Highly crosslinked polyethylene in posterior-stabilized total knee arthroplasty: in vitro performance evaluation of wear, delamination, and tibial post durability.

Recent advances in total knee arthroplasty (TKA) include the development of highly crosslinked polyethylene (HXPE). To assess the suitability of using HXPE in posterior-stabilized TKA, knee simulator wear testing and a novel tibial post durability test were performed on a modern posterior-stabilized implant design with both conventional polyethylene (CPE) and HXPE materials. The laboratory testing reproduced clinically relevant wear and tibial post damage mechanisms. For the designs tested, wear volume was reduced by 67% to 75% for aged HXPE compared with aged CPE. Components of HXPE also demonstrated superior tibial post durability compared with the CPE design, despite the use of unaged material to represent best-case CPE tibial post strength. With appropriate design considerations, HXPE can be successfully incorporated into a posterior-stabilized TKA.

Wear, delamination, and fatigue resistance of melt-annealed highly crosslinked UHMWPE cruciate-retaining knee inserts under activities of daily living.

The wear, delamination, and fatigue resistance of artificially aged gamma irradiation-sterilized conventional polyethylene (CPE) and gas-plasma-sterilized melt-annealed highly crosslinked polyethylene tibial inserts (HXPE) were compared. Six CPE and 12 HXPE (six irradiated at 58 kGy and six at 72 kGy) left knee inserts were wear tested for 5.5 million cycles (Mc) under loads and motions that mimic activities of daily living, such as walking, chair rise, stair ascent, and deep squatting. Another six HXPE (72 kGy) and six CPE inserts were also tested under conditions that could produce severe delamination for 8 Mc. Ten other knees (five 72 kGy HXPE and five CPE) were subjected to posterior edge loading fatigue testing for 5 Mc. The HXPE inserts had an average wear rate reduction of about 80% relative to their CPE counterparts during all activities. All of the CPE inserts delaminated and fractured during high cycle deep squat (152 degrees flexion) motions, while all the HXPE remained intact. None of the HXPE inserts delaminated after 8 Mc, while all of the CPE inserts developed delamination damage within 1.5-5.8 Mc of delamination testing. All CPE inserts developed subsurface cracks and delamination within 2.8 Mc during posterior edge loading fatigue studies, while none of the HXPE inserts showed cracking or delamination after 5 Mc. These results show that aged HXPE has higher wear and fatigue resistance than aged CPE, and offers potential long-term advantages for young active patients with sustained activities of daily living.

Characterization of a highly cross-linked ultrahigh molecular-weight polyethylene in clinical use in total hip arthroplasty.

This article reports on a commercially available extensively cross-linked ultrahigh molecular-weight polyethylene (HXPE) produced by subjecting molded GUR 1050 ultrahigh molecular-weight polyethylene (UHMWPE) to 100 +/- 10 kGy of electron beam radiation followed by melt annealing and sterilization by gas plasma. When compared to contemporary conventional molded GUR 1050 UHMWPE sterilized by 37 kGy of gamma radiation, the HXPE material has enhanced wear properties, has no detectable free radicals, and is resistant to oxidation and oxidative-related material property changes. The relative wear improvement of the HXPE is maintained in the presence of bone cement or alumina particles. The HXPE produced greater than 90% fewer wear particles in all size ranges and statistically significantly (P < .0001) smaller average-size particles than did the conventional UHMWPE.

The use of functional analysis in evaluating knee kinematics.

The importance of understanding the six-degrees-of-freedom kinematics of the knee during ambulatory activities was examined in the context of the function of total knee arthroplasty. Studies of knee kinematics during walking, stair climbing, and a deep flexion squat indicate that knee kinematics is activity-dependent. A comparative study of patients and healthy subjects during stair climbing indicates the importance of maintaining the function of the posterior cruciate ligament. A second study used walking kinematics derived from patient testing as input to a wear simulator. There was increased wear relative to standard simulator input that was related to the slip velocity at the contact surface. Finally, results from a study of deep flexion indicate that substantial femoral rotation is required during deep flexion activities. The current study shows the importance of studying in vivo knee kinematics for future enhancement in the treatment of the arthritic knee.