Advances in refractive surgery: 1975 to the present

Small incision lenticule extraction (SMILE) lenticule thickness readout compared to change in axial length measurements with the IOLMaster

PURPOSE

To compare theoretical values from the small incision lenticule extraction (SMILE) lenticule thickness readout with change in axial length measurements taken with the IOLMaster.

METHODS

We prospectively studied 214 eyes from 107 patients undergoing bilateral SMILE surgery for myopia or myopic astigmatism between December 2014 and May 2017 at an ophthalmological practice in Singapore. All eyes were examined pre-operatively and 1 and 3 months post-operatively with the IOLMaster following SMILE surgery. Achieved lenticule thickness was taken as the change in axial length after surgery. A linear mixed-effects model was used to examine changes in axial length, spherical equivalent and acuity over time. The relationships between change in axial length and theoretical lenticule thickness and spherical equivalent were examined with multiple linear regression analyses, and model prediction was assessed with adjusted R² statistics.

RESULTS

Mean (95% confidence interval [CI]) spherical equivalent pre-operatively was - 5.25 (95% CI - 5.38 to - 5.12) diopters (D), at 1 month was 0.04 (95% CI - 0.09 to 0.17) D (p < 0.001), and at 3 months was - 0.02 (95% CI - 0.15 to 0.11) D (p < 0.001). Mean (95% CI) pre-operative axial length was 27,726 (95% CI 25,595 to 25,857) μm. Post-operative axial length at 1 month was significantly shorter at 25,595 (95% CI 25,464 to 25,726) μm (p < 0.001) with no change thereafter (p = 0.647). Pre-operative mean ± standard deviation (SD) refractive target was 0.24 (± 0.3) D, and mean difference between target and post-operative spherical equivalent at 1 month was 0.20 D (95% CI 0.16 to 0.25 D, p < 0.001). Multiple regression analysis showed that change in axial length at 1 month was, on average, 5% lower than theoretical lenticule thickness, indicating an average difference of 5.4 μm (95% CI 5.2 to 5.6 μm). Preoperative spherical equivalent predicted negative association with change in axial length at 1 month (β = - 14.8, 95% CI - 18.2 to - 11.3, adjusted R² = 0.457, p < 0.001).

CONCLUSIONS

Calculated lenticule thickness values were less than expected, and post-operative refractive outcomes at 1 month showed a slight under-correction. Further research in this area is needed to validate these findings.

Induction of Controlled Wound Healing with PMMA Segments in the Deep Stroma in Corneas of Hens

Purpose

Polymethylmethacrylate (PMMA) segments are normally used in additive surgery to treat both corneal ectasia post laser-assisted in situ keratomileusis and keratoconus. The aim of this work was to develop an experimental animal model to induce wound healing in the deep stroma in corneas of hens.

Methods

PMMA segments were implanted in the right eyes of 40 adult hens without suture in the wound incision. Animals were randomized for 5 time points to histopathology study (1, 3, 15, 30, and 90 days) being clinically evaluated during the experiment.

Results

Thirty-four eyes (85%) had a successful clinical outcome and intraoperative mistakes occurred in 6 eyes (15%), including anterior chamber perforation resulting in abscess (1), excess of lamellar dissection with segment migration (3), and peripheral incisions evolving with neovascularization (2). At 24 hours, all the epithelial injuries were completed in integrity, but a persistent stromal incision, with a fish mouth form, was observed until day 15. Corneal edema disappeared at the fifth day. Haze, keratocyte cell death, keratocyte proliferation, myofibroblast differentiation, and new matrix production were observed in length around the segment.

Conclusions

Wound healing was induced in the deep corneal stroma by means of PMMA segment implantation in a new animal model developed in hens.

Evaluation of Changes in Human Corneas After Femtosecond Laser-Assisted LASIK and Small-Incision Lenticule Extraction (SMILE) Using Non-Contact Tonometry and Ultra-High-Speed Camera (Corvis ST)

PURPOSE

To evaluate theoretical biomechanical advantages of human corneas treated with small incision lenticule extraction (SMILE) compared with femtosecond laser-assisted LASIK (FS-LASIK) Patients and methods: In a prospective, comparative, non-randomized, consecutive case series patients with moderate to high myopia and/or astigmatism underwent corneal refractive surgery. Patients either received standard FS-LASIK or SMILE. Preoperatively and up to 3 months postoperatively data were analyzed including tomography with Pentacam HR, Goldmann tonometry, non-contact tonometry, and deformability of the cornea using an ultra-high-speed camera (Corvis ST). Data were analyzed and p < 0.05 was considered as statistically significant (t-test).

RESULTS

Seventy-three patients (128 eyes) were treated. Forty-eight eyes of 29 patients underwent FS-LASIK and 80 eyes of 44 patients underwent SMILE. Preoperative spherical equivalent (SE) was -3.23 ± 1.64 D (FS-LASIK group) and -4.83 ± 1.63 D (SMILE group) (p < 0.0001). Almost all parameters obtained by Corvis ST between preoperative and postoperative measurements showed significant changes after refractive surgery. Significant changes were seen between both groups postoperatively. However, matched pair subgroup analysis (n = 69; 26 eyes FS-LASIK; 43 eyes SMILE) of eyes with initially equal pachymetry, intraocular pressure, SE, and difference of pre- to postoperative pachymetry (p>0.05), showed no significant changes in parameters measured with Corvis ST (p>0.05).

CONCLUSIONS

Corneal biomechanical parameters measured preoperatively with Corvis ST showed significant differences postoperatively in total and in both groups. In subgroup analysis with homogenous groups, FS-LASIK showed no significant changes in biomechanical data measured with Corvis ST compared with SMILE.

Assessment of axial length before and after myopic LASIK with the IOLMaster

PURPOSE

To assess the relationship between theoretical ablation depth and axial length change after LASIK with the IOLMaster (Carl Zeiss Meditec, Dublin, CA).

METHODS

Ninety-nine eyes were examined preoperatively and 1 and 3 months after LASIK.

RESULTS

Mean ± standard deviation (SD) spherical equivalent before LASIK was -4.06 ± 1.91 diopters (D). Mean ± SD ablation depth was 83.13 ± 30.31 μm. Mean ± SD postoperative axial length of 25.11 ± 0.14 mm at 1 month was significantly shorter than mean ± SD preoperative axial length of 25.20 ± 0.14 mm (P < .001) with no subsequent change thereafter (P = .450). An increase in ablation depth of 1 μm led to a decrease in axial length of 0.00118 ± 0.00005 mm. Ablation depth correlated strongly with change in axial length (adjusted R(2) = 0.9039).

CONCLUSIONS

The IOLMaster showed a decrease in axial length after LASIK that correlated well with theoretical ablation depth.

Comparison of the predictability, uniformity and stability of a laser in situ keratomileusis corneal flap created with a VisuMax femtosecond laser or a Moria microkeratome

This prospective study compared the predictability, uniformity and stability of laser in situ keratomileusis corneal flap thickness created by a femtosecond laser or a classic microkeratome. Twenty-five femtosecond laser (VisuMax, Carl Zeiss Meditec) flaps and 38 microkeratome (Moria M3) flaps were measured using anterior segment optical coherence tomography at 1 week, 1 month and 6 months postoperatively. Flap thickness at seven points on each of the four meridians was calculated. At 6 months, VisuMax flaps showed better prediction than Moria flaps for central flap thickness. The standard deviation within individual flaps was smaller for VisuMax flaps and their index of symmetry was better. The mean thicknesses among the four eccentricities in the VisuMax flaps were the same, while Moria flaps were thicker at the 3-mm radius compared with the centre. The VisuMax femtosecond laser created corneal flaps with better predictability and uniformity, and similar reproducibility and stability, compared with the microkeratome.

Repeatability of laser in situ keratomileusis flap thickness measurement by Fourier-domain optical coherence tomography

PURPOSE

To evaluate the repeatability of Fourier-domain optical coherence tomography (OCT) measurements of the thickness of femtosecond laser-created laser in situ keratomileusis (LASIK) flaps.

SETTING

Doheny Eye Institute, University of Southern California, Los Angeles, California, USA.

DESIGN

Case series, evaluation of diagnostic technology.

METHODS

In this consecutive series, Fourier-domain OCT (RTVue) was used to measure flap thickness 1 week after LASIK. Flaps were created with a Pulsion 60 kHz femtosecond laser programmed for 110 μm flap thickness. Each eye was scanned 2 times with a radial pachymetry pattern and 1 time with a horizontal line scan. Flap thicknesses were measured at 6 positions across the corneal flap (ie, ± 0.5 mm, ± 1.5 mm, and ± 2.5 mm from the center on the horizontal and vertical meridians). The within-grader flap thickness repeatability and between-grader reproducibility were calculated by pooled standard deviations (SDs).

RESULTS

Twenty-one eyes were measured. The mean flap thickness measurements were highly predictable at all positions. Thickness SDs varied from 5.3 to 9.5 μm and uniformity, from 121.7 to 126.5 μm. The within-grader repeatability was 3.3 to 6.4 μm based on the same image measured at different times and 4.7 to 7.4 μm for different images. The between-grader reproducibility was 4.0 to 9.0 μm. There was no statistically significant asymmetry between the nasal side and the temporal side, the superior side and the inferior side, or the pericentral area and the central area of the corneal flap.

CONCLUSIONS

The femtosecond laser created LASIK flaps with uniform and predictable thicknesses. Fourier-domain OCT gave highly repeatable flap-thickness measurements.

FINANCIAL DISCLOSURE

Drs. Rosas Salaroli, Zhang, Branco Ramos, and Allemann have no financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Effects of femtosecond laser-assisted multilayer intrastromal ablation in the midperipheral cornea

We describe the femtosecond laser-assisted midperipheral multilayer intrastromal ablation technique, which combines the benefit of radial keratotomy, ie, not disturbing the optical zone, with currently available technology to correct myopia. The technique was performed in 2 patients, both of whom experienced a decrease in mean corneal power. In 1 patient, the manifest refraction improved, with a 1-line increase in uncorrected visual acuity 1 day postoperatively. The multilayer intrastromal ablation technique, which flattens the cornea and decreases corneal power, can be used to correct moderate myopia without injuring the corneal epithelium.

Advances in technologies for laser-assisted in situ keratomileusis (LASIK) surgery

Laser-assisted in situ keratomileusis (LASIK) has become the most widely used form of refractive surgery today. The objective of this surgical technique is to modify the anterior corneal shape by ablating tissue from the stroma by means of the excimer laser after creating a hinged corneal flap. This way, we are able to change the refractive status of the patient, providing better unaided vision. Continuous improvements in the original technique have made the surgical procedure safer, more accurate and repeatable. These progressions are due to the development of novel technologies that are the responsible for new surgical instrumentation, which makes the surgical procedure easier for the surgeon, and better excimer laser ablation algorithms, which increase the optical quality of the ablation and thus the safety of the vision correction procedure. This article aims to describe the more relevant advances in LASIK that have played an important role in the spread and popularity of this technique.

Outcomes of radiofrequency in advanced keratoconus

PURPOSE

To evaluate the use of radiofrequency energy to correct advanced keratoconus.

SETTING

Universidade Federal de Minas Gerais, Belo Horizonte, and Universidade de Ciências da Saúde de Alagoas, Maceió, Brazil.

METHODS

In this prospective comparative study, radiofrequency was applied to 25 eyes of 21 consecutive patients. One group comprised patients with a K-reading between 54.0 diopters (D) and 58.0 D; 8 thermal spots were placed at the 4.0 mm optical zone. The other group comprised patients with a K-reading greater than 58.0 D; 16 spots were applied at the 4.0 mm and 5.0 mm optical zones. The minimum follow-up was 18 months in all patients. Differences between preoperative and postoperative uncorrected visual acuity, best spectacle-corrected visual acuity (BSCVA), manifest refraction, and K-readings were clinically and statistically evaluated.

RESULTS

At end of the 18-month follow-up, the mean BSCVA in the 8-spot group improved from 20/100 (0.71+/-0.25 logMAR) preoperatively to 20/40 (0.32+/-0.11 logMAR) and in the 16-spot group, from 20/200 (1.03+/-0.30 logMAR) to 20/60 (0.62+/-0.22 logMAR). The mean manifest refractive spherical equivalent (MRSE) improved from -7.70 D+/-5.20 (SD) preoperatively to -6.82+/-4.41 D after 18 months in the 8-spot group and from -11.33+/-6.70 to -8.38+/-5.12 D, respectively, in the 16-spot group. The mean best contact lens-corrected visual acuity was 20/30 (0.18+/-0.24 logMAR) in the 8-spot group and 20/40 (0.31+/-0.19 logMAR) in the 16-spot group. A dense corneal scar was seen in 1 patient in the 16-spot group at the 6-month follow-up.

CONCLUSIONS

Radiofrequency appeared safe for the treatment of advanced keratoconus. Contact lens fitting was stable in all cases.

A Longitudinal Study of LASIK Flap and Stromal Thickness with High-speed Optical Coherence Tomography

OBJECTIVE

To assess corneal anatomic changes after LASIK with a high-speed corneal and anterior segment optical coherence tomography (CAS-OCT) system.

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Fifty-one eyes of 26 healthy persons undergoing LASIK.

METHODS

The CAS-OCT prototype operated at a 1.3-mum wavelength and 2000 axial scans/second. The corneas were scanned with a flap profile pattern (horizontal line, 512 axial scans) and a flap map pattern (4 radials, 256 axial scans each). Both patterns are 8 mm long and are centered on the corneal vertex. LASIK flaps were created using either a mechanical microkeratome (Hansatome; Bausch & Lomb, Inc., Rochester, NY) or a femtosecond laser (Pulsion; IntraLase Corp., Irvine, CA). Intraoperative pachymetry was performed using a 50-MHz ultrasound probe. Three OCT scans were obtained on preoperative and post-LASIK visits up to 6 months. An automated algorithm was developed to process the OCT images and to calculate corneal, flap, and stromal bed thickness profiles and maps. The profiles and maps were divided into central (diameter, <2 mm), pericentral (2-5 mm), and transitional (5-7 mm) zones for analysis.

MAIN OUTCOME MEASURES

Corneal, flap, and stromal bed thicknesses as determined by OCT and ultrasound pachymetry.

RESULTS

The flap interface was best detected in the pericentral zone. One week after surgery, the repeatability of OCT flap and stromal bed thickness measurement was 2 to 7 microm by pooled standard deviation for zones inside a 5-mm diameter. The central flap thickness in 24 Hansatome eyes with a 180-microm setting was 143+/-14 microm by OCT and 131+/-17 microm by ultrasound. In the 8 IntraLase cases with a 120-microm setting, it was 156+/-11 microm by OCT and 160+/-19 microm by ultrasound. Eyes with other settings also were analyzed. There were small systematic changes in flap thickness up to 1 week and bed thickness up to 3 months.

CONCLUSIONS

We have developed a method for using high-speed OCT to measure LASIK flap thickness after surgery. The measurement is noncontact, rapid, and repeatable. Profile and map measurements provide more information than point measurements previously demonstrated. This could be valuable for planning LASIK enhancement and characterizing microkeratome performance.

High-speed optical coherence tomography for management after laser in situ keratomileusis

PURPOSE

To report applications of optical coherence tomography (OCT) in the management of laser in situ keratomileusis (LASIK) related problems.

SETTING

Doheny Eye Institute and Department of Ophthalmology, Keck School of Medicine of the University of Southern California, Los Angeles, California, USA.

METHODS

Five patients referred for LASIK-related problems were enrolled in a prospective observational study. Clinical examination, ultrasound (US) pachymetry, Placido ring slit-scanning corneal topography (Orbscan II, Bausch & Lomb), and high-speed corneal OCT were performed.

RESULTS

In cases of regression and keratectasia, OCT provided thickness measurements of the cornea, flap, and posterior stromal bed. Locations of tissue loss and flap interface planes were identified in a case with a recut enhancement complication. The information was used to determine whether further laser ablation was safe, confirm keratectasia, and manage complications. Optical coherence tomography measurements of central corneal thickness agreed well with US pachymetry measurements (difference 6.4 microm +/- 11.7 [SD]) (P = .026), while Orbscan significantly underestimated corneal thickness (-67.5 +/- 72.5 microm) (P = .17).

CONCLUSIONS

High-speed OCT provided noncontact imaging and measurement of LASIK anatomy. It was useful in monitoring LASIK results and evaluating complications.

Comparison of Corneal Thickness Measured With Optical Coherence Tomography, Ultrasonic Pachymetry, and a Scanning Slit Method

PURPOSE

To investigate the relationship between 1310-nm optical coherence tomography (OCT), an ultrasound pachymeter, and the Orbscan II (Bausch & Lomb, Rochester, NY) in corneal thickness measurements before and after LASIK.

METHODS

Twenty-eight eyes of 14 patients with a mean age of 39.9 +/- 8.6 years underwent LASIK for correction of myopia with or without astigmatism. Central corneal thickness was measured with real-time 1310-nm OCT and Orbscan preoperatively and 1 day, 1 week, and 1 month postoperatively, and with an ultrasound pachymeter preoperatively and 1 month postoperatively.

RESULTS

Measurements of corneal thickness by all instruments correlated well with one another pre- and postoperatively (r value range: 0.87 to 0.94, P<.05); however, there were significant differences among these methods (ANOVA: P<.005). Baseline readings for ultrasound and Orbscan were similar, as were readings 1 month postoperatively for OCT and Orbscan (P>.05). Thickness measured by OCT was thinner than for other instruments preoperatively (P<.005) and thicker than Orbscan postoperatively (P<.005). Significant corneal thinning (P<.001) was found with all study devices 1 day after laser ablation. Significant corneal thinning (P<.005) was found with OCT and Orbscan 1 week compared to 1 day postoperatively, and thickening (post hoc: P<.001) was evident at 1 month compared to 1 week postoperatively.

CONCLUSIONS

Although the instruments correlated well in measuring corneal thickness after LASIK, there were significant differences. All instruments indicated corneal swelling and recovery during the first postoperative week and thickening at 1 month.

Laser applications for corneal disease

The clinical use of the carbon dioxide (CO2) laser and diode laser is increasing in veterinary medicine. New applications for their use are being explored, including ophthalmic applications. The use of lasers for small-animal corneal disease is fairly limited due to several factors. The ideal laser for corneal use is the excimer laser due to its extremely precise photoablative capability. However, the excimer laser is unlikely ever to become practical for veterinary use. The frequency of corneal disease in small animals in which tissue ablation is indicated is relatively low. And for most of these diseases, routine surgical techniques work as well or better than laser ablation. The CO2 laser can be used on corneal tissue, but must be used very cautiously so as not to ablate too deeply, creating serious scarring or perforation. There are also concerns regarding its effect on corneal nerves, stromal collagen, and corneal endothelium. The CO2 laser can be very effective in ablating limbal tumors with corneal extension. The use of the laser is less invasive, technically less difficult, and faster because of excellent hemostasis. The diode laser, due to its high melanin absorption, can be used effectively to ablate epibulbar melanomas with corneal stromal invasion.

Reliability of Corneal Thickness and Endothelial Cell Density Measures

PURPOSE

We evaluated the reliability and agreement between the Orbscan, an ultrasonic pachymeter (Humphrey 855), and the Konan SP 9000-LC in terms of central corneal thickness. The Konan was also used to study the reliability and agreement between endothelial cell density measures.

METHODS

Twenty-five normal subjects were examined on two occasions (mean separation = 9 +/- 5 days) by a single examiner using all three instruments for central corneal pachymetry. The Konan Center Method and a manual counting method were performed by two examiners to determine endothelial cell density. Reliability and agreement were assessed by calculating the 95% limits of agreement (LoA) and intraclass correlation coefficients (ICC).

RESULTS

For corneal pachymetry test-retest reliability, the 95% limits of agreement were -20 to +17 microm for the ultrasound, -27 to +22 microm for the Konan, and -13 to +13 microm for the Orbscan. There was fair-to-good agreement between the pachymeters (intraclass correlation coefficients range = 0.85 to 0.92). For endothelial cell density test-retest reliability, the 95% limits of agreement for the Konan Center Method was -498 to +530, and -482 to +333 cells/mm2 for examiners 1 and 2, respectively. The test-retest 95% limits of agreement for the manual overlaid grid method was -355 to +355, and -535 to +670 cells/mm2 for examiners 1 and 2, respectively.

CONCLUSIONS

The reliability and agreement of the Orbscan and Konan corneal pachymeters was good, although the reliability of the Konan for estimating endothelial cell density was fair, at best.

Lasers in ophthalmology

Laser technology continues to progress with the addition of new lasers, new delivery systems, and new applications. The introduction of lasers to veterinary ophthalmology has radically changed the level of care that we can provide to our patients. The development of the diode laser has particularly had an impact on veterinary ophthalmology. The diode's affordability, portability, and broad applications for veterinary patients have allowed laser surgery to become a routine part of veterinary ophthalmology practice. Educating the public and veterinary community in available laser techniques will generate improved ophthalmic care and provide more data on which to build future applications.

Intraocular Lens Implantation and Laser in situ Keratomileusis (Bioptics) to Correct High Myopia and Hyperopia With Astigmatism

PURPOSE

To analyze the refractive outcome of moderate to high myopic and hyperopic patients with astigmatism who underwent programmed refractive surgery; first lens phacoemulsification with intraocular lens implantation and 3 months later, laser in situ keratomileusis (LASIK).

METHODS

Four men and eight women (22 eyes) with a mean age 47.3 years (range, 38 to 75 yr), and an average spherical equivalent refraction of -11.76 D and +5.22 D and (range, -17.50 to +8.50 D) underwent two refractive procedures. First, phacoemulsification of the lens with a self-sealing incision through clear cornea on the steepest topographic axis and implant of a monofocal intraocular lens in the bag was performed by two experienced surgeons. Second, LASIK was performed with the Nidek EC-5000 excimer laser and the Moria LSK-One microkeratome, by one surgeon. Eyes were divided into two different groups. In the first group, the IOL implanted was calculated to leave the eye slightly myopic, with final correction to be achieved with LASIK. In the second group, the IOL implanted was calculated to achieve emmetropia, correcting any residual refractive error with the laser.

RESULTS

After surgery, mean spherical equivalent refraction was +0.26 D (range, -0.375 to +1.50 D). Predictability of refractive outcome: 0 to -1.00 D, 63.63%; +0.25 to +1.00 D, 31.80%; +1.25 to +2.00 D, 4.54%. Mean residual refractive astigmatism was 0.30 D (range, 0 to 1.50 D). Uncorrected visual acuity of 20/20 or better was achieved in 18.3% of eyes; 20/40 or better in 81.8%. No eyes lost two or more Snellen lines of visual acuity and no adverse effects were observed.

CONCLUSIONS

Bioptics (phacoemulsification with IOL implantation followed 3 months later by LASIK with the Nidek EC-5000 excimer laser) for correction of moderate to high myopia and hyperopia, with astigmatism, enabled us to treat the total refractive error and adjust final outcomes.