Open access peer-reviewed chapter

Modern Refractive Lenticular Femtosecond Laser Corneal Surgery for Correction of Myopia and Myopic Astigmatism

Written By

Maja Bohač, Mateja Jagić, Doria Gabrić, Lucija Zerjav, Smiljka Popović Suić and Iva Dekaris

Submitted: 11 March 2022 Reviewed: 04 May 2022 Published: 15 June 2022

DOI: 10.5772/intechopen.105159

From the Edited Volume

Refractive Surgery - Types of Procedures, Risks, and Benefits

Edited by Maja Bohač and Mateja Jagić

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Abstract

Small-incision lenticule extraction (SMILE) is becoming the procedure of choice in treating myopia and myopic astigmatism. With great comparability in terms of visual outcome with the femtosecond laser-assisted in situ keratomileusis (FsLASIK) procedure, the method is characterized by better patient satisfaction and less postoperative dry eye induction. Moreover, it has the advantages of better eye surface stability and biomechanical strength compared to FS-LASIK. The method is now globally accepted among refractive surgeons. Patients suitable for the procedure must meet criteria for keratorefractive procedures generally. Our current clinical experience suggests that the lenticule extraction procedure delivers promising refractive results in terms of predictability, efficacy, and safety.

Keywords

  • lenticule extraction
  • SMILE
  • LASIK
  • femtosecond laser
  • myopia
  • refractive surgery

1. Introduction

LASIK is the most commonly used corneal refractive surgical procedure to treat ametropia worldwide [1, 2]. Compared to earlier microkeratome variant, femtosecond laser-assisted laser in situ keratomileusis (FsLASIK) provides precise flap creation achieving better morphological stability. Even so, flap related complications, induction of higher-order aberrations, as well as biomechanical corneal instability are still present [3, 4, 5]. When ablating stroma between 10 and 30% of depth, LASIK is estimated to reduce the tensile strength of the stroma by about 35% [6, 7, 8].

In recent years, the lenticle extraction method has gradually become popular as a potential alternative for traditional LASIK and PRK procedures. The femtosecond laser-assisted corneal procedure known as small-incision lenticule extraction (SMILE) was first described by Sekundo et al. in 2008 [9] and after larger series followed, the procedure became clinically available in 2011. Using an ultrashort pulse laser system, procedure delineates contour of tissue volume that needs to be excised in order to accomplish refractive correction. It is a flapless procedure where two precise intrastromal planar sections are created by femtosecond laser forming the lenticule that is manually extracted through a superiorly (nasal/temporal) placed small 2–5 mm length incision after careful dissection from the pocket. When removing intrastromal lenticule, corneal shape is altered without Bowman’s membrane disruption, therefore procedure offers biomechanical stability of the cornea, especially in treatment of higher levels of myopia and astigmatism [6, 9]. Since there is no flap creation, lenticule extraction procedure rules out formerly known risks in LASIK procedures, such as flap creation complication and dislocation [6, 7, 8, 10, 11, 12].

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2. Small-incision lenticule extraction

Recently, two emerging alternatives have been introduced in the market: CLEAR using Z8 by Ziemer, Switzerland [13, 14, 15] and SmartSight using ATOS by SCHWIND eye-tech-solutions, Germany [16, 17].

CLEAR (Corneal Lenticule Extraction for Advanced Refractive correction) treatment is an additional treatment program from FEMTO LDV Z8, which is a multipurpose laser (cataract surgery, corneal transplantation, flap creation for LASIK, tunnel/pocket creation for inlays, arcuate incision). In the technical aspect, it works under pulse energies below 100 nJ with a repetition rate above 20 MHz and a spiral raster laser pattern [15]. Besides eye-tracking guided centration, the laser system has intraoperative OCT, which is predominantly used for cases of corneal transplantation, tunnel creation for inlays, and cataract surgery. The ability to create two side-cuts potentially reduces the learning curve for less experienced surgeons since tunnels guide directly to the anticipated plane of the lenticule (anterior or posterior) [13, 14].

SmartSight treatment profile by SCHWIND ATOS, without using side cuts, does not have a minimal thickness (as in SMILE) and includes lenticule tapering toward the periphery, a refractive progressive transition zone, to achieve minimal refractive regression by reducing epithelial remodelling [17]. The laser works in the plasma-mediated ablation regime, slightly above the threshold for laser-induced optical breakdown, and below the photodisruption regime. It works under pulse energy below 100 nJ, with spot spacing >4 μm and track spacing ~3 μm, with a repetition rate up to 4 MHz, and an asymmetric scanning pattern. The laser system has cyclotorsion control, where it incorporates a video-based eye registration from the diagnostic image along with an eye-tracker guided centration to improve the predictability of the astigmatic corrections (Figure 1).

Figure 1.

Video-based eye registration (cyclotorsion control) from the diagnostic image along with an eye-tracker guided centration inside the Schwind ATOS.

When forming and extracting lenticule in SMILE procedure from anterior half of the stroma, the tensile corneal strength is reduced by 55% while this effect is less profound in the case of lenticule formed in deeper stromal layers [7]. Therefore, extent of changes in biomechanical corneal properties is depending on the lenticule volume and location (depth) in the cornea [7, 8, 18].

The differences between SMILE and FsLASIK are potential sources that could influence the final refractive and overall optical performance of the eye after surgery by inducing unwanted astigmatism. Moreover, there has been an increasing awareness and understanding of the change in higher-order optical aberrations following corneal refractive surgery over the last two decades. It is widely accepted that higher-order aberrations should be either maintained after surgery at preoperative levels or modified to improve the overall optical and visual performances of the eye [19, 20, 21, 22].

2.1 Indications

Indications for lenticule extraction adhere to the guidelines for all corneal refractive surgical procedures [23].

Prior to the decision if the patient meets the criteria for refractive surgery complete ophthalmologic examination is needed. The examination includes uncorrected distance visual acuity, corrected distance visual acuity, manifest and cycloplegic refraction, corneal tomography, corneal and ocular aberrometry, tonometry, slit lamp, and dilated funduscopic examination.

Patients with stable refraction, myopia up to −10.00 D, and astigmatism up to 5 D or SE up to 12.50 D with sufficient corneal thickness and normal tomography are considered eligible candidates. As the most common contraindications would be considered: abnormal corneal topography, signs of progressive preoperative corneal thickness <480 μm or calculated residual stromal bed thickness <275 μm, scotopic pupil wider than 7.5 mm, dry eye, inflammation of ocular adnexa and periocular area, active autoimmune disease or connective tissue diseases.

2.2 Surgical procedure

The surgery is performed under topical anesthesia. After standardized cleaning with 2.5% povidone-iodine and sterile draping, an eyelid speculum is used to keep the eye open. After positioning patient on the surgical bed, and connecting the surgical cone (disposable interface) to suction ports, the patient is instructed to fixate the light target when the eye is aligned with the cone. When centration coincides with the visual axis and there is visible matching of corneal vertex (from corneal tomography), suction can be applied, followed by treatment initialization and laser ablation immediately after complete suction is achieved. Caps can be 100–150 μm thick and incisions are usually positioned superotemporal with width between 2.5 and 3.2 mm. The optical zone selected depends on the scotopic pupil size and attempted correction. Automatic suction release occurs upon completion of lenticule formation. After identifying both anterior and posterior lenticular surface with thin blunt spatula, separation of the lenticule and extraction through the side cut is performed. In order to detect any residual material or tears, lenticule tissue is thoroughly inspected.

2.3 Clinical results

In two separated studies we were evaluating outcomes, safety, efficacy, and predictability of small-incision lenticule extraction procedures performed at different laser systems. For treating myopia and myopic astigmatism. In first study, ReLEx SMILE procedure was performed on VisuMax from Zeiss, with comparing refractive and visual outcomes with FsLASIK procedure performed on VisuMax for flap creation and Schwind Sirius 750s for excimer ablation at one-year period. The second study was conducted on Atos for Schwind eye-tech-solutions, performing SmartSight lenticule extraction procedure. During a three-month follow up refractive, wavefront, and topographic outcomes were evaluated. The results of both studies are presented below.

2.3.1 Smile vs FS LASIK

2.3.1.1 Astigmatism

There was a significant difference in the magnitude of astigmatism between the SMILE and the FsLASIK groups one year after the surgery [24]. Postoperatively, the amount of any astigmatism revealed by subjective refraction results from a combination of the treated astigmatism coupled with the effects of postoperative healing. In the SMILE group, we encountered more residual manifest astigmatism compared with the FsLASIK group. Vector analysis of astigmatism did not show any difference between the two groups prior to surgery. Both mean J0 and J45 values were slightly lower in the FsLASIK group in comparison with the SMILE group indicating that astigmatism is less prevalent after FsLASIK (Figures 25). This indication is further supported by the slightly higher surgically induced astigmatism values following SMILE compared with FsLASIK. Both techniques of vector analysis show that individual differences between the vector value pre- and postoperative were strongly correlated with the preoperative vector values. This is encouraging indicating that for individual cases the postoperative astigmatic vector values can be predicted with precision using the preoperative astigmatic value in both SMILE and FsLASIK. The Thibos' method of vector analysis [25], clearly points out that within the SMILE group the correlation between ΔJ45 and preoperative J45 (0.792) tended to be lower in comparison with the counterpart in the FsLASIK group (0.924). This suggests that the precision of controlling a change in astigmatism with FsLASIK is superior compared with SMILE.

Figure 2.

Change in J0 vector value in each case treated with SMILE procedure. Significant association between the change in J0 (ΔJ0) and preop J0 value presented as linear regression. The least squares line: ΔJ0 = 1.015J0 + 0.040 (R = .861, N = 89, P < .001).

Figure 3.

Change in J45 vector value in each case treated with SMILE procedure. Significant association between the change in J45 (ΔJ45) and preop J45 value is presented as linear regression. The least squares line: ΔJ45 = 1.082J45 + 0.019 (R = .792, N = 89, P < .001).

Figure 4.

Change in J0 vector value in each case treated with FsLASIK procedure. Significant association between the change in J0 (ΔJ0) and preop J0 value is presented as linear regression. The least squares line: ΔJ0 = 0.952J0 − 0.005 (R = .921, N = 92, P < .001).

Figure 5.

Change in J45 vector value in each case treated with FsLASIK procedure. Significant association between the change in J45 (ΔJ45) and preop J45 value. Is presented as linear regression. The least squares line: ΔJ45 = 0.962J45 − 0.002 (R = .923, N = 92, P < .001).

Turning to the mean target and surgically induced astigmatism values, in the FsLASIK group the target and surgically induced astigmatism values were nearly identical. This can only occur when the residual astigmatism is almost totally nullified. In the SMILE group, the mean surgically induced astigmatism was significantly higher than the target induced astigmatism (−0.57 D and −0.41 D respectively). This indicates that the SMILE procedure tends to overcorrect and even induce astigmatism. The centration is different for both SMILE and FsLASIK procedures, wherein SMILE, procedure is centred on the visual axis and FsLASIK is centred on the corneal vertex. In the event that the intersection of the corneal surface and the visual axis does not coincide with corneal apex, a smaller amount of unwanted astigmatism may be predicted [26]. Given the procedure centration on corneal vertex, this should more likely occur after FsLASIK. Other factors must be responsible for the increased astigmatism after SMILE.

In Figures 6 and 7 vector diagrams demonstrate the unwanted induced astigmatism that occurred in some cases, where surgically induced astigmatism values appear more dispersed from the central point in the SMILE group compared with the FsLASIK group. Of a total of 89 eyes treated with SMILE procedure, at one-year postop we found three cases where astigmatism increased by 0.75 D and 10 cases where astigmatism increased by 0.50 D. The results of astigmatic corrections after SMILE differ among authors. Some authors reported no significant differences in postoperative astigmatism between SMILE and FsLASIK, and no significant increases in astigmatism [27, 28]. On the other hand, others reported more favourable outcomes after FsLASIK [29]. In addition, Kunert et al. [30] and Qian et al. [31] reported up to 1.00 D overcorrection of astigmatism and an overall undercorrection of high astigmatism after the SMILE procedure. None of the available reports mentions or discusses cases where astigmatism becomes manifest during the postop period. Unexpected postoperative astigmatism following a SMILE procedure could, to some extent, be explained by insufficient intraoperative centration, decentration of refractive lenticule ablation profile relative to the visual axis, dislodged fragments from the lenticule (although we did not encounter any), and the impact of any epithelial hyperplasia during the postoperative period. The lower incidence of astigmatism in the FsLASIK group may be linked to the advanced eye-tracking devices designed to compensate for any cyclotorsional effect and eye movements during the excimer laser ablation [32]. For the SMILE procedure centration was achieved manually after instructing the patient to fixate a blinking green light and locking the laser procedure about the visual axis using suction ports [6]. Slight tilting of the lenticule, in association with any decentration, would further contribute to any unexpected postop astigmatism.

Figure 6.

Polar diagram showing target and surgically induced astigmatic values for the SMILE group. The targeted surgically induced astigmatism data points are shown as empty circles and filled dots respectively, with semicircles from −2 DC to 0 (central point) in 0.5DC steps and from 0°to 90°and 180° (right to left) in 30° steps.

Figure 7.

Polar diagram showing target and surgically induced astigmatic values for the FsLASIK group. The target and surgically induced astigmatism data points are shown as empty circles and filled dots respectively, with semicircles from −2 DC to 0 (central point) in 0.5DC steps and from 0°to 90°and 180° (right to left) in 30° steps.

2.3.1.2 Higher order aberrations (HOAs)

At one-year postop, significant differences between the two groups were found for all higher-order aberrations (HOAs). Coma, trefoil, and spherical aberration (SA) tended to be lower in the FsLASIK group compared with SMILE. In the SMILE group, a significant increase in postoperative SA was revealed while there were no differences for coma or trefoil. For the FsLASIK group, significant changes in coma and trefoil were observed but not for SA. The changes in the mean values of some HOAs were statistically significant, but their clinical relevance is open to question. Figures 813 show there are highly significant correlations between changes in coma, trefoil, and SA in individual cases when compared with preoperative values. The results of these linear regressions can be used to predict the likely change in an HOA we can expect to encounter after surgical intervention on an individual case-by-case basis. For example, Figures 8 and 9 show preoperative values for coma below 0.15 μm are not expected to change greatly after either SMILE or FsLASIK. The magnitude of coma is predicted to fall by approximately 0.14 μm after either procedure when the preop value is in the region of 0.30 μm. Turning to Figures 12 and 13, when the preoperative SA is of the order +0.10 μm the postoperative value should reduce by nearly 50% after either SMILE or FsLASIK. However, if the preoperative was −0.10 μm the predicted postoperative value after SMILE is +0.010 μm and +0.002 μm after FsLASIK. Thus, when refractive surgery is the desired option, it would be advisable to treat highly aberrated eyes with FsLASIK.

Figure 8.

Change in coma value in each case treated with SMILE procedure. Significant association between the change in coma (y) and preop coma (x) value presented as linear regression. The least squares line: y = 0.847x − 0.094 (R = .562, N = 89, P < .001).

Figure 9.

Change in coma value in each case treated with FsLASIK procedure. Significant association between the change in coma (y) and preop coma (x) value presented as linear regression. The least squares line: y = 0.688x − 0.034 (R = .743, N =92, P < .001).

Figure 10.

Change in trefoil value in each case treated with SMILE procedure. Significant association between the change in trefoil (y) and preop trefoil (x) value presented as linear regression. The least squares line: y = 0.793x − 0.057 (r = .515, N = 89, P < .001).

Figure 11.

Change in trefoil value in each case treated with FsLASIK procedure. Significant association between the change in trefoil (y) and preop trefoil (x) value presented as linear regression. The least squares line: y = 0.741x − 0.027 (R = .618, N = 92, P < .001).

Figure 12.

Change in spherical aberration (SA) value in each case treated with SMILE procedure. Significant association between the change in SA (y) and preop SA (x) value presented as linear regression. The least squares line: y = 0.832x − 0.027 (R = .779, N = 89, P < .001).

Figure 13.

Change in spherical aberration (SA) value in each case treated with FsLASIK procedure. Significant association between the change in SA (y) and preop SA (x) value presented as linear regression. The least squares line: y = 0.428x + 0.004 (R = .545, N = 92, P < .001).

Our results conflict with other published reports. Wu et al. [33] reported the magnitude of all higher-order aberrations increased after either SMILE or FsLASIK. However, after surgery, the average values for SA and horizontal coma were lower in the SMILE group compared with the FsLASIK group. Lin et al. [34] also reported increases in all ocular higher-order aberrations after both SMILE and FsLASIK but, with significantly lower values of SA and coma after the SMILE procedure. Others report that contrast sensitivity improved after SMILE implying more favorable high order aberration profiles [6, 28]. Our experience does not support previous reports because we found SA increased after SMILE with coma and trefoil reduction after the FsLASIK. The differences between some reports may be due to several factors such as geographical factors. For example, the work of Wu et al. [33] Lin et al. [34], and Liu et al. [35] were based in Southeast Asia, and the work by Ganesh et al. [36] was based in India. Our results were obtained predominantly from Caucasian eyes. The differences in the outcomes between studies can result from a variety of reasons including genetic factors. However, results based on studies in other territories are concordant with the findings from Asia [6, 27, 37].

2.3.1.3 Conclusion

In conclusion, our experience with both procedures yields satisfactory visual acuity results. However, FsLASIK offers a marginally improved outcome as indicated by the residual high order aberrations and astigmatism.

2.3.2 SmartSight lenticule extraction on SCHWIND ATOS

2.3.2.1 Efficacy and safety

The short-term changes at three-month follow-up of the efficacy and safety of lenticule extraction treatments using the SmartSight profile were analyzed.

The main difference and advantage of SCHWIND ATOS and SmartSight at this time of development is the low energy delivered to the cornea since the laser works slightly above the threshold for the laser-induced optical breakdown with energies between 80 and 100 nJ. In addition, the laser also possesses features such as cyclotorsion control and eye-tracker guided centration. Lack of the abovementioned technologies was one of the main drawbacks for the surgeons in transition from excimer laser-based procedures to lenticular extraction and was often emphasized as the main shortcoming in the treatment of a higher amount of astigmatism.

The analysis revealed promising results after the treatment. The unaided vision was expected to improve overall. Most of the outcome measures showed significant improvement compared to the preoperative status. The improvement in visual acuities was significant (Figures 1416).

Figure 14.

Standard graphs for reporting outcomes in laser vision correction: Cumulative Snellen Visual acuity.

Figure 15.

Difference between UDVA and CDVA.

Figure 16.

Accuracy of MRSEq to intended target (D).

2.3.2.2 Refractive outcome and keratometry

An excellent refractive outcome was observed in terms of manifest refraction, but this was only partly confirmed by the objective refraction and the topographical changes. This suggests that manifest refraction may be more forgiving in terms of exactly determining the accuracy of the treatments, but at the same time, UDVA is the main driver for patient satisfaction. CDVA loss of two lines occurred only in a single eye (Figure 17).

Figure 17.

Change in Snellen lines of CDVA.

At three months after the surgery, for the change in wavefront refraction or corneal keratometry 68% of eyes were within 0.5D from target (Figures 18 and 19), with 63% and 58% of eyes within 0.5D from target astigmatism for wavefront refraction and corneal keratometry, respectively (Figures 20 and 21). The angle of error was within 25° from the attempted astigmatism axis in 60% and 42% of the eyes for wavefront refraction and corneal keratometry, respectively (Figure 22).

Figure 18.

Wavefront refraction vs. attempted SEQ (D).

Figure 19.

Accuracy of SEQ to intended target (D).

Figure 20.

Scattergram of achieved change in wavefront refraction vs attempted correction of the astigmatism.

Figure 21.

Percentage of eyes within intended target of postoperative astigmatism.

Figure 22.

Angle of error from attempted astigmatism axis.

Previous publications, like recent studies by Sideroudi et al. [38] and Ganesh et al. [39], suggest that undercorrection in SMILE can be associated with forward shifting of posterior corneal surface that leads to posterior curvature steepening. Opposite to our findings, some works report lower changes observed in keratometries than in refraction. This could be due to using simpler models and not considering difference in refractive indices (used for keratometry) and actual refractive corneal index, the effect of central tissue removal on refraction, or effect of the vertex distance on planned refraction (spectacle plane to corneal plane). Taking this into consideration, The SmartSight profile involves tapering the lenticule toward the edge to achieve smoothing of the transition zone from treated to the untreated cornea in an attempt to reduce the biomechanical changes and epithelial remodelling on the edge of the treatment. It is determined as refractive progressive transition zone, similar to the one used in the SCHWIND AMARIS ablation profiles, ranging from 0.2 mm to 0.8 mm, determined by corneal curvature gradient and also induced by correction.

In this study at three months, the scattergram of achieved change in wavefront refraction vs. achieved change in keratometry readings of the SEQ showed a very good correlation (Figure 23), with 75% eyes within 0.75D (Figure 24).

Figure 23.

Scattergram of achieved change in wavefront refraction vs achieved change in keratometry readings of the SEQ.

Figure 24.

Agreement of change in SEQ between wavefront refraction and keratometry readings.

2.3.2.3 Corneal and ocular wavefront (aberrations)

Corneal aberrations slightly increased after the treatment, but the change of ocular aberrations was very minor and non-significant (Figures 25 and 26). This may confirm the relatively neutral behaviour in terms of aberrations reported from other refractive lenticule extraction techniques, as well as be indicative of adequate centration. SA was less positive when measured with ocular aberrations than for corneal aberrations. Postoperative corneal SA increased more than ocular SA, remaining stable at three months follow-up. The RMS higher-order aberrations increased, both for corneal and ocular aberrations, with corneal aberrations showing systematically higher inductions HOA than the ocular counterparts (Figure 27). Corneal topography and aberrometry revealed an induction of positive SA associated with an increase in the RMS higher-order aberrations.

Figure 25.

Preoperative and postoperative corneal wavefront aberrations.

Figure 26.

Preoperative and postoperative ocular wavefront aberrations.

Figure 27.

Change in postoperative HOAs from preoperative baseline.

2.3.2.4 Conclusion

A limitation of this work is that only 50 eyes of 31 consecutive patients completed the three-months follow-up and were included for analyses. Another limitation is the retrospective nature of the study. Several confounding factors may be argued in our review, we have considered both eyes of the patients.

These clinical results are presented based on a three-month clinical follow-up, which is considered minimal for establishing notable clinical significance in refractive surgery. In literature, however, there are results with shorter follow-ups reported for determining the time-course of visual recovery. Studies with longer follow-ups and a greater number of clinical cases will shed light on the durability of performance and allow for further nomogram refinement to improve outcomes.

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3. Conclusions

When achieving excellent clinical visual outcomes in refractive surgery, it is often difficult to demonstrate that novel procedures like lenticule extraction are superior to the standardized LASIK procedure. Up to this point, comparable outcomes in terms of refractive predictability, efficacy, and safety at minimum of three months were found, also theoretical biomechanical advantage of lenticule extraction over Fs. LASIK was described in the literature. Still, a longer learning curve for the surgeons, more frequent suction loss occurrence, prolonged visual recovery, and complicated enhancement treatment have been observed when comparing lenticule extraction to traditional Fs. LASIK. Aforementioned requires further enhancement and refinement of the procedure. Given the increasing clinical use over the last decade, lenticule extraction treatment has continuously been optimized and improved through multiple iterations. Introduction of new laser platforms such as CLEAR and SmartSight, with different energy levels, repetition rates and spot spacing has significantly improved visual outcomes. Precisely, combining high frequency and low energy profile for smooth cutting results in lenticule surface that could provide better clinical performance and optical quality for each laser platform. SmartSight treatment includes even a refractive progressive transition zone tapering the lenticule towards the edge of the transition zone to reduce epithelial remodelling and, therefore refractive regression. Additionally, eye tracking, the centring according to pupil, vertex or defined offset by surgeon, and the video-based cyclotorsion compensation are particularly helpful in astigmatism correction. More studies involving a larger number of patients with longer follow-up will evaluate if new profiles and laser platforms can improve already achieved good visual outcomes after lenticule extraction.

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Conflict of interest

The authors declare no conflict of interest.

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Appendices and nomenclature

LASIK

laser in situ keratomileusis

SMILE

small-incision lenticule extracton

FsLASIK

femtosecond laser-assisted in situ keratomileusis

PRK

photorefractive keratectomy

CLEAR

corneal lenticule extraction for advanced refractive correction

OCT

optical coherence tomography

D

diopter

DC

diopter cylinder

SE, SEq

spherical equivalent

HOA

higher order aberration

SA

spherical aberration

UDVA

uncorrected distance visual acuity

CDVA

corrected distance visual acuity

RMS

root mean square

OW

ocular wavefront

CW

corneal wavefront

nJ

nano Joule

MHz

mega Hertz

μm

micrometer

J0

vector of astigmatism power at axis of 90° and 180°, so-called Cartesian or with-the-rule astigmatism

J45

vector of astigmatism power at axis of 45° and 135°, so called oblique astigmatism

ΔJ45

overall change in value of J45

ΔJ0

overall change in value of J0

R

value that indicates a linear correlation between variables

P

measure of the probability that an observe difference could have occurred

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Written By

Maja Bohač, Mateja Jagić, Doria Gabrić, Lucija Zerjav, Smiljka Popović Suić and Iva Dekaris

Submitted: 11 March 2022 Reviewed: 04 May 2022 Published: 15 June 2022