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Table of Contents
Year : 2021  |  Volume : 3  |  Issue : 1  |  Page : 15

Wavefront-guided photorefractive keratectomy after LASIK for high myopia

1 Department of Opthalmology, Paulista Ophthalmological Unit (UPO), São Bernardo do Campo; Department of Ophthalmology, Sorocaba Ophthalmological Hospital, Sorocaba, Brazil
2 Department of Ophthalmology, State University of Campinas, Campinas, Brazil
3 Department of Optics and Visual Science, Federal University of Sao Paulo, São Paulo, Brazil
4 Department of Opthalmology, Paulista Ophthalmological Unit (UPO), São Bernardo do Campo, Brazil

Date of Submission14-Feb-2020
Date of Acceptance16-Apr-2021
Date of Web Publication18-May-2021

Correspondence Address:
Dr. Livia Cristina Rios
Rua Joel Ribeiro 330, Jardim Emilia 18031005, Sorocaba, SP
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/pajo.pajo_69_20

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In this report, we discuss a case of a high myopic patient who underwent mechanical LASIK surgery in 2008 that respected the Randleman criteria for ectasia risk but incurred in a percentual thickness alteration (PTA) over 40%. The patient underwent reoperation in 2016 to correct the residual refractive error with wavefront-guided photorefractive keratectomy. At the time of the first surgery, the concept of PTA did not exist. Currently, a PTA that exceeds 35%–40% correlates with an increased risk of ectasia. We reviewed the literature focusing on the differences between the current rationale for post-LASIK enhancement and the selected strategy at the first reintervention. The purpose of this provocative case report is to emphasize the importance of personalized surgery in reoperations, always aiming for the welfare and best vision for the patient.

Keywords: Ectasia, keratoconus, laser, LASIK, risk

How to cite this article:
Rios LC, Barcelos R, Mendonça Junior AC, Silva PG, Rodrigues PF, Moscovici BK. Wavefront-guided photorefractive keratectomy after LASIK for high myopia. Pan Am J Ophthalmol 2021;3:15

How to cite this URL:
Rios LC, Barcelos R, Mendonça Junior AC, Silva PG, Rodrigues PF, Moscovici BK. Wavefront-guided photorefractive keratectomy after LASIK for high myopia. Pan Am J Ophthalmol [serial online] 2021 [cited 2022 May 25];3:15. Available from: https://www.thepajo.org/text.asp?2021/3/1/15/316303

  Introduction Top

One of the main causes of reversible visual impairment in the world is refractive error and the objective of refractive surgery is to allow spectacle independence.[1],[2],[3]

High ametropias, especially with older models of excimer lasers, have always been a challenge due to the increase in high-order aberrations and a greater chance of residual ametropia. 1-5 In this case report, we show a tall myopic patient undergoing surface retreatment, guided by aberrometry, years after LASIK surgery.

  Case Report Top

A 26-year-old healthy male physician with static refraction of −8. 00 DS −2.25 DC 175° in the right eye and −8.50 DS −2.25 DC 180° in the left eye underwent specific tests for refractive surgery screening. He presented peripheral lattice degeneration in both eyes, without rupture; however, otherwise, his eyes were healthy. The optical biometer intraocular lens (IOL) MASTER (Zeiss, Jena, Germany) showed an axial length of 25.88 mm in OD and 25.93 mm in OS. The anterior surface and aberrometry were analyzed with Orbscan II corneal tomography (Bausch and Lomb®, Rochester, New York, USA) and OPD scan-III aberrometer (NIDEK, Japan), respectively [Figure 1]a and [Figure 1]b. The Orbscan showed a pachymetry of 548 μm in OD and 553 μm in OS, maximum keratometry of 47.5 diopters (D) in both eyes, and a superior-inferior asymmetry (I/O index) of 0.1 D in both eyes. The OPD scan showed high-order aberrations (HOAs) of 0.373 μm in OD and 0.414 μm in OS with 6 mm pupil diameter. Therefore, a LASIK with a microkeratome was planned.
Figure 1: (a) Corneal tomography (2008) – OD and OS. (b) Aberrometry (2008) – OD and OS

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First surgery

The Hansatome™ Bausch and Lomb microkeratome (Bausch and Lomb, Rochester, New York, USA) was used, with a planned flap thickness of 160 μm. The suction ring was 8.5 mm.

The LASIK surgery was performed with NIDEK EC-5000CXIII excimer laser (NIDEK, Bunkyo City, Japan), which has a beam size of 2 mm × 9 mm, a scanning slit with Gaussian profile, a pulse frequency of 5–50 Hz, and a 200 Hz eye tracker. A small optical zone of 5.5 mm was planned to minimize corneal consumption according to Munnerlyn formula.[1] The total treatment area was 7.0 mm with an ablation depth of 110 μm in both eyes.

There were no complications during the early and mid-term postoperative follow-up.[1] Thirty days after surgery, the subjective refraction was −0.75 DS in OD and −0.25 DS −1.00 D 45° in OS and after 6 months presented − 1.00 DS in OD and −0.75 DS − 1.00 DC 45° in OS.

Late postoperative period after the first surgery

Eight years after the first surgery, the patient had residual myopia and astigmatism and desired enhancement. He presented −1.5 DS −0.50 DC 180° in OD and −1.25 DS −1.50 DC 045° in OS, with 20/20 BCVA. His dominant eye was OD, and although he was not presbyopic yet, he was comfortable using monovision regardless of dominance.

The patient performed iDesign® aberrometry (Johnson and Johnson, New Jersey, USA), Scheimpflug Pentacam® corneal tomography (Oculus, Wetzlar, Germany), and Visante® optical coherence (Carl Zeiss AG, Oberkochen, Germany). His examinations demonstrated a pachymetry of OD 464 μm and OS 460 μm [Figure 2]a and HOAs of 0.87 μm in OD and 0.82 in OS μm [Figure 2]b. The flap thickness measured by the OCT showed 110 μm in OD and 160 μm in OS, and the epithelial map showed increased pachymetry at the center. The comparative Pentacam between 2010 and 2016 showed no changes of the following parameters: anterior axial map (keratometry), posterior elevation best fit sphere (BFS), and pachymetry maps. Evaluating patient needs, monovision was programmed to minimize spectacle dependency.
Figure 2: (a) Corneal tomography (2016) – OD and OS. (b) Aberrometry (2016) – OD and OS

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Second surgery

Wavefront-guided photorefractive keratectomy (WG-PRK) was performed in the OS with corneal deepithelialization assisted by 20% alcohol for 20 s. At the end of the procedure, mitomycin C (MMC) was applied for 2 min. In the postoperative period, the patient was satisfied and presented static refraction of +0.75 DS with 20/20 visual acuity and stability [Figure 3]. He also improved the HOA measurement in OS with a decrease of the root mean square (RMS) value, from 0.82 to 0.64.
Figure 3: (a) Profile ablation for the second surgery in OS (wave-guided). (b) Improvement of high-order aberration in OS after personalized surgery with RMS value from 0.82 to 0.64

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The WG-PRK was performed with VISX SR4 excimer laser (Johnson & Johnson, New Brunswick, EUA), which has a beam size of 0.65–6.5 mm with a variable spot Gaussian profile, a 10–20 Hz frequency, and a 60 Hz eye tracker. Surgical settings were as follows: 160 Hz fluence, an optical zone of 6.7 mm × 6 mm, and a total treatment zone of 8 mm. HOAs were analyzed considering a pupil diameter of 6.33 mm, and the depth of ablation was 41 μm [Figure 3]. At the most recent examination, 12 years after the first surgery, and 4 years after the second surgery, the patient refraction is stable, with no signs of corneal ectasia and 20/20 vision [Figure 4], [Figure 5], [Figure 6], [Figure 7].
Figure 4: (a) Corneal tomography (2017) OD and OS – postoperative. (b) Differential topography (2017) – OS pre and postoperative

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Figure 5: Comparative 2 last examinations in OD with no signs of corneal decompensation

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Figure 6: Topographic map showing a larger optical zone in OS

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Figure 7: Epithelial pachymetry maps

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  Discussion Top

The case presented here is challenging and didactical because it involves three controversial aspects regarding refractive surgery core concepts. First, according to the current knowledge on post-LASIK ectasia, this case violates not a single threshold risk factor for ectasia, but at least three of them (age, dioptric correction, and percentual thickness alteration [PTA]). Second, there is the issue of determining if this is a case of residual ametropia, myopic progression, or treatment regression. At last, the patient also required treatment enhancement. There are several strategies (PRK, flap lift, and inner flap ablation) and ablation profiles (WFO, WFG, and TG) that can be combined in LASIK retreatment. Despite several alternatives available, there is no gold standard treatment for reoperations; moreover, ectasia risk must also be taken into consideration when planning enhancements. We present a brief review of the literature regarding these aspects and correlate it with the findings present in this case.

Risk factors and risk score systems

At the time of the first surgery, femtolasik technology was not yet available, and LASIK with microkeratomes was widely performed. The recommended residual stromal bed (RSB) safety threshold was 250 μm, and Randleman's criteria to analyze the risk of developing ectasia were the standard of care to plan refractive surgery.[2],[3],[4]

The Ectasia Risk Score System developed by Randleman et al. helps identify preoperative patients at high risk of ectasia. The risk categories are based on cumulative score points, such as low (0–2 points), moderate (3 points), and high (4 points).[3],[4],[5],[6],[7] Abnormal topography is the most significant risk factor for postoperative ectasia. Other risk factors include thin preoperative corneal thickness, younger age, and greater attempts at correction.[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20] According to the Randleman's criteria, the patient in question was classified as a high-risk category in which he obtained four points including 26 years of age and Manifest Refraction Spherical Equivalent (MRSE) between −8 and −10 D; however, he presented a normal topography pattern and normal corneal thickness.

According to the surgical plan, the expectation was to find a difference between the preoperative and postoperative pachymetry of 110 μm in both eyes. Intriguingly, we found a difference of 90 μm in OD and 103 μm in OS, which was not compatible with the emmetropia in both eyes. This difference may be attributed to the lower reproducibility of the pachymetry in the Orbscan device.[21] The RMS increased after the surgery [Figure 8]: from 0.37 μm to 0.86 μm in the right eye and from 0.41 μm to 0.86 μm in the left eye.
Figure 8: (a) Scheimpflug (2016) – Comparative examinations OD and OS (2010–2016). (b) Aberrometry (postoperative) OD and OS

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Another possible score risk system is the brain cornea, which uses machine learning data to give a risk score for refractive surgery. This tool is available online and the user provides the following information of the patient: age, ablation depth, thinner point, flap thickness, index of height decentration, and Belin-Ambrosio (BAD-D). Those indexes were used based on Randleman's criteria and those with better results of sensitivity and specificity for detecting cases susceptible to ectasia.[22] In 2014, Santhiago et al. described PTA as a new metric to detect the risk of corneal ectasia in patients with normal topography.[8],[9] They postulated that the amount of biomechanical change following LASIK is affected by a combination of factors such as preoperative corneal thickness, ablation depth, and flap thickness. After reviewing previously diagnosed ectasia cases and calculating their PTA, the authors established a direct correlation between the risk of ectasia and PTA. Recently, PTA has been validated as an ectasia screening tool, a value that surpasses 35% yields a 100% sensitivity for ectasia, while a value above 40% delivers the maximum coverage for sensitivity/specificity under the receiver operating characteristic (ROC) curve.[7]

In a recent study by Chan et al., they compared three risk scoring systems.[20] They retrospectively compared the PTA with two other ectasia risk score systems: ERSS[12] and SCORE. Differently from ERSS and PTA, SCORE consists of a software program linked to the Topography System Orbscan IIZ (Bausch and Lomb, Technolas Perfect Vision GmbH) designed to detect keratoconus based on both topography and tomography data.[10],[11],[12] This comparison showed that the PTA system had the smallest area under the ROC curve of 0.557 against 0.844 and 0.911 for ERSS and SCORE, respectively. The results presented by Chan highlight the limitations of PTA as a standalone tool for ectasia risk evaluation. Conversely, they also showed the importance of combining all three systems to enhance sensitivity but at the cost of specificity. The problem of this comparison is that PTA is not a screening tool, only a risk factor; therefore, the comparison has limited value.

Recently, the concept of percentage of volume alteration (PVA) was also suggested by Gatinel et al.[13] The comparison of PVA with PTA was performed with theoretical calculations for LASIK treatments. The difference consists in the concept that PVA is the relative volume of the cornea altered by treatment, unlike PTA, where the relative corneal thickness altered by treatment is calculated. The PTA, with its one-dimensional metric, allows the judgment of the impact of the treatment at the thinnest point of the cornea, while the PVA parameters are reasonable for the characterization of treatment in a three-dimensional metric.[14]

Besides all this, the use of microkeratomes has low predictability, decreasing the reliability of the PTA, even with the procedure being performed by an experienced surgeon. The use of PTA in surgical programming with a microkeratome may promote an increased risk of ectasia if this difference results in a thicker than expected flap. In this patient, with the same device, with the same experienced surgeon, there was a 50 μm difference between the eyes. Solomon et al. studied the variability of microkeratomes and found 78.4% variability between them. In a study with only Hansatome microkeratome, with intended 160 μm flap thickness, it was obtained a mean flap thickness of 116.4 ± 19.8 μm.[23],[24] The PTA value was not known during the preoperative assessment of the first procedure, but it was possible to retrospectively calculate it based on previous examinations. The PTA value was 49.2% and 48.8% for the right and left eye, respectively, which would constitute a clear risk for ectasia and a solid reason to contraindicate surgery. Unfortunately, we do not have the data necessary to calculate the PVA retrospectively, in this case.

Although the patient presented PTA> 40% and a high risk of ectasia according to Randleman's score, he did not develop corneal ectasia. In other words, despite having a high-risk factor, not all patients will experience post-LASIK ectasia.

The screening of refractive surgery has advanced too much in recent years, mainly with studies with deep and machine learning, comparing patient data with population averages. The main indexes used based on this technology are the ART MAX, BAD-D index, and percentage thickness increase. The use of artificial intelligence manages to further improve these indexes, such as Pentacam Random Forest Index.[22],[25]

Studies with the Galilei tomographer also have indexes based on artificial intelligence, like this one that uses posterior asphericity asymmetry index, corneal volume, and inferosuperior asymmetry. Another interesting tool in the same device is the cone location magnitude index that compares the asymmetry for curvature, elevation, or thickness. The majority of these indexes were not available at the time of the first surgery.[25],[26]

Finally, the biomechanical study of the cornea can bring valuable information in the screening of refractive surgery patients, although it is also not available at the time of our case. Even more, associated with tomographic indexes, this association is called the tomographic biomechanical index, with sensitivity and specificity close to 90%.[27]

All of these indexes and risk scores are used to assist medical decision-making and should not be analyzed in isolation as they are not 100% effective.

Myopic regression/progression

The long-term cellular responses of the cornea can promote regression of the postoperative refraction programmed in procedures such as LASIK, PRK, and SMILE. In many cases, this response requires additional treatment that also carries additional risks. One of the main causes of optical regression is epithelial dysfunction since the corneal epithelium can smooth stromal irregularities.[15] The corneal stroma also undergoes longitudinal morphological changes in response to ablation with an excimer laser, which can lead to refractive regression.[15]

Studies have shown that the thickness of epithelium in LASIK increases significantly between 3 and 9 months of myopic ablation, with corresponding visual regression.[16] In myopic treatments, we have epithelial hyperplasia with epithelial thickening, greater in higher corrections. This epithelial increase is a bit greater in the mid periphery than the center. Theoretically, in myopic regression, we would have a substantial epithelial increase at the center.[15],[16] Optical regression after refractive treatments is multifactorial and involves the corneal epithelium, inflammatory cascades, limbic stem cells, and corneal stroma. Evidence shows that hyperopic treatments have a higher frequency of optical regression than myopic treatments. This epithelial response is directly proportional to the amount of the correction. Higher corrections have been associated with an increased postoperative concentration of cytokines in the ocular surface, such as epidermal growth factor, insulin growth factor, transforming growth factor β, hepatocyte growth factor, and keratinocyte growth factor, which in turn may lead to stimulation of limbal stem cells and result in epithelial hyperplasia.[15] Finally, there are no available data that indicate the important role of epithelial behavior in myopic regression in myopic LASIK treatments.[28]

Besides, stromal biomechanics can play a key role in regression. Due to the biomechanical forces based on the stromal collagen network, instantaneous subtraction of this tissue can lead to unwarranted changes, even with decreased epithelial disruption compared to PRK. Stromal keratocytes are unable to reconstruct the extracellular collagen matrix, and the cornea changes its shape and resistance to traction.[28] With the removal of stromal tissue, the posterior cornea, which is less rigid, can, theoretically, became steeper in some treatments designed to flatten the cornea, also resulting in optical regression.[15] Even more, it is difficult to separate myopic regression (flattening loss caused by excimer laser) from myopic progression (increased myopia due to other factors) Myopic regression after LASIK is characterized by gradual, complete, or incomplete loss of primary correction, limiting the predictability, efficiency, and long-term stability of the surgery.[15],[16] With the improvement of techniques and greater experience of surgeons, there was a decrease in myopic regression, but this phenomenon still presents itself as a challenge for the surgeon. In addition to the increased incidence in high-grade corrections, another factor that can influence myopia increase after LASIK is the axial length of the eye due to myopic progression.[16] A retrospective study by Gab-Alla, with 1219 patients, suggests that the axial length >26 mm is a risk factor for post-LASIK myopic regression and that it should be assessed before LASIK and before LASIK enhancement. He considered myopic regression when he found deviation >0.5D in the cyclopegic refraction after 2 years of LASIK surgery follow-up.[16] The study demonstrated that the preoperative high axial length ≥26 mm increases the risk of myopic regression after LASIK. In the study, 25.13% of the patients had axial length ≥26 mm, and all of these patients presented myopic regression with varying degrees. There is a controversy about axial length changes, although some researchers suggest that at 13 years of age, the axial length has already reached the size of an adult.[16] Other studies showed that axial growth changes can occur up to young adults and even into adulthood.[29] This relationship between the axial length and the biomechanical alterations of the cornea after LASIK still needs further studies because it has not been completely elucidated. The study has several limitations but supports the idea that high axial length in the postoperative period significantly increases the risk of myopic regression after LASIK and that it should be one of the measures to be evaluated in the preoperative period.[16]

In our case, we found greater epithelial hyperplasia in the left eye, corresponding to what we would expect for a LASIK postoperative period, with a greater increase in the mid periphery, not corresponding to the most common profile in myopic regression due to epithelial hyperplasia. In the first surgery, the transitory epithelial hyperplasia justifies the refractive instability in the 1st month, and the difference between the intended and achieved keratometry justifies the final residual error.

Even presenting an axial length of almost 26mm, a possible factor of myopic progression, the patient did not have any axial length changes.

Retreatment after LASIK

Currently, one of the most challenging scenarios in refractive surgery is to deal with patients unhappy with their outcomes after LASIK, which often includes reinterventions. Several techniques for lifting LASIK flaps have been described, allowing a subsequent intrastromal ablation.[30] However, there may be greater technical difficulty in lifting them and complications such as interface epithelial growth, which can occur in up to 13% of the cases as shown in one study conducted by Ortega-Usobiaga et al.[31]

Some patients may have thin residual beds, and the use of the laser can worsen the condition and lead to the development of ectasias.[8],[9] Other viable techniques include the creation of a new flap, surface ablation over the flap, phakic IOL implantation, or phacoemulsification with an IOL. The latter is often reserved for larger refractive errors or cases where the laser is no longer possible, such as thin corneas or insufficient residual bed. Studies comparing flap re-cutting or lifting and ablation of the RSB have shown that the latter is more effective in producing better long-term results, despite its disadvantages (such as increased epithelial growth compared to the virgin cornea and the inherent risk of the procedure such as diffuse lamellar keratitis and stretch marks). Both of them produces even more reduction on the corneal biomechanics.[30],[32],[33],[34]

The creation of a new flap can lead to the appearance of serious complications if it interferes with the previous flap. The lifting of an old flap created a long time ago can be difficult, especially in the absence of information about the procedure performed previously, and the surgeon can find an irregular flap or absence of stroma that is sufficient for further ablation. Lifting the anterior flap was associated with an increased postoperative risk of corneal haze.[31]

Surface ablation has some advantages: since it does not damage corneal nerves in the RSB, induces fewer symptoms of dry eye; can treat any base membrane dysfunction and micro striae, which improves the patient's visual quality; does not lead to epithelial ingrowth; promotes less alteration in corneal biomechanics. The main disadvantages are the increased recovery time and the possibility of haze.[30],[32],[33],[34]

A study carried out with patients with previous LASIK surgery, retreated with flap lift or PRK assisted by alcohol between June 2004 and June 2005 shows that in the postoperative period, the flaps had a smoother surface and fewer streaks in the ablated area, indicating that the removal of Bowman's membrane is beneficial in the treatment of micro striae. In this study, all eyes were retracted with ablations guided by the wavefront generating a reduction in flap-induced HOA.[30],[32],[33],[34]

The results of this study indicate that the treatment of residual ametropia after LASIK by surface ablation in the flap is safe and effective if the amount of Spherical Equivalent (SE) correction is not very high, thus suggesting this technique for corrections of <−1.50 D of residual myopia.[30],[32],[33],[34]

Currently, there is a preference for surface surgery in reoperations, since the stroma does not heal perfectly and the lamella, theoretically, has no biomechanical power.[30]

It is crucial to avoid violation of the flap interface, not more than the thickness of the flap (discounting epithelium), in surface ablations, to prevent stromal scarring. Therefore, the assessment of flap thickness, using imaging examinations, is an essential item in surgical planning.

It is important to accurately determine the RSB thickness before retouching to avoid calculation errors and/or misinterpretations that may promote overestimation or underestimation, which increases the risk of ectasia and the percentage of enhancement contraindication. The most accurate methods for this measurement are very high-frequency digital ultrasound (VHFDU), confocal microscopy, optical coherence tomography, and Scheimpflug tomography. The comparative analysis between these instruments demonstrated higher RSB thickness values reported OCT in contrast to VHFDU measurements; therefore, these measurements should be individualized due to the bias of each diagnostic method. If there is any doubt superficial ablation should be performed.[35]

High-order aberrations

Eyes that have a large amount of HOA show deterioration in visual quality. Conventional refractive surgery tends to induce HOA while correcting low-order aberrations.[18],[19]

The introduction of HOAs is a common complication of corneal refractive procedures. The development of femtosecond lasers and the introduction of wavefront ablation systems help to minimize the induction of HOAs, but the results are often variable and not always notably superior to conventional systems.

A prospective, randomized study of 74 eyes of 37 patients who underwent LASIK retreatment showed that personalized surgeries achieved superior results compared to the same treatment for virgin eyes in terms of correction of ocular aberrations and improvement of visual quality; this fact is due to the already submitted eye already presenting major changes of a high order, which generates better results.[19],[36]

Since the OPD scan had previously shown increased HOA, based on the increased efficacy of WFG treatments for LASIK enhancements, this was the strategy of choice for this case. We found the reduction of HOA from 0.8 to 0.6 μm in RMS results.

  Conclusion Top

In summary, a safe, effective, and stable outcome with corneal wavefront-customized PRK enhancement in association with MMC was achieved for post-LASIK residual myopia with a high risk of ectasia based on both ERSS and PTA. There was a significant improvement in the postoperative visual acuity both with and without correction and a reduction in corneal HOA, while no signs of ectasia were detected over the combined 12 years of follow-up.

This anecdotal case demonstrates the limitations regarding the specificity of currently available methods. Hence, further research focusing on specificity improvement of these score systems is warranted to avoid “false positives” while screening possible candidates for refractive surgery.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient (s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]


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