Scleral Sag Factor Video 12


This video describes the fitting of Scleral Lens using your Medmont E300 Corneal Topographer and Scleral Sag Factor. OCT Studies at Pacific University led
by Patrick Caroline. Have shown us that the Peripheral Corneal, Limbus, and Sclera begin to form a straight line or tangent shape, the Cornea maybe curved, however; at
approximately 10 Millimeters the eye begins to form this tangent shape which runs past the Cornea, through the Limbus, and onto the Sclera in the majority of eye shapes. When measuring the angles at the 15 Millimeter Chord Diameter, the
inner circle, and at the 20 Millimeter Chord Diameter, we can see that the eye is fairly symmetrical in angle around the clock at the 15 Millimeter Chord. There’s approximately 1.7 Degrees of Differential at the15 Millimeter Chord. However, at the 20 Millimeter Chord, the outer ring, the eye has far more Asymmetry, there’s 6.6 Degrees of differential around the
clock on the Sclera. And that can be seen in these bar graphs where the 10 to 15 Millimeter Chord shows a fairly tight distribution of angles around the clock. However at the 15 to 20 Millimeter Chord the eye begins to show a greater degree of Asymmetry. If we focus our Scleral Lens fitting at the 15 Millimeter Chord we can presume that the eye is fairly
symmetrical around-the-clock. Which leads us to designing lenses by
Corneal Topography. Now what is the average angle that exists in the Sclera? And on 55 Normal Eyes a study was performed at Pacific
University to measure the average angle, and that worked out to be 38.4 Degrees on 55 Normal Eyes. Compared to 55 Keratoconic Eyes. The average angle was very similar at 38.25, so the angles of the Peripheral Cornea, Limbus, and Sclera, from a 10 to 15 Millimeter Chord, are primarily the same between Normal and Keratoconic patients. If we convert this to measurement of Sagittal Depth,
what is the depth of the eye from a 10 to 15 Millimeter Chord in 55 Normal Eyes? And that was 1997 Microns or approximately 2000 Microns of depth. Compared with the 55 Diseased Eyes the depth was very similar because the angle
is very simple, 1970 Microns, very close to an average
of 2000 Microns for both Normal and Diseased Eyes. We’re were the eyes different? They were different in the Cornea, 55 Normal Eyes measured on average 1748 Microns, whereas the Keratoconic Patients showed more Sagittal Depth, 1960 Microns. If our Corneal Topographer can measure the Sagittal Depth from a 10 Millimeter Chord to the peak of the eye, then we have a very powerful tool for
predicting the initial Scleral Lens required, because we know that the average Sagittal Depth, of the eye, from
a 10 to 15 Millimeter Chord, is approximately 2000 Microns and the Medmont will be capable of measuring that Sagittal Depth. Simply go to your ‘Analysis’ and ‘Details’ window, set your ‘Chord’ at 10 Millimeters Diameter, then measure the ‘Weighted Average Height’, the second column and that will provide you with the Sagittal Depth of Eye. Then you take your Corneal Sagittal Depth, 1906 Microns, add it to your Scleral Sag Factor, your
average of 2000 Microns of depth from 10 to 15 Millimeters, plus our Apical Clearance, and that gives you the required Sagittal Depth of Lens, if you’re Scleral Lens lands very close to a 15 Millimeter Chord. Like this one does. And this is the method that we can use to more accurately
predict the initial Scleral Lens required for your patients. When doing this from the Medmont Studio software we could put onto a theoretical Corneal GP on this patient and determine if this is a good candidate for a Corneal GP or if a Scleral Lens is required. If we put the lens on the Geometric
Center, and look at the fluid underneath the lens on the graph down below. There’s landing at six o’clock and at twelve o’clock, however; across the Vertical Meridian there’s a significant amount of fluid. If we click our cursor on this point there is 230 Microns of fluid on the Nasal side on the Temple side 257 Microns, and that is observable on the graph down below were the amount of fluid underneath the contact lens across this Horizontal Meridian is excessive, and this would cause lateral displacement of the lens, bubbles form above 90 Microns, and we have over 200 Microns on both Nasal and Temple sides. So this lens clearly would struggle to be centered and would the patient would struggle to find comfort. So therefore, lets fit this patient with a Scleral Lens. We select our map. go to, ‘Analysis’, and ‘Details’, set a Chord Diameter at10 Millimeters,
to measure 10 Millimeters across the eye, we click on the, ‘Flat’ button, and observe the, ‘Weighted Average Height’, the Sagittal Depth of the eye, and across the Flat Meridian, the Vertical
Meridian, the eye measures 1716 Microns. We might click on the ‘Steep’ Meridian to
determine the highest Axis of Depth on this eye, and this works out to be 1800 Microns across the Horizontal or Steep Meridian of this eye. Generally it’s better to take the higher
of the two values because you would prefer to have your Scleral Lens go on eye with more depth than required rather than less depth or touch. If the lens has touch than it needs to be removed with a higher Sagittal Depth. with lens placed on eye. So it’s always better to put on a lens with more depth or to error on the higher side of Sagittal Depth. Therefore, we would take this Corneal Sagittal Depth go back to are formula, take our Corneal Sag, add it to the
average 2000 Microns for Scleral Sag Depth, plus your Apical Clearance, and that gives
you the calculation of lens that you require. And again this calculation is based on a 15 Millimeter Chord Diameter because the studies at Pacific University were
performed at 15 Millimeters. If your Scleral Lens lands very near to a 15 Millimeter diameter this formula will be a highly accurate way
of determining your initial lenses. However, if your Scleral Lens does not land
and a 15 Millimeter Chord, then use the Corneal Sagittal Depth from your Medmont Topographer knowing
that 1750 Microns is the normal death of Cornea. if you have a measurement of 2000 Microns then you know you should go up higher in Sagittal Depth in your trial lens set

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