Engineering Failure Analysis 18 (2011) 1415–1423

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Engineering Failure Analysis

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Failure analysis of retrieved UHMWPE tibial insert in totalknee replacement

S. Liza a, A.S.M.A. Haseeb a,⇑, A.A. Abbas b, H.H. Masjuki a

a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Department of Orthopaedic Surgery, Faculty of Medicine Building, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 22 November 2010Received in revised form 7 April 2011Accepted 9 April 2011Available online 22 April 2011

Keywords:PolyethyleneFailure analysisTotal knee replacementWearDelamination

1350-6307/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.engfailanal.2011.04.001

⇑ Corresponding author. Tel.: +60 3 7967 5204; faE-mail address: haseeb@um.edu.my (A.S.M.A. Ha

a b s t r a c t

This study involves the failure analysis of an ultra high molecular polyethylene (UHMWPE)tibial insert from Apollo� Total Knee System, which was removed after 10 years of servicefrom 70 years old female patient. The tibial insert was investigated by using a stereoscope,scanning electron microscope (SEM), infinite focus microscope (IFM) and energy dispersespectroscopy (EDS) to characterize the morphology and composition of the bearing surface.Differential scanning calorimetry (DSC) and Fourier transform spectroscopy (FTIR) wereemployed to characterize the degradation and crystallinity of the component. Gel-perme-ation chromatography (GPC) was used to measure the polyethylene tibial insert molecularweight. Results showed that the failure of total knee replacement (TKR) was associatedwith high grade wear and oxidation degradation. Surface delamination, scratch marks, pit-ting, folding, and embedded third body particles were observed on the retrieved UHMWPEtibial surface. Pit depth as large as 60 lm was measured on the surface. The damage fea-tures observed on the UHMWPE tibial insert suggested degradation is due to fatiguerelated wear and is oxidation-induced. Overall results show that the UHMWPE tibial insertwhich was retrieved from a patient who is active and but not overweight underwent deg-radation of material properties and high grade wear during 10 years of service.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Ultra high molecular weight polyethylene (UHMWPE) has been used as a load bearing and articulating counterface in to-tal knee replacement (TKR) for the past 40 years [1]. Polyethylene wear is considered a major limitation of long term successof TKR [2,3]. Analysis of retrieved polyethylene components has been carried out by a number of researchers to reveal theevidence of the TKR failure [4–9]. Their work has helped to characterize and analyse the failure due to wear of polyethylenetibial insert against metal femoral component. Recent studies have dealt with a number of polyethylene tibial inserts fromthe TKR. The groups of retrieved tibial insert were analyzed by using Hood’s grading scale system. The surface of tibial insertwas divided in 10 regions and the wear damage on the surface was measured based on the extent and severity of the sevendamage modes (pitting, scratching, burnishing, embedded particulate debris, abrasion, permanent deformation and surfacedelamination). Then, the total damage was achieved by sum the score for all seven modes across all the ten regions. The re-sults then were compared by considering different factors which could cause their failure.

Previous study by Engh et al. [4] compared the wear mode of two different designs of tibial insert, namely mobile andfixed bearing knee inserts. They found that TKR with increased contact areas between metal and polyethylene componentscan result in reduced contact stresses and decreased wear. Ashraf et al. [10], in their retrieval analysis, determined the wear

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1416 S. Liza et al. / Engineering Failure Analysis 18 (2011) 1415–1423

rate of 18 retrieved polyethylene tibial inserts by measuring the linear penetration and volumetric wear using a coordinatemeasuring machine. They found a linear relationship between both the volumetric and linear penetration with the time ofimplantation.

Apart from retrieval study in a group, there was also a case study on one retrieved polyethylene tibial insert. Diabb et al.[11], investigated the cause of failure of a 6 month implanted tibial inserts. In their study, they focused more on the changesof the properties of polyethylene that lead to the failure of polyethylene tibial insert.

The present study attempted to establish the causes that led to the surface damage of polyethylene tibial insert from aknee prosthesis which was retrieved after 10 years in vivo. The patient was an elderly housewife who was a communityambulator, and was active and not overweight. Analysis on the worn surface polyethylene will provide information aboutthe mechanism and process of the polyethylene degradation after 10 years of service.

2. Material and methods

2.1. Retrieved specimens

In 1999, a 70 years old female patient received a fixed bearing, posterior stabilized Apollo� Total Knee System (SulzerOrthopaedics, Austin, Texas) for the treatment of severe osteoarthritis of the right knee. Ten years postoperatively, the pa-tient returned to the clinic complaining of pain which was increasing in severity. She also complained of increasing insta-bility and deformity of the knee during walking. Radiographs of the knee performed at this time showed that the tibialtray was in a severe varus position, with reduction of the medial joint space (Fig. 1a). The right total knee replacementwas revised shortly after the consultation. Intra-operatively, the tibial prosthesis was found to be loose. There was no evi-dence of infection. The failed knee replacement was replaced with the new implant as shown in Fig. 1b. Fig. 2 shows theretrieved knee prosthesis consisting of three components which are femoral component, UHMWPE tibial insert and tibialtray. The femoral component and tibial tray were made of cobalt–chromium (CoCr) alloys and tibial insert was made fromhighly cross-linked UHMWPE.

2.2. Surface degradation evaluation

Surface degradation features of UHMWPE component were evaluated using stereomicroscope, scanning electron micro-scope (Quanta 200 FESEM, FEI) and Alicona infinite focus microscope (IFM). IFM not only provided high resolution topo-graphical and true colour information but could also measure the roughness average (Ra) of the polyethylene. Acomparison of the Ra values from both lateral and medial regions was done. Scanning electron microscope (Quanta FEG-SEM with Oxford EDX), used to investigate the worn surface characterization of the retrieved sample. Energy dispersiveX-ray (EDX) microanalysis was performed to find out the component of the surface features.

2.3. Oxidation characterization

Fourier transform infrared (FTIR) spectroscopy was performed on a Perkin-Elmer Spectrum 100 FTIR Spectrometer.FTIR was used to characterize the extent of oxidative degradation in the polyethylene tibial insert. The sample for FTIR

Fig. 1. Radiographs of the right knee of the patient (a) at presentation 10 years after primary total knee replacement (arrow pointing at decreased jointspace at medial joint line with tibial tray in varus position) and (b) anteroposterior and lateral radiographs of the right knee post revision knee replacement.

Femoral Component

UHMWPE Tibial Insert

Tibial Tray

Fig. 2. Knee prosthesis components after retrieval from the patient (cleaned).

S. Liza et al. / Engineering Failure Analysis 18 (2011) 1415–1423 1417

characterization was prepared by cutting the UHMWPE sample into approximately 1 mm thin section. The FTIR measure-ments were performed on two distinct regions of the thin section. These are the oxidized surface layer and the oxidized bulkregion. Scans of thin sections were acquired at 2 cm�1 intervals from 600 cm�1 to 4000 cm�1.

2.4. Crystallinity measurement

The crystallinity of the UHMWPE tibial insert was measured using differential scanning calorimeter (DSC) (DSC 820,Mettler-Toledo Inc., Columbus, OH). The sample for DSC characterization was prepared by using a 5 mm inner diameterserrated coring bit at the weight bearing region. Heating runs were conducted from 30 �C to 200 �C at rate of 10 �C/minon samples weighing about 5.6 mg. The sample was sealed in an aluminium sample pan and used for the measurementin the standard DSC mode. Sample crystallinity was determined by comparing the total heat of melting DH (the area underthe endotherm) to the total heat of fusion of fully crystalline UHMWPE (DHf = 293 J/g) [12]. The percentage crystallinity wascalculated as 100 DH/DHf.

2.5. Molecular weight measurement

Gel-permeation chromatography (GPC) analysis (ALLIANCE GPCV-200) equipment was made in order to measure thepolyethylene molecular weight of the tibial insert. Molecular weight analysis was conducted with a 50 mg of polyethylenedissolved in trichlorobenzene at 140 �C.

3. Results

The wear damage analysis of the articulating surface of the UHMWPE tibial insert showed different types of features.Fig. 3b and c displays the wear surface of retrieved UHMWPE at 40� magnification under the stereoscope. Light scratchingmarks, oriented in the vertical direction in the micrographs are noted on both the lateral and medial compartments.

Further observation was done in the SEM and the micrographs are shown in Fig. 4. Close observation of the surface atdifferent areas revealed a number of features. Scratches (Fig. 4a), pitting (Fig. 4b), folding (Fig. 4c), and surface delamination(Fig. 4d) can be observed as a result of the wear of the component during service. Such features were seen all over thesurface.

As can be seen from Fig. 4c, there were some ‘white particles’ scattered on the worn surface. The EDX analysis (Fig. 5)confirmed the presence of Ca and P in these particles. Since hydroxyapatite Ca10(PO4)6(OH)2 is an important componentof the human bone [13], the presence of Ca and P in these particles suggests that these particles are bone fragments.

1 cm1 cm

(a)

(b) (c)

Fig. 3. (a) General view of UHMWPE and optical microscopic images of the surface of tibial insert, (b) lateral compartment and (c) medial compartment.

Delamination

(a)

Pit

(b)

(c) (d)

Fig. 4. SEM images of UHMWPE tibial plate surface indicating different features of wear damage: (a) scratches, (b) pits, (c) folding and (d) delamination.

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Images of the sample surface were taken by using infinite focus microscope (IFM) (Fig. 6). These images (Fig. 6a and b)revealed the occurrence of the features described above in a more clear way. In addition, IFM also allows the quantificationof surface data and provide the dimensional surface profiles (Fig. 6c and d). For example, deep pits were revealed at the bot-tom right corner of Fig. 6c and upper left corner of Fig. 6d. Pit depths as large as 60 lm and 35 lm were obtained.

The surface roughness of UHMWPE tibial component was also measured by IFM. The surface roughness data, which arelisted in Table 1, suggest that there is no significant difference between lateral and medial roughness.

Fig. 5. (a) SEM image showing white particles scattered on the worn surface of UHMWPE tibial insert, (b) a typical EDX spectrum on the white particlesindicating the presence of calcium (Ca) and phosphorus (P).

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Fig. 7 shows the FTIR spectra taken on material at the surface and bulk region of the UHMWPE samples. There was nosignificant difference between the two regions. Prominent peaks due to absorption at 2915 and 2848 cm�1 (CH2 stretching),1462 and 1471 cm�1 (CH2 and CH3 bending) and 719 and 730 cm�1 (linear chain of at least 4 CH2 groups) are clearly visiblein Fig. 7. Additionally, there are a number of small absorption peaks at surface region which can be assigned to oxidationproducts. These are carboxylic acid (C@O) at 1697.03 and 1300.69 cm�1, ketone (C@O) at 1716 cm�1, ester (C@O) at1748.26 cm�1 and alcohol (C@O) at 3361.09 cm�1 [14,15]. The band at 1166.54 cm�1 can be mainly assigned to PO�4

[13]. This suggests that the sample contained a small amount of bone fragment which was indicated previously by EDXanalysis.

In the present study, the crystallinity and molecular weight measurement of the UHMWPE tibial insert were carried outin order to determine whether there was a change after the insert was implanted for 10 years. Fig. 8 shows the melting endo-therm for the retrieved UHMWPE tibial insert. The heat of fusion for this sample was 164.85 J/g. It was obtained by integrat-ing the area under the endothermic peak. The value of the average crystallinity obtained from DSC of the UHMWPE tibialinsert was 56.6%.

Finally, the molecular weight of the retrieved UHMWPE tibial insert was measured by gel-permeation chromatography(GPC). Fig. 9 shows the distribution of molecular weight of UHMWPE tibial insert after the tibial insert was implanted for10 years. The molecular weight (Mw) of UHMWPE tibial insert was 130,104 g/mol.

4. Discussion

Orthopaedic literature contains many articles describing the clinical performance of knee replacement with various de-signs and materials [4–8]. According to Hood et al. [6], there are seven surface wear damage modes on the articulating sur-face of retrieved tibial insert. The wear damage modes are pitting, scratching, burnishing, embedded particulate debris,abrasion, permanent deformation and surface delamination. In the present study the most common features on the articu-lating surface of the retrieved polyethylene tibal insert was delamination. This was followed by scratches, pitting, folding,and third body particles (bone fragment). Ho et al. [16] suggested that high-grade wear consisted of pitting, scratching,and delamination. Thus, the wear damage on the 10 years implanted UHMWPE tibial insert in this current study can be clas-sified as a high-grade wear.

Delamination and pitting which are related to fatigue wear were reported by Bradford et al. [17] as the most commonfeature of damage for total knee replacement. Medel et al. [18] suggested that the fatigue wear mechanism was predominantin the knee implant system due to lower conformity and higher contact stress between the femoral and tibial component.Studies show that the formation of large flakes of wear debris known as delamination was initiated by cracks from the cyclicstress at the contact surface of polyethylene [19]. Similarly, Burnett et al. [20] suggested that the fatigue wear modes (delam-ination and cracking) are resulted from the combination of higher stresses and reduced mechanical properties. They also re-ported that the reduction in mechanical properties such as loss of toughness is due to oxidation. It has been observed that theoxidative degradation of polyethylene can lead to a decrease in wear resistance and mechanical properties [15,21,22].

Collier et al. [8] observed that delamination is correlated with zone of high oxidation. They found that subsurface zonewhich highly oxidized could be related to the delamination. They proposed that the reduction in mechanical properties suchas decrease in tensile strength and low elongation, led to the formation of visible subsurface white zone and resulted delam-ination and cracking. In the present study, the presence of oxidation product such as acid carboxylic, ketone, ester and alco-hol from FTIR analysis confirms that oxidation degradation had occurred in the UHMWPE insert. Repetitive cyclic loading onthe insert during daily activities over 10 years have resulted in the delamination and pitting in the present retrieved sample.

Table 1Surface roughness of wear UHMWPE component surface.

Top (lm) Left (lm) Bottom (lm) Right (lm) Average

Lateral 1.931 1.753 1.242 1.722 1.662Medial 1.843 2.204 1.343 1.454 1.711

Fig. 6. IFM images taken on wear surface of UHMWPE tibial inset; (a and b) surface images and (c and d) 3D surface images corresponding dimensionalsurface profiles.

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1166

.54

cm-1

1300

.69

cm-1

1697

.03

cm-1

1716

cm-1

1748

.26

cm-1

3361

.09

cm-1

1166

.85

cm-1

1300

.51

cm-1

1697

.24

cm-1

1748

.67

cm- 1

3361

.29

cm-1

1716

.2cm

-1

Surface region

Bulk region

1716

cm-1

Fig. 7. IR spectrum recorded on the surface and bulk region of UHMWPE samples.

Fig. 8. A plot of heat flow versus temperature for UHMWPE tibial insert.

Fig. 9. The distribution of molecular weight of UHMWPE tibial insert after being implated for 10 years.

S. Liza et al. / Engineering Failure Analysis 18 (2011) 1415–1423 1421

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The gamma sterilization method has significant effect on delamination of the polyethylene surface. Severe delaminationwas found on the retrieved polyethylene surface with gamma radiation in air and in nitrogen [8,23]. No delamination wasfound on the gas plasma sterilized polyethylene insert [4]. The degradation does not occur immediately after sterilization butover time. It has been reported that degradation occurs over a period prior to implantation (shelf ageing) and in vivo services[21,24]. Sterilization by gamma irradiation in air has been shown to have the potential to accelerate the oxidation of poly-ethylene components [25]. It is suggested that gamma sterilization have played a role in the degradation of the retrievedinsert.

A correlation between implantation time and presence of delamination was found by Collier et al. [8]. Retrieved sampleswith short implantation duration, less than 4 years, displayed signs of delamination or cracking on 17–27% of cases. In con-trast, 65% of the retrieved tibial components displayed a signs of delamination or cracking for durations greater than 4 years.

Apart from delamination, pitting, scratching and folding were also observed in the present sample. Garcia et al. [5] havefound that pitting and scratching were the most predominant damage modes on the surface damage of 40 rotating-platformTKAs. Third body particles have been reported by Crowninshield et al. [7] and Muratoglu et al. [26] as a main cause of for-mation multiple surfaces folding, pitting and scratching. Crowninshield et al. [7] found that the third body particles wereembedded in and dislodged from the polyethylene surface, causing the formation of pitting during the motion and loadingat knee prosthesis. In addition, scratches were formed when third body particles were pushed along the articular surface andleaving tracks of deformation within the polyethylene surface. Hirakawa et al. [27] suggested that small debris particlesattributed to the formation of scratches and larger debris particles which produce by polyethylene fatigue attributed tothe delamination formation. These findings clearly showed the formation of surface damage modes were related to eachother.

The absence of surface burnishing on the UHMWPE surface distinguishes this work from the previous reports. The ab-sence of burnishing wear can be caused by the design of knee implant. Engh et al. [4] have found that the knee implant withfixed bearing system have small percentage of the surface being burnished when compared with the mobile bearing systemdue to small contact area. In this present study, the Apollo� Total Knee system which is fixed bearing system did not exhibitburnishing on the polyethylene surface.

In the present study, the crystallinity and molecular weight measurement of the UHMWPE tibial insert were carried outin order to determine whether there is a change after the insert was implanted for 10 years. Kurtz [28] suggested that thedegree of crystallinity for medical grade polyethylene should be within 39–75%. Thus in this study, the crystallinity fallswithin the suggested range. Since the crystallinity data of UHMWPE tibial insert prior to implantation was also not available,no comment can be made on the role of crystallinity on the performance of the present implant.

However, it is expected that there is a change in the molecular weight of UHMWPE after implanted for 10 years. In thepresent case, the molecular weight of UHMWPE was measured as 130, 104 g/mol. This result is approximately ten times low-er than the actual average molecular weight of a medical grade polyethylene which is 1.5 � 106 g/mol [28]. Diabb [11] alsoreported on the reduction of molecular weight after retrieval. They found that the molecular weight of the polyethylene tib-ial insert just after 7 months was 166,855 g/mol. The degradation in molecular weight of retrieved UHMWPE tibial insert inthe present case suggest that polyethylene properties have changed. This is believed to have contributed to the weardamage.

5. Conclusions

The following conclusions can be drawn from the present study.

� High-grade wear and oxidation degradation are associated with the failure of the 10 years implanted UHMWPE tibialinsert.� The most prominent wear damage mode identified in the retrieved tibial insert is delamination. This is followed by pit-

ting, folding and scratching. The pit depth as large as 60 lm was measured on the surface. Third body particles also occuron the insert surface.� The retrieved UHMWPE tibial insert showed low molecular weight, indicating that polyethylene properties have changed

in vivo which contributed to wear damage.

Acknowledgements

The authors would like to thank University Malaya Medical Centre (UMMC) for supplying the sample. This research wassupported by Postgraduate Research Grant (PPP), Grant No: PS079-2009B from University of Malaya.

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