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Refractive Errors, Myopia, and Presbyopia
Published in Ching-Yu Cheng, Tien Yin Wong, Ophthalmic Epidemiology, 2022
Ka Wai Kam, Chi Pui Pang, Jason C. S. Yam
In summary, myopia is more prevalent among both children and adults in East Asians, i.e., Japanese, Koreans, and Chinese, than in other populations, while South Asians like Indians and Malays are more myopic than Caucasians. Nevertheless, there are cautions in interpretations of reported data. First, there are age and cohort effects. Second, the prevalence of high myopia was higher in teenagers than adults mostly in cross-sectional studies conducted in different periods. High prevalence of myopia or high myopia in teenagers indicates that the myopia boom will arrive later in the working age. Third, many studies reported non-cycloplegic refraction, which may lead to over-estimations, as cycloplegic refraction should be the gold standard, especially among children. Fourth, both axial length and cornea curvature should be reported. The ratio of axial length to corneal radius of curvature (AL/CR), which is highly correlated to spherical equivalent but not affected by cycloplegia, is an important parameter, especially in studies without cycloplegic refraction.
The Anatomy of Joints Related to Function
Published in Verna Wright, Eric L. Radin, Mechanics of Human Joints, 2020
The discussion thus far has related to the relatively gross movements of synovial joints that may be observed during clinical examination, when exposed by surgery in the operating theater or by dissection in the anatomy classroom, or by modest biomechanical investigation. At this level of description, the shapes of articular surfaces are commonly likened to sections of simple geometrical shapes (plane surface, cylinder, cone, ellipsoid, and sphere; Fig. 15A). Close biomechanical examination of articular contours suggests that this is an approximation of variable crudity and that all synovial articular surfaces describe sections or combinations of sections of ovoids [as the name suggests, an ovoid looks like a hen’s egg with little and big ends; it may or may not be symmetrical about its long axis—a simple (Fig. 15B) or compounded (Fig. 15C) ovoid, respectively] (73,74). The radii of curvature may vary from very large (giving a surface that is virtually planar) to very small (giving a highly curved, almost spheroidal surface when radii in different planes are similar and relatively constant, as on the head of the femur). Large variations in radius of curvature may occur upon the same articular surface, as in the femoral condyles. It may be possible to classify all joint surfaces into one of two basic forms: (1) an ovoid joint proper in which a convexoconvex surface (Fig. 15B or C) mates with a concavoconcave surface; and (2) a sellar (saddle) joint, in which both surfaces are concavoconvex (Fig. 15D), (i.e., of concave section in one plane and convex section in a plane perpendicular to this) (73,74).
Vitreoretinal surgical anatomy
Published in A Peyman MD Gholam, A Meffert MD Stephen, D Conway MD FACS Mandi, Chiasson Trisha, Vitreoretinal Surgical Techniques, 2019
The adult globe is a slightly asymmetric sphere with an average axial length of 24 mm (Fig. 1.2).1 The vertical and horizontal diameters are slightly smaller, at 23 and 23.5 mm, respectively.2,3 The radius of curvature of the cornea is steeper (8 mm) compared with the sclera (12 mm). The average corneal diameter is 10.6 mm and the depth of the anterior chamber is 3.15 mm. The pars plana, which meets the retina at the ora serrata, has a width of 4–4.5 mm.
Non-Orthogonal Refractive Lenses for Non-Orthogonal Astigmatic Eyes
Published in Current Eye Research, 2019
Ahmed Abass, Bernardo T. Lopes, Steve Jones, Lynn White, John Clamp, Ahmed Elsheikh
The design method is based on the fact that a surface with a certain power 5 Each meridian was constructed by a series of nodes that were equally spaced along the meridian length. At these nodes, the optical powers of the Back-surface were Figure 3) and the corresponding focal lengths were Figure 4(Aa)). The design is then completed by moving the corresponding meridian nodes on the front-surface outwards until the focal lengths at all nodes were equal to the desired focal length Figure 4(Ab)). An optimisation process was then carried out to reposition the front meridian nodes in the anterior-posterior direction such that most of the refracted light rays fall on the desired focal point. In case the designer needed to create the back-surface of the lens by setting its radius of curvature Equation 2.
Comparison of Javal-Schiøtz keratometer, Orbscan IIz and Pentacam topographers in evaluating anterior corneal topography
Published in Clinical and Experimental Optometry, 2023
Jennifer M Turner, Christine Purslow, Paul J Murphy
Since the J-S measures the axial radius of curvature,17 axial curvature was used for all measurements. Axial topographic profiles for the Orbscan and Pentacam, in horizontal and vertical meridians, were sampled from the output display screen of each instrument using a transparent overlay reference grid to sample the reported radius of curvature at 0.5 mm intervals. Hovering the cursor over a specific point on the screen prompted a display of the radius of curvature of that location. Data were collected for up to 12 mm in the horizontal and 8 mm in the vertical meridians. The profile limit for each subject was determined by the available data, which was limited by the tear film reflective properties, the presence of eyelashes and by the eyelid position.
Manufacture of custom‐made spectacles using three‐dimensional printing technology
Published in Clinical and Experimental Optometry, 2020
Emre Altinkurt, Nihan Aksu Ceylan, Umut Altunoglu, Gozde Tutku Turgut
The patient underwent bilateral lens aspiration and anterior vitrectomy surgery. Intraocular lens implantation was not performed because of bilateral microcornea (horizontal diameter of the corneas was 8.5-mm). Post‐operative refraction was R: +19.00/+0.50 × 100 and L: +20.00/+1.25 × 60. Mean anterior corneal radius of curvature was R: 7.42-mm and L: 7.11-mm. The patient could not use contact lenses in the post‐operative period for optical rehabilitation because of tight lids, narrow interpalpebral aperture and microcornea. Similarly, due to microcephaly, the patient could not wear paediatric spectacles (Figure 1A); therefore, it was decided to produce custom‐made frames for spectacles. The prescription was R: +22.00 D and L: +24.00 D.