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Significance of Intraocular Lens Power Calculation

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Significance of Intraocular Lens Power Calculation Br J Ophthalmol: first published as 10.1136/bjo.71.11.850 on 1 November 1987. Downloaded from British Journal of Ophthalmology, 1987, 71, 850-853 Significance of intraocular lens power calculation MOHINDER SINGH AND ALIAS DAHALAN From the Department of Ophthalmology, National University ofMalaysia, Kuala Lumpur, Malaysia SUMMARY A total of 94 patients underwent extracapsular cataract extraction and insertion of Sinsky style two-loop posterior chamber intraocular lenses. Forty-six eyes received a standard power TOL and 48 eyes were given a preoperatively calculated IOL. A significant difference was found in the two groups with regard to the postoperative refractive error and uncorrected visual acuity. Posterior chamber intraocular lens (TOL) implanta- metery as well as corneal curvature measurements. tion is rapidly becoming the preferred method of With the increasing availability of lens implants in visual rehabilitation after cataract surgery. Patients the third world, a great majority of patients will have opting for intraocular lenses nowadays demand the their aphakia corrected by intraocular lenses in the best optical correction. To achieve ideal and near future. A-scan biometry is unlikely to be within copyright. optimum results, estimation of correct IOL power is the reach of many practising ophthalmologists in necessary. The precision of such calculations these countries. Very few studies have compared the depends mostly on the accuracy of ultrasonic bio- outcome of implanting a standard powered and an individually tailored posterior chamber IOL.' We report such a study with a view to assessing the Correspondence to Mohinder Singh, FRCS, 37 Norman Road, significance of calculating the IOL power pre- Ilford, Essex IGI 2NH. operatively. http://bjo.bmj.com/ 10 9 8 7 6 on September 25, 2021 by guest. Protected 3 2 ....I I I. I. I aI a I I I I I I I I I I I I . * * I I * IaI. I I mI 16 17 18 19 20 21 22 23 24 25 DIOPTRES Fig. 1 Range ofintraocular lenses used in the measured group. 850 Br J Ophthalmol: first published as 10.1136/bjo.71.11.850 on 1 November 1987. Downloaded from 851 I V v v 7z 30 p/ , Ir II - "Z Z aZ ~--- IV e Ir Ir Ira U ±vr l EMNETROPIA +2.25 +1.25 +0.25 0 -0.25 -1.25 -2.25 -3.5 -6. 5 l I 4 1~ I +3.00 +2.00 +1.00 -1.00 -2.00 -3.00 Fig. 2 Difference between thepostoperative spherical equivalent refraction and emmetropia. Unmeasured group n = 46. 17I copyright. http://bjo.bmj.com/ 17O Fig. 3 Difference between thepostoperativespherical equivalent refraction andpredicted refraction. Measured group n=48. / on September 25, 2021 by guest. Protected ---- - - - V X -0.25 -12 orj,Ir/-- -0 ,/ zz DIOPTRES FROM PREDICTED REFRACTION +1.25 +0.25 0 -0. 25 -1.25 -1.75 +1.50 +1.00 -1.00 -1.50 -2.00 Br J Ophthalmol: first published as 10.1136/bjo.71.11.850 on 1 November 1987. Downloaded from 852 Mohinder Singh andAlias Dahalan Material and methods Table 1 Postoperative visualacuity Ninety-four patients were operated upon under Vision Uncorrected Corrected general anaesthesia and with a Zeiss operating Measured Unmeasured Measured Unmeasured microscope. A standard extracapsular cataract group group group group extraction was the McIntyre performed by using 6/5 1 0 10 8 manual aspiration system. A Sinsky style two-loop 6/6 4 2 23 22 posterior chamber IOL was inserted by the same 6/9 8 5 12 14 surgeon (MS). No viscoelastic material was used in 6/12 15 8 3 2 this series. The limbal wound was closed with six to 6/18 14 l l 0 0 6/24 4 3 0 0 seven 10/0 monofilament sutures. 6/36 2 9 0 0 The first group of consecutive 46 patients received 6/60 0 8 0 0 a standard power IOL of 19-5 dioptres, ±0.5 dioptre, depending on availability. The second group of consecutive 48 patients had their IOL power esti- Fig. 3 shows the distribution of the difference mated preoperatively. Axial length measurements between the spherical equivalent refraction and were done with Cooper Vision Ultrascan digital A, predicted refraction for the measured group. In this which uses an applanation transducer. The corneal group 83*3% of the cases fell within 1 D of the curvature was measured with a Bausch and Lomb predicted refraction and 100% within 2 D. 66&6% of keratometer. The IOL power required to produce 1 eyes in this group showed postoperative refraction dioptre myopia was calculated by computing the within 2 D of myopia (Fig. 4). 87-5% of cases were SRK formula.23 within 1 D of the expected refraction. All patients were refracted between 8 and twelve weeks after surgery. The postoperative residual Discussion spherical equivalent refraction was determined for every pseudophakic eye. Patients having postsurgical Calculation of IOL power preoperatively helps to copyright. astigmatism greater than 2 dioptres were not con- achieve a desired refraction and avoid unexpected sidered for the study. high ametropia.4 It has been shown to be useful even for planned ametropia in those patients who Results have a large refractive error in the fellow eye.' The accuracy of IOL power estimation in this study is The 'unmeasured' group consisted of 46 patients. similar to that found in previously reported series.67 Their ages varied between 28 and 77 years, mean 57.8 Our study has shown that patients who received a years. The 'measured' group consisted of 48 cases. calculated IOLs achieved much better uncorrected http://bjo.bmj.com/ Their ages ranged from 35 to 80 years, mean 59-3 visual acuity than those in the unmeasured group, years. The intraocular lenses implanted in the though there was little difference in the corrected measured group ranged from 16 to 25 dioptres visual acuity in both groups. Unexpected high refrac- (Fig. 1). tive error was not present in the measured group, The postoperative visual acuity is shown in Table whereas two such cases were encountered in the 1. The postoperative corrected visual acuity was unmeasured group. Similar observations have been similar in the two groups. However, there was a reported by other workers.468 Our data show that the significant difference in the uncorrected postopera- margin of refractive error can be significantly on September 25, 2021 by guest. Protected tive visual acuity in the two groups (p<0O5). 58-3% of reduced by preoperative determination of lens the patients in the measured group could see 6/12 or power. better uncorrected as compared with only 32*6% in Ultrasonic lens power estimation is not available to the unmeasured group. Only 12.5% of cases in the every implant surgeon in the developing countries. measured group had uncorrected vision below 6/18, Insertion of a standard power lens and correction of whereas 43-5% of patients in the unmeasured group the residual pseudophakic refractive error by a had vision worse than 6/18. contact lens or spectacles is the only other alternative Fig. 2 shows the distribution of the difference to practise. Intraocular lens power can also be between the spherical equivalent refraction and determined clinically if the surgeon can evaluate the emmetropia for the unmeasured group. 52% of the patient's basic refraction. Binkhorst9 has outlined the patients fell within 1 D of emmetropia, 95-6% within possible sources of error in the clinical determination 3 D. There were two patients outside this range of lens power. It is extremely difficult if not imposs- measuring -3-5 D and -6-5 D postoperative residual ible to obtain the basic refraction in most cases in an pseudophakic refractive error. Asian population. However, reasonably good results Br J Ophthalmol: first published as 10.1136/bjo.71.11.850 on 1 November 1987. Downloaded from Significance ofintraocular lens power calculation 853 17 15 /~ 7 copyright. I,- --/,F- - I0 Ir Ir Ir.6 Ir 11m Ee1, - . -/4, .I I / I 5 'I - DIOPTRES FROM ENMETROPIA P+0 0 -0.25 -1.25 -2. 25 .25 +1 .00 -1.00 -2.00 -3.00 Fig. 4 Difference between thepostoperativespherical equivalentrefraction and emmetropia. Measured group n=48. http://bjo.bmj.com/ have been reported by implanting a standard power tive refractive results with and without intraocular lens power calculation. BrJ Ophthalmol 1986; 70: 22-5. IOL. I In our measured group the majority (68%) of 2 Retzlaff J. A new intraocular lens calculation formula. J Am the lenses inserted ranged from 18 to 205 dioptres. Intraocul Implant Soc 1980; 6: 148. An appropriate range of lenses most suitable for the 3 Sander DR, Kraff MC. Improvement of intraocular lens power local population can be reasonably determined by calculation using empirical data. JAm Intraocul Implant Soc 1980; 6: 263. conducting a pilot study using preoperative ultra- on September 25, 2021 by guest. Protected 4 Percival P. Lens power calculation - is it necessary? Trans sonography coupled with the knowledge of post- Ophthalmol Soc UK 1983; 103: 577-9. operative residual pseudophakic refractive error. 5 Hillman JS. Intraocular lens power calculation for planned Hence the routine use of lens calculation may be ametropia: a clinical study. BrJ Ophthalmol 1983; 67: 255-8. 6 Hillman JS. The selection of intraocular lens power by calculation dispensed with if the resources to carry it out are and by reference to the refraction - a clinical study. Trans limited. A careful selection of patients might reduce Ophthalmol Soc UK 1982; 102: 495-7. unexpected surprises but will not eliminate them, and 7 Hoffer KJ. Accuracy of ultrasound intraocular lens calculation. both the patient as well as the surgeon should be Arch Ophthalmol 1981; 99: 1819-23.
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