AT Corneal Topography I 2023-24 Slides PDF

Advanced Techniques 6LMS0096 2023-24 Semester A Dr Sheila Rae 1  Optometry is becoming increasingly complex in terms of the spectacle, contact lens and surgical options available to manage refractive error and ocular pathology  Increasing complexity of treatments means...

Advanced Techniques 6LMS0096 2023-24 Semester A Dr Sheila Rae 1  Optometry is becoming increasingly complex in terms of the spectacle, contact lens and surgical options available to manage refractive error and ocular pathology  Increasing complexity of treatments means that much more detailed quantification of ocular parameters is required  Instrumentation advances mean easy to use imaging techniques are available  The skill of the optometrist is shifting towards data interpretation and decision making 2  Detailed knowledge of corneal shape in the normal and abnormal eye  Theory of corneal topography ▪ Different instrument types  Measurement  Interpretation of the results  Corneal topography in refractive error management  Corneal topography in anterior segment disease 3  We can’t work out the radii of curvature of the anterior and posterior surfaces, the refractive index and the thickness from only the power  We can work out the power of a surface or lens if we know ▪ The radii of curvature of the anterior and posterior surfaces ▪ The refractive index ▪ The thickness 4  kerato : to do with the cornea  metry / meter: to do with measurement  scope: to view or look  topography : to do with height or shape  Keratometer / keratometry : measurement of the cornea  Ophthalmometer: measurement of ocular parameters ▪ Old style name for keratometer  Video keratographer / keratoscope: used in place of corneal topographer in some countries  Corneal topographer / topography : measurement of height and shape of the cornea 5  The cornea needs to be a smooth regular surface for refraction ▪ Loss of this smooth surface, such as in corneal disease results in blurred vision  The emergence of rigid corneal contact lenses as a viable means of vision correction in the mid 20thC meant there was a role for the keratometer in optical practices ▪ Not unusual as late as 1980s to fit rigid contact lenses without one  Early Javal Schiotz style keratometers from 1881 6  The cornea needs to be a smooth regular surface for refraction  The cornea contributes significantly to the eye’s refractive power ▪ Purposefully changing corneal curvature and shape can alter the total refractive power of the eye ▪ ? Which other structures could be changed to alter refractive power ▪ ? Why chose to change corneal shape and not other ocular structures to modify Rx 7  Cornea provides around 2/3rds of the eye’s refractive power in the unaccommodated eye ▪ Tear film actually is the first refracting surface of the eye  Steeply curved surface required for this ▪ Much higher radius of curvature than spectacle lenses ▪ +2D plano convex spec lens has a radius of curvature of around 250mm ▪ +45D cornea has a radius of curvature of around 7.5mm 8  Emmetropisation is the process of co-ordinated eye growth to aim for no refractive error  Optical components involved are ▪ Corneal curvature ▪ Anterior chamber depth ▪ Crystalline lens curvature ▪ Crystalline lens thickness ▪ Axial length (posterior chamber length) 9 As well as being toroidal, the cornea also suffers from inherent spherical aberration (SA)  An optical aberration common Point focus to all steeply curved surfaces Point spread function  Can be +ve SA or –ve SA, depending on whether peripheral rays fall in front or behind the retina ▪ Centre of an image is clear, edges are fuzzy 10  Spherical aberration (SA) is a ‘4th order’ higher order aberration in the Zernike classification system Spherical aberration ▪ Rotationally symmetric aberration 0 ▪ Classified as Z4 or term 12  The high curvature of the cornea results in +ve SA Point spread function  The anterior corneal surface is aspheric to offset some of the SA 11 Corneal +ve SA is offset by –ve crystalline lens SA  The lens has less –ve SA with aging  Total ocular SA therefore becomes more +ve with aging, but Effect of aberrations on retinal image quality is also dependent on pupil size  Poorer retinal image with larger pupil  As pupil gets smaller with age, this counteracts the increase in SA 12 Cornea is typically a prolate ellipse  Radius of curvature flattens towards the periphery  Amount of flattening varies from person to person  Need to describe the apical radius (r0) and the rate of flattening 13  Corneal asphericity can be described using Q values or eccentricity values ▪ Q is the coefficient of asphericity  Q is zero for a sphere  Q between -1 and zero for prolate ellipses  Most corneas are spheres or prolate ellipses ▪ If Q is closer to zero, closer to a sphere 14  Corneal asphericity can be described using ▪ Q values ▪ Shape factor (SF) ▪ Eccentricity values (e)  Shape factor (SF) = - Q  Eccentricity e = - √Q or Q = - e2 ▪ So where Q is –ve, SF or e will be +ve 15  For Q values ▪ Prolate -1 < Q < 0 ▪ Sphere Q=0 ▪ Oblate 0 < Q < 1 16  Topographers often give an averaged Q, e or SF across the cornea ▪ Corneas however are often not symmetrical ▪ The rate of flattening is often much greater near the periphery; the central cornea is often near spherical  Q, e or SF may be specified across a certain cord diameter, e.g. 5mm, 8mm or 10mm  Which diameter is most relevant depends on the type of refractive surgery or CL to be fitted 17  Q, e or SF may be specified across a certain cord diameter, e.g. 5mm, 8mm or 10mm  If LASIK is using an ablation zone of 5mm, then Q across that diameter would be relevant  If designing a CL with a 10mm diameter, then Q across that diameter would be relevant 18 19  No significant correlation between central corneal  Central corneal curvature curvature and eccentricity  Consistent or variable  Many variables that are rate of flattening related  Corneal symmetry  ? All these variables affect  Corneal diameter corneal shape, so why  Scleral radius select soft contact lenses  Anterior chamber depth based only on central corneal curvature 20  The flattening periphery of the prolate ellipse reduces +ve spherical aberration  Also limits the change in curvature needed at the corneal- scleral junction ▪ Central cornea r = 7.85mm ▪ Peripheral cornea r = 9.00 – 9.50mm ▪ Sclera r = 13.00 - 15.00mm  Corneal-scleral topography is of increasing interest in CL fitting ▪ Understanding soft lens fit and comfort ▪ Especially hybrid, semi-scleral rigid, soft multifocals, scleral lenses 21   Corneal-scleral junction nearly smooth in some Px  In others, there is more of an angle  Angle often varies nasal vs. temporal  A smooth transition would be 180°, a less smooth one