Comparison of asymmetric and symmetric centration strategies for photorefractive keratectomy in myopic astigmatism patients

doi:10.21203/rs.3.rs-4887429/v1

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Comparison of asymmetric and symmetric centration strategies for photorefractive keratectomy in myopic astigmatism patients

https://doi.org/10.21203/rs.3.rs-4887429/v1

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Purpose:

To compare differences in clinical outcomes of symmetric offset (SO) and asymmetric offset (AO) centration strategies in photorefractive keratectomy (PRK) in patients with myopia or myopic astigmatism.

Methods:

Forty refractive surgery candidates who visited Farabi Eye Hospital’s refractive surgery clinic from May to August 2022 were enrolled in the study. We randomly assigned one of each patient’s eyes to the AO and the other one to the SO group using random blocks. Patients were followed for four months. Ablation profile, Visual acuity (VA), and higher order aberrations (HOA) were evaluated. Effective optical zone (EOZ) and its circularity index were calculated automatically by a MATLAB-based algorithm.

Results:

There was no significant difference between the two groups' total ablation volume and maximum and central ablation depth (all P > 0.05). The two groups showed significant differences in minimum ablation depth, with a higher value for SO than AO (0.03 ± 0.03 and 0.01 ± 0.01, respectively, P-value < 0.001). There wasn’t a significant difference between postoperative mean RMS of higher order aberrations (HOA) among AO and SO groups (all P > 0.05). The EOZ was 6.046 ± 0.63 in the SO group and 6.047 ± 0.68 in the AO group (P value = 0.61, paired t-test).

Conclusions:

In conclusion, symmetric and asymmetric offset centration strategies result in safe and effective refractive correction. There was no significant difference between the two groups' higher-order aberration, visual acuity, refraction, and postoperative total ablation volume.

Health sciences/Diseases

Health sciences/Health care

Health sciences/Medical research

Photorefractive keratectomy

PRK

Corneal vertex

Myopia

Astigmatism

Pupil offset

There are controversies regarding the optimum center for laser vision correction. Due to imperfections in the optical system of the human eye, the line of sight doesn’t match the visual axis of the eye. A decentration in laser ablation may result in high-order aberrations and decreased visual quality. Therefore, clinicians must opt for the best centration strategy. The three most popular centration techniques applied by various devices are pupil centration, symmetric pupil offset centration and asymmetric pupil offset centration ¹. Pupil centration (PC) is shown to have acceptable results in myopic patients ², but the instability of pupils according to changing light conditions disfavors this method ¹. An advantage of this technique is the reduction of total ablation volume ³. Symmetric offset (SO) or coaxial light method uses the corneal vertex (the first Purkinje on the anterior corneal surface) image as a center of ablation and excellent results are anticipated as this centration is closer to the real visual axis ^4–7. The asymmetric pupil offset (AO) considers the corneal vertex as a centration point but the edges of the optical zone cover the pupil boundaries ⁸.

Several studies compared the visual and refractive outcomes of AO and SO in laser-assisted in situ keratomileusis (LASIK) and transepithelial photorefractive keratectomy (TransPRK) on hyperopic patients since the introduction of AO in 2012 ^9–11. However, the effectiveness of AO has not been properly evaluated in myopic astigmatism patients. There is only one study in the literature comparing the visual and ablating outcomes of AO with SO in LASIK for myopic and myopic-astigmatism patients ¹². To the best of our knowledge, there is not any precedent report comparing the outcomes of AO and SO in myopic-astigmatism patients who were treated by PRK. Hence, this prospective clinical trial was designed to report the safety and efficacy of the AO centration strategy and compare its outcome with the SO ablation profile.

This randomized, single-blind study compared visual, refractive, and topographic outcomes of asymmetric versus symmetric centration strategies in myopic astigmatism patients at Farabi Eye Hospital. This prospective study adhered to the tenets of the Declaration of Helsinki and the protocol of the study was approved by the Medical Ethics Committee of Tehran University of Medical Sciences (IR.TUMS.FARABIH.REC.1400.067). In this study, 40 patients were recruited (14 males, 26 females) between May and August 2022. All included patients signed an informed consent. Inclusion criteria were 18–45 years old, preoperative manifest spherical equivalent (MRSE) of less than − 10.00, refractive cylinder of -6.00 D or less, and preoperative CDVA 20/30 or better. Patients with intraocular pressure (IOP) > 21, a history of recent hard or soft contact lens wear, a history of systemic disease, and a history of ophthalmic surgery or trauma were excluded.

Two surgeons (GL, PA) collaborated in this study and considered their patients for enrolment. At the first visit, patients underwent Sirius topography, aberrometry, pupillometry, and manifest and full cycloplegic refraction. The included patients were scheduled for correction of their refractive error by the photorefractive keratectomy (PRK) method using SCHWIND AMARIS 1050, RS. On the day of surgery, one of each patient's eyes was randomly assigned to the AO group using random blocks by the operator of the SCHWIND platform, while the other was allocated to the SO group. Surgeons and patients were blind to the allocation. Patients were visited on the postoperative day, the next week, the next month, and after four months.

Surgical technique:

The target refraction was individualized for all included patients. The optical zone was considered 7 mm for all patients; however, if the device’s software recommends an optical zone of less than 7, we set the optical zone to 6.7 mm. Both eyes of all patients were ablated with the same optical zone. The surgery procedure was as follows. After local anesthesia with 0.5% propoxyphene tetracaine eye drops (Sina Darou, Tehran, Iran), the epithelium was removed by alcohol debridement by a blunt spatula. Laser ablation was conducted with the excimer laser (SCHWIND AMARIS 1050, SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany). This device is an excimer laser with a 1050 Hz pulse repetition. Then mitomycin-C (MMC) 0.02% was applied based on the depth of ablation, followed by irrigation with a balanced sterile solution.

Post-operative Assessment:

Post-operative data, including total ablation volume, maximum ablation depth, minimum ablation depth, central ablation depth, and total ablation zone, were extracted directly from the SCHWIND device. At each follow-up, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), and manifest refraction were collected. At 4-month visits, Sirius wavefront aberrometry and topography were performed. Corneal wavefront aberrations were measured from the central 6 mm corneal zone and reported in the Zernike coefficient of aberrations format. Changes in post-operative aberration in comparison with preoperative values were calculated and analyzed.

Image processing:

We calculated angle lambda based on anterior chamber depth, corneal central thickness, and the pupil center offset using the method proposed by Gharieb et al. ¹³. As there is a little difference between the angle kappa (the angle between the pupillary and visual axis) and angle lambda (the angle between the pupillary axis and line of sight), to avoid readers’ confusion, we use angle kappa term throughout this paper ¹⁴. We utilized preoperative and follow-up anterior tangential maps to calculate the effective optical zone (EOZ). The EOZ represents the corneal surface area marked by a change of 0.25 D on the anterior tangential curvature difference map ¹⁵. For this purpose, we employed MATLAB software, version 2021b (The MathWorks). Preoperative and postoperative anterior tangential map CSV files generated by the Sirius topography system were exported to MATLAB to compute the tangential curvature difference map. The region with a difference of less than ± 0.25 diopter was regarded as the cutting boundary. All the boundaries were rechecked manually and in cases of multiple crosses over zero, manually corrected by the expert. Images from 6 eyes were excluded due to the low quality of reconstructed images. Subsequently, we created a mask from the region enclosed by this boundary and computed the area in square millimeters (mm2) (Figure.). EOZ diameter was calculated by the mean diameter of every 1 degree of the EOZ zone.

Statistical Analysis:

Results are expressed as mean ± standard deviation. The sample size of 35 was calculated with power = 90%, and significance level = 0.05 to detect a difference of higher order aberrations based on the results previous study by Li et al. The t-test was applied to compare mean preoperative values among the eyes of each patient. Post-operative values were compared using paired t-test with Bonferroni correction of P- values. All data analyses were performed using SPSS software (version 27, IBM Corp).

Eighty eyes of 40 patients with myopic astigmatism underwent PRK. The mean age was 31 ± 7 years. 26 patients were female. The preoperative characteristics for both symmetric and asymmetric groups are shown in Table 1. The MRSE was − 2.6 ± 1.39D in the AO and − 2.6 ± 1.35 SO (P-value = 0.98, t-test). The angle kappa was 2.51 ± 1.83 degrees in the asymmetric offset group (AO) and 3.01 ± 2.21 symmetric offset group (SO) (P-value = 0.28, t-test). There wasn’t any significant difference between the preoperative aberrometry of the two groups (all P > 0.05). Also, preoperative pupillometry of patients showed insignificant differences between the scotopic, mesopic, and photopic pupil dimeters of the two groups (all P > 0.05).

Table 1

Preoperative demographic, refractive, topographic, and higher order aberration data of symmetric offset group and asymmetric offset group. Abbreviation: AO, asymmetric offset; SO, symmetric offset; MRSE, manifest refractive spherical equivalent; CDVA, corrected distance visual acuity, and RMS, root mean square.
	AO	SO	Sig
Sphere	-1.83 ± 1.56	-1.93 ± 1.5	0.79
Cylinder	-1.54 ± 0	-1.34 ± 0	0.471
MRSE	-2.6 ± 1.39	-2.6 ± 1.35	0.98
Moderate myopia	64.86%	67.57%	0.806
Low myopia	35.14%	32.43%	0.806
High astigmatism	27.03%	29.73%	0.797
Low astigmatism	72.97%	70.27%	0.797
CDVA(LogMAR)	0.008 ± 0.02	0.002 ± 0.01	0.272
Angle Kappa	2.51 ± 1.83	3.01 ± 2.21	0.28
scotopic pupil size (mm)	5.5 ± 0.93	5.5 ± 0.78	0.97
mesopic pupil size (mm)	4.32 ± 0.83	4.29 ± 0.81	0.89
photopic pupil size (mm)	3.55 ± 0.59	3.53 ± 0.69	0.88
Trefoil horizontal (RMS)	-0.2 ± 0.13	-0.16 ± 0.14	0.6
Coma vertical (RMS)	0.12 ± 0.23	0.06 ± 0.19	0.15
Coma horizontal (RMS)	0.03 ± 0."	-0.01 ± 0.22	0.10
Trefoil vertical (RMS)	-0.04 ± 0.11	-0.02 ± 0.1	0.80
Spherical aberration (RMS)	0.25 ± 0.11	0.25 ± 0.12	0.70
Total higher order aberrations (RMS)	1.75 ± 0.98	1.78 ± 1.09	0.90
Sphere Target	0.18 ± 0.2	0.17 ± 0.2	0.89
Cylinder Target	0 ± 0	0 ± 0
Pupil decentration X (mm)	0.03 ± 0.19	-0.06 ± 0.18	0.02
Pupil decentration Y (mm)	0.04 ± 0.14	0.01 ± 0.12	0.40
Total Pupil offset (mm)	0.27 ± 0.15	0.25 ± 14	0.48

Table 2 summarized the postoperative ablation profiles of patients. The total ablation volume was not significantly different between the two groups (P = 0.49, paired t-test). There was also no significant difference between the maximum and central ablation depth of the two groups (all P > 0.05, Table 2). The two groups showed significant differences in minimum ablation depth, with a higher value for SO than AO (0.03 ± 0.03 and 0.01 ± 0.01, respectively; P-value < 0.001).

Table 2

Ablation characteristics of symmetric offset group and asymmetric offset group. Abbreviation: AO, asymmetric offset; SO, symmetric offset.
	SO	AO	Sig
Optical zone (mm)	6.99 ± 0.07	6.98 ± 0.08	0.46
Total ablation volume	1745.06 ± 827.99	1624.26 ± 687.84	0.49
Ablation depth max	63.16 ± 27.18	61.24 ± 24.02	0.74
Ablation depth central	61.51 ± 26.51	59.81 ± 24.45	0.77
Ablation depth min	0.03 ± 0.03	0.01 ± 0.01	0.000
Total ablation zone (mm)	8.2 ± 0.28	8.19 ± 0.26	0.9

The mean follow-up time of these patients was 119.06 ± 28.08 days. The mean postoperative MRSE was 0.02 ± 0.21D and 0.02 ± 0.25 in SO and AO, respectively (P-value = 0.8, paired t-test). There wasn’t a significant difference between postoperative mean RMS of higher order aberrations (HOA) among AO and SO groups (all P > 0.05, Table 3). The EOZ diameter was 6.046 ± 0.63 in the SO group and 6.047 ± 0.68 in the AO group (P value = 0.83, paired t-test). The EOZ center offset was 0.21 ± 0.13 63 in the SO group and 0.27 ± 0.14 in the AO group (P value = 0.83, paired t-test).

Table 3

Postoperative demographic, refractive, topographic, and higher order aberration data of symmetric offset group and asymmetric offset group. Abbreviation: AO, asymmetric offset; SO, symmetric offset; MRSE, manifest refractive spherical equivalent, CDVA, corrected distance visual acuity, and RMS, root mean square; EOZ, effective optical zone.
	SO	AO	Sig
Sphere	0.14 ± 0.25	0.16 ± 0.34	0.45
Cylinder	-0.22 ± 0.33	-0.26 ± 0.38	0.3
MRSE	0.02 ± 0.21	0.02 ± 0.25	0.8
CDVA (LogMAR)	0.004 ± 0.01	0.01 ± 0.03	0.19
EOZ area (mm²)	29.30 ± 6.22	29.46 ± 6.81	0.9
EOZ (diameter)	6.046 ± 0.63	6.047 ± 0.68	0.9
EOZ centre offset (mm) (from pupil centre)	0.21 ± 0.13	0.27 ± 0.14	0.09
Trefoil horizontal (RMS)	-0.19 ± 0.16	-0.17 ± 0.16	0.62
Trefoil vertical (RMS)	-0.03 ± 0.15	-0.04 ± 0.12	0.24
Coma vertical (RMS)	0.23 ± 0.25	0.16 ± 0.22	0.14
Coma horizontal (RMS)	0 ± 0.37	-0.02 ± 0.3	0.9
Spherical aberration (RMS)	0.36 ± 0.17	0.34 ± 0.17	0.78
Total higher order aberrations (RMS)	1.1 ± 0.42	0.99 ± 0.32	0.18
Trefoil (RMS)	-0.06 ± 0.2	-0.02 ± 0.22	0.47
Δ Coma vertical (RMS)	-0.1 ± 0.18	-0.1 ± 0.15	0.48
Δ Coma horizontal (RMS)	0.03 ± 0.2	0 ± 0.13	0.6
Δ Trefoil vertical (RMS)	-0.01 ± 0.16	0.01 ± 0.11	0.58
Δ Spherical aberration (RMS)	-0.11 ± 0.17	-0.08 ± 0.16	0.48

Figure 1 shows the standard outcome charts of refractive surgery. 89% and 95% of the eyes in the AO and SO groups achieved better than 20/20 UDVA at four months postoperatively (P-value = 0.3, Chi-square). 3% of eyes in the AO and SO groups had worse postoperative CDVA compared with preoperative CDVA. 8% and 3% of eyes in AO and SO groups had better postoperative CDVA compared with preoperative values (P-value = 0.6, Chi-square). The postoperative MRSE ranging from + 0.14 to + 0.50 D was 19% in the AO, and 14% in SO groups (P-value = 0.6, Chi-square). The rate of postoperative MRSE ranging from − 0.14 to -0.50 D was 8% in both AO and SO groups (P-value = 0.6, Chi-square).

Different ways to centre laser vision correction methods include using the pupil centre, corneal vertex, or finding a point in between while taking both into account^16–18. Asymmetric offset ablation profile in SCHWIND excimer laser is a type of combined centration that uses an oversized ablation zone centered on the corneal vertex and then fits the outer boundaries of the ablation area with a backward method and tilt removal to an area centered on the pupil center. Another option in the SCHWIND excimer laser is the symmetric offset method, which uses the corneal vertex as a centration point. It has been shown that both symmetric (SO) and asymmetric offset (AO) centration studies resulted in acceptable visual and refractive outcomes in LASIK surgery for hyperopic patients ^9,10. In a retrospective study by Ortueta et al., the safety and efficacy of the AO centration strategy were demonstrated in TransPRK surgery for hyperopia and mixed hyperopic patients. While it has been shown that angle kappa is less significant in myopic eyes ¹⁴, there is still a need to provide evidence on the safety and efficacy of AO vs SO in myopic eyes. In addition, it's important to note that angle kappa can also be substantial in myopic eyes, as demonstrated by our findings, which revealed that angle kappa ranged from 0.01 to 8.35 with a mean of 2.76 ± 2.02 (Table 1).

Both asymmetric and symmetric offset strategies were shown to have similar safety and efficacy (Fig. 2). In accordance with our report, in a study by Ortueta et al. on 47 eyes in the SO and 51 eyes in the AO groups, both centration techniques resulted in similar safety and efficacy profiles ¹¹. The post-operative accuracy of manifest spherical equivalent was similar between the two groups (92% and 89% had a postoperative MRSE within ± 0.50 D in the AO and SO groups, respectively). The previous studies achieved similar 95% postoperative MRSE accuracy using a SCHWIND excimer laser ¹⁹. We could not demonstrate clinical implications of the theoretically suggested superiority of AO over SO in terms of higher-order aberrations (HOA) in myopic astigmatism patients (8) as our results showed that there were not any significant differences between clinically important HOA, including coma, spherical aberration, trefoil and total RMS among two groups.

The results of this study revealed that there wasn’t a significant difference in the maximum and central ablation depth of SO and AO groups. Moreover, although the total ablation volume was higher in the SO group, the difference between the two groups didn’t reach statistical significance. Interestingly, the minimum ablation depth was higher in the SO group. In contrast with our findings, Li et al. showed that the maximum depth was higher in the SO group (44 eyes) than in the AO group (46 eyes) (90.6 ± 24.8 vs. 89.6 ± 23.7, P = 0.005) ¹². The insignificance of the maximum and central ablation depth between SO and AO groups in our results could be explained by the fact that the maximum ablation depth is correlated with the square of the optical zone diameter times the attempted refractive error ²⁰. As in this study, there was no significant difference between the planned optical zone (POZ) of the two groups; the slight difference caused by different centration strategies did not show any significant difference in final ablation depth and volume. Additionally, in the theoretical explanation of the asymmetric offset technique by Arba-Mosquera et al., they claimed that this method is identical to SO except that it reduces the ablation depth by removing the tilt component ⁸. The current study demonstrated that the AO laser vision correction will result in lower minimum ablation depth, not the central and maximum depth. This lower minimum ablation depth points out the added peripheral area to cover the pupil diameter in the AO method. Therefore, in myopic refractive correction, where the maximum ablation occurs on the central cornea and the peripheral optical zone ablated minimally, we shouldn’t expect a significant difference between these two methods' visual and refractive outcomes. However, in hyperopic laser ablation, the results may be different and require further research to compare the outcomes of these two centration strategies.

The optical zone (OZ) represents the segment of the cornea that light traverses to project images onto the retina. For optimal visual results post-refractive surgery, the laser targets an area encompassing both the POZ and a neighboring transition zone. The POZ is responsible for the main refractive adjustments, while the treatment zone (TZ) is essential in mitigating sudden shifts in corneal shape. Ideally, the POZ should align perfectly with the Effective Optical Zone (EOZ), which is the actual corneal area reshaped during the surgical procedure. However, this is often not the case, and EOZs are reported to be smaller than POZs in a clinically relevant manner. The discrepancy between the POZ and EOZ can have a significant impact on the success of the surgery. This reduction in the EOZ compared to the POZ has been reported to be around 20% and may be attributed to various factors^21–23. These include changes in corneal biomechanics, wound healing processes, alterations in corneal topography, and, for excimer-based procedures, a decrease in laser energy efficiency in the peripheral cornea^23,24.

The observed reduction in EOZ relative to POZ may have connections with various corneal factors. Changes in corneal biomechanics, as discussed by Damgaard, might contribute to this trend²⁵. Additionally, the process of wound healing could influence the EOZ measurements post-intervention²⁶. Alterations in corneal topography might also play a role in the reduction seen in EOZ²². Furthermore, a decreased consumption of laser energy in the peripheral cornea could be a contributing factor²⁷. These associations suggest that the reduction in EOZ is not merely a measurement variance but potentially indicative of underlying corneal changes post-procedural intervention. Understanding these relationships is critical for enhancing the precision of corneal procedures and for tailoring post-operative care to optimize patient outcomes.

With the automation of the EOZ determination process, it's pertinent to reflect upon prior research methods for defining the EOZ. Historically, studies utilized the difference map of the tangential anterior, coupled with mouse tracking across 12 half corneal meridians (spaced every 30°), centering on the corneal vertex. By manually setting points on each half-meridian, the distance between them was calculated. The mean distance spanning all six meridians then provided the EOZ diameter¹⁵. In a different approach, a study introduced the Region-Growing Algorithm to define the EOZ²⁸. Adding to the methodologies, another approach presented an automatic calculation of EOZ for the tangential curvature map. This technique calculates the EOZ area by examining image pixels, specifically where the boundary between two colors exists. This area is then converted to square millimeters, from which the diameter is derived.

In our series of patients, we didn’t find a significant difference between the EOZ of the AO and SO groups. In addition, we didn’t find a significant difference between postoperative higher-order aberrations of the two groups. Also, we calculated the circularity index of EOZ. The similarity between the circularity index of two ablation profiles could explain the similarity between higher-order aberrations of the method. In contrast with our findings, Li et al. found the EOZ diameter was significantly higher in SO than the AO group (5.01 ± 0.22, 4.96 ± 0.15; SO, and AO respectively, (P value = 0.01)¹². They also found coma and trefoil were higher in AO than SO. This difference could be explained by the fact that in our series all the patients have similar optical zones (Table 1). Additionally, Li et al. only reported the diameter of manually calculated EOZ which could be imprecise ²⁹. As Arba Mosquera et al. explained the main difference between AO and SO centration strategies is about the peripheral ablation zone that is ablated regarding the pupil boundaries⁸. However, it seems that in myopic astigmatism patients, these two centration studies didn’t show a significant difference, and the optical zone adjustment is the key variable. Further comparative studies are required to evaluate the results of AO and SO on hyperopic and mixed astigmatism patients.

The strength of the current study is that many personal confounding factors, including age and sex, were compensated due to the method of group allocation. We randomly assigned one eye of each patient to one group and the other one to another group. However, this study had several limitations. First, we only included myopic and myopic astigmatism patients. Second, in our series, there weren’t any extreme myope patients (myopia of more than 8 diopters) (The mean preoperative MRSE in our patients was − 2.66, and the maximum MRSE was − 6.63). Third, the potential confounding effects of epithelium were not studied. Finally, we followed patients for 4 months. Hence, further studies with longer follow-up periods should be done.

In conclusion, both symmetric and asymmetric offset centration strategies result in safe and effective refractive correction in myopic astigmatism patients. We have not found any significant difference between the postoperative total ablation volume of the two groups. The higher-order aberration, visual acuity, and refraction were similar in the mean 4-month follow-up period.

conflicts of interest

The authors have no conflicts of interest.

Author Contribution

Farhad Salari.Mahsan Samadi. Mehrnaz Atighehchian and Zahra Montazerian wrote the main manuscript text and Hemed Ghassemi.Parisa Abdi.Golshan Latifi and Samuel Arba Mosquera prepared figures 1-2.All authors reviewd the manuscript.

Data Availability

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request

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Comparison of asymmetric and symmetric centration strategies for photorefractive keratectomy in myopic astigmatism patients

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