Oxidation of Second Generation Sequentially Irradiated and Annealed Highly Cross-Linked X3™ Polyethylene Tibial Bearings

Oxidation and Damage Mechanisms of Second-Generation Highly Cross-Linked Polyethylene Tibial Inserts

BACKGROUND

After clinical introduction in 2005, sequentially annealed, highly cross-linked polyethylene (SA HXLPE) was studied for retrievals with short implantation times; however, long-term follow-ups are lacking. The objective of this study was to examine and compare the revision reasons, damage mechanisms, and oxidation indices of SA HXLPE and conventional gamma inert-sterilized (Gamma Inert) ultra-high-molecular-weight polyethylene tibial inserts implanted for >5 years.

METHODS

There were 74 total knee arthroplasty tibial inserts (46 SA HXLPEs, 28 Gamma Inerts) implanted for >5 years (mean 7 ± 2 years) retrieved as part of a multicenter retrieval program. Cruciate-retaining implants comprised 44% of the SA HXLPEs and 14% of the Gamma Inerts. Patient factors and revision reasons were collected from revision operating notes. A semiquantitative scoring method was used to assess surface damage mechanisms. Oxidation was measured using Fourier transform infrared microscopy according to American Society for Testing and Materials 2102. Differences between cohorts were assessed with Mann-Whitney U-tests.

RESULTS

Loosening (Gamma Inert: 17 of 28, SA HXLPE: 15 of 46) and instability (Gamma Inert: 6 of 28, SA HXLPE: 15 of 46) were the most common revision reasons for both cohorts. The most prevalent surface damage mechanisms were burnishing, pitting, and scratching, with burnishing of the condyles being higher in Gamma Inert components (P = .022). Mean oxidation was higher in the SA HXLPE inserts at the articulating surface (P = .002) and anterior-posterior faces (P = .023). No difference was observed at the backside surface (P = .060).

CONCLUSIONS

Revision reasons and surface damage mechanisms were comparable in the Gamma Inert and SA cohorts. Further studies are needed to continue to assess the in vivo damage and clinical relevance, if any, of oxidation in SA HXLPE over longer implantation times, particularly for implants implanted for more than 10 years.

Sequentially Irradiated and Annealed Highly Cross-Linked Polyethylene: Clinical and Radiographic Outcomes in Total Knee Arthroplasty at 10-Year Follow-up

Highly cross-linked polyethylene (HXLPE) has become the preferred bearing surface in total hip arthroplasty. However, its acceptance in total knee arthroplasty (TKA) has not been as robust because of concerns pertaining to wear and its impact on implant failure. Therefore, this multicenter study was purposed to evaluate the 10-year (1) radiographic outcomes; (2) complications; and (3) implant survivorship in patients with TKA receiving a sequentially irradiated and annealed HXLPE. A retrospective, multi-center study was performed on 139 patients (171 TKAs) who underwent primary TKA with HXLPE and possessed a minimum of 10-year follow-up. Radiographs were analyzed for radiolucencies along the implant-fixation interface using the Modern Knee Society Radiographic Evaluation System. Kaplan-Meier analysis determined implant survivorship when the end points were revision for polyethylene wear and polyethylene revision for any reason. Sixteen TKAs (9.9%) demonstrated periprosthetic linear radio-lucencies. Seventeen TKAs (9.9%) required additional surgeries, 9 (5.3%) of which were revisions, with 1 (0.6%) TKA requiring revision because of polyethylene wear. Other causes of revision included instability (1.8%), infection (1.6%), and arthrofibrosis (1.6%). The mean time to revision was 5.9 years (range, 0.1-11.1 years). Survivorship pertaining to polyethylene revision for wear was 99.4%, whereas all-cause polyethylene revision was 94.7%. This study in patients undergoing primary TKA using a second-generation HXLPE demonstrated excellent results with respect to polyethylene wear characteristics and strength with a 99.4% survivorship at 10 years. [Orthopedics. 2023;46(2):e111-e117.].

Endurance Behavior of Cemented Tibial Tray Fixation Under Anterior Shear and Internal-External Torsional Shear Testing: A New Methodological Approach

BACKGROUND

Early total knee arthroplasty failures continue to surface in the literature. Cementation technique and implant design are two of the most important scenarios that can affect implant survivorship. Our objectives were to develop a more suitable preclinical test to evaluate the endurance of the implant-cement-bone interface under anterior shear and internal-external (I/E) torsional shear testing condition in a biomechanical sawbones.

METHODS

Implants tested included the AS VEGA System PS and the AS Columbus CR/PS (Aesculap AG, Germany), with zirconium nitride (ZrN) coating. Tibial implants were evaluated under anterior shear and I/E torsional shear conditions with 6 samples in 4 test groups. For the evaluation of the I/E torsional shear endurance behavior, a test setup was created allowing for clinically relevant I/E rotation with simultaneous high axial/tibio-femoral load. The test was performed with an I/E displacement of ±17.2°, for 1 million cycles with an axial preload of 3,000 N.

RESULTS

After the anterior shear test an implant-cement-bone fixation strength for the AS VEGA System tibial tray of 2,674 ± 754 N and for the AS Columbus CR/PS tibial tray of 2,177 ± 429 N was determined (P = .191). After I/E rotational shear testing an implant-cement-bone fixation strength for the AS VEGA System PS tray of 2,561 ± 519 N and for the AS Columbus CR/PS tray of 2,824 ± 515 N was resulted (P = .39).

CONCLUSION

Both methods had varying degrees of failure modes from debonding to failure of the sawbones foam. These two intense biomechanical loading tests are more strenuous and more representative of clinical activity.

Evaluation of Safety and Medium-Term Functional Outcomes of a Medial Fixed-Bearing Unicompartmental Knee Arthroplasty with Ultra-Highly Cross-Linked Polyethylene

The primary objective of this study was to evaluate the in vivo safety of a unicompartmental knee arthroplasty design with sequentially annealed cross-linked polyethylene by evaluating reoperation rate, in particular those related to excessive polyethylene wear or breakage. The secondary objective was to examine functional outcomes via standardized questionnaires. This was a 5-year institutional review board-approved prospective single-surgeon case series of the first 152 consecutive patients with symptomatic medial unicompartmental osteoarthritis implanted with a partial knee replacement between May 2010 and December 2014. Study participants were asked to complete the Knee injury and Osteoarthritis Outcome Score (KOOS) and Western Ontario and McMaster Universities Osteoarthrtis Index (WOMAC) questionnaires at preoperation and 2 to 5 years postoperation. Major complications and all reoperations were recorded and we produced Kaplan-Meier survivorship curves with the end point of revision to TKA. Pre- and postoperative differences for KOOS and WOMAC were evaluated by paired t-tests. The median length of follow-up was 7.2 (0-9.72) years. Seven patients required revision surgery to TKA (4.9% of patients): four with progression of arthritis in other compartments, two for infection, and one for loosening of the femoral component and subsequent progression of pain. There were no failures of polyethylene. Survival of cohort was 99.3 and 97.9% at 2 and 5 years, respectively. Patients significantly improved (p-value < 0.001) between preoperative assessment and at 2 years, with no decline at 5 years postoperation. These preliminary midterm results with this fixed-bearing design and cross-linked polyethylene were encouraging with no catastrophic failures of polyethylene. Patient reported outcomes were significantly improved and revision rates were acceptable and lower than registry reported results.

Massive Osteolysis in a Modern Total Knee Prosthesis

Case

An 82-year-old woman underwent right total knee replacement with a sequentially irradiated and annealed highly cross-linked polyethylene insert. At 9 years, she was found to have a massive femoral osteolysis with an impending fracture.

Conclusion

This case demonstrates a rare occurrence of massive femoral osteolysis, requiring revision surgery, with a sequentially irradiated and annealed highly cross-linked polyethylene.

Synergy between vitamin E and D-sorbitol in enhancing oxidation stability of highly crosslinked ultrahigh molecular weight polyethylene

Oxidative stability of radiation crosslinked ultrahigh molecular weight polyethylene (UHMWPE) artificial joints is significantly improved by vitamin E (VE), but there is a dilemma that VE hinders crosslinking and thus jeopardizes the wear of UHMWPE. In this effort, we proposed an efficient strategy to stabilize UHMWPE under limited antioxidant contents, where VE and D-sorbitol (DS) were used as the primary antioxidant and the secondary antioxidant respectively. For non-irradiated blends with fixed antioxidant contents, oxidative stability accessed by oxidation induction time (OIT) of VE/DS/UHMWPE blends was superior to that of VE/UHMWPE blends, while DS/UHMWPE blends showed no increase in OIT. The cooperation between DS and VE exhibited a synergistic effect on enhancing the oxidative stability of UHMWPE. Interestingly, the irradiated VE/DS/UHMWPE blends showed comparable OIT but a significantly higher crosslink density than the irradiated VE/UHMWPE blends. The crystallinity, melting point, and in vitro biocompatibility of the blends were not affected by VE and DS. The quantitative relationships of mechanical properties, oxidation stability, crystallinity and crosslink density were established to unveil the correlation of these key factors. The overall properties of VE/UHMWPE and VE/DS/UHMWPE blends were compared to elucidate the superiority of the antioxidant compounding strategy. These findings provide a paradigm to break the trade-off between oxidative stability, crosslink density and mechanical properties, which is constructive to develop UHMWPE bearings with upgraded performance for total joint replacements. STATEMENT OF SIGNIFICANCE: VE-stabilized UHMWPE is the most commonly used material in total joint replacements at present. However, oxidation and wear resistance of VE/UHMWPE implants cannot be unified since VE reduces the efficiency of radiation crosslinking. It limits the use of VE. Herein, we proposed a compounding stabilization by the synergy between VE and DS. The antioxidation capability of VE was revived by DS, thus enhancing the oxidation stability of unirradiated UHMWPE. The irradiated VE/DS/UHMWPE exhibited similar oxidation stability but higher crosslink density than irradiated VE/UHMWPE, which is beneficial to combat wear of UHMWPE and to inhibit the occurrence of osteolysis. This synergistic antioxidation strategy endows the UHMWPE joint material with good overall performance, which is of clinical significance.

Influence of radiation conditions on the wear behaviour of Vitamin E treated UHMWPE gliding components for total knee arthroplasty after extended artificial aging and simulated daily patient activities

The long term performance of total knee arthroplasty (TKA) with regards to the bearing materials is related to the aging behaviour of these materials. The use of highly crosslinked materials in hip arthroplasty improved the clinical outcome. Nevertheless, the outcome for these materials compared to conventional UHMWPE (ultra-high molecular weight polyethylene) remains controversial in TKA and alternative bearing materials may be advantageous to improve its outcome in the second and third decade. The aim of this study is the evaluation of the influence of radiation conditions on the wear behaviour of Vitamin E blended UHMWPE gliding components for TKA by simulation of extended aging and high demanding daily patient activities. For a medium radiation dose (30 kGy), the influence of the irradiation type (E-beam or Gamma radiation) and the thermal conditions (room temperature (RT) or heated to 115 °C) are evaluated in comparison to non-irradiated material. Significant influences on the wear behaviour were found for the radiation source and temperature during irradiation. Furthermore, no relevant degradation of the tested materials was observed after extended artificial aging. There was a good correspondence between the wear pattern in this study and retrievals.

Early failure of sequentially annealed polyethylene in total knee arthroplasty

Improvements in the processing of polyethylene have led to a dramatic reduction in wear rates in total hip arthroplasty. This led to the adoption of modern highly cross-linked polyethylene in total knee arthroplasty (TKA). However, the differences in modes of wear and failure between total hip arthroplasty and TKA have tempered expectations regarding similar decreases in polyethylene-related complications in TKA. We present a case of early catastrophic failure of a modern sequentially irradiated and annealed highly cross-linked polyethylene insert only 5 years after contemporary cementless TKA.

Influence of Irradiation Temperature on Oxidative and Network Properties of X-Ray Cross-Linked Vitamin E Stabilized UHMWPE for Hip Arthroplasty

Previous studies have shown that increased cross-link density, reduced free radicals, and increased antioxidant grafting resulting from electron-beam irradiation at elevated temperatures improved the wear performance and the oxidative stability of vitamin E blended UHMWPE. The current study explores the impact of elevated irradiation temperature on vitamin E blended UHMWPE using X-ray. We hypothesize that the effects of temperature would be similar to those observed after electron-beam irradiation due to the relatively high dose rate of X-rays. Two X-ray doses of 80 and 100 kGy and two irradiation temperatures, that is, room temperature and 100°C were considered. The reference was Vitelene®, a vitamin E stabilized polyethylene cross-linked with 80 kGy by e-beam at 100°C. Oxidation index and oxidation induction time, as well as cross-link density, gel fraction, and trans-vinylene index, were determined, as the oxidative and network properties are decisive for the long-term implant performance. Gel fraction and oxidation induction time were significantly improved subsequently to warm irradiation in comparison with the material irradiated at room temperature. In conclusion, X-ray irradiation at elevated temperatures resulted in an increase of cross-linking and oxidative resistance of vitamin E stabilized polyethylene comparable to those of e-beam irradiated UHMWPE.

Oxidation, Damage Mechanisms, and Reasons for Revision of Sequentially Annealed Highly Crosslinked Polyethylene in Total Knee Arthroplasty

BACKGROUND

Sequentially annealed, highly crosslinked polyethylene (HXLPE) has been used clinically in total knee arthroplasty (TKA) for over a decade. However, little is known about the revision reasons; its surface damage mechanisms; or its in vivo oxidative stability relative to conventional polyethylene. We asked whether retrieved HLXPE tibial inserts exhibited: (1) similar revision reasons; (2) improved resistance to surface damage; and (3) improved oxidative stability, when compared with conventional gamma inert sterilized polyethylene inserts.

METHODS

A total of 456 revised tibial inserts were collected in a multicenter retrieval program between 2000 and 2016. The implantation time for the HXLPE components was 1.8 ± 1.8 years, and for the control inserts it was 3.4 ± 2.7 years. Revision reasons were assessed based on medical records, radiographs, and examinations of the retrieved components. Surface damage was assessed using a semi-quantitative scoring method. Oxidation was measured using Fourier transform infrared spectroscopy.

RESULTS

The tibial inserts in both cohorts were revised most frequently for loosening, infection, and instability. The most commonly observed surface damage modes were burnishing, pitting, and scratching. Oxidation of the HXLPE inserts was, on average, low and similar to the control inserts at the bearing surface and the stabilizing post.

CONCLUSIONS

We observed evidence of in vivo oxidation in both HXLPE and control tibial inserts. We found no association between the levels of oxidation and the clinical performance of the HXLPE tibial components. The findings of this study document the revision reasons, surface damage modes, and oxidative behavior of sequentially annealed HXLPE for TKA.

Influence of conformity on the wear of total knee replacement: An experimental study

Wear of total knee replacement continues to be a significant factor influencing the clinical longevity of implants. Historically, failure due to delamination and fatigue directed design towards more conforming inserts to reduce contact stress. As new generations of more oxidatively stable polyethylene have been developed, more flexibility in bearing design has been introduced. The aim of this study was to investigate the effect of insert conformity on the wear performance of a fixed bearing total knee replacement through experimental simulation. Two geometries of insert were studied under standard gait conditions. There was a significant reduction in wear with reducing implant conformity. This study has demonstrated that bearing conformity has a significant impact on the wear performance of a fixed bearing total knee replacement, providing opportunities to improve clinical performance through enhanced material and design selection.

The use of highly crosslinked polyethylene in total knee arthroplasty

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. Cite this article: Bone Joint J 2017;99-B:996-1002.

Ultra-High Molecular Weight Polyethylene: Influence of the Chemical, Physical and Mechanical Properties on the Wear Behavior. A Review

Ultra-high molecular weight polyethylene (UHMWPE) is the most common bearing material in total joint arthroplasty due to its unique combination of superior mechanical properties and wear resistance over other polymers. A great deal of research in recent decades has focused on further improving its performances, in order to provide durable implants in young and active patients. From "historical", gamma-air sterilized polyethylenes, to the so-called first and second generation of highly crosslinked materials, a variety of different formulations have progressively appeared in the market. This paper reviews the structure-properties relationship of these materials, with a particular emphasis on the in vitro and in vivo wear performances, through an analysis of the existing literature.

High temperature homogenization improves impact toughness of vitamin E-diffused, irradiated UHMWPE

Diffusion of vitamin E into radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is used to increase stability against oxidation of total joint implant components. The dispersion of vitamin E throughout implant preforms has been optimized by a two-step process of doping and homogenization. Both of these steps are performed below the peak melting point of the cross-linked polymer (<140°C) to avoid loss of crystallinity and strength. Recently, it was discovered that the exposure of UHMWPE to elevated temperatures, around 300°C, for a limited amount of time in nitrogen, could improve the toughness without sacrificing wear resistance. We hypothesized that high temperature homogenization of antioxidant-doped, radiation cross-linked UHMWPE could improve its toughness. We found that homogenization at 300°C for 8 h resulted in an increase in the impact toughness (74 kJ/m² compared to 67 kJ/m² ), the ultimate tensile strength (50 MPa compared to 43 MPa) and elongation at break (271% compared to 236%). The high temperature treatment did not compromise the wear resistance or the oxidative stability as measured by oxidation induction time. In addition, the desired homogeneity was achieved at a much shorter duration (8 h compared to >240 h) by using high temperature homogenization. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1343-1347, 2017.

Vitamin E stabilised polyethylene for total knee arthroplasty evaluated under highly demanding activities wear simulation

As total knee arthroplasty (TKA) patients are getting more active, heavier and younger and structural material fatigue and delamination of tibial inserts becomes more likely in the second decade of good clinical performance it appears desirable to establish advanced pre-clinical test methods better characterizing the longterm clinical material behaviour. The questions of our study were 1) Is it possible to induce subsurface delamination and striated pattern wear on standard polyethylene TKA gliding surfaces? 2) Can we distinguish between γ-inert standard polyethylene (PEstand.30kGy) as clinical reference and vitamin E stabilised materials (PEVit.E30kGy & PEVit.E50kGy)? 3) Is there an influence of the irradiation dose (30vs 50kGy) on oxidation and wear behaviour? Clinical relevant artificial ageing (ASTM F2003; 2weeks) of polyethylene CR fixed TKA inserts and oxidation index measurements were performed by Fourier transform infrared spectroscopy prior testing. The oxidation index was calculated in accordance with ISO 5834-4:2005 from the area ratio of the carbonyl peak (between 1650 and 1850cm^-1) to the reference peak for polyethylene (1370cm^-1). Highly demanding patient activities (HDA) measured in vivo were applied for 5million knee wear cycles in a combination of 40% stairs up, 40% stairs down, 10% level walking, 8% chair raising and 2% deep squatting with up to 100° flexion. After 3.0mc all standard polyethylene gliding surfaces developed noticeable areas of progressive delamination. Cumulative gravimetric wear was 355.9mg for PEstand.30kGy, 28.7mg for PEVit.E30kGy and 26.5mg for PEVit.E50kGy in HDA knee wear simulation. Wear rates were 12.4mg/mc for PEstand.30kGy in the linear portion (0-2mc), 5.6mg/mc for PEVit.E30kGy and 5.3mg/mc for PEVit.E50kGy. In conclusion, artificial ageing of standard polyethylene to an oxidation index of 0.7-0.95 in combination with HDA knee wear simulation, is able to create subsurface delamination, structural material fatigue in vitro, whereas for the vitamin-E-blended materials no evidence of progressive wear, fatigue or delamination was found.

STATEMENT OF SIGNIFICANCE

As total knee arthroplasty patients are getting more active, heavier and younger and structural material fatigue and delamination of polyethylene tibial inserts becomes more likely in the second decade of good clinical performance, it appears desirable to establish advanced pre-clinical test methods better characterizing the longterm clinical material behaviour. Various studies reported in literature attempted to artificially create delamination during in vitro knee wear simulation. We combined artificial ageing to clinically observed oxidation of gamma inert and vitamin E stabilised polyethylene inserts and highly demanding patient activities knee wear simulation based on in vivo load data. With this new method we were able to create clinically relevant subsurface delamination and structural material fatigue on standard polyethylene inserts in vitro.

Grafted Vitamin-E UHMWPE may increase the durability of posterior stabilized and constrained condylar total knee replacements

The suitability of grafted vitamin-E highly crosslinked polyethylene (VE-HXPE) for use in posterior stabilized (PS) and constrained condylar knee (CCK) applications has not been explored. We hypothesized that VE-HXPE performs better than conventional and crosslinked polyethylene under clinically relevant conditions. PS tibial post fracture resistance under adverse shear loading conditions, CCK tibial post resistance to torsional fatigue, delamination resistance under high stress, and wear resistance were evaluated. Grafted VE-HXPE exhibits (1) 10% and 57% improved PS post fatigue strength compared to conventional PE (CPE) and remelted HXPE; (2) 45% improved CCK post fatigue strength compared to CPE; (3) Greater than 36× the delamination resistance of CPE; and (4) 96% and 73% wear reduction compared to CPE and HXPE. VE-HXPE performed well under clinically relevant in vitro conditions. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1789-1798, 2017.

Multicenter Study of Highly Cross-linked vs Conventional Polyethylene in Total Knee Arthroplasty

BACKGROUND

Despite substantial interest and use of highly cross-linked polyethylene (HXLPE) in total knee arthroplasty (TKA), outcomes remain largely unknown. The purpose of this study is to compare HXLPE and conventional polyethylene at 4- to 5-year follow-up.

METHODS

A prospective multicenter study of 307 posterior-stabilized TKAs (168 conventional and 139 HXLPE) was performed. Short-Form-6D, Short-Form 36, Knee Society Score, Lower Extremity Activity Score, health-related quality of life outcomes, and radiographs were collected preoperatively and at routine postoperative intervals.

RESULTS

Two hundred twenty-four patients obtained a minimum 4- to 5-year follow-up for analysis. There were no osteolysis or polyethylene failures in either group. Although both conventional and HXLPE poly groups showed significant improvements in all measures from preoperative baselines (P < .05), the XLPE group had slightly greater mean Knee Society Score function (P = .04), Lower Extremity Activity Score (P = .03), and Short-Form 36 Physical Composite Score (P = .03) scores and a greater improvement in Short-Form 6D health-related quality of life of 0.16 points (d = 1.02, 95% CI: 0.01-1.11) at latest follow-up.

CONCLUSION

The study findings support comparative safety and outcomes of HXLPE related to mechanical failure or osteolysis in the midterm. However, longer-term follow-up is warranted to assess whether wear and mechanical properties of HXLPE are maintained in vivo.