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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 20  |  Issue : 4  |  Page : 144-147

Effect of continuous intrastromal corneal ring (MyoRing) implantation on intraocular pressure measurements in patients with keratoconus


Department of Ophthalmology, Assiut University, Assiut, Egypt

Date of Submission20-May-2019
Date of Acceptance31-Aug-2019
Date of Web Publication17-Dec-2019

Correspondence Address:
MD Mahmoud Abd El-Radi
Department of Ophthalmology, Assiut University Hospital, Assiut University, 6th Floor, 71515 Assiut
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/DJO.DJO_25_19

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  Abstract 


Purpose The aim of this study was to document the effect of implanting continuous intrastromal corneal ring (MyoRing) on intraocular pressure (IOP) measurements by Goldmann applanation tonometer (GAT).
Patients and methods Patients with moderate keratoconus with keratometric (K) readings between 48 and 55 D, corneal thinnest location on pentacam of more than 380 μm, and clear cornea underwent MyoRing implantation using a femtosecond laser system as per standard care. IOP was measured over the central cornea using GAT at baseline and after surgery by 1 week, 1, and 6 months, with the readings of month 6 being used in statistical analysis.
Results A total of 30 eyes of 30 patients were enrolled in the study. The mean preoperative IOP was 14.3 mmHg, whereas the mean postoperative IOP was 13.9 mmHg, with no statistical significance (P>0.05). The mean preoperative pachymetry at the center of the cornea was 442.4 μm, whereas the mean postoperative pachymetry at the center of the cornea was 438 μm, with no statistical significance (P>0.05).
Conclusion Implantation of MyoRing in patients with keratoconus resulted in a mild decrease in IOP measurements that was neither clinically nor statistically significant.

Keywords: intraocular pressure, intrastromal corneal ring (MyoRing), keratoconus


How to cite this article:
El-Radi MA, Saleh MG. Effect of continuous intrastromal corneal ring (MyoRing) implantation on intraocular pressure measurements in patients with keratoconus. Delta J Ophthalmol 2019;20:144-7

How to cite this URL:
El-Radi MA, Saleh MG. Effect of continuous intrastromal corneal ring (MyoRing) implantation on intraocular pressure measurements in patients with keratoconus. Delta J Ophthalmol [serial online] 2019 [cited 2020 Sep 22];20:144-7. Available from: http://www.djo.eg.net/text.asp?2019/20/4/144/273322




  Introduction Top


Keratoconus is a progressive corneal degenerative disease resulting in corneal thinning, steeping, and protrusion. Visual acuity significantly drops owing to myopia and astigmatism, which becomes irregular with progression of the disease [1].

The plan for management of keratoconus depends on different factors including the presenting best-corrected visual acuity, the degree of corneal thinning, and keratometric readings [2]. Rigid gas permeable contact lenses (RGPs) represent a conservative option to correct corneal irregularity and astigmatism in keratoconus, but they do not stop the progression of the disease [3]. Corneal collagen cross-linking is now considered as the treatment of choice in mild to moderate cases of keratoconus and is proven to halt the disease progression by being able to strengthen the corneal collagen fibers [4]. Deep anterior lamellar or penetrating keratoplasty is now resorted to only in advanced cases of keratoconus with marked deterioration of visual acuity [5],[6].

The implantation of intrastromal corneal ring (ICR) segments (e.g. INTACS, Ferrara, and Keraring) is indicated for cases with moderate keratoconus to flatten and partially regularize the steep irregular keratoconic corneas [7],[8]. A modification of corneal ring segments used in treatment of keratoconus was suggested by Daxer in the form of a continuous circular ring segment ‘MyoRing’ which is formed of poly-methyl methacrylate and is available with different ring thickness (250–350 μm) [9],[10].

Corneal structural and viscoelastic properties are affected in corneal diseases especially those affecting corneal thickness as in keratoconus. Intraocular pressure (IOP) measurement in patients with keratoconus represents a challenge for ophthalmologists owing to corneal thinning. IOP measurement and the central corneal thickness (CCT) may be affected by the insertion of intrastromal ring segments. However, the effect of implantation of intrastromal rings on corneal biomechanics and consequently the IOP measurements was neither widely investigated nor clearly understood [11],[12],[13].

In this study, we compared the IOP measurements before and after implantation of continuous ICRs (MyoRing) in patients with keratoconus.


  Patients and methods Top


This is a prospective interventional study that was conducted at TIBA Eye Center (private practice) where 30 eyes of 30 patients were included.

All patients were fully informed about the study concerning the aim, possible benefits, and complications of MyoRing implantation procedure and signed a written informed consent. The study was conducted under the tenets of the Declaration of Helsinki and after approval of the Ethical Committee of the Faculty of Medicine, Assiut University, Assiut, Egypt.

Patients with progressive keratoconus with keratometric (K) readings between 48 and 55 D, stage II and III Amsler–Krumeich classification [14],[15], corneal thinnest location on pentacam of more than 380 μm, and clear cornea were included.

Exclusion criteria were patients with advanced keratoconus (stage IV with Kmax>55 D), corneal scars, and previous corneal surgeries such as corneal collagen cross-linking or post-LASIK ectasia.

All patients underwent a thorough ocular examination including uncorrected and corrected distant visual acuity, slit lamp, and fundus examination.

Preoperative pentacam (Oculus pentacam; Oculus GmbH, Wetzlar, Germany) was ordered to ensure diagnosis and severity of keratoconus, in addition to CCT measurements.

Goldmann applanation tonometer (Haag-Streit GAT, Köniz, Switzerland) was the tonometer used in this study for measuring the IOP before and after MyoRing insertion. Three consecutive GAT readings were recorded, and the average was calculated. Correction tables that take into consideration low corneal thickness were used to compensate for false low IOP values [16] and the corrected values were used in statistical analysis. All Goldmann tonometers were checked to ensure accurate calibration. Preoperative assessment and IOP measurements were taken by the same examiner (M.G.).

Surgical technique of MyoRing insertion

All patients underwent MyoRing implantation (manufactured by DIOPTEX Forschungs-, Entwicklungs-und Vertriebs; GmbH DIOPTEX, Munich, Germany) using a femtosecond laser system (FS-200 wavelight; Alcon, Fort Worth, USA). The device used 200-kHz repetition rate, 1030-nm wavelength and a 5-μm spot size to create an intrastromal corneal pocket of 8-mm diameter and a depth of nearly 80% of the corneal thickness at the thinnest location with a superior tunnel incision at 90° of 4-mm width.

The choice of the appropriate MyoRing was according to the nomogram recommended by the manufacturer. All cases were done by the same surgeon (M.R.).

Postoperative treatment with topical antibiotic–steroid eye drops was prescribed for a week and topical preservative-free lubricants for a month.

IOP (Goldmann applanation tonometer) was measured for all patients at 1 week, 1, and 6 months postoperatively.

Follow-up pentacam was ordered 6 months postoperatively, and the CCT was reassessed.

Statistical analysis

Statistical analysis was carried out by SPSS version 20.0 (SPSS Inc., Chicago, Illinois, USA). Shapiro–Wilk test was used to test data for normal distribution and showed that data were nonparametric. Wilcoxon signed rank test was used to compare mean IOP readings before and after surgery. P value was set at 0.05.


  Results Top


The study included 30 eyes of 30 patients. The mean age was 21.7±6.5 years, with 60% males and 40% females ([Table 1]).
Table 1 Demographic data of the studied patients

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The mean preoperative corneal thickness was 442.4±26.2 μm. Six months after implantation of MyoRing, the mean corneal thickness was 438±25.8 μm ([Table 2]).
Table 2 Pachymetric and intraocular pressure measurements before and after implantation of MyoRing

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The mean preoperative IOP was 14.3±1.55 mmHg. Six months after implantation of MyoRing, the mean IOP was 13.9±1.41 mmHg ([Table 2]).

These changes in both pachymetric values and IOP measurements were statistically insignificant (P>0.05).


  Discussion Top


Goldmann applanation tonometry is still considered as the ‘gold standard’ for measuring the IOP [17]. Several corneal variables including thickness, curvature, biomechanical properties, and hydration influence IOP measurement by GAT. Decreased CCT, change in corneal curvature, and loss of corneal rigidity are considered contributing causative factors for the observed decline in IOP after many refractive corneal procedures including LASIK and RK [18],[19],[20].

Accurate IOP measurement is very important in patients after corneal refractive surgery because their eyes are often treated with topical corticosteroid drops for weeks or months which may lead to IOP rise in susceptible individuals. The latter rise might be theoretically masked by an underestimation effect of refractive surgery on Goldmann measurements of IOP [21].

Corneal ring implants do not notably affect the CCT. Moreover, a direct effect of those implants on the corneal rigidity was not shown in previous studies [22]. This may explain why changes in IOP measurements are minimal.

This study showed that the mean IOP, measured by GAT at corneal center, decreased after MyoRing implantation in keratoconic eyes. However, the change was neither statistically nor clinically relevant.

Previous studies have shown the effects of implanting ICRs on IOP in different models and using different IOP measuring devices. Kreisberg et al. [23] investigated IOP after implantation of a 360° ICR in eye-bank eyes. The authors used manometric controlled pressure (between 10 and 40 mmHg) and a pneumotonometer and found no significant difference in IOP before and after implantation. Tran et al. [24] compared GAT and pneumotonometry after implantation of Intacs (Addition Technology Inc., Des Plaines, IL, USA). There was no difference in measurements at the center of the cornea. Paracentrally, over the device, GAT measured an elevated IOP of 40–60 mmHg, whereas the pneumotonometer showed no increase in IOP. The elevation measured by GAT was probably caused by locally increased corneal stiffness associated with the underlying rings.

This study has some limitations such as small sample size and the lack of enough prior research studies on the topic to be compared with our results.


  Conclusion Top


Implantation of MyoRing slightly lowered IOP measurements by GAT over the central cornea. The change in IOP was neither statistically nor clinically significant.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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