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 Table of Contents  
ORIGINAL ARTICLE
Year : 2020  |  Volume : 21  |  Issue : 1  |  Page : 19-24

Corneal higher order aberrations after intrastromal corneal ring segment implantation for keratoconus


Department of Ophthalmology, Faculty of Medicine, Beni-Suef University, Beni-Suef, Egypt

Date of Submission16-Jul-2019
Date of Decision21-Nov-2020
Date of Acceptance04-Dec-2019
Date of Web Publication28-Feb-2020

Correspondence Address:
MBBCh Mohamed A Mekky
7H Taha Hussein Street, Elnozha El Gadeda, Cairo 11843
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/DJO.DJO_35_19

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  Abstract 


Background Keratoconus is the most common primary corneal ectatic disease. It is a progressive, noninflammatory, and localized paraxial stromal thinning of the cornea, which often results in bilateral and asymmetrical corneal distortion and anterior corneal protrusion. Patients with corneal protrusion often develop high myopia and irregular astigmatism, resulting in significant impairment of visual acuity.
Purpose The aim of this study was to assess changes of higher order aberrations (HOAs) induced by the implantation of intrastromal corneal ring segments (ICRS) in keratoconus.
Patients and methods In the current study, a prospective analysis of HOAs, measured by using Pentacam, before and after ICRS implantation was done in a nonrandomized consecutive series of 20 eyes having progressive grade II/III keratoconus (Amsler-Krumeich classification).
Results At 1 month postoperatively, a statistically highly significant reduction was found in keratometric readings. The flattest keratometric reading (K1) improved from 46.42±3.08 D preoperatively to 43.25±3.45 D postoperatively, which is highly significant (P<0.01). The steepest keratometric reading (K2) improved from 49.96±3.29 D preoperatively to 46.04±3.15 D postoperatively, which is highly significant (P>0.01). The total HOA root mean square improved from 1.55±0.54 preoperatively to 1.26±0.38 postoperatively, with a statistically significant difference (P<0.05). Primary coma improved from 1.27±0.54 preoperatively to 0.87±0.27 postoperatively, which is highly significant (P>0.01). In addition, astigmatism root mean square changed from 2.42±1.52 preoperatively to 1.98±1.31 postoperatively. Trifoil and quadrifoil showed nonsignificant changes (P>0.05).
Conclusion ICRS (Keraring) implantation using femtosecond laser is an effective method for improvement and stabilization of optical, keratometric, and aberrometric measures in keratoconic eyes. Most of the improvement in HOAs was achieved after 1 month and nearly locked up or changed insignificantly till the third month.

Keywords: higher order aberrations, intrastromal corneal ring segment implantation, keratoconus, Pentacam


How to cite this article:
Saif MY, Saeed MA, Mekky MA. Corneal higher order aberrations after intrastromal corneal ring segment implantation for keratoconus. Delta J Ophthalmol 2020;21:19-24

How to cite this URL:
Saif MY, Saeed MA, Mekky MA. Corneal higher order aberrations after intrastromal corneal ring segment implantation for keratoconus. Delta J Ophthalmol [serial online] 2020 [cited 2020 Apr 6];21:19-24. Available from: http://www.djo.eg.net/text.asp?2020/21/1/19/279720




  Introduction Top


Keratoconus is a progressive disease that usually appears during puberty or the second decade of the life and normally progresses for the following two decades until it stabilizes. In severe cases, corneal scarring further contributes to vision loss [1].

Approximately 50% of normal fellow eyes will progress to keratoconus within 16 years. Both eyes are affected eventually, at least on topographical imaging, in almost all cases. It can be graded by the highest axis of corneal power on keratometry as mild (<48 D), moderate (48–54 D), or severe (>54 D). Presentation is commonly with features initially in only one eye [2].

Middle Eastern, and Central Asian ethnicity is considered a risk factor for keratoconus. In Egypt, in a study conducted in Beni-Suef, Egypt, keratoconus was found in 1.7% of patients seeking refractive surgeries. This result is comparable to white people in Asia (Iran), but the incidence is higher in a study conducted in Malaysia [3] and in another study in Saudi Arabia [4].

A genetic predisposition to keratoconus is well documented with increased incidence in some familial groups and numerous reports of correspondence between monozygotic twins. Approximately 6–23.5% of patients with keratoconus have a positive family history [5].

Diagnosis of keratoconus has greatly improved from simple clinical diagnosis with the advent of modern imaging modalities. These diagnostic devices have allowed us to diagnose the disease much earlier, and newer treatment modalities have been used. There are varieties of diagnostic imaging tools to diagnose subtle abnormalities in corneal curvature, thickness, and tissue architecture like photographic placido disk studies, keratometry, photokeratoscopy, and finally, computer-assisted videokeratoscopy [1].

Over the past decade, outcome data have accumulated for new interventions in keratoconus treatment, which promise to reduce transplantation rates significantly, delay disease progression, and save many patients from long-term reliance on rigid contact lens wear. These interventions include corneal collagen cross-linking, intrastromal corneal ring segments (ICRS), topographic photorefractive keratectomy, and phakic intraocular lens implantation. None of these recent treatment modalities are applicable to advanced (stage IV) disease with corneal scarring, in which corneal transplantation is indicated by deep anterior lamellar keratoplasty up to penetrating keratoplasty [6].

This study was designed to assess changes in higher order aberrations (HOAs), using Pentacam, which were induced by the implantation of ICRS in keratoconus.


  Patients and methods Top


This is a prospective analysis of a nonrandomized consecutive series of 20 eyes of 20 patients having progressive grade II/III keratoconus (Amsler-Krumeich classification) [7] for whom ICRS were implanted. This was followed by cross-linking after the end of the study to control any further progression. The study started since March 2017 and had follow-ups. It adhered to the tenets of the declaration of Helsinki published by the WMA. A written informed consent was signed by all patients to participate in the study and for publication of the study results. The study was approved by the Local Ethical Committee of Beni-Suef University Faculty of Medicine.

Inclusion criteria

  1. This study was conducted on patients undergoing ICRS implantation; Keraring (Mediphacos, Belo Horizonte, Brazil) with a diameter of 5 mm and arc/thickness of 210/250.
  2. Patients should have a clear cornea and contact lens intolerance.
  3. Patients should be aged from 15 to 45 years old.
  4. All patients should have complete data for preoperative and postoperative follow-up.


The patients had the following topographical data on the sagittal curvature map:
  1. Steep keratometry (K) more than or equal to 45 D.
  2. Maximal K reading ‘K-max’ more than or equal to 45 D.
  3. Curvature power at the superior point ‘S’ was greater than that at the inferior point ‘I’ (S-I) more than or equal to 2.5 D.
  4. I-S more than or equal to 1 D.


Exclusion criteria

The following were the exclusion criteria:
  1. Patients who were previously diagnosed as having keratoconus grade IV more tha or equal to 58 D or with central corneal thickness less than 350 µm.
  2. Patients who previously underwent laser refractive surgery.
  3. Patients who have not completed 2 weeks without wearing contact lenses.
  4. Patients with corneal scarring or corneal dystrophies.
  5. Patients with severe eye dryness and atopy.
  6. Patients undergoing other refractive surgeries other than laser keratorefractive surgeries.


Preoperatively, all patients were subjected to general and ocular history taking (history of atopy or other ocular allergies, contact lens wearing, previous ocular surgeries or trauma and family history of keratoconus). Evaluation of tear film was done by Schirmer’s test and ocular surface by slit-lamp examination. Evaluation of ocular motility and alignment was also performed.

Evaluation of corneal topography and aberrometry was done by Scheimpflug imaging system, Sirius Pentacam (CSO, Costruzione Strumenti Oftalmici, Florence, Italy). Aberration root mean square (RMS) values were calculated for a 5-mm pupil and recorded; higher order RMS, primary coma (computed for the Zernike terms Z31), and trefoil-like (computed for Zernike terms Z33) aberrations were recorded.

Surgical technique

The procedure was performed under topical anesthesia using benoxinate eye drops (Benox; EPICO Inc., Cairo, Egypt). Sterilization of both eyes was performed with povidone–iodine, followed by draping and application of a speculum to the eye to be treated.

The pupillary center was marked by gentian violet on the surface of the cornea using a Sinskey hook. Peripheral pachymetry in Pentacam results was reviewed in all cases, to ensure sufficient corneal thickness and appropriate depth of ICRS placement. A disposable Intralase suction ring was inserted to minimize any excessive decentration and to make the cornea planate to fixate the eye and to help to maintain the precise distance from the laser head to focal point. The creation of the intrastromal tunnel with the femtosecond laser (Femto Abbott IFS; Abbott Medical Optics, Santa Ana, California, USA) was completed within 15 s with minimal manipulations of the cornea.

The incision axis corresponded to the axis of the steepest keratometric reading, and the tunnel depth was 80% of the thinnest corneal location. The ring was inserted into the tunnel, using a special forceps and maneuvered until the desired position, by means of a special hook. Once placed in the pocket, the ICRS flattens the cornea. The access to the pocket was self-sealing and did not require suturing.

Kerarings vary in thickness from 150 to 350 mm. The segment ranges from 90–210° of an arc. We used one ring with an inner diameter of 5 mm and an outer diameter of 5.8 mm centered on this mark: arc 210 and thickness 250. We used the nomogram provided by the Keraring manufacturer. The choice of insertion of segments was determined according to the distribution of ectatic area on the corneal surface, whereas the thickness of the segment was determined according to the distribution of the ectatic area as well as the sphero-cylinder values.

Postoperatively, all patients were prescribed gatifloxacin and dexamethasone eye drops four times daily for 1 week. The patients’ follow-up was at 1 and 3 months. Postoperative evaluation and data collection were performed by corneal topography with the rotating Scheimpflug device (Pentacam) to evaluate the corneal topography.


  Results Top


A total 20 patients with a mean age 28.45±9.20 years (range, 15–45 years) were included. Six (30%) eyes belonged to females, and 14 (70%) were males ([Table 1]).
Table 1 Demographic data of the studied cases

Click here to view


At 1 month postoperatively, a statistically highly significant reduction was found in keratometric readings. Flat k (K1) improved from 46.42±3.08 D preoperatively to 43.25±3.45 D postoperatively, which is highly significant (P<0.01). Steep k (K2) improved from 49.96±3.29 D preoperatively to 46.04±3.15 D postoperatively, which is highly significant (P>0.01). The total HOA RMS improved from 1.55±0.54 preoperatively to 1.26±0.38 postoperatively, which improves the quality of vision, and this is statistically significant (P<0.05). Primary coma improved from 1.27±0.54 preoperatively to 0.87±0.27 postoperatively, which is highly significant (P>0.01). In addition, astigmatism RMS changed from 2.42±1.52 preoperatively to 1.98±1.31 postoperatively, whereas trifoil and quadrifoil showed nonsignificant changes (P>0.05), as shown in [Table 2].
Table 2 Comparison between preoperative and 1-month postoperative data

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At 3 month postoperatively, a statistically highly significant reduction was found in keratometric readings compared with the preoperative readings. K1 improved from 46.42±3.08 D preoperatively to 42.98±3.3 D postoperatively, which is highly significant (P<0.01). K2 improved from 49.96±3.29 D preoperatively to 45.82±3 D postoperatively, which is highly significant. The total HOA RMS improved from 1.55±0.54 preoperatively to 1.16±0.45 postoperatively, and this is also highly significant. In addition, the primary coma improved from 1.27±0.54 preoperatively to 0.79±0.3 postoperatively, which is highly significant. The astigmatic RMS changed from 2.24±1.52 preoperatively to 2±1.31 postoperatively, which is nonsignificant. Similarly, trifoil and quadrifoil showed nonsignificant changes (P>0.05), as shown in [Table 3].
Table 3 Comparison between preoperative and third month postoperative data

Click here to view



  Discussion Top


The current study analyzed the corneal aberrometric findings from 20 keratoconus eyes, for whom, ICRS implantation was done. Regarding corneal curvature, a significant central flattening was achieved by the reduction of keratometric readings. The changes in anterior corneal aberrations were also evaluated, as the decrease of vision in patients with keratoconus is not only caused by sphero-cylindrical refractive errors (low order aberrations) owing to elevated corneal curvature but also to a significant extent by higher order ones. This is clearly explained by the fact that spectacles in most cases are not able to provide full correction.

To do so, we reported the patients’ preoperative and postoperative changes in topographic and aberrometric data 1 and 3 months after ICRS implantation. There was a significant reduction comparable to previous studies.

In 2007, a study by Shabayek and Alió [8] was done to report the outcome after implantation of ICRS (Keraring) aided by femtosecond laser for the correction of keratoconus of 21 eyes of 16 consecutive keratoconic patients. The Keraring implantation significantly increased the uncorrected visual acuity and the best-corrected visual acuity and decreased the spherical equivalent and the average keratometric values. There was no significant difference between the third and sixth month follow-up data. In addition, 40% (eight eyes with a relatively low RMS of total HOA) showed a nonsignificant increase in the RMS of total HOA, whereas 60% (12 eyes with a relatively higher RMS of total HOA≥3.0 µm) showed a significant decrease in the RMS of the total HOA owing to a significant reduction in coma aberrations. This was similar to the current study, as the total HOA showed highly significant improvement in the coma aberration, mainly except for the long-term follow-up till 6 months.

In 2007, Coskunseven et al. [9] reported the results of ICRS (Keraring) implantation using a femtosecond laser (IntraLase Corp., Irvine, California, USA) in 32 keratoconic patients (50 eyes). They found that ICRS (Keraring) implantation using femtosecond laser for tunnel creation was a minimally invasive procedure for improving visual acuity (both uncorrected visual acuity and best-corrected visual acuity) and topographic findings in keratoconic patients. This is different from the present study in that we used the topographic findings postoperatively to assess the degree of visual improvement postoperatively, as the relationship between HOAs and visual function is complicated and not fully understood. Applegate et al. [10] analyzed visual acuity as a function of Zernike mode and level of RMS error. They demonstrated that all aberrations were not equivalent, in that different Zernike polynomial coefficients with the same amount of RMS error result in varying degrees of degradation in high and low contrast acuity. In normal participants, the RMS wavefront error appears to have a low correlation to visual acuity, whereas in irregular corneas, they found HOAs to be closely related to visual acuity, especially in keratoconic eyes. In the current study, specific HOAs were chosen, being the most sensitive and related to the quality of vision. Besides, we preferred to deal with individual rather than total mean values. We found that there was a marked improvement in total HOAs RMS especially in coma RMS, which is highly significant, with nonsignificant change in trefoil and quadrifoil.

In 2010, Pinero et al. [11],[12] held a study to characterize the refractive, keratometric, and corneal aberrometric effect of a specific type of ICRS as a function of its thickness and the preoperative conditions of the cornea. A total of 72 consecutive keratoconic eyes of 57 patients ranging in age from 15 to 68 years were retrospectively analyzed and included in the study. All cases had a diagnosis of keratoconus and had undergone implantation of a 160° arc-length Keraring segment, by femtosecond laser technology. Correlations between ring segment thickness and several clinical parameters were investigated. They found significant reductions in central curvature, corneal astigmatism, and coma aberration after surgery (P≤0.03). Moderate and limited correlations were found between ring segment thicknesses and changes in mean keratometry and HOAs. So, the selection of the ring segment to implant in keratoconus should be based not only on refraction and subjective appearance of the corneal topographic pattern but also on corneal aberrometry. This highly customized selection would allow a more predictable outcome. This is similar to the present study which found that there was a reduction in HOAs, but we did not compare different ring segments in thickness.

In 2010, Bühren et al. [13] performed a prospective study on the effect on HOAs after implanting the ICRS along the coma meridian. This study included 12 consecutive eyes of 10 patients with up to grade III keratoconus as determined by the Amsler scale. A single segment of the Keraring was implanted in all patients along the coma axis using a manual technique, and patients were followed-up for 6 months. They found that there was a significant improvement in uncorrected distance visual acuity and best-corrected visual acuity in these patients. Interestingly, however, we did not see a significant decrease in cylindrical and spherical error. Instead, they saw a significant decrease in coma (18% reduction) and total HOAs (21.1% reduction), suggesting that an improvement of HOAs can also improve vision in patients with keratoconus. This supports our results, although we used femtolaser rather than manual technique for implantation.

In 2012, Simoceli et al. [14] performed a study to describe the wavefront changes induced by the implantation of Ferrara ICRS in 96 eyes of 92 patients with keratoconus grades I, II, and III. Ocular aberrations were examined using the Nidek OPD scan wavefront aberrometer. They found that Ferrara ICRS improved CDVA and UDVA in all groups, decreasing total aberrations, coma, and tilt, while increasing other HOAs. In the present study, we used Keraring, but we almost had the same results except for tilt improvement and increasing other HOAs. This might be owing to using a different device to assess corneal aberrations and we also had only 20 eyes in the current study.

In 2018, EL-Kasaby [15] performed a study to compare visual acuity, refraction, and topographic corneal changes after implantation of MyoRing versus Ferrara ring segment for management of keratoconus. He found that both MyoRing and Ferrara ring segments were effective modalities for treatment of grades II–IV keratoconus, but MyoRing had a superior capability in improving the keratoconus parameters as well as in halting the progression of the disease. This is similar to the present study in improving the topographic parameters, but our patients did collagen cross-linking to protect against any further progression after Keraring implantation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Matalia H, Swarup R. Imaging modalities in keratoconus. Indian J Ophthalmol 2013; 61:394.  Back to cited text no. 1
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Abu-Amero KK, Kalantan H, Al-Muammar AM. Analysis of the VSX1 gene in keratoconus patients from Saudi Arabia. Mol Vis 2011; 17:667.  Back to cited text no. 4
    
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Shabayek MH, Alió JL. Intrastromal corneal ring segment implantation by femtosecond laser for keratoconus correction. J Ophthalmol 2007; 114:1643–1652.  Back to cited text no. 8
    
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Coskunseven E, Kymionis GD, Tsiklis NS, Atun S, Arslan E, Jankov MR et al. One-year results of intrastromal corneal ring segment implantation (KeraRing) using femtosecond laser in patients with keratoconus. Am J Ophthalmol 2008; 145:775–779.  Back to cited text no. 9
    
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Applegate RA, Ballentine C, Gross H, Sarver EJ, Sarver CA. Visual acuity as a function of Zernike mode and level of root mean square error. ‎Optom Vis Sci 2003; 80:97–105.  Back to cited text no. 10
    
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Pinero DP, Alio JL, Teus MA, Barraquer RI, Uceda-Montanés A. Modeling the intracorneal ring segment effect in keratoconus using refractive, keratometric, and corneal aberrometric data. Invest Ophthalmol Vis Sci 2010; 51:5583–5591.  Back to cited text no. 12
    
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Bühren J, Kook D, Yoon G, Kohnen T. Detection of subclinical keratoconus by using corneal anterior and posterior surface aberrations and thickness spatial profiles. Invest Ophthalmol Vis Sci 2010; 51:3424–3432.  Back to cited text no. 13
    
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Simoceli VP, Torquetti L, Simoceli RA, Ferrara P. Wavefront aberrations prior to and post intrastromal corneal ring segment implantation in keratoconus. J Cataract Refract Surg 2012; 3:141–145.  Back to cited text no. 14
    
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  [Full text]  



 
 
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  [Table 1], [Table 2], [Table 3]



 

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