Biomedical subjects

L E Amaya

Publications and source records attributed to L E Amaya.

11 recordsLinked to original sources

PubMed1997

Causes and mechanisms of cerebellar infarction in young patients.

BACKGROUND AND PURPOSE: The incidence of cerebellar infarction in a series of patients with stroke is approximately 1.5%. The average patient age in most reported series is 62 years. The most common etiologies in this age group are atherosclerosis and cardiac embolism. The aim of this study was to determine the causes and mechanisms of cerebellar infarction in patients younger than 40 years. METHODS: We analyzed retrospectively the clinical and radiological data from 21 men and 16 women with cerebellar infarction admitted to our hospital from January 1986 to December 1996. The patients had been studied extensively to determine the etiology of the cerebellar infarction. RESULTS: In the 37 patients (mean age, 30 years), 29 infarcts were limited to one territory (15 in the posteroinferior cerebellar artery [PICA]; 14 in the superior cerebellar artery); 8 had nonterritorial infarctions. The most common stroke mechanisms in each territory were as follows: PICA: nonatherosclerotic vasculopathic (67%), cardioembolic (20%), and hematologic and cryptogenic (each 7%); superior cerebellar artery: cardioembolic (42%), cryptogenic (31%), migrainous (21%), and nonatherosclerotic vasculopathic and hematologic (each 7%); and mixed territory: nonatherosclerotic vasculopathic (50%), cryptogenic (25%), cardioembolic (12%), and hematologic (12%). CONCLUSIONS: The most common mechanism of cerebellar infarctions was arterial occlusion as a result of intracranial vertebral artery dissection (40%), mainly with PICA involvement. Embolism from a cardiac source resulted primarily from patent foramen ovale and rheumatic valvular disease. Hematologic disturbances and migraine were responsible for a few cases.

Adolescent↗

PubMed1993

Myopic astigmatism and simple myopia correction.

A new technique for the treatment of myopic astigmatism and simple myopia correction is described. The technique is based on bioengineering concepts previously studied by the authors that accurately and mathematically describe the transformation of an ellipsoidal surface (astigmatic eye) into a spheroidal one (nonastigmatic eye) by means of minimal cuts (keratotomies). The surgical procedure is described using selected cases to illustrate the alternatives the model can handle. The model itself is not presented because the calculations were done by computer.

Adolescent↗

PubMed1986

Mechanical behavior of the sclera.

Induced deformations, similar to the posterior pole deformations due to advanced myopia, were studied in a series of 30 rabbits. The induced deformations were produced by means of a specially designed cup. An analysis of the applicability of the shell theory is made, and it is concluded that the thin shell theory is applicable to the mechanical study of the eyes. The basic experiment consisted of introducing a controlled deformation at different pressure levels; a wide range of types of increments and times of substained deformations were studied. It is concluded that the scleral tissue is an elastic material and has a fairly high 'Creep' rate. The material properties indicate that the eye can reaccommodate induced pressure, and thus glaucoma and myopia could be present without external indications of 'high pressure' in the eye.

Accommodation, Ocular↗

PubMed1985

Mathematical model of geometry of the eye.

A computer program was written in order to construct a geometric model of the human eye, having as a base some subroutines from the SAP IV program designed by the University of California. It was written in Fortran. In order to generate the proposed geometry, it was necessary to divide the eye into three zones, each one corresponding to a plane geometric figure, since the human eye does not correspond to any one shape. When the three zones are joined together, they form an eyeball.

Humans↗

PubMed1985

Corneal curvature modification by scleral surgery: preliminary results.

The preliminary results of a scleral technique designed to correct mixed and myopic compound astigmatism are given, and the mathematical basis of this technique is explained. The amount of scleral resection per diopter of astigmatism is indicated. This procedure steepens the flattest meridian and flattens the meridian localized at 90 degrees from the first one.

Astigmatism↗

PubMed1984

Experimental modification of the corneal curvature by means of scleral surgery.

Most surgical procedures for correcting astigmatism are done on the cornea. The present article shows the modification of the corneal curvature by scleral surgery. The experiments were conducted on rabbit eyes. The most spherical eyes were chosen according to the keratometric measurements. A geometrical analysis of the astigmatism shows that beginning with an ellipse (image of an astigmatic eye in the corneascope), by modifying the main radii of the ellipse, while the perimeter is maintained constant a perfect circle is thus obtained. This analysis enables us to calculate the amount of sclera surgery required to obtain a given correction for a given degree of astigmatism. Four scleral procedures were investigated: folds, resections, imbrications, and sclerotomies; the results show that the first three procedures steepen the meridian where the surgery is performed while flattening simultaneously the ortogonal meridian. The main advantages of scleral surgery vs corneal surgery for correction of astigmatism are: (a) the cornea remains untouched, (b) the surgical area will be protected by the conjunctiva, (c) the stitches are left permanently, and (d) larger astigmatic errors can so be corrected.

Animals↗

PubMed1980

Bio-structural model of the human eye.

As part of a bio-engineering approach to study myopia, a structural model of the eye is proposed. The model is based on structural laws as employed by civil engineers, and on mechanical properties of the sclera and vitreous previously analyzed by the authors. The described structural behavior relates many loose clinical observations and indicates that progressive myopia should be considered a mechanical problem due to an abnormal vitreous pressure rather than an optics problem. From this bio-engineering approach, vitrectomy is suggested as the appropriate solution.

Biomechanical Phenomena↗

PubMed1980

Mechanical analog for the study of the internal forces of the eye.

As part of the bio-engineering approach to study myopia and also as a complement to the bio-structural model of the eye presented by the authors, an analog for the study of internal forces is analyzed. The behavior of the model indicates not only that the forces involved in the development of progressive myopia, but those of pre- and post-trabecular glaucomas could be studied and related with such an analog model, supporting the author's feelings that many medical problems could be successfully explained by engineering methods and techniques.

Biomechanical Phenomena↗

PubMed1980

Mechanical behavior of the iris-lens-zonule-ciliary body system.

The mechanical behavior of athe iris-lens-zonule-ciliary body (ILZC) system under intraocular pressure variations, measured with Scan A, is studied on a series of 20 rabbit eyes. The experiments indicate that the ILZC system behaves like a partition between the anterior and vitreous chambers and not like a supporting structural member. The observed behavior is related to previous clinical observations and to the biostructural model of the eye described in a previous paper by the authors.

Animals↗

PubMed1979

Interaction between anterior and vitreous chambers.

As part of the search for a bioengineering model of the eye, the interaction between instantaneous changes of pressure in the anterior and vitreous chambers are studied. A description of the experiments as well as data evaluation is given. The results indicate a linear relationship between the chambers and a diaphragm effect of the system lens-iris-zonula-ciliar body.

Animals↗

PubMed1979

Mechanical behavior of the vitreous.

A description of the experiments of the mechanical behavior of the vitreous body is presented. The experiments were performed with a photoelastic material surrounding both rabbit and human eyes. The results show that the vitreous body transmits forces isotropically as any newtonian fluid. Tests done on rabbits' eyes with a previous vitrectomy agree with the previous statements. Finally, a hypothesis of the effect of pressure on the posterior pole in myopia is given, stressing the changes clinically observed.

Animals↗