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ORIGINAL ARTICLE
Year : 2019  |  Volume : 32  |  Issue : 2  |  Page : 665-671

Comparing the corneal biomechanical stability after small incision lenticule extraction and laser-assisted in situ keratomileusis for myopic correction using an ultra-high-speed camera (Corvis ST)


1 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Shebeen El-Kom, Egypt
2 Department of Ophthalmology, Ministry of Health, Damietta, Egypt

Date of Submission10-Nov-2017
Date of Acceptance09-Jan-2018
Date of Web Publication25-Jun-2019

Correspondence Address:
Hossam F El-Barbry
3-Elmaahad Elazhari Street, Damietta
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mmj.mmj_761_17

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  Abstract 

Objectives
The aim of this study was to invest igate the differences in corneal deformation parameters after femtosecond laser small incision lenticule extraction (SMILE) and laser-assisted in situ keratomileusis (LASIK).
Background
Applanation time and deformation amplitude [as measured with the CorVis ST (CST)] may be useful in assessing corneal biomechanical changes after corneal refractive surgery. The aim of this study was to evaluate the corneal biomechanical changes using the CST in eyes with LASIK and SMILE.
Participants and methods
This is a small prospective clinical interventional series study carried out from March 2016 to April 2017 on 50 eyes divided equally into two groups. The first group included eyes that were subjected to the LASIK procedure using the EX500 Allegretto excimer laser platform and the second group included eyes that were subjected to the SMILE procedure using the VisuMax 500 kHz laser system; the CST measured the corneal biomechanical changes before and after the procedures.
Results
Using CST, the deformation amplitude increased significantly in both groups. It was also noted that the mean percentage of change of the deformation amplitude was nearly two times higher in group I (0.13 ± 0.03) than group II (0.07 ± 0.01) (P < 0.001). First applanation time showed a significant decrease from preoperative to postoperative values in both groups (P < 0.001). Also, the highest concavity time, radius, and peak distance showed a significant decrease from preoperative to postoperative values in both groups.
Conclusion
Both LASIK and SMILE considerably decreased the corneal biomechanical properties, with greater reduction in the LASIK group.

Keywords: biomechanics, Corvis ST, laser-assisted in situ keratomileusis, small incision lenticule extraction


How to cite this article:
Ahmed KE, El-Gohary SM, El-Barbry HF. Comparing the corneal biomechanical stability after small incision lenticule extraction and laser-assisted in situ keratomileusis for myopic correction using an ultra-high-speed camera (Corvis ST). Menoufia Med J 2019;32:665-71

How to cite this URL:
Ahmed KE, El-Gohary SM, El-Barbry HF. Comparing the corneal biomechanical stability after small incision lenticule extraction and laser-assisted in situ keratomileusis for myopic correction using an ultra-high-speed camera (Corvis ST). Menoufia Med J [serial online] 2019 [cited 2019 Sep 21];32:665-71. Available from: http://www.mmj.eg.net/text.asp?2019/32/2/665/260918




  Introduction Top


The structural and reparative properties of the cornea are essential to its function as a resilient, yet transparent, barrier to intraocular injury. Because the cornea is also the scaffold for the major refractive surface of the eye, any mechanical or biological response to injury will also influence optical performance. Consequently, the same mechanisms responsible for preserving ocular integrity can undermine the goals of achieving predictable and stable visual outcomes after refractive surgery[1]. Previous studies already reported a significant reduction in corneal resistance after laser-assisted in situ keratomileusis (LASIK) surgery[2]. The goal of research in this setting is to improve outcomes and reduce complications by discerning details of the biomechanical and wound-healing pathways, identifying measurable predictors of individual responses[1]. In this context, the possibility of standardizing the measurement of corneal tissue deformation degree, induced by refractive surgery procedures, would be essential to determine the predictability in the development of complications such as keratectasia and to compare different surgical procedures in terms of biomechanical and tissue stability. Corneal biomechanical properties have been a focal point of research not only because of the influence on intraocular pressure assessment but also for assessing the influence on corneal structural integrity after refractive surgery[3],[4].

The Oculus CorVis ST (Oculus Optikgeräte; OCULUS, Wetzlar, Germany) is a noncontact high-speed tonometer, supported by Scheimpflug Technology, (Santen Pharmaceutical Co. Ltd, Osaka, Japan) designed to obtain in-vivo measurements of corneal biomechanical properties; this device enables monitoring of corneal deformation response to a symmetrically metered air pulse. The device depicts the time required to applanate the cornea with the air puff, and the time of the first inward applanation is directly proportional to the intraocular pressure (IOP), which ranges from 1 to 60 mmHg. IOP and central corneal thickness (CCT) are obtained during one measurement process. Additional Corvis ST (CST) parameters are measured in time (ms), length (ml), and velocity (m/s) of the first (air puff flattens cornea) and second (interruption of air puff results in 'reformation' of cornea) corneal applanation; furthermore, peak distance, radius, and deformation amplitude (ml) of the highest corneal concavity during the measurement process are measured[5]. The point of maximum corneal concavity is automatically captured and generates several corneal biomechanical parameters[5],[6]. The complete visualization of the deformation process can be shown in a captured video image [Figure 1]. The introduction of femtosecond laser in ophthalmology led to a new era of refractive procedures. The method of flap creation whether by microkeratome or by femtosecond laser had an effect on corneal biomechanics as reported in several studies[7],[8]. With recent advances, the small incision lenticule extraction (SMILE) procedure now allows for the removal of corneal stromal lenticules of predetermined thickness without the need to create a flap[9],[10]. How this flapless procedure affects corneal biomechanics has not been fully investigated as yet. The aim of this study was to investigate the differences in corneal biomechanical stability in patients with myopia and myopic astigmatism before and following SMILE and LASIK using an ultra-high-speed camera (Corvis ST).
Figure 1: Snapshots of the video output of the Corvis ST.

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  Participants and Methods Top


This is a small prospective clinical interventional series study carried out from March 2016 to April 2017 on 50 eyes of 28 patients equally divided into two groups; group I underwent the wavefront optimized LASIK procedure and group II underwent the femto SMILE procedure. We studied corneal biomechanics before and 3 months after refractive surgery. The patients' age ranged from 20 to 38 years; they had moderate to high myopia (spherical equivalent refraction from −4.00 to −7.00 diopters (D), with or without astigmatism from up to 2.50 D), stable for at least 1 year, and a corrected distance visual acuity of 20/25 or better. Patients with A CCT less than 470 μm in SMILE and less than 500 μm in LASIK, a calculated postoperative residual stromal bed of less than 300 μm, a calculated deformation amplitude of more than 1.2 mm, a presence of keratoconus, pregnancy, or breastfeeding, and any other ocular pathological conditions were excluded from the study.

Informed consents for retrospective data analysis were obtained from the candidates and the study was approved by the local ethics committee. Tenets of the Declaration of Helsinki were followed. Wavefront optimized LASIK procedures were performed using the myopic astigmatism algorithm of the EX500 Allegretto excimer laser platform (WaveLight GmbH, Erlangen, Germany). This algorithm uses a 6.50 mm optical zone with a 1.25 mm blend zone. All flaps were created using the Sub Bowman keratectomy SBK automated disposable microkeratome with a 90 μm head (Moria, Antony, France). The VisuMax 500 kHz laser system (Carl Zeiss Meditec, Jena, Germany) platform was used for the SMILE surgeries. The spot distance was 3 μm for lamellar cuts and 2 μm for side cuts. The spot energy was set to 130 nJ in all patients. The minimum lenticule side cut thickness was set to 10 μm. The lenticule side cut angle was 130°, the incision side cut angle was 70°, and the optical zone was 6.5 mm as SMILE is not performed in patients with a mesopic pupillary diameter greater than 6.5 mm. A small-sized cone was used in all patients; cap diameter was set to 7.5 mm. A cap of 90 μm was used in all patients. Topical steroids (fluorometholone 0.1%; Santen Pharmaceutical Co. Ltd.) were used initially eight times daily and tapered for a period of 20 days. Topical antibiotics (ofloxacin ophthalmic solutions 0.5%; Santen Pharmaceutical Co. Ltd.) were used four times daily for 7 days. Tears Naturale (hypromellose 2910, dextran 70, glycerol eye drops; Alcon Laboratories Inc., Fort Worth, Texas, USA) were used four times daily for 30 days postoperatively. Visual acuity assessment and slit lamp examination were performecd on the first day, one week, one month and three months after surgery. CST parameters were measured at three months postoperative.

The CST was used to measure the first applanation (A1) time, highest concavity (HC) time, second applanation (A2) time, A1 and A2 lengths, HC radius of curvature, peak distances, and deformation amplitude [Table 1].
Table 1: Parameters derived from Corvis ST

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Statistical analysis

Statistical analysis was carried out using SAS software (version 9.1; SAS Institute Inc., Cary, North Carolina, USA). The arithmetic mean and SDs were calculated for both groups. Sample size was calculated on the basis of previous studies[11],[12], using Med Calc statistical software (Ostend, Belgium), assuming the area under receiver operating characteristic curve to be 0.80, an α of 0.05, and a power of 80.0%. The changes in, IOP, A1 time, HC time, A2 time, A1 length, A2 length, HC radius, HC peak distance, and deflection amplitude that were defined as the differences between the preoperative and postoperative parameter, and then the percentage of change of these parameters were calculated for comparison. To check for normal distribution, the Kolmogorov–Smirnov test was used. Comparisons of the means of normally distributed data were performed using the t-tests for paired samples (for preoperative and postoperative comparison) and an unpaired t-test for two means (for comparison between the two groups). A P value less than 0.05 was considered statistically significant.


  Results Top


The study included 50 eyes of individuals seeking refractive surgery. The preoperative characteristics of both groups showed no significant statistical differences between them in age, preoperative sphere, cylinder, spherical equivalent, and CCT [Table 2]. The CST showed no significant difference between the preoperative and postoperative IOP in both groups, with the postoperative IOP being lower than the preoperative values (P < 0.001). Deformation amplitude showed a significant increase from preoperative to postoperative values in both groups. There was a nearly two-fold increase in the mean percentage of change of the deformation amplitude in the LASIK group (P < 0.001 in group I, P = 0.005 in group II). First applanation time showed a significant decrease from preoperative to postoperative values in both groups (P < 0.001). Also, HC time, HC radius, and peak distance showed a significant decrease from preoperative to postoperative values in both groups, whereas the A2 time showed a significant decrease only in group I (P < 0.001) and no significant change in group II (P = 0.189). The A1 and A2 lengths showed no significant postoperative decrease in both group I and II, respectively [Table 3]. The mean postoperative A1 time, HC time, and A2 time were slower in group II (7.22 ± 0.25, 17.11 ± 0.35, 22.04 ± 0.86 ms, respectively) than in group I (7.14 ± 0.21, 16.99 ± 0.47, 21.06 ± 0.31 ms, respectively), denoting slower movement of the cornea in reponse to air puff (i.e., stiffer cornea) [Table 4] [Table 2] shows the corneal biomechanical parameters preoperatively and postoperatively in LASIK (group I) and SMILE (group II).
Table 2: Mean values and range of age, sphere, cylinder, spherical equivalent, and preoperative central corneal thickness in both groups

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Table 3: Corneal biomechanical parameters preoperatively and postoperatively in laser-assisted in situ keratomileusis (group I) and small incision lenticule extraction (group II)

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Table 4: Difference in deformation amplitude and first applanation time, preoperatively, and 3 months postoperatively in laser-assisted in situ keratomileusis (group I) and small incision lenticule extraction (group II)

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  Discussion Top


There is a rapid deveoplment of technologies of corneal refractive surgery to meet the rising expectations of doctors and patients regarding achieving the besst visual acuity. Through meticulous preoperative assessment, examination, and the use of state-of-the-art corneal topography and wavefront analysis, the rate of intraoperative and postoperative complications has decreased markedly. Iatrogenic stresia is one of the postoperative complications with its etiopathogenesis not clearly understood. Although many precautions are taken, there is still a small estimated risk of 0.04–0.6% that LASIK can lead to corneal ectasia[13]. The cornea is a very complex anisotropic tissue. The strength of the central cornea depends on interlamellar proteoglycan bonding, whereas anterior and peripheral strength depend on branching and interlacing of lamellae. The anterior stroma is ~25% stiffer than its posterior counterpart[13]. One of the most important aspects of the preoperative examination of LASIK candidates is to screen cases at risk for progressive ectasia. Keratectasia occurs after LASIK, even in the absence of known risk factors. Such cases are referred to as having high susceptibility or predisposition to develop ectasia, such as forme fruste keratoconus. The corneal stroma undergoes a two-step process of delamination and interfibril fracture, which is clinically characterized by thinning and bulging of the cornea[14]. Several refractive procedures tend to alter the optical as well as the anatomical configurations of the cornea. New refractive procedures need to be fully investigated not only from the visual point of view but also from the safety point of view. Corneal biomechanics describes the response of corneal tissue to forces applied to them and entails interactions between the externally applied force, the intrinsic properties of the cornea as well as the IOP. Softer tissues tend to stretch or deform greater than stiffer tissues. The complexity of assessment of the in-vivo corneal biomechanics is affected by several factors, thus hindering the use of simple or single factors as predictive biomechanical tools. We used the CST as it seems to be more useful in delineating true biomechanical differences than the ORA[15] and examined specific parameters for biomechanical stability. Hon and Lam[16] measured corneal biomechanical properties with the CST and reported that a thinner cornea was associated with a higher corneal deformation and that the measurement of deformation amplitude was an indicator of the biomechanical properties in normal patients; it was one of the most reliable corneal parameters and deserved clinical attention during the CST measurements on eyes with or without disease[17]. In our study, the mean values of postoperative CCT and manifest refraction spherical equivalent (MRSE) were not statistically different between the two groups. Moreover, the potentially confounding factors including age and preoperative CCT and MRSE were adjusted statistically. Thus, we could analyze corneal deformation parameters independent of these confounders. In our study, the average age of the patients was 25.98 ± 3.50 years (range: 20–38) in group I and 27.24 ± 3.77 years (range: 21–36) in group II. The average of postoperative MRSE was − 5.39 ± 1.22 (range: −4.0 to −7.75) diopter in group I, and the average of postoperative MRSE was − 5.49 ± 1.15 (range: −4.0 to −8.0) diopter in group II. The average preoperative central cornea thickness was 525.38 ± 16.29 (range: 506–567) μm in group I and the average preoperative central cornea thickness was 531.88 ± 16.29 (range: 513–603) μm in group II. The average tissue ablated was 68.2 ± 22.3 (range: 41–89) μm in group I and the average lenticule thickness was 70.1 ± 19.2 (range: 52–98) μm in group II. The preoperative values of deformation amplitude were within normal ranges in all patients. We found a weak correlation between corneal biomechanics and the age of the patient. This differs from other studies[18],[19] which reported a change in corneal biomechanics with age, and this difference may be explained by the close age distribution in our study group being of adult and middle age ctegories without children and elderly population. A statistically significant moderate correlation was found between CCT and preoperative levels of deformation amplitude and A1 time, supporting findings in other studies[11],[12],[20]. The CST showed no significant difference between the preoperative and postoperative IOP in both groups, with the postoperative IOP being lower than the preoperative value, supporting findings in other studies[11],[12]. Deformation amplitude showed a significant increase from preoperative to postoperative values in both groups. There was a nearly two-fold increase in the mean percentage of change of the deformation amplitude in the LASIK group, denoting much lower biomechanical change (different from another study[20] that found a five-fold increase in the LASIK group). These differences in the biomechanical behavior between both groups in our study can be explained by three factors. First, the microkeratome creates a meniscus flap extending deeper into the peripheral stronger corneal layers, thus severing more biomechanically vital collagen bundles. Second, there is a different healing pattern between the two gorups with more inflammation with the femto SMILE group, resulting in stronger fibrotic scarring as reported in previous studies[7],[8]. Third, there was a difference in the flap to cap diameter between the two groups as flaps tended to be higher than the transition zones in the LASIK group (>8.5 mm), whereas the usual flap to cap diameter in the femto SMILE cases was less than 8 mm, thus salvaging the stronger peripheral collagen bundles. [Figure 2] shows CST corneal biomechanical changes in both groups, with group I on the right and group II on the left. Note the higher deflection amplitude in group I. First, applanation time, HC time, HC radius, and peak distance showed a significant decrease from preoperative to postoperative values in both groups, whereas the A2 time showed a significant decrease only in group I and no significant change in group II. The A1 and A2 lengths showed no significant postoperative decrease in both groups I and II. These results are consistent with previous studies[11],[12]. It was also found that the mean postoperative A1 time, HC time, and A2 times were slower in group II (7.34 ± 0.25, 17.07 ± 0.55, and 22.04 ± 0.86, respectively) than in group I (7.25 ± 0.31, 16.15 ± 0.68, and 20.95 ± 0.44, respectively), denoting slower movements of the cornea in respect to the air puff (i.e., stiffer cornea) [Table 4]. The limitations of the current study include small sample size and lack of follow up. Also, the study did not include follow-up. The study was based on the assumption that most of the biomechanical changes after refractive procedures occur within 1 week of surgery as reported in previous studies. Also, one of the factors affecting the corneal biomechanics is the state of corneal dryness. This further adds to the difficulties encountered as some patients seeking refractive surgeries are already contact lens intolerant, with a significant degree of dry eye.
Figure 2: Corvis ST (CST) corneal biomechanical changes in both groups, with group II on the left and group I on the right. Note the higher deflection amplitude in group I.

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  Conclusion Top


Applanation time (applanation 1) and deformation amplitude (as measured with the CorVis ST tonometer) may be useful in assessing corneal biomechanical changes after corneal refractive surgery. It was noted that the mean percentage of change of the deformation amplitude was nearly two times higher in the LASIK group (0.13 ± 0.03) than the SMILE group (0.07 ± 0.01). The mean postoperative A1 time, HC time, and A2 times were slower in group II (7.34 ± 0.25, 17.07 ± 0.55, and 22.04 ± 0.86, respectively) than in group I (7.25 ± 0.31, 16.15 ± 0.68, and 20.95 ± 0.44, respectively), denoting greater reduction in biomechanical stability in the LASIK technique.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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