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 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.
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.
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