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2. Linking EyeSpace to your topographer

2. How to use OCT scans to evaluate scleral lens fits

11. Scleral TroubleshootingCharl Laas

OCT scans are very helpful to evaluate the overall fitting relationship of the EyeSpace Scleral lens to the eye. To maximise the use of the instrument, follow these useful tips when performing OCT scans for scleral lens fitting:

  1. Use the OCT 'Corneal Angle' tool to evaluate the fit.
  2. The aim is to capture the lens profile on the eye. Since most current OCT's cannot measure the full diameter of the lens in one scan, it becomes necessary to measure multiple zones of the lens to get a full overview of the fit.

  1. To assess the fit always measure along the flat and steep meridian of the lens and not by default the horizontal and vertical meridian of the eye. Scleral shapes, like corneal shapes, can exhibit with-the-rule, against-the-rule or oblique toricity. As a result, the flat meridian of the lens, marked by the line engravings on the lens, can settle in the horizontal, vertical or oblique meridians of the eye.
  2. When performing the scan, the patient must look in the primary position of gaze where possible. When the eye turns the scleral lens can shift position which alters the lens-to-eye fitting relationship.
  3. Move the eyelids out of the way when performing vertical scans.
  4. For trial fitting allow the scleral lens to settle for at least 15 minutes before performing the scan. Aim for a central Post Lens Tear Thickness (PLTT) of 200 microns.
  5. For follow up scans the lens must be in the eye for at least 3 hours. Aim for a central PLTT of 150 microns.

Central Scans

The patient must look straight ahead.

Locate the two engraving lines on the EyeSpace Scleral Toric lens which marks the flat meridian and scan along this axis. Scanning along this axis will provide the PLTT profile of the central flat meridian.

Next, turn the OCT Corneal Angle tool 90 degrees away from the flat meridian of the lens. This scan provides the PLTT profile of the central steep meridian.

The image below shows an example of a central OCT scan taken over the flat, and in this case horizontal, meridian. With this OCT it is possible to measure the central PLTT and also judge the BOZR alignment with the central horizontal curvature of the cornea.

The iVue OCT has an extra map called the Vault map. This map does a clearance assessment and provides a PLTT thickness map over the central 6.00 mm of the eye.

Mid-Peripheral Scans

The patient must look straight ahead.

Scan the mid-peripheral zone along the flat and steep meridians of the lens. Use these scans to measure the PLTT at the junction where the back optic zone radius (BOZR) meets the Z-Zone. These PLTT values are used to determine if the BOZR should be steepened or flattened relative to the corneal shape.

In the OCT image below, taken over the mid-periphery of the lens, the central PLTT is 216 microns whereas the outer edge of the BOZR is 223 microns showing good alignment with the central corneal shape.

At the BOZR and Z-Zone junction, the PLTT increases to 318 microns which is not a sign the BOZR is too flat but rather a result of the graft shape suddenly steepening where the donor button attaches to the host tissue. This shape change can be isolated to this geographical area only. It is, therefore, necessary to evaluate all four quadrants of the mid-peripheral zone to get a full overview of the lens-to-cornea fit relationship.

You can also use the mid-peripheral OCT scans to determine if the back optic zone diameter (BOZD) is correct. For instance, in central keratoconus, the BOZD can be reduced to improve the lens-to-cornea fitting relationship in the mid-periphery.

In corneal grafts, the BOZD can be increased or decreased to match up with the donor button size. In the example below of a corneal graft, providing all four quadrants of the mid-peripheral zone look the same, the BOZD can be decreased to improve the lens-to-cornea fitting relationship.

SLZ Scans

The patient must look straight ahead where possible.

Scan the scleral landing zone (SLZ) in all four quadrants along the flat and steep meridians of the lens.

Use these scans to confirm the EyeSpace Scleral lens is not bearing on the limbal zone of the eye and to evaluate the angle of the landing zone relative to the angle of the sclera.

The OCT image below shows the nasal SLZ scan of the right eye. In the image, it is easy to observe the limbal zone and confirm the lens is not bearing on this area.

Next, evaluate the SLZ and judge the angle of the SLZ relative to the limbal-sclera angle. If the SLZ is 'heel' bearing the angle should be steepened, If the SLZ is 'toe' bearing the SLZ angle must be flattened.