Decentration of intraocular lenses (IOLs) is a major complication of pseudophakia and the most common cause of explantatión in the U.S.1 With the increasing usage of multifocal lenses and IOLs, the risk of decentration should be examined since it can cause severe visual impairment. Various studies on IOL decentration that use different methods of measurement have been published.2"6 In pathologic studies of eyes obtained after death, decentration was calculated as the distance between the center of the IOL and that of the ciliary ring.2,3 In clinical studies, decentration was calculated with the aid of Scheimpflug images4 or Purkinje images.5 Other authors obtained their results by either looking at anterior segment photographs6 or by simple observation of decentration. Most of these studies analyzed Pseudophakie eyes with posterior chamber IOLs. The differences found were related to the type of capsulotomy (ie, capsulorhexis), loop fixation (ie, in the bag, sulcus, asymmetrical), and IOL characteristics (ie, design, size, material of the haptics).2"8
To our knowledge, there is no published review of decentration found with Worst-Fechner iris claw IOLs9 in myopic phakic eyes, and the IMAGEnet analogic digital system for measuring the decentration of IOLs has not been used before.
We have developed a method, the PérezTbrregrosa Harto (PTH) system, to quantify IOL decentration of Worst-Fechner iris claw IOLs implanted in myopic phakic eyes, using the IMAGEnet digital system.
MATERIALS AND METHODS
We studied IOL decentration in 22 phakic eyes of 14 high myopic patients after iris claw lens implantation. The optical zone had a 5 -millimeter diameter with a 3.5-millimeter total length of the haptics. All surgery was performed by the same surgeon (J.L.M.).
Patients at least 18 years of age were selected according to the following criteria: Preoperative myopia of at least -9.00 diopters (D) and stable refraction with change of less than - 0.50 D for at least 18 months before surgery. Most patients conveyed that they wanted to undergo surgery because of contact-lens intolerance and to improve visual acuity for occupational needs. A minority of patients felt handicapped by their visual corrective devices. All patients signed an informed consent before undergoing surgery.
The operation was performed in patients who presented with a preoperative mean spherical equivalent of -14.70 ± 3.79 D (range, -9.00 to -23.50 D). The mean age at the time of surgery was 34.09 ± 9.55 years (range, 23 to 48 years).
A complete ophthalmic examination was performed including: slit-lamp microscopy, Goldmann applanation tonometry, retinal examination, and refraction with and without cycloplegia (with streak retinoscopy and autorefractometry - Canon RK-I, Canon, Tnkyo, Japan). The spectacle-corrected visual acuity was measured, both preoperatively and postoperatively, under standard conditions using the Snellen chart, visual acuity was computed in decimal notation. Biometrical measures were performed by ?-scan ultrasonic ocular echography (Nidek echo-scan U8-1000 and DGH ultrasonic), and lens calculation was obtained following the formula and tables of Van der Heijde et al.10 Noncontact specular microscopy (Topcon SLr7E,7F) with video image system (IMAGEnet Topcon Córp, 1988) was used to examine the corneal endothelium on all patients.
The surgical technique, and the evaluations before and after surgery were conducted as described in previous reports.11·12 Before surgery, pilocarpine eye drops 2% were instilled twice, 30 minutes before surgery, to achieve stable intraoperative myosis; this enabled us to use the pupil as a reference for centration of the IOL. Patients were examined on the first postoperative day, and at 1 and 3 weeks after surgery. Examination schedules were then continued at 1, 3, 6, and 12 months, and then yearly thereafter. At each postoperative visit, all eyes underwent visual acuity measurement, refraction, slit-lamp microscopy, and applanation tonometry.
All of the eyes with the IOLs were photographed using a retinal camera (lbpcon, TRC-50X), long after the postoperative medical regime was completed; therefore, the pupils were not dilated (mean follow up, 4.68 ± 2.5 months after surgery; range, 2 to 11 months). The images obtained were incorporated at the IMAGEnet digital system with imageprocessing software (IMAGEnet Instruction Manual, Version 3.30, Topcon Corporation, 1990). We asked the patients to look straight ahead, and measured eye fixation by centering the circular light reflexion rings on the positioned cornea.
IMAGEnet enabled the examiner to make different circles and measure any distance within the image on the screen. The unit of measurement provided by the IMAGEnet system is the pixel, an unspecified unit which has no correlation with the amplified eye image on screen and, therefore, needs to be explained. Once we took measurements in pixels, we converted them into millimeters, using the standard size of a Worst-Fechner iris claw IOL as a reference (8.5 mm of total diameter and 5 mm of optic diameter). In each case, we measured the total diameter of the IOL in pixels (8.5 mm), and used this known distance to calculate any other distance with a simple equation: D = (8.5 X L) + W
Where: D = distance in millimeters of a given measurement; L = distance in pixels of a given measurement; W = diameter in pixels of the WorstFechner iris claw IOL.
Decentration with respect to the cornea center was considered to be the distance between the center of the IOL and that of the cornea on screen. Decentration with respect to the pupil center was considered to be the distance between the center of the IOL and that of the pupil on screen. With IMAGEnet, we traced the circles of the esclero-corneal limbus, the IQL, and the pupil on the screen. Then, the centers of the cornea, the pupil, and the IOL were marked, and the distance between the IOL center and either the pupil center or the cornea geometric center were measured in pixels. The measurement of the decentration was calculated in millimeters using the aforementioned equation after measuring the diameter of the IOL. In every Case, we used the IMAGEnet digital system to: mark the centers of the cornea, IOL optic, and the pupil; and measure in pixels the distance of decentration and the diameter of the lens (8.5 mm). Then, we computed the decentration in millimeters using a calculator (Figs 1 to 4).
Data was stored in the data base Lotus 1-2-3 version 2.2, and the statistical data process was performed using SPSS version 4.0 statistical software. Bidirectional analysis of the variables was performed with lineal correlation of Pearson, ? < .01 indicated statistical significance.
Figure 1 : The pupil center is marked so that the circle drawn around it lies just over pupil margin (white circle).
Figure 2: The IOL center is found, which is the center of a circle (arrow) that runs over the edge of the IOL.
Figure 3: IMAGEnet digital system provides the distance between the two centers (arrows) in pixels. All of the measurements appear on the right top corner.
Figure 4: The diameter of the IOL is measured in pixels (8.5 mm) between arrows.
The Table shows the decentration in millimeters with respect to both the corneal center and the pupil center for each of the 22 phakic eyes. Results were ordered beginning with the largest IOL decentration with respect to the pupil center.
The mean IOL decentration with respect to the corneal center was 0.51 mm (range, 0 to 0.8374 mm) with a standard deviation of 0.25 mm. We found that in 4 eyes, decentration was less than 0.3 mm; in 6 eyes, more than 0.3 mm and less than 0.5 mm; in 9 eyes, more than 0.5 mm and less than 0.8; and in 3 eyes, more than 0.8 mm.
The mean decentration with respect to the pupil center was 0.47 mm (range, 0.182 to 0.9341 mm) with a standard deviation of 0.29 mm. We observed that in 7 eyes, decentration was less than 0.3 mm; in 6 eyes, more than 0.3 mm and less than 0.5 mm; in 6 eyes, more than 0.5 mm and less than 0.8; and in 3 eyes, more than 0.8 mm.
We found a statistically significant (Pearson's test, p < .01) positive correlation between the IOL decentration with respect to the corneal and pupil centers. Those IOLs having smaller decentration with respect to the pupil center also showed smaller decentration with respect to the cornea center.
Perfect centration of IOLs gives better optical results, most likely avoids complications, and reflects the ability and surgical technique of the surgeon.13 There have been many reports published about IOL decentration in the posterior chamber,28 but, to our knowledge, there are none about iris claw lens in phakic myopic eyes. This may be due to the low number of implantations that use the iris claw lens.
This article is the first to study centration with the Worst-Fechner iris claw IOL. Image analysis was performed with the help of the IMAGEnet digital system. So far, the best method to assess the IOL decentration has not yet been established. Other authors have based their decentration reports on postmortem pathologic findings,2,3 complex clinical studies (Scheimpflug4 and Purkinje images5,14,15), anterior segment photographs,6 and simple observation of decentration.
Optical decentration and fixation points at the iris are more easily identified with the iris claw lens than with posterior chamber IOLs, since access to the whole IOL is complete with the iris claw lens.
We observed most Worst-Fechner iris claw IOLs to have strong fixation to the iris and overall excellent support. However, difficulties in the surgical technique arise during the enclavating procedure of the haptics in the iris. Consequently, it is difficult to obtain a perfect IOL centration.
The IMAGEnet digital system allowed us to study IOL decentration by providing anterior segment images. To enhance result reliability, we took several measurements in each photograph. Since IMAGEnet provides measurements in pixels, we converted them into millimeters (Worst-Fechner iris claw IOL diameter = 8.5 mm). The same lens diameter measured various lengths in pixels because the size of the image from the photograph depended upon the magnification and focusing distance used in each particular case.
At first, we calculated Worst-Fechner IOL decentration with respect to the cornea center because this had been the reference taken in previous reports6 that assessed posterior chamber IOL decentration in Pseudophakie eyes. The surgeon used the entrance pupil, one of the main concepts in centering refractive procedures, as a reference. We think it is better to study IOL decentration with respect to the pupil center than to the cornea geometrical center because the eye is not a centered optical system.
However, there were little differences between both methods, and a statistically significant positive correlation existed between IOL decentration with respect to the centers of the cornea and pupil. In 59% of the IOLs, we found a decentration of less than 0.5 mm with respect to the pupil center. In the cornea center, such decentration was observed in 45.5% of the IOLs.
The most important advantages we found with our method were: easy access; objectivity and accuracy of the measurements; possibility of studying decentration and other measurements (ie, the distance from the iris claw lens to the limbus); and computer ability to save and display images at any time.
The main disadvantages are equipment cost and management of software.
In our clinical experience, we did not find postoperative changes in IOL position, provided that proper fixation to the iris was achieved at the time of surgery.
If we assume a standard 4-millimeter entrance pupil, since the diameter of the optic of the IOL is 5 mm, we think that a 1-millimeter displacement of the IOL could be allowed without expecting any clinical significant visual disturbance. In such a case, despite the IOL being decentered by 1 mm, 4 mm of the entrance pupil would be covered with the optic of the IOL, resulting in good foveal vision. As the entrance pupil size varies with ambient lighting, the optic of the IOL should ideally be at least the size of the entrance pupil under dim illumination.
Measurement of the Decentration of Worst Claw Lenses in Phakic Myopic Eyes
Our study on 22 eyes with Worst-Fechner iris claw IOLs for high myopia revealed a decentration with respect to the pupil of 0.47 ± 0.29 mm (range, 0.182 to 0.9341 mm). Decentration was not more than 1 nun in any of the eyes. Therefore, if we assumed a standard 4-millimeter entrance pupil, no serious visual impairment due to decentration could be expected. We intend to increase the present series and add other findings, such as evaluating the IOL-endothelial contact by studying point of fixation of the IOL to the iris and the distance from the loop to the anterior chamber angle. This will also enable us to calculate before surgery the most suitable site for lens fixation to the iris to prevent further decentration and associated risks.
We have developed the PTH system, using the IMAGEnet digital system to accurately assess the decentration of Worst-Fechner iris claw IOLs implanted in myopic phakic eyes.
Further studies are underway to develop this method and to assess the decentration of other IOLs and refractive procedures.
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Measurement of the Decentration of Worst Claw Lenses in Phakic Myopic Eyes