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The Journal of Arthroplasty Vol. 26 No. 8 2011

Posterior Condyle Surface Damage on RetrievedFemoral Knee Components

Colin D.C. Burnell, BSc, MD, FRCSC,* Jan-M. Brandt, Dipl.-Ing., PhD,*Martin J. Petrak, MSc, PEng,* and Robert B. Bourne, MD, FRCSCy

Abstract: Twenty-two retrieved femoral knee components were identified with posterior condylesurface damage on average at 99° flexion (range, 43°-135° flexion). Titanium alloy materialtransfer and abrasive surface damage were evident on cobalt-chromium alloy femoral componentsthat were in contact with titanium alloy tibial trays. Surface damage on the retrieved Oxiniumfemoral components (Smith and Nephew, Inc, Memphis, Tenn) that were in contact with titaniumalloy tibial trays showed gouging, associated with the removal and cracking of the oxide andexposure of the zirconium-niobium alloy substrate. Cobalt-chromium alloy femoral componentsthat were in contact with cobalt-chromium alloy tibial trays showed abrasive wear. Contactbetween the femoral component and tibial tray should be avoided to prevent surface damage tothe femoral condyles, which could potentially accelerate polyethylene wear in vivo. Keywords:retrieval analysis, total knee arthroplasty, surface damage, wear, polyethylene, Oxinium.© 2011 Elsevier Inc. All rights reserved.

Total knee arthroplasty is well known to provideexcellent pain relief and improvement in quality of lifefor patients with arthritis. With increasing life expec-tancy and the expansion of indications to includeyounger, more active patients, it has become essentialto maximize the longevity of such implants [1-3].However, the longevity of total knee arthroplasties hasbeen limited due to osteolysis induced by polyethylene(PE) wear particles in combination with changes in fluidpressure around the implant interface [4-5]. Efforts toimprove implant survival have largely been focused onimproving component fixation, the quality of the PEinsert [6], and the tibial tray locking mechanism [7]. As aresult, much of the literature on the in vivo performanceof total knee arthroplasty components has been focusedon wear patterns of the PE, with some reports on thedamage that can occur to the bearing surface of thefemoral component during in vivo use [3,8-11].

e *Concordia Joint Replacement Group, Concordia Hip and Kneeinnipeg, Manitoba, Canada, R2K 2M9; and yDivision of

c Surgery, London Health Sciences Centre, University of Westernndon, Ontario, Canada, N6A 5A5.ted May 31, 2010; accepted March 4, 2011.nflict of Interest statement associated with this study can beoi:10.1016/j.arth.2011.03.011.requests: Jan-M. Brandt, Dipl.-Ing., PhD, Concordia Jointnt Group, Concordia Hip and Knee Institute, Suite 310-cordia Ave, Winnipeg, Manitoba, Canada, R2K 2M9.Elsevier Inc. All rights reserved.

403/2608-0054$36.00/0016/j.arth.2011.03.011

146

Surface damage on the femoral bearing surface canincrease the surface roughness and accelerate PE wear[12-16], possibly promoting osteolysis and makingexpensive revision surgery necessary [17]. In an effortto address the issue of counterface roughness, the use offemoral components composed of a wear-resistantoxidized zirconium-niobium alloy has been introduced(Oxinium [OxZr]; Smith and Nephew, Inc, Memphis,Tenn) as an alternative to contemporary cobalt-chromium(CoCr) alloy femoral components. Through a thermaldiffusion process in heated air, the surface of thezirconium-niobium alloy (Zr-2.5%Nb; ASTM F-2384) istransformed into an oxide ceramic with a gradienthardened surface underneath, imparting to the implantthe beneficial wear properties of a ceramic componentwith less risk of fracture as seen in full ceramiccomponents [18-25]. The oxide represents a wear-resistant surface transformation, rather than a wear-resistant coating and, thus, is suggested to be resistanttoward both chipping or delamination [26]. The oxide ofOxZr is about 5 μm thick and exhibits high wettability andhigh nanohardness, which decreases adhesive wear andabrasive wear at the interface, respectively [26-28].Although the oxide has a high hardness, the underlyingzirconium-niobium alloy substrate is comparatively soft,with a Young modulus less than half than that of CoCralloy (approximatley 98 GPa vs approximately 230 GPa).When scratchedwith third-body abrasive particles, suchasbone cement debris, CoCr alloy surfaces tend to form atypical peak- and valley-type deformation of the surface.In contrast, OxZr has been shown to form mainly valleys

0

Fig. 2. (A) Typical surface damage observed on the retrievedfemoral component of the retrieved Oxinium femoral compo-nents from the Genesis II knee system after being in contactwith a Ti alloy tibial tray (OxZr/Ti). (B) Schematic indicatingthe flexion range where the surface damage was observed on

Surface Damage on Retrieved Femoral knee Components � Burnell et al 1461

under in vitromicroabrasivewear conditions [29-30]. Theabsence of material buildup along the edges of thescratches has translated into reduced PEwear in simulatortesting when compared with similarly roughened CoCralloy bearing components [29-30]. Retrieval analysis ofOxZr femoral heads that were revised for dislocation hasshown that contact of the head with the margin of thetitanium (Ti) alloy acetabular cup has the potential tocreate significant damage to the oxide, with plasticdeformation of the underlying substrate and a moretypical peak and valley surface damage profile [31-34].During implantation of an OxZr femoral component, it

was noticed by one of the authors (C.B.) that apparentsurface damage on the posterior condylar region hadoccurred at the time of insertion of the PE insert whenthe tibial tray came in contact with the femoralcomponent as the tibia was subluxed anteriorly at highflexion. In fact, such unintended contact was identifiablein the technique manual of one of the implant systems(Fig. 1) and was deemed rather typical for all total kneesystems rather than being specific to only 1 manufac-turer. The surface damage was clearly visible due to thecontrast in color between the oxide (black) and theunderlying substrate material (gray). Most of theapparent surface damage on the femoral condyles wasidentified macroscopically as mainly scratching andgouging (Fig. 2A). Further inspection showed similarsurface damage to be present on retrieved CoCr alloyfemoral components, although the surface damageappeared less visible due to the lack of contrast at thehighly polished CoCr surface.

Fig. 1. Arrow pointing toward the possible contact thatoccurred between the femoral component and the tibial trayduring primary knee arthroplasty, consequently causing thesurface damage on the femoral bearing. This type of surfacecontact was rather typical for all total knee systems thanspecific to only 1 manufacturer. Adapted from the PrimarySurgical Technique Guide for the Genesis II total knee system(Smith and Nephew, Inc).

the retrieved femoral components. The starting point of surfacedamage on the medial and lateral condyles was referred to as“lower flexion”; the end point of surface damage on the medialand the lateral condyle was referred to as “higher flexion.”Both points “lower flexion” and “higher flexion” weremeasured on each retrieved femoral component using aprotractor and, thus, defined the range of surface damage.

The aim in this retrieval study was to characterize thetype of surface damage on the posterior condyle regionof femoral components of total knee arthroplasties usingscanning electron microscopy and surface profilometry.We hypothesized that unintended contact between thefemoral condyles and the posterior aspect of the tibialtray may generate damage features that could poten-tially have adverse effects on PE wear in vivo.

Materials and MethodsFifty femoral components were retrieved from 50

consecutive total knee revisions surgeries that wereperformed by 4 different surgeons at the Concordia Hipand Knee Institute (Winnipeg, Manitoba, Canada) and 1surgeon at the London Health Sciences Centre (London,Ontario, Canada). The retrievals were visually inspectedto determine the frequency of surface damage on the

1462 The Journal of Arthroplasty Vol. 26 No. 8 December 2011

posterior condyles. Surface damage at the periphery ofthe femoral condyles surfaces was ignored, as suchdamage was deemed likely to represent retrieval artifact.Consequently, 22 retrieved femoral components from22 patients were identified with posterior condylesurface damage and were then selected for furtherretrieval analysis.The selected 22 femoral components were of 3

different knee systems; there were 5 retrievals fromthe NexGen total knee system (Zimmer Inc, Warsaw,Ind), 12 retrievals from the Genesis II total knee system(Smith and Nephew, Inc), and 5 retrievals from theDuracon total knee system (Stryker Inc, Mahwah, NJ).The Genesis II femoral components were either made ofcast CoCr alloy (ASTM F75) or of OxZr. The NexGenknee system and the Genesis II knee system both camewith a cast Ti alloy tibial tray (ASTM F1472). Thefemoral component and the tibial tray of the Duracontotal knee system were both manufactured from CoCralloy (ASTM F75).Patient and relevant data were compiled from patient

records and the Ortech Online Database [35]. Theprimary diagnosis was osteoarthritis in 19 patients andrheumatoid arthritis in 1 patient; records were unavail-able for 2 patients. There were 9 male patients and 13female patients. Of the 22 femoral components, 15 werefrom right knee arthroplasties, and 7 were left kneearthroplasties. The mean implantation period for theimplants was 28.6 months (range, 4.5-99.2 months).The mean patient body mass index at time of revisionsurgery was 35.95 kg/m2 (range, 24.5-47.7 kg/m2).Reason for revision included instability (n = 6), stiffness(n = 5), infection, (n = 5), aseptic loosening (n = 2), postfracture, severe arthrofibrosis, patella maltracking, andpainful malalignment. None of the components wererevised because of wear-related failure of the PE insert orosteolysis. The primary total knee arthroplasties wereperformed by 11 different surgeons. There were 4 PEinserts of cruciate-retaining design, 17 of a standardposterior-stabilized design, and 1 of a posterior-stabilized,high-flexion design. The range of motion before revisionsurgery was available for 15 patients and measuredbetween 5.5° in extension (range, 0°-20°) and 85.9° inflexion (range, 20°-125°).The location of the surface damage on the retrieved

femoral components was assessed first by placing thecomponent in 0° flexion on a nonabrasive flat surface(Fig. 2B). The femoral components were then rotatedmanually on the condyles to the point when thedamaged surface of the posterior aspect approachedthe nonabrasive flat surface. This altered position wasthen measured using a digital protractor to approximatethe location of surface damage on the condyles andrepresented the degree of flexion where the surfacedamage started (referred to as “lower flexion”), assumingthe femoral components were initially implanted at 0°

flexion. The femoral component was further rotateduntil the trailing end of the damaged surface portionapproached the nonabrasive flat surface (referred to as“higher flexion”). The mean and the range of flexionangles of the apparent surface damage were reported forboth the medial condyle and the lateral condyle.The retrieved femoral components were further

classified into CoCr alloy femoral components thatcame in contact with Ti alloy tibial trays (referred to asCoCr/Ti material contact), Oxinium femoral compo-nents that came in contact with Ti alloy tibial trays(referred to as OxZr/Ti material contact), and CoCralloy femoral components that came in contact withCoCr alloy tibial trays (referred to as CoCr/CoCrmaterial contact). Such categories were deemed bene-ficial for the consecutive illustration, as a typical surfacedamage was shown for each combination of femoralcomponent material and tibial tray material at thecontact zone.The surface damage features on the retrieved 22

femoral components were inspected using a scanningelectron microscope (Model 6460 LV; JOEL, Ltd, Tokyo,Japan). The images were either taken in secondaryelectron mode or in backscattered electron mode (BSEmode). The BSE mode was considered more advanta-geous than the secondary electron mode, as it permittedthe detection of possible material deposits that hadcompositions different from the substrate material of thefemoral component. Such differencewould be evident inthe BSEmode, as the visible responsewas directly relatedto the atomic number of the surface materials. Forexample, the zirconium-niobium alloy substrate of anOxZr femoral component would appear brighter thanthe oxide on an image taken in BSE mode, thuspermitting a more conclusive surface analysis. Inaddition to the BSE mode, energy dispersive spectro-metry (EDS) (Genesis X-ray Microanalysis; EDAX, Inc,Mahwah, NJ) was applied to identify the chemicalcomposition of specific areas of interest, especially inthe case of suspected material transfer.Surface damage features were quantified by measure-

ment of surface roughness using a contact profilometerwith a 2-μm radius diamond stylus (Surfcom 1800D; CarlZeiss, Oberkochen, Germany). Seven different surfaceroughness parameters were determined for each case.Least square curve fit and a cutoff wavelength of 0.25 mmwere chosen for calculating the roughness parametersusing the Surfcom software (TIMS, Version 7.03; CarlZeiss, Oberkochen, Germany). Between 3 and 10 mea-surements per damaged surface area were taken, and thearithmetic mean was calculated. The number of measure-ments possible in each damaged area was limited by itssize, realizing that asmanymeasurements as possiblewererequired to accurately reflect the surface roughness,especially in a highly damaged region with increasedsurface variability [36].

Table 1. Implant Characteristics and Surface Damage Location on the Retrieved Femoral Components (Mean)

Implant System (Manufacturer)

FemoralComponentMaterial

Tibial TrayMaterial

Surface Damage Location (°, Flexion)

Medial Condyle Lateral Condyle

Lower Flexion Higher Flexion Lower Flexion Higher Flexion

NexGen (Zimmer, Inc) CoCr alloy Ti alloy 91.6 ± 20.1 123.6 ± 4.2 94.3 ± 14.4 113 ± 15.3Genesis II (Smith and Nephew, Inc) CoCr alloy Ti alloy 74.2 ± 19.1 112 ± 9.1 87.5 ± 10.6 120 ± 7.1Genesis II (Smith and Nephew, Inc) OxZr Ti alloy 73.3 ± 11.1 107 ± 11 80.6 ± 13.7 111.8 ± 11.7Duracon (Stryker, Mahwah, NJ) CoCr alloy CoCr alloy 90 ± 5 122.4 ± 10.8 82.5 ± 9.6 117.5 ± 21.8

Fig. 3. Backscattered electron image of the retrieved CoCralloy femoral component after being in contact with a Ti alloytibial tray (case 10, CoCr/Ti; Genesis II knee system). Energydispersive spectrometry confirmed Ti alloy material transferonto the CoCr bearing surface. Gouging of the CoCr alloyfemoral component by the Ti alloy tibial tray was also observed.

Surface Damage on Retrieved Femoral knee Components � Burnell et al 1463

Surface profilometry traces were obtained, startingfrom the visually apparent nondamaged area goingthrough the damaged area and ending in the visuallyapparent nondamaged area. As such, the evaluationlength for each scratch was different, as the width of thedamaged area varied from one scratch to another. Thesurface roughness parameters Ra, Rpm, Rsk, Rpk, andRvk depend on the evaluation length and how much ofthe surface portion was damaged and how much wasnot damaged in the profilometry trace. This was deemedto be due to the fact that the profilometer calculated Rafor every 0.25-mm interval (cutoff wavelength) andreported the average for the entire evaluation length.Roughness parameters such as Rp and Rv wereindependent of the evaluation length because theserepresent the maximum peak height and valley depthnoted in the entire evaluation length.With the Ti alloy tibial trays, material transfer to the

femoral component surface contributed to the Rp value,in addition to any plastic deformation or piling up of thesubstrate material. With the CoCr alloy tibial trays,where material transfer was not identified, the Rp valuerepresented mainly piling up of the substrate materialalong the edge of the gouge. Surface roughness traceswere taken for CoCr/Ti material contact, OxZr/Timaterial contact, and CoCr/CoCr material contact toillustrate the general surface damage characteristics.The measured values for multiple surface roughness

parameters were compared using a statistical softwareprogram (SPSS, version 17; SPSS Inc, Chicago, Ill). TheKolmogorov-Smirnov test was applied to test whetherthe values of the surface roughness parameters in eachgroup of material contact (CoCr/Ti, OxZr/Ti, and CoCr/CoCr) were normally distributed. Normality was con-firmed in each group of material, and the analysis ofvariance with Fisher least squared test as the post hocmethod was applied for unequal sample sizes tocompare the means between 3 groups.

ResultsAll 22 femoral components selected for further

retrieval analysis showed surface damage on either themedial condyle or the lateral condyle (Table 1). In 16cases, surface damage was observed on both condyles ofthe retrieved femoral components. Surface damage ofthe medial femoral condyle was seen on 20 retrievals;

lateral femoral surface damage was only seen on 14retrievals. The articular surface of each of the PE insertsshowed areas where the original machining marks hadbeen altered by surface damage.The surface damage on the medial condyles started

on average at 81° flexion (low-flexion range, 43°-110°flexion) and ended on average at 115° flexion (high-flexion range, 95°-135° flexion). The surface damageon the lateral condyles started on average at 85°flexion (low-flexion range, 65°-105° flexion) andended on average at 115° flexion (high-flexionrange, 90°-135° flexion). A typical appearance of thesurface damage of the CoCr/Ti material contact isshown for case 10 (Fig. 3). Ti alloy material transferwas seen on all femoral components in this group ofCoCr/Ti material contact.Similar abrasive damage features were observed on

the OxZr/Ti material contact of case 16 (Fig. 4). Ti alloymaterial transfer on the OxZr surface was also identifiedusing EDS analysis. The oxide was damaged, in someareas even completely removed, and the zirconium-niobium alloy substrate was clearly visible. Plasticdeformation of the underlying substrate did produce

Fig. 4. (A) Backscattered electron image of the retrieved OxZrfemoral component after being in contact with the Ti alloytibial tray (case 16, OxZr/Ti; Genesis II total knee system). Thelighter gray area represents the area where the Ti alloy tibialtray had gouged the femoral condyle and, thus, the oxide andexposing the Zr-2.5%Nb substrate. (B) Backscattered electronimage of a different area on case 16, where EDS confirmed thepresence of Ti alloy material transfer.

Fig. 5. (A) Secondary electron image of the retrieved OxZrfemoral component after being in contact with the Ti alloytibial tray (case 11, OxZr/Ti material contact; Genesis II kneesystem), showing obvious surface cracks on the severeplastically deformed OxZr surface at high magnification(original magnification ×400) on the retrieved OxZr femoralcomponent of case 16. Detached oxidized Zr-2.5%Nb was notobserved on any of the OxZr retrievals.

1464 The Journal of Arthroplasty Vol. 26 No. 8 December 2011

many microcracks in the oxide (Fig. 5). However, thesemicrocracks were not accompanied by any delaminationor flaking of the oxide.Under higher magnification, both the amount and the

severity of scratches were visually more apparent on theCoCr/Ti material contact compared with the CoCr/CoCr

Table 2. Surface Roughness Parameters Measured on the RetrievComponent/Tibial Tray Material Contact

Femoral Component/Tibial Tray Material Contact

Su

Ra Rpm Rp

CoCr/Ti 0.3 ± 0.1 0.8 ± 0.3 2.5 ±OxZr/Ti 0.6 ± 0.4 † 1.8 ± 1.1 † 4.5 ±CoCr/CoCr 0.4 ± 0.1 1.3 ± 0.5 3.9 ±

Mean and 95% CI.† Statistically significantly different (P b .05).

material contact. In some cases, EDS showed materialtransfer of Fe and Cr in some areas, most likely becauseof contact with other operating instruments made ofstainless steel.The mean and 95% confidence intervals (CIs) for the

surface roughness parameters of each material contactcategory were calculated (Table 2). Surface roughnessparameters Ra (P = .009; 95% CI, −0.684 to −0.112 μm),Rpm (P = .013; 95% CI, −1.786 to −0.242 μm), Rpk (P =.026; 95% CI, −2.024 to −0.146 μm), and Rvk (P = .029;95% CI, −2.044 to −0.123 μm) were found to bestatistically significantly higher for the OxZr/Ti materialcontact compared with the CoCr/Ti material contact.Based on the present sample size, no statistically

significant difference was found between the surfaceroughness parameters for the CoCr/Ti material contactcompared with the surface roughness parameters for theCoCr/CoCrmaterial contact (Ra [P= .231; 95%CI,−0.506to 0.130 μm], Rpm [P = .272; 95% CI, −1.322 to 0.394μm], Rp [P = .239; 95% CI, −3.838 to 1.016 μm], Rv[P = .540; 95% CI, −5.856 to 3.164 μm], Rsk [P = .483;

ed Femoral Components Categorized Based on Their Femoral

rface Roughness Parameters (μm)

Rv Rsk Rpk Rvk

1.2 2.6 ± 1.5 0.3 ± 0.4 0.9 ± 0.5 0.8 ± 0.52.9 6.2 ± 5.9 0.2 ± 1.4 2.0 ± 1.2 † 1.9 ± 1.3 †

1.2 3.9 ± 1.7 −0.1 ± 0.6 1.6 ± 0.5 1.7 ± 0.5

Surface Damage on Retrieved Femoral knee Components � Burnell et al 1465

95% CI, −0.725 to 1.479 μm], Rpk [P = .177; 95%CI, −1.744 to 0.344 μm], and Rvk [P = .081; 95%CI, −2.001 to 0.127 μm]). Similarly, no statisticallysignificant difference was found between the surfaceroughness parameters for the OxZr/Ti material contactcompared with the surface roughness parameters forthe CoCr/CoCr material contact (Ra [P = .212; 95%CI,−0.130 to 0.549μm],Rpm[P= .224; 95%CI,−0.367 to1.467 μm], Rp [P = .618; 95%CI, −1.966 to 3.224 μm], Rv[P = .326; 95% CI, −2.501 to 7.141 μm], Rsk [P = .644;95% CI, −0.914 to 1.442 μm], Rpk [P = .479; 95%CI, −0.731 to 1.501 μm], and Rvk [P = .787; 95%CI, −0.999 to 1.284 μm]).Characteristic surface traces taken on the CoCr/Ti

material contact, the OxZr/Ti material contact, and theCoCr/CoCr material contact clearly showed definedpeaks and valleys. Additional retrieval analysis com-bined with surface profilometry was deemed necessaryto gain additional surface roughness data and strengthenthe statistical analysis.

DiscussionTwenty-two retrieved femoral components (44%) from

50 retrieved total knee arthroplasties showed evidence ofposterior condyle surface damage. The location of surfacedamage was independent of the implant system and mayhave been unintentionally caused during implantation ofthe implant components. In particular, the weight-bearing condyles of the femoral components may haverested on the posterior aspect of the cemented tibial tray athigh flexion when the tibial PE inserts were about to belocked into place. Such circumstance may indicate thatthis type of surface damagemay have its origin in the usedoperating technique (Fig. 1).Increased surface roughness of the femoral components

has the potential to accelerate PE wear in vitro [6].Muratoglu et al [6] used retrieved CoCr alloy femoralcomponents with an Ra and Rp value ranging from 0.1 to0.2 μm and 0.32 to 0.77 μm, respectively, and suchclinically relevant surface roughness was shown to pro-mote PE wear. The industry standard for new, never-implanted femoral metallic bearing surfaces (ASTM F2083-08) suggests an Ra value less than 0.1 μm, which islower than the measured Ra value in the damaged areasof the retrieved femoral components in the presentstudy. However, the effects of surface roughness on PEwear rate in vivo are not completely understood. Thecorrelation of surface roughness parameters such as Raand Rp with PE wear rates varies between studies andmay be influenced by differences in study design such aslubrication, loads applied, and the degrees of freedomof motion involved [37-39]. Despite the surface damageidentified on the retrieved femoral components, noneof the patients in the present study were revised forPE wear or osteolysis. This may be due to the fact that thedamaged areas on the femoral condyles were located

quite far posterior, limiting the number of cycles duringwhich the peaks of the surface damage could have beenable to come in contact with the PE insert, or becausethe overall damaged area represents only a fraction ofthe femoral weight-bearing surface. In all likelihood,there are multiple factors such as implant design, align-ment, patient activity, and other aspects of surgicaltechnique, which combine to determine surface damageand PE wear [40-42].Surface profilometry shows that surface roughness

parameters Ra, Rpm, Rsk, Rpk, and Rvk are statisticallysignificantly higher for the OxZr/Ti material contactcompared with the CoCr/Ti material contact. There wasno statistically significant difference between these sameroughness parameters for the CoCr/Ti material contactcompared with the CoCr/CoCr material contact or theOxZr/Ti material contact compared with the CoCr/CoCrmaterial contact. As previously stated, the direct com-parison between surface roughness parameters wassomewhat impractical given that the evaluation lengthof the surface damage differed between the obtainedsurface roughness traces. A shorter evaluation lengthwould increase the weight of a more significant peak orvalley, whereas the reverse would be true with a longerevaluation length. Rp and Rv parameters were notsignificantly different between the material combina-tions in the present study but were higher than valuesreported for new, never-implanted femoral components[41,43]. Statistically significant differences betweenseveral roughness values could not be obtained giventhe large SD and small sample size. Knee simulator weartesting was deemed necessary to further investigate theeffect of surface damage identified in the present studyon PE wear and wear particle generation. Although theabsolute PE wear volume may not be increased by aroughened counterface, the shape and size of the PEwear particles produced may be altered and possiblybecome more biologically active and, thus, turn into aprecursor for osteolysis [13,44,45].Scanning electron microscopy of the damaged areas

revealed that the surface damage on the CoCr alloyfemoral component at the point of contact with the Tialloy tibial tray appeared less severe than the surfacedamage on the OxZr femoral component at the point ofcontact with the Ti alloy tibial tray. However, the CoCralloy femoral components showedmore scratching of thesurface surrounding the scratch, likely because of third-body damage from sources such as Ti alloy transferparticles, bone cement debris, or bone fragments [46]. TheOxZr femoral components did not reveal such scratching,confirming the resistance of the OxZr toward third-bodyabrasive wear. Although in some areas the oxide of theOxZr was completely removed and the substrate materialwas clearly visible, there was no evidence of delaminationor chipping of the oxide along the edges of the scratches.These findings confirmed the ability of OxZr to sustain

1466 The Journal of Arthroplasty Vol. 26 No. 8 December 2011

localized surface damage without leading to catastrophicfailure of the oxide. Brittle cracking in the oxide followedby the removal of highly abrasive OxZr particles has beenpreviously observed [26,32-34], and this was suggested tobe due to plastic deformation of the underlying substrate.It is a reasonable possibility that some of the missing OxZrwas shed in form of highly abrasive particles and,combined with the increased surface roughness, couldpotentially accelerate PE wear in the long term in vivo.In any case, we acknowledge that it is impossible to

exactly state when the observed surface damage on thefemoral components occurred. The femoral compo-nents could have come into contact with the tibialtrays at different stages during primary surgery or atrevision surgery. However, we suspect that most sur-face damage on the posterior aspect on the femoralcomponents stemmed from direct contact between thefemoral component and the tibial tray at high flexion,at the time when the PE insert was placed onto thetibial tray. Based on the outpatient records, somefemoral components were retrieved without removingthe PE insert from the tibial tray at revision surgery,thus limiting the potential for contact with the posteriorrim of the tibial tray at high flexion. The extent ofgouging, scratching, and material transfer identifiedwould seem to indicate that substantial surface pressurewas involved. Indentations on the posterior rim of thetibial tray locking mechanism indicated that the tibialtray was positioned anterior to the posterior femoralcondyles when the inciting surface damage hadoccurred. Recently, Heyse et al [47] reported similarsurface damage features in their study on retrievedOxZr femoral components; and this supports thefindings of the present study.In summary, the present study emphasized the need

to exercise care in the handling and insertion of totalknee arthroplasty components during surgery. Damageto the integrity of the femoral bearing surface fromunintended contact with the tibial tray at high flexioncould have an adverse effect on PE wear and promotewear particle–induced osteolysis in the long term. Oneof the manufacturers (Smith and Nephew, Inc) currentlysuggests to temporarily place a polymeric plate coveron the tibial tray to avoid unintended contact of thefemoral component with the tibial tray during sur-gery. This polymeric plate cover can be subsequentlyremoved so that the PE inserts can be locked onto thetibial tray. In addition, the surgeon should attempt tolock the PE insert onto the tibial tray without excessiveanterior translation of the tibia whenever possible.Because all types of femoral bearing surface materialsexperienced surface damage in the present retrievalanalysis, all manufacturers should consider supplyinga type of protective device as part of the standard instru-mentation kit to prevent surface damage to the femoralcondyles during implantation. Although the effects of

the identified surface damage on PE wear remainunknown, care should be taken to avoid contactbetween the femoral component and tibial tray of anymaterial combination.

AcknowledgmentsThe authors thank Mr Vivek Pawar, MSc, and Mr Kyle

Hubbard, BSc (Smith and Nephew, Inc), for performingthe surface analysis on the retrieved femoral compo-nents. Funding for this study was obtained from theAlexander Gibson Fund, University of Manitoba, Win-nipeg, Manitoba, Canada.

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