The In Vivo Performances Of Highly Crosslink UHMWPE In The Bearing Surfaces Of Total Knee Arthroplasty

Background： Highly cross-linked ultra-high molecular weightpolyethylene (XLPE) has demonstrated superior wear performancecompared to conventional polyethylene (PE) in total hip arthroplasty(THA), with follow-up studies beyond10years showing few or nopatients with osteolysis. XLPE was more recently introduced into totalknee arthroplasty (TKA) following in vitro knee simulator findings oflower wear rates compared to those found with conventional PE. Thus far,however, the clinical benefit of XLPE remains unclear. Additionalconcerns exist for the use of XLPE in TKA. The larger stresses and morecomplex stress distributions in tibial TKA polyethylene componentscompared to polyethylene acetabular components in THA must beconsidered in light of the alterations in material properties caused by thecrosslinking process. Evaluation of surface damage and oxidativeproperties of retrieved XLPE TKA tibial components showed littleimprovement with the use of XLPE. A major limitation of existingretrieval studies is the lack of controlled conditions between groups.Implant design and length of implantation are important variables thatmay bias a comparison of the benefit of XLPE and conventional PE tibialcomponents based on materials alone. Also, none of the investigations onXLPE damage were done in3D, and differences in damage patternsbetween materials were not examined. Therefore, a comprehensive investigation of retrieved crosslink TKAinserts should be proposed to achieve a further understanding of thesecomponents.Moreover, the unexpected clinical outcomes of XLPE tibial insertsshould be having further attentions. Crosslinking processes were involvedin the manufacturing of UHMWPE to achieve better performances inwear resistance of TKA bearing surfaces. The wear resistance ofpolyethylene will be increased with higher local crosslink density inmaterial. However, the wear performances of TKA inserts could beweakened due to in vivo decreases of crosslink density in XLPE, and thischanging may derived from the effect of local loading and oxidation.Thus far, it still remains unclear of the relationship among mechanicalloading, oxidation level, and crosslink density in crosslink polyethylene.Therefore in the current study, we applied the techniques of damagemapping, swell ratio test, and FTIR test to assess the relationship amongthe effect of loading, oxidation, and crosslink density in retrieved XLPETKA inserts.Objective： In the current study we compared the surface damagesand dimensional changes of retrieved crosslink and conventional TKAinserts to investigate if there is any advantage in clinical performance wasassociated with XLPE in knee. To further explain the mechanism of invivo changing in wear resistance of XLPE TKA bearings, the evaluationsof the relationship among the effect of loading, oxidation, and crosslinkdensity were also performed.Method: The current study was consisting of assessment ofprosthesis damages, and investigating of crosslink density in wearresistance. In the first experiment, we matched44XLPE to44conventional-PE inserts from HSS retrieval database with identical manufacture, implant design, size, and similar length of implantation.Surface damage on the articular surfaces was subjectively Hood gradedand digitally mapped to determine the percent damaged area of eachdamage mode.3D dimensional changes that had occurred as a result ofimplantation were determined by comparing laser scans of29retrievedinserts with size-matched pristine inserts.In the second experiment, we chose40retrieved XLPE tibial insertsfrom HSS retrieval database. The criteria include: the implants shouldhave been placed into-22℃freezer within6months after retrieval;visible damaged/undamaged regions should be identified on the plateauof each insert. Samples were cut from loaded/unloaded surface andsubsurface regions of each insert respectively. Techniques of swell ratiotest and FTIR test were applied to obtain the crosslink density andoxidation index in each region. Finally, the relationship among loading,oxidation level, and crosslink density were evaluated.Result: In the first experiment, the damage scores and percentdamaged areas were not different between the matched XLPE andconventional-PE inserts. However, XLPE inserts showed greater articularsurface dimensional changes (p=0.03). Within the same design, deviationpatterns were consistent between the two materials; however, as expected,the location of the dimensional changes differed among designs. In thesecond experiment, we found a significant lower crosslink density in theloaded surface than any other region (p<0.001). On the contrary, theoxidation index was significant higher in the loading surface region(p<0.001).Negative correlation has been discovered between crosslink densityand average oxidation index in the loaded surface (p<0.001), but the sametrend has not been found in any other area. The unloaded surface region presented higher oxidation level than its controlled subsurface region(p<0.001), but no crosslink density change was demonstrated in this area.Conclusion: In summary, this large retrieval study demonstratedonly minor differences in accumulated surface damage and dimensionalchanges between matched groups of highly cross-linked and conventionalpolyethylene TKA inserts of the same designs, suggesting no short termbenefit of XLPE in knee implants. While by investigating the relationshipamong the effect of in vivo loading, crosslink density, and oxidationindex of retrieved crosslink TKA inserts, we found the in vivo wearresistance of total knee bearing may be influenced by local loading andoxidation. Long-term clinical and longer-term retrieval studies willelucidate any clinical advantages of using XLPE tibial components.