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Contact lenses
Published in Pablo Artal, Handbook of Visual Optics, 2017
In a clinical setting, the optical power of the eye is typically determined by performing a subjective refraction. This procedure involves the iterative application of spherical and cylindrical lenses in front of the eye to determine the optimal visual acuity at distance. The lenses are mounted in front of the eye, in a trial frame, at a distance that replicates the position of a pair of spectacles to be worn by the patient. This is known as the spectacle plane, and the distance to the anterior corneal surface, typically 12–13 mm, is known as the vertex distance. When fitting a contact lens, clinicians must compensate for the vergence change that occurs from the spectacle plane to the anterior corneal plane to ensure the retinal image remains in optimal focus. While this can be calculated for each individual case, using the formula (Bennett 1974)
Management of open‐angle glaucoma by primary eye‐care practitioners: toward a personalised medicine approach
Published in Clinical and Experimental Optometry, 2021
Jack Phu, Ashish Agar, Henrietta Wang, Katherine Masselos, Michael Kalloniatis
Prostaglandin analogues, being more frequently prescribed, have primarily local side‐effects, but these may also have cosmetic implications for patients of different ages. Most notably, the sunken upper eyelid is often associated with an aged appearance, and has been the trigger for cosmetic surgical treatments.225,226 The eyelashes may also decrease in length, thickness and darkness (pigmentation) with age, which may be ‘treated’ by prostaglandin treatment;227 for some patients there may therefore be a perceived cosmetic benefit to prostaglandin use.228 However, some patients may also develop poliosis of the eyelashes and hair growth elsewhere on the face,229–231 which are conversely cosmetically undesirable.232 From a practical perspective, these cosmetic changes may present challenges to spectacle wearers (where eyelash lengthening affects the back vertex distance), and from the perspective of a clinician, the periorbitopathy may present a challenge for performing applanation tonometry.233
Aniseikonia and anisometropia: implications for suppression and amblyopia
Published in Clinical and Experimental Optometry, 2019
Jayshree South, Tina Gao, Andrew Collins, Jason Turuwhenua, Kenneth Robertson, Joanna Black
If aniseikonia is not evident without correction and is caused solely by the spectacle correction, then equal magnification lenses can be prescribed. This can be done by adjusting the spectacle magnification induced by each lens (Equation 2) so that both lenses induce a similar amount of magnification, resulting in a relative spectacle magnification ratio close to one, or zero induced aniseikonia (see Table 3 for worked example). However, in cases where some inherent anatomical, retinally induced, or neurological aniseikonia exists, it is important to correct the subjectively measured aniseikonia. That is, the correction should neutralise the combined effects of all internal and optically induced types of aniseikonia, so that the patient may perceive equal‐sized images and achieve optimal binocular vision. Spectacle magnification can be manipulated by adjusting lens base curves, centre thicknesses and/or refractive indices. The magnification induced by the lens Power Factor, which is determined by back vertex power and vertex distance,1965 usually cannot be easily altered if the refractive error is to be fully corrected. Equation 2: Spectacle magnification equation.
Scleral Cross-Linking Using Glyceraldehyde for the Prevention of Axial Elongation in the Rabbit: Blocked Axial Elongation and Altered Scleral Microstructure
Published in Current Eye Research, 2019
Xiao Lin, Rajeev K. Naidu, Jinhui Dai, Xingtao Zhou, Xiaomei Qu, Hao Zhou
The right eyes of all rabbits were randomly divided into three groups by randomized number generated by a calculator: the cross-linking group (CL, n = 5), and the untreated control group (UC, 12 Eyes in the CL and NCL group were treated with a −8.00 Diopter spherical lens monocularly over the course of two weeks (Figure 1A). Rabbits in the UC group did not receive any treatment at all. The concave lens (4 cm diameter, −8.00 Diopter, polymethyl methacrylate) was placed in front of each right eye at a vertex distance of approximately 7 mm. The cross-linking effects were achieved by a sub-Tenon’s injection of 0.15 ml 0.5 M glyceraldehyde dissolved in physiologic saline solution (DL-glyceraldehyde from Sigma-Aldrich; isotonic sodium chloride solution 0.9% from Braun Melsungen AG) to the right eyes of rabbits in the CL group at days 0, 2, 4, 6, 8, 10, and 12 (Figure 1B). This method of sub-Tenon’s injection was previously described by Wollensak, Gregor and Iomdina, Elena.8,9 Throughout the experiment, the externally applied lens systems were inspected and adjusted if necessary at four-hour intervals throughout the day to ensure the sutures in the lens system were firmly in place and the lenses were clean and free of debris. Animals without lens in front of eye appropriately during inspection were excluded from the analysis.