Noel Alpins

A new method of analyzing vectors for changes in astigmatism.

ABSTRACT This method of astigmatism analysis recognizes the need to define an astigmatism goal, thus allowing the surgeon to obtain precise, separate measures of the magnitude and the angle of surgical error. From this, the surgeon can evaluate what surgery may be required to achieve the initial preoperative goal. An index that measures surgical success is adjusted for the level of preoperative astigmatism. The resulting data allow statistical comparison of multiple surgeries and techniques. This method also assists in resolving the case when spectacle and corneal astigmatism do not coincide.

Astigmatism analysis by the Alpins method.

Purpose: To determine the effectiveness of correcting astigmatism by laser refractive surgery by a vectorial astigmatism outcome analysis that uses 3 fundamental vectors: target induced astigmatism vector (TIA®), surgically induced astigmatism vector, and difference vector, as described by the Alpins method. Methods: A data set of 100 eyes that had laser in situ keratomileusis to correct myopia and astigmatism (minimum preoperative refractive astigmatism 0.75 diopter) was analyzed. The data included preoperative and 3 month postoperative values for manifest refraction and standard keratometry. Using the ASSORT® or VectrAK® analysis program, individual and aggregate data analyses were performed using simple, polar, and vector analysis of astigmatism and an analysis of spherical change. Statistical analysis of the results was used for means and confidence limits, as well as to examine the differences between corneal and refractive astigmatism outcomes. Results: At an individual patient level, the angle of error was found to be significant, suggesting variable factors at work, such as healing or alignment. A systematic error of undercorrection of astigmatism is prevalent in the treatment of these 100 patients by a factor of between 15% and 30%, depending on whether refractive or corneal values are examined. Spherical correction showed systematic undercorrection of 11%, and parallel indices demonstrated it to be more effective than the astigmatic correction. Conclusion: This method of astigmatism analysis enables the examination of results of astigmatism treatment measured by both refractive and corneal measurements using vector analysis. By examining individual vector relationships to the TIA (ie, the correction index, index of success, and flattening index), a comprehensive astigmatism analysis is completed. Each index provides information necessary for understanding any astigmatic change. Astigmatic outcome parameters are more favorable when measured by subjective refractive than objective corneal methods.

New method of targeting vectors to treat astigmatism

Purpose: To describe a method for optimizing the treatment of astigmatism using vector analysis of both refractive and corneal topographic values. Setting: Cheltenham Eye Centre, Melbourne, Australia. Methods: This study evaluated a method of vector analysis for planning surgery that uses both preoperative topographic and refractive values and determined how to select the relative treatment emphasis to be given to each. In addition, the significance of the phenomenon of ocular residual astigmatism (ORA) was explored. Its presence provides an inherent limitation on eliminating astigmatism from the eye’s optical system. Results: Various comparisons of preoperative and ORA values are plotted in a series of 100 excimer laser photoastigmatic refractive keratectomy patients. These ORA values are equivalent to the expected corneal astigmatism resulting from surgery where treatment is performed by refractive astigmatism values alone. A theoretical example is given in which the corneal astigmatism remaining from surgery is reduced by giving less emphasis to completely eliminating refractive astigmatism and consequently greater emphasis to completely eliminating topographic astigmatism. Conclusion: Using vectors in astigmatism surgery enables the incorporation of topography and refractive values into the surgical plan. This would achieve a greater reduction in corneal astigmatism and potentially a better visual outcome than using refractive astigmatism values alone.

Vector analysis of astigmatism changes by flattening, steepening, and torque

Purpose: To understand the effect of astigmatism surgery by analyzing astigmatic changes according to their component parts. Setting: Cheltenham Eye Centre, Melbourne, Australia. Methods: The component parts of the astigmatic changes considered were flattening, steepening, clockwise torque, and counterclockwise torque. Calculations to determine the astigmatic change were performed by vector analysis using rectangular coordinates after doubling the astigmatism and surgical vector axes. A reference axis was used for the resolution of astigmatic change to ascertain its effect along a selected meridian. Results: When correcting astigmatism, the orientations of incisional (tissue addition) or nonincisional (tissue ablation) techniques in any cornea are at right angles to each other. Since differences exist in the measured astigmatism depending on whether it is measured topographically or by manifest refraction, an on‐axis correction in one will occur with an off‐axis effect in the other. The net result is a reduced flattening effect and a proportionately increased torque effect for the offaxis component. Conclusion: When treatment is applied off one of the four primary axes, the treating vector can be resolved into its component parts of flattening, steepening, and torque. Analyzing changes in this way provides a uniform means of assessing astigmatic changes for all types of cataract and refractive surgery and quantifies the flattening effect when treatment is applied off the intended meridian.

Excimer laser correction of myopic astigmatism

ABSTRACT The excimer laser allows the controlled ablation of corneal tissue to correct refractive error. By using a combination of spherical and slit apertures, it is possible to correct both myopia and astigmatism. We report the results of 139 consecutive eyes that had photoastigmatic refractive keratectomy (PARK) for myopic astigmatism (myopia ≤15.00 diopters [D] with astigmatism ≤6.00 D) and compare these results with 107 consecutive and concurrent eyes that received photorefractive keratectomy (PRK) for myopia (≤‐15.00 D). The same excimer laser was used by 27 different surgeons. All patients were followed for at least three months. In the PARK group, 68% were within ±1.00 D at six months and 77% were within ±2.00 D. In the PRK group, these figures were 87% and 97%, respectively. Uncorrected visual acuity of 20/40 or better was achieved in 72% of PARK and 90% of PRK patients at six months. Minor adverse reactions occurred in 6% of PARK and 11 % of PRK patients. No significant surgeon effect was seen. Photoastigmatic refractive keratectomy provides a realistic approach to the surgical correction of myopic astigmatism and is comparable to PRK in safety and efficacy.

Customized photoastigmatic refractive keratectomy using combined topographic and refractive data for myopia and astigmatism in eyes with forme fruste and mild keratoconus

PURPOSE: To examine the outcomes of photoastigmatic refractive keratectomy using corneal and refractive parameters for myopia and astigmatism in eyes with forme fruste and mild keratoconus. SETTING: Private practice, Melbourne, Australia. METHODS: Photoastigmatic refractive keratectomy was performed with a Star 1 or Star 2 laser (Visx) in 45 eyes with forme fruste or mild keratoconus using the Alpins vector planning technique. Inclusion requirements were best corrected visual acuity (BCVA) 20/40 or better, no slitlamp signs of keratoconus, mean keratometry less than 50.00 diopters (D), and corneal and refractive stability for at least 2 years. RESULTS: Thirty‐two eyes had follow‐up of 5 years and 9 eyes, of 10 years. Preoperatively, the mean refractive astigmatism was −1.39 DC ± 1.08 (SD) (range 0.45 to −5.04 DC) and the mean corneal astigmatism was 1.52 ± 1.18 D (range 0.35 to 4.75 D) by manual keratometry and 1.70 ± 1.42 D (range 0.32 to 5.32 D) by topography. Twelve months postoperatively, the mean refractive astigmatism was −0.43 ± 0.40 D and the mean corneal astigmatism was 1.05 ± 0.85 D by keratometry and 1.02 ± 0.83 D by topography. At 12 months, the uncorrected visual acuity was 20/20 or better in 56% of eyes and 20/40 or better in all eyes. The BCVA was 20/20 or better in 89% of eyes and 20/30 or better in all eyes. Seven eyes had a loss of BCVA, and 16 eyes had a gain. There were no cases of keratoconus progression. CONCLUSIONS: Photoastigmatic refractive keratectomy in eyes with forme fruste and mild keratoconus was safe and effective for myopia and astigmatism in carefully selected patients with refractive and corneal stability. The incorporation of the corneal astigmatism data into the applied treatment parameters may improve visual and total astigmatism results.

Treatment of irregular astigmatism

Purpose: To treat irregular astigmatism by applying separate appropriate treatments in each of the two distinct hemidivisions of the cornea. Setting: Cheltenham Eye Centre, Melbourne, Australia. Methods: Two general surgical strategies are presented. The first applies the principles of optimization separately to each corneal hemidivision to achieve the maximum reduction in astigmatism when measured topographically and refractively. The second is for targeting symmetrical orthogonal topographic goals for each semimeridian to create the regular state in differing ways. These are performed in one of the following ways: without changing refractive astigmatism; by reducing the associated ocular residual astigmatism; by shifting the less favorably placed topography semimeridian to the other more favorably located one; by shifting both topographic semimeridians to more favorably located sites. This is an alternative when a potential improvement in the best corrected visual acuity is sought and the maximum reduction of astigmatism is not the priority. Results: The calculated treatments necessary to achieve various improved astigmatic states, together with each of their respective separate refractive astigmatism targets, are presented. A single refractive astigmatism value for the entire cornea is also calculated by vector summations Conclusion: Consideration of each of the two distinct hemidivisions of the eye enables improved treatment of irregular astigmatism, potentially resulting in improved visual outcomes.

Clinical outcomes of laser in situ keratomileusis using combined topography and refractive wavefront treatments for myopic astigmatism

PURPOSE: To evaluate outcomes of laser in situ keratomileusis (LASIK) guided by wavefront alone versus wavefront plus topographic data. SETTING: NewVision Clinics, Cheltenham, Australia. METHODS: Twenty‐one eyes (14 patients) were distributed into 2 groups in a prospective double‐masked study. One group was treated by wavefront parameters alone (WF, n = 11), and the other, by wavefront combined with topography values (WF&VP, n = 10) using vector planning. All treatments were performed using Visx Star S4 CustomVue software. In the WF&VP group, the treatment profile was calculated using simulated keratometry readings from the Humphrey Atlas topography and 2nd‐order Zernike coefficients defocus 4 and astigmatism 3 and 5 from the WaveScan wavefront display of the entire eye. RESULTS: Mean corneal astigmatism preoperatively was 1.07 diopters (D) ± 0.54 (SD) in the WF group and 1.50 ± 0.87 D in the WF&VP group. At 6 months, it was 0.67 ± 0.57 D (39% reduction) and 0.83 ± 0.55 D (44% reduction), respectively. The WF&VP group had a greater reduction in horizontal coma. The mean gain in low‐contrast visual acuity under mesopic conditions was 0.06 in the WF group and 0.11 in the WF&VP group and the mean gain in high‐contrast visual acuity, 0.02 and 0.05, respectively. Two patients reported a change in the preferred eye postoperatively to the eye treated using vector planning. No result demonstrated statistical significance. CONCLUSION: The WF&VP group had greater reduction in corneal astigmatism and better visual outcomes under mesopic conditions than the WF group and equivalent higher‐order aberrations.

Astigmatic change 1 year after excimer laser treatment of myopia and myopic astigmatism

Purpose: To evaluate the surgically induced astigmatism (SIA) 1 year after excimer laser photorefractive astigmatic keratectomy (PARK) and photorefractive keratectomy (PRK). Setting: Royal Victorian Ear and Eye Hospital, Melbourne, Australia. Methods: This study comprised 333 PARK patients and 155 PRK patients treated with a VISX 20/20 excimer laser and followed prospectively for 12 months. Vector analysis of the change in astigmatism was used to calculate the SIA in the PRK group and the percentage of astigmatism corrected in the PARK group. Results: Among patients with low cylinders astigmatic correction varied greatly, particularly in those treated for large amounts of myopia. The spherical PRK treatments yielded a mean induced postoperative astigmatism of 0.47 diopter. There was a linear relationship between this inadvertent SIA and increasing myopia. Conclusion: Excimer laser surgery for myopia creates a low degree of random, unpredictable SIA that may be the result of irregular epithelial thickening during postoperative healing. This creates a background noise of astigmatic change upon which the targeted astigmatic correction is superimposed.

Predictability of excimer laser treatment of myopia and astigmatism by the VISX Twenty-Twenty

Background: To determine the predictability of excimer laser photorefractive keratectomy (PRK) to correct myopia, astigmatism, or both between −1.00 and −19.00 diopters (D). Setting: Royal Victorian Eye and Ear Hospital, East Melbourne, Australia. Methods: This study comprised 1218 consecutive eyes treated with a VISX TwentyTwenty excimer laser and followed prospectively for 12 months. Low myopia was treated with one ablation zone (6.0 mm), high myopia with two ablation zones (5.0 and 6.0 mm), and extreme myopia with three ablation zones (4.5, 5.0, and 6.0 mm). Maximum spherical treatment was 15.00 D at the corneal plane. Data were analyzed to determine the predictability of the postoperative outcomes by preoperative refraction. Results: Nine hundred eighty eyes (80.5%) were available for the 12 month follow‐up. The predictability of refraction and uncorrected and best corrected visual acuities progressively decreased with increasing myopia, although a comparable percentage of spherical correction was achieved at each diopter of myopia. The likelihood of losing lines of best corrected visual acuity and corneal haze increased with increasing myopia. Conclusion: These data can be used to counsel patients of likely outcomes of excimer laser PRK to correct myopia.