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Bettlach, Switzerland

Kohut G.,Hopital Cantonal | Dallmann F.,Mathys Ltd. Bettlach
Journal of Biomechanics | Year: 2012

The notching phenomenon is one of the major concerns with reversed total shoulder arthroplasty. Repetitive contact between the humeral implant and the scapula (mechanical notching) produces progressive abrasion of the implant if the moving part is made of polyethylene. Its debris may then lead to active osteolysis (biological notching). Inversion of bearing materials, i.e. Glenosphere made of polyethylene and humeral Inlay made of metal, aims at the reduction of this phenomenon. However, the question arises if the tribological behavior would then be different. On an experimental setup, the gravimetric wear of both material configurations was measured after loading and moving over 500,000 cycles. The abrasion of the polyethylene Inlay due to mechanical notching was calculated by means of 3D CAD models with different notching stages. The loss of mass due to gravimetric wear was compared to the loss of mass caused by mechanical notching. After 500,000 cycles the measured amount of wear of the polyethylene components was between 8 and 10. mg for both tribological pairings. The calculated loss of mass of the polyethylene Inlay caused by mechanical notching ranged from 73 to 3881. mg. The results of this study indicate that the gravimetric polyethylene wear in the estimated life-time is very low and not significantly different between both material configurations. However, the polyethylene abrasion due to mechanical notching in the configuration with polyethylene Inlay is by far more important than any gravimetric wear. These results support the continued use of inverted bearings in reversed total shoulder arthroplasty. © 2011 Elsevier Ltd.

Lerf R.,Mathys Ltd. Bettlach | Zurbrugg D.,Niutec AG | Delfosse D.,Mathys Ltd. Bettlach
Biomaterials | Year: 2010

Wear and oxidative degradation may limit the life span of UHMWPE implants. Cross-linking and stabilisation by vitamin E are proposed to overcome wear and degradation. The present investigation takes a close look to the oxidative behaviour of cross-linked and stabilised UHMWPE. First, the consolidated vitamin E stabilised UHMWPE was qualified in terms of microstructure and homogeneity of the distribution of the additive to be suitable for oxidation profiles over the entire section. Then cross-linked samples with five different concentrations of vitamin E (nil to 1.0%) underwent two different ageing protocols. The first was under pressurized oxygen at 70 °C, as defined in the ASTM F 2003 standard with a prolonged period of 60 days, the second was in 5% aqueous hydrogen peroxide solution with iron (III) chloride as catalyst at 50 °C. The first accelerated ageing protocol showed that a vitamin E concentration as low as 0.05% is effective to protect irradiated highly cross-linked UHMWPE against oxidation when exposed direct to oxygen. Vitamin E stabilised, highly cross-linked UHMWPE exhibits therefore no oxidation potential origination from the irradiation treatment. Analysis of samples treated by the second chemical ageing yielded, that vitamin E is effective to prolong initial stability against a supplementary attack of hydrogen peroxide and reactive radicals. The time period of stability against the aggressive hydrogen peroxide solution increases with increasing vitamin E content. However, even 0.05% have a marked stabilisation effect. Therefore, such small additions of vitamin E are effective to protect the UHMWPE material against a supplementary exposure to in vivo oxidation after the irradiation treatment. In conclusion, vitamin E shields cross-linked UHMWPE for orthopaedic application against oxidation in the heat of consolidation, during irradiation treatment and finally while implanted in the human body. © 2010 Elsevier Ltd. All rights reserved.

Badertscher R.P.,University of Zurich | Lerf R.,Mathys Ltd. Bettlach | Delfosse D.,Mathys Ltd. Bettlach | Adlhart C.,University of Zurich
Polymer Degradation and Stability | Year: 2012

Today, UHMWPE implants are stabilized with α-tocopherol and cross-linked by irradiation in order to reduce wear. Little is known about the structural transformation of the antioxidant α-tocopherol upon irradiation. In the present investigation, the major irradiation reaction products of α-tocopherol dissolved at 0.1 wt.% in liquid model hydrocarbons were characterized spectroscopically and by independent synthesis. We observed only a single product group, namely phenolic alkyl ethers formed by radical recombination of a phenoxyl radical with a secondary alkyl radical. The irradiation dose is the parameter which controls the amount of consumption of α-tocopherol. At a dose of 27.5 kGy, 31-34% of α-tocopherol was transformed into the corresponding ether, while at 97.9 kGy, the degree of transformation was 68-76%. The observed ether formation in the liquid model hydrocarbons explains two significant observations for the α-tocopherol stabilized polymers, namely depletion of the α-tocopherol's phenol group upon irradiation and "grafting", i.e. formation of a chemical bond between the polymer and its antioxidant. © 2012 Elsevier Ltd. All rights reserved.

Wuttke V.,Mathys Orthopaedie GmbH | Witte H.,TU Ilmenau | Kempf K.,Mathys Orthopaedie GmbH | Oberbach T.,Mathys Orthopaedie GmbH | Delfosse D.,Mathys Ltd. Bettlach
Biomedizinische Technik | Year: 2011

Ceramic-on-ceramic articulations are a frequently used bearing for total hip replacements. This success mainly is due to their excellent tribological properties. Ceramics can withstand high pressure loads due to its brittleness but only low bending stresses. A ceramic ball head fracture is the result of subcritical crack growth. This kind of fracture in vivo can abet by damage or contamination of the stem cone. The main goal of this work is to provide a risk assessment of different possible damage mechanisms and contaminations that may result in lower fracture strength of a ceramic ball head. To simulate potential causes, different types and dimensions of metal wire, foils, hair, and lubricants were inserted between the ceramic ball head and the metal cone of the stem. The test results clearly show that fracture strength is negatively influenced by most of the inhomogeneities between the cone and the head because they increase the peak stresses acting on a part of the ceramic ball head. The results of this article clearly confirm the demand for an undamaged taper fit "free of contamination" between the ceramic head and the metal cone during implantation. © 2011 by Walter de Gruyter Berlin Boston 2011.

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