Abstract
Purpose
The present study compares the efficacy of refractive surgery using third-generation and sixth-generation laser vision correction procedures in terms of postoperative visual acuity. The goal of the authors was to determine whether advances in laser technology had improved patients' uncorrected visual acuity, as measured at six-month follow-ups.
Materials/Methods
Results were reported from one of Europe's leading laser eye clinics, which has performed an outstandingly high number of treatments (over 100,000). The results of the clinic's first 10,000 treatments and most recent 10,000 treatments were evaluated.
Results
The analysis was performed by treated dioptric range and age group. The raw six-month visual acuity results show a statistically significant improvement over the last 10,000 interventions. The treatments resulted in significant improvements in all groups compared to the previous technology. With the new devices, visual acuity increased to above 1.0 in young myopes. The best results were seen in patients between 18 and 45 years of age, in the dioptric range between −1.0 D and −6.0 D.
Conclusions
It can be concluded that advances in technology improved refractive outcomes in all patient groups. This conclusion has excellent reliability and predictability due to the particularly high number of cases.
Introduction
Refractive excimer laser surgery to improve visual acuity is a well-known and accepted procedure today. Continuous technological developments have been made over the last 30 years, with the appearance of new types of devices and new surgical techniques, together with parallel advances in diagnostic and surgical equipment. There are now six generations of excimer laser systems for corneal vaporisation and corneal reshaping [1].
Corneal refractive surgery began in 1987 in Europe and the United States in parallel. In Europe that year, radial keratotomy (RK) was performed by Theo Seiler using the first generation of excimer laser [2], while in the U.S., Marguerite McDonald performed a myopic surface ablation in a blind eye, which was later enucleated and pathologically assessed [3].
Radial keratotomy incisions made using an excimer laser produced similar refractive results to the original RK procedures carried out using a diamond knife. Surface ablation (photorefractive keratectomy, or PRK) showed higher predictability and safety compared to RK, thus for a long time PRK became the gold standard [4, 5]. Wound healing issues following the use of early laser technology necessitated the development of other methods, such as LASIK, Epi-LASIK, LASEK, and FemtoLASIK [6–8].
While the long-term refractive results appear to be similar regardless of the surgical method used, they may be more profoundly affected by the different generations of excimer lasers. The first generation of excimer lasers operated with large beam technology (Summit, VISX), leaving central island phenomena that caused under-correction and other visual complaints [9]. Scanning slit and flying spot technology, with the use of the third generation of excimer lasers, showed significantly higher predictability and safety in myopic and hyperopic eyes. Subsequent generations of excimer lasers still operate with flying spot beam delivery, which remains the optimal technology [10, 11]. Table 1 presents the different generations of excimer lasers [1].
The different generations of excimer lasers
| Excimer laser | Features |
| 1st generation | Pre-clinical phase (Touton, VISX, Summit). This period was experimental from a technical and biological point of view, thus the technical level was rudimentary and unpredictable. |
| 2nd generation | Broad beam, fixed optical zone. A laser beam corresponding to the full treatment diameter was used, with internal masks or diaphragms corresponding to the dioptre treated. The removal of tissue vapours was not usually possible, thus so-called central islands formed due to absorption by the masking. |
| 3rd generation | Scanning-slit technology, multi-zone treatments. This was an improved broad beam technique with a variable optical zone. A laterally moving laser beam with a slit cross-section scanned the area to be treated. |
| 4th generation | Flying spot lasers, built-in eye trackers, and hyperopia treatments. A beam with a cross-section of a few millimetres, typically with a Gaussian energy distribution, produced the desired refractive result in unit shots scattered in space and time. The dispersed ablation reduced the uniform temperature rise and the masking effect of the tissue vapour released at the moment of ablation. |
| 5th generation | Wavefront-guided and optimised treatments. Aberrometry measurements are taken into account during treatment. Treatment is planned by computer, based on a preoperative aberrometry map of the eye. |
| 6th generation | High-repetition-rate laser and automatic eye-tracking system. The main objectives are to control as many environmental variables as possible; select optimal pupil size; improve ablation profiles; continuously monitor cyclotorsion and eye movements, with the corresponding correction of the laser beam; and continuously monitor corneal thickness during therapy in some systems. |
Materials and methods
The laser outpatient clinic selected for the study has been performing corneal laser vision correction procedures continuously since 1998, always using the highest technical standards available. All treatments have taken place in the operating room at the Sasszem Klinika. In both the initial and recent periods, the surgeries were performed by five ophthalmologists, all of them trained to perform laser surgery. The specialists in both periods had equally high qualifications. The only differences between the treatments offered in the two periods lies in the type and level of sophistication of the diagnostic tools, the technical capabilities of the laser devices, and precise laser control during laser firing.
The total number of treatments performed at the clinic since 1998 exceeds 100,000. The first 10,000 treatments were carried out using third-generation scanning-slit technology (using an Aesculap-Meditec MEL 60 laser). Although these excimer laser devices had already achieved high predictability, the treatment was inconvenient, since a beam-shielding mask weighing 80–100 g had to be placed on the surface of the cornea, and the treatment took about 60 s for the correction of a refractive error of −4.0 dioptres, for example.
For the second group of 10,000 evaluated treatments, a sixth-generation high-repetition-rate flying spot laser with eye-tracking technology (SCHWIND AMARIS 500, SW Version: 6.1.2117.8001) was used. This is a non-contact procedure that takes about 18 s for −4.0 dioptre correction.
In the present study, the six-month follow-up results for the two different treatments were grouped for comparability by dioptric range:
- -Low myopia (low myop): between −1.0 D and −3.0 D
- -Medium myopia (medium myop): between −3.0 D and −6.0 D
- -High myopia (high myop): above −6.0 D
- -Low hyperopia (low hyperop): up to +3.0 D
- -High hyperopia (high hyperop): above +3.0 D.
The results of the six-monthly follow-ups are shown in Table 2. The six-monthly average visus, the number of eyes, and the standard deviation are given for each group. Further subdivision by age has also been applied, as this may also affect postoperative results.
Results of the six-monthly follow-ups after refractive treatment
| Results of the first 10,000 treatments (six-monthly follow-up raw visus results) | Results of the last 10,000 treatments (six-monthly follow-up raw visus results) | ||||||||
| Age | Visus group | Eye, n | Postop visus | Age | Visus group | Eye, n | Postop visus | ||
| Mean | SD | Mean | SD | ||||||
| 18–40 | low myop | 2,598 | 1.04 | 0.03 | 18–40 | low myop | 3,419 | 1.17 | 0.13 |
| medium myop | 4,092 | 1.01 | 0.03 | medium myop | 3,886 | 1.15 | 0.15 | ||
| high myop | 1,724 | 0.82 | 0.07 | high myop | 1,475 | 1.08 | 0.20 | ||
| low hyperop | 62 | 0.80 | 0.04 | low hyperop | 14 | 0.99 | 0.18 | ||
| high hyperop | 47 | 0.79 | 0.07 | high hyperop | na | na | na | ||
| 18–40 total | 8,523 | 0.98 | 0.05 | 18–40 total | 8,794 | 1.15 | 0.15 | ||
| 41–65 | low myop | 189 | 0.94 | 0.04 | 41–65 | low myop | 240 | 1.01 | 0.29 |
| medium myop | 351 | 0.88 | 0.04 | medium myop | 523 | 0.97 | 0.31 | ||
| high myop | 342 | 0.63 | 0.08 | high myop | 185 | 0.89 | 0.29 | ||
| low hyperop | 398 | 0.95 | 0.04 | low hyperop | 214 | 0.95 | 0.32 | ||
| high hyperop | 197 | 0.79 | 0.05 | high hyperop | 44 | 0.84 | 0.26 | ||
| 41–65 total | 1,477 | 0.84 | 0.06 | 41–65 total | 1,206 | 0.96 | 0.30 | ||
| Sum total | 10,000 | 0.96 | 0.05 | Sum total | 10,000 | 1.13 | 0.19 | ||
Notes. na: not available.
In the present study, the results obtained with the third and sixth generation of excimer laser are compared.
Data analysis
No sampling was performed. The first 10,000 subjects from two time periods, 1998–2002 and 2017–2019, were included in the study. The comparison was based on the results of the six-monthly follow-up measurements, which were carried out using the same method in both cases.
To compare means, MedCalc Statistical Software version 22.0.16 was used (MedCalc Software bv, Ostend, Belgium; https://www.medcalc.org; 2020). Mean differences with 95% CI, standard error, t-value, and P-value were reported. Hedges's g was also calculated as an effect size measurement. The level of significance was set at α = 0.05.
Results
The results for the two periods are shown in Table 2. The follow-ups were performed by conventional visus testing.
To summarise, there was a statistically significant difference between the first (third-generation) and the last (sixth-generation) 10,000 treatments, regardless of age, t (19,998) = 86.528, P < 0.001, g = 1.22. The effect of the difference between the treatments was higher in the younger age group, t (17,315) = 99.413, P < 0.001, g = 1.51, compared to the 41–65 age group, t (2,681) = 15.010, P < 0.001, g = 0.44 (see Table 3).
Differences between the first and last 10,000 treatments
| Age | Visus group | Difference | SE | 95% CI | t | P | g |
| 18–40 | A-Low myop | 0.13 | 0.00 | 0.125, 0.135 | 49.969 | <0.001 | 1.30 |
| B-Medium myop | 0.14 | 0.00 | 0.135, 0.145 | 58.485 | <0.001 | 1.31 | |
| C-High myop | 0.26 | 0.01 | 0.25, 0.27 | 50.485 | <0.001 | 1.79 | |
| D-Low hyperop | 0.19 | 0.02 | 0.14, 0.24 | 7.669 | <0.001 | 2.27 | |
| E-High hyperop | na | – | – | – | – | – | |
| 18–40 Total | 0.17 | 0.00 | 0.167, 0.173 | 99.413 | <0.001 | 1.51 | |
| 41–65 | A-Low myop | 0.07 | 0.02 | 0.03, 0.11 | 3.293 | 0.001 | 0.32 |
| B-Medium myop | 0.09 | 0.02 | 0.06, 0.12 | 5.408 | <0.001 | 0.37 | |
| C-High myop | 0.26 | 0.02 | 0.23, 0.29 | 15.534 | <0.001 | 1.41 | |
| D-Low hyperop | 0.00 | 0.02 | na | 0.000 | 1.000 | 0.00 | |
| E-High hyperop | 0.05 | 0.02 | 0.01, 0.09 | 2.515 | 0.012 | 0.43 | |
| 41–65 Total | 0.12 | 0.01 | 0.10, 0.14 | 15.010 | <0.001 | 0.44 | |
| Total | 0.17 | 0.00 | 0.166, 0.174 | 86.528 | <0.001 | 1.22 | |
Notes. SE: standard error; 95% CI: 95% confidence interval for the difference; t: t-value; P: P-value; g: Hedges's effect size measurement.
In the 18–40 age group, the effect of the difference between the treatments was highest in the high myopia and low hyperopia groups. In the 41–65 age group, there was no significant difference between the treatments in the low hyperopia group. Similarly to the younger group, the effect of the difference between the treatments was highest in the high myopia group (Fig. 1).
Differences, with 95% CI, between the first and last 10,000 treatments in patients aged 18–40 and 41–65 years (A-Low myop, B-Medium myop, C-High myop, D-Low hyperop, E-High hyperop)
Citation: Developments in Health Sciences 7, 1; 10.1556/2066.2024.00063
In recent years, hyperopia of around +3.0 D has not been treated with excimer laser. Instead, clear lens exchange is suggested to patients as a treatment modality [12].
Discussion
The real value of Table 2 is that the analysis of postoperative results is based on a huge amount of data. To our knowledge, no analysis of such a large amount of data has previously been published in the literature [13–19].
The tables contain a basic set of descriptive statistics for data evaluation. In the latter 10,000 cases, the standard deviation is larger. This is not a real problem, as the standard deviation means that visual acuity is around 1.0 for patients in the lower improvement range too, which is the average visual acuity of a healthy young adult. Based on the self-reported data, patients were satisfied with the results of their treatment.
With respect to patients between 41 and 65 years of age, in both the low myopia and medium myopia groups the difference was not statistically significant, while in the high myopia group the difference was statistically significant. In the low hyperopia group, the difference was not statistically significant, while in the high hyperopia group the difference was statistically significant.
Conclusions
With its extremely high number of cases of laser corneal refractive surgery, the clinic is able to provide a remarkable amount of data, making it possible to assess whether the technical devices used influence treatment results. The analysis of the two groups of 10,000 treatments shows that the reliable third-generation laser systems, operating at the earlier technical level, already delivered good results with high predictability. The results of the comparison demonstrate the increased effectiveness of the sixth-generation laser systems. Overall, among the more recent 10,000 cases, the average visus of treated patients increased from 0.96 to 1.13 in the low myopia group. The improvement was most pronounced among patients aged between 18 and 40, treated for refractive errors of between −1.0 D and −6.0 D. Overall, it can be concluded that the improvement in technology was justified, as it resulted in the improved effectiveness of laser refractive vision correction.
Authors' contribution
MD: study conception and design, SzM: data collection, CsSz: draft manuscript, BH: analysis, JT: interpretation of results, ZZN: study conception and revise manuscript. All authors approved the final manuscript.
Ethical approval
The study was conducted in accordance with the Declaration of Helsinki and according to the requirements of all applicable local and international standards.
Conflicts of interest
The authors declare no conflict of interest. No financial support was received for this study.
ZZN and JT are members of the Editorial Board of the journal.
Acknowledgements
We would like to thank the management and staff of the Sasszem Klinika for providing us with extremely valuable treatment data.
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