[Calculating the diameter of the anterior chamber before implanting an artificial lens].

Abstract

Anterior chamber lenses sized correctly and positioned properly yield excellent results. Problems reported with anterior chamber implants are related to improper length/or lens placement to the scleral spur. In clinical routine examinations the "white to white data plus one mm" determine the overall length of an anterior chamber lens. However, variability of limbal anatomy causes great variations in estimations of corneal diameter. Approximation of corneal profile can be achieved by using ellipsoid or paraboloid functions. These data together with ultrasound measurements and keratometer readings serve as prerequisites for computations. For biometry an Ocuscan DBR 400-ST unit is used; the corneal refraction (r0) is measured with an automatic keratometer (Humphrey). The peripheral measurements are performed 13.5 degrees nasally and temporally (r1, r2). Using Euler's formula the horizontal radius is calculated (R0, R1, R2). Taking the constant epsilon 2 (epsilon 2 = r2-r0(2)/r2sin2 phi) the diameter can be calculated with the formula h = square root of (2ap-p2)(1-epsilon 2). Model A: two asymmetric halves are computed, the addition gives the corneal diameter (H = h1 + h2). Model B and C: only one symmetric half is computed, the double distance gives the corneal diameter (H = 2 h). The distance p is taken from biometry data; the distance from the anterior corneal surface to the posterior lens surface (ACD + LE) should be multiplied with the factor 0.32: p = (ACD + LE) 0.32. 250 eyes were examined comparing the optical data with the calculated results of our corneal model; the mean value for p was 2.4811 millimeter. Based on a control system the computer eliminated 35 eyes (14%). In the rest group of 215 eyes (100%) the difference to the optical measurements was not greater than +/- 0.25 mm in 181 calculations (84.2%). We noticed a tendency to predict too short internal diameters with external methods. Using optical measurements 1.25 millimeters should be added assuring correct position of the haptics. Using our computer-program for calculations of corneal diameter new anatomic conditions in eyes with abnormal dimensions may be detected.