The mantra in real estate is “location, location, location.” In orthokeratology (ortho-k), an ideal result depends on centration, centration, centration. Certainly, the primary outcome goals of ortho-k are to provide excellent vision and to maintain excellent corneal health, both of which require ideal fitting characteristics.
Determination of Lens Specifications
Lens Diameter In addition to an updated refraction, internal/external eye health evaluation, and corneal topography, the most significant measurement to create ideal centration with ortho-k is horizontal visible iris diameter (HVID). Accurate HVID can be measured in a few different ways: 1) manually estimate with hand-held rulers (Figure 1); 2) using the caliper tool on your topographer (Figure 2); and 3) using a contact lens of known diameter (Figure 3).
Once you determine the HVID, the ideal lens diameter to create optimal centration is 92% to 95% of the HVID. For example, a patient who has an HVID of 11.8mm would likely have excellent centration with a lens diameter of 10.8mm to 11.0mm (11.8 x 0.92 = 10.8).
Other Specifications Once the diameter is determined, another parameter affecting centration is the alignment curve(s). This area balances the weight of the lens peripherally to help keep it properly positioned. Standard ortho-k designs with symmetric alignment curves can often produce excellent results.
With the elevation topography map, you can assess the location and magnitude of astigmatic height differences. If corneal astigmatism is confined to the central cornea and the vertical/horizontal elevation difference is less than 25 microns at an 8.0mm diameter, a lens with a spherical alignment zone will likely show uniform 360° bearing and a well-centered lens. But, if the astigmatism continues further toward the limbus and shows higher vertical/horizontal elevation differences, toric or asymmetric alignment curves may be required to achieve ideal centration.
What’s Your Elevation?
Because successful ortho-k depends on creating an optimal balance of fluid forces under the lens, overall sagittal depth of the lens is important. Remember to look at the elevation topography map to understand what you are trying to accomplish. Successful ortho-k requires a lens with the proper sagittal depth to create the ideal fitting relationship over patients’ unique corneal elevations.
Whether you prescribe ortho-k based on diagnostic lens evaluation or topography-based software, you need to understand the lens diameter, balanced fluid forces, and elevation for an ideal result. CLS