The elliptical contact lens for eccentricity values higher than 0.45
In case of a standard elliptical contact lenses that are dimensional stable, there is a limitation to the available eccentricity and makes these lenses partially usable and in many cases gives moderate results. The choice between one or two eccentricities is certainly not sufficient to obtain a correct fit. The problems, gradual closure with too little eccentricity of the contact lens and poor centering and excessive deposits with too great eccentricity, are of course known from practice. Changing the basic curve radius is not a correct solution and causes other problems. The concept, a comfortable, dimensional stable contact lens, requires that the value of the lens eccentricity is in the right proportion to the corneal eccentricity. Especially with a spherical or almost spherical cornea, an elliptical contact lens is not always used without risk; the tear flow is then too low to ensure metabolism of the cornea in the longer term.
In case of monofocal lenses and a correction between +3.50 and -3.50.
Cornea astigmatism between 0.50 and 1.50
E-values higher than 0.45
B.C.R. 6.00 – 10.00 per 0.05 ascending
Power +/-40.00 per 0.25 ascending
Thickness 0.20 bij -3.00
Lens diameter 8.10 – 12.50 per 0.10 ascending
Eccentricity Ascending as stated
In case of low corneal astigmatism, approximately 1.00 dpt., the B.C.R. is taken 0.10 mm flatter than the average K-values to achieve sufficient edge lift. The difference between the lens and cornea eccentricity can therefore be better 0.20mm.
With a higher corneal astigmatism up to 2.25 dpt., a surcharge of 0.05 mm is sufficient for the B.C.R. and 0.15 greater eccentricity. The use of an elliptical contact lens with a corneal astigmatism of 2.50 dpt. and higher is neither useful nor comfortable; a peripheral toric contact lens is then the better choice.
Choice of diameter
1.5 mm smaller than the vertical cornea diameter.
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