Excimer laser 6th generation: state of the art and refractive surgical outcomes
© El Bahrawy and Alió; licensee BioMed Central. 2015
Received: 20 June 2014
Accepted: 11 February 2015
Published: 1 March 2015
After nearly three decades of innovation in excimer laser, today we are presented with a state of the art generation targeting minimally invasive refractive surgery with high speed laser, faster trackers, pupil monitoring systems and better customization profiles. These systems are capable of delivering better treatments with less induced postoperative high order aberrations. The results reported by many authors had confirmed the superiority in efficiency and safety profiles of this generation compared to previous generations. Still, current technology is facing major challenges in the correction of high hyperopic errors and in presbyopic treatments, with upgrades in ablation centration and thermal control needed, which will ensure better biomechanical results, as a step closer to perfection in refractive surgery.
KeywordsRefractive surgery Excimer laser 6th Generation
The birth and evolution of excimer laser refractive surgery
The concepts of modern refractive surgery witnessed its breakthrough when Professor Jose I. Barraquer described in 1949 his coined technique of keratomeliosis, setting the foundation for all following innovation in this field. The name excimer laser came as an abbreviation of “excited dimer”, introduced by the Russian, Nikolay Basov, in 1970 using a xenon dimer gas . Few years later, the argon-fluoride excimer laser was developed and was first tried on an organic tissue by IBM scientists. The introduction of excimer laser to be used in the human eye was done by Stephen Trokel as a precise and safe tool of corneal shaping, these concepts later defined the refractive techniques widely used now, when Marguerite McDonald under the supervision of Steve Kaufmann, performed the most commonly used epithelium removal technique photorefractive keratectomy (PRK). Peyman, presented the first patency of using excimer laser as a corneal refractive tool, and it was accepted in Jun 1989 (personal correspondence Gholam Peyman). Following Ioannis Pallikaris, among others, introduced the most widely used and commonly accepted technique of laser in situ keratomeliosis (LASIK) in 1990 .
FDA Indications for LASIK and PRK: 
Less than −14.0 D with or without astigmatism between −0.50 and −5.00 D
Up to −12.0 D with or without astigmatism up to −4.00 D
Up to + 5.00 D with or without astigmatism up to +3.00 D
Up to +5.00 D with or without astigmatism up to +4.00 D
Astigmatism up to 6.00 D, cylinder is greater than sphere and of opposite sign.
Features of the successive generations of Excimer Lasers
Pre-clinical (Touton, VISX, Summit)
Broad beam laser, fixed optical zone
Broad beam laser, variable optical zone, multizone treatment
Flying spot laser, built in tracker, hyperopic treatment
Customised wavefront (guided, optimised) treatments
• Faster ablation rates & tracking systems
• Lower biological interaction
• More variables under control
• Pupil size
• Advanced ablation profiles
• Ciclotorsion control
• Online pachymetry
Sixth generation excimer lasers
Comparison between 6 th generation Excimer platforms
Slit scanning + variable spot-size scanner
6 scans/sec.60 Hz max
160 mJ/cm2 - 450 mJ/cm2
10 x2 mm scanning slit (1 mm for customized and hyperopia)
Spot size (cornea)
0.95 mm (1.2 mm)
Optical zone (OZ)
4 - 10 mm
4.5 mm - 8 mm
Aspheric (aberration free)
Munnerlyn with aspheric tansition zone
Aspheric (including Q-value)
Transition zones adjustable
Yes (TEC = torsion error correction)
Cyclo-Torsion, Sampling Rate
Ablation depth per shot (cornea)
0.42 μm - 0.68 μm
Ablation volume per shot (cornea)
110 pl -220 pl
Ablation depth per diopter (6.5 mm OZ)
Time per diopter (6.5 mm OZ)
Ablation depth -5dpt/OZ = 6 mm
65 μm (12 s)
Eye tracking system
Active video tracking (SMI)
Active video tracking (SMI)
Active video tracking (SMI)
Eye tracker response time
Static ±15° Dynamic ± 7°
X-Y & Z-tracking
Yes - integrated
Centration of pupil
Automatic, user defined
FluenceTest needed every:
Before every treatment day
Fluence & Calibration
Automatic and objective
Manual and subjective
Manual and subjective
Capable of customized ablation
Method used for wavefront
Hartmann - Shack
Corneal Wavefront Analyzer/CSO
Yes (Corneal wavefront)
Yes (topographic based on Zernike)
264 × 144 × 136 cm(including patient bed)
137 × 151.6 × 147 cm
120 × 145 × 130 cm (without patient bed)
Weight (without patient bed)
265 kg (without bed and gas) patient bed 188 kg
Website: last visited May 2014
The reduction of induced aberrations, is a critical trend in modern laser refractive surgery, 6th generation platforms feature advanced ablation profiles, with the reduction of spot size as a key factor of control of the induced aberration; which is 0.54 mm and 0.68 mm for Schwind Amaris and Allegretto Eye-Q respectively, compared to a spot size of 0.8 mm or more for conventional excimer lasers. Also, these profiles are able to correct pre-existing optical aberration through integrated customized and wavefront ablation technology. The efficiency of the previous features requires extremely accurate laser spot placement, in which the eye tracker latency time is of only 1.6 milliseconds (ms). A conventional laser platform eye tracker will have a capturing rate of 60 to 330 Hz, able to detect the pupil position at 4000 Hz, with a response time of 36 ms, clearly not fast enough for a high speed laser platform of a speed reaching 700 Hz. The new five dimensional turbo speed tracker have an acquisition speed of 1050 Hz generating a response rate less than 3 ms with unique rotational balance, tracking both the pupil and the limbus .
A conventional eye tracker adjusts eye movements into an X and Y-axis linear movement, and lasers are able to follow eye rolling, through translation of these linear movements into rotations with the help of an eye model, so that these generated rotations can be followed and compensated. Modern eye trackers do not only follow these horizontal and vertical displacements of the eye but also track the cyclotorsional rotations. These cyclorotations can be classified into static cyclorotation component (SCC); occurs when the patients move from an upright to a supine position and a dynamic cyclorotation component (DCC); which occurs during the treatment procedure .
The high repetition rate of excimer lasers may result in shorter interval between laser pulses on the same area of the cornea, increasing the thermal load on the cornea and resulting in thermal damage, as a safety feature, a recent generation laser will use an intelligent thermal effect control to significantly reduce the heating effect on the cornea, the area around the applied laser spot is blocked for a certain time allowing the cornea to cool down, this will generate a dynamic thermally optimized distribution of the laser pulses, with enough time for each spot to cool down between pulses. The thermal load from refractive corrections using a 500 Hz laser with a fluence of 500 mJ/cm2 was recorded with an infrared thermography camera to analyze and evaluate each single in vivo measurement; the overall maximum temperature change induced by the refractive ablations was ≤4°C, the increase in the peak temperature of the ocular surface never exceeded 35°C, and this low thermal load was independent of the amount of correction the eye achieved .
Another safety feature is the automatic monitoring of the pupil size, as illumination is automatically adjusted in such a way that the pupil is exactly the same size at the start of the treatment as it was on the preoperative examination. Finally, the integrated online pachymetry in these state of the art platforms will display the corneal thickness in real time, with the ability to monitor the targeted measurements before and after flap lifting as well as during and after laser ablation, which is documented in the treatment log at the end of the procedure [9-11].
Outcomes of 6th generation lasers
Outcomes of the AMARIS-SCHWIND 6 th generation excimer laser
Number of patients
Number of eyes
Mean age (years)
Fellow up (months)
Tomita et al. 
34 ± 8 (18–65)
−4.89 ± 2.12 (−0.5 to −11.63)
0.66 ± 0.20
96.6 % 20/20
94.1% ±0.5 D
Tomita et al. 
33.9 ± 7.89
−5.02 ± 2.17 (−2.75 to 11.50)
0.70 ± 0.23
96.9% 0.0 LogMAR
88.4% ±0.5 D
Vega-Estrada et al. 
36.65 ± 10.80
−8.39 ± 0.93
0.95 ± 0.8
89.6% ±1.00 D
0.11 ± 0.26 LogMAR
Alió et al. 
≥ − 8.5
84.3% ±0.5 D
Arba-Mosquera et al. 
−4.27 ± 1.62 (−7.38 to −1.38)
0.425 ± 0.129 (P < 0.01)
0.47 ± 0.72 lines (P < 0.05)
−0.07 ± 0.25 (−0.63 to +0.50)
Alió et al. 
+5.64 ± 0.93 (3.50 to 7.88)
−0.44 ± 0.22 (P = 0.00)
70.37% ±0.5 D
Arbelaez et al. 
+3.02 ± 2.06 (+0.13 to +5.00) +1.36 ± 1.61 (Ast.) (0.00 to 5.00)
↑ 0.18 (P < 0.05)
89% ±0.5 D
94% ±0.5 (Ast.)
Alió et al. 
Mixed Ast. > 3.0
65.3% ±1.0 D
Arbelaez et al. 
−0.14 ± 0.31
97% ±0.5 D
65% No changes
+0.25 ± 0.37 (Ast.)
Arbelaez et al. 
−3.13 ± 1.58
−0.69 ± 0.67 (Ast.)
96% ±0.5 D
The small spot hyperopic laser in situ keratomileusis (H-LASIK) ablation at the periphery of the cornea producing certain degrees of steepness. This treatment modality had several limitations as decentration, decrease in best corrected visual acuity (BCVA), high frequency for the need of retreatments, residual refractive errors and induction of astigmatism due to the high levels of corneal aberrations as negative spherical aberration, all cause loss of the efficiency of treatments and changes in biomechanics of the cornea . Our studies included a sample of 51 eyes in 28 patients of a MSE of +5.64 ± 0.93 D followed up for 6 months, we reported an efficacy of 70.37% within ±0.5 D of target refraction, the HOA postoperatively was −0.44 ± 0.22 μm, with a efficacy and safety index of 0.85 and 0.94 respectively . Another 6 months retrospective follow up of 51 eyes with a spherical equivalent of more than 5.5 D showed significant increase in corneal root mean square (RMS) HOA, RMS spherical aberration (SA) and RMS coma were also observed six months after surgery (p < 0.01). Corneal asphericity for the 4.5 mm (Q45) and 8 mm (Q8) of corneal diameter also changed significantly during the postoperative period (p < 0.01). Strehl ratio change was not statistically significant (p = 0.77) . Arbelaez et al. published a study with MSE +3.02 ± 2.06 D (astigmatism was +1.36 ± 1.61 D), they reported 6 months follow up with a mean postoperative increase in HOA of 0.18 μm (P < 0.05), 89% were within ±0.5 D of the target refraction and 94% were ±0.5 D of target astigmatism . Kanellopoulos used a larger sample of 202 eyes with a longer follow up of 2 years to demonstrate the safety and efficacy of topography guided ablation using a 400 Hz WaveLight excimer laser by (Alcon Laboratories, Fort Worth, TX). In his study the MSE was +3.04 ± 1.75 D, the results showed that the mean refraction spherical equivalent was ±0.50 D of target refraction in over 80% of cases, with an increase in the RMS of 15% .
Patients’ satisfaction after refractive surgery, wavefront guided or not, is primarily dependent on the successful treatment of lower order aberrations of the sphere and cylinder of the eye. LASIK has been successful in the correction of mild to moderate myopic astigmatism, but with limited reports on the efficacy, predictability and safety of it in higher myopic astigmatism in the terms of astigmatic correction of HOA, with limitation of retreatments needed . We reported the 3 months follow up of 52 eyes of mixed astigmatism of more than 3 D, in which the efficacy was 65.3% within ±1.0 D of target refraction . Earlier, Arbelaez et al. published a number of reports, with an average of 1.26 ± 3.29 D of astigmatism, and with 6 months follow up found an increase of 0.09 μm with a residual astigmatism 0.50 ± 0.26 D (P < 0.0001) [12,22,23]. The mean decrease of astigmatic magnitude in these reports was 93%, indicating a slight under correction of the preoperative astigmatism, but with marked improvements from the 36% to 91% reported with the use of older excimer laser platforms [27,28]. Also, recent reports showed a 100% efficacy within ±0.25 D after 12 months . Topography-guided hyperopic astigmatism correction showed a correction of a mean preoperative cylinder value of −1.24 ± 1.41 D to the respective postoperative value of −0.35 ± 0.25 D .
New platforms had shown similar satisfactory results with PRK. Aslanides et al. in his 2 years follow up of 80 eyes with mild to high myopia and myopic astigmatism found 91% to be within ±0.5 D of target refraction, but reported a statistically significant increase in postoperative coma (+0.12 μm) and spherical aberration (+0.14 μm) compared to preoperative values (P < 0.001, both cases) when performed using the SCHWIND AMARIS [29,30].
Challenges facing excimer laser refractive surgery today
In spite of the astonishing progress in the field of laser refractive surgery, a number of challenges are still facing the technology. One of the most critical is the limitations in the visual and optical outcomes in patients with high hyperopic refractive errors, as most authors reported a significant induction of the corneal HOA, most significantly in the RMS coma [19,20].
The efficacy of the excimer laser technology even with latest platforms in the treatment of presbyopia is still the subject of great debate. PresbyLASIK has been described in three different approaches to create multifocality: the transitional multifocality, the central presbyLASIK and the peripheral presbyLASIK. Both central and peripheral techniques reportedly obtained adequate spectacle independence for near and for far with a degree of neuroadaptation process needed in peripheral techniques. An intentional increase in coma aberrations was noticed in transitional multifocality giving it a range of very limited use and outcomes. The level of scientific evidence we already have is enough to consider presbyLASIK as a useful tool in the correction of presbyopia . Epstein et al. reported a 4 years follow up of 103 patients who underwent peripheral presbyLASIK, 89% of hyperopes and 92% of myopes was completely spectacle independent, with distance unaided visual acuity of 20/20 in 67.9% of hyperopes and 70.7% in myopes. They also reported that the overall increase in the HOA was manifested to a greater extent in hyperopic cases . Other programs as The Supracor presbyopia procedure showed good near visual acuity outcomes over 6 months follow up, but loss of corrected distance visual acuity (CDVA) occurred in 39.1% of eyes, also reported in PresbyMAX . At 1 year, 70% of patients achieved uncorrected distance visual acuity (UDVA) 0.1 logMAR or better, 84% patients obtained uncorrected near visual acuity (UNVA) 0.1 logRAD or better, and 83% of eyes were within 0.75 D of defocus .
Ablation centration is a major issue in the excimer laser development, the decentration of ablation can lead to under correction and irregular astigmatism, which is most important in hyperopic patients [35,36], who tend to have a larger angle kappa values . There are four main methods of centration in laser refractive surgery that has been suggested in literature; center of the pupil, coaxially sighted corneal light reflex (CSCLR), corneal vertex normal and between the pupillary and visual axis . Many reports had demonstrated that pupil-centered and vertex-centered treatments provide similar visual and optical outcomes. However, in eyes showing large temporal pupil decentration, pupil-centered ablation seemed to produce a lower amount of coma and consequently, a reduced loss of BCVA compared with vertex-centered patients [26,38,39].
One of the other challenges is the limitation facing wavefront-customized treatment. Despite decreases in spot size of up to 0.5 mm and thermal damage control limitations still exist, mainly through the biomechanical changes induced by the wound healing patterns . It was reported that one month after treatment, corneal hysteresis and the corneal resistance factor decreased significantly from 10.44 to 9.3 mmHg and from 10.07 to 8.13 mm Hg, respectively . So, when looking to customization as the planning of the most optimum ablation pattern specifically for each individual eye based on its diagnosis and visual demands, the best approach is a sophisticated pattern . As wavefront guided treatment may increase the HOA up to 100%, as the induction related to the baseline levels of HOAs, is more significant in patients with less than 0.3 μm or greater than 0.3 μm of HOA. But results of this customized treatment still showed promising results as Arbelaez et al. reported an average change in coma from 0.38 μm to 0.31 μm (a 19% decrease) (P = 0.04), in trefoil from 0.35 μm to 0.12 μm (a 66% decrease) (P = 0.0005), and in spherical aberration from +0.14 μm to +0.08 μm (a 48% decrease) (P = 0.02) . Furthermore, when comparing a small spot scanning laser and a variable spot scanning laser, Yu et al. reported in his study of 50 patients, in which one eye of each patient was treated by the small spot laser using Allegretto Wave Eye-Q system and the other with the variable-spot scanning laser of Visx Star CustomVue S4 IR system; the small-spot scanning laser group had significantly less spherical aberration (0.12 versus 0.15) and significantly less mean total higher-order RMS (0.33 μm versus 0.40 μm) .
Patient satisfaction with the results of treatment with the recent excimer laser was remarkable. Kyprianou et al. reported a 100% satisfaction in 32 patients, with average age of 31.9 years and a preoperative MSE of −3.05 D. This was evaluated by a questionnaire consisting of 21 questions. Patients were most satisfied in questions concerning quality of vision, distance vision, when watching TV and driving during daytime and during the night .
In summary, the latest generation of excimer laser platforms had introduced a large number of features as faster laser, smaller spot size, a high speed tracker, pupil monitoring and online pachymetry, all of which provided superior treatment with significant improvement of induced post operative HOA and control of thermal damage. This technology is still facing major limitations in terms of high hyperopic, presbyopic treatments, along with difficulties in laser centration along with the limitation of the customized treatments, generated by the biomechanical patterns of wound healing .
This study has been supported in part by a grant from the Spanish Ministry of Health, InstitutoCarlos III, Red Temática de Investigación Cooperativa en Salud “Patología ocular del envejecimiento, calidad visual y calidad de vida”, Subproyecto de Calidad Visual (RD07/0062) and a grant from the Spanish Ministry of Economy and Competitiveness, Instituto Carlos III, Red Temática de Investigación Cooperativa en Salud (RETICS) “Prevención, detección precoz y tratamiento de la patología ocular prevalente, degenerativa y crónica”. Subprograma “dioptrio ocular y patologías frecuentes” (RD12/0034/0007).
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