Though there have been many large-scale U.S. Food and Drug Administration trials of the devices used for refractive surgery, it takes the small research groups to drill down deeper into the various effects of such procedures as LASIK and PRK. To find out about the latest research on many facets of refractive surgery, from corneal inlays to laser procedures, read on.

Preop Considerations

In an effort to improve centration during refractive procedures, researchers from New Jersey performed a study to get an idea of the location of the corneal apex.

In the study, the investigators determined the corneal apex of 68 eyes of 41 refractive surgery patients. They used the location of three possible proxies for the true corneal apex derived from the patients’ preoperative Pentacam maps: Kmax; thinnest local pachymetry and maximum anterior elevation compared to the geographic center. Kmax and the thinnest local data points were directly output by Pentacam, while the maximum anterior elevation was estimated by directing the cursor over the map and recording the x and y locations.

They found that all three measures were inferotemporal to the geographic center of the cornea. Specifically, the average Kmax was located 0.006 ±0.989 mm (r: -2.71 to 3.33) temporal and 0.663 ±1.55 mm inferior (r: 3.67 to 2.23) to the geographic center. Thinnest local pachymetry was 0.377 ±0.284 mm (r: -0.53 to 1.03) temporal and 0.197 ±0.249 mm (r: -1.13 to 0.16) inferior to the center. Maximum anterior elevation was located 0.186 ±1.01 mm (r: -2.92 to 2.14 mm) temporal and 0.368 ±0.635 mm (r: -1.74 to 1.05 mm) inferior compared to the map center. The researchers say that the average of these three proxies of the true corneal apex suggests that the average apex location is 0.19 mm temporal and 0.409 mm inferior to the geometric apex and, of the three indicators, thinnest pachymetry data was the least variable.4195

Researchers in Paris say that, though the predicted values of corneal removal as calculated preop by the AMO CustomVue system were higher than the actual postop values, the values are predictable.

The study included 60 eyes of 30 myopes who had wavefront-guided LASIK with flap creation using the IntraLase laser (corneal flap: 120 µm) and a refractive ablation using the Visx Star S4 IR. At one month postoperatively, inclusion criteria were uncorrected visual acuity 20/20 and ±0.50 D sphere. The physicians measured corneal pachymetry with Visante optical coherence tomography and Pentacam preoperatively and then at one month postoperatively. Central corneal ablation was calculated by making the subtraction of postop pachymetry from preop. The researchers compared these values to the theoretical values planned by the CustomVue profile of ablation. They also measured the corneal flap thickness and the stromal posterior bed thickness using OCT and compared them to the theoretical expected values.

Preoperatively, the mean value of CustomVue theoretical corneal photoablation was 76 ±14 µm. Mean value of corneal thinning obtained on OCT was 68 ±11 µm. The mean value of corneal thinning obtained by Pentacam was 72 ±20 µm. The difference was significant (p=0.03) between CustomVue and OCT but not significant (p=0.45) between CustomVue and Pentacam, say the researchers. The average thickness of the corneal flap was 123.6 ±1.8 µm. The mean theoretical value calculated for the posterior stromal bed preop was 350 ±35 µm, while, postop, the mean value of the posterior bed on OCT was 354 ±39 µm. The difference wasn’t significant (p=0.17). The researchers say that secondary epithelial hyperplasia from the central corneal flattening may be behind the differences between the preop theoretical values and those measured postop.5654

After the results of a small case study, researchers from Los Angeles say refractive surgeons should be aware of a possible contraindication for LASIK in a group of patients that was previously above suspicion.

The surgeons say that, though the rapid deposition of dystrophic corneal deposits in the lamellar interface following LASIK surgery has been reported almost exclusively in individuals of Korean ancestry with granular corneal dystrophy, type II (Avellino corneal dystrophy), they’ve seen two cases of accelerated deposition of the transforming growth factor beta-induced protein (TGFBIp) after LASIK in two individuals of non-Korean ancestry.

In the first case, a 51-year-old Chinese man with no clinically evident corneal opacities and without a family history of a corneal dystrophy underwent an uncomplicated bilateral LASIK procedure. Two years after surgery, dense polymorphic opacities developed in the lamellar interface, resulting in reduced visual acuity and glare.

In the second case, a 48-year-old Japanese woman with an extensive family history of presumed granular corneal dystrophy underwent uncomplicated bilateral LASIK and an enhancement procedure one year later in each eye. Three years after the enhancement procedure, the patient noted decreased vision and significant glare secondary to confluent granular interface opacities. Laser PTK on the stromal bed eight years after the enhancement procedure didn’t improve the patient’s symptoms. Genetic screening of TGFBI in both patients revealed p.Arg124His, which is diagnostic of granular corneal dystrophy, type II.

The researchers recommend that, prior to refractive surgery, genetic analysis be performed in any patient who has a history of TGFBI dystrophy in his family, since dystrophic deposits may not appear until after LASIK is performed.4649

Surgeons from Italy say that, in their particular area, when patients inquire about refractive surgery, their level of knowledge is inadequate, and they need a very thorough informed consent to set them straight.

The surgeons asked 200 consecutive refractive surgery candidates to fill out a standardized questionnaire before receiving any type of refractive surgery information from the outpatient center’s staff. Most of them, 87 percent, had very little information about the risks of refractive surgery and 72 percent expected to get perfect results. Ninety-four percent were interested in surgery for functional reasons, with the rest motivated by cosmetic reasons.

They found that a third of the patients had been referred by an ophthalmologist, 28 percent had their first exposure to refractive surgery through the media and 39 percent had been informed about the surgery by friends. Sixty-seven percent decided on their own to make an appointment for a surgical evaluation, 5.5 percent were prompted by friends to do so and 27.5 percent say other sources, including the Internet, motivated them to make the appointment.

The researchers say that, because the patients have such high expectations but low knowledge of the risks, it’s imperative that the refractive surgery team fill them in on both of these areas beforehand.5370

Postop Outcomes

Researchers from Canada and the United Arab Emirates say they’ve found some differences in outcomes between thin-flap LASIK with the IntraLase and wavefront-guided surface treatments. One of the authors is a consultant for Abbott Medical Optics.

In the retrospective study, the surgeons analyzed 240 surface ablation patients and 137 patients who underwent thin-flap LASIK (AMO’s iLASIK). Further, the surface-ablation group was divided by random selection into two equal groups (120 each) who either had an epithelial flap created and retained postop (wASA) or who didn’t have such a flap (nASA).

The researchers say there was no significant difference in the efficacy of the treatment of sphere and cylinder between ASA and iLASIK, even when considering wASA and nASA separately. Spherical aberration increased significantly in all groups postop (iLASIK=0.038 ±0.098 µm, wASA=0.112 ±0.083 µm and nASA=0.111 ±0.094 µm) with the iLASIK surgery having a significantly smaller increase than the ASA groups (p<0.001).
Improvements in uncorrected acuity weren’t significantly different between the iLASIK and ASA groups, even when considering wASA and nASA separately. Uncorrected minimum angle of resolution at three months was significantly better (p=0.002) for the wASA group (1.14 ±0.19 min. arc, closer to 20/20) than the other two groups (nASA=1.18 ±0.23 and iLASIK=1.22 ±0.19 min. arc, closer to 20/25 than the wASA group). The higher-order aberration results mirrored SA results across the groups, say the investigators.4208

Researchers from Houston’s Baylor College of Medicine have quantified the effect of refractive surgery on the size of the optical zone postop.

The investigators reviewed 72 eyes that underwent wavefront-guided myopic PRK and 30 eyes that underwent either hyperopic LASIK or hyperopic PRK with the AMO CustomVue system. Subjects that had wavefront measurements with
≥ 6 mm pupil before and at least three months after LASIK or six months after PRK were included. Assuming full correction of 2nd-order aberrations, using the Zernike Tool program, the researchers calculated the polychromatic modulation transfer function at 9 cyc/deg with Stiles-Crawford effect for 2- to 6-mm pupils. The functional optical zone was defined as the zone over which the MTF at 9 cyc/deg was ≥0.18. The researchers say that this criterion is consistent with 20/20 or better vision.

In the 72 eyes that underwent myopic PRK, the amount of refractive correction was -3.71 ±1.75 D (r: -0.63 to -7.69 D). Ninety-nine percent of eyes (71/72) had a functional optical zone ≥6 mm preoperatively, but only 65 percent (47/72) had a FOZ ≥6 mm postoperatively. For one eye with a preop FOZ <6 mm (5.8 mm), the postop FOZ was 5.5 mm.

In the 30 eyes that underwent hyperopic LASIK/PRK, the refractive correction was 0.84 ±0.57 D (r: 0.35 to 1.76 D). Seventy-seven percent of eyes (23/30) had a FOZ ≥6 mm preop, and 73 percent (22/30) had FOZ ≥6 mm postop. For seven eyes who had a preop FOZ <6 mm, five had an increased postop FOZ of at least 6 mm.

In both groups, the researchers say that the postop FOZ decreased with increasing magnitude of 4th-order spherical aberration and higher-order aberrations (p<0.05), indicating that aberrations play an important role in determining the size of the FOZ.4212

In a retrospective study, researcher’s at Brazil’s University of São Paulo have compared the spherical aberration and contrast sensitivity outcomes of wavefront-guided LASIK and wavefront-guided PRK.

The researchers reviewed the charts of 70 eyes (35 patients) who underwent custom LASIK or custom PRK for myopia up to 5 D and astigmatism up to 1.5 D.

The mean pretreatment spherical aberration was 0.13 ±0.07 µm in the LASIK group and 0.12 ±0.08 µm in the PRK group (p=0.344). In the LASIK group, the mean SA was 0.19 ±0.10 µm after 12 months postoperatively, and in the WFG-PRK group, 0.17 ±0.11 µm (p=0.089). Photopic contrast sensitivity improved significantly for most frequencies for both techniques, say the researchers. No significant improvement was found for most mesopic contrast sensitivity frequencies for either group.

The physicians say that, despite SA induction, photopic contrast sensitivity has improved after WFG-LASIK and WFG-PRK without a significant difference between them, and that changes in low levels of SA root mean square wavefront error poorly predict visual performance.3969

Researchers from Greece say that PRK has an effect on the sensitivity of the peripheral visual field.

Researchers evaluated 32 eyes of 18 patients preop and at three months postop following PRK for the correction of myopia (mean: -2.75 ±0.97 D) at an optical zone of 6.5 mm with the Allegretto 400 (Alcon/WaveLight) laser. They evaluated the visual fields using a full threshold procedure with stimulus size III (26 arc min.) and a background illumination of 10 cd/m2. They performed the measurements under cycloplegia. For the central degrees of the optical field (1 to 22 degrees) the refracted error was completely corrected. The researchers say that the marked cycloplegic effect necessitated the use of a +3 D auxiliary lens to cover the distance of the stimulus (33 cm).

The investigators say that the detection threshold value exhibited a decrease ranging from 0.5 to 9 dB (mean threshold decrement: 2.65 ±1.85 dB) for the periphery of the optical field (30 to 40 degrees). This change was statistically significant. In particular, in the peripheral 40 degrees of the visual field the threshold mean slope, which indicates field progression, changed with a depression of 3.86 ±2.65 dB. No statistically significant difference was observed for the central visual field (0 to 22 degrees).

The physicians say that, since visual field loss can occur due to disorders of the eye such as glaucoma, it’s important to take into account whether a patient has undergone refractive surgery in the past.4205

Cross-linking

Researchers in Switzerland and Italy say that, after 18 months, corneal cross-linking appears to be maintaining its effectiveness in cases of ectasia.

In the study, 23 eyes of 23 patients with progressive ectasia were treated with cross-linking composed of creation of an epithelial abrasion, application of riboflavin 0.1% and then irradiation by UVA light.

Eighteen months after cross-linking, mean best-corrected acuity had improved from 0.47 ±0.29 to 0.77 ±0.21 on the decimal scale (a little worse than 20/40 to just below 20/25 on the Snellen chart) (p<0.05). The mean UCVA and spherical equivalent refraction didn’t show significant differences at 18 months compared to baseline. The topography maps exhibited reduced steepest points as early as four months following treatment, as well as stability of keratometry readings over at least six months. Mean central corneal thickness decreased significantly (p<0.05) from 437 ±45 µm prior to cross-linking to 422 ±38 µm at 18 months. Similarly, an evaluation of the mean thinnest point pachymetry showed a significant decrease from 428 ±47 µm to 412 ±44 µm. Corneal volume also decreased significantly (p<0.05) from a preoperative 57.44 ±2.58 mm3 to 55.80 ±3.25 mm3. Although mean spherical aberration and mean corneal coma decreased during the 18 months, the differences were not statistically significant when compared with preop data, say researchers.2863

Corneal Inlays

ReVision Optics researchers say that, after two years, the company’s PresbyLens corneal inlay has remained safe and biocompatible.

The PresbyLens is a hydrogel corneal inlay that’s implanted beneath a LASIK-style flap for the treatment of presbyopia. In this study, investigators implanted 1.5-mm PresbyLenses in the eyes of eight emmetropic presbyopes (mean age 52 years; r: 45 to 60). Their mean preop spherical equivalent was +0.31 D (r: 0 to +0.75 D), and they used a mean near add of +2.03 D (r: +1.5 to +2.5 D). At the two-year follow-up point, visual acuity was measured using an Optec 6500 Vision Tester and a slit-lamp exam was performed to determine corneal clarity. In a companion study, 12 Yucatan mini pigs were implanted with the PresbyLens in a microkeratome-created corneal pocket. The researchers performed slit-lamp exams throughout the study and histology on sacrificed pigs at six months and one year.

In the human study, all eyes implanted retained 20/32 or better uncorrected near acuity at two years postop, though no eyes achieved 20/40 or better at near preoperatively. The mean gain in UCNVA was 3.6 lines (r: 1.6 to five lines). The mesopic uncorrected distance acuity was minimally affected with a maximum of three lines lost in the implanted eye. On average, UCDVA in photopic conditions was 1.2 lines better than in mesopic conditions. Binocular mesopic UCDVA was 20/25 or better for all patients.

The slit-lamp exam revealed clear corneas with very mild edge haze. In the porcine study, the implanted eyes remained clear without any reaction to the inlay, say the investigators. They add that slit-lamp exam at one year revealed clear corneas and histology data suggest that the PresbyLens inlay appears to be inert.813

In other PresbyLens research, researchers at ReVision Optics and a surgeon from Mexico say there may be slight differences in outcomes depending upon the type of flap the inlay is implanted beneath.

In the study, the investigators cut LASIK-style corneal flaps in 39 presbyopic emmetropes using a mechanical microkeratome (Hansatome, Bausch + Lomb) and in 20 patients using the IntraLase laser. The mean preoperative spherical equivalent was 0.20 D for the microkeratome patients and 0.07 D for the IntraLase group. All patients received a 2-mm diameter PresbyLens.

The investigators say the only statistically significant finding was that postop uncorrected near visual acuity was better for the IntraLase group (p<0.05), at LogMAR 0.4 ±0.09 (a little worse than 20/20) vs. 0.15 ±0.15 for the microkeratome group (a little worse than 20/25). However, distance acuity was better and more improved for the microkeratome group, albeit at a non-statistically significant level (p<0.1). OCT images showed no difference in mean flap bed radii of curvatures. The microkeratome radius averaged 7.8 ±0.27 mm and the Intra-Lase averaged 7.9 ±0.17 mm. Despite the small differences, the researchers say that both flap-making methods allowed the PresbyLens to yield good visual results.2867

Dr. Probst is the national medical director for TLC Vision.