All-Polyethylene Versus Metal-Backed Tibial Components-An Analysis of 27,733 Cruciate-Retaining...

All-Polyethylene Versus Metal-BackedTibial Components—An Analysis of 27,733Cruciate-Retaining Total Knee Replacementsfrom the Swedish Knee Arthroplasty Register

Asgeir Gudnason, MD, Nils P. Hailer, MD, Annette W-Dahl, RN, PhD, Martin Sundberg, MD, PhD, and Otto Robertsson, MD, PhD

Investigation performed at the Department of Orthopedics, Institute of Surgical Sciences, Uppsala University Hospital, Uppsala, and the Swedish KneeArthroplasty Register, Department of Orthopedics and Clinical Sciences, Lund University Hospital, Lund, Sweden

Background: Currently, the use of metal-backed tibial components is more common than the use of all-polyethylenecomponents in total knee arthroplasty. However, the available literature indicates that all-polyethylene tibial componentsare not inferior to the metal-backed design. We hypothesized that there would be no difference in the ten-year survival ratebetween all-polyethylene and metal-backed tibial components of a specific design in a large nationwide cohort.

Methods: In the Swedish Knee Arthroplasty Register, we identified 27,733 cruciate-retaining total knee replacementsusing the press-fit condylar prosthesis with either metal-backed or all-polyethylene tibial components inserted from 1999to 2011. Unadjusted survival functions were calculated with the end points of revision for any reason, revision due toinfection, and revision due to reasons other than infection, and the differences between the groups were investigated withthe log-rank test. Cox proportional hazard models were fitted to analyze the influence of various covariates on the adjustedrelative risk of revision.

Results: The median duration of follow-up was 4.5 years (range, zero to 12.9 years). Of all total knee replacements,16,896 (60.9%) were in women and 10,837 (39.1%) were in men. Metal-backed components were used in 16,011 totalknee arthroplasties (57.7%) and all-polyethylene in 11,722 total knee arthroplasties (42.3%). With revision for any reasonas the end point, the all-polyethylene tibial component had slightly superior, unadjusted ten-year survival compared withthe metal-backed component: 97.2% (95% confidence interval [CI], 96.7% to 97.7%) compared with 96.6% (95% CI,96.2% to 96.9%; p = 0.002). Cox multiple regression analysis adjusting for age group, sex, and patellar resurfacingshowed that all-polyethylene components had a reduced risk of revision for any reason (relative risk = 0.75; 95% CI, 0.64to 0.89) and a reduced risk of revision due to infection (relative risk = 0.63; 95% CI, 0.46 to 0.86). Patellar resurfacing andmale sex increased the risk of revision due to infection (relative risk = 2.22 [95% CI, 1.37 to 3.62] and 2.21 [95% CI, 1.66to 2.94], respectively).

Conclusions: These all-polyethylene tibial components were at least as good as or superior to metal-backed tibialcomponents with respect to implant survivorship at ten years in cruciate-retaining total knee replacements. We concludedthat these less expensive all-polyethylene tibial components can be safely and effectively used in total knee arthroplasty.

Level of Evidence: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support ofany aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission ofthis work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Noauthor has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influencewhat is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online versionof the article.

Peer Review: This article was reviewed by the Editor-in-Chief and one Deputy Editor, and it underwent blinded review by two or more outside experts. It was also reviewedby an expert in methodology and statistics. The Deputy Editor reviewed each revision of the article, and it underwent a final review by the Editor-in-Chief prior to publication.Final corrections and clarifications occurred during one or more exchanges between the author(s) and copyeditors.

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COPYRIGHT � 2014 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED

J Bone Joint Surg Am. 2014;96:994-9 d http://dx.doi.org/10.2106/JBJS.M.00373

The tibial component in total knee arthroplasty can be anall-polyethylene component or a modular unit using apolyethylene liner mounted on a metal tray. All-polyethylene

components were originally used in the historical Total Condylartotal knee arthroplasty system, for which excellent long-termsurvival rates have been reported1,2. However, during the devel-opment of novel designs, a number of biomechanical studies haveshown that metal-backed tibial components improved the distri-bution of forces between the implant and the cement3,4. Further,increases in dynamic strain under all-polyethylene compared withmetal-backed tibial components have been described5.

In modern practice, modular metal-backed components aremore commonly used than all-polyethylene tibial components,although a number of arguments in favor of the all-polyethylenedesign remain valid. The all-polyethylene tibial component is lessexpensive than its modular counterpart6, and several randomizedtrials and smaller cohort studies have indicated that the all-polyethylene tibial component is at least equivalent to the modulardesign in function, knee motion, primary stability, and long-termimplant survival7-12. These favorable findings also apply to patientswith rheumatoid arthritis or obesity13,14. A recent meta-analysisconcluded that ‘‘the AP [all-polyethylene] tibial component wascomparable with or better than the MB [metal-backed] tibialcomponent in TKA [total knee arthroplasty].’’15

However, small and insignificant differences described inmedium-sized cohort studies may add up to clinically relevantdifferences in implant survival when larger populations arestudied. It was our aim to extract data from a large nationwideknee arthroplasty registry to investigate whether identical all-polyethylene and metal-backed tibial designs perform equallywell with regard to revision rates. We hypothesized that therewould be no difference in ten-year survival between all-polyethyleneand metal-backed tibial components of a specific design withrespect to revision for any reason. The secondary end point wasrevision due to infection.

Materials and MethodsThe Swedish Knee Arthroplasty Register

The Swedish Knee Arthroplasty Register (SKAR) was initiated in 1975 andsubsequently evolved into a nationwide register, which has been repeatedly

validated16-18

. In Sweden, each resident is assigned a personal identificationnumber that makes it possible to link the index operation to further revisionsurgery and—through linkage to the National Population Register—includeinformation on dates of death or emigration.

Study Population and Inclusion CriteriaWe analyzed survival rates for all-polyethylene and metal-backed tibial com-ponents, restricting the analysis to the Press-Fit Condylar prosthesis (PFCSigma; DePuy, Johnson & Johnson, Warsaw, Indiana). In Sweden, this is themost commonly used total knee prosthesis with an optionally available all-polyethylene design. Data were extracted from the SKAR, and 33,729 primarytotal knee arthroplasties performed using the PFC Sigma knee prosthesis from1999 to 2011 were found to be eligible. To reduce the influence of potentialconfounders, we excluded all other diagnoses but primary osteoarthritis of theknee. We also excluded uncemented total knee arthroplasty and posterior-stabilizeddesigns, implying that only cruciate-retaining designs were included in theanalysis. Thus, all remaining 27,733 total knee replacements had a nonporouscemented femoral component and identically designed, curved metal-backed or

all-polyethylene tibial component. The polyethylene components for both all-polyethylene and metal-backed designs were manufactured and sterilized in anidentical manner. Follow-up started on the day of the index operation and ended atrevision surgery, death, emigration, or December 31, 2011, whichever camefirst. Revision surgery in the SKAR is defined as exchange, addition, or removalof any component of the prosthesis. Procedures that do not involve any com-ponent exchange are registered, but that registration is not considered reliable.

StatisticsContinuous descriptive statistics used means, median values, ranges, and 95%confidence intervals (CIs) where appropriate. A p value of <0.05 was consideredsignificant. Survival functions were calculated with the end points of revisionfor any reason, revision due to infection, and revision due to reasons other thaninfection according to the Kaplan-Meier method, and differences between groupswere investigated with the log-rank test. Cox proportional hazard models werefitted in order to analyze the influence of various covariates on the relative risk(RR) of revision, with the end points described above. The covariates of patellarresurfacing, sex, age group (less than forty-five years, forty-five to fifty-four years,fifty-five to sixty-four years, sixty-five to seventy-four years, seventy-five to eighty-four years, and eighty-five years or older) and type of tibial component were firstentered as singular variables in order to calculate crude RR, followed by mutuallyadjusted models including all covariates. Model diagnostics included calculation ofthe correlation coefficient between transformed survival time and the scaledSchoenfeld residuals. A total of 27,733 total knee arthroplasty components wereinserted into 23,657 patients, and 4076 patients had received bilateral procedures.On the basis of the published literature

19-22, we assumed that the inclusion of both

joints in patients managed with bilateral procedures has a negligible effect on survivalestimates in an observational study of this size, but additional cluster analyses in-cluding personal identification as a cluster variable were performed. These clusteranalyses generated robust estimates that were generally very similar to those that hadbeen calculated without taking bilaterality into account (data not shown).

Source of FundingThe study had no external source of funding.

Results

The median duration of follow-up was 4.5 years (range, zeroto 12.9 years). Of all total knee arthroplasties, 16,896 (60.9%)

were in women and 10,837 (39.1%) were in men. The mean age atthe time of the index surgery was 70.5 years (range, 18.4 to ninety-six years). The distribution in various age groups is given in a tablein the Appendix. Metal-backed components were used in 16,011total knee arthroplasties (57.7%) and all-polyethylene, in 11,722total knee arthroplasties (42.3%). We found a slightly higherproportion of all-polyethylene tibial components in women and inyounger patients, and older patients were overrepresented in themetal-backed group (see Appendix). Only 1117 (4%) of all total

TABLE I Age Groups and Kaplan-Meier Survival Analysis

Age Groups (yr)10-Year Survival

Rate (%)95% Confidence

Interval

<45 92.8 0.86-1.00

45-54 93.6 0.92-0.96

55-64 94.8 0.94-0.96

65-74 97.2 0.97-0.98

75-84 98.2 0.98-0.99

‡85 98.5 0.98-0.99

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TH E J O U R N A L O F B O N E & JO I N T SU R G E RY d J B J S . O R G

VO LU M E 96-A d NU M B E R 12 d J U N E 18, 2014AL L-PO LY E T H Y L E N E VE R S U S M E TA L -BAC K E D TI B I A L

CO M P O N E N T S —AN AN A LY S I S O F 27,733 TKR S

knee arthroplasties received patellar resurfacing, with resurfacingbeing more frequent in the metal-backed group (see Appendix).Six hundred and thirty-two total knee arthroplasties had beenrevised: 416 of those revisions were in the metal-backed groupand 216 in the all-polyethylene group. The most common reason

for revision was infection (195 knees), and 431 knees were revisedfor various reasons other than infection (data were missing for sixrevisions).

With revision for any reason as the end point, unadjustedten-year survival of the implants was 96.6% (95% CI, 96.2% to

TABLE II Cox Regression Analysis of Risk Factors for Revision for Any Reason

Covariates Unadjusted RR* 95% CI Adjusted RR* 95% CI P Value RR†

ProsthesisMetal-backed 1 1All-polyethylene 0.77 0.66-0.91 0.75 0.64-0.89 0.001

SexFemale 1 1Male 1.01 0.86-1.19 0.97 0.82-1.14 0.693

Patellar resurfacingNo 1 1Yes 1.07 0.75-1.53 1.08 0.76-1.55 0.669

Age group (yr)

<45 2.29 0.85-6.15 2.31 0.86-6.21 0.97045-54 2.10 1.55-2.85 2.12 1.57-2.87 <0.00155-64 1.67 1.39-2.01 1.68 1.39-2.03 <0.00165-74 1 175-84 0.74 0.59-0.92 0.73 0.59-0.91 0.05‡85 0.64 0.36-1.10 0.64 0.36-1.14 0.13

*Adjusted risk ratios (RR) were calculated with 95% confidence intervals (CIs) using a Cox proportional hazard model. †P values refer to adjustedRR.

TABLE III Cox Regression Analysis of Risk Factors for Revision Due to Infection

Covariates Unadjusted RR* 95% CI Adjusted RR* 95% CI P Value†

ProsthesisMetal-backed 1 1All-polyethylene 0.61 0.45-0.83 0.63 0.46-0.86 0.003

SexFemale 1 1Male 2.22 1.67-2.95 2.21 1.66-2.94 <0.001

Patellar resurfacingNo 1 1Yes 2.15 1.32-3.49 2.22 1.37-3.62 0.001

Age-group (yr)

<45 3.30 0.81-13.43 3.45 0.86-14.26 0.08145-54 1.31 0.70-2.47 1.40 0.74-2.63 0.30355-64 1.18 0.83-1.69 1.20 0.84-1.70 0.32665-74 1 175-84 0.87 0.61-1.24 0.90 0.63-1.28 0.556‡85 0.78 0.32-1.93 0.86 0.35-2.14 0.752

*Adjusted risk ratios (RR) were calculated with 95% confidence intervals (CIs) using a Cox proportional hazard model. †P values refer to adjustedRR.

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96.9%) for the metal-backed tibial component and 97.2% (95%CI, 96.7% to 97.7%) for the all-polyethylene tibial components(p = 0.002) (Fig. 1). Ten-year survival with revision due to in-fection as the end point was 99.0% (95% CI, 98.8% to 99.2%) forthe metal-backed components and 99.1% (95% CI, 98.8% to99.5%) for the all-polyethylene component (p = 0.002). Usingrevision for reasons other than infection as the end point, ten-year survival was 97.6% (95% CI, 97.3% to 97.9%) for the metal-backed tibial components and 98.1% (95% CI, 97.8% to 98.4%)for the all-polyethylene tibial components (p = 0.15). Unadjustedten-year survival according to the age groups (less than forty-fiveyears, forty-five to fifty-four years, fifty-five to sixty-four years,sixty-five to seventy-four years, seventy-five to eighty-four years,and eighty-five years or older) showed increasing implant sur-vival with increasing age (Table I).

Cox multiple regression analysis was performed with thecovariates of patellar resurfacing, sex, age group, and type oftibial component. All-polyethylene tibial components had alower risk of revision for any reason with an adjusted RR of0.75 (95% CI, 0.64 to 0.89) compared with metal-backed tibialcomponents (Table II).

With revision for any reason used as the end point, survivalanalysis showed that patellar resurfacing and sex did not influencethe risk of revision. With revision due to infection used as the endpoint, all-polyethylene tibial components had a lower risk of re-vision compared with metal-backed components, with an adjustedRR of 0.63 (95% CI, 0.46 to 0.86). Patellar resurfacing and male sex

increased the risk of revision due to infection (Table III). All-polyethylene components had a slightly reduced risk of revision forreasons other than infection with an adjusted RR of 0.82 (95% CI,0.69 to 1.00) compared with metal-backed tibial components, butthis finding was of borderline clinical importance and significance.

Discussion

All-polyethylene tibial components showed improved overallimplant survival compared with metal-backed tibial com-

ponents. This finding is mainly due to the fact that total kneearthroplasties with all-polyethylene tibial components are atlower risk of revision due to infection. In contrast, the risk ofrevision for reasons other than infection did not show asignificant difference between the groups. Our findings are inagreement with published literature on this topic23-26.

Our finding that the risk of revision due to infection washigher for metal-backed components than for all-polyethylenecomponents was not unexpected. Early infections after totalknee arthroplasty can be successfully treated by debridementand implant retention27. In metal-backed total knee arthro-plasty, the polyethylene liner is mostly exchanged during re-vision surgery and all procedures involving an extraction orchange of an implant are reported to the SKAR. In contrast,early revisions because of infection at the site of all-polyethylenetotal knee replacements do not usually encompass extraction orexchange of implants and such procedures are, per definition,not reported to the SKAR.

Fig. 1

Ten-year survival of the implants, with revision for any reason as the end point. The shaded areas indicate the 95% confidence intervals.

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However, debridement of infected tissues combinedwith retention of the total knee arthroplasty components doesnot achieve infection control as efficiently as debridementcombined with component exchange28,29. Consequently, it islikely that not all knees treated with debridement because ofearly infection after total knee arthroplasty in our cohort hada successful outcome and may have resulted in delayed, moreextensive revision surgery that would have been consequentlyreported to the SKAR. We found no difference between thegroups with regard to the time to revision because of infection(data not shown). In conclusion, the question of whether all-polyethylene total knee replacements are less prone to in-fection cannot be clearly answered on the basis of our registerdata.

Regression analysis of the risk of revision for any reasondid not show any difference between men and women. How-ever, men are at higher risk of revision due to infection than arewomen. This is in agreement with several reports on sex-specificdifferences with regard to the survival of total knee replace-ments30,31, including one by the SKAR32.

All-polyethylene and metal-backed tibial componentswere described by Toman et al.33 as equivalent with regard tosurvival rate in patients with a body mass index (BMI) of<37.5 kg/m2. Since BMI has been registered in the SKARonly since 2009, we were unable to include this factor in ouranalysis.

Insertion of patellar components was performed in onlya minority of the knees in our study. This is common practicein Sweden, where only 3% of all total knee arthroplastiesinclude patellar resurfacing32. In our study population, pa-tellar resurfacing was unequally distributed between the twogroups of all-polyethylene and metal-backed tibial compo-nents, but the covariate of patellar resurfacing exerted noinfluence on overall implant survival. However, patellar re-surfacing seemed to slightly increase the risk of revision be-cause of infection. The issue of whether patellar resurfacingshould be performed continues to be debated34,35.

Limitations of our study include the above-mentionedregistration bias concerning revision surgery due to infec-tion. Further problems with respect to register studies ingeneral are associated with incomplete registration of revi-sion surgery, but completeness of the SKAR data is 94% withregard to revision procedures18. Another problem is the lackof data related to patient outcome, although such informa-tion is now prospectively collected from a few surgical unitsin the SKAR. Some factors with a putative influence on outcome,such as BMI or medication with corticosteroids, statins, or bis-phosphonates, are not registered and may skew our results. Thesame applies to medical comorbidities such as diabetes mellitus.However, we have no reason to believe that such factors wereunequally distributed between the two investigated groupsof total knee replacements. Observation times were notidentical in the groups, with the all-polyethylene group having aslightly shorter length of follow-up than the metal-backed group.However, both the Kaplan-Meier survival analysis and the Coxregression analysis compensate for differences in observation

times. A potential weakness is that individual surgeons are onlyregistered voluntarily, with data generally reported to the reg-istry on a per surgical unit basis.

One of the strengths of our study is the large cohort oftotal knee replacements included in our analysis. Such num-bers render type-II errors improbable, but this poses theproblem of finding significant, but clinically irrelevant, differ-ences. We believe that the main conclusion that can be derivedfrom our analysis is that the all-polyethylene tibial componentperformance is at least equal to that of the metal-backed design.Our main focus is not on the small, significant difference be-tween the two components, but rather on the fact that, in thislarge cohort, the two components seem to perform equallywell. We also restricted our analysis to implants that areidentical in all aspects, with the exception of the presence orabsence of metal backing. This is not the case in some otherstudies devoted to the investigation of all-polyethylene com-ponents compared with metal-backed tibial components, po-tentially allowing differences in polyethylene wear and affectingthe subsequent risk of revision36. We also restricted our analysisto implants designed for cruciate-retaining surgery and ex-cluded all PFC Sigma components that are posterior stabi-lized. The inclusion of posterior-stabilized designs couldhave skewed our results since such components are subjectto a greater amount of volumetric wear than are noncon-strained components, which in turn could influence the riskof revision37,38.

In conclusion, our data indicate that the all-polyethylenetibial component can be safely and effectively used in totalknee arthroplasty. We concluded that the use of the all-polyethylene design is at least as good as, or better than, theconventional metal-backed tibial component in the PFC Sigmaknee prosthesis with regard to overall ten-year survival of theimplant.

AppendixTables showing data on the age and sex distribution of thepatients and the use of resurfaced patellar components

are available with the online version of this article as a datasupplement at jbjs.org. n

Asgeir Gudnason, MDNils P. Hailer, MDDepartment of Orthopedics,Institute of Surgical Sciences,Uppsala University Hospital,SE-751 85, Uppsala,Sweden. E-mail address: [email protected]

Annette W-Dahl, RN, PhDMartin Sundberg, MD, PhDOtto Robertsson, MD, PhDThe Swedish Knee Arthroplasty Register,Department of Orthopedics and Clinical Sciences,Lund University,SE-221 85, Lund, Sweden

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