Wear, Debris, and Biologic Activity of Cross-linked Polyethylene in the Knee

Abstract

Cross-linked polyethylene currently is being introduced in knee prostheses. The wear rates, wear debris, and biologic reactivity of non cross-linked, moderately cross-linked, and highly cross-linked polyethylene have been compared in multidirectional wear tests and knee simulators. Multidirectional pin-on-plate wear studies of noncross-linked, moderately cross-linked (5 Mrad), and highly cross-linked (10 Mrad) polyethylene showed a 75% reduction in wear with the highly cross-linked material under kinematics found in the hip, but only a 33% reduction under wear in kinematics representative of the knee. In knee simulator studies, with the fixed-bearing press-fit, condylar Sigma cruciateretaining knee under high kinematic input conditions, the wear of 5 Mrad moderately cross-linked polyethylene was 13 ± 4 mm 3 per 1 million cycles, which was lower (p < 0.05) than the wear of clinically used, gamma vacuum foil GUR 1020 polyethylene (23 ± 6 mm 3 /1 million cycles). For the lowcontact stress mobile-bearing knee, the wear of moderately cross-linked polyethylene was 2 ± 1 mm 3 per 1 million cycles, which was lower (p < 0.05) than GVF GUR 1020 polyethylene (5 ± 2 mm 3 /1 million cycles). The wear debris isolated from the fixed-bearing knees showed the moderately crosslinked material had a larger percentage volume of particles smaller than 1 µm in size, compared with GVF GUR 1020 polyethylene. Direct cell culture studies of wear debris generated in sterile wear simulators using multidirectional motion showed a increase (p < 0.05) in tumor necrosis factoralpha levels and reactivity for GUR 1050 cross-linked polyethylene debris compared with an equivalent volume of noncross-linked GUR 1050 polyethylene. The use of crosslinked polyethylene in the knee reduces the volumetric wear rate. However, the clinical significance of reduced fracture toughness, elevated wear in abrasive conditions, and the elevated tumor necrosis factor-alpha release from smaller more reactive particles warrant further investigation.Intentionally cross-linked polyethylene (PE) has been introduced extensively into hip prostheses during the last 5 years. Initial laboratory studies in hip simulators indicated that with high levels of cross-linking produced by 10 Mrad of irradiation, virtually 0 wear was produced. 2,13,14 More recently, both clinical and simulator studies of highly cross-linked PE done in more physiologically relevant conditions 5 showed a reduction (p < 0.05) in wear compared with conventional materials, with finite wear rates in the range of 5 to 10 mm 3 per 1 million cycles. Investigation of the wear debris from hip simulator studies has shown that moderately cross-linked PE has a larger percentage number and volume of smaller particles, compared with PE that has not been cross-linked. 4 These differences can only been shown in studies that analyze the whole size range of PE debris, and this is shown more clearly when the volume of wear debris rather than number of particles is analyzed. These observations are particularly important, as i...

Surface Damage Is Not Reduced With Highly Crosslinked Polyethylene Tibial Inserts at Short-term

“…The improvement in wear performance in XLPE tibial inserts is supported by wear simulator experiments [12,22,33,34], but evidence of in vivo success is required for a comprehensive assessment. In our study, retrieval analysis was used to compare the accumulation of damage in vivo of conventional and highly crosslinked PE tibial plateaus.…”

“…For example, knee simulator studies use gravimetric methods to measure the amount of PE worn from the articulating surfaces [12,22,33,34]. Of course this approach cannot be applied to retrieved implants because the original weights of the implants are unknown and the implants may show added weight through embedded thirdbody particles.…”

“…XLPE was more recently introduced into TKA after in vitro knee simulator findings of lower wear rates compared with those found with conventional PE [12,22,33,34]. Thus far, however, the clinical benefit of XLPE remains unclear.…”

The John Charnley Award: An Accurate and Sensitive Method to Separate, Display, and Characterize Wear Debris: Part 1: Polyethylene Particles

“…With followup exceeding 12 years, the incidence and extent of osteolysis have been markedly lower for hip arthroplasties using crosslinked PEs [5,27,28]. This outcome contradicts the predictions by some early studies [11,18,25], which indicated the osteolytic potentials with elevated crosslinking could be as high as or higher than for historical PEs. Our results indicated this misprediction was in great part due to inherent limitations of the protocols used to isolate and characterize the PE wear particles generated in laboratory wear simulations.…”

Wear is Reduced in THA Performed with Highly Cross-linked Polyethylene

“…A recent study [34] raised concerns that these morphologic characteristics of the wear particles might affect macrophage response and osteolysis. These concerns are reinforced by the results of studies that show increased in vitro bioactivity with the HCLPE particles [16,26]. Only longer followup studies will clarify these questions.…”