Martin Brummund

Three-dimensional finite element modeling of the human external ear: Simulation study of the bone conduction occlusion effecta)

A linear three-dimensional (3D) elasto-acoustic finite element model was used to simulate the occlusion effect following mechanical vibration at the mastoid process. The ear canal and the surrounding soft and bony tissues were reconstructed using images of a female cadaver head (Visible Human Project(®)). The geometrical model was coupled to a 3D earplug model and imported into comsol Multiphysics (COMSOL(®), Sweden). The software was used to solve for the sound pressure at the eardrum. Finite element modeling of the human external ear and of the occlusion effect has several qualities that can complement existing measuring and modeling techniques. First, geometrically complex structures such as the external ear can be reconstructed. Second, various material behavioral laws and complex loading can be accounted for. Last, 3D analyses of external ear substructures are possible allowing for the computation of a broad range of acoustic indicators. The model simulates consistent occlusion effects (e.g., insertion depth variability). Comparison with an experimental dataset, kindly provided by Stenfelt and Reinfeldt [Int. J. Audiol. 46, 595-608 (2007)], further demonstrates the models accuracy. Power balances were used to analyze occlusion effect differences obtained for a silicone earplug and to examine the increase in sound energy when the ear canal is occluded (e.g., high-pass filter removal).

Biomechanical assessment of the stabilization capacity of monolithic spinal rods with different flexural stiffness and anchoring arrangement.

BACKGROUND Spinal disorders can be treated by several means including fusion surgery. Rigid posterior instrumentations are used to obtain the stability needed for fusion. However, the abrupt stiffness variation between the stabilized and intact segments leads to proximal junctional kyphosis. The concept of spinal rods with variable flexural stiffness is proposed to create a more gradual transition at the end of the instrumentation. METHOD Biomechanical tests were conducted on porcine spine segments (L1-L6) to assess the stabilization capacity of spinal rods with different flexural stiffness. Dual-rod fusion constructs containing three kinds of rods (Ti, Ti-Ni superelastic, and Ti-Ni half stiff-half superelastic) were implanted using two anchor arrangements: pedicle screws at all levels or pedicle screws at all levels except for upper instrumented vertebra in which case pedicle screws were replaced with transverse process hooks. Specimens were loaded in forward flexion, extension, and lateral bending before and after implantation of the fusion constructs. The effects of different rods on specimen stiffness, vertebra mobility, intradiscal pressures, and anchor forces were evaluated. FINDING The differences in rod properties had a moderate impact on the biomechanics of the instrumented spine when only pedicle screws were used. However, this effect was amplified when transverse process hooks were used as proximal anchors. INTERPRETATION Combining transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff-half superelastic) rods provided more motion at the upper instrumented level and applied less force on the anchors, potentially improving the load sharing capacity of the instrumentation.

In-vitro assessment of the stabilization capacity of monolithic spinal rods with variable flexural stiffness: Methodology and examples.

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. To assess the stabilization capacity of such a spinal rod, in vitro experiments on porcine spine models are carried out. This paper describes the methodology followed to evaluate the effect of Ti-Ni rods compared to conventional titanium rods. Validation of the methodology and examples of results obtained are also presented.

Ti–Ni Rods with Variable Stiffness for Spine Stabilization: Manufacture and Biomechanical Evaluation

A new concept of monolithic spinal rods with variable flexural stiffness is proposed to reduce the risk of adjacent segment degeneration and vertebral fracture, while providing adequate stability to the spine. The variability of mechanical properties is generated by locally annealing Ti–Ni shape memory alloy rods. Ten-minute Joule effect annealing allows the restoration of the superelasticity in the heated portion of the rod. Such processing also generates a mechanical property gradient between the heated and the unheated zones. A numerical model simulating the annealing temperature and the distributions of the mechanical properties was developed to optimize the Joule-heating strategy and to modulate the rod’s overall flexural stiffness. Subsequently, the rod model was included in a finite element model of a porcine lumbar spine to study the effect of the rod’s stiffness profiles on the spinal biomechanics.

A simplified axi-symmetric finite element model of the human outer ear to determine the earplug induced auditory occlusion effect

Earplugs are a frequently used short-term solution for hearing conservation in the workplace environment. Due to limited auditory comfort, however, workers often only wear them for short periods of time and become prone to hearing loss. An important source of discomfort is the auditory occlusion effect, which expresses itself through the distortion of the wearers voice and the amplification of physiological noises upon earplug insertion. Simplified numerical modeling can help to better assess and design earplugs, because it requires few system resources and is simpler in terms of numerical and experimental implementation than an equivalent complex model. This work describes a novel coupled linear elasto-acoustic two dimensional finite element (FE) model of the human outer ear. The model comprises the auditory canal as well as the bony, cartilaginous, and skin tissues whose material parameters were approximated using literature findings. The outlined model can compute the transfer functions between the so...

Impact of anchor type on porcine lumbar biomechanics: Finite element modelling and in-vitro validation

Background: Rigid posterior implants used for spinal stabilization can be anchored to the vertebrae using pedicle screws or screws combined with transverse process hooks. In the present study, a finite element model of a porcine lumbar spine instrumented with screws and hooks is presented and validated. Methods: The porcine lumbar spine model was validated using in‐vitro measurements on six porcine specimens. Validation metrics included intervertebral rotations (L1 to L6) and nucleus pressure in the topmost cranial instrumented disc. The model was used to compare the biomechanical effect of anchor types. Findings: Good agreement was observed between the model and validation experiments. For upper transverse hooks construct, intervertebral rotations increased at the upper instrumented vertebra and decreased at the adjacent level. Additionally, nucleus pressures and stress on the annulus decreased in the adjacent disc and increased in the upper instrumented disc. The pull‐out forces predicted for both anchor configurations were significantly lower than the pull‐out strength found in the literature. Interpretation: These numerical observations suggest that upper transverse process hooks constructs reduce the mobility gradient and cause less stress in the adjacent disc, which could potentially reduce adjacent segment disease and proximal junction kyphosis incidence without increasing the risk of fixation failure. Future work needs to assess the long‐term effect of such constructs on clinical and functional outcomes. HighlightsRigid instrumentations are used for spinal fusion.Proximal junction failure are frequent complications associated with rigid constructs.Transverse hooks at the end of the construct can reduce the stress concentration.A finite element model of hybrid construct was developed and validated.Transverse hooks decrease the mobility gradient at the end of the construct.Transverse hooks decrease the stress concentration on the adjacent segments.

Implementation of a 3D porcine lumbar finite element model for the simulation of monolithic spinal rods with variable flexural stiffness.

Monolithic superelastic-elastoplastic spinal rods (MSER) are promising candidates to provide (i) dynamic stabilisation in spinal segments prone to mechanical stress concentration and adjacent segment disease and (ii) to provide fusion-ready stabilization in spinal segments at risk of implant failure. However, the stiffness distributions along the rods longitudinal axis that best meet clinical requirements remain unknown. The present study is part of a mixed numerical experimental research project and aims at the implementation of a 3D finite element model of the porcine lumbar spine to study the role of MSER material properties and stiffness distributions on the intradiscal pressure distribution in the adjacent segment. In this paper, preliminary intradiscal pressure predictions obtained at one functional spinal unit are presented. Due to a lack of porcine material property data, these predictions were obtained on the basis of uncalibrated human vertebral disc data which were taken from the literature. The results indicate that human annulus and nucleus data predict experimental porcine in vivo and in vitro data reasonably well for the compressive forces of varying magnitudes.

Implementation of a simplified, artificial external ear test fixture for measurement of the earplug induced auditory occlusion effect

Earplugs remain a frequently used short-term solution for occupational hearing conservation. Due to comfort limitations, as induced by, e.g., the occlusion effect, workers often only wear earplugs for limited amounts of time and are likely to develop professional hearing loss. The occlusion effect expresses itself in the low frequencies through an altered perception of the wearer’s own voice and the amplification of physiological noises that occur upon earplug insertion. While many studies examined the occlusion effect experimentally, no study was found that attempted to implement an artificial external ear model dedicated to the measurement of the objective occlusion effect. A simplified external ear test fixture can help to better assess and design earplugs, because it allows standardized experimental testing. This work describes the implementation of a cylindrical artificial test fixture of the human outer ear that comprises the auditory canal as well as the bony, cartilaginous, and skin tissues that a...

Impact of spinal rod stiffness on porcine lumbar biomechanics: Finite element model validation and parametric study:

A three-dimensional finite element model of the porcine lumbar spine (L1–L6) was used to assess the effect of spinal rod stiffness on lumbar biomechanics. The model was validated through a comparison with in vitro measurements performed on six porcine spine specimens. The validation metrics employed included intervertebral rotations and the nucleus pressure in the first instrumented intervertebral disc. The numerical results obtained suggest that rod stiffness values as low as 0.1 GPa are required to reduce the mobility gradient between the adjacent and instrumented segments and the nucleus pressures across the porcine lumbar spine significantly. Stiffness variations above this threshold value have no significant effect on spine biomechanics. For such low-stiffness rods, intervertebral rotations in the instrumented zone must be monitored closely in order to guarantee solid fusion. Looking ahead, the proposed model will serve to examine the transverse process hooks and variable stiffness rods in order to further smooth the transition between the adjacent and instrumented segments, while preserving the stability of the instrumented zone, which is needed for fusion.