m of w e a r d e p e n d e d on the extent and position of the m o s t oxidized region of the polyethylene. Specimens oxidized for 0, 2, and 4 days s h o w e d no signs of delamination. Specimens oxidized for 6 a n d 8 days d e l a m i n a t e d u n d e r the test conditions described. The 8-day oxidized specimens catastrophically d e l a m i n a t e d after only 20,000 sliding cycles. In these specimens, the plastic delamin a t e d f r o m b o t h the articulating surface a n d the surface n e x t to the m e t a l baseplate. Cracks propagated t h r o u g h the subsurface oxidized bands, w h i c h w e r e present on b o t h the u p p e r and lower surfaces (Fig. 7). Six-day oxidized specimens d e l a m i n a t e d after an average of 200,000 sliding cycles. Specim e n s aged for 6 days d e l a m i n a t e d less catastrophically, a n d small blisters similar to those seen on retrieved samples at the onset of d e l a m i n a t i o n could be identified u n d e r the w e a r track. Sections t a k e n t h r o u g h the plastic perpendicular to the w e a r track

Effect of Oxidation on Delamination of Polyethylene

•

Bell et al.

285

Oxidat ion Pat io 2

Fig. 6. Profile of oxidation versus depth for artificially oxidized ultrahigh-molecularweight polyethylene.

3

4

5

6

1000 Depth ( g m ) 2000 u-*

6 Days 8 Days 10 Days

3000 -

s h o w e d subsurface cracks that w e r e propagating t h r o u g h the oxidized subsurface b a n d but h a d not joined with the surface of the plastic (Fig. 8). Polyethylene artificially aged for 2 a n d 4 days did not delaminate, but in these specimens, a significant increase in surface w e a r was apparent. Figure 9 shows the change in v o l u m e of the w e a r track f r o m this test. For all specimens, there is an initially large v o l u m e t r i c change, w h i c h can be attributed to creep of the plastic. This is followed by reduced volumetric

Fig. 7. Chlorosulfonic acid-treated thin sections through ultrahigh-molecular-weight polyethylene sample that was aged for 8 days prior to wear testing for 20,000 sliding cycles. Note the catastrophic loss of polyethylene (A) and the cracks that propagate through the subsurface oxidized band (B).

wear, w h i c h can be attributed to the loss of material. Even so, after only 200,000 cycles, a significant difference in the v o l u m e of the w e a r track b e t w e e n control and samples aged for different lengths of time can be demonstrated. The volumetric change in the w e a r track is significantly larger with increased aging. Figure l0 shows the weight loss of UHMWPE for the samples that w e r e aged for varying periods. Initially, there was a significant increase in w e a r rate as m e a s u r e d by weight loss for samples aged 6 days c o m p a r e d with control samples and specimens aged for 2 and 4 days. In this test, samples did not develop a subsurface b a n d until 6 days of aging and, consequently, d e l a m i n a t i o n did not occur on specimens aged less t h a n 6 days. The higher gravimetric w e a r rate observed within the first 200,000 cycles for samples aged 4 and 6 days can be attributed to the w e a r of the highly oxidized surface

Fig. 8. Chlorosulfonic acid-treated thin section through ultrahigh-molecular-weight polyethylene sample that was aged for 6 days prior to wear testing for 200,000 sliding cycles. Note the formation of small subsurface cracks within the subsurface oxidized band. This band is less evident than that seen on specimens aged for 8 days. The crack at this time has not propagated through to the surface of the component, which had not delaminated.

286

T h e J o u r n a l of A r t h r o p l a s t y Vol. 13 No. 3 April 1998 150

o

a---•

E =

~

100

>

~

........~

Untreated 4 days

............~ - -

6 clays

............~'~'~

8 days

50-

0"7

.

0

200000

.

.

.

400000

600000

Number

of

800000

Cycles

Fig. 9. Volumetric change during pin-on-plate wear test Ior aged and unaged specimens.

layer. Thin sections taken t h r o u g h the wear track after 200,000 cycles show h o w this oxidized surface layer has w o r n through.

Mechanical Testing Results from tensile testing (Fig. I1) s h o w e d a significant decrease in ultimate tensile strength with increased aging. Specimens oxidized for 8 days were e x t r e m e l y brittle and broke w i t h o u t a region of plastic flow. A significant increase was seen for

strain at break with 4- and 6-day aged specimens compared with the n o n a g e d specimens (P < .05). The 8-day specimens had significantly lower strain at break t h a n all other groups (P < .05) (Fig. 12). Rotating b e a m fatigue tests s h o w e d a significant reduction in fatigue life with increasing oxidation time in all groups (Fig. 13). The typical fatigue failure appearance of fractured surfaces with initiation sites, beach marks, and overload regions was evident w h e n investigated u n d e r the scanning electron microscope (Fig. I4).

0.05 -

0.04 -

.~

Fig. 10. Average weight loss of aged and unaged specimens in the pin-on-plate wear test.

0.03

O _J

E: ._~

......... 0.02

O

0.01

0.00

-0.01

i

i

200000 400000 Number of Sliding Cycles

J

600000

Untreated Oxidized 2 days Oxidized 4 days Oxidized 6 days

Effect of Oxidation on Delamination of Polyethylene •

Bell et al.

287

Z

=

600

C ~•

500

•

400

C

•"

300

E

200

=

100

~

0 •

IRRADIATEDONLY

[]

OXIDIZED 4 DAYS

[]

OXIDIZED8 DAYS

Fig. I I . Ultimate tensile strength of ultrahigh-molecular-weight polyethylene specimens aged for 0, 4, and 8 days in various states of oxidation.

Fatigue crack growth rates could not be obtained for 8-day oxidized specimens, as these were so brittle they fractured at the points of loading. Comparison of irradiated and 4-day oxidation showed a statistically significant increase in crack growth rate

for both PolyHi (P < .026) andRCH 1000 (P < .006) compared with nonaged control specimens. Comparison of nonaged (control) RCH I000 with nonaged Poly Hi showed the latter to have a significantly slower crack growth rate (P = .12). This

4

3 i--

2

E

, m m

0

0

96

144

192

TIME (HRS) Fig. 12. Effect of oxidation on ultimate strain of ultrahigh-molecuIar weight polyethylene.

288

The Journal ofArthroplasty Vol. 13 No. 3April 1998 300000-

250000"

200000

o

150000

E =

Z

100000'

50000

0 Irradiated

Oxidized 96 hours

Oxidized 192 hours

Fig. I3. Effect of aging on the fatigue Iife of ultrahigh-molecular-weight polyethylene.

difference was m o r e a p p a r e n t once specimens w e r e oxidized, w i t h Poly Hi having a crack g r o w t h rate a p p r o x i m a t e l y an order of m a g n i t u d e less t h a n that of RCH 1000 (P < .016) (Fig. 15).

Discussion Oxidation of UHMWPE sterilized in air by g a m m a irradiation has b e e n s h o w n to increase with implantation time. The effects of ionizing irradiation on

Fig. 14. Scanning electron micrograph of fatigue-fractured (Wholer [G. T. G. Engineering, Loughborough, UK] rotating beam test) surface of 6-day-aged specimen showing beach marks and intitiation sites.

oxidation of p o l y m e r s is well understood. Sterilizing p o l y e t h y l e n e by g a m m a irradiation leads to scission of the long m o l e c u l a r chains and p r o d u c t i o n of free radicals within the p o l y m e r matrix. D e p e n d i n g on the e n v i r o n m e n t , these free radicals either crosslink the p o l y m e r or react with oxygen, resulting in degradation of the material. Diffusion of o x y g e n into the p o l y m e r matrix a n d diffusion of free radicals out of the p o l y m e r are t i m e - d e p e n d e n t processes [17,18], and u n d e r certain conditions, this can lead to the d e v e l o p m e n t of a region of high subsurface oxidation that contours the shape of the p o l y e t h y l e n e c o m p o n e n t [11]. In our study a n d in other studies, this region of highly oxidized plastic occurs at a d e p t h of a b o u t 0.5-1.5 m m b e l o w the surface. This is particularly i m p o r t a n t for k n e e arthroplasties because the m a x i m u m shear stresses coincide with the region of m a x i m u m oxidation. The objective of this study was to discover w h e t h e r the d e v e l o p m e n t of a subsurface oxidized b a n d w o u l d lead to d e l a m i n a t i o n of tibial c o m p o n e n t s . Several studies h a v e tried to correlate increased oxidation of p o l y e t h y l e n e acetabular cups with an increase in the w e a r rate. N u m e r o u s studies h a v e s h o w n that the in vivo w e a r of acetabular cups, as m e a s u r e d b y p e n e t r a t i o n of the femoral head, is linear after the first 1 or 2 years, indicating that an increase in w e a r rate due to t i m e - d e p e n d e n t oxidation does not occur. In addition, Li et al. f o u n d n o correlation of in vivo w e a r of acetabular cups with the presence of a subsurface oxidized b a n d in the

E f f e c t of Oxidation on Delamination of Polyethylene

•

Bell et al.

289

4.00e-3 =---=,

o

,=

3.00e-3

.....

\ \

Fig. 15. Crack growth rates during cyclic loading in compact tension specimens for aged and unaged RCH 1000 (GUR 412) and Poly Hi (GUR 415) ultrahigh-molecular-weight polyethylene.

o-~

Irradiated R C H 1 0 0 0

OxidizedPoly Hi

~

-

irradiated P01y Hi

Oxidized RCH 1000

\

k.

2.00e-3

-

1.00e-3

-

o

==

O.OOe+O

i

O

O O O

0 Q Q

0 0 Q

0 O 0

O 0 0

0 0 0 CO

Number of Cycles

same specimens after retrieval [12]. Fisher et al. s h o w e d an increased surface w e a r rate in acetabular cups that w e r e shelf-stored but did not d e m o n s t r a t e d e l a m i n a t i o n [19]; however, Sutula et al. indicated that in retrieved acetabular cups, cracking of these c o m p o n e n t s was associated w i t h a subsurface oxidized b a n d [11]. Our study shows that oxidation of UHMWPE tibial c o m p o n e n t s can be caused either by stresses i m p o s e d during e v e r y d a y activity or by postirradiation changes leading to the d e v e l o p m e n t of this subsurface band. In the majority of retrieved samples investigated in this study, d e l a m i n a t i o n cracks p r o p a g a t e d t h r o u g h this band. This observation f r o m retrieved specimens was s u p p o r t e d b y artificially aging UHMWPE, w h i c h induces a subsurface oxidized b a n d that resulted in the d e l a m i n a t i o n of plastic in w e a r tests that simulated the sliding conditions of TKAs. In a g r e e m e n t w i t h data presented by Sanford and S a u m [131, we h a v e s h o w n that artificial aging for a shorter period produces surface oxidation. In the w e a r test, we s h o w e d that d e l a m i n a t i o n of c o m p o n e n t s occurred only w h e n the subsurface oxidized b a n d was present a n d that d e l a m i n a t i o n cracks p r o p a g a t e d t h r o u g h this band. In samples in w h i c h UHMWPE was oxidized at the surface (in specimens aged for short periods of time), an increase in surface oxidation was apparent, producing increased surface w e a r but w i t h o u t delamination. Our results h a v e s h o w n that increasing oxidation b y artificially aging c o m p o n e n t s reduces the fatigue life of polyethylene. Without subsurface oxidative degradation, delamination does not occur in w e a r tests, but f r o m our fatigue tests, it is still possible to induce fracture in n o n o x i d i z e d samples. This suggests that in terms of d e l a m i n a t i o n

failure, oxidation of the plastic reduces the operational limits of the material. In addition, the tensile strength of the p o l y e t h y e l e n e is reduced with increasing oxidation, w h i c h also appears to significantly embrittle this material. Work by Rimnac et al. has s h o w n that UHMWPE m a n u f a c t u r e d by different processes propagates fatigue cracks at different rates [20]. Our study also shows that different UHMWPEs m a y be m o r e susceptible to oxidation, resulting in increased fatigue crack p r o p a g a t i o n and reduced life in some materials. As the artificial aging technique does not a p p e a r to alter the crystallinity of the plastic in the subsurface region similar to changes seen in some retrieved TKAs, it m u s t be concluded that oxidation per se is responsible for reducing the fatigue life of UHMWPE. A n u m b e r of studies h a v e investigated other m e t h o d s of sterilization that reduce the oxidation of the plastic [ 2 ] - 2 3 ] . These either are alternative m e t h o d s of sterilization or involve g a m m a irradiation of p o l y e t h y l e n e u n d e r conditions that reduce oxidation and favor crosslinking. In addition, postirradiation t r e a t m e n t , such as annealing [24] of UHMWPE, has b e e n s h o w n to further reduce the generation of free radicals while p r o m o t i n g crosslinking. These m e t h o d s reduce the d e v e l o p m e n t of this subsurface oxidized b a n d in polyethylene, w h i c h w o u l d m e a n a reduction in the incidence of delamination w e a r in tibial c o m p o n e n t s . It is envisaged that research will shift f r o m the p r e v e n t i o n of d e l a m i n a t i o n w e a r in k n e e arthroplasties to the p r e v e n t i o n of surface-associated wear. These problems m a y well be similar to those faced in the

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d e v e l o p m e n t of w e a r - r e s i s t a n t s u r f a c e s for t o t a l h i p arthroplasties.

Acknowledgment W e t h a n k t h e E n g l i s h M e d i c a l D e v i c e s A g e n c y for t h e c o n t r i b u t i o n t h e y h a v e m a d e to this study.

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