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| ORIGINAL ARTICLE |
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| Year : 1991 | Volume
: 39
| Issue : 2 | Page : 50-52 |
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Prediction of intra ocular lens power in 100 patients using the ophthasonic a scan unit
NK Limdi, KJ Sheth
Department of Ophthalmology, Baroda Medical College, Gujarat, India
Correspondence Address:
N K Limdi
Department of Ophthalmology, Baroda Medical College, Gujarat
India
 Source of Support: None, Conflict of Interest: None  | Check |
PMID: 1916979 
The aim of this study was to determine the range of ametropias encountered by using the computerized Ophthasonic A-Scan. The SRK formula was used to compute the IOL power using axial length and keratometry as inputs. The accuracy of prediction of IOL power was calculated by noting the disparity between the expected post-operative refraction and the actual refraction obtained after one and a half months of IOL implantation. The results obtained were encouraging; the maximum deviation from the expected result being +/- 3.2 D. Keywords: Biometry, SRK Formula, Axial length, Keratometry
How to cite this article:
Limdi N K, Sheth K J. Prediction of intra ocular lens power in 100 patients using the ophthasonic a scan unit. Indian J Ophthalmol 1991;39:50-2 |
| Introduction | |  |
Intra ocular lens implantation in cataract surgery is a rapidly evolving art to which ultrasonic biometry adds the finishing touch. The pre-operative calculation of IOL power today, is an established ophthalmological technique which is being adopted by an increasing number of ophthalmic surgeons the world over. The precision in 100 power calculations pre-operatively give more predictable and accurate post-operative results, which is the goal one strives to achieve. Is a smaller amount of ametropia worth the time and expense of a precise biometry?
Is ultrasonic biometry a precise and reliable tool for calculating IOL power? This study aims to determine the efficacy of ultrasonic biometry in calculating IOL power.
| Material and methods | | |
The post-operative refraction was evaluated in 100 patients undergoing cataract extraction with posterior chaber IOL implantation. The patients were selected irrespective of their age and sex. Keratometry was carried out on an Appaswamy Keratometer (Baush and Lomb type) and readings were recorded in dioptres. The axial length was measured with the Opthasonic A-scan ultrasonic unit with a built in microprocessor for determining the axial length of the eye, computing the IOL power, to analyse the echo returns, put out audio signals and automatically print the valid measurements while they are being taken. The microprocessor analyses the amplitude of lens and retinal spikes to use as criteria to ensure proper
probe alignment.
The IOL power was calculated by using the SRK formula. This was done by using the Programme - 4 in the Ophthascan unit in which the SRK formula is incorporated. The basic formula is
P = A - 2.5L - 0.9K, where,
P = Power of lens for emmetropia
A = A Constant of lens to be implanted L = Axial Length
K = Average of Keratometry readings.
A correction for `short eyes' has been developed by the authors for use with the SRK Emmetropia Programme. The following relationship will apply to the calculated power automatically.
AL 20mm: 2D added to IOL
AL 20mm to 21 mm: 1.50D added to estimated IOL power.
AL 21 mm to 22mm: 0.75D added to estimated IOL power.
where AL is the axial length.
Depending on the patient's occupation and refraction of the other eye, the desired post-operative refraction of the patient was derived. In general, the aim was to make the patient myopic, by 1 to 2 D, so that they would have good near vision without correction.
At the end of one and a half months, retinoscopy was performed and correction was given for distance and near. The difference between the expected post-operative refraction and the spherical correction for distance was determined. This gave the deviation of the postoperative refraction from the expected value. The results were tabulated and analysed.
| Results | | |
The maximum corrected post-operative visual acuity for distance was 6/6 and the minimum was finger counting at 5 feet. Both patients with visual acuity less than 6/60 were young males. One patient had a traumatic cataract, the other had retinitis pigmentosa.
The average keratometry readings show a trend towards the myopic side. The average axial length 23.19 mm falls in the normal range. The average IOL power 20.91 D is close to the standard power of PC IOL ie. 21.0 D.
The accuracy of prediction is equally good in all three groups. This is because of the correction factors incorporated in the programme.
Sanders, Retzlaff and Kraff in their study using the SRK formula had the following results [3].
| Discussion | | |
The results obtained in this study are comparable to those obtained by other authors [1][2],[3]. In 99% patients the error of prediction was within ± 3.OD. The maximum error encountered was +3.2 D. These results can be further refined by using immersion instead of applanation for measurement of axial length and by calculating the personal A-constant for each surgeon and lens style.
Therefore, the answer to the question - "Is a smaller amount of ametropia worth the time and expense of a precise biometry?" is obvious . Biometry is definitely essential, because the primary reason for IOL implantation is the reduction of high aphakic ametropia. As the calculation of IOL power can be done with no risk to the patient, there are no medical reasons for implanting any "Standard" lens.
| References | | |
| 1. | Thomas Oslen, Carl Uggerhoj Andersen, Hans Jorgen Plesner. Computerized intra ocular lens calculation: Clinical results and predictability." Br. J. Ophthalmol Vol.71, Jan-Jun'87.  |
| 2. | Jose L. Menezo. Vincente Chaqoues, Miguel Harto. The SRK regression formula in calculating the dioptric power of intraocular lens. Br. J. Ophthalmol Vol. 68, 235-237, 1984. |
| 3. | Donald R. Sanders. John Retzlaff. Manus Kraff. Comparison of SRK II and other second generation formulae. J. Cat. Refract Surg. Vol.14 March, 1988. |
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6], [Table - 7]
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