Accuracy of lens power calculations with the biconvex and meniscus intraocular lenses.
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Equipment Design↗
Biomedical subjects
T Olsen
Publications and source records attributed to T Olsen.
[Control of refraction in cataract surgery].
The optical problems associated with cataract surgery are reviewed and methods to control the refractive state of the eye after the operation are presented. The clinical benefit of a calculation of intraocular lens (IOL) power from measurements of corneal curvature and eye length is demonstrated in a series of operated patients from two Danish eye departments. It is concluded, that the postoperative refraction can be controlled with a reasonable accuracy. Large and unexpected deviations in the postoperative refraction are avoided in this way, and in a number of cases it is possible to reduce the need for spectacles after the operation.
Cataract Extraction↗
Immersion versus contact technique in the measurement of axial length by ultrasound.
The axial length was measured by A-scan in 60 cataractous patients using a non-contact (immersion) and a contact technique. The mean axial length was found to be 23.49 mm with the immersion technique and 23.35 mm with the contact technique, using the same Sonometrics transducer probe. The difference of 0.14 mm (+/- 0.19) was significant at the 0.01 level. The difference should be taken into account when evaluating the accuracy of IOL calculation, and may call for different empirical corrections of the axial length with different ultrasonic equipment.
Aged↗
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.
Aged↗
Computerised intraocular lens calculation: clinical results and predictability.
The predictability of a theoretical, computer-assisted IOL calculation method and of the Sanders-Retzlaff-Kraff (SRK) method was evaluated from pre- and postoperative biometry in 110 cataractous patients subjected to a routine IOL calculation. With the theoretical method and the preoperative data the refraction was on the average 0.72 D (SD 0.78) more myopic than expected, an error which could be corrected for by (1) substituting the assumed postoperative chamber depth by the actual mean value and (2) adding 0.16 mm to the preoperative axial length. With the postoperative data the error was 0.05 D (SD 0.78) (p greater than 0.05). By the SRK method the refraction was 0.15 D (SD 0.71) and 0.37 D (SD 0.69) more hyperopic than expected with the pre- and the postoperative data respectively. These offset errors could be corrected for in either case by adjusting the assumed A constant in the SRK formula. With appropriate empirical corrections for offset errors the theoretical and the SRK method were similar in accuracy, that is, about 82% of the predictions within +/- 1 D, 99% within +/- 2 D, and 100% within +/- 3 D error. The variation in postoperative refraction after computerised IOL calculation was about one-third of the variation previously seen after implantation of standard power IOL.
Aged↗
Measuring the power of an in situ intraocular lens with the keratometer.
A method to measure the power of an intraocular lens (IOL) in situ using an extended range keratometer and a depth measuring apparatus is described. With this method, the IOL power was measured in ten patients with an anterior chamber IOL and in ten patients with a posterior chamber IOL and compared with the labeled power. The total mean error (measured power minus labeled power) was found to be 0.01 diopters (D) with a standard deviation of 0.22 D. The largest error was 0.40 D.
Cornea↗
Calculating axial length in the aphakic and the pseudophakic eye.
The axial length of 149 eyes (84 pseudophakic and 65 aphakic) was calculated by refractive means and compared with the axial length determined by ultrasound. The mean error (+/- SD) between the measured and the calculated axial length (measured minus calculated) was 0.02 mm (+/- 0.51) and 0.13 mm (+/- 0.54) for the aphakic and the pseudophakic eyes, respectively. The mean error was not significantly different from zero (P greater than .05). Calculation of the axial length may represent a valuable alternative to biometry for the aphakic and the pseudophakic eye.
Aged↗
Oxygen transport in newborn piglets with pulmonary hypertension.
Because of the neonate's susceptibility to pulmonary hypertension (PHN) and his inefficiency in invoking the compensatory mechanisms often used by adults to maintain stable levels of O2 consumption (VO2) in the face of changes in O2 delivery (DO2) and metabolic demand, we have attempted to define the O2 handling capabilities of the newborn piglet affected with various types of pulmonary vasoconstriction. Hemodynamically similar levels of PHN were generated in 18 newborn piglets (six through group B beta-hemolytic Streptococci infusion; six through hypoxia; six through hypercarbia) and O2 transport was studied. At 60 min VO2 was similar in all groups, although DO2 was different (10.7 +/- 6.7, 7.2 +/- 1.6, and 21.7 +/- 8.9 ml/kg.min, in the septic, hypoxic, and hypercarbic groups, respectively). Extraction efficiency varied in an inverse fashion (43 +/- 12%, 72 +/- 12%, and 27 +/- 16%, in the septic, hypoxic, and hypercarbic groups, respectively). Supply dependency and a critical DO2 were observed in the septic and hypoxic PHN groups (18.4 and 12.2 ml/kg.min, respectively). Both of these were elevated as compared to healthy adult levels. Hypercarbic pulmonary hypertension was supply independent at the levels studied; however, DO2 remained elevated in these animals and may never have reached the critical DO2 level.
Animals↗
Radial keratotomy for pseudophakic myopia.
Cataract extraction with implantation of a standard power IOL occasionally gives rise to considerable post-operative myopia. When operation of the second eye is indicated, the question may arise whether the patient should be made strongly myopic in both eyes, be made anisometropic, or have the already implanted IOL exchanged for a weaker one. We report here on 3 patients made isometropic and low-grade myopic after radial keratotomy in the myopic pseudophakic eye followed by cataract extraction and implantation of an IOL of predicted power in the second eye.
Aged↗
Pre- and postoperative refraction after cataract extraction with implantation of standard power IOL.
The pre- and postoperative refraction results are reported in 99 patients receiving an anterior chamber lens of standard 19.0 D power after intracapsular cataract extraction. The mean refraction after the operation was -0.76 D, SD 2.13. Apart from eight patients with suspected lenticular myopia, 5% of the patients ended up with a refraction that differed more than +/- 5D from the preoperative value. This variation was due to variation in the power of the biological lens removed at surgery, the mean value of which was 22.8 D, SD 3.3. As a measure of the aniseikonia induced at surgery, the ocular magnification was calculated to increase 2.7%, SD 4.2%, as compared with the preoperative value. The variation should be considered in the discussion of whether a preoperative biometry and calculation of appropriate IOL power are recommended or not.
Aged↗
Theoretical vs empirical prediction of aphakic refraction.
The aphakic refraction was predicted from preoperative and postoperative keratometry and axial length determinations in 43 patients subjected to cataract extraction without intraocular lens implantation using a theoretical and an empirical (Sanders, Retzlaff, Kraff [SRK]) method of calculation. The error (mean +/- SD; observed value minus expected value) of the theoretical vs the empirical method was 0.02 +/- 0.76 diopters (D) vs 0.66 +/- 0.72 D, respectively, when the preoperative data were used in the calculations, and 0.21 +/- 0.80 D vs 0.84 +/- 0.79 D, respectively, when the postoperative data were used in the calculations. The mean error was significantly different from zero (predicted values were lower than observed values) for the SRK method but not for the theoretical method. The mean error of the SRK method could be eliminated by adjusting the offset constant in the regression formula.
Aged↗
The swelling pressure of the human corneal stroma as determined by a new method.
The swelling pressure of 115 human corneas was determined using a modified electronic balance modified to simultaneously record the swelling force and the thickness of the stroma. The swelling force was found to follow a straight-line dependence on the stromal thickness when plotted in a double logarithmic scale, which means that the swelling pressure of each cornea could be expressed by a power fit of the form SP = aTb, where SP is swelling pressure, T stromal thickness, and a and b are constants of the cornea. In 45 control corneas swollen in 0.9% NaCl, pH 7.4, 0.01 M Hepes buffer, the mean value (+/- S.D.) of 'a' and 'b' were 7.09 mmHg mm-1 (+/- 2.96) and -3.48 (+/- 0.20), respectively. This corresponded to a mean swelling pressure of 84.0 mmHg at a standard stromal thickness of 0.5 mm. In paired experiments, the swelling pressure was found to be influenced insignificantly (P greater than 0.05) by a number of conditions, including lowering the pH to 4.0, increasing the temperature to 37 degrees C, and increasing the NaCl concentration to 9%. A significant correlation was found between the swelling pressure and the dry weight of the specimen (P less than 0.05), indicating a considerable biological variation of the swelling pressure. It is shown, that this variation may explain the normal variation in human corneal thickness in vivo.
Cornea↗
Theoretical approach to intraocular lens calculation using Gaussian optics.
A computer-assisted method of calculating intraocular lens (IOL) power using formulas according to Gaussian optics is described. The method is more exact than current theoretical formulas, especially in calculating corneal power and in dealing with principal planes, and tends to give somewhat higher values for IOL power. One advantage of the theoretical approach is that the eye can be analyzed optically in a conventional manner, not only for the IOL power of emmetropia but also for the power of the biological lens, the total refractive power of the eye, and the magnification of the entire system. The relation to other theoretical formulas is discussed.
Humans↗
Theoretical, computer-assisted prediction versus SRK prediction of postoperative refraction after intraocular lens implantation.
The postoperative refraction prediction was evaluated in 99 patients with a 19.0-diopter standard power anterior chamber lens using a theoretical, computer-assisted method based on Gaussian optics and an empirical intraocular lens calculation method based on the SRK formula. Both the theoretical and the empirical predictions accorded with the observed values in the near emmetropia region. In the prediction of ametropia, the SRK method showed a tendency to predict refractions that were lower than the actual values in hyperopic eyes and higher than the actual values in myopic eyes. The bias of the SRK method may be due to the use of an erroneous refraction factor that converts the ametropia to the deviation in implant power from the emmetropia power. By introducing an individual refraction factor, the accuracy of the SRK method approached that of the theoretical method.
Aged↗
Diabetic retinopathy after 3 years' treatment with continuous subcutaneous insulin infusion (CSII).
Twenty-four insulin-dependent juvenile diabetics with no or minimal background retinopathy were randomly allocated to conventional insulin therapy (CIT) or continuous sc insulin infusion (CSII) administrated by a portable pump. At the present 3 year follow-up, there was one drop-out in the CSII group. Although the metabolic control was significantly better in the CSII patients, both groups improved significantly in metabolic control during the observation period. After 3 years, the HbA lc level was 7.4% +/- 1.2 (+/- SD) in the CSII patients and 8.6% +/- 1.6 in the CIT patients (P less than 0.01). As compared to the status at the beginning of the study, a progression of diabetic retinopathy (criteria: development of more than 2 microaneurysms) was observed in 4 of 11 in the CSII group and in 5 of 12 in the CIT group (P greater than 0.05). None of the patients were seen to develop soft exudates or neovascularisations. This study confirms the impression gained from a previous one-year follow-up of similar progression of retinopathy in patients on CSII and CIT.
Adult↗
Prediction of intraocular lens position after cataract extraction.
A formula is presented for calculating the postoperative position of an anterior chamber intraocular lens (IOL) from preoperative data on chamber depth, lens thickness, corneal curvature and diameter. The formula was arrived at by regression analysis of preoperative and postoperative data on 80 patients receiving an anterior chamber lens after intracapsular cataract extraction. The significant predictors, in order of importance, were the height of the cornea, regarded as a section of a sphere (P less than 0.001), the preoperative chamber depth (P less than 0.01), and the preoperative lens thickness (P less than 0.05). The axial length did not show a significant correlation with postoperative chamber depth (P greater than 0.05). Using the regression formula, the postoperative chamber depth could be predicted a posteriori with an accuracy of +/- 0.22 mm (SD) and a correlation coefficient of 0.58. The method described may be applied to other types of IOLs and may increase the accuracy of IOL power calculations.
Aged↗
On the calculation of power from curvature of the cornea.
The theoretical basis for the calculation of corneal refractive power from anterior curvature is considered. It is shown that the power can be calculated with sufficient accuracy from one simple formula provided the refractive index of the 'cornea' is 1.3315. It is suggested that keratometer readings should be calibrated with this value in order to increase the accuracy in intraocular lens calculation.
Cornea↗
Reflectometry of the precorneal film.
In order to gain information about the thickness of the oily layer of the precorneal film, a clinical method of measuring the reflectivity of the precorneal film was developed. The method entails the use of a slit-lamp photometer to measure the reflectivity at 2 selected wavelengths, 500 and 700 nm, at 20 degrees incidence. Based on a physical model of the oily layer acting as a thin dielectric film, a theory is given on the interpretation of the results in terms of the thickness of the oily layer. In 10 normal subjects a mean reflectivity (+/- SD) of 4.06% (+/- 0.83) and 3.16% (+/- 0.79) was found at 500 and 700 nm, respectively. A significant positive correlation between the measurements at the 2 wavelengths was found. It is shown, that these results are consistent with a thickness of about 40 nm, as being the most probable thickness of the oily layer. This value seems to be in accordance with other experimental data. Objective reflectometry may provide an important new tool for the study of the anteriormost part of the ocular surface.
Computers↗