Since the beginning of refractive surgery, a wide variety of procedures have been investigated by ophthalmologists for the correction of refractive error.1,2 The most successful among these include LASIK (a term used by Pallikaris et al3,4), photorefractive keratectomy5–7 (PRK), laser epithelial keratomileusis8–12 (LASEK), and more recently, thinflap femto-LASIK or sub-Bowman’s keratomileusis (SBK).13 The success of the femtosecond laser as an alternative flap cutting tool14 is due to its precise photodisruption of tissue with minimal collateral tissue damage and inflammation at a preset depth.15 Most recently, femtosecond laser intrastromal ablation has been proposed as a minimally invasive method for correcting ametropia without the creation of a flap and use of an excimer laser.16,17
Although excimer laser ablation has had modest success in treating hyperopia,2,18–20 many other prior procedures, such as hexagonal keratotomy, automated lamellar keratoplasty, epikeratophakia, thermokeratoplasty, and keratophakia were not successful and, for the most part, have been abandoned.2,18 Among these, only conductive keratoplasty went beyond the correction of low to moderate hyperopia to be the first to gain United States Food and Drug Administration approval for the correction of presbyopia.21–23
With the recent introduction of presbyopia as the newest indication for refractive surgery, a great deal of attention has been focused on the quest for developing an ideal presbyopic solution. In 2005, the estimated global impact of presbyopia was 1.04 billion people with over half of these not having adequate near vision correction, and 410 million being listed as visually impaired (94% in developing countries).24 In addition, more than 26 million people alone in the United States suffer from this condition,25 affecting almost everyone over the age of 51.26 Without widespread success in correcting presbyopia, the global disability is expected to grow to 563 million people by 2020.24
Because of this expanding global impact, a wide host of technologies and techniques are being pursued to achieve surgical correction of presbyopia,27,28 such as multifocal corneal ablation,29,30 pseudoaccommodative cornea treatment,31 intracorneal inlays,32 refractive lens exchange,33 phakic multifocal intraocular lenses,34,35 and anterior ciliary sclerotomy.36 Despite these efforts, a number of limitations have prevented widespread acceptance of surgical presbyopia correction. Concerns regarding optical and visual distortion,37 induced corneal ectasia, pain, haze, delay in visual recovery, anisometropy with monovision, regression of effect, decline in uncorrected distance vision, and the inherent risks with invasive techniques all have played a limiting role in finding the ideal solution.
In an effort to overcome some of these limitations, we report a new, minimally invasive method of intrastromal correction, which offers a painless and faster postoperative recuperation than surface ablation techniques, and yet also avoids flap cutting of the strongest anteriormost corneal fiber as with LASIK or thin-flap LASIK. It avoids intraoperative surgical risks, and because it is entirely intracorneal, it is termed INTRACOR.
Patients and Methods
The INTRACOR procedure is performed using the TECHNOLAS femtosecond laser system (Technolas Perfect Vision GmbH, Munich, Germany), which delivers a completely intrastromal customized pattern of laser pulses into the cornea to induce a local reorganization of the biomechanical forces and change in corneal shape. The entire pattern of applied laser energy depends on the patient’s refractive error, so that it not only improves uncorrected near visual acuity (UNVA), but also corrects and improves uncorrected distance visual acuity (UDVA) in eyes with low ametropia.
The TECHNOLAS femtosecond laser delivers a therapeutic ultrashort laser pulse of 600 to 700 fsec duration, 1053±10 nm wavelength, and a maximum of 6 μJ energy to achieve the high peak power for photodisruption at any and all targeted locations within the cornea. The basic pattern for presbyopia correction is a series of femto-disruptive cylindrical rings that are delivered beginning within the posterior stroma, at a variable distance from Descemet’s membrane, and extending anteriorly through the mid-stroma to an anterior location at a predetermined, fixed distance beneath Bowman’s layer. The pattern of laser delivery is entirely intrastromal, without impacting either the endothelium, Descemet’s membrane, Bowman’s layer, or epithelium at any point throughout the procedure (different from incisional keratotomy by avoiding the need for superficial healing and risk of perforation, infection, ectasia, or late overcorrection). The net effect is a central steepening of the anterior corneal surface, not in the shape of a steep central island, but rather as a multifocal hyperprolate, corneal shape with an ideal, pupil-dependent aberration pattern. The variable refractive power of the central cornea enhances the depth of focus, improving near vision, while maintaining distance vision at nearly the same acuity and photopic refraction. The cylindrical pattern works best with a mild hyperopic, near emmetropic, preoperative refraction, and can be modified with additional pulses in eyes with low myopia, hyperopia, or astigmatism. Ultimately, the number, spacing, and size of the intrastromal cylindrical rings were varied to correct a greater amount of preoperative hyperopia, and it was necessary to add some intrastromal radial femto-disruptions to correct low myopia or astigmatism. The intrastromal laser treatment was performed rapidly with treatment times between 18 and 30 seconds.
In this initial case series, we enrolled all qualified presbyopic patients who had a spherical equivalent manifest refraction ranging between +1.75 and −0.50 diopters (D), with a cylinder of up to −1.50 D. All eyes were followed postoperatively for at least 6 to 12 months. Exclusion criteria included pseudophakia; previous LASIK, PRK, or other refractive surgery; keratoconus or other corneal ectasia; and any corneal opacifying disease. All treatments were performed by the same surgeon (L.A.R.) at Centro Oftalmológico Colombiano in Bogotá, Colombia, beginning in October 2007. Informed consent was obtained and signed by all patients.
Each eye was examined preoperatively and at defined periods after the procedure (1, 3, 6, 9, and 12 months). During those examinations, examiners were unmasked and monocular measurements were obtained, masking the fellow eye, including manifest refraction, UNVA, and best distance corrected near visual acuity obtained by Jaeger readings at 40 cm with the ETDRS chart. Postoperative depth of focus testing at 6 months was performed by determining the range from minimum (near) to maximum (far) distance in which patients reported a good, sharp focus when testing near vision with distance correction. Uncorrected intermediate visual acuity (UIVA) was obtained at 60 cm distance by Jaeger readings, whereas UDVA and corrected distance visual acuity (CDVA) were obtained by Snellen readings, all under consistent photopic conditions. All visual acuity values were expressed using decimal notation. Other measures included corneal pachymetry, obtained by the thinnest point when using Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany) and ultrasound pachymetry (Sonogage Inc, Cleveland, Ohio); endothelial cell density obtained by specular microscopy (SP-3000P; Topcon, Tokyo, Japan); anterior and posterior corneal asphericity obtained by Pentacam; photopic and mesopic contrast sensitivity obtained by Optec 6500P Vision Tester (Stereo Optical Co Inc, Chicago, Ill); and ocular aberrations obtained both by Wavefront Supported Custom Ablation (WASCA; Carl Zeiss Meditec, Jena, Germany) and ray tracing (Tracey Technologies, Houston, Tex) aberrometers. Individual corneal biomechanics were measured by corneal resistance factor (CRF) and corneal hysteresis (CH) obtained by the Ocular Response Analyzer (Reichert Instruments, Depew, NY).
Eighty-three eyes of 45 patients aged 44 to 67 years (mean age: 52.7±6.0 years) underwent intrastromal refractive surgery for presbyopia. Follow-up ranged from 6 to 12 months (mean follow-up: 7.87 months). All 83 eyes had at least 6-month follow-up, whereas 27 eyes had 9-month follow-up and 22 eyes had 12-month follow-up.
Preoperatively, the average UNVA of all eyes was 0.27±0.1 (range: J11 to J7). On the first postoperative day, UNVA increased significantly with continued improvement at 1-month follow-up to 0.87±0.15 (range: J1 to J3, n=83 eyes), at 3 months to 0.88±0.13 (range: J1 to J3, n=83), at 6 months to 0.92±0.12 (range: J1 to J2, n=83), at 9 months to 0.96±0.08 (range: J1 to J2, n=27), and at 12 months to 1.0 (J1, n=22). After surgery, 76 (91.6%) of the treated eyes showed UNVA of 0.8 (J2) or better and 58 (69.9%) eyes achieved UNVA of 1.0 (J1). No eyes showed decrease in UNVA at any follow-up examination.
By 6 months postoperatively, the mean value for UIVA at 60 cm was 0.89 (20/20 to 20/25). Postoperative depth of focus with distance corrected near vision was from 26.1 cm (range: 10 to 40 cm) to 45.8 cm (range: 30 to 60 cm).
With regard to distance outcomes, UDVA also improved for most eyes, with the greatest improvement noted at the time of last follow-up. Before surgery, the average UDVA of the 83 eyes was 0.86±0.2 (range: 20/20 to 20/30). At 1 month after surgery, it improved to 0.95±0.24 (range: 20/16 to 20/30, n=83 eyes), at 3 months to 0.97±0.24 (range: 20/16 to 20/30, n=83), at 6 months to 0.98±0.21 (range: 20/16 to 20/30, n=83), at 9 months to 1.02±0.22 (range: 20/16 to 20/25, n=27), and at 12 months to 1.07±0.21 (range: 20/16 to 20/25, n=22). Despite the fact that most eyes at 6 months postoperative gained from 1 to 6 Snellen lines (43 eyes, 51.8%) or remained unchanged (24 eyes, 28.9%), 16 (19.3%) of 83 eyes showed a mild decrease in UDVA, due to a minimal myopic shift. Most of these 16 eyes (13 eyes, 15.7%) lost 1 line (2 of these eyes were followed to 12 months and remained with the same 1-line loss), 2 (2.4%) eyes lost 2 lines (1 of these eyes was followed to 12 months and remained with the same loss), and 1 (1.2%) eye lost 3 lines, which was followed to 9 months, remaining with the same loss.
Mean values of UNVA and UDVA at every observation period are shown in Figure 1. This figure shows a significant shift of UNVA towards smaller Jaeger letters and, despite those 16 eyes that lost 1 or more distance lines, a shift of UDVA to smaller Snellen letters.
Figure 1. Change over Time of the Mean near and Distance Uncorrected Visual Acuity (UCVA) Following the INTRACOR Procedure. Number of Eyes Is Shown in Parentheses at Every Follow-Up Time.
At last follow-up, 9 (10.8%) eyes had either UNVA or UDVA of 0.67 (J3 and 20/30) or less. The remaining 74 (89.2%) eyes were 0.8 (J2 and 20/25) or better simultaneously for both UNVA and UDVA.
Regarding CDVA, at 6 months, 2 (2.4%) eyes lost 1 line and another 2 (2.4%) eyes lost 2 lines from 1.33 (20/15) to 0.8 (20/25). The total follow-up for these later 2 eyes was 6 months, so it is unknown whether visual acuity changed after that time. The remainder of eyes were stable (64 eyes [77.1%]) or gained 1 line (15 eyes [18.1%]). At 9 months, the 2 eyes that previously lost 1 line at 6 months gained this line again and even another eye gained 1 line. Finally, at 12 months, none of the 22 eyes lost lines in CDVA and 3 (3.6%) eyes gained 1 line. At 6, 9, and 12 months, all 83, 27, and 22 eyes, respectively, gained 1 or more lines (up to 8 lines) in distance corrected near visual acuity.
Figure 2 shows the change in mean values of CDVA and corrected near visual acuity (CNVA) at every observation period. This figure, similar to Figure 1, also shows a significant shift of the distribution of both measurements towards smaller Jaeger and Snellen letters, respectively. The disparity between CDVA and distance corrected near visual acuity is related to the limitation of our testing of near visual acuity to 1.0 or J1 (ie, J1+ not available), whereas distance visual acuity could be tested beyond 1.0 (ie, better than 20/20).
Figure 2. Change over Time of the Mean Best Corrected Distance Visual Acuity (BCDVA) and Best Distance Corrected near Visual Acuity (BDCNVA) Following the INTRACOR Procedure. Number of Eyes Is Shown in Parentheses at Every Follow-Up Time.
Figures 3 and 4 show the changes in spherical equivalent refraction, comparing pre- and postoperative values. Figure 3 shows the mean value for all eyes up to 12-month follow-up, whereas Figure 4 shows the comparison for each eye, preoperative versus 6 months postoperative. Both figures depict a general tendency for a minimal myopic shift with the procedure (just 0.30 D in spherical equivalent refraction over 1 year). The mean cylinder value decreased slightly at the last follow-up, from 0.47 D preoperatively to 0.39 D at 12 months postoperatively. Figure 5 shows this change in cylinder refraction before and after surgery.
Figure 3. Change over Time of the Mean Spherical Equivalent Refraction for All Eyes Following the INTRACOR Procedure.
Figure 4. Change in Spherical Equivalent Refraction Following the INTRACOR Procedure when Comparing Preoperative vs 6 Months Postoperative Values for Each of the 83 Eyes.
Figure 5. Change over Time of the Mean Cylinder Refraction for All Eyes Following the INTRACOR Procedure.
Mean pre- and postoperative values at 6 months for corneal thickness were 541.4 μm and 540.8 μm, respectively, and for endothelial cell density were 2392 cell/mm2 and 2345 cell/mm2, respectively. Both photopic and mesopic contrast sensitivity were nearly unchanged when comparing mean pre- and postoperative values (Table).
Table: Contrast Sensitivity Before and After INTRACOR in 83 Eyes
Ocular aberrations were measured pre- and postoperatively using WASCA and Tracey aberrometers with a pupil size of 5.5 mm. Mean changes after surgery in total and higher order aberrations are shown in Figure 6, whereas specific aberrations (spherical, coma, and trefoil) with each aberrometer are shown in Figures 7 and 8. These figures show a relative stability of the total and higher order aberrations when testing with both aberrometers. However, a shift in primary spherical aberration (4,0) toward negative values and secondary spherical aberration (6,0) toward positive values was noted, which was our goal for obtaining good near vision with a hyperprolate corneal shape. Some differences in measured coma and trefoil aberrations were noted with one aberrometer versus the other.
Figure 6. Comparative Change of Total and Higher Order (HO) Aberrations Following the INTRACOR Procedure when Using WASCA and Tracey Aberrometers. RMS = Root-Mean-Square
Figure 7. Pre- and Postoperative Comparison of Spherical (SPH), Coma, and Trefoil Aberrations, Using the WASCA Aberrometer Following the INTRACOR Procedure.
Figure 8. Pre- and Postoperative Comparison of Spherical (SPH), Coma, and Trefoil Aberrations, Using the Tracey Aberrometer Following the INTRACOR Procedure.
Corneal biomechanical properties were measured using the Ocular Response Analyzer. In most eyes, a mild reduction in CRF was noted from 9.55±1.58 to 8.97±1.30 and relative stability in CH from 9.69±1.29 to 9.71±1.16. Figures 9 and 10 show these postoperative changes in each eye and Figure 11 shows mean values, comparing CRF and CH pre- and postoperatively.
Figure 9. Change in Corneal Resistance Factor (CRF) in Each of the 83 Eyes, Comparing the Pre- and Postoperative Findings Following the INTRACOR Procedure.
Figure 10. Change in Corneal Hysteresis (CH) in Each the 83 Eyes, Comparing the Pre- and Postoperative Findings Following the INTRACOR Procedure.
Figure 11. Mean Values of Corneal Resistance Factor (CRF) and Corneal Hysteresis (CH) Pre- and Postoperatively Following the INTRACOR Procedure.
Corneal asphericity was also measured pre- and postoperatively, with mean changes depicted in Figure 12. This figure shows that both anterior and posterior corneal asphericity changed in the same proportion.
Figure 12. Mean Anterior and Posterior Corneal Asphericity, Comparing Pre- and Postoperative Values Following the INTRACOR Procedure.
No complications were found during or after the procedure, and all patients were generally pleased with their results with none using spectacles. The symptomatic complaint of halos was reported by every patient, starting the day after surgery, but this improved with time: at 6-month follow-up, 30% of patients reported halos whereas at 9 and 12 months this improved to 15% and 3%, respectively.
The concept of an entirely intrastromal femtosecond laser procedure that reorganizes biomechanical forces to create an ideal hyperprolate corneal shape is highly attractive for the management of presbyopia. The potential advantages of such a procedure are: 1) intrastromal delivery without breaking the epithelium, 2) avoidance of pain and inflammation from the exposed ocular surface, 3) speed of recovery due to the absence of surface wound healing, 4) titratability of laser delivery for managing low ametropia and asphericity differences, and 5) stability of refractive outcome by preserving the strongest, anterior corneal fibers. However, whenever a new procedure is introduced, the potential disadvantages must also be considered and studied. These may include: 1) dissatisfaction with the hyperprolate aberration pattern, 2) diffractive effects from the paracentral laser pulse delivery, 3) high dependability on proper centration and alignment, and 4) progression or loss of effect over time due to changes in the biomechanical corneal forces. Although these potential disadvantages may play a role in the clinical success or failure of this new procedure, they can also be more fully characterized and/or managed with large scale clinical investigation and refinement. They are, for the most part, beyond the scope of this initial article, yet we will attempt to address them with our discussion of safety.
One of the most important indicators of long-term success when considering any new technique is that of safety. The intrastromal correction of presbyopia offers improvements in safety with the preservation of the corneal epithelium, Bowman’s layer, and anterior stromal fibers. Yet, best corrected visual acuity is the paramount metric of clinical safety. Any approach for improving near vision must consider not only the distance, but CNVA. Because the viability of a presbyopic solution is also dependent on efficacy, we consider an even more demanding metric of best distance corrected near visual acuity when examining CDVA. Finally, the UNVA should be improved, whereas the UDVA should be either unaffected or, at worst, only minimally affected.
In this study, intrastromal correction of presbyopia was both safe and effective, with all eyes gaining UNVA, and achieving good UIVA as well. Uncorrected distance visual acuity was affected by a mild myopic shift, which was effective in reducing mild, pre-existing hyperopia in some patients. This myopic shift, however, led to a mild myopic outcome in previously emmetropic patients, whereas those with pre-existing myopia also received radial intrastromal femtosecond laser incisions to concurrently correct their myopia. Only 19.3% lost one or more lines (mostly just one) of UDVA, and almost 89.2% of eyes achieved 0.8 (J2 and 20/25) or better vision, simultaneously for both UNVA and UDVA. In addition, the mean CDVA, which initially remained stable after the procedure, continued to show improvement over time with only 2 (2.4%) eyes losing two lines at 6 months. No (0%) eye lost one or two lines of CDVA at 1 year, but because this did not include the two eyes with a loss at 6 months, further follow-up of these eyes is required. Distance corrected near visual acuity not only improved initially after the procedure, but continued to improve over time with 22 (100%) eyes seeing J2 or better when best distance corrected at 12-month follow-up. Good depth of focus was noted at 6 months, represented in the range of near 26.1 cm to far 45.8 cm. However, we must consider that this study included patients in their mid to late 40s (range: 44 to 67 years, mean: 52.7±6.0 years) who are expected to demonstrate more accommodative reserve than patients older than 60 years.
Biomechanically, the intrastromal laser pattern induced a hyperprolate change in corneal shape, which subsequently led to the beneficial refractive effect on both near and distance visual acuity. Despite these changes, the CH was unaffected postoperatively at a measurement of 9.7, whereas CRF showed a mild decrease from 9.55 to 8.97. This small decline is not a reason for concern, because even a popular procedure such as LASIK, for example, reveals a postoperative decline in CRF and even CH by a much greater amount, with values as low as 6.0 or 5.38,39 Unfortunately, no good diagnostic test of corneal biomechanics currently exists, aside from the Ocular Response Analyzer, to characterize the biomechanical change in further detail.
The asphericity of the hyperprolate corneal shape changed in the same proportion both anteriorly and posteriorly, with good reproducibility among eyes and stability over time. The corneal asphericity with LASIK or PRK usually changes just in one surface, or when the change is in both surfaces, it is not proportional.
In an effort to further understand the biomechanically induced change in asphericity with INTRACOR and the multifocality of its outcome, aberrometry was obtained with two different methods, using both the WASCA and Tracey aberrometers. The magnitude of the total and higher order aberrations changed little after the procedure, except for that of primary and secondary spherical aberration. The primary (4,0) spherical aberration shifted toward a negative value, whereas the secondary (6,0) spherical aberration shifted in a positive direction. This combination of moderate negative spherical aberration with positive secondary spherical aberration reveals an ideal aberration profile for enhancing depth of focus. In addition to the spherical aberration changes, differences in measured coma and trefoil aberrations were observed, but these values showed inconsistencies between the two aberrometers. Most likely, the WASCA aberrometer obtains the more accurate values in comparison with the Tracey aberrometer, as our results with the former were more closely aligned to the desired outcome.
Corneal thickness remained nearly unchanged after surgery, revealing no subtraction or ablation of the corneal stroma, as with LASIK, PRK, or other ablation surgeries. Also, no significant changes in endothelial cell count were seen, nor was there a change in the photopic and mesopic contrast sensitivity. Although we did not have a formal questionnaire, all patients said they would have the surgery again. Every patient reported halos after surgery, especially at night, but a minimal number had difficulty while driving at night. After 6 months, many patients no longer reported experiencing these halos, and after 12 months, almost all patients were asymptomatic. In addition, patients reported a reduction of UNVA in dim lighting. However, this is to be expected, because of the larger pupil with a smaller percentage contribution from the central steep, hyperprolate shape under these conditions.
INTRACOR is a minimally invasive, intrastromal correction of presbyopia using the TECHNOLAS femtosecond laser system. It is a promising procedure, with safe, effective, and favorable visual results that seem to improve during the first 12 months of follow-up. According to Jaeger chart testing, the procedure has significant potential to improve near vision by several lines, with progressive improvement in eyes followed up to and beyond the first 6 months. Although, more than half the eyes gained one or up to six lines of UDVA and another one-third of eyes remained stable, only one-fifth lost one or more lines, due to a mild myopic shift in distance refraction. As a safety concern, there was a loss of two lines of CDVA in 2 (2.4%) of 83 eyes at 6-month follow-up, but none of the 22 eyes followed for 12 months lost any lines, suggesting improvement of CDVA with time. Symptomatically, the complaint of halos at night was experienced by all patients immediately after the procedure, but also improved with time. The technique lacks the disadvantages of some other corneal refractive surgical techniques, with regard to postoperative pain, inflammation, haze, and biomechanical instability, due to the preservation of the corneal epithelium and anterior stromal fibers. Further studies with a larger number of eyes and longer follow-up are recommended to more fully characterize this technology and support the outcomes reported herein.
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Contrast Sensitivity Before and After INTRACOR in 83 Eyes
|Contrast Sensitivity % (cycles/degree)|