13:00
Tissue Mechanics & Biomaterials
13:00
15 mins
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DO MODELING CHOICES HAVE A LARGE INFLUENCE ON THE PREDICTION OF FRETTING WEAR AT THE CONE-HEAD INTERFACE?
Thom Bitter, Dennis Janssen, Wim Schreurs, Gerjon Hannink, Tim Marriott, Imran Khan
Abstract: Introduction: In total hip arthroplasty, a potential cause of failure of some large diameter metal-on-metal devices is fretting corrosion at the head-stem modular junction, which can be simulated using Finite Element Analysis (FEA). Results of FEA simulations may be affected by material properties, contact models and contact definitions. However, in the literature, a wide spread of coefficients of friction (CoF)1,2,3 are used. We present an extensive assessment of the variability of contact models, contact parameters and assembly forces, and their effect on pull-off forces, contact pressures, micromotions and fretting potential. To provide more accurate results, our simulations were validated against experimental data. Materials and Methods: Bimetric stems with Type 1 tapers and Magnum taper adaptors (Biomet UK Ltd) were assembled using forces of 4 and 15 kN, and then disassembled to determine the pull-off forces. A model of the same stem and adaptor was created in Abaqus 6.13-3. Elastic-plastic material properties were based on several uniaxial experiments. Different contact algorithms were tested: general contact, contact pairs, node-to-surface and surface-to-surface. Other parameters that were varied were the CoF (0.15-0.3-0.5), and numerical damping. A load according to ISO 7206-6 was applied, while measuring micromotions and contact pressures. To compare the results, a wear score based on Archard’s4 law was used. Results: The contact models and damping had no significant effect on the pull-off forces and wear scores. The CoF, which can be measured, and assembly force, which depends on surgical technique, had a significant effect on the pull-off force and the wear score. When either one or both increased, an increase in pull-off force and decrease in wear score was observed. Discussion: Using the correct CoF and assembly force is a prerequisite to get accurate predictions. The application of damping had no effect, but can be used to make the model more stable. A higher CoF seemed to produce less wear, although clinically, fretting wear consists of several types of wear in addition to the mechanical wear examined in the current study. Without full validation, studies using different CoFs and assembly forces cannot be directly compared. Hence, to minimize the uncertainties in a simulation, the coefficient of friction should be determined experimentally for each contact pair. References: 1Fessler H, Proc Inst Mech Eng H. 1989; 2Shareef N, Biomaterials. 1996; 3Zhang T, J Strain Anal Eng Des. 2013; 4Archard JF, Proc R Soc A Math Phys Eng Sci. 1956;
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13:15
15 mins
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PEEK FEMORAL COMPONENT IN TKA: BONE REMODELLING AND DEVICE PERFORMANCE DURING GAIT AND SQUATTING
Lennert de Ruiter, Dennis Janssen, Adam Briscoe, Nico Verdonschot
Abstract: Introduction: The mechanical performance of an all- polymer femoral implant in total knee arthroplasty (TKA) has been studies in a clinical trial with a 10-year follow-up period [1]. The polyacetal-on-polyethylene device could not prove its efficacy but positive trends were observed. In this study we return to a polymer-on-polymer bearing couple by replacing the conventional CoCr femoral component by polyetheretherketone (PEEK-OPTIMA®). We wanted to investigate its use during normal gait and deep squat to determine reconstruction safety and bone remodelling.
Methods: A cemented cruciate retaining femoral component was used according to an existing TKA design. Two Finite Element models were created. A deep squat model was based on the Oxford Knee Rig and ranges between 40 and 155 degrees of flexion. It was used to evaluate the structural integrity of the femoral component and the cement mantle. The gait simulation was modelled according to the ISO 14243-1 standard and was used to determine the bone remodelling as that is most dependent on frequent, low level loading.
Results: The PEEK femoral component showed lower compressive (110 vs. 120 MPa) and tensile (98 vs. 250 MPa) stresses than the CoCr implant. Relative to yield stress however, PEEK reaches up to 90-98 percent. The bone cement showed more similarity between both configurations. Under compression (60 vs. 70 MPa) and tension (16 vs. 17 MPa) the PEEK and CoCr cases were numerically similar. The bone strain energy patterns during gait showed different patterns between CoCr and PEEK. Both the pattern and the amount were similar between PEEK and the intact case and differed markedly from CoCr.
Discussion: For the implant itself, normalized loads were substantially higher for PEEK than for CoCr. However, indications are present that could explain such high stresses to have been caused by non-physiological sources, such as computational (contact) artefacts. Additionally, analyses also show that only a small section of the implant is at risk. Therefore, it is not possible to say whether the entire reconstruction would fail. The similarity of cement mantle stresses in the CoCr and PEEK configurations do not necessarily mean equivalence in safety. The success of CoCr implants might be partially explained by the stress protection of the underlying cement, which should be studied for its contribution. On bone remodelling the coarse nature of the ‘intact’ model, requires caution when interpreting the results. A more sophisticated model could provide a more detailed assessment, but it appears that in the PEEK construct bone strains are more similar to an intact knee joint than for conventional CoCr, indicating a reduced post-operative bone remodeling stimulus.
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13:30
15 mins
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IN-VIVO ASSESSMENT OF THE LOAD AND LOAD-ABILITY OF HUMAN SKIN IN RELATION WITH THE DEVELOPMENT OF PRESSURE ULCERS. A PILOT-STUDY
Iris Hoogendoorn, Jasper Reenalda, Bart Koopman, Hans Rietman
Abstract: Pressure ulcers are a significant problem in health care, affecting the quality of life, morbidity and mortality of patients, and increase health care costs. A pressure ulcer is defined as a localized injury to the skin and/or underlying tissue, usually over a bony prominence, as a result of pressure or pressure in combination with shear [1].
The combination of pressure and/or shear results in stresses and strains inside the tissue. The distribution of these stresses and strains is dependent on the morphology and mechanical properties of the different tissue layers and on the shape and properties of the supporting surface. Directly, these stresses and strains can lead to tissue damage but indirectly they can also lead to changes in the microcirculation causing ischemia and tissue damage [2]. The exact aetiology of pressure ulcers is still under debate[3], but if there is an imbalance between the mechanical load and the individual susceptibility and tolerance, the damage threshold will be exceeded and a pressure ulcer can develop [4].
Early identification of patients at risk for developing pressure ulcers and early detection of tissue damage preliminary to ulcer formation is therefore essential to reduce the prevalence, impact and costs of pressure ulcers, and to improve the quality of life of patients. This requires more insight into the relation between the load applied to the skin and load ability of the skin and underlying tissue, which may prevent tissue injury and prevent pressure ulcers.
The aim of this study is to investigate the load-ability of the skin in-vivo at different anatomical locations. Oxygenation and perfusion are measures to estimate tissue viability and can therefore be used to estimate the effect of mechanical loading on the skin. To investigate the effect of mechanical loading on the skin in-vivo, a measurement device has been build that can apply pressure in combination with a shear force with an indenter at the skin. The amount of pressure and/or shear is measured using load cells.
The ‘Oxygen to See’ (O2C) will be used to assess the tissue viability before, during and after the mechanical loading. The O2C combines laser Doppler flowmetry and white light photo spectroscopy to measure perfusion and oxygenation of tissue up to a maximum depth of 10 mm. The probes of the O2C are custom integrated in the indenter head to monitor the effect of mechanical loading of the skin.
The effect of mechanical loading will be analysed at different anatomical sites during loading and after loading by assessment of the hyperaemic peak and recovery time of the oxygenation and perfusion parameters to baseline. Statistical analyses will be used to compare results between subjects, anatomical locations and different magnitudes of mechanical loading. Measurements will be made at healthy subjects and spinal cord injured patients.
Preliminary results of the research will be presented.
REFERENCES
[1] European Pressure Ulcer Advisory Panel and National Pressure Ulcer Advisory Panel, “Prevention and treatment of pressure ulcers: quick reference guide”, Washington DC: National Pressure Ulcer Advisory Panel, 2009.
[2] E. Linder-Ganz and A. Gefen, “The effects of pressure and shear on capillary closure in the microstructure of skeletal muscles”, Ann. Biomed. Eng., Vol. 35, pp. 2095–107, (2007).
[3] S. Coleman, C. Gorecki, E. A. Nelson, S. J. Closs, T. Defloor, R. Halfens, A. Farrin, J. Brown, L. Schoonhoven, and J. Nixon, “Patient risk factors for pressure ulcer development: systematic review”, Int. J. Nurs. Stud., Vol. 50, pp. 974–1003,( 2013).
[4] S. Coleman, J. Nixon, J. Keen, L. Wilson, E. McGinnis, C. Dealey, N. Stubbs, A. Farrin, D. Dowding, J. M. G. a. Schols, J. Cuddigan, D. Berlowitz, E. Jude, P. Vowden, L. Schoonhoven, D. L. Bader, A. Gefen, C. W. J. Oomens, and E. A. Nelson, “A new pressure ulcer conceptual framework”, J. Adv. Nurs., Vol. 70, pp. 2222–2234, ( 2014).
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13:45
15 mins
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A MEDIUM THROUGHPUT DEVICE TO STUDY THE EFFECTS OF COMBINATIONS OF SURFACE STRAINS AND FLUID-FLOW SHEAR STRESSES ON CELLS
Ravi Sinha, Séverine Le Gac, Nico Verdonschot, Albert van den Berg, Bart Koopman, Jeroen Rouwkema
Abstract: We report a 96-well-plate-sized device to screen for the effects on cells of all combinations of five equibiaxial surface strains (2-20% range) and five fluid flow shear stresses (0.18-0.33 Pa range), each combination in four replicates (100 units).
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14:00
15 mins
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IN VITRO DEGRADATION OF MAGNESIUM METAL MATRIX COMPOSITES CONTAINING BREDIGITE
Sina Naddaf Dezfuli, Sander Leeflang, Jie Zhou
Abstract: Ti is currently the most popular material for bone fracture fixation devices. One of the disadvantages of using Ti and other metals is that they are too strong and too stiff, causing the stress-shielding effect. Often, a second invasive surgery is needed to remove the implant after the healing process is completed. Biodegradable plates and screws can eliminate the need for implant removal operations. In recent years, much attention has been paid to developing Mg and its alloys for orthopedic applications. These materials possess densities and elastic moduli closer to those of the human bone than other metallic biomaterials for permanent implants. However, Mg corrodes too rapidly in physiological environments, which has halted the advances towards its clinical applications. Moreover, Mg lacks bioactivity to promote cell growth and speed up the healing process.
The degradation rate of Mg can be reduced and its bioactivity enhanced by adding a bioactive ceramic agent, e.g., bridigite Ca7Mg(SiO4)4 with proven bioactivity, to Mg to form Magnesium Metal Matrix Composites (Mg-MMCs). With powder metallurgy (P/M) techniques, the maximum amount of bioceramic addition has been limited to 15 vol.%, above which ceramic particles tend to form agglomerates, negatively affecting mechanical properties [1, 2].
The present study aimed at exploring the possibility of adding bredigite particles up to 40 vol.% to a Mg powder and determine the benefits in terms of the reduction in degradation rate and formation of bone-like apatite (Ca-P-containing compounds) on composite surface.
Mg-MMCs with 0, 10, 20, 30 and 40 vol.% of bredigite were prepared from powder mixtures using a vacuum hot press. Bredigite particles were uniformly distributed in all the Mg-MMCs. No structural disintegrity could be observed under optical microscope, as shown in Fig. 1.
The degradation rates of Mg-MMCs were determined by measuring the amount of evolving H2 during immersion tests in the DMEM cell culture medium (Dulbecco's Modified Eagle's Medium) for up to 24 h and the amounts of ions released or lost using an inductively coupled plasma atomic emission spectroscopy (ICP-AES).
Fig. 2 compares the amounts of H2 after 24 h immersion in DMEM. Clearly, H2 evolution decreased with increasing volume fraction of bredigite, confirming the benefits from adding bredigite to Mg. Mg-40Br exhibited the lowest amount of H2, corresponding to the lowest rate of degradation.
Fig. 3 shows the amounts of ions (Mg, Si, Ca and P) released from samples to DMEM or lost from DMEM over time. All the samples released increasing amounts of Mg over time, indicating gradual degradation. Mg-40Br consistently released the least amounts of Mg, confirming the results of H2 measurement. The substantial differences in Mg release between Mg-0Br and Mg-20Br demonstrated the effect of bredigite in slowing down the degradation of Mg. Ca and P should be treated together since they form Ca-P-containing precipitates on sample surface. The losses of Ca and P in DMEM over time imply the deposition of Ca-P compounds on sample surface. A maximum amount of Ca was lost to pure Mg after 24 h, while P losses were very similar between all the samples. Considering the fact that bredigite contained about 42 wt.% of Ca, the loss of Ca in DMEM leading to Ca-P deposition must have been counteracted by Ca release from the composites as a result of bredigite degradation.
In conclusion, Mg-MMCs with large volume fractions of bredigite were successfully made using the (P/M) technique. The in vitro degradation tests in DMEM showed decreasing amounts of H2 with increasing volume fraction of bredigite, confirming the beneficial effect of bredigite in slowing down the degradation of Mg. After 24 h, the amount of free Ca in DMEM was larger for the composites with larger fractions of bredigite, suggesting the release of Ca ions to compensate for the loss of Ca for Ca-P precipitation on composite surface.
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14:15
15 mins
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RECTIFIER AUTOMATIC IMPEDANCE MATCHING FOR BIOMEDICAL IMPLANTS
Gustavo Martins, Wouter Serdijn
Abstract: Wireless power transfer or harvesting has enabled several applications, including RFID and low-power remote sensing [1-3]. It is also a suitable method for providing power to electronic devices implanted in the human body or attached to a patient's skin, as it prevents the need for uncomfortable cables or surgeries to replace batteries. Wireless power receivers are usually characterized by a high frequency input signal with low available power.
In implanted devices, the antenna or coil alignment and its distance from the skin, as well as the composition of the tissue layers that separate it from the outside world, are hard to predict. All these factors influence the amount of power available to the implanted device., which takes the AC/DC power converting chain away from the optimum operating point for which it was designed. Basically, a change in available input power changes the optimum rectifier DC load and its input impedance. An adaptive power conversion chain can be designed to compensate those variations, thus increasing the system operating range and reliability.
In the proposed approach, a DC-DC switched converter modulates the rectifier output load [4] while the input impedance matching circuit is simultaneously adapted in order to track the maximum power transfer point. This technique increases the input power range over which the rectifier converts with high efficiency. Simulation results shows that the rectifier can operate with power conversion efficiencies greater than 50% for an input power ranging from -22 dBm to +2 dBm.
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