Introduction and Aims: The enduring success of the low friction arthroplasty advanced by Sir John Charnley may be appreciated by the fact that almost 700,000 primary and revision hip and knee arthroplasties were performed in 2003 in the US. Despite this success, the advent of wear debris generation leading to osteolysis and fixation failure are of growing concern, particularly with the increased graying and activity levels of our society. This paper addresses these concerns.

Method: Enhanced polyethylenes have been cleared by the FDA, but in the absence of clinical data supporting their safety and effectiveness. Only recently have short-term clinical experiences begun to emerge. Their proclaimed advantage lies in the reduction of wear debris generation through enhanced cross-linking of the polymer chains coincident with elimination of oxidation through the manufacturing process. These processes, however, vary in the amount and type of radiation used, the extent of polymer remelting and endpoint sterilisation methodology.

Results: Changes in the mechanical properties of these materials, particularly in their reduced resistance to fatigue crack propagation raises concerns about their long-term suitability in both hip and knee components where locking mechanisms offer foci for stress risers. Although it is claimed that the propensity for crack initiation is reduced, recent case reports suggest rapid fatigue failure once a crack has occurred. Material integrity, in this regard, is seen to be influenced by locking attachments, component positioning and highly cross-linked polyethylene choice.

Conclusion: It is concerning that 62% of acetabular polyethylene liners are manufactured from one of the highly cross-linked polyethylenes currently available. The clinical sequel to this is not manifest by the small number of short-term reports citing their efficacy. Cost as well as patient selection and the unknown clinical realities of long-term series reporting are concerns with these materials that only in vivo time will elucidate.

Introduction and Aims: Dual surface articulation characterises mobile bearing knee (MBK) designs, thereby increasing the potential for polyethylene damage. This hypothesis was investigated for eight, contemporary MBK designs.

Method: Both joint simulator and computational evaluations were performed on contemporary MBK systems. As part of an FDA clinical investigation a knee joint simulator was designed that successfully cycled three pairs of a MBK design for five million walking cycles at room temperature under saline conditions. Additionally, a finite element analysis was developed to determine the potential for abrasion, delamination and pitting for eight designs during walking gait.

Results: For the MBK simulator evaluation, the mean wear volume was 125mm^3 (range 75–175mm^3). This result is comparable to evaluations performed on fixed bearing designs in contemporary knee simulators utilising bovine serum for lubrication. The surface and subsurface stress distributions measured displayed a wide variation in both magnitude and location. Several of the designs utilised their available articulating interface optimally, lowering the magnitude of the stresses, while others produced edge-loading scenarios with relatively high stresses.

Conclusion: Dual surface articulation between a polyethylene insert and metallic femoral and tibial tray components is a consequence of MBK designs. These studies determined that the potential for polymer damage in optimally aligned MBK systems is significantly less than their fixed plateau counterparts and remains so as long as edge-loading is avoided. Further, MBK designs require high-quality polyethylene and precision manufacturing of the metallic components to ach

Introduction and Aims: Modular acetabular designs are widely used in THA procedures and now accommodate highly cross-linked polyethylene liners. However, polymer processing influences material properties, including a decrease in resistance to crack propagation. This study comparatively evaluated locking mechanism integrity of three modular acetabular designs, which employ conventional and highly cross-linked polyethylene liners.

Method: Locking mechanism integrity was established for both conventional and highly cross-linked polymers through push-out (n=3) and lever-out (n=3) testing of fully seated liners. When possible, liners were reinserted and forcibly disassembled.

Results: The push out and lever out strengths measured for the highly cross-linked polyethylene acetabular liners in this study indicated that short-term disassociation of these components is no more likely than that for the conventional polyethylene liners of each design. Student t-tests confirmed the null hypothesis. In addition, when compared to the strengths of clinically successful modular designs none of the systems evaluated presents any great risk of short-term disassociation. Significant reductions in locking mechanism strength after liner reinsertion were also noted for both polymers.

Conclusion: Short-term static disassociation failure of highly cross-linked polyethylene liners were found to be equivalent to conventional polyethylene liners. Given their touted improvement in wear reduction, they would appear to be a reasonable alternative in the young patient requiring THA. Secondarily, neither conventional nor highly cross-linked polymers should be re-inserted for any reason at the time of surgery.