Bernard Chen

Evaluation of trajectories and contact pressures for the straight nucleus cochlear implant electrode array - a two-dimensional application of finite element analysis.

A two-dimensional (2D) finite element analysis has been used in this study to model the insertion of the Nucleus electrode array with different stiffness properties in order to evaluate the propensity of damage by visualizing the predicted trajectories and by comparing the buckling stresses and the contact pressures at the tip (and its distribution along the length) of the electrode array. Previous temporal bone studies have shown that damage during insertion of an electrode array around the basal turn of the cochlear spiral could be related to the design and the stiffness properties of the electrode array. However, it is difficult to evaluate different designs of electrode arrays purely by experimental methods as the experimental conditions and their results are difficult to reproduce. Three electrode arrays with different mechanical properties, i.e. uniform stiffness, graded stiffness, and a soft tip have been modelled. Buckling stress and contact pressure at the tip of the electrode array were found to be highest for the arrays with uniform stiffness. The contact pressures at the tip of the electrode array appeared strongly influenced by the stiffness profile and were optimal for graded stiffness. The results indicate the importance of the electrode array design and stiffness properties in minimizing trauma. However, there are a number of limitations in the present 2D evaluation which will require further analysis using a three-dimensional model to obtain definitive results.

Development of a steerable cochlear implant electrode array

Development of a new steerable electrode array with embedded nitinol shape memory alloy actuators is described in this paper. The risk of damaging the basilar membrane during insertion of electrode array into the human cochlear is expected to be significantly reduced with the ability to redirect the tip of the electrode array at the critical hook region. The final position of this electrode array can also be adjusted to lie beneath the basilar membrane inside the scala tympani for delivery of neurotrophins (growth factors). The bending behaviour of the steerable electrode array and its trajectories during insertion into the scale tympani were predicted using a 3D finite element model. Results from the model have shown that the new electrode array can be steered through the critical ‘hook’ region and be accurately positioned for delivery of neurotrophins.

Finite element analysis of damage by cochlear implant electrode array's proximal section to the basilar membrane.

Hypothesis This study aims to examine the mechanism of damage to the basilar membrane caused by the proximal section of the cochlear implant electrode array. Background The electrode array has been found to severely damage the basilar membrane. Most previous studies on cochlear implant insertion damage largely focused on the injury by the front section (tip) of the electrode array to the membrane. Little attempt has been made to investigate the damage caused by the array’s proximal section. Methods A computational model using the finite element method has been developed for assessing the likelihood of the damage based on two criteria: 1) frequency of contact between the proximal section of the electrode array and the upper wall of the scala tympani where the basilar membrane is located, and 2) magnitude of the associated shear stresses at the contact areas. The model has been validated and used for studying the effect of electrode array’s stiffness properties on the damage. Results The proximal section of the contour array is most likely to hit the basilar membrane, compared with its previous versions (the straight array and the single wire electrode). In terms of shear stress magnitude, the proximal section of the contour array exerts higher stresses on the scala tympani’s upper wall and, thus, is more likely to damage the basilar membrane, compared with that of the straight array. Conclusion Results from this study are useful for cochlear implant surgeons in better understanding the mechanism of damage by the electrode array’s proximal section to the basilar membrane and in establishing advanced insertion techniques for reducing the damage (in particular, the results strongly support the “advance off-stylet” technique). The outcomes of the study also are beneficial for cochlear implant designers in selecting appropriate stiffness profiles for future electrode arrays, which are expected to cause minimal damage to the basilar membrane (a new design of the contour array with stiffness increasing from the front to the proximal section is highly recommended).

Complete Efficiency Analysis of Epicyclic Gear Train With Two Degrees of Freedom

Epicyclic gear trains (EGTs) are important mechanical transmissions with many applications. For optimal design and operation of these gear trains, it is necessary to obtain complete efficiency maps of such transmissions. The efficiency of a two degrees of freedom (two-dof) EGT is derived based on the internal power flow and virtual power flow patterns. Expressions for the efficiencies in different operating conditions are obtained and verified by three special conditions.

A numerical scheme of convex yield function with continuous anisotropic hardening based on non-associated flow rule in FE analysis of sheet metal

A non-associated flow rule (NAFR) model is developed by adopting the convex function YLD-2004 as the yield stress function and the plastic potential function. The yield stress function coefficients are continuously updated which are associated with the change of directional uniaxial yield stress and biaxial yield stress to simulate the anisotropic hardening behaviour. That was achieved by implementing the numerical identification procedure of coefficients into stress integration procedure. The coefficients of plastic potential function are constant and identified by the uniaxial and biaxial r-value. This constitutive model is capable of describing anisotropic hardening and yield behaviour of strongly textured aluminium alloy sheet metal. In this paper, the model was implemented into the FE code via ABAQUS subroutine to predict the deep drawn cup earing and directional flow stresses of the AA5042-H2 aluminium alloy. The new anisotropic hardening model shows better agreement with experiments compared with the isotropic hardening model.

Novel implant for peri-prosthetic proximal tibia fractures

BACKGROUND Repair of peri-prosthetic proximal tibia fractures is very challenging in patients with a total knee replacement or arthroplasty. The tibial component of the knee implant severely restricts the fixation points of the tibial implant to repair peri-prosthetic fractures. A novel implant has been designed with an extended flange over the anterior of tibial condyle to provide additional points of fixation, overcoming limitations of existing generic locking plates used for proximal tibia fractures. Furthermore, the screws fixed through the extended flange provide additional support to prevent the problem of subsidence of tibial component of knee implant. METHODS The design methodology involved extraction of bone data from CT scans into a flexible CAD format, implant design and structural evaluation and optimisation using FEM as well as prototype development and manufacture by selective laser melting 3D printing technology with Ti6Al4 V powder. RESULTS A prototype tibia implant was developed based on a patient-specific bone structure, which was regenerated from the CT images of patients tibia. The design is described in detail and being applied to fit up to 80% of patients, for both left and right sides based on the average dimensions and shape of the bone structure from a wide range of CT images. CONCLUSION A novel tibial implant has been developed to repair peri-prosthetic proximal tibia fractures which overcomes significant constraints from the tibial component of existing knee implant.

Effects of Lumbar Spine Assemblies and Body-Borne Equipment Mass on Anthropomorphic Test Device Responses During Drop Tests

When simulating or conducting land mine blast tests on armored vehicles to assess potential occupant injury, the preference is to use the Hybrid III anthropomorphic test device (ATD). In land blast events, neither the effect of body-borne equipment (BBE) on the ATD response nor the dynamic response index (DRI) is well understood. An experimental study was carried out using a drop tower test rig, with a rigid seat mounted on a carriage table undergoing average accelerations of 161 g and 232 g over 3 ms. A key aspect of the work looked at the various lumbar spine assemblies available for a Hybrid III ATD. These can result in different load cell orientations for the ATD which in turn can affect the load measurement in the vertical and horizontal planes. Thirty-two tests were carried out using two BBE mass conditions and three variations of ATDs. The latter were the Hybrid III with the curved (conventional) spine, the Hybrid III with the pedestrian (straight) spine, and the Federal Aviation Administration (FAA) Hybrid III which also has a straight spine. The results showed that the straight lumbar spine assemblies produced similar ATD responses in drop tower tests using a rigid seat. In contrast, the curved lumbar spine assembly generated a lower pelvis acceleration and a higher lumbar load than the straight lumbar spine assemblies. The maximum relative displacement of the lumbar spine occurred after the peak loading event, suggesting that the DRI is not suitable for assessing injury when the impact duration is short and an ATD is seated on a rigid seat on a drop tower. The peak vertical lumbar loads did not change with increasing BBE mass because the equipment mass effects did not become a factor during the peak loading event.

Evaluation of a New Design to Improve the Flexibility of the Nucleus Standard Straight Array Cochlear Implant

Cochlear implants can successfully provide auditory information for bilaterally profoundly deaf patients by electrically stimulating auditory nerve fibres via an electrode array, which is surgically implanted into the scala tympani of the cochlea. It is therefore important that the electrode array does not cause damage to the fine intracochlear structures during the process of insertion, as this can result in the loss of spiral ganglion cells, which are necessary for the implant to evoke auditory percepts. There is strong evidence that trauma and damage during insertion of electrode arrays into the human cochlea are related to the stiffness of the electrode array. Previous studies were conducted to experimentally determine the stiffness properties of electrode arrays using three-point flexural bending and buckling tests. In this paper, the design of Nucleus straight electrode array is modified to give a greater flexibility to further reduce the risk of trauma to delicate structures of the cochlea during surgical insertion of the electrode array. This is achieved by reducing the cross-sectional area of the electrode array at selected positions over its length. Improvements in the flexibility of the new straight electrode array and the bending behavior at its tip have been demonstrated using Finite Element analyses. Loads applied to the tip of the electrode array at different angles with respect to the longitudinal axis of the electrode array showed that the modified design caused the tip to be more flexible and therefore better able to curl around the inner spiral of the scala tympani and thus less likely to penetrate the basilar membrane during insertion. Loads applied at other positions along the electrode array showed that bending occurred more readily using the modified design thereby reducing the friction and shear stresses at the contact interface between the electrode array and the delicate cochlea structures.