INTRODUCTION

Tibiofemoral contact at the base of the articular surface spine in posterior-stabilized total knee arthroplasty (TKA) implants can lead to spine fracture [1]. Revision TKA implants also have an articular surface spine to provide sufficient constraint when soft tissues are compromised. While some revision TKA designs have metal reinforcement in the articular surface spine, others rely solely on a polyethylene spine. This study used finite element analysis (FEA) to study the effect of metal reinforcement on stresses in the spine when subjected to posteriorly directed loading.

Introduction and Aims: Due to relative motion that can occur between the polyethylene articular surface and tibial tray, backside wear of modular tibial components can be a significant contributor to wear in TKR. This study examines the backside wear performance of a tibial component system from both a laboratory and clinical perspective.

Method: Polyethylene components, CR and PS, from the NexGen knee system (Zimmer Inc.) were evaluated for backside wear. These components were identified on the back surface by the manufacturer with engraved lettering of a depth ranging from 20 to 30 micrometers. Twenty-seven components retrieved after 24 to 80 months in-situ were evaluated along with six components having undergone three million cycles of laboratory knee function simulation. Backside wear was quantified by engraving mark depth and screw hole recess penetration measurements utilising a New View 5000 scanning white light interferometer (Zygo). The severity of third-body abrasion was also recorded.

Results: This particular knee system utilised a peripheral rail and dovetail polyethylene locking mechanism which demonstrated little relative polyethylene to tibial tray motion during joint function simulation. Simulator testing produced backside wear of 6.4 micrometers/million cycles or 4.5 mm3/million cycles. This backside wear represented 30% of total component wear as measured gravimetrically. Backside wear in the clinically retrieved components was sufficient to completely remove the manufacturer’s engraving marks on only three of 27 components. The remaining 24 components all experienced backside wear insufficient to remove all engraving. The severity of third-body abrasion (typically bone cement) was generally associated with greater backside wear. Two of the three clinically retrieved components with worn-through lettering had evidence of significant third-body wear. In 11 clinically retrieved components (utilised on tibial trays with screw holes), backside wear was measured by comparing engraving mark depth in unworn polyethylene areas over screw recesses with engraving mark depth in areas of polyethylene contact with the tibial tray. These components demonstrated 14 micrometers of wear at an average of 37 months in-situ or 4.4 micrometers per year. None of the retrieved components were clinically associated with osteolysis.

Conclusion: In this particular tibial component system, backside wear was moderate for both the joint simulator and clinically retrieved specimens. Backside wear does not appear to be the major contributor of total polyethylene wear in this implant system. The presence of third-body particles contributed to greater wear.