IOL Power Calculation

OKULIX is particularly suited for intraocular lens (IOL) calculation ⁴, ⁵. Axial lengths can be entered either manually or by a computer link to the measuring device. Interfaces exist to the AL100, AL2000, AL3000 and UD6000 from Tomey. Other links are in preparation. It should be kept in mind that OKULIX uses optical lengths, not acoustical ones. Measuring devices may produce values with a systematic bias. Therefore, the values from different devices have to be transformed. The transformation is based on comparing measurements with artificial eyes or on the outcome of patient’s prediction errors. Also optical devices such as the IOLMaster (Zeiss) are included.

IOL Data

The program package includes a compilation of the mostly implanted IOL of the marked leaders. This data base is permanently actualized. In this data base, an IOL is represented formally by it’s type and power (in dpt). For the calculation inside OKULIX, the IOL is physically characterized by the curvature radii, the refractive index and the central thickness. This is necessary in order to utilize the higher calculation accuracy (compared to all "formulae"), because the visual impression with a specific IOL is not unambiguously characterized only by the refractive power, which, in addition, is only well defined in the terminology of Gaussian optics.

Anterior Chamber Depth

Postoperative anterior chamber depth (ACD) principally cannot be exactly calculated from the data of preoperative measurements, because it depends on the individual shrinkage of the capsular bag. In order to make this point more transparent to the user, the input for ACD inside OKULIX is separated from the other program branches. In most of the other IOL calculation formulae, this problem is hidden to the user, so that he has no information about the error contribution of ACD error to the overall error. Particularly, we would like to point to the difference between exact calculation on the one hand and estimation on the other hand. Moreover, it should be distinguished between mean ACD for a particular lens type and most probable ACD for an individual eye. Mean ACD has been calculated numerically from the A-constant of SRK formulae ¹¹ (see tab.11). This approach was chosen because for the A-constants a large data basis exists. For some of the IOL postoperative ACD has been measured with laser interferometry. These values fit highly accurately with the values calculated from the A-constants. For the estimation of the most probable individual ACD an algorithm is used which contains only the axial length and the manufacturer’s IOL parameters together with the mentioned mean ACD values of the IOL-type. In a study at the University Eye Hospital Mainz this procedure was more accurate than other methods known from the literature ⁸. The calculated ACD is a proposal to the user which can be accepted or modified. Particularly for very short eyes and / or very high IOL powers, results depend strongly on the ACD. The user, however, can simulate the amount of a variation in the single case. Based on the calculation of the mean ACD from the A-constants, the results in terms of emmetropic IOL power for a mean eye are identical for OKULIX and for the SRK formulae.

Spherical Aberration

Spherical aberration of the human eye mostly causes a myopisation, increasing with pupil width. Among others, it depends on the asphericity of the cornea, the asphericity of the IOL and the ratio of the IOL curvature radii called "shape factor". All these influences are exactly taken into account in OKULIX. They can be best graphically visualized with the simulated Landolt’s rings. In order to obtain at least an estimate of the amount of the effect, the refraction deficit of an IOL is given not only paraxially, but also for the "best focus" for a pupil width of 2.5mm.

Phakic Intraokular Lenses

Also phakic IOL can be calculated with OKULIX. To do so, preoperative refraction must be known in addition to phakic anterior chamber depth and axial length. First, the vertex radii of the crystalline lens are calculated assuming an refractive index of 1.42. This value is in the upper physiological range. It has been chosen for safety reasons in order to overestimate the vertex radii rather than to underestimate them. The available space for the implantation therefore is underestimated too. These assumptions influence the resulting IOL power only marginally. The thickness of the crystalline lens has to be measured. In case of missing data 3mm is assumed. However, thickness variations in the physiological range are negligible with respect to the resulting IOL power. The IOL proposed by OKULIX is displayed in a scaled drawing of the anterior eye segment. The user can modify the position (ACD) of the IOL thus causing a variation of the best fitting power. If anterior chamber IOL are to be calculated for aphakic eyes, the corresponding button has to be activated. In addition, anterior and posterior surfaces of the IOL may be exchangofed by one another. I this case the button IOL reverse has to be activated.

Toric IOL

So-called "toric IOL" for the correction of the corneal astigmatism can be selected in OKULIX in the same way as rotationally symmetric IOL. The best and safest approach is using the topography of the eye. The mark on the IOL defining the meridian of lowest refractive power has to be aligned with the corneal meridian of highest refractive power shown in red in the OKULIX topography. In case of phakic, toric IOL also subjective or objective refraction can be used if the astigmatism of the crystalline lens is to be corrected in addition. However, the astigmatism of the crystalline lens is not constant in time. Therefore, this approach should be applied with care. Possible implantation errors with toric IOL can be quantified in the retinal image module, button image param. when the cylinder power of the correction glass is not zero. Output of this calculation is the angle by which the IOL should be rotated, and, assuming the IOL being rotated by this angle, the astigmatic power difference in the IOL plane which corresponds to the vector sum of the corneal cylinder and the power of the correction glass in the corneal plane. The spherical power of the correction glass is not taken into account, but, of course, the results depend on the angle of the cylinder axis. This function can be easily tested when the proposed angle of flattest IOL meridian (= steepest corneal meridian) in the input field of the IOL power selection module is not accepted, but another (wrong) value is entered instead. After selection of the button best correction glass the angular difference between the ideal value and the input value is calculated, i.e. the angle, by which the IOL should be rotated. Rounding errors may cause little differences, because the power values of the correction glasses calculated with the option best correction glass are rounded to integer multiples of 0.25D.

Table 11: IOL Types

• ØOpt: Optical cross diameter [mm].
• sph: spherical power range in diopters
• ast: astigmatic power range (start, steps, max.)
• n: index of refraction
• ACD_O: mean ACD value as used in OKULIX [mm].