Comparison of conventional and highly crosslinked UHMWPE patellae evaluated by a new in vitro patellofemoral joint simulator.

A series of in vitro patella damage tests were developed using a knee simulator to investigate electron-beam-irradiated and -melted ultra-high molecular weight polyethylene and conventional ultra-high molecular weight polyethylene at the patellofemoral articulation. Three different simulations were created: (i) normal gait with optimal component alignment, (ii) stair climbing with optimal component alignment, and (iii) stair climbing with 4 degrees of femoral component internal rotation to simulate a component malalignment condition. In the last two simulations, all patellae were artificially aged. In normal gait, the unaged conventional and highly crosslinked patellae demonstrated similar behavior. In both stair climbing tests, unlike the aged highly crosslinked components, the aged conventional patellae developed cracks by 2 million cycles. These results demonstrate the potential advantage of highly crosslinked polyethylene for the patella.

Highly cross-linked polyethylene: the debate is over--in the affirmative.

Highly radiation-cross-linked, subsequently melted ultrahigh molecular weight polyethylene (XLP) has rapidly gained wide acceptance in total joint arthroplasty as a means to markedly reduce wear and the associated periprosthetic osteolysis. Several laboratory studies have shown negligible wear of XLP in both hip and knee for durations simulating 20 years of in vivo service. Three clinical studies involving 3 different demographic groups and 2 different measurement techniques have found that the femoral head penetration after bedding-in is less than 10 mm/y. Retrieval acetabular and tibial specimens up to 3 years after insertion have confirmed the absence of wear and oxidation. With in vivo experience now over 5 years, it appears that XLP is the material of choice in total joint arthroplasty.

Effect of oral alendronate on net bone ingrowth into canine cementless total hips.

Alendronate has been shown to prevent osteolysis in a canine total hip arthroplasty (THA) model. However, the effect of alendronate on bone ingrowth and remodeling around canine cementless hip replacement components remains unclear. We hypothesized that oral alendronate would increase net bone ingrowth into the porous surface of THA components when intimate bone/implant contact exists. To test this hypothesis, six mature dogs received unilateral cementless THA and were treated with five milligrams of oral alendronate daily from day seven to day eighty-four postoperatively. Six comparable dogs served as controls. Quantitative analysis of net bone ingrowth into the porous surface did not show a significant difference between the two groups, (control 7.9 +/- 1.5, alendronate 7.5 +/- 1.4, P < 0.6). These results did not support the hypothesis that oral alendronate would increase net new bone formation into the porous surface of THA components. Key words: alendronate, bone ingrowth, total hip arthroplasty.

Third-body wear testing of a highly cross-linked acetabular liner: the effect of large femoral head size in the presence of particulate poly(methyl-methacrylate) debris.

The hip simulator wear performance of an electron beam cross-linked and subsequently melted ultrahigh molecular weight polyethylene against femoral heads of 28-, 38-, and 46-mm diameter in the presence of poly(methyl-methacrylate) particulate debris was contrasted with that of conventional polyethylene against a 46-mm diameter head. Over 5 million cycles of testing, the average wear rate of the conventional polyethylene liners was 29.3 +/- 3.0 mg per million cycles. All highly cross-linked components exhibited marked reduction in wear, with the highest wear measuring 0.74 +/- 0.85 mg per million cycles. This study, using a clinically relevant third-body material, showed the electron beam cross-linked material to be far more resistant to this third-body wear than conventional ultrahigh molecular weight polyethylene, even when very large diameter femoral heads were used.

Knee-simulator testing of conventional and cross-linked polyethylene tibial inserts.

We compared the resistance to delamination and to adhesive/abrasive wear of conventional and highly cross-linked polyethylene tibial inserts of a cruciate-retaining total knee design using a knee simulator. Both groups were tested after aggressive, accelerated aging, and 1 set of conventional inserts was studied without aging. Aging oxidized the conventional, but not the highly cross-linked, inserts. The simulated normal gait testing lasted for 5 and 10 million cycles for the conventional and highly cross-linked inserts, respectively. Aged conventional inserts showed delaminations, whereas none were observed in the unaged conventional and aged cross-linked inserts. Wear rates measured by the gravimetric method were 9 +/- 2 mm3, 10 +/- 4 mm3, and 1 +/- 0 mm3 per million cycles; by the metrologic method, they were 8 +/- 1 mm3, 9 +/- 2 mm3, and 3 +/- 0 mm3 for the unaged conventional, aged conventional, and aged highly crosslinked inserts, respectively. In the test model used, oxidation led to delamination, whereas increased cross-link density resulted in reduced adhesive/abrasive wear of tibial inserts.

Surface analysis of early retrieved acetabular polyethylene liners: a comparison of conventional and highly crosslinked polyethylenes.

In vivo wear behavior of 16 highly cross-linked and 19 conventional polyethylene acetabular explants, at an average in vivo duration of 6 months were studied. Highly cross-linked groups showed machining marks from the original manufacturing process in some areas and extensive scratching and some polishing of the articulating surfaces. The conventional group showed greater loss of machining marks, scratching, and polishing. Representative samples were melted to allow recovery of plastic deformation and to show true removal of material caused by wear. Melt-recovery experiments consistently showed the disappearance of surface scratches and the restoration of the original machining marks in the highly cross-linked explants. In the conventional group, few of the scratches were eliminated and only limited restoration of the machining marks was apparent. These observations support the hypothesis that the early in vivo scratching of highly cross-linked polyethylene acetabular liners is primarily caused by plastic deformation.

The efficacy of BMP-2 to induce bone ingrowth in a total hip replacement model.

The purpose of this canine total hip arthroplasty (THA) study was fourfold: (1) to quantify the efficacy of rhBMP-2 in a carrier (alphaBSM) versus alphaBSM alone, and versus untreated controls to induce bone formation across a defect between a porous acetabular component and host bone; (2) to quantify whether rhBMP-2/alphaBSM improves bone growth into the porous surface beneath that defect; (3) to quantify the efficacy of rhBMP-2/alphaBSM in inducing bone ingrowth into the porous layer at points of intimate bone-implant contact; and (4) to determine whether rhBMP-2/alphaBSM placed in the lateral uncovered aspect of the porous acetabular component promotes de novo bone formation. Fifteen dogs were sacrificed 12 weeks after uncemented THA. Five dogs received rhBMP-2/alphaBSM, five dogs received alphaBSM, and five dogs were controls. In contrast to the controls in which no bone filled the defect, the rhBMP-2/alphaBSM induced defect filling and full bone formation in the underlying porous coating. The alphaBSM produced an intermediary response. However, no increase in new bone formation occurred at sites of intimate bone porous surface contact. Recombinant bone morphogenetic protein-2/alphaBSM promoted defect filling and bone ingrowth into the porous coating beneath the defect region, both of potential value in future total joint replacement surgery.

Effect of radiation, heat, and aging on in vitro wear resistance of polyethylene.

Radiation cross-linking increases the wear resistance of polyethylene used in total hip replacement. Radiation also generates residual free radicals, which are detrimental to long-term properties of polyethylene. Two approaches are used to stabilize the residual free radicals and terminally sterilize the components. One is postirradiation annealing with gas sterilization and the other is postirradiation melting with gamma sterilization in nitrogen. The hypothesis of the current study is that postirradiation annealing followed by gamma sterilization in nitrogen will result in more free radicals in polyethylene than gamma sterilization either in air or in nitrogen alone. To test this hypothesis, concentration of residual free radicals was quantified in polyethylene that was annealed and gamma sterilized in nitrogen and control polyethylenes gamma sterilized in air versus in nitrogen. Three crosslinked polyethylenes that were melted and gas sterilized also were included in the study. The effects of residual free radicals were studied by accelerated aging. Oxidation levels and weight loss in bidirectional pin-on-disk tests were determined before and after aging. Polyethylene that was subjected to postirradiation annealing and gamma sterilization resulted in 58% more residual free radicals than control polyethylenes. Weight loss of the annealed polyethylene increased by 16-fold on accelerated aging and had three times higher oxidation levels than that measured in control polyethylenes after aging. In contrast, polyethylenes that were stabilized with postirradiation melting and terminally gas sterilized showed no detectable residual free radicals. Accelerated aging did not affect the weight loss and oxidation levels of melted polyethylenes.

Third-body wear of highly cross-linked polyethylene in a hip simulator.

The wear performance of a radiation cross-linked melted ultrahigh-molecular-weight polyethylene (UHMWPE) articulating against 28-mm cobalt chrome femoral heads in the presence of third-body particulate debris was investigated in a hip simulator and compared with the wear of conventional UHMWPE. Particles of aluminum oxide or bone cement containing barium sulfate were added to the serum. In the presence of aluminum oxide particles, the incremental wear rates of conventional UHMWPE averaged as high as 149 +/- 116 mg/million cycles compared with 37 +/- 38 mg/million cycles for the highly cross-linked components. The difference in the average weight loss was statistically significant at P <.01. With bone cement particles, the conventional UHMWPE components had an average incremental wear rate of 19 +/- 5mg/million cycles, and the wear rate of the highly cross-linked UHMWPE components was 0.5 +/- 0.7 mg/million cycles.

Aggressive wear testing of a cross-linked polyethylene in total knee arthroplasty.

Recently, highly cross-linked polyethylenes with high wear and oxidation resistance have been developed. These materials may improve the in vivo performance of polyethylene components used in total knee arthroplasty. To date, the in vitro knee wear testing of these new polyethylenes has been done under conditions of normal gait. However, their critical assessment also must include aggressive in vitro fatigue and wear testing. In the current study, an aggressive in vitro knee wear and device fatigue model simulating a tight posterior cruciate ligament balance during stair climbing was developed and used to assess the performance of one type of highly cross-linked polyethylene tibial knee insert in comparison with conventional polyethylene. The highly cross-linked inserts and one group of conventional inserts were tested after sterilization. One additional group of conventional inserts was subjected to accelerated aging before testing. The articular surfaces of the inserts were inspected visually for surface delamination, cracking, and pitting at regular intervals during the test. The aged conventional polyethylene inserts showed extensive delamination and cracking as early as 50,000 cycles. In contrast, the unaged conventional and highly cross-linked polyethylene inserts did not show any subsurface cracking or delamination at 0.5 million cycles. The appearance and location of delamination that occurred in the aged conventional inserts tested with the current model previously have been observed in vivo with posterior cruciate-sparing design knee arthroplasties with a tight posterior cruciate ligament.

Gradient crosslinking of UHMWPE using irradiation in molten state for total joint arthroplasty.

Increased crosslink density increases the wear resistance of ultra-high molecular weight polyethylene (UHMWPE) acetabular components used in total hip reconstructions. However, increasing crosslink density can reduce the mechanical properties of UHMWPE. Other researchers have tried to limit the crosslinking to a superficial layer on the articulating surfaces to retain the properties of the virgin polymer. We present here a method of producing a gradient of crosslink density across the acetabular component. Acetabular liners with 26 mm inner diameter were machined from the UHMWPE. The liners were then irradiated at 140 degrees C in the molten state of the polymer using a 2 MeV electron beam with limited penetration of the effects of radiation into polyethylene. The liners were then machined to an inner diameter of 32 mm to remove the radiation induced oxidized surface layer. The limited penetration of the e-beam resulted in a gradient of crosslink density with a crosslink density of 0.15 +/- 0.01 mol/dm3 near the articulating surfaces and 0.12 +/- 0.01 mol/dm3 near the backside. The concentration of the trans-vinylene unsaturations decreased gradually as a function of depth from the articulating surfaces to the backside of the liners. The wear resistance of the melt-irradiated liners was contrasted with those of conventional liners using the Boston hip simulator. The gravimetric wear rate was 27 +/- 5 mg million cycles with the conventional liners, while the melt-irradiated acetabular liners did not show any weight loss.