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Biomedical subjects

A F Mak

Publications and source records attributed to A F Mak.

At least 37 records · Page 2Linked to original sources
PubMed1998

Finite element modelling of a residual lower-limb in a prosthetic socket: a survey of the development in the first decade.

A review is presented of the existing finite element models developed from 1987 to 1996 for the biomechanics of lower-limb prostheses. Finite element analysis can be a useful tool in investigating the mechanical interaction between the residual limb and its prosthetic socket, and in computer-aided design and computer-aided manufacturing of prosthetic sockets. Various assumptions and simplifications are made in these models to simplify the actual problem with complex geometry, material properties, boundary and interfacial conditions, as well as loading situations. The analyses can provide the information on the stress distribution at the stump/socket interface and within the residual limb tissues. More recently, nonlinear models have been developed taking into consideration the process of socket rectifications, the slip/friction conditions and material large deformation. The models so far developed have provided some basic understanding of the biomechanics. Comparison of the predictions of these models with experimental measurements indicated that the predicted stresses were within the ranges measured, although one-to-one correspondence was difficult to achieve. Further research is still required in order to improve these models to obtain higher precision in the results taking into account nonlinear and dynamic effects.

Artificial Limbs↗
PubMed1998

Biomedical gait evaluation of the immediate effect of orthotic treatment for flexible flat foot.

Flexible flat foot subjects attending the prosthetics and orthotics units come with prescriptions from orthopaedic surgeons for arch supports. Usually a pair of thermoformed plastic inserts are fabricated and fitted to treat the patients. However the effect of the orthotic treatment is not yet clear. A motion analysis system with two video cameras placed on the lateral and rear sides of the subject together with one force platform was used to investigate the immediate effects of the orthotic treatment. The force platform collected force data and the two cameras captured two-dimensional displacement data of the lower limb. Eight subjects, all having an arch index (AI) larger than 3.0 participated in the study. For each subject, three successful steps on the force platform were videotaped for both the shod (with shoe only) and the orthotic (with shoe and orthosis) conditions. The kinetic variables were normalized to individual body weight and averaged for each subject. A Paired t-test was conducted to analyse sample means of matched pairs between the shod and the orthotic conditions. The results showed changes in displacement data with relatively little change in the collected force data. The modified UCBL shoe insert evaluated significantly affected the orientation and movements of the subtalar joint, ankle joint and knee joint. These immediate effects reduced the degree and duration of abnormal pronation during the stance phase and thus had the potential for decreasing strain in the plantar ligaments and reducing abnormal tibial rotation which may be therapeutic for the foot.

Ankle Joint↗
PubMed1998

Development of a simple approach to modify the supporting properties of seating foam for pressure relief.

Pressure sores are a major problem frequently encountered by persons who use wheelchairs. Custom seat design, including contoured seating and various modular seating devices, has become an important option for pressure relief, especially when the market size is small and the variations of individual needs and requirements are large. An alternative approach in custom seating design for pressure relief is proposed in this paper. Holes were drilled in foam cushions to lower their supporting properties, particularly at the high pressure areas. This technique is evaluated systematically in this article. The scope of this study included: 1) a comparison of the foam material properties before and after such modifications, and 2) an evaluation of the static and dynamic degradation behaviors of the foams before and after modifications. It was found that the compression load (C-L) of the foam could be reduced by up to 46% using this simple drilling technique, while the material removed was only up to 28% by volume. It was also found that this approach would not significantly compromise the static and dynamic degradation behaviors of the foam; that is, such modification apparently did not dramatically shorten the lifespan of the foam material. Simple hole-drilling seems to be an effective approach to altering the supportive properties of foam cushions for pressure relief.

Biomechanical Phenomena↗
PubMed1997

Deformation-induced hierarchical flows and drag forces in bone canaliculi and matrix microporosity.

Existing theories for interstitial flows in bone have only examined the contributions from different flow systems separately, such as the flows through the microporosity, the canaliculi, and the Haversian canals. An overall model encompassing the hierarchical microstructure is important to our understanding of the actual physics of flows in bone. The flow-induced drag forces and streaming electrical potentials could interact with the osteocytes to effect biological responses. A finite element model was developed to study the contributions from various hierarchical flow channels in bone. Cortical bone is modelled as a fully hydrated biphasic poroelastic material with a superposing network of one-dimensional channels radiating from the Haversian canals representing the canaliculi. Interfacial cross-flows between these one-dimensional channels and the neighbouring poroelastic matrix are driven by the pressure differences between the matrix and the channel. The model was subjected to stress fields simulating uniform compression and pure bending. The effects of the interfacial permeability and the solid content within the channels on the drag forces in the channels were assessed. Abrupt changes in these drag forces occurred as the channel solidity approached that of the microporosity. The results were quite sensitive to the interfacial permeability, i.e. the interconnectivity between the canalicular system and the matrix microporosity. This biomechanical model should be useful to the study of mechanotransduction in bone.

Biomechanical Phenomena↗
PubMed1997

Estimating the effective Young's modulus of soft tissues from indentation tests--nonlinear finite element analysis of effects of friction and large deformation.

A nonlinear finite element model was developed to investigate the biomechanics of indentation, particularly the influence of friction and large deformation on the calculation of the effective Young's modulus from the cylindrical, flat-ended indentation test of soft tissues. A new kappa table was given for calculation of the effective Young's modulus to account for the effects of layered geometry with consideration of the larger deformation. The results indicate that the effect of friction on the calculation of Young's modulus becomes significant with a large aspect ratio and with a large Poisson's ratio. It is found that the factor kappa increases almost proportionally to the increase of the indentation depth, especially obvious with a larger Poisson's ratio v and a larger aspect ratio a/h.

Elasticity↗
PubMed1997

Evaluation of a simple approach to modify the supporting property of seating foam cushion for pressure relief.

To provide pressure relief at the high pressure areas of the buttocks, small holes were drilled on the polyurethane (PU) foam cushion under the ischial tuberosities areas. This modification was done to locally lower the supporting property of the cushion. This approach could be easily implemented in the clinical setting. Measurements of interfacial pressures at the ischial tuberosities on such modified cushions for eight nondisabled subjects showed an average pressure 25% lower than those measured on foam cushions without modification (p < 0.01). The maximum pressure was 30% lower after modification (p < 0.01) The peak pressure gradient also showed a 38% decrease on modified foam compared to those measured on unmodified foam cushions (p < 0.01). Simple hole drilling could potentially provide an effective approach to enhance the pressure-relief characteristics of foam cushions.

Equipment Design↗
PubMed1997

Three-dimensional motion analysis of the voluntary elbow movement in subjects with spasticity.

The purpose of this study was to quantitatively compare the difference in voluntary upper extremity movements between subjects with and without spasticity. Eight normal subjects (mean 26.7 +/- 2.8 years, four males and four females) and seven subjects with spasticity (mean 25.9 +/- 4.3 years, three males and four females) were involved in this study. The subjects sat in an adjustable chair and performed two voluntary tasks involving the elbow joint. Task A was to move the hand between two touch-plates which were mounted 28 cm apart on the surface of the table. Task B was to flex and extend the elbow joint in the sagittal plane with the forearm in neutral position. Reflective markers were attached on the shoulder, the elbow and the wrist. A Peak5 video-based motion analysis system was used to record the positions of the markers in the three-dimensional (3-D) space during the movement tasks. A set of quantitative parameters were used to document the elbow movement. The results revealed that in comparison to normal subjects, subjects with spasticity exhibited a higher average jerk, a larger standard deviation of the coordinates of the markers along the movement path, a larger standard deviation of the angle between the plane of the elbow joint and the horizontal plane, and a longer 3-D path length. The characteristics of spastic elbow movement and the usage of quantitative parameters were discussed.

Adult↗
PubMed1997

Change of foot size with weightbearing. A study of 2829 children 3 to 18 years of age.

The change of foot length and width with age has been reported in a few anthropometric studies in the literature. However, the relationship with body height rarely is reported, and the dynamic effect of weightbearing on foot size has not been documented. In this series, 2829 children 3 to 18 years of age of equal gender distribution were included in the study. The foot length and width on weightbearing and nonweightbearing were measured with a special precision electronic caliper. The foot length and width were found to increase linearly from the age of 3 years until 12 years in girls and 15 years in boys. This was followed by a phase during which the increase plateaued. The foot length and width increased significantly on weightbearing at all ages in both genders with a mean of 2.1 to 4.4 mm or 3.1% to 4.8%, respectively. The foot length and width also were found to correlate significantly with the body height in both genders, with a correlation coefficient of 0.96 to 0.98. No significant differences were found between the sizes of the dominant and nondominant foot in either gender.

Adolescent↗
PubMed1996

Posture effect on seating interface biomechanics: comparison between two seating cushions.

OBJECTIVE: This study aimed at investigating the effects of sitting posture on ischial pressure and pelvic orientation for two types of cushions. DESIGN: Two types of seating devices, Roho and Polyurethane (PU) Foam cushions, six predefined postures, and two subject groups, Normal and Paraplegic, were tested. Ischial pressure and pelvic orientation were monitored. The sequence in which the cushions were tested were randomized, and the postures were tested according to a preplanned sequence. SETTING: The experiments were carried out in the laboratory of a rehabilitation engineering centre in Hong Kong. PARTICIPANTS: Six paraplegic subjects (referred sample) and 8 normal volunteers were tested. At the time of study, no subjects showed any signs of pressure sore or other health problems. MAIN OUTCOME MEASURES: It was hypothesized that sitting posture could significantly affect pelvic orientation as well as pressure distribution. It was also hypothesized that cushion selection was a critical factor in compensating the adverse effects of postures on pressure distribution. RESULTS: With the paraplegic group, the maximum ischial pressure recorded on Roho for various postures ranged from 88 mmHg for the Trunk-Bent-Forward (PF) posture to 146 mmHg for the Trunk-Bent-Right (PR) posture, whereas on PU foam cushions, the values ranged from 106 mmHg for the PF posture to 221 mmHg for the PR posture. With the paraplegic subjects sitting with their trunks bent laterally, it was found that the mean pressure difference between the left and right ischial tuberosities was more prominent on the PU foam cushion than on the Roho. CONCLUSION: Sitting posture can significantly affect pelvic orientation and ischial pressure. This study was also showed that the Roho cushion was significantly more efficient in compensating the adverse effects of sitting posture on pressure distribution.

Biomechanical Phenomena↗
PubMed1996

An ultrasound indentation system for biomechanical properties assessment of soft tissues in-vivo.

An ultrasound indentation system for biomechanical assessment of soft tissues in vivo was developed. The pen-size, hand-held probe was composed of an ultrasound transducer and a load cell. The ultrasound transducer was at the tip of the probe serving also as the indentor. The thickness and deformation of the soft tissue layer were determined from the ultrasound echo. A compressive load cell was connected in series with the ultrasound transducer to record the force response. A validation experiment was performed on porcine tissues. Force and deformation acquired with the present system was in good comparison with those obtained from a Housfield material testing machine. Material constants were obtained via a curve-fitting procedure by predicting the force transient response from the deformation-time data using a quasilinear viscoelastic model. In addition, deformation in the fat and in the muscle could be differentiated. The potential applications of this type of indentation probes are many. The specific application of this current development is for stump tissue assessment in the design of prosthetics.

Adipose Tissue↗
PubMed1996

A finite element analysis of the load transfer between an above-knee residual limb and its prosthetic socket--roles of interface friction and distal-end boundary conditions.

A nonlinear finite element model was developed to investigate the manner of the load transfer between the above-knee residual limb and its prosthetic socket. The model was two-dimensional (2-D) with consideration of nonlinear boundary conditions and large deformation. Interface elements were used to simulate the friction/slip conditions between skin and the socket. Different distal-end conditions, including air cushion distal-end loading, were simulated to explore their influences on the load transfer.

Amputation Stumps↗
PubMed1995

Evaluation of an active seating system for pressure relief.

In the first part of this study, the inflation-pressure and interface-pressure profiles of an active cushion system, the Talley active air bellows cushion, were examined continuously for one complete working cycle using the dynamic pressure monitor. The relationship between the inflation pressure and the interface pressure was explored. A well-defined relationship was found in the areas directly over the air bellows. In the second part of this study, the pressure-relieving characteristics of the active cushion were assessed quantitatively and compared to two types of passive cushions--the Roho high-profile air floatation cushion and the polyurethane (PU) foam cushion. Eight non-disabled subjects were positioned on the active cushion at two inflation-pressure levels--30 mmHg and 60 mmHg, or on the Roho or the PU foam cushions. Interface pressures were recorded using the Oxford pressure monitor. For the active cushion it was shown that the higher the inflation pressure was, the better the pressure-relieving characteristics seemed to be. In general, the pressure-relieving characteristics of the active cushion were not as good as those of the passive cushions being tested. The active cushion could alter the pressures over the ischial tuberosities cyclically but the amount of pressure alternation depended on the relative position of the ischial tuberosities and the air bellows.

Adult↗
PubMed1994

A biphasic poroelastic analysis of the flow dependent subcutaneous tissue pressure and compaction due to epidermal loadings: issues in pressure sore.

A layer of skin and subcutaneous tissue on a bony substratum was modeled as a homogeneous layer of biphasic poroelastic material with uniform thickness. The epidermal surface and the bony interface were taken to be impervious. The soft tissue on the bony interface was assumed either fully adhered or completely free to slide on the bone. The cases for surface pressure loadings and displacement controlled indentations were simulated. The resultant biomechanical responses of the layer, including the transient tissue hydrostatic pressure and the tissue compaction, were presented. A new hypothesis is offered to interpret the threshold pressure-time curve for pressure sores in term of the time required for a particular area in the tissue layer to reach a critical compaction for a given level of applied pressure.

Biomechanical Phenomena↗
PubMed1994

Biomechanical assessment of below-knee residual limb tissue.

In vivo indentation properties of the residual limb tissues of a group of senior subjects with below-knee (BK) amputation were measured and compared with those of nondisabled young adults. It was found that differences attributable to site variations, states of muscular activity, and the differences between the nondisabled young group and the group consisting of the seniors with amputation were all noted to be highly significant statistically. However, paired comparison between the residual limb and the sound contralateral limb of the senior group showed no significant difference. Residual limb properties measured right before the first prosthetic measurements and fittings and those measured at the first outpatient follow-up were found to be slightly different.

Adult↗
PubMed1989

Nonlinear viscoelastic properties of articular cartilage in shear.

The strain dependence of the intrinsic viscoelastic properties of the cartilage matrix in shear was investigated. Stress relaxation experiments were performed on bovine articular cartilage at shear strains ranging from approximately 3% to 16%. The tissue was found to exhibit nonlinear strain-dependent viscoelastic behavior, with the nonlinearity occurring primarily in the short-time transient during stress relaxation. In addition, the equilibrium stress was found to fit a quadratic relation with strain. This relationship was noted to be nearly linear with strain from 3% to 16%. The instantaneous stress was seen to be highly nonlinear, and followed a cubic relationship with applied shear strain. Fung's quasilinear theory can be used to describe the stress relaxation response over the range of strains examined when a nonlinear regression is performed to determine an "average" normalized relaxation function. Alternately, strain dependence can be incorporated into the model to describe and predict more accurately the strain-dependent stress relaxation response.

Animals↗
PubMed1987

Biphasic indentation of articular cartilage--I. Theoretical analysis.

A mathematical solution has been obtained for the indentation creep and stress-relaxation behavior of articular cartilage where the tissue is modeled as a layer of linear KLM biphasic material of thickness h bonded to an impervious, rigid bony substrate. The circular (radius = a), plane-ended indenter is assumed to be rigid, porous, free-draining, and frictionless. Double Laplace and Hankel transform techniques were used to solve the partial differential equations. The transformed equations and boundary conditions yielded an integral equation of the Fredholm type which was analyzed asymptotically and solved numerically. Our asymptotic analyses showed that the linear KLM biphasic material behaves like an incompressible (v = 0.5) single-phase elastic solid at t = 0+; the instantaneous response of the material is governed by the shear modulus (mu s) of the solid matrix. The linear KLM biphasic material behaves like a compressible elastic solid with material properties defined by those of the solid matrix, i.e. (lambda s, mu s) or (mu s, v s) as t----infinity. The transient viscoelastic creep and stress-relaxation behavior, 0 less than t less than infinity, of this material is controlled by the frictional drag (which is inversely proportional to the permeability k) associated with the flow of the interstitial fluid through the porous-permeable solid matrix. For given values of the Poisson's ratio of the solid matrix v s and the aspect ratio a/h, where a is the radius of the indenter and h is the thickness of the layer, the creep behavior with respect to the dimensionless time H Akt/a2 is completely controlled by the load parameter P/2 mu sa2 and the stress relaxation behavior is completely controlled by the rate of compression parameter R0 = kH A/V0h where H A = lambda s + 2 mu s and the equilibrium strain u0/h. This mathematical solution may now be used to describe an indentation experiment on articular cartilage to determine the intrinsic material properties of the tissue, i.e. permeability k, and the elastic coefficients of the solid phase (lambda s, mu s) or (mu s, v s).

Cartilage, Articular↗
PubMed1987

Electromechanical potentials in cortical bone--I. A continuum approach.

An electrokinetic model to characterize the electromechanical effect in cortical bone has been developed using the basic principles of the biphasic theory of porous materials and a simple model for permeability and charge distribution for cortical bone. The model is developed analytically in Part I of this paper and is shown to account qualitatively for the principal experimental results reported to date. Part II of this paper concerns experimental analysis of this model, reporting results of low frequency testing of the dynamic characteristics of stress-generated potentials. Quantitative analysis of these results indicates that the microporosity of bone, made up of the channels around the hydroxyapatite encrusting the collagen matrix, is the compartment responsible for the electromechanical effects in fluid-saturated cortical bone. This microporous compartment would seem to be the obvious source of the electrokinetic effect, because it has the greatest surface area in bone and constitutes the rate limiting fluid flow compartment in deformation-induced fluid flow at low frequency.

Animals↗