Topographical Data

The BE Retainer is fit from topographical data of corneal shape and sagittal height values. These required measurements for BE Retainer calculation and fitting exclude the keratometer as a useful tool.  Secondly, a topographer is necessary to evaluate the corneal shape changes and BE Retainer Optimal Orthokeratology fit following over-night wear.  The keratometer will simply not provide adequate information for Optimal Orthokeratology follow-up and evaluation of fit and corneal changes.

The recommended topographer for the BE Retainer design is the Medmont E300.

 It is also imperative that the topographer is as accurate as possible.  Accurate data entered into the BE Retainer software program results in successful first fit results.  Inaccurate data entered in the BE Retainer software program results in numerous re-trials and therefore, loss of time, effort and money.  In regards to the topographical data entered into the BE Retainer software program, “Garbage in, then, Garbage out”, in terms of results.

The topographer you use to fit and manage your patient needs to provide you with the following data or information:

  1. Apical Radius Calculation

  2. Sagittal Height or Eccentricity (E-Value) over a specific chord diameter, = 9.35mm cord diameter (diameter of data collection across the cornea)

  3. Axial/Tangential Maps

  4. Subtractive/Difference Maps

Apical Radius

The calculation of the radius of curvature at the apex of the cornea: As all topographers must estimate this figure, the more accurate the algorithm, the more accurate the data for fitting BE Retainers.  An error of 0.01mm in apical radius (Ro) = 2um in sagittal height error is acceptable.

Sagittal Height

The height (in microns) of the cornea over a specified cord diameter:

The BE Retainer is fit to a desired apical clearance in order to achieve the correct squeeze film pressures.  For successful first fit results, the topographer must measure the height of the cornea in microns (0.001mm) over a cord length/diameter of no less than 9.35mm.  The standard BE Retainer (11.0mm) comes in contact with the cornea at a diameter of 9.35mm.  If the measured height is accurate over this cord length, the correct apical clearance will be achieved, resulting in a successful fitting BE Retainer.

Eccentricity (E-Value)

The “rate of flattening” of the cornea from the apex to the periphery:

E-value is used as an alternative if your topographer does not calculate sagittal height.  E-value also gives you a quick reference of a patient’s potential for OK therapy.  Ie.  the lower the e-value, the lower the potential for refractive change and visa versa.  However, sagittal height is preferred to e-value as the calculation of eccentricity assumes a constant rate of flattening.  We know that the cornea does not flatten at a constant rate, therefore, using E-value for the calculation of BE Retainers introduces error which results in inaccurate apical clearance and inaccurate fitting/acuity.  Remember, “garbage in, garbage out”. See image below.

Axial, Tangential and Subtractive/Difference Maps

Standard topography maps for evaluating the cornea and shape changes:

Axial power or curvature maps provide orthokeratologists with most of the significant information on corneal shape and is most commonly referred to for fitting and follow-up.  Tangential power or curvature maps show small changes of corneal curvature at localized points or areas in greater detail.  Subtractive/Difference maps determine the effects of the BE Retainer Optimal Orthokeratology retainer wear by comparing pre-therapy and post therapy plots and determining treatment zone size and position.