Biomedical subjects

A Gefen

Publications and source records attributed to A Gefen.

27 records · Page 2Linked to original sources

PubMed2001

Computational tools in rehabilitation of erectile dysfunction.

Erectile dysfunction (ED) is defined as the inability to achieve and maintain an erection adequate for satisfactory intercourse. It is a common problem among approximately 50% of men between the ages of 40 and 70. Erectile dysfunction is not only stressful to both the affected individual and his partner, but it can also negatively affect self-esteem. Biomechanical models have recently been developed to study both the structural and hemodynamic factors involved in normal and pathological erectile conditions. These computational models, which are reviewed in the present paper, allow for better understanding of the mechanisms acting in ED and provide a suitable basis for development of state-of-the-art interdisciplinary treatment approaches aimed to improve the quality of life for these men.

Adult↗

PubMed2001

Simulations of foot stability during gait characteristic of ankle dorsiflexor weakness in the elderly.

Falls are common among the elderly and often cause injuries. They most frequently occut during walking and are associated with the chronic deterioration in neuromuscular and sensory systems, as well as with ankle dorsiflexor muscular weakness and lowered endurance of these muscles to fatigue. In the present study, a three-dimensional (3-D) finite element model of the structure of the foot was utilized to determine the effects of ankle dorsiflexor muscle weakness on the structural stability of the foot and, consequently, on the risk of falls during gait. The medial-lateral tendency of instability of the foot during gait in such conditions of weakness was analyzed by means of this model to identify the most important muscles used in controlling foot stability in affected individuals. The values of the eccentricity of the center of pressure under the heel during foot placement were used to indicate the degree of foot stability. The computational analysis indicated that it is the tibialis anterior muscle's weakness that dramatically decreases foot stability. Clinical investigation is now needed to correlate the significance of tibialis anterior muscle weakness with other known risk factors affecting the tendency to falls among the elderly, e.g., deterioration of sensory abilities. Rehabilitation practitioners and physical therapists may apply the present analytic approach to evaluate the stability of a foot before treatment and compare the predicted with the actual therapeutic results in terms of optimization of foot-ground pressure.

Accidental Falls↗

PubMed2001

Analysis of mechanical stresses within the alveolar septa leading to pulmonary edema.

Mechanical ventilation has been associated with pulmonary edema in the clinical setting, but the pathophysiological mechanisms of this process have not been clearly defined. Experimental studies have shown that high transpulmonary pressures resulting from ventilation may damage the capillary walls, thereby leading to edema. Knowledge of the stress distribution within the alveolar septa would be an important step in understanding this phenomenon. A newly developed saline-filled alveolar sac model was utilized for analysis of septal stresses in young and aging healthy lungs, in order to examine their vulnerability to pulmonary edema during ventilation. Significant stress concentrations were shown to develop near highly curved regions (small local radii of less than 4 mum in a lung inflated to 80% could be as high as 25 times that of average septal stresses. The combination of elevated stress sites that are formed in the stiffer parenchyma of the aging lung, together with the cyclic loading of ventilation, may explain the gaps and breaks previously observed in pulmonary edema.

Exercise↗

PubMed2000

A biomechanical model of Peyronie's disease.

Peyronie's disease is a pathological condition of the penis which is characterized by localized ossification of the tunica albuginea. A common symptom of the chronic stage is penile deformity during erection, which is frequently associated with pain and erectile dysfunction. A two-dimensional biomechanical model of the penis was applied to study the development of Peyronie's disease by simulating the mechanical stress distribution which would result from the interaction of the ossified tunical tissue with other penile soft tissues. The model was solved by using commercial finite element software for a characteristic erectile pressure. The results demonstrate that Peyronie's plaques may induce intensified stresses around the penile nerves and blood vessels, up to double those in the normal penis. These elevated stresses may cause a painful sensation of neural origin or ischemia in regions of compressed vascular tissue. Severe penile deformities have been shown to develop if Peyronie's plaques develop only around one of the corpora cavernosa due to the non-homogeneous resistance of the tunica to expansion during erection. The present model can be clinically applied as an aid in the planning process of reconstructive surgery or insertion of a prosthesis.

Biomechanical Phenomena↗

PubMed2000

Predicting penile size during erection.

The aim of this prospective study was to identify clinical and engineering parameters of the flaccid penis for prediction of penile size during erection. Dorsal and ventral penile lengths, as well as base and tip circumferences were measured in flaccid states, gently stretched states and at full erection resulting from intracavernosal injection of prostaglandin E1 in 55 patients. The forces required to stretch the penis were measured by a specially designed gauge and regression relationships of the measured dimensions were calculated. An engineering model was developed to analyze differences between results obtained during stretching and erection, as well as to approximate the optimal force values which should be applied during the stretching part of the clinical evaluation of penile size. The ratio between the flaccid to stretched penile lengths was shown to be the best predictor for the ventral elongation from flaccid to erect penile lengths. The engineering analysis predicted that a minimal tension force of approximately 450 g during stretching of the penis is required to reach the potential erection length. The stretching forces exerted by the urologist in the clinical setting were experimentally shown to be significantly (P<0.01) less than this value. The values of the relative and absolute elongations of the stretched penis at its ventral aspect provide reliable estimations of its potential maximal elongation during erection. The model designed for this study may obviate the use of intracavernosal injections for estimating penile length during erection.

Adult↗

PubMed2000

Optimization of design and surgical positioning of inflatable penile prostheses.

The interaction between the cylinders of an inflatable penile prosthesis (IPP) and the surrounding tissues during IPP-aided erection may result in local elevated stresses. These stresses may reach values that can obstruct penile blood vessels and cause ischemia and/or stimulate nerves around the operation site, thereby inducing sensations of pain. A new numerical model was used to analyze penile stresses postimplantation of different IPP types, in order to optimize prosthesis design and surgical positioning by enabling minimal stress transfer to dorsal blood vessels and nerves. The results suggest that intraluminal pressures should be maintained at low levels (about 80 kPa) while cylinder thickness and stiffness should be kept just high enough (approximately 15% of the radius and 1000 MPa, respectively) to eliminate deleterious cylinder-tissue contact stresses. Smaller prosthetic cylinders, i.e., occupying about 45% of the cavernosal space, may be advantageous in terms of reducing dorsal stresses, but lower penile rigidity should be expected. A significant decrease of dorsal stresses can also be achieved by encouraging the surgeon to position the cylinders toward the lower part of the corpora. The numerical simulations indicate that circular cylinders may allow greater biomechanical compatibility of the IPP with the penis structure than elliptic ones, and this should be a subject for clinical investigations.

Biomechanical Phenomena↗

PubMed2000

Biomechanical analysis of the three-dimensional foot structure during gait: a basic tool for clinical applications.

A novel three-dimensional numerical model of the foot, incorporating, for the first time in the literature, realistic geometric and material properties of both skeletal and soft tissue components of the foot, was developed for biomechanical analysis of its structural behavior during gait. A system of experimental methods, integrating the optical Contact Pressure Display (CPD) method for plantar pressure measurements and a Digital Radiographic Fluoroscopy (DRF) instrument for acquisition of skeletal motion during gait, was also developed in this study and subsequently used to build the foot model and validate its predictions. Using a Finite Element solver, the stress distribution within the foot structure was obtained and regions of elevated stresses for six subphases of the stance (initial-contact, heel-strike, midstance, forefoot-contact, push-off, and toe-off) were located. For each of these subphases, the model was adapted according to the corresponding fluoroscopic data, skeletal dynamics, and active muscle force loading. Validation of the stress state was achieved by comparing model predictions of contact stress distribution with respective CPD measurements. The presently developed measurement and numerical analysis tools open new approaches for clinical applications, from simulation of the development mechanisms of common foot disorders to pre- and post-interventional evaluation of their treatment.

Biomechanical Phenomena↗

PubMed1999

Stresses in the normal and diabetic human penis following implantation of an inflatable prosthesis.

The prevalence of impotence in diabetes mellitus ranges as high as 75%. The implantation of an inflatable penile prosthesis (IPP) is frequently carried out to restore erectile function. However, clinical studies have demonstrated that severe post-implantation penile pain during erection is a common complication in diabetic men. A biomechanical model of the penis/prosthesis complex is developed, based on cross-sectional anatomy, to simulate the internal stress distribution due to interaction of the prosthesis with both normal and diabetic penile tissues. The material properties of the model components are adopted from experimental data. The model is solved by using commercial finite-element software for a characteristic inflation loading of the penile prosthesis. Elevated structural stresses during erection are found in the dorsal aspect of the tunica albuginea (normal 5.1-31.5 kPa, diabetic 5.1-70 kPa post-implantation). Following IPP implantation, stresses in the diabetic penis are almost as twice as high as those in the normal one and can cause a painful sensation owing to nerve stimulation or to ischaemia in regions of compressed vascular tissue.

Diabetic Angiopathies↗

PubMed1999

Analysis of stress distribution in the alveolar septa of normal and simulated emphysematic lungs.

The alveolar septum consists of a skeleton of fine collagen and elastin fibers, which are interlaced with a capillary network. Its mechanical characteristics play an important role in the overall performance of the lung. An alveolar sac model was developed for numerical analysis of the internal stress distribution and septal displacements within the alveoli of both normal and emphysematic saline-filled lungs. A scanning electron micrograph of the parenchyma was digitized to yield a geometric replica of a typical two-dimensional alveolar sac. The stress-strain relationship of the alveolar tissue was adopted from experimental data. The model was solved by using commercial finite-element software for quasi-static loading of alveolar pressure. Investigation of the state of stresses and displacements in a healthy lung simulation yielded values that compared well with experimentally reported data. Alteration of the mechanical characteristics of the alveolar septa to simulate elastin destruction in the emphysematic model induced significant stress concentrations (e.g., at a lung volume of 60% total capacity, tensions at certain parts in an emphysematic lung were up to 6 times higher than those in a normal lung). The combination of highly elevated stress sites together with the cyclic loading of breathing may explain the observed progressive damage to elastin fibers in emphysematic patients.

Animals↗