D. W. Van Citters

In Vivo Oxidation in Remelted Highly Cross-Linked Retrievals

BACKGROUND Elimination of free radicals to prevent oxidation has played a major role in the development and product differentiation of the latest generation of highly cross-linked ultra-high molecular weight polyethylene bearing materials. In the current study, we (1) examined oxidation in a series of retrieved remelted highly cross-linked ultra-high molecular weight polyethylene bearings from a number of device manufacturers and (2) compared the retrieval results with findings for shelf-stored control specimens. The hypothesis was that radiation-cross-linked remelted ultra-high molecular weight polyethylene would maintain oxidative stability in vivo comparable with the stability during shelf storage and in published laboratory aging tests. METHODS Fifty remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners and nineteen remelted highly cross-linked ultra-high molecular weight polyethylene tibial inserts were received after retrieval from twenty-one surgeons from across the U.S. Thirty-two of the retrievals had been in vivo for two years or more. Each was measured for oxidation with use of Fourier transform infrared spectroscopy. A control series of remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners from three manufacturers was analyzed with electron paramagnetic resonance spectroscopy to measure free radical content and with Fourier transform infrared spectroscopy to measure oxidation initially and after eight to nine years of shelf storage in air. RESULTS The never-implanted, shelf-aged controls had no measurable free-radical content initially or after eight to nine years of shelf storage. The never-implanted controls showed no increase in oxidation during shelf storage. Oxidation measurements showed measurable oxidation in 22% of the retrieved remelted highly cross-linked liners and inserts after an average of two years in vivo. CONCLUSIONS Because never-implanted remelted highly cross-linked ultra-high molecular weight polyethylene materials had no measurable free-radical concentration and no increase in oxidation during shelf storage, these materials were expected to be oxidation-resistant in vivo. However, some remelted highly cross-linked ultra-high molecular weight polyethylene retrievals showed measurable oxidation after an average of more than two years in vivo. This apparent departure from widely expected behavior requires continued study of the process of in vivo oxidation of ultra-high molecular weight polyethylene materials.

In vivo oxidation in retrieved highly crosslinked tibial inserts

The current study determined (I) the environment where oxidation in a series of retrieved, HXL UHMWPE tibial inserts occurred (in vivo or postexplant); and (II) the effect of fabrication variables (irradiation source, irradiation dose) and thermal processing after irradiation (annealing or remelting) on oxidation resistance. Hypotheses examined are (1) HXL UHMWPE tibial inserts have potential to oxidize in vivo, and (2) annealed HXL UHMWPE oxidizes at a higher rate in vivo than remelted HXL UHMWPE. Highly crosslinked UHMWPE tibial inserts (87), received by an IRB-approved retrieval laboratory from 20 surgeons at 10 institutions across the U.S., were analyzed from 2005 to 2011. Fourier transform infrared spectroscopy was used to determine oxidation and trans-vinylene index for each retrieved insert. Measured oxidation that was maximum subsurface was found in 56% of all HXL tibial inserts. This maximum oxidation correlated significantly with in vivo time, trans-vinylene index, and thermal processing after irradiation. Articular oxidation rate correlated with crosslinking irradiation dose and thermal processing after irradiation. Retrieved below-melt annealed tibial inserts had significantly higher articular oxidation rates than remelted tibial inserts (p < 0.001). Articular oxidation rates correlated positively with cross-linking irradiation dose and postirradiation thermal processing. Edge oxidation correlated with postirradiation thermal processing. Oxidation of HXL UHMWPE may have clinical implications for tibial inserts, since loss of UHMWPE toughness resulting from oxidation has led to fatigue damage in gamma-sterilized tibial inserts.

Oxidation of Ultra-High Molecular Weight Polyethylene and Its Influence on Contact Fatigue and Pitting of Knee Bearings

Tibial hearings of total knee replacements are generally made from ultra-high molecular weight polyethylene (UHMWPE). Failure of those hearings has been found to result in many cases from subsurface crack initiation and propagation. Two types of subsurface-originated failure modes are investigated in this study: macroscopic contact fatigue and microscopic pitting. The cracks responsible for both modes of failure initiate in polyethylene material that has been embrittled by oxidation; this oxidation is an undesirable outcome of the gamma irradiation (in air) used to sterilize the tibial bearings. Accelerated aging was used in this work to achieve oxidation levels similar to those found in retrieved bearings. Tribotesting of the aged bearing materials was carried out under simulated service conditions using a rolling/sliding tester. Macroscopic fatigue cracks and microscopic surface pitting developed in UHMWPE specimens tested on the rolling/sliding tester, and the damage was similar to that found in retrieved tibial bearings. The fatigue cracks invariably initiated in the embrittled oxidized layer, and the number of cycles before initiation of the fatigue cracks was dependent on the oxidation level and on the contact stress. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

Delamination of a highly cross-linked polyethylene liner associated with titanium deposits on the cobalt-chromium modular femoral head following dislocation

Retrieval studies of total hip replacements with highly cross-linked ultra-high-molecular-weight polyethylene liners have shown much less surface damage than with conventional ultra-high-molecular-weight polyethylene liners. A recent revision hip replacement for recurrent dislocation undertaken after only five months revealed a highly cross-linked polyethylene liner with a large area of visible delamination. In order to determine the cause of this unusual surface damage, we analysed the bearing surfaces of the cobalt-chromium femoral head and the acetabular liner with scanning electron microscopy, energy dispersive x-ray spectroscopy and optical profilometry. We concluded that the cobalt-chromium modular femoral head had scraped against the titanium acetabular shell during the course of the dislocations and had not only roughened the surface of the femoral head but also transferred deposits of titanium onto it. The largest deposits were 1.6 microm to 4.3 microm proud of the surrounding surface and could lead to increased stresses in the acetabular liner and therefore cause accelerated wear and damage. This case illustrates that dislocations can leave titanium deposits on cobalt-chromium femoral heads and that highly cross-linked ultra-high-molecular-weight polyethylene remains susceptible to surface damage.

The use of highly crosslinked polyethylene in total knee arthroplasty

&NA; Advances in polyethylene (PE) in total hip arthroplasty have led to interest and increased use of highly crosslinked PE (HXLPE) in total knee arthroplasty (TKA). Biomechanical data suggest improved wear characteristics for HXLPE inserts over conventional PE in TKA. Short‐term results from registry data and few clinical trials are promising. Our aim is to present a review of the history of HXLPEs, the use of HXLPE inserts in TKA, concerns regarding potential mechanical complications, and a thorough review of the available biomechanical and clinical data.

Failure analysis of orthopaedic implants

Abstract: It is generally accepted that failure of total joint arthroplasty can be broadly attributed to patient factors, procedure factors or device factors. Whereas patient and procedure factors account for many revisions in any year, the majority of revisions are due to prosthesis failure. Because manufacturers do not proactively seek to evaluate retrieved devices, which could demonstrate long-term problems, it is imperative that retrieval centers exist to identify industry-wide problems that may impact thousands of patients. This chapter examines the history of retrieval laboratories, the typical failure modes of implants and the tools used to evaluate retrieved devices.

WEAR EVALUATION OF CROSSLINKED UHMWPE FOR KNEE PROSTHESES USING A MULTI- STATION ROLLING-SLIDING TRIBOTESTER

Tribology-related failure of total knee replacement prostheses can lead to risky and expensive revision surgery. Recent changes in polyethylene processing may reduce the incidence of, or even eliminate contact fatigue failure related to bearing oxidation, but devices are still subject to failure modes relating to wear or non-oxidation related fatigue. Contemporary materials claim wear reduction and improved oxidation resistance through various crosslinking and annealing steps, but clinical wear performance of these materials in the knee is difficult to determine. A device incorporating clinically relevant motions and stress states has been used in this work to compare the wear properties of crosslinked polyethylene materials subjected to different irradiation and sterilization conditions. It can potentially be used to analyze the wear behavior of polyethylene materials from different manufacturers, allowing for better prediction of relative performance in vivo.Copyright