Outcome Of Refractive Surgery Research Articles

A statistical model of the aberration structure of normal, well‐corrected eyes

A statistical model of the wavefront aberration function of the normal, well-corrected eye was constructed based on normative data from 200 eyes which show that, apart from spherical aberration, the higher-order aberrations of the human eye tend to be randomly distributed about a mean value of zero. The vector of Zernike aberration coefficients describing the aberration function for any individual eye was modelled as a multivariate, Gaussian, random variable with known mean, variance and covariance. The model was verified by analysing the statistical properties of 1000 virtual eyes generated by the model. Potential applications of the model include computer simulation of individual variation in aberration structure, retinal image quality, visual performance, benefit of novel designs of ophthalmic lenses, or outcome of refractive surgery.

Patient outcomes of refractive surgery: The refractive status and vision profile

Purpose To evaluate the performance of a questionnaire, the Refractive Status and Vision Profile (RSVP), in the assessment of patient outcomes following refractive surgery. Setting Patients recruited from 5 refractive surgery practices. Methods The RSVP was self-administered by patients before and 2 to 6 months after bilateral refractive surgery. Information on uncorrected visual acuity (UCVA), refractive error, and self-reported satisfaction with vision was also collected. Changes in total RSVP scores and in the scores of RSVP subscales ( concern, functioning, driving, symptoms, optical problems, glare, and trouble with corrective lenses) were assessed. The relationship between change in the RSVP and subscale scores was assessed in relation to change in traditional clinical measures. The responsiveness of the RSVP to clinically meaningful changes in patients’ vision was measured by calculating its effect size. Results One hundred seventy-six patients completed baseline and postoperative RSVPs and had bilateral refractive surgery. Postoperatively, 92.0% of patients had a UCVA of 20/40 or better in at least 1 eye. Fifteen percent had some worsening in the total RSVP score, and there was substantial variation in the proportion of patients who had worsening in particular subscale scores, ranging from 7.0% who reported worsening in trouble with corrective lenses to 41.5% who reported worsening in driving. Change in satisfaction with vision following surgery was correlated with change in the overall RSVP and subscale scores but not with change in refractive error. A significant worsening in 3 or more RSVP subscales was independently associated with an almost 6-fold (odds ratio 5.84, 95% confidence interval: 1.88,18.13) likelihood of patient report of dissatisfaction with vision, after adjusting for age, sex, preoperative refractive error, and postoperative UCVA. Low scores (ie, minimal dysfunction) on 2 of the RSVP subscales ( physical functioning and optical problems) at baseline were predictive of poor postoperative patient outcomes. The RSVP was very sensitive to the intervention of refractive surgery (effect size of 1.2 to 1.4). Conclusions The RSVP was able to detect clinically relevant changes in functional status and quality of life after refractive surgery. Change in the RSVP score was correlated with change in patient report of satisfaction and was predictive of postoperative patient satisfaction. The RSVP provides a valuable new metric to assess outcomes of refractive surgery.

Power vector analysis of the optical outcome of refractive surgery

Purpose To demonstrate the power vector method of representing and analyzing spherocylindrical refractive errors. Setting School of Optometry, Indiana University, Bloomington, Indiana, USA. Methods Manifest and keratometric refractive errors were expressed as power vectors suitable for plotting as points in a 3-dimensional dioptric space. The 3 Cartesian coordinates ( x, y, z) of each power vector correspond to the powers of 3 lenses that, in combination, fulfill a refractive prescription: a spherical lens of power M, a Jackson crossed cylinder of power J 0 with axes at 90 degrees and 180 degrees, and a Jackson crossed cylinder of power J 45 with axes at 45 degrees and 135 degrees. The Pythagorean length of the power vector, B, is a measure of overall blurring strength of a spherocylindrical lens or refractive error. Changes in refractive error due to surgery were computed by the ordinary rules of vector subtraction. Results Frequency distributions of blur strength ( B) clearly demonstrate the effectiveness of refractive surgery in reducing the overall blurring effect of uncorrected refractive error. Power vector analysis also revealed a reduction in the astigmatic component of these refractive errors. Paired comparisons revealed that the change in manifest astigmatism due to surgery was well correlated with the change in keratometric astigmatism. Conclusions Power vectors aid the visualization of complex changes in refractive error by tracing a trajectory in a uniform dioptric space. The Cartesian components of a power vector are mutually independent, which simplifies mathematical and statistical analysis of refractive errors. Power vectors also provide a natural link to a more comprehensive optical description of ocular refractive imperfections in terms of wavefront aberration functions and their description by Zernike polynomials.

Effect of age on the refractive outcome of myopic photorefractive keratectomy

Purpose To evaluate the effect of age on the outcome of myopic photorefractive keratectomy (PRK). Setting University-based refractive surgery practice. Methods A retrospective analysis of 197 eyes that had PRK for myopia was performed. Four groups of patients were compared: Group 1, patients <30 years (n = 35); Group 2, patients 31 to 40 years (n = 56); Group 3, patients 41 to 50 years (n = 47); Group 4, patients >50 years (n = 59). The percentage of eyes with an uncorrected visual acuity of 20/20 or better and 20/40 or better and the percentage of eyes with spherical correction within ±0.5 and ±1.0 diopter (D) of the attempted correction were derived for each age group. The percentage of patients overcorrected and undercorrected by 1.0 D or more in each age group was also calculated. Results The difference between the percentage of patients who achieved a visual acuity of 20/20 or better in Group 4 and in the younger groups was statistically significant at 3 months ( P = .02) but not at 6 months ( P = .70) and 12 months ( P = .55). The difference between the percentage of patients who achieved an acuity of 20/40 or better was not significant at any time. The difference between the percentage of patients who were within ±0.5 D of intended correction in Group 4 and in the younger groups was statistically significant at 3 months ( P = .001), 6 months ( P = .006), and 12 months ( P = .008); the difference between the percentage of patients who were within ±1.0 D of intended correction was not significant at any time. The difference between the percentage of patients overcorrected and undercorrected by 1.0 D or more in Group 4 and in the younger groups was not significant at any time. Conclusions In the 3 younger groups, age did not significantly affect visual outcome or predictability. However, there was a significant difference between Group 4 and the 3 younger groups in predictability of the refractive outcome at 3, 6, and 12 months. Age may play a role in the outcome of refractive surgery.

Refractive keratotomy in keratoconus suspects.

We utilized videokeratography to determine the frequency of keratoconus suspects in a refractive surgery population, and compared the results of refractive keratotomy in these patients to those of patients with normal corneal topography. The records of 67 consecutive patients (120 eyes) undergoing refractive keratotomy were reviewed retrospectively. Quantitative measurements of relative inferior corneal steepening (inferior-superior value, I-S value) were obtained from preoperative videokeratography. Three of 120 eyes (3 of 67 patients, 4.5%) were identified as keratoconus suspects based on I-S (inferior-superior) values greater than +1.74 (range, +1.76 to +2.00), which represented two standard deviations (+/- 0.66) above the mean (+0.42). All three of these eyes achieved postoperative uncorrected visual acuity of 20/30 or better. Of those eyes with normal preoperative topography (I-S value +1.74 or less), 90 of 112 (80%) achieved postoperative uncorrected visual acuity of 20/30 or better, and 105 of 112 (94%) achieved 20/40 or better. Although long-term follow up is needed to confirm stability of refractive surgery outcome, it appears that keratoconus suspects can obtain acceptable results with refractive keratotomy. Studies on larger patient populations using videokeratography are needed to better define keratoconus, keratoconus suspects, and normal variants of corneal topography and to determine how eyes with abnormal topography respond to refractive keratotomy.

Measurement of Elastic Modulus of the Central Bovine Cornea by Means of Holographic Interferometry. Part II. Results

The structural and elastic properties of the cornea play an important role in the outcome of corneal refractive surgery. A way to describe the elastic properties is to calculate Young's modulus. The tangent Young's modulus (Eit) represents the first part of the viscoelastic response of the intact cornea due to intraocular pressure increase. The tangent Young's modulus (Eit) of the central cornea of 16 fresh enucleated intact bovine eyes determined from the immediate response was calculated by means of holographic interferometry. The stress was an increase of the intraocular pressure. In 16 eyes the change of intraocular pressure was 10 Pa (group I), starting with an intraocular pressure of 1340 Pa (about 10 mm Hg). In 6 of these 16 eyes there was a second pressure increase of 20 Pa starting at the same intraocular pressure of 1340 Pa (about 10 mm Hg) (group II). The mean value for the Young's modulus in group I was 1.58 x 10(5) Pa. For group II the Young's modulus was 1.60 x 10(5) Pa. The difference was not significant. The values for the Young's moduli of the cornea of intact eyes are about 100 times smaller than calculations based on experiments with strips of the cornea. It is necessary to consider the localization of the corneal tissue to avoid influences of corneal hydration or dehydration and to define the existing intraocular pressure if Young's modulus of the cornea is to be calculated.

Corneal topography: a new photokeratoscope.

The assessment of the topography of the cornea is an important modality in the management of surgical procedures complicated by astigmatism. Additionally, the outcome of corneal refractive surgery is often best illustrated by photokeratoscopy. Currently available photokeratoscopes suffer from a number of deficiencies that include (i) the lack of topographic detail from the central cornea, (ii) the difficulty in transporting these units, (iii) the cost of such units. A photokeratoscope which addresses these shortcomings is described.