John Z. Wu

Analysis of effects of friction on the deformation behavior of soft tissues in unconfined compression tests.

Frictionless specimen/platen contact in unconfined compression tests has traditionally been assumed in determining material properties of soft tissues via an analytical solution. In the present study, the suitability of this assumption was examined using a finite element method. The effect of the specimen/platen friction on the mechanical characteristics of soft tissues in unconfined compression was analyzed based on the published experimental data of three different materials (pigskin, pig brain, and human calcaneal fat). The soft tissues were considered to be nonlinear and viscoelastic; the friction coefficient at the contact interface between the specimens and platens was assumed to vary from 0.0 to 0.5. Our numerical simulations show that the tissue specimens are, due to the specimen/platen friction, not compressed in a uniform stress/strain state, as has been traditionally assumed in analytical analysis. The stress of the specimens obtained with the specimen/platen friction can be greater than those with the frictionless specimen/platen contact by more than 50%, even in well-controlled test conditions.

How to determine the permeability for cement infiltration of osteoporotic cancellous bone

Cement augmentation is an emerging surgical procedure in which bone cement is used to infiltrate and reinforce osteoporotic vertebrae. Although this infiltration procedure has been widely applied, it is performed empirically and little is known about the flow characteristics of cement during the injection process. We present a theoretical and experimental approach to investigate the intertrabecular bone permeability during the infiltration procedure. The cement permeability was considered to be dependent on time, bone porosity, and cement viscosity in our analysis. In order to determine the time-dependent permeability, ten cancellous bone cores were harvested from osteoporotic vertebrae, infiltrated with acrylic cement at a constant flow rate, and the pressure drop across the cores during the infiltration was measured. The viscosity dependence of the permeability was determined based on published experimental data. The theoretical model for the permeability as a function of bone porosity and time was then fit to the testing data. Our findings suggest that the intertrabecular bone permeability depends strongly on time. For instance, the initial permeability (60.89 mm(4)/N(*)s) reduced to approximately 63% of its original value within 18 seconds. This study is the first to analyze cement flow through osteoporotic bone. The theoretical and experimental models provided in this paper are generic. Thus, they can be used to systematically study and optimize the infiltration process for clinical practice.

Inadequate placement of osteochondral plugs may induce abnormal stress-strain distributions in articular cartilage --finite element simulations.

The transplantation of osteochondral (cartilage-bone) plugs is an alternative approach to treat local, full thickness cartilage defects in young patients. It is technically difficult to control the amount of the press fit tolerance and the position of the osteochondral (OC) plug in the recipient hole. Inadequate placement of the OC plugs may produce abnormal stress and strain distributions within the cartilage, and thus influence the regeneration of the injured cartilage site and the maintenance of opposing, healthy cartilage surfaces. In the present study, the influence of press fit tolerance and the placement of the OC plug on the joint contact mechanics was simulated using finite element methods. The joint was assumed to be axi-symmetric with a spherical femur and tibia and a cylindrical OC plug. Our simulations showed that small misplacements of the OC plug induced abnormal tension in the articular cartilage of the opposing, healthy cartilage surface. Such tension might induce unpredictable adaptations, or possibly degenerations, in the opposing cartilage layer. The contact stress profiles in the joint were predicted to change discontinuously across the plug/recipient interface, even when the plug was perfectly placed in the recipient hole, i.e., the plugs surface was aligned with the recipient surface. For a fixed coefficient of friction and a fixed fit tolerance, the maximal sliding force was predicted to vary with the size of the plug and reached a maximum at a specific plug diameter. The present simulations should be helpful for the design of instruments for osteochondral transplantation and placement of OC plugs, for understanding articular cartilage adaptation following osteochondral repair, and for providing insight into the mechanics at the transplant/recipient interface where proper integration of the plug into the joint is most problematic.

Simultaneous determination of the nonlinear-elastic properties of skin and subcutaneous tissue in unconfined compression tests.

Background/aims: The compressive nonlinear‐elastic properties of soft tissues are usually determined using unconfined compression tests. To determine the nonlinear‐elastic behavior of skin and subcutaneous tissue using a conventional approach, the skin and subcutaneous tissue had to be separated before testing. Using such an approach, measurement errors may be increased as a consequence of the reduced specimen dimensions and cumulative experimental errors. In the present study, we propose a novel method to determine the nonlinear‐elastic behaviors of the skin and the subcutaneous tissue simultaneously using specimens of skin/subcutaneous composites.

Modeling of time-dependent force response of fingertip to dynamic loading

An extended exposure to repeated loading on fingertip has been associated to many vascular, sensorineural, and musculoskeletal disorders in the fingers, such as carpal tunnel syndrome, hand-arm vibration syndrome, and flexor tenosynovitis. A better understanding of the pathomechanics of these sensorineural and vascular diseases in fingers requires a formulation of a biomechanical model of the fingertips and analyses to predict the mechanical responses of the soft tissues to dynamic loading. In the present study, a model based on finite element techniques has been developed to simulate the mechanical responses of the fingertips to dynamic loading. The proposed model is two-dimensional and incorporates the essential anatomical structures of a finger: skin, subcutaneous tissue, bone, and nail. The skin tissue is assumed to be hyperelastic and viscoelastic. The subcutaneous tissue was considered to be a nonlinear, biphasic material composed of a hyperelastic solid and an invicid fluid, while its hydraulic permeability was considered to be deformation dependent. Two series of numerical tests were performed using the proposed finger tip model to: (a) simulate the responses of the fingertip to repeated loading, where the contact plate was assumed to be fixed, and the bone within the fingertip was subjected to a prescribed sinusoidal displacement in vertical direction; (b) simulate the force response of the fingertip in a single keystroke, where the keyboard was composed of a hard plastic keycap, a rigid support block, and a nonlinear spring. The time-dependent behavior of the fingertip under dynamic loading was derived. The model predictions of the time-histories of force response of the fingertip and the phenomenon of fingertip separation from the contacting plate during cyclic loading agree well with the reported experimental observations.

Three-dimensional finite element simulations of the mechanical response of the fingertip to static and dynamic compressions

The analysis of the mechanics of the contact interactions of fingers/handle and the stress/strain distributions in the soft tissues in the fingertip is essential to optimize design of tools to reduce many occupation-related hand disorders. In the present study, a three-dimensional (3D) finite element (FE) model for the fingertip is proposed to simulate the nonlinear and time-dependent responses of a fingertip to static and dynamic loadings. The proposed FE model incorporates the essential anatomical structures of a finger: skin layers (outer and inner skins), subcutaneous tissue, bone and nail. The soft tissues (inner skin and subcutaneous tissue) are considered to be nonlinearly viscoelastic, while the hard tissues (outer skin, bone and nail) are considered to be linearly elastic. The proposed model has been used to simulate two loading scenarios: (a) the contact interactions between the fingertip and a flat surface and (b) the indentation of the fingerpad via a sharp wedge. For case (a), the predicted force/displacement relationships and time-dependent force responses are compared with the published experimental data; for case (b), the skin surface deflection profiles were predicted and compared with the published experimental observations. Furthermore, for both cases, the time-dependent stress/strain distributions within the tissues of the fingertip were calculated. The good agreement between the model predictions and the experimental observations indicates that the present model is capable of predicting realistic time-dependent force/displacement responses and stress/strain distributions in the soft tissues for dynamic loading conditions.

Biodynamic Response of Human Fingers in a Power Grip Subjected to a Random Vibration

BACKGROUND Knowledge of the biodynamic response (BR) of the human hand-arm system is an important part of the foundation for the measurement and assessment of hand-transmitted vibration exposure. This study investigated the BR of human fingers in a power grip subjected to a random vibration. METHOD Ten male subjects were used in the experiment. Each subject applied three coupling actions to a simulated tool handle at three different finger grip force levels. RESULTS AND CONCLUSIONS The BR is practically independent of the hand coupling actions for frequencies at or above 100 Hz. Above 50 Hz, the BR is correlated to finger and hand sizes. Increasing the finger coupling force significantly increases the BR. Therefore, hand forces should be measured and used when assessing hand-transmitted vibration exposure. The results also show that under a constant-velocity vibration, the finger vibration power absorption at frequencies above 200 Hz is approximately twice that at frequencies below 100 Hz. This suggests that the frequency weighting specified in the current ISO 5349-1 (2001) may underestimate the high frequency effect on vibration-induced finger disorders.

Effect of Fluid Boundary Conditions on Joint Contact Mechanics and Applications to the Modeling of Osteoarthritic Joints

The long-term goal of our research is to understand the mechanism of osteoarthritis (OA) initiation and progress through experimental and theoretical approaches. In previous theoretical models, joint contact mechanics was implemented without consideration of the fluid boundary conditions and with constant permeability. The primary purpose of this study was to investigate the effect of fluid boundary conditions at the articular surfaces on the contact mechanics, in terms of load sharing and fluid flow properties using variable permeability. The tested conditions included totally sealed surfaces, open surfaces, and open surfaces with variable permeability. While the sealed surface model failed to predict relaxation times and load sharing properly, the class of open surface models (open surfaces with constant permeability, and surfaces with variable permeability) gave good agreement with experiments, in terms of relaxation time and load sharing between the solid and the fluid phase. In particular, the variable permeability model was judged to be the most realistic of the three models, from a biological and physical point of view. This model was then used to simulate joint contact in the early and late stages of OA. In the early stages of OA, the model predicted a decrease in peak contact pressure and an increase in contact area, while in the late stages of OA, peak pressures were increased and contact areas were decreased compared to normal. These findings agree well with experimental observations.

Dynamic interaction between a fingerpad and a flat surface: experiments and analysis

Many neural and vascular diseases in hands and fingers have been related to the degenerative responses of local neural and vascular systems in fingers to excessive dynamic loading. Since fingerpads serve as a coupling element between the hand and the objects, the investigation of the dynamic coupling between fingertip and subjects could provide important information for the understanding of the pathomechanics of these neural and vascular diseases. In the present study, the nonlinear and time-dependent force responses of fingertips during dynamic contact have been investigated experimentally and theoretically. Four subjects (2 male and 2 female) with an average age of 24 years participated in the study. The index fingers of right and left hands of each subject were compressed using a flat platen via a micro testing machine. A physical model was proposed to simulate the nonlinear and time-dependent force responses of fingertips during dynamic contact. Using a force relaxation test and a fast loading test at constant loading speed, the material/structural parameters underlying the proposed physical model could be identified. The predicted rate-dependent force/displacement curves and time-histories of force responses of fingertips were compared with those measured in the corresponding experiments. Our results suggest that the force responses of fingertips during the dynamic contacts are nonlinear and time-dependent. The physical model was verified to characterize the nonlinear, rate-dependent force-displacement behaviors, force relaxations, and time-histories of force responses of fingertips during dynamic contact.

Effectiveness of a new method (TEAT) to assess vibration transmissibility of gloves

Abstract A test method based upon total effective acceleration transmissibility (TEAT) is proposed to study the vibration isolation performance of anti-vibration gloves. The vibration transmission characteristics of three different gloves are investigated under predominantly axial vibration using the proposed method and the procedure outlined in ISO-10819 (Mechanical Vibration and Shock—Hand–Arm Vibration—Method for the Measurement and Evaluation of the Vibration Transmissibility of Gloves at the Palm of the Hand, International Standard Organization, Geneva, Switzerland, 1996). The measured data are systematically analyzed to illustrate the measurement and evaluation errors arising from misalignments of the response accelerometer within the palm-held adaptor, unintentional non-axial vibration caused by the vibration exciter and dynamics of the coupled hand–handle system. The degree of adaptor misalignment, estimated from the measured data, was observed to vary from 5.9° to 59.6°. Such variations could cause measurement errors in excess of 20%. The vibration transmission characteristics of selected gloves, evaluated using the proposed method, are compared with those derived from the standardized method to demonstrate the effectiveness of the TEAT approach. From the results, it is concluded that the TEAT method, based upon vector sums of both the source and response accelerations, can effectively account for the majority of the measurement errors, and yield more repeatable and reliable assessments of gloves. Relevance to industry Prolonged exposure to hand-transmitted vibration has been related to the possible occurrence of several health disorders by affecting the bones, joints, muscles and nervous system. The epidemiological studies show that millions of industrial workers are being exposed to hand vibration throughout the world. It is thus vital to develop improved methods for assessment of effectiveness of anti-vibration gloves. The proposed methodology could be applied to assess the effective vibration attenuation performance of anti-vibration gloves, and it could contribute towards developing an improved test method.