ABSTRACT
The refraction factor was evaluated by using the theoretical formula for intraocular lens power calculation. This evaluation was made for several combinations of lens position, postoperative refraction, corneal power, axial length and lens design. The calculated values range from 1.19 to 1.51. Refraction factor was found to increase with posterior position of the lens, corneal power and algebraic increase of postoperative refraction. Position of the lens is the most important factor. The following values can be used practically: 1.25 for anterior chamber lenses and meniscal lenses; 1.33 for posterior chamber plano-convex lenses; 1.50 for posterior chamber biconvex lenses.
Subject(s)
Lenses, Intraocular , Refraction, Ocular , Humans , MathematicsABSTRACT
After having recalled the principles of lens power calculation, we analyse in detail the parameters of the (Formula: see text). Corneal power K read on keratometers has to be modified, for it is about one diopter too strong. Evaluation of the anterior chamber depth C is analysed and corrected of 0.08 mm for the 1st C and of 0.06 mm for the 2nd C. Ultrasonic axial length L should be increased by the retinal thickness; the error induced by neglecting this correction is concealed by the overestimation of K in usual formulas. The equivalent power of the lens De is slightly underestimated relative to back surface power supplied by the manufacturer. The various formulas available in the Literature are commented and compared. A new formula is proposed, which is more particularly adapted to contact ultrasound probes, such as Digiecho II, which seem to under-evaluate L and C values.
