This is where we turn to the multiple topography maps that in an earlier post I suggested you capture, because through averaging we can obtain a more reliable measure and remove potentially poorly captured maps. Hopefully your corneal topography software will do this for you, otherwise, it is not too hard to work out by adding up the individual K values obtained from each map and then dividing by the number of maps that you have measured. This is where you can also remove from your calculation any measured K values that stand out as being different from the others, and then recalculate the average.
Computer aided lens selection
In a previous post, I described how computers can be used to model the eye with this model the used to calculate optimum lens selection of an inventory or trial lens or to design an empirical lens. The sophistication of the software will depend on the lens design in question. At its simplest, it will just replace the equivalent lookup table or slide rule for inventory designs, right up to more recent designs that calculate what are typically described as ‘free form’ designs where the lens is individually calculated to more closely match the unique corneal contours for each eye. For these designs, it would be extremely cumbersome to calculate the initial fit or subsequent changes without computers. Regardless of complexity, my suggestion is that you should adopt and use a software solution if it is available for your favored lens design.
The most accurate corneal shape information to enter into computer software is the raw corneal topography data in the form of an export file that is then loaded into the lens design software. Alternative approaches are to enter single or multiple sag heights at chords specified by the software or the best fit ellipse values calculated by the topographer software. Of these entering sag heights (weighted average height if available) will give the most accurate representation of corneal shape for the lens design being fit. It is the measurements from the flat corneal meridian that the software will require when fitting a standard (spherical) lens design, so take care to ensure it is the flat and not steep meridian that is being measured.
About Paul
Dr Paul Gifford is a co-founder of Eyefit, an information resource to assist contact lens practitioners in all modes of practice. Learn more about him here.
