Computerised intraocular lens calculation: clinical results and predictability

Valeurs biométriques de l’œil du sujet africain de race noire et hypothèse du rôle dans certaines pathologies

INTRODUCTION

The international literature seldom refers to eye measurements of the black African patient. Therefore, the various patterns where biometry elements are used as constituents deserve reconsideration as they may not correspond to the eye of the black patient of our region. This study provides true measurements for the black African patient in Ivory Coast. It also establishes hypotheses by extrapolating the role of measurements in pathologies such as glaucoma.

MATERIAL

and method: The study included 325 eyes of 217 male and female patients. All the patients underwent ocular keratometrics and echo biometrics to measure the depth of the anterior chamber, the width of the lens, and the total axial length with the ultrascan biometer using the contact method in A echometrics.

RESULTS

The mean keratometry in women was significantly higher than in men (43.99 +/- 1.62 diopters vs 43.46 +/- 1.45 diopters). The anterior chamber was deeper in men (2.69 mm +/- 0.54 vs 2.53 mm +/- 0.48). The men's eyes were significantly longer, with an axial length of 23.26 mm +/- 1.07 vs 22.56 mm +/- 0.90.

CONCLUSION

Significant biometric differences between men's and women's eyes, on the one hand, and between black African and white patients' eyes, on the other hand, can be observed. The reasons for these differences are several: they are natural, socioeconomic, and technical. If standard implant of a different power for black and white patients can be imagined, can we also infer that the biometrics of a black African patient predisposes him to a disorder such as glaucoma more than a white patient? Further in-depth studies could provide an answer to this question.

Further Investigation of the Tolerance and Mechanical Adjustability of the *Acri.Tec AR-1 PC/IOL in Rabbit Eyes: An Intraocular Lens with Reversibly Adjustable Optical Power

PURPOSE

To examine the tolerance and mechanical function of an adjustable intraocular lens (IOL) in rabbit eyes.

METHODS

Implantation of the *Acri.Tec AR-1 PC/IOL into 14 rabbit eyes. Manipulation of the lens 8 weeks after implantation in order to change the refractive power. Follow-up for up to 5 months. Histopathologic examination of the eyes.

RESULTS

Implantation and mechanical adjustment of the PC/IOL were possible. Eyes healed normally. No difference between eyes containing the *Acri.Tec AR-1 PC/IOL and eyes containing the control PC/IOL could be detected with respect to signs of inflammatory reaction, corneal transparency, intraocular pressure and histopathologic appearance. Histopathologic examination of the eyes showed that the *Acri.Tec AR-1 PC/IOL did not cause any damage in rabbit eyes.

CONCLUSION

The *Acri.Tec AR-1 PC/IOL is well tolerated in rabbit eyes for extended periods of time, suggesting that this PC/IOL should be well tolerated in the long run. Surgical adjustment of the adjusting element can be performed with little effort several weeks after implantation.

Investigation of the Safety of an Intraocular Lens with Reversibly Adjustable Optical Power: The *Acri.Tec AR-1 PC/IOL

PURPOSE

To report the safety of a newly developed intraocular lens (PC/IOL) with reversibly adjustable refractive power (*Acri.Tec AR-1 PC/IOL) after implantation into adult human eyes. The surgical setup and specially designed instruments required for implantation will be evaluated. The examined parameters included: stability of the IOL during implantation, positioning of the IOL, rotation of the IOL, integrity of the posterior lens capsule, central visual acuity, morphology of the eye, clinical signs of inflammation, intraocular pressure, refraction.

METHODS

The study was carried out on 35 human eyes after using the Acri.Tec AR-1 PC/IOL after lens removal through phacoemulsification; as control, the implantation of conventional PC/IOL into the second eye of 29 patients was used. The mean follow-up was 9 + 6 months. An initial adjustment surgery of the *Acri.Tec AR-1 PC/IOL was carried out 2 weeks after implantation in 2 patients; the follow-up of these eyes was 3 and 6 months.

RESULTS

Implantation was possible without any problem. Eyes healed normally. No difference between eyes containing the *Acri.Tec AR-1 PC/IOL and eyes containing conventional PC/IOLs could be detected with respect to visual acuity, stabilization of refractive error, signs of inflammatory reaction, slitlamp microscopic aspect of the eye and intraocular pressure. As intended, initial adjustment surgery successfully changed the mydriatic refraction from +1.0 resp. +0.75 dpt to 0 resp. -0.5 dpt. 26 bilaterally pseudophakic patients including 2 patients having had adjustment surgery had more than 3 months follow-up. Mean visual acuity in the AR-1 PC/IOL group at 3 months (0.76 +/-0.24) did not differ from mean visual acuity in the pseudophakic control group (0.74 +/-0.23). Adjusted eyes had a visual acuity of 1.0. No adverse effects occurred.

CONCLUSION

Initial data suggest that the *Acri.Tec AR-1 PC/IOL can be safely implanted into the capsular bag of adult human eyes and can beadjusted after implantation, thereby achieving the desired change of refraction. Further exploration of the potential usefulness of this type of reversibly adjustable PC/IOL as a tool for refractive optimization is warranted.

Intraocular lens with reversibly adjustable optical power

PURPOSE

To evaluate whether it is technically feasible and safe to implant and adjust an intraocular lens (IOL) with reversibly adjustable refractive power designed to correct residual postoperative refractive error.

SETTING

Animal study facility, Klinikum Mannheim, University of Heidelberg, Heidelberg, Germany.

METHODS

An Acri.Tec AR-1 posterior chamber IOL (PC IOL) was implanted in pig cadaver eyes and in rabbit eyes after the crystalline lens was removed by phacoemulsification. The IOL was manipulated to change the refractive power. The outcome measures were the stability of the IOL during implantation, IOL positioning and rotation, the ability to move the adjustment device after the IOL had been implanted for several weeks, clinical signs of inflammation, and changes in the histopathologic appearance of the eye.

RESULTS

Implantation and adjustment of refractive power were possible. The eyes healed normally. There was no difference between eyes with the AR-1 PC IOL and eyes with a control IOL in inflammatory reaction, corneal transparency, or histopathologic appearance.

CONCLUSION

The AR-1 PC IOL was easy to implant and well tolerated in rabbit eyes. Surgical adjustment of the adjusting element was performed with little effort several weeks after implantation of the IOL.

Older age as risk factor for deviation from emmetropia in pseudophakia

PURPOSE

To find risk factors for deviation from emmetropia after cataract surgery in clinical practice.

METHODS

We evaluated the refractive outcome in 106 patients who had underone phacoemulsification and in-the-bag IOL placement 115 +/- 10 days after surgery. Postoperative optical correction and refractive error (diopters of spherical equivalent--ED) were related to age and sex, pre-operative axial length and keratometric diopter power, and operative incision technique.

RESULTS

Emmetropia was achieved in 15% of cases; 65% of eyes needed a myopic correction, averaging = 0.46 +/- 0.91 ED. The refractive error was 0.74 +/- 0.61 ED (< or = 1 ED in 77% of cases, < or = 2 ED in 97%). Both optical correction and refractive error were correlated to older age at the time of surgery (p=0.002 and p=0.001, respectively). Astigmatism appeared greater in clear-cornea incision than in limbar incision cases (p=0.05).

CONCLUSIONS

The higher refractive error in patients aged over 73 years suggests that age may be a risk factor for deviation from emmetropia after cataract surgery.

Measurement of optic disc size: equivalence of methods to correct for ocular magnification

AIMS

To compare methods available to correct the magnification of images that result from the optics of the eye and identify errors, and source of error, of the methods.

METHODS

11 methods were applied to ocular biometry data from three independent cohorts. Each method was compared with the method of Bennett, which uses most biometric data. The difference between each method and Bennett's is the "error" of the method. The relation between the error and axial length, ametropia, and keratometry was explored by linear regression analysis.

RESULTS

Methods using axial length had the lowest mean (+0.5 to +2.6%) and standard deviation (0.6 to 1.2%) of errors. Of methods using keratometry and ametropia only, the lowest mean (-1.4% to +4.4%) and standard deviation (2.9 to 4.3%) of errors was found for a new method described in this paper, and that used by the Heidelberg retina tomograph (HRT). The highest mean error (+2.2 to +7.1%) was found for Littmann's method. Littmann's correction was larger than the HRT's by 3.5 to 3.7%. The mean difference between the new and HRT methods and the "abbreviated axial length" method of Bennett is -1.3 to +2.0%. The error of the "keratometry and ametropia" methods is related to axial length.

CONCLUSIONS

Methods using axial length are most accurate. The abbreviated axial length method of Bennett differs little from more detailed calculations and is appreciably more accurate than methods using keratometry and ametropia alone. If axial length is unknown, the new and the HRT methods give results closest to the abbreviated axial length method.

Factors influencing the accuracy of the SRK formula in the intraocular less power calculation

Several intraocular lens (IOL) power calculation formulas (either theoretical or empirical) are used to determine the emmetropic IOL power) The Sanders-Retzlaff-Kraff (SRK) linear regression formula is among the most widely recognized empirical ones. In the present study intraocular lens power calculation aiming at emmetropia was performed, using SRK formula, in 145 cataractous eyes undergoing lens implantation. The final refraction was evaluated at 8 to 12 months after surgery. The purpose of this study was the identification and quantitative evaluation of the factors which influence significantly the accuracy of SRK in the intraocular lens power calculation. The following factors were studied: (1) the error in preoperative biometry with regard to the difference between post and preoperative axial length measurements, (2) the position of the implantation of the intraocular lens (anterior versus posterior chamber), (3) the intraocular lens style, (4) the intraocular lens power level, (5) the preoperative corneal astigmatism, (6) the surgically induced corneal astigmatism, and (7) the postoperative astigmatism. Multiple regression and stepwise regression analysis showed a strong correlation (R2 = 0.65; p < 0.001) between postoperative refractive error (Rf) and error in preoperative biometry (delta AL), surgically induced corneal astigmatism (SIA) and postoperative astigmatism (Ap) only. This correlation is expressed by the following equation: Rf = 0.07 -2.55 delta AL -0.42 SIA + 0.34 Ap. This equation indicates the quantitative effect of each factor on the accuracy of the SRK formula, by defining the pattern of the fluctuations of the amount or state (myopic or hyperopic) of refractive error induced by changes of variables delta AL, SIA and Ap.

Refractive results after phacoemulsification and ECCE. A comparative study

The refractive results were evaluated in 79 patients undergoing cataract extraction by phacoemulsification using a 6-7 mm tunnel incision, and compared with a group of 77 patients undergoing planned extracapsular cataract extraction (ECCE) by the same surgeon. A mean increase in the keratometric cylinder of 0.05D and 0.52D was found in the phacoemulsification and the extracapsular cataract extraction group, respectively. This was significantly different from zero for the extracapsular cataract extraction group (p < 0.05) but not for the phacoemulsification group (p > 0.05). By vector analysis, the mean surgically induced astigmatism was 0.91D and 1.36D in the phacoemulsification and the extracapsular cataract extraction group, respectively (p < 0.01). The IOL power prediction error (spectacle plane) was found to be 0.17D (+/- 0.69 SD) in the phacoemulsification group and 0.02 D (+/- 0.79 SD) in the extracapsular cataract extraction group, respectively. We conclude that phacoemulsification improves the surgical control of the refractive outcome of cataract surgery.

Accuracy of the newer generation intraocular lens power calculation formulas in long and short eyes

The accuracy of two newer generation theoretical intraocular lens (IOL) power calculation formulas and of the empirical SRK I and II formulas was evaluated in a series of 500 IOL implantations including a series of unusually long and short eyes. The prediction error of the theoretical formulas was found to be largely unaffected by the variation in axial length and corneal power, while the prediction of the SRK I formula was less accurate in the short and long eyes. The prediction of the SRK II formula was more accurate than the SRK I in that no systematic offset error with axial length could be demonstrated. However, because of a relatively larger scatter in the long eyes and a significant bias with the corneal power, the absolute error of the SRK II formula was higher than that of the theoretical formulas in the long eyes. The higher accuracy of the newer generation theoretical formulas was attributed to their improved prediction of the pseudophakic anterior chamber depth.

Theoretical versus SRK I and SRK II calculation of intraocular lens power

The predictability of a theoretical, computerized (PC-assisted) intraocular lens (IOL) power calculation method and of the Sanders-Retzlaff-Kraff [SRK] I and II methods was evaluated from preoperative and postoperative biometry in 202 cataractous patients who had extracapsular cataract extraction (ECCE). The theoretical method resulted in the lowest range and standard deviation of the error, and the highest correlation coefficient between the observed and the predicted refraction (P less than .05). The superiority of the theoretical approach was most clearly demonstrated when the postoperative measurements were used in the predictions (P less than .001). This demonstrated the potential accuracy of the formula used and the importance of incorporating methods to predict the IOL position after surgery. If the prediction of the IOL chamber depth was properly corrected for the axial length dependence, a high prediction accuracy could be obtained in short as well as in long eyes.

The accuracy of ultrasonic determination of axial length in pseudophakic eyes

The axial length was calculated by refractive means in 206 pseudophakic eyes and compared with the axial length measured by ultrasound using a Kretz 7200 MA A-scanner or a Sonometrics DBR 400 scanner. The difference (+/- SD) between the calculated and the measured axial length was 0.13 mm (+/- 0.54) and 0.08 mm (+/- 0.28) using the Kretz and Sonometrics equipment, respectively (P less than 0.01). The lower variation between the calculated and the measured axial length observed with the Sonometrics scanner indicated this instrument to be the most accurate. The calculated axial length may be used in future studies on the clinical accuracy of ultrasonic equipment.