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Weissenburg in Bayern, Germany

Morlock M.M.,TU Hamburg - Harburg | Bishop N.,TU Hamburg - Harburg | Kaddick C.,EndoLab GmbH
Orthopade | Year: 2011

Replacement of the hip joint has become an exceptionally successful procedure since the inauguration of the low friction principle by Charnley. Aseptic osteolysis and joint dislocation have been addressed by the development of wear-optimized materials and the introduction of larger heads. As an increase in head diameter against polyethylene causes wear increase, larger hard-on-hard bearings were introduced, which exhibit reduced wear and reduced dislocation risk with increasing head diameter. These findings were derived from standard simulator testing, not sufficiently considering the risk of fluid film breakdown under adverse conditions, which can cause a dramatic increase in wear and friction proportional to the head diameter. Such adverse conditions can occur clinically in patients due to several factors and have caused the presently observed unexpected problems with these new designs. Standardized preclinical testing has to be viewed as a minimum requirement but certainly not as a guarantee for the clinical success of new materials and designs even if the testing is adapted to the current patient requirements, which is presently not the case. The future of tribology lies in the prevention of adverse conditions in patients, the improvement and optimized use of proven existing materials and not in the use of new materials. © 2011 Springer-Verlag.

Haider H.,University of Nebraska Medical Center | Kaddick C.,EndoLab GmbH
Journal of ASTM International | Year: 2012

Some well-known mobile bearing designs have had truly excellent long-term clinical results. Their lower constraint and ability for some self-alignment might have helped reduce the shear forces and torques transmitted to the prosthesis-bone interface, thereby lowering the risk of loosening. However, the most commonly assumed benefit of mobile bearings is the reduction in wear due to less contact stress and reduced cross shear. In a rotating platform, wear can be reduced because the rolling/sliding motion is separated from the transverse rotational motion, which reduces cross-shear. Although it has not been categorically proven clinically, such lower wear expectations with mobile bearings might have influenced the thinking of some total knee replacement (TKR) designers and test engineers. This paper amalgamates in vitro TKR wear results from two separate laboratories (in Nebraska and Germany) to present the largest data set ever published on wear, across the widest variety of fixed and mobile bearing TKR designs. Many hundreds of TKR samples were tested with largely similar methodologies using the ISO 14243-1 force-control method. These tests covered 133 different fixed and mobile bearing designs and materials, in total (bicondylar) and unicompartmental forms, and of a wide range of sizes. Clear differences in wear resulted with known superior bearing materials. This illustrates how sensitive and capable of discriminating between low and high wearing implants the force-control wear testing methodology is. However, between both labs, and across all tests, no statistically significant differences were found in wear overall between fixed and mobile bearings. Therefore, the wear of mobile bearing knees is not necessarily less than that of fixed bearings. In both, it depends on the detailed design and materials of the TKR. Testing appears to be necessary with all implant designs, regardless of the history of clinically successful predicates of seemingly similar generic design. Copyright © 2012 by ASTM International.

Pourzal R.,University of Duisburg - Essen | Catelas I.,University of Ottawa | Theissmann R.,University of Duisburg - Essen | Kaddick C.,EndoLab GmbH | Fischer A.,University of Duisburg - Essen
Wear | Year: 2011

Biological effects of wear products (particles and metal ions) generated by metal-on-metal (MoM) hip replacements made of CoCrMo alloy remain a major cause of concern. Periprosthetic osteolysis, potential hypersensitivity response and pseudotumour formation are possible reactions that can lead to early revisions. To accurately analyse the biological response to wear particles from MoM implants, the exact nature of these particles needs to be characterized. Most previous studies used energy-dispersive X-ray spectroscopy (EDS) analysis for characterization. The present study used energy filtered transmission electron microscopy (TEM) and electron diffraction pattern analysis to allow for a more precise determination of the chemical composition and to gain knowledge of the crystalline structure of the wear particles.Particles were retrieved from two different test rigs: a reciprocating sliding wear tribometer (CoCrMo cylinder vs. bar) and a hip simulator according to ISO 14242-1 (CoCrMo head vs. CoCrMo cup). All tests were conducted in new born calf serum (30 g/l protein content). Particles were retrieved from the test medium using a previously published enzymatic digestion protocol.Particles isolated from tribometer samples had a size of 100-500nm. Diffraction pattern analysis clearly revealed the lattice structure of strain induced hcp e{open}-martensite. Hip simulator samples revealed numerous particles of 15-30nm and 30-80nm size. Most of the larger particles appeared to be only partially oxidized and exhibited cobalt locally. The smallest particles were Cr2O3 with no trace of cobalt. It optically appeared that these Cr2O3 particles were flaking off the surface of larger particles that depicted a very high intensity of oxygen, as well as chromium, and only background noise of cobalt. The particle size difference between the two test rigs is likely related to the conditions of the two tribosystems, in particular the difference in the sample geometry and in the type of sliding (reciprocating vs. multidirectional).Results suggest that there may be a critical particle size at which chromium oxidation and cobalt ionization are accelerated. Since earlier studies have shown that wear particles are covered by organic residue which may act as a passive layer inhibiting further oxidation, it would suggest that this organic layer may be removed during the particle isolation process, resulting in a change of the particle chemical composition due to their pyrophoric properties. However, prior to being isolated from the serum lubricant, particles remain within the contact area of head and cup as a third-body. It is therefore possible that during that time, particles may undergo significant transformation and changes in chemical composition in the contact area of the head and cup within the tribological interface due to mechanical interaction with surface asperities. © 2011 Elsevier B.V.

Schwiesau J.,Aesculap AG | Schilling C.,University of Stuttgart | Kaddick C.,EndoLab GmbH | Utzschneider S.,Ludwig Maximilians University of Munich | And 5 more authors.
Medical Engineering and Physics | Year: 2013

The objective of our study was the definition of testing scenarios for knee wear simulation under various highly demanding daily activities of patients after total knee arthroplasty. This was mainly based on a review of published data on knee kinematics and kinetics followed by the evaluation of the accuracy and precision of a new experimental setup. We combined tibio-femoral load and kinematic data reported in the literature to develop deep squatting loading profiles for simulator input. A servo-hydraulic knee wear simulator was customised with a capability of a maximum flexion of 120°, a tibio-femoral load of 5000. N, an anterior-posterior (AP) shear force of ±1000. N and an internal-external (IE) rotational torque of ±50. Nm to simulate highly demanding patient activities. During the evaluation of the newly configurated simulator the ability of the test machine to apply the required load and torque profiles and the flexion kinematics in a precise manner was examined by nominal-actual profile comparisons monitored periodically during subsequent knee wear simulation. For the flexion kinematics under displacement control a delayed actuator response of approximately 0.05. s was inevitable due to the inertia of masses in movement of the coupled knee wear stations 1-3 during all applied activities. The axial load and IE torque is applied in an effective manner without substantial deviations between nominal and actual load and torque profiles. During the first third of the motion cycle a marked deviation between nominal and actual AP shear load profiles has to be noticed but without any expected measurable effect on the latter wear simulation due to the fact that the load values are well within the peak magnitude of the nominal load amplitude. In conclusion the described testing method will be an important tool to have more realistic knee wear simulations based on load conditions of the knee joint during activities of daily living. © 2012 IPEM.

Grupp T.M.,Aesculap AG | Grupp T.M.,Ludwig Maximilians University of Munich | Yue J.J.,Yale University | Garcia R.,Aventura | And 5 more authors.
European Spine Journal | Year: 2015

Introduction: The objective of our in vitro study was to introduce a test method to evaluate impingement in lumbar spinal disc arthroplasty in terms of wear, contact pattern, metal ion concentration and particle release. Material and Method: Impingement wear simulation was performed on a 6-station spinal wear simulator (Endolab, Germany) on a lumbar spinal disc system (activ® L Aesculap AG, Germany) using four different protocols specific to impingement in flexion, in extension, in lateral bending and in combined flexion bending. Impingement contact stress is intentionally created by applying an angular displacement of +2° in addition to the intended range of motion in the impingement direction, whereas a bending moment of 8 Nm remains constant during the impingement phase (plateau). Results: An average volumetric wear rate of 0.67 mm3/million cycles was measured by impingement under flexion, of 0.21 mm3/million cycles under extension, of 0.06 mm3/million cycles under lateral bending and of 1.44 mm3/million cycles under combined flexion bending. The particle size distribution of the cobalt-chromium wear particles released by impingement in flexion (anterior), extension (posterior), lateral bending (lateral) and combined flexion bending (antero-lateral) revealed that most of the detected cobalt-chromium particles were in a size range between 0.2 and 2 µm. Conclusion: The impingement wear simulation introduced here proved to be suitable to predict in vivo impingement behaviour in regard to contact pattern seen on retrieved devices of the activ® L lumbar disc arthroplasty design in a pre-clinical test. © 2014, Springer-Verlag Berlin Heidelberg.

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