It has become possible to accurately calculate lenses in RGPD on almost any cornea with minimal error by calculating eccentricity on chord equal to lens diameter by Average Saggital Height of Medmont, however, it was noted that the central clearance of some lenses, even with the maximum correspondence of the remaining zones, is insufficient.
In order to understand this situation, I decided to compare the elliptical model used in the design with the RGP Designer with a more accurate approximation of the topographer (Zernike polynomials).
20 random Medmont images were taken from different patients with healthy corneas and not using contact correction. Next, each of the following parameters were determined:
1. Visible Iris Diameter Iris.Radius*2
2. Total diameter of required lens Iris.Radius*2-13. Chord of comparison
4. Eccentricity on chord equal TD using height data sqrt(1-(2*(337.6/FlatK)*(AverageCornealHeight(Flat_Angle, rnTD/2)*-1000/1000)-(rnTD/2)^2)/((AverageCornealHeight(Flat_Angle, rnTD/2)*-1000/1000)^2))
5. Saggital height of elliptical model on chord of comparison (((337.6/Flat_K)-sqrt((337.6/Flat_K)^2-(1- rnExTDSag^2)*(((rnChord)/2)^2)))/(1- rnExTDSag^2))*1000
6. Saggital height calculated by topographer (Zernike polynomials) on chord of comparison (AverageCornealHeight(Flat_Angle, ((rnChord)/2)) * -1000)
7. Difference between 5 and 6
6. Saggital height calculated by topographer (Zernike polynomials) on chord of comparison (AverageCornealHeight(Flat_Angle, ((rnChord)/2)) * -1000)
7. Difference between 5 and 6
Then, the sagittal height of the elliptical model was compared with the sagittal height actually determined by topographer on the chords corresponding to the landing zones, and then the difference was calculated.
The studied chords (marked with red lines). Counting from the diameter of the selected lens.
It was found that on the -8.8 and -6.8 chords, the cornea according to the polynomial model is on average lower by 1 and 2 microns, respectively, which is not clinically significant. On the -5 chord (TLT end O.Z.) the difference is 5 ± 1.7 microns. And the landing zone is 9 ± 2.2, 14 ± 2.8, 11 ± 3.8, respectively. The difference between the two models will be clearly visible, if we make these averaged changes to the RGPD designer of a conventional 6 zone OK lens.
Average difference between elliptical model and polynomial in RGP designer
In fact, the cornea in the landing zone is lower than expected by the elliptical model. This explains the decrease in central clearance, as the lens falls through in the landing zone.
However, the decrease does not always occur at the maximum value of the revealed difference, since the design of the landing zone may not be completely congruent with the cornea, as for example in such cases:
The revealed differences between the mathematical models used in RGPD designer and polynomial model of Medmont, and the ability to determine them automatically by software create the prerequisites for routine supercastomization of the landing zone. Also, it determines the possible future development of the RGPD Designer, for example, the ability to import a RAW file and building a lens from sagittal height data.
P.S.
Full algorithm of fitting with saggital height coming soon.
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