Evaluation of sequentially crosslinked ultra-high molecular weight polyethylene

The Divergence of Wear Propagation and Stress at Steep Acetabular Cup Positions Using Ceramic Heads and Sequentially Cross-Linked Polyethylene Liners

The aim of the present wear simulator study was to assess the effect of steep acetabular cup positions on the wear propagation of highly cross-linked-PE (HX-PE) liners. Furthermore, a finite element analysis (FEA) was performed in order to calculate the stress within the HX-PE material in case of steep cup positions under physiological loadings. The higher stress in the HX-PE at a steep acetabular cup position did not result in increased wear in the present wear simulator study. The gravimetrical wear rates at normal (45°) and steep cup inclinations (75°) showed wear amounts of 3.15±0.27mg and 2.18±0.31mg per million cycles (p=0.028), respectively. However, FEA revealed clear increase in stress at the HX-PE liners with respect to steep cup positions.

Microstructural modifications induced by accelerated aging and lipid absorption in remelted and annealed UHMWPEs for total hip arthroplasty

Three types of commercially available ultra-high molecular weight polyethylene (UHMWPE) acetabular cups currently used in total hip arthroplasty have been studied by means of Raman micro-spectroscopy to unfold the microstructural modification induced by the oxidative degradation after accelerated aging with and without lipid absorption. The three investigated materials were produced by three different manufacturing procedures, as follows: irradiation followed by remelting, one-step irradiation followed by annealing, 3-step irradiation and annealing. Clear microstructural differences were observed in terms of phase contents (i.e. amorphous, crystalline and intermediate phase fraction). The three-step annealed material showed the highest crystallinity fraction in the bulk, while the remelted polyethylene is clearly characterized by the lowest content of crystalline phase and the highest content of amorphous phase. After accelerated aging either with or without lipids, the amount of amorphous phase decreased in all the samples as a consequence of the oxidation-induced recrystallization. The most remarkable variations of phase contents were detected in the remelted and in the single-step annealed materials. The presence of lipids triggered oxidative degradation especially in the remelted polyethylene. Such experimental evidence might be explained by the highest amount of amorphous phase in which lipids can be absorbed prior to accelerated aging. The results of these spectroscopic characterizations help to rationalize the complex effect of different irradiation and post-irradiation treatments on the UHMWPE microstructure and gives useful information on how significantly any single step of the manufacturing procedures might affect the oxidative degradation of the polymer.

Highly cross-linked polyethylene may not have an advantage in total knee arthroplasty

BACKGROUND

Long-term results after total knee replacement (TKR) with conventional and compression-molded polyethylene (PE) have been excellent. The introduction of highly cross-linked polyethylene (XLPE), which has demonstrated superior wear properties in total hip replacement (THR), has led to its recent use in TKR. However, the knee has a unique biomechanical environment characterized by large contact stresses and shear forces and differs from the highly conforming articulation (and primarily abrasive and adhesive wear) found in THR. For this reason, XLPE, with its decreased fatigue resistance and toughness compared to PE, may not be the best material to withstand these unique forces.

QUESTIONS

This review and evaluation of the literature aims to answer the following questions. What are the advantages and disadvantages of XLPE in TKR? Does its success in THR ensure a favorable outcome in TKR? Does the increased cost of XLPE justify its use in TKR?

METHODS

A systematic literature review of MEDLINE, Science Direct, and Google Scholar databases was performed searching for advantages and disadvantages of XLPE in TKR. We found 18 biomechanical in vitro investigations and 3 clinical studies comparing conventional and XLPEs. We included levels I through IV published articles in peer-reviewed journals in English language.

RESULTS

Several in vitro studies found XLPE to have significantly better wear properties compared to conventional PE. However, the two clinical investigations that directly compared conventional PE and XLPE found no difference in clinical or radiographic outcomes. Additionally, clinical studies with long-term follow-up on TKR with conventional PE did not find wear-induced osteolysis to be a major cause of failure. Four studies did find cost to be significantly higher for XLPE compared to conventional PE.

CONCLUSIONS

Based on our review, we concluded that (1) the material properties of XLPE reduce adhesive and abrasive wear, but not the risk of crack propagation, deformation, pitting, and delamination found in TKR; (2) wear-induced osteolysis in TKR has not been found to be a major cause of failure at long-term follow-up; (3) mid-term follow-up studies show no difference in any recorded outcome measure between conventional PE and XLPE; and (4) XLPE is two to four times the cost of conventional PE without an improvement in clinical or radiographic outcomes. For these reasons, we currently cannot recommend the use of XLPE in TKR. Conventional compression-molded polyethylene with its outstanding long-term results should remain the material of choice in TKR.

Wear testing and particle characterisation of sequentially crosslinked polyethylene acetabular liners using different femoral head sizes

Larger femoral heads lead to a decreased risk of total hip dislocation and an improved range of motion. However, the larger diameter is associated with increased wear rates. The low wear rates of crosslinked polyethylene opens up the possibility of using larger heads. The aim of this experimental study was to evaluate the wear of conventional non-crosslinked versus sequentially crosslinked polyethylene liners in combination with different ceramic head sizes (28, 36, 44 mm). Wear testing was performed in a hip simulator according to ISO 14242. Wear particles from the polyethylene liners were characterized after wear testing. The wear measurements revealed a significant increase in the wear of crosslinked polyethylene liners with larger heads. By sequential crosslinking, however, the gravimetric wear using larger heads was reduced to a fractional amount of the wear using conventional polyethylene. Significant differences were observed for particle morphology but not for the number of particles when comparing non-crosslinked and crosslinked polyethylene.

Comparison of one-step and sequentially irradiated ultrahigh-molecular-weight polyethylene for total joint replacements

Structure and properties of several ultrahigh-molecular-weight polyethylenes (UHMWPEs), which were crosslinked either by standard, single-step irradiation or by newer, several-step irradiation (sequential irradiation), were compared. To verify the results, the same characterization was carried out for commercially available UHMWPE liners made of sequentially irradiated polymer (X3™, Stryker, Mahwah, NJ) and one-step irradiated polymer (denoted as PE-IMC, produced by Beznoska, Kladno, Czech Republic). The structure was studied by a number of spectroscopic, diffraction, thermal, and microscopic methods. Mechanical properties were assessed by small-punch, microhardness and, wear testing. Our results suggested that sequential irradiation followed by annealing did not lead to unusual structure at the molecular or supermolecular level. Consequently, all measured mechanical properties, including wear resistance, were comparable with one-step irradiated UHMWPEs.

Survival of hard-on-hard bearings in total hip arthroplasty: a systematic review

BACKGROUND

Improvements in prosthetic materials, designs, and implant fixation for THA have led to bearing surface wear being the limitation of this technology. Hard-on-hard bearings promise decreased wear rates and increased survival. However, there may be different survival rates based on bearing materials, manufacturing technologies, and femoral component designs. Additionally, survival rate variability may be based on study design.

QUESTIONS/PURPOSES

We determined survival rates and study levels of evidence and quality for the following bearings: stemmed metal-on-metal THA, metal-on-metal hip resurfacing, ceramic-on-ceramic THA, and ceramic-on-metal THA.

METHODS

We performed a systematic review of the peer-reviewed literature addressing THA hard-on-hard bearings. Quality for Level I and II studies was assessed.

RESULTS

The four Level I or II second-generation stemmed metal-on-metal THA studies reported between 96% and 100% mean survival at 38 to 60 months. The two Level I hip resurfacing studies reported 94% and 98% mean survival at 56 and 33 months. The four Level I studies of ceramic-on-ceramic THA reported survival from 100% at mean 51 months to 96% at 8 years.

CONCLUSIONS

While hard-on-hard bearing survival rates have generally been variable with earlier designs, contemporary implants have demonstrated survival of 95% or greater at followup of between 3 and 10 years. Some variability in survival may be due to differences in surgical technique, component positioning, and implant designs. As bearing designs continue to improve with modified materials and manufacturing techniques, use will increase, especially in young and active patients, though concerns remain about the increased reports of adverse events after metal-on-metal bearings.

Tradeoffs amongst fatigue, wear, and oxidation resistance of cross-linked ultra-high molecular weight polyethylene

This study evaluated the tradeoffs amongst fatigue crack propagation resistance, wear resistance, and oxidative stability in a wide variety of clinically-relevant cross-linked ultra-high molecular weight polyethylene. Highly cross-linked re-melted materials showed good oxidation and wear performance, but diminished fatigue crack propagation resistance. Highly cross-linked annealed materials showed good wear and fatigue performance, but poor oxidation resistance. Moderately cross-linked re-melted materials showed good oxidation resistance, but moderate wear and fatigue resistance. Increasing radiation dose increased wear resistance but decreased fatigue crack propagation resistance. Annealing reduced fatigue resistance less than re-melting, but left materials susceptible to oxidation. This appears to occur because annealing below the melting temperature after cross-linking increased the volume fraction and size of lamellae, but failed to neutralize all free radicals. Alternately, re-melting after cross-linking appeared to eliminate free radicals, but, restricted by the network of cross-links, the re-formed lamellae were fewer and smaller in size which resulted in poor fatigue crack propagation resistance. This is the first study to simultaneously evaluate fatigue crack propagation, wear, oxidation, and microstructure in a wide variety of clinically-relevant ultra-high. The tradeoff we have shown in fatigue, wear, and oxidation performance is critical to the material's long-term success in total joint replacements.