Biomedical subjects

Arthur Ho

Publications and source records attributed to Arthur Ho.

10 recordsLinked to original sources

PubMed2006

Predicting the performance of accommodating intraocular lenses using ray tracing.

PURPOSE: To predict and compare the amount of accommodation achievable by pseudophakic accommodating intraocular lenses (IOLs) using optical ray-tracing analysis. SETTING: Computational laboratory. METHODS: Two-element IOLs (2E-IOL, with mobile front or back optical elements) were compared with single-element IOLs (1E-IOL). Modeling using computer-assisted ray tracing of both IOL types assumed lens elements were equiconvex/equiconcave. The 4 possible combinations of configurations representing a wide range of varying positive and negative power (up to +40 diopters [D]) of front and back optical elements were evaluated. RESULTS: The 1E-IOLs offered limited amplitude of accommodation with axial shift (approximately 1.2 D/mm). For 2E-IOLs, configurations with high positive-power front elements returned the best amplitude of accommodation (up to approximately 3.0 D/mm when the front element power was +40 D). CONCLUSIONS: Considering the maximum potential amounts of axial shifts available, 1E-IOLs were predicted to provide 1.0 D of accommodation or less and 2E-IOLs were predicted to provide up to 3.0 D to 4.0 D depending on design configuration and amount of axial shift achievable. Potential issues relating to accommodative aniseikonia and spherical aberration have been identified.

Accommodation, Ocular↗

PubMed2006

Assessment of the strength of minicapsulorhexes.

PURPOSE: To evaluate the effect of age, size, position, and species on the strength of minicapsulorhexes. SETTING: Surgical Suite and Laser Laboratory, Ophthalmic Biophysics Center, Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, Miami, Florida, USA. METHODS: Capsulorhexes 0.7 to 2.3 mm in diameter were made centrally or peripherally in 35 eye-bank eyes and 32 rabbit eyes. A custom-made instrument stretched the capsulorhexes until rupture. Load and stretch at rupture were recorded. RESULTS: Maximum load and stretch were 26.3 mN +/- 20.3 (SD) and 50% +/- 18% for central and 50.8 +/- 20.5 mN and 69% +/- 17% for peripheral capsulorhexes in eye-bank eyes and 19.8 +/- 15.2 mN and 38% +/- 13% for central and 13.5 +/- 9.5 mN and 30% +/- 7% for peripheral capsulorhexes in rabbit eyes. Peripheral capsulorhexes were stronger and more elastic than central capsulorhexes in eye-bank eyes, and maximum load and stretch increased statistically with the capsulorhexis diameter. CONCLUSIONS: Peripheral minicapsulorhexes were more resistant to rupture than central capsulorhexes in eye-bank eyes, probably because of increased lens capsule thickness at the periphery. An increase in capsulorhexis diameter increased the resistance to rupture.

Adult↗

PubMed2006

In vitro dimensions and curvatures of human lenses.

The purpose of this study was to determine dimensions and curvatures of excised human lenses using the technique of shadowphotogrammetry. A modified optical comparator and digital camera were used to photograph magnified sagittal and coronal lens profiles. Equatorial diameter, anterior and posterior sagittal thickness, anterior and posterior curvatures, and shape factors were obtained from these images. The data were used to calculate lens volumes, which were compared with the lens weights. Measurements were made on 37 human lenses ranging in age from 20 to 99 years. These showed that lens dimensions and the anterior radius of curvature increase linearly throughout adult life while posterior curvature remains constant. The relative shape (or aspect ratio) of the posterior lens is unchanged through adult life since both equatorial diameter and posterior thickness increase at the same rate. The ratio of anterior thickness to posterior thickness is constant at 0.70. It is suggested that in vivo forces alter the apparent location of the lens equator, that the in vitro lens shape corresponds to the maximally accommodated shape in vivo and that the shapes of the accommodated and unaccommodated lens progressively converge toward each other due to lens growth with age, with a convergence point located near the age of total loss of accommodation (55-60 years). Together, these observations provide additional support for the Helmholtz theory of accommodation.

Accommodation, Ocular↗

PubMed2005

A novel method for assessing variations in visual acuity after the blink.

A novel instrument was developed to assess the visual acuity in correlation with time after blink. The method uses an optical blink detector which triggers the display of a random target symbol on a computer monitor. The delay time between blink and symbol display varies randomly from blink to blink, the exposure duration is fixed at 100 ms. The subject has to identify the correct symbol, a score is kept of the correct responses in correlation to delay time. The recovery of visual acuity after blink was measured for subjects wearing two types of bifocal contact lenses and a control group of non-contact lens wearers. All groups required between 200 and 500 ms to achieve their best visual acuity. Subjects wearing the translating bifocal lenses recovered their vision significantly faster than the subjects wearing no lens or the simultaneous vision bifocal lens.

Adult↗

PubMed2004

Osmolality and buffering agents in soft contact lens packaging solutions.

AIM: The study determined the effects of packaging solution osmolality and buffering agent on soft contact lens parameters. METHODS: One lens type from each FDA contact lens material category was equilibrated to phosphate buffered saline (PBS) or borate buffered saline (BBS), at three osmolality concentrations: 270, 310 and 414 mOsmol/kg. Lens diameter (LD), base curve (BOZR) and back vertex power (BVP) were measured and compared to nominal packaging label values. RESULTS: Statistically significant differences were found in all three parameters across all osmolality levels for both buffering agents. Etafilcon A showed the largest amount of parameter change, followed by alphafilcon A and balafilcon A. Lotrafilcon A was the least affected. LD of alphafilcon A lenses (greater than +/- 0.2 mm) and BVP of lotrafilcon A and etafilcon A lenses (greater than +/- 0.25 D) measured outside the International Standards Organization (ISO) tolerance from their packaging labels when measured in solutions that mimicked their packaging solutions. CONCLUSION: Osmolality and buffering agents influence lens parameters. Packaging solutions can vary the parameters of some lens types from their nominal value to outside the tolerance range set by ISO.

Journal Article↗

PubMed2004

Radius of curvature and asphericity of the anterior and posterior surface of human cadaver crystalline lenses.

The purpose of this study was to measure the shape of the anterior and posterior surface of human cadaver lenses in situ using a corneal topography system. Measurements were performed on 13 pairs of eyes using the PAR Corneal Topography System (PAR-CTS). The age of the donors ranged from 46 to 93 years, with an average age of 76.4 years. Anterior lens topography was measured after excision of the cornea and iris. Posterior lens topography was measured after sectioning the posterior segment and adherent vitreous. The PAR-CTS files providing raw surface height were exported for analysis. In each surface, 18 meridians separated by 10 degrees were fitted using conic sections to obtain values of the apical radius of curvature (R) and shape factor (p). The average apical radius of curvature and asphericity were R=10.15+/-1.39mm and p=4.27+/-1.39 for the anterior surface and R=-6.25+/-0.79mm and p=-0.64+/-1.85 for the posterior surface. A significant variation of the radius of curvature and shape factor as a function of the meridian angle (lens astigmatism) was found in some lenses. Contrary to previous findings, the anterior lens surface was found to steepen toward the periphery.

Aged↗

PubMed2004

Peripheral light focusing as a potential mechanism for phakic dysphotopsia and lens phototoxicity.

Our aim was to examine secondary image formation in the anterior segment caused by peripheral light focusing (PLF) in the human cornea, and in particular the crystalline lens. Non-sequential ray-tracing (OptiCAD) was applied to an anatomically based human eye model, which incorporates a gradient index crystalline lens. For analysis of the limbal effect, we varied the incident angle from 100 to 122 degrees, while for the crystalline lens effect, the incident angle was varied from 60 to 90 degrees. The corneal shapes studied included central radii from 7.4 to 8.2 mm with a range of shape factors. In each case, we computed the peak and average intensities, and the area of exposure at the limbus or lens periphery. The computation was repeated with a previous model eye for comparison. For the limbal effect, a peak intensity gain of x22.5 was found at an incident angle of 104 degrees which compares well with previous results. The average intensity gain at this angle was x7.5 over an area of 0.23 mm2. Steeper corneal curvature produced a greater PLF effect. For the crystalline lens effect, maximum UVA (365 nm) intensity gain peaked at x8.6 at 84 degrees with average intensity gain of x2.3. The area of UVA exposure peaked at 4.7 mm2 at 70 degrees. A relatively wide range (30 degrees ) of incident angles produced peak PLF gains of x3 or more in the lens. Significant focusing of light is directed to the nasal limbus, and to a lesser extent to the crystalline lens over a broad range of incident angles. PLF in the nasal cornea is reduced by an order of magnitude when a UV-blocking soft contact lens is used. The concentration levels and intraocular sites of PLF action on UV and visible light suggest a new mechanism of phakic dysphotopsia and lens phototoxicity.

Anterior Chamber↗

PubMed2003

Aberration correction with soft contact lens: is the postlens tear film important?

PURPOSE: A number of factors may influence the performance of soft contact lenses designed to correct the aberrations of the eye. Although the effect of many of these factors has been investigated, the effect of postlens tear film has not. We estimated the optical effect of the postlens tear film by using simplified models of postlens tear thickness profiles. METHODS: Three models were analyzed, including (1) a worst-case scenario before lens settling, (2) representative components of thickness profile based on Zernike-like forms, and (3) a series of undulations (ripples) across the postlens tear film. A range of thicknesses of the tear film was studied. Ray-tracing analyses were conducted on all models to calculate the modulation transfer function response between 0 and 100 cycles per degree and Strehl ratio. RESULTS: The results show that in only a few scenarios does the reduction in optical performance because of the postlens tear film approach those introduced by conventional spherical surface soft contact lenses. CONCLUSION: The postlens tear film can introduce discernable amounts of optical degradation in the lens-wearing eye. However, the amount of this effect is likely to be small compared with other sources, and benefits afforded by the use of aberration-corrected soft contact lenses should not be severely affected.

Computer Simulation↗

PubMed2003

Prevention of the adverse photic effects of peripheral light-focusing using UV-blocking contact lenses.

PURPOSE: Peripheral light-focusing (PLF) is an occult form of ultraviolet radiation (UVR) hazardous to the human eye. In PLF, obliquely incident light is refracted from the peripheral cornea to concentrated sites inside the anterior segment. In the current study, the directionality of this phenomenon for UVR and whether PLF is established in outdoor settings exposed to sunlight were investigated. The protection provided by a UV-blocking contact lens was also evaluated. METHODS: UVA and UVB sensors were placed on the nasal limbus of an anatomically based model eye. The temporal limbus was exposed to a UV light source placed at various angles behind the frontal plane. PLF was quantified with the sensor output. The ensemble was mounted in the orbit of a mannequin head and exposed to sunlight in three insolation environments within the region of Sydney, Australia. PLF for UVA and UVB was determined with no eyewear or with sunglasses and commercially available soft contact lenses, with and without UV-blocking capability. RESULTS: The intensity of UVA peaked at approximately 120 degrees incidence, the level at which the UVB response was also at its maximum. The intensification of UVA was up to x18.3. The intensity of PLF for UVA and UVB was reduced by an order of magnitude by a UV-blocking contact lens, whereas a clear contact lenses had a much lesser effect. Only the UV-blocking contact lens achieved a significant effect on UVA and UVB irradiance in the urban, beach, and mountain locales (P < 0.056). CONCLUSIONS: The results identify another type of sunlight hazard: the peripheral focusing of obliquely incident light. UVR from albedo (reflected ambient light) is capable of establishing PLF in the anterior segment, but this can be shielded by UV-blocking soft contact lenses. Sunglasses may be unable to shield oblique rays, unless side protection is incorporated. Contact lenses can offer UVR protection against all angles of incidence, including the peak-response angle. They can also protect the eye in settings in which the wearing of sunglasses is not feasible or convenient.

Contact Lenses↗

PubMed2002

Ablation profiles for wavefront-guided correction of myopia and primary spherical aberration.

PURPOSE: To calculate ablation profiles for wavefront-guided correction of ametropia and primary spherical aberration. SETTING: Bascom Palmer Eye Institute, University of Miami, Miami, Florida, USA. METHODS: The primary spherical aberrations of the ocular surfaces of an aspheric eye model were calculated before and after simulated ablations for correction of myopia ranging from 0 to 10 diopters. The corneal asphericity to correct primary spherical aberration and the corresponding ablation profiles were also calculated. RESULTS: The corneal asphericity factor that produces zero primary spherical aberration ranges from -0.45 to -0.47. The calculated ablation profiles are parabolic in first approximation, and the ablation depth varies linearly with the amount of correction. To control residual primary spherical aberration with a tolerance of one-quarter wavelength, the precision of the ablation must range from 0.2 to 0.3 microm. CONCLUSIONS: Ocular aberrometry and corneal topography can be used to calculate ablations for the correction of ametropia and primary spherical aberration. Precise control of postoperative spherical aberration appears to be feasible in theory.

Algorithms↗