(1/63) The shape of the aging human lens: curvature, equivalent refractive index and the lens paradox.
Scheimpflug slit images of the crystalline lens are distorted due to the refracting properties of the cornea and because they are obliquely viewed. We measured the aspheric curvature of the lens of 102 subjects ranging in age between 16 and 65 years and applied correction for these distortions. The procedure was validated by measuring an artificial eye and pseudophakic patients with intraocular lenses of known dimensions. Compared to previous studies using Scheimpflug photography, the decrease of the radius of the anterior lens surface with age was smaller, and the absolute value for the radius of the anterior and posterior lens surface was significantly smaller. A slight decrease of the posterior lens radius with age could be demonstrated. Generally, front and back surfaces were hyperbolic. Axial length was measured of 42 subjects enabling calculation of the equivalent refractive index of the lens, which showed a small, but highly significant decrease with age. These new findings explain the lens paradox and may serve as a basis for modelling the refractive properties of the lens. (+info)
(2/63) Measuring higher order optical aberrations of the human eye: techniques and applications.
In the present paper we discuss the development of "wave-front", an instrument for determining the lower and higher optical aberrations of the human eye. We also discuss the advantages that such instrumentation and techniques might bring to the ophthalmology professional of the 21st century. By shining a small light spot on the retina of subjects and observing the light that is reflected back from within the eye, we are able to quantitatively determine the amount of lower order aberrations (astigmatism, myopia, hyperopia) and higher order aberrations (coma, spherical aberration, etc.). We have measured artificial eyes with calibrated ametropia ranging from +5 to -5 D, with and without 2 D astigmatism with axis at 45 degrees and 90 degrees. We used a device known as the Hartmann-Shack (HS) sensor, originally developed for measuring the optical aberrations of optical instruments and general refracting surfaces in astronomical telescopes. The HS sensor sends information to a computer software for decomposition of wave-front aberrations into a set of Zernike polynomials. These polynomials have special mathematical properties and are more suitable in this case than the traditional Seidel polynomials. We have demonstrated that this technique is more precise than conventional autorefraction, with a root mean square error (RMSE) of less than 0.1 micro m for a 4-mm diameter pupil. In terms of dioptric power this represents an RMSE error of less than 0.04 D and 5 degrees for the axis. This precision is sufficient for customized corneal ablations, among other applications. (+info)
(3/63) The management of orbital cysts associated with congenital microphthalmos and anophthalmos.
AIMS: To study the management of the orbital cysts present in a group of patients with anophthalmos and microphthalmos. METHODS: A retrospective study of 34 patients (40 orbits) treated for orbital cyst associated with microphthalmos and anophthalmos. RESULTS: The two largest treatment groups comprised 17 orbits (42.5%) where the cyst was removed surgically and 17 orbits (42.5%) where the cyst was retained and conformers were used. The remaining cases comprised two orbits (5%) where the cyst was aspirated initially; two orbits (5%) with large cysts which will need to be excised after further orbital growth; one orbit (2.5%) in which a silicone expander was used initially, and one orbit (2.5%) in which a mildly microphthalmic eye had some vision and was monitored but required no surgery. CONCLUSION: In this study 33 out of 34 patients had a good cosmetic result which illustrates that the orbital cyst in microphthalmos or anophthalmos performs a useful role in socket expansion and that the majority of patients with this condition can expect a good cosmetic outcome. (+info)
(4/63) Autogenous temporalis fascia patch graft for porous polyethylene (Medpor) sphere orbital implant exposure.
BACKGROUND: Temporalis fascia has been recommended for hydroxyapatite sphere exposure. The aim of this study was to identify potential risk factors for exposure of porous polyethylene (Medpor) sphere implants and evaluate the use of autogenous temporalis fascia as a patch graft for exposure. METHODS: A retrospective review of consecutive cases of porous polyethylene sphere orbital implant exposure. RESULTS: Five cases presented between May 2000 and October 2001 (three males, two females; mean age 44.5 years). Three had enucleation (two with primary implants) and two had evisceration (one with primary implant). Exposure occurred in one primary, two secondary, and two replacement implants. Orbital implant diameter was 20 mm in four cases and 16 mm in one case (contracted socket). The mean time from implantation to exposure was 23 months (range 0.7-42.6). Three patients had secondary motility peg placement before exposure. The average time from last procedure (sphere implant or peg insertion) to exposure was 3 months (range 0.7-12.6). Four patients required surgical intervention, of which three needed more than one procedure. Autogenous temporalis fascia grafting successfully closed the defect without re-exposure in three of these four patients. The grafts were left bare in three patients, with a mean time to conjunctivalise of 2.4 months (range 1.6-3.2). CONCLUSIONS: Exposed porous polyethylene sphere implants were treated successfully with autogenous temporalis fascia graft in three of four patients. This technique is useful, the graft easy to harvest, and did not lead to prolonged socket inflammation, infection, or extrusion. (+info)
(5/63) A newly developed peripheral anterior chamber depth analysis system: principle, accuracy, and reproducibility.
AIM: To develop a new, non-contact system for measuring anterior chamber depth (ACD) quantitatively, and to investigate its accuracy as well as interobserver and intraobserver reproducibility. METHODS: The system scanned the ACD from the optical axis to the limbus in approximately 0.5 second and took 21 consecutive slit lamp images at 0.4 mm intervals. A computer installed program automatically evaluated the ACD, central corneal thickness (CT), and corneal radius of curvature (CRC) instantly. A dummy eye was used for investigating measurement accuracy. The effects of CT and CRC on the measurement results were examined using a computer simulation model to minimise measurement errors. Three examiners measured the ACD in 10 normal eyes, and interobserver and intraobserver reproducibility was analysed. RESULTS: The ACD values measured by this system were very similar to theoretical values. Increase of CRC and decrease in CT decreased ACD and vice versa. Data calibration using evaluated CT and CRC successfully reduced measurement errors. Intraobserver and interobserver variations were small. Their coefficient variation values were 7.4% (SD 2.3%) and 6.7% (0.7%), and these values tended to increase along the distance from the optical axis. CONCLUSION: The current system can measure ACD with high accuracy as well as high intraobserver and interobserver reproducibility. It has potential use in measuring ACD quantitatively and screening subjects with narrow angle. (+info)
(6/63) Integrated hydroxyapatite implant and non-integrated implants in enucleated Asian patients.
INTRODUCTION: This study compares the outcome and complications of integrated hydroxyapatite implant and non-integrated orbital implants following enucleation in Asian patients. MATERIALS AND METHODS: This is a retrospective study of enucleated patients with coralline hydroxyapatite implants versus non-integrated implants (acrylic, glass and silicone) at the Singapore National Eye Centre from January 1991 to December 2000. The outcomes measured were implant migration, extrusion, socket infection, conjunctival dehiscence and implant exposure. Statistical analysis was done using the 2-sample t-test. RESULTS: Twenty-one patients had the hydroxyapatite implant and 38 non-integrated implants (27 acrylic, 9 glass and 2 silicone). The mean duration of follow-up was 2.7 years and 4 years for the hydroxyapatite implant and non-integrated implants respectively. Three patients with pre-existing severe socket contracture before enucleation surgeries were excluded from the study. Four cases of implant migration, 4 cases of implant extrusion and 3 cases of socket infection were encountered; all were sockets fitted with non-integrated implants. There was a higher rate of conjunctival dehiscence for sockets with hydroxyapatite implants (6 out of 21) compared to sockets with non-integrated implants (3 out of 35). This was statistically significant (P = 0.048). CONCLUSIONS: Implant complications of migration, extrusion and socket infection were found in non-integrated implants and none in coralline hydroxyapatite implants, which had a significantly higher rate of conjunctival dehiscence. Most of these were easily managed with only a small number progressing to implant exposure. (+info)
(7/63) Myoconjunctival enucleation for enhanced implant motility. result of a randomised prospective study.
PURPOSE: Implant motility and cosmetic appearance of the eye after enucleation have remained major considerations whenever mycoconjunctival enucleation is planned. Despite major advances in surgical technique and the availability of a variety of implant materials and shapes, there is lack of consensus on the best material and type of implant. This study was conducted to compare routine and myoconjunctival techniques of enucleation in terms of motility index. METHODS: Thirty consecutive patients scheduled for enucleation for non-malignant indications were randomly assigned to one of the two groups- routine and myoconjunctival techniques. The postoperative results were compared for the motility of implant and the complications. RESULTS: The two techniques showed no difference in rates of complications, but implant motility was significantly better with the myoconjunctival technique. Implant extrusion or migration was not seen. CONCLUSION: We recommend the use of myoconjunctival enucleation for enhanced motility and cosmesis. (+info)
(8/63) Simulation of artificial vision, III: do the spatial or temporal characteristics of stimulus pixelization really matter?
PURPOSE: In preceding studies, simulations of artificial vision were used to determine the basic parameters for visual prostheses to restore useful reading abilities. These simulations were based on a simplified procedure to reduce stimuli information content by preprocessing images with a block-averaging algorithm (square pixelization). In the present study, how such a simplified algorithm affects reading performance was examined. METHODS: Five to six volunteers with normal vision were asked to read full pages of text with a 10 degrees x 7 degrees viewing window stabilized in central vision. In a first experiment, reading performance with off-line and real-time square pixelizations was compared at different resolutions. In a second experiment, off-line square pixelization was compared with off-line Gaussian pixelization with various degrees of overlap. In a third experiment, real-time square pixelization was compared with real-time Gaussian pixelization. RESULTS: Results from the first experiment showed that real-time square pixelization required approximately 30% less information (pixels) than its off-line counterpart. Results from the second experiment, using off-line processing, revealed a restricted range of Gaussian widths for which performances were equivalent or significantly better than that obtained with square pixelization. The third experiment demonstrated, however, that reading performances were similar in both real-time pixelization conditions. CONCLUSIONS: This study reveals that real-time stimulus pixelization favors reading performance. Performance gains were moderate, however, and did not allow for a significant (e.g., twofold) reduction of the minimum resolution (400-500 pixels) needed to achieve useful reading abilities. (+info)