Spectacle Plane Research Articles

Measuring corneal power for intraocular lens power calculation after refractive surgery: Comparison of methods

Purpose To find a more accurate and predictable method for intraocular lens (IOL) power calculation in eyes after refractive surgery. Setting Department of Ophthalmology, Kangnam St. Mary’s Hospital, Seoul, Korea. Methods The accuracy of the following methods for calculating IOL power in 132 eyes after PRK or LASIK was compared: manual keratometry, hard contact lens, refraction-derived keratometry at the corneal plane, and the refraction-derived keratometry at the spectacle plane. Based on this comparison, the IOL power was calculated in the 2 eyes of a patient using refraction-derived keratometry at the spectacle plane with the SRK II formula. Cataract surgery with IOL implantation was then performed. Result The largest corneal power values were obtained using a manual keratometer and the smallest using refraction-derived keratometry at the spectacle plane ( P < .001). In the patient having cataract surgery with IOL implantation, near target refraction was achieved with minimal error in IOL power. Conclusions If the corneal power is known before refractive surgery, the use of the smallest value of those obtained using refraction-derived keratometry and the hard contact lens method is recommended. However, if the corneal power before refractive surgery is unknown, the use of the hard contact lens method is recommended.

Correcting high astigmatism with piggyback toric intraocular lens implantation

An 86-year-old man presented for cataract surgery with corneal astigmatism of 5.12 diopters (D). After cataract extraction with small-incision techniques, 2 toric plate-haptic silicone intraocular lenses (IOLs) were implanted in the capsular bag, each with a 3.50 D cylinder add (2.30 D at the spectacle plane). Six weeks postoperatively, corneal astigmatism was 3.38 D at 70 degrees and refractive astigmatism was 1.00 D at 20 degrees. Uncorrected visual acuity was 20/40. No IOL rotation was observed. Implantation of piggybacked toric lenses may be a viable option for correcting moderate to high astigmatism.

Effect of a standard paired arcuate incision and augmentation sutures on postkeratoplasty astigmatism

Purpose To analyze the efficacy of a standardized paired arcuate incision and augmentation suture technique in the treatment of various levels of post-penetrating keratoplasty (PKP) astigmatism. Setting Lions Eye Institute, Royal Perth Hospital, Perth, Australia, and University of Dundee Department of Ophthalmology, Dundee, United Kingdom. Methods A standardized paired arcuate incision and paired augmentation suture technique was used to treat 34 eyes with post-PKP astigmatism ranging from −3.50 to −20.00 diopters (D) at the spectacle plane. The technique consisted of paired arcuate incisions of 3 clock hours, 480 μm deep in the graft–host junction, and 2 pairs of augmentation 10-0 nylon sutures. Results The mean preoperative cylinder was −9.14 D ± 4.38 (SD) and the mean postoperative cylinder, −3.59 ± 1.92 D at the corneal plane after a mean follow-up of 50 ± 43 weeks. This represents an empirical reduction in mean cylinder of 5.55 D (60.7%). The Alpins correction index (surgically induced astigmatism [SIA] divided by target induced astigmatism) was calculated for each case, and the mean was 1.01 ± 0.34, with a median of 0.91. Approximately 53.1% of cases achieved a correction index between 0.80 and 1.20, and the correction index correlated poorly with the initial magnitude of cylinder. A direct numerical relationship between SIA and the initial magnitude of cylinder was observed, although a standard surgical procedure was used in all cases. Conclusion A simple standardized technique using paired arcuate incisions in the graft–host junction with paired augmentation sutures reduces the amount of cylinder in proportion to the magnitude of the preoperative cylinder and effectively reduces post-PKP astigmatism.

A new method for determining prismatic effects in cylindrical spectacle corrections.

This study presents a new method of finding differential prismatic effects in an anisometropic spectacle correction containing cylindrical lenses. The calculations are based on dynamic spectacle magnifications rather than Prentice's rule. If an imaginary circular object is considered, cylindrical lenses will produce elliptical far point images, which can be superimposed on the spectacle plane for comparison. The difference between left and right ellipses, in any meridian, then represents the distance the eyes have to diverge in order to fuse the object of regard. This distance can be translated into prism diopters of differential prismatic effect. Although the conventional methods for finding this effect often result in very large errors, the new method can be performed with great accuracy. In part, this is because it uses the actual eccentricities of the two eyes rather than an assumed average eccentricity. Moreover, the method includes considerations of base curves and center thickness. Contrary to the classical methods, it can thus be applied to clinically realistic lenses.

Excimer laser correction of astigmatism with multipass/multizone treatment

Purpose: To evaluate the accuracy of excimer laser correction of myopic astigmatism by multipass/multizone photoastigmatic refractive keratectomy (PARK).Setting: Tertiary referral ophthalmic hospital with an associated private laser facility.Methods: This study comprised a consecutive series of 332 eyes of 289 patients who were followed for 6 months. All patients were 18 years or older, had stable myopic astigmatism (up to a −19.0 diopters [D] spherical equivalent [SE] at the spectacle plane), and had a best corrected visual acuity of at least 20/60 in both eyes. All eyes were treated with a VISX Twenty-Twenty excimer laser. The correction was divided between ablation zones using a multipass/multizone treatment paradigm based on the amount of myopia and astigmatism. Patients were examined 1 week, and 1, 3, and 6 months after surgery.Results: Analysis of the mean percentage of spherical correction across the range of myopic preoperative SEs treated demonstrated 90% correction for most amounts of myopic astigmatism. Eyes with low myopia (mean preoperative SE ≤ −5.0 D) treated with ≤ −1.0 diopter cylinder (DC) of astigmatism achieved a mean percentage of spherical correction of 91% versus 93% in eyes with high myopia (>−5.0 D mean preoperative SE). Eyes with low myopia treated with >−1.0 DC of astigmatism achieved a mean percentage spherical correction of 90% versus 89% in eyes with high myopia. The differences between the two groups were not statistically significant. Patients with high relative cylinder (>80% of total sphere treated) achieved comparable results. Analysis of the astigmatic component of the treatment, independent of the spherical result, showed a trend toward overcorrection in the high myopia group with less than −1.0 DC and a mean astigmatic correction of 89 and 98%, respectively, in the low and high myopic astigmatism groups. The mean angle of error was +2.0 degrees.Conclusion: Multipass/multizone PARK for myopic astigmatism demonstrated a high degree of predictability and stability with desirable results for low and high levels of astigmatism across the range of myopic astigmatism treated by surface ablation.

Astigmatism and the toric intraocular lens and other vertex distance effects

An intraocular lens with a given cylindrical power will correct a variable amount of cylinder at the spectacle plane. This variability depends not only on the vertex distance but also on the degree of ametropia. The amount of spectacle cylinder corrected will be more for myopes and less for hyperopes. Anterior corneal cylinder (as determined by the keratometer) is a more reliable way of estimating the cylinder needed at the intraocular lens plane.

Intraocular lens power calculation for cataract surgery after photorefractive keratectomy for high myopia.

To assess intraocular lens (IOL) power calculations in patients undergoing cataract surgery after excimer laser photorefractive keratectomy (PRK) for myopia. Four eyes of two patients underwent phacoemulsification with IOL implantation after PRK for myopia. The estimated refractive error that would have been induced had the IOL predicted for emmetropia been implanted was calculated using SRK-II, SRK/T, Holladay, and Binkhorst formulas. Manual keratometry and videokeratography-simulated keratometry values measured before surgery were used. Keratometry values calculated by subtracting the refractive change induced by the excimer laser PRK from the manual keratometry or videokeratography-simulated keratometry values measured before PRK were also used. Both spectacle and corneal plane calculations were performed. Manual keratometry and videokeratography-simulated keratometry values underpredicted the IOL power. Corneal plane manual or videokeratography refraction-derived keratometry calculations were most accurate using the SRK/T formula, while spectacle plane calculations were most accurate using the SRK-II formula. In both methods the calculated refractive error was within 0.52 diopters (D) for the emmetropic lens power predicted. Statistical analysis was not performed. Refraction-derived keratometric values provided the most accuracy in calculating IOL powers. Our results suggest the SRK/T formula was the most accurate for corneal plane calculations, while the SRK-II formula was the most accurate for spectacle plane calculations.

Myopic shift after cataract removal in childhood.

Children who have had cataract removal tend to have decreasing hyperopia (myopic shift) as they grow older. We wondered if the rate of myopic shift could be determined by age at surgery, cataract type, glaucoma, or other factors. We studied 156 aphakic eyes of children who had cataract surgery before age 10 and documented refractions for more than 3 years. Refraction was corrected with contact lenses and spectacles; glaucoma was managed with medicine and surgery. Stepwise multiple regression was used to analyze differences in the rate of myopic shift between subgroups. The average refraction tended to follow a logarithmic decline with age (P < 0.01, R2 = 0.97). The average rate of myopic shift (the slope of spectacle plane refraction vs log of age, where age is in years and log is base 10) was -5.5, with a standard deviation of 3.8. Age at surgery had a small but statistically significant effect on the rate (P < 0.01, R2 = 0.04). No other studied factor reached statistical significance. However, among the 86 eyes with cataract removal after age 6 months, age at surgery was not as significant (P = 0.21), and unilateral cataract eyes tended to have a greater rate than bilateral cataract eyes (-7.7 vs -5.7; P = 0.05, R2 = 0.05). Aphakic refraction tends to follow a logarithmic decline with age. The rate of myopic shift is determined partly by age at surgery and whether the cataract was unilateral or bilateral, although the effects are small. A wide variation in the rate of myopic shift exists. The following factors made little difference in the rate: cataract type, glaucoma, sex, side, and best corrected visual acuity.

Instruments/techniques

We developed a system of coaxial video cameras that records monocular eye position and scene, and superimposes these images using a digital video mixer. We mounted miniature video cameras above and below a cube beam-splitting prism in the spectacle plane. An infrared emitting diode was imaged in the cornea to locate eye position. The technique was accurate to about 0.5 degrees within 15 degrees of primary gaze; however, we see its main advantages as being its low cost and simple design that, for some applications, does not require complex computer analysis and data manipulation. With improved camera optics, it has the potential for helmet mounting and use remote from a recording console. We used the instrument to monitor a reader's eye position when using low vision devices, and see applications of the technique in the field of visual ergonomics and sports vision.

Experimental Verification of Computational Methods to Calculate Magnification in Refractive Surgery

To determine the correlation between measured and computed magnification caused by a change in the plane of correction from the spectacle plane to the corneal plane in myopic refractive surgery. Fourteen patients who underwent radial keratotomy and five normal volunteers served as subjects. Anticipated relative magnification was computed and measured by means of a direct-comparison eikonometer. Measured values were correlated to the anticipated magnification effects determined by computation. Measured and computed magnifications were highly correlated (r = .891). Magnification induced by refractive surgery can increase visual acuity in excess of 1 line for myopic corrections or similarly decrease visual acuity for hyperopic corrections. Magnification effects can be modeled accurately by means of computational methods. When clinical studies are designed to evaluate refractive surgery, ignoring the effects of magnification is similar to saying that visual acuity can be measured with and without the aid of a magnifier and the results directly compared.

Computer Program to Calculate Vectorial Change of Refraction Induced by Refractive Surgery

After refractive surgery, there may be changes in the spherical and the cylindrical components of the ametropia. In astigmatism, there may be variations in both the power and the axis. The best way to describe change in astigmatism caused by surgery is by vectorial analysis, which indicates the changes in power and the axis. However, in clinical practice, this procedure is limited by the tedious calculations required to establish the oblique cylinder combinations. We propose a computer program for the calculation, by vectorial analysis, of the refractive changes induced by surgery. The program allows for reference to refractive data or keratometric readings. When the refractive data are considered, it is also possible to obtain the transposition of the correction values from the spectacle plane to that of the cornea. Finally, the program enables the user to calculate the coupling ratio. Some selected surgical cases are discussed by way of example. We think that this program will be a simple and accurate means of evaluating the results of refractive and corneal surgery.

On the Stiles-Crawford effect and ocular imagery.

To study the modifying effect on spherical aberration of the retinal directional sensitivity (the Stiles-Crawford effect), ocular imagery was simulated by computer ray-tracing in a phakic and a pseudophakic model eye. The Stiles-Crawford effect was found to reduce the distance between the paraxial focus and the effective least blur focus. The modifying effect on spherical aberration was found to be insignificant for pupil sizes below 4 mm, and to increase in magnitude above this size. For an 8 mm pupil, the effective focus was found to move 0.3 mm and 0.2 mm toward the paraxial focus for the phakic and the pseudophakic eye, respectively. This corresponded to a shift of 1.0 D and 0.6 D in the spectacle plane, respectively. The modifying effect of the retinal directional sensitivity should be taken into account in the theory of ocular refraction and in objective keratometry and refractometry.

Intraocular Lens Power Calculations for Multifocal Intraocular Lenses

We performed calculations for anterior and posterior chamber intraocular lenses, determining the necessary power for emmetropia and 3 diopters of myopia for various axial lengths and corneal powers. Our results demonstrate that to achieve an effective add of 3.00 diopters at the spectacle plane (reading distance of 33 cm), the fabricated add (labeled add) on an intraocular lens must vary from 3.3 diopters for an anterior chamber lens in a patient with a flat cornea, to 5.1 diopters for a posterior chamber lens in a patient with a steep cornea. For most patients, targeting for an effective add of 3 diopters is appropriate, but factors such as availability of fabricated adds from the manufacturer and unusual patient requirements for near vision, may alter this target. We reviewed alternatives with monofocal lenses, such as monovision and compound myopic astigmatism.

Artificial pupils and Maxwellian view

When control of the pupil size is required, the simplest method is to use a physical artificial pupil or aperture that is placed in the spectacle plane. In some clinical applications (e.g., the potential acuity meter) an optical artificial pupil is imaged in the plane of the natural pupil by a Maxwellian view optical system. We compared visual performance with physical and Maxwellian artificial pupils by measuring the effects of the pupil diameter (0.5-5 mm in range) and defocus (5-D myopia to 4-D hyperopia) on minimum angles of resolution (MAR's) and on angular blur disk diameters. For pupil diameters down to ~ 2.0 mm there were no meaningful differences between the visual resolution that is obtained with the physical and the Maxwellian pupils. At the smallest diameter (0.5 mm) the physical artificial pupils caused the MAR to increase because of the diffraction limitation on resolution, and defocus no longer affected MAR. With the small Maxwellian pupils vision did not become diffraction-limited so that maximum resolution could still be obtained. MAR was still affected by defocus. The angular blur disk diameters measured with the smaller Maxwellian pupils were slightly but significantly larger than those found with physical artificial pupils. For physical artificial pupils, field-of-view restrictions may result from vignetting with the eye pupil. Thus small physical artificial pupils can act as pinholes causing resolution to become impaired but insensitive to defocus. Also vignetting by the eye pupil can restrict the field of view. Small optical artificial pupils from Maxwellian viewing do not impair resolution, and the resolution may remain sensitive to defocus. The eye pupil does not cause any field restriction, although, if small, it may filter higher spatial frequencies out of the retinal image.

Contrast sensitivity as a function of focus in patients with the diffractive multifocal intraocular lens

Contrast sensitivity as a function of focus and visual acuity as a function of contrast were investigated in 19 patients with a diffractive multifocal intraocular lens and compared with 19 control patients with a conventional monofocal implant. The contrast sensitivity of the multifocal patients followed a bimodal curve with a maximum sensitivity at the far focus and a second peak at the near focus, corresponding to about +3 diopters in the spectacle plane. The maximum sensitivity of the multifocal group was 0.14 log units lower than the control group (PÅ.05). In the near region, the contrast sensitivity of the multifocal patients exceeded that of the control group from + 2 diopters and inward. No difference in distance visual acuity was found with high contrast letters. With intermediate contrast letters, the visual acuity of the multifocal patients was lower than that of the control group (PÅ.05).

Working distance comparison of plus lenses and reading telescopes.

Low vision aids used in the spectacle plane may be either plus lenses or telescopes focused for near. The plus lens has a shorter working distance than a near point telescope of the same equivalent power. A relationship between the anterior focal length (fv) of the near point telescope and the equivalent focal length (fe) of a thin lens of equal power can be derived from the formula fv = (mag) (fe). This formula provides a basis for determining what equivalent powers are equal and interchangeable between the two types of aids. Furthermore, the advantages of the increased working distance afforded by the near point telescope can be defined.

Myelinated Retinal Nerve Fibers Associated with Ipsilateral Myopia, Amblyopia, and Strabismus

Four patients had extensive unilateral myelinated nerve fibers associated with ipsilateral myopia, amblyopia, and strabismus. Their profound visual impairment, exotropia, and the early age onset of symptoms indicated that the amblyopia may have been organically caused. Prognosis is poor for even partial correction but good results have been obtained with intense therapy that includes full correction of the refractive error in each eye and extraocular muscle surgery if cosmetically necessary. In patients with axial myopia, images of a similar size will be produced by placing a lens of the correct power at the spectacle plane. In younger patients, a contact lens should not be used to correct the refractive error because it creates an undesirable anisometropia. Younger patients who have parafoveal fixation and no strabismus respond best to amblyopia therapy.

Linear regression of lens movement with refractive state using ciliary ganglion stimulation in the cat.

Simultaneous measurements of lens motion and spectacle plane refractive changes in the cat due to ciliary ganglion stimulation were made. The measurements were made for stimulation frequencies ranging between 10 and 60 pulses per second. A linear regression analysis reveals that the forward movement of the lens and spectacle plane refraction changes are linearly related with correlation coefficients ranging between 0.87 and 0.98. Using five cat eyes as a basis, it was found that the average maximum lens motion is 0.57 mm, average maximum refractive change is 1.72 diopters and average slope of the D vs lens movement curve is 3.55 D/mm.