Journal of Refractive Surgery

Original Article 

Effect of Conus Eccentricity on Visual Outcomes After Intracorneal Ring Segments Implantation in Keratoconus

Zisis Gatzioufas, MD, PhD; Georgios D. Panos, MD, PhD; Mohamed Elalfy, MD; Aye Khine, MD; Samer Hamada, MD; Damian Lake, MD; Nikos Kozeis, MD, PhD; Miltos Balidis, MD, PhD

Abstract

PURPOSE:

To investigate the potential impact of cone eccentricity on visual outcomes after Keraring (Mediphacos, Belo Horizonte, Brazil) implantation for keratoconus.

METHODS:

Nineteen eyes from 19 patients with keratoconus who underwent femtosecond laser–assisted Keraring implantation for keratoconus were included in this retrospective study. Uncorrected visual acuity (UDVA), corrected visual acuity (CDVA), keratometric readings, central corneal thickness, maximum keratometric distance from corneal apex (DKmax), corneal thinnest point from corneal apex (DTh), and coma were evaluated preoperatively and 6 months after the Keraring implantation. DKmax and DTh were used as metrics reflecting the eccentricity of the cone.

RESULTS:

UDVA, CDVA, keratometric readings, and coma improved at 6 months postoperatively. However, there was no correlation between DKmax or DTh and visual outcomes at 6 months postoperatively.

CONCLUSIONS:

The data did not show any impact of the cone eccentricity on visual outcomes after Keraring implantation for keratoconus at 6 months postoperatively.

[J Refract Surg. 2018;34(3):196–200.]

Abstract

PURPOSE:

To investigate the potential impact of cone eccentricity on visual outcomes after Keraring (Mediphacos, Belo Horizonte, Brazil) implantation for keratoconus.

METHODS:

Nineteen eyes from 19 patients with keratoconus who underwent femtosecond laser–assisted Keraring implantation for keratoconus were included in this retrospective study. Uncorrected visual acuity (UDVA), corrected visual acuity (CDVA), keratometric readings, central corneal thickness, maximum keratometric distance from corneal apex (DKmax), corneal thinnest point from corneal apex (DTh), and coma were evaluated preoperatively and 6 months after the Keraring implantation. DKmax and DTh were used as metrics reflecting the eccentricity of the cone.

RESULTS:

UDVA, CDVA, keratometric readings, and coma improved at 6 months postoperatively. However, there was no correlation between DKmax or DTh and visual outcomes at 6 months postoperatively.

CONCLUSIONS:

The data did not show any impact of the cone eccentricity on visual outcomes after Keraring implantation for keratoconus at 6 months postoperatively.

[J Refract Surg. 2018;34(3):196–200.]

Intracorneal ring segments (ICRS) are small polymethylmethacrylate devices implanted into the cornea to alter its geometry in a manner that will enhance its refractive properties and thereby improve visual acuity. The concept of ICRS implantation was first proposed by Burris1 in 1978 for treatment of myopia. Colin et al.2 introduced the use of ICRS implantation for the management of keratoconus in 2000. Since then, many different types of ICRS with variable thickness, geometry, and diameter have been developed and used for restoring visual acuity in patients with keratoconus. Their implantation was initially managed mechanically, but femtosecond laser–assisted implantation has been gradually replacing the conventional mechanical technique.3

The mechanism of action in ICRS is simple. They act as spacer elements between the collagen fibers of the corneal stroma and induce an arc-shortening effect, resulting in flattening of the central corneal area.4 It has been shown that the flattening effect is directly proportional to the thickness of the ICRS and inversely proportional to the corneal diameter.5 Several studies documented that implantation of ICRS decreases keratometric readings, spherical equivalent, and cylinder, reduces higher order aberrations, and improves uncorrected (UDVA) and corrected distance visual (CDVA) acuity in patients with keratoconus.6–10 Moreover, ICRS present good long-term results and their beneficial effect is preserved to some extent after ICRS explanation.11–13 The complication rate is relatively low and mostly includes postoperative complications such as infection, ICRS displacement/migration, ICRS extrusion, corneal scarring, and corneal vascularization.10,14

There are now implantation nomograms, most of them offered by the ICRS manufacturers, indicating the appropriate ring segment characteristics for each individual case and suggesting in a customized manner the most suitable parameters for ICRS implantation. There are also certain indications and contraindications regarding the patient selection to maximize the safety and efficacy of the treatment.10,15 However, there are still many gray zones in the preoperative assessment. Which prognostic factors could influence the clinical outcome or predict the success/failure rate of this treatment method?

The aim of our study was to investigate whether the eccentricity of the cone correlates with the final visual outcome after ICRS implantation for stable keratoconus.

Patients and Methods

This was a retrospective, interventional cohort study. Our study included 19 patients with stable keratoconus who underwent unilateral Keraring (Mediphacos, Belo Horizontale, Brazil) implantation with the use of a femtosecond laser (LDV Z6; Ziemer, Port, Switzerland). All patients had keratoconus stage 2 or 3 according to the Amsler–Krumeich classification. None of the treated eyes had undergone corneal cross-linking in the past and they all had stable keratoconus for a period longer than 12 months prior to ICRS implantation. Patients were enrolled in the study dataset on the basis of the following criteria: age older than 21 years, contact lens intolerance, CDVA of less than 0.2 logMAR (0.63 Snellen), and corneal thickness more than 400 microns in the area of ICRS implantation, plus absence of central corneal scars, ocular surface disease, allergic eye disease, previous ocular surgery, or any other ocular pathology except for keratoconus. Pregnant or breast feeding women were excluded from the study cohort.

A complete ophthalmic examination was performed preoperatively and postoperatively, including UDVA, CDVA, manifest refraction, spherical equivalent (SE), keratometry (K) readings (in diopters [D]), central corneal thickness (μm), maximum keratometric distance from corneal apex (DKmax), and thinnest point distance from corneal apex (DTh). Corneal topography was evaluated using the Scheimpflug camera (Pentacam; Oculus Optikgeräte, Wetzlar, Germany). Diagnosis of keratoconus was facilitated by corneal topography and corneal elevation mapping, as evaluated by Scheimpflug imaging (Pentacam). Visual acuity was measured using Snellen notation and then converted to logMAR for statistical analysis. DKmax and DTh were used as indicators of the cone eccentricity and have been calculated with the aid of the Pentacam-derived x and y values for Kmax and thinnest point, respectively, using the Pythagorean theorem:

Distance from the center = x2+y2

The standard nomogram of the ICRS provided by the manufacturers was used for selection of the appropriate ring segment in each individual case and calculation of the implantation parameters. This nomogram is based on the keratoconus pattern morphology (nipple, bow-tie, or oval cone), the keratoconus pattern symmetry (percentage of ectatic area located on one side of the steep corneal meridian), and the amount of corneal astigmatism and manifest refraction (manifest sphere diopters). All patients had one ring segment implanted in the flat meridian adjacent to the cone, with a diameter of 5 mm and variable thickness between 150 and 300 μm. Intrastromal tunnels were created using a femtosecond laser. After topical anesthesia with proparacaine hydrochloride 0.5%, the corneal apex was marked with ink under the surgical microscope (OPMI Lumera 700; Carl Zeiss AG, Jena, Germany). A suction ring of 9.5 mm was applied and, after applanation, a corneal tunnel of 1.3 mm width was created with the femtosecond laser in the 5-mm zone and 80% corneal depth, as well as a single, radial, 2.7-mm corneal incision at the tunnel starting point. The corneal tunnel length was equal to the ring segment arc length plus 10°. Ring segments were inserted into the tunnel with the aid of special forceps. Postoperatively, patients were prescribed chloramphenicol 0.5% eye drops for 2 weeks and fluorometholone 0.1% eye drops for 4 weeks. Scheduled follow-up was at postoperative 1 day, 1 week, and 1, 3, and 6 months.

All patients provided their written consent prior to ICRS implantation and the tenets of the Declaration of Helsinki were fully respected. The local institutional review board committee approved this study.

Statistical Analysis

Normality of the data distribution was tested using the Shapiro–Wilk test and parametric and non-parametric tests were applied accordingly. All results are presented as mean ± standard deviation. A P value of less than .05 was considered statistically significant. All statistical analyses were performed using MedCalc software (version 15; MedCalc, Ostend, Belgium).

Results

Nineteen eyes (10 left and 9 right eyes) of 19 patients were included. Demographic data are presented in Table 1.

Demographic Data of the Study Group

Table 1:

Demographic Data of the Study Group

Significant improvement was observed in all parameters examined in this study between baseline and follow-up at 6 months apart from the CCT and the DTh. All results are summarized in Table 2.

Summary of the Visual, Refractive, and Corneal Topographic and Tomographic Outcomes After ICRS Implantation in Patients With Keratoconus

Table 2:

Summary of the Visual, Refractive, and Corneal Topographic and Tomographic Outcomes After ICRS Implantation in Patients With Keratoconus

No correlation was observed between preoperative DKmax and UDVA improvement (Pearson's correlation coefficient, r = 0.2739; P = .2565) or preoperative DKmax and CDVA improvement (Pearson's correlation coefficient, r = 0.08701; P = .7232). Moreover, no correlation was observed between DKmax difference and UDVA improvement (Pearson's correlation coefficient, r = 0.1958; P = .4217) or DKmax difference and CDVA improvement (Pearson's correlation coefficient, r = 0.02175; P = .9296). Similarly, no correlation was observed between preoperative DTh and UDVA improvement (Pearson's correlation coefficient, r = 0.3142; P = .1902) or preoperative DTh and CDVA improvement (Pearson's correlation coefficient, r = 0.4083; P = .0827).

Discussion

There is evidence that the flattening effect of ICRS is directly proportional to the thickness of the implanted segment and inversely proportional to the corneal diameter at the implantation site.16 The efficacy of ICRS in managing keratoconus has been well documented. ICRS reduce spherical equivalent and keratometric readings, higher order aberrations, and particularly coma, thereby improving UDVA and CDVA.6,7,12,17 The increase in visual acuity is attributed to the improvement of the aberrometric profile and mainly to the reduction of coma, as a result of the regularized corneal geometry after ICRS implantation.7,12 Moreover, ICRS implantation significantly improved the contact lens tolerance, as shown by numerous studies.17–19

Several factors have been identified to be associated with the visual outcomes after ICRS implantation. Vega-Estrada et al.7 reported that poor preoperative visual acuity is a good prognostic factor for significant visual improvement, whereas Alió et al.20 suggested that significant visual improvement is less likely in advanced keratoconus (stage 4). Advanced keratoconus has also been linked to low predictability of the keratometric and visual outcomes after ICRS implantation.21 Peña-García et al.22 proposed that alignment of the refractive and keratometric axes (angle < 15°) is another positive prognostic factor for successful visual outcomes after ICRS implantation.

It is well documented that significant flattening of the central cornea occurs 6 months after ICRS implantation.6 The reduction in keratometry is evident in all grades of keratoconus, but the largest keratometric amelioration is observed in cases with a high degree of disease severity, as shown by Ertan and Kamburoglou.23 Moreover, Boxer Wachler et al.24 reported that patients with advanced keratoconus demonstrated the greatest decrease in spherical equivalent, which correlates well with the amount of flattening in the central cornea. The corneal flattening effect of ICRS shows an excellent long-term stability11,25 and, interestingly, it is preserved to some extent even after ICRS removal.26 However, the refractive improvement reverses after ICRS explantation, with subsequent visual deterioration.26

The aim of our study was to investigate whether the cone eccentricity influences the visual outcomes after ICRS implantation. Central cones have higher simulated keratometry values at 3 mm and anterior corneal higher order aberrations compared to peripheral cones.27 It has also been documented that the pericentral cornea flattens more than the central cornea after ICRS implantation, thereby maintaining the prolate shape of the corneal optical zone.28,29 Shetty et al.30 showed that cone location appears to affect visual acuity after combined topography-guided photorefractive keratectomy and corneal cross-linking, with better results for cones within a central 2-mm zone. However, Greenstein et al.31 suggested that more topographic flattening occurs in central cones rather than peripheral cones after corneal cross-linking.

In our study, there was improvement of keratometric readings, UDVA, CDVA, and coma after Keraring implantation, but we did not observe any correlation between the visual outcomes and the eccentricity of the cone, as reflected by the DKmax and DTh. Both DKmax and DTh decreased significantly postoperatively. Of course, our study has certain limitations. The number of eyes included in our study was relatively small. Moreover, our study included both central (within a 2-mm zone) and peripheral (outside a 2-mm zone) cones without being able to differentiate the impact of eccentricity for these two distinct subgroups (low number of eyes).

Our data do not show any influence of the cone eccentricity on the final visual outcome after Keraring implantation for keratoconus. Larger studies are required to investigate further the effect of conus location on ICRS outcomes and validate these results.

References

  1. Burris TE. Intrastromal corneal ring technology: results and indications. Curr Opin Ophthalmol. 1998;9:9–14. doi:10.1097/00055735-199808000-00003 [CrossRef]
  2. Colin J, Cochener B, Savary G, Malet F. Correcting keratoconus with intracorneal rings. J Cataract Refract Surg. 2000;26:1117–1122. doi:10.1016/S0886-3350(00)00451-X [CrossRef]
  3. Shabayek MH, Alió JL. Intrastromal corneal ring segment implantation by femtosecond laser for keratoconus correction. Ophthalmology. 2007;114:1643–1652. doi:10.1016/j.ophtha.2006.11.033 [CrossRef]
  4. Silvestrini TA, Mathis ML, Loomas BE, Burris TE. A geometric model to predict the change in corneal curvature from the intrastromal corneal ring (ICR). Invest Ophthalmol Vis Sci.1994;35:2023.
  5. Burris TE, Baker PC, Ayer CT, Loomas BE, Mathis ML, Silvestrini TA. Flattening of central corneal curvature with intrastromal corneal rings of increasing thickness: an eye-bank eye study. J Cataract Refract Surg. 1993;19(suppl):182–187. doi:10.1016/S0886-3350(13)80404-X [CrossRef]
  6. Piñero DP, Alió JL. Intracorneal ring segments in ectatic corneal disease: a review. Clin Experiment Ophthalmol. 2010;38:154–167. doi:10.1111/j.1442-9071.2010.02197.x [CrossRef]
  7. Vega-Estrada A, Alió JL, Brenner LF, et al. Outcome analysis of intracorneal ring segments for the treatment of keratoconus based on visual, refractive, and aberrometric impairment. Am J Ophthalmol. 2013;155:575–584. doi:10.1016/j.ajo.2012.08.020 [CrossRef]
  8. Coskunseven E, Kymionis GD, Tsiklis NS, et al. One-year results of intrastromal corneal ring segment implantation (KeraR-ing) using femtosecond laser in patients with keratoconus. Am J Ophthalmol. 2008;145:775–779. doi:10.1016/j.ajo.2007.12.022 [CrossRef]
  9. Kaya V, Utine CA, Karakus SH, Kavadarli I, Yilmaz OF. Refractive and visual outcomes after Intacs vs Ferrara intrastromal corneal ring segment implantation for keratoconus: a comparative study. J Refract Surg. 2011;27:907–912. doi:10.3928/1081597X-20110906-03 [CrossRef]
  10. Giacomin NT, Mello GR, Medeiros CS, et al. Intracorneal ring segments implantation for corneal ectasia. J Refract Surg. 2016;32:829–839. doi:10.3928/1081597X-20160822-01 [CrossRef]
  11. Torquetti L, Ferrara G, Almeida F, et al. Intrastromal corneal ring segments implantation in patients with keratoconus: 10-year follow-up. J Refract Surg. 2014;30:22–26. doi:10.3928/1081597X-20131217-02 [CrossRef]
  12. Vega-Estrada A, Alió JL, Brenner LF, Burguera N. Outcomes of intrastromal corneal ring segments for treatment of keratoconus: five-year follow-up analysis. J Cataract Refract Surg. 2013;39:1234–1240. doi:10.1016/j.jcrs.2013.03.019 [CrossRef]
  13. Yeung SN, Lichtinger A, Ku JY, Kim P, Low SA, Rootman DS. Intracorneal ring segment explantation after intracorneal ring segment implantation combined with same-day corneal collagen crosslinking in keratoconus. Cornea. 2013;32:1617–1620. doi:10.1097/ICO.0b013e3182a738ba [CrossRef]
  14. Coskunseven E, Kymionis GD, Tsiklis NS, et al. Complications of intrastromal corneal ring segment implantation using a femtosecond laser for channel creation: a survey of 850 eyes with keratoconus. Acta Ophthalmol. 2011;89:54–57. doi:10.1111/j.1755-3768.2009.01605.x [CrossRef]
  15. Vega-Estrada A, Alió JL. The use of intracorneal ring segments in keratoconus. Eye Vis (Lond). 2016;3:8. doi:10.1186/s40662-016-0040-z [CrossRef]
  16. Ertan A, Colin J. Intracorneal rings for keratoconus and keratectasia. J Cataract Refract Surg. 2007;33:1303–1314. doi:10.1016/j.jcrs.2007.02.048 [CrossRef]
  17. Alió JL, Shabayek MH, Artola A. Intracorneal ring segments for keratoconus correction: long-term follow-up. J Cataract Refract Surg. 2006;32:978–985. doi:10.1016/j.jcrs.2006.02.044 [CrossRef]
  18. Carrasquillo KG, Rand J, Talamo JH. Intacs for keratoconus and post-LASIK ectasia: mechanical versus femtosecond laser-assisted channel creation. Cornea. 2007;26:956–962. doi:10.1097/ICO.0b013e31811dfa66 [CrossRef]
  19. Shetty R, Kurian M, Anand D, Mhaske P, Narayana KM, Shetty BK. Intacs in advanced keratoconus. Cornea. 2008;27:1022–1029. doi:10.1097/ICO.0b013e318172fc54 [CrossRef]
  20. Alió JL, Shabayek MH, Belda JI, Correas P, Diez Feijoo E. Analysis of results related to good and bad outcomes of Intacs implantation for keratoconus correction. J Cataract Refract Surg. 2006;32:756–761. doi:10.1016/j.jcrs.2006.02.012 [CrossRef]
  21. Zare MA, Hashemi H, Salari MR. Intracorneal ring segment implantation for the management of keratoconus: safety and efficacy. J Cataract Refract Surg. 2007;33:1886–1891. doi:10.1016/j.jcrs.2007.06.055 [CrossRef]
  22. Peña-García P, Alió JL, Vega-Estrada A, Barraquer RI. Internal, corneal, and refractive astigmatism as prognostic factors for intrastromal corneal ring segment implantation in mild to moderate keratoconus. J Cataract Refract Surg. 2014;40:1633–1644. doi:10.1016/j.jcrs.2014.01.047 [CrossRef]
  23. Ertan A, Kamburoglu G. Intacs implantation using a femtosecond laser for management of keratoconus: comparison of 306 cases in different stages. J Cataract Refract Surg. 2008;34:1521–1526. doi:10.1016/j.jcrs.2008.05.028 [CrossRef]
  24. Boxer Wachler BS, Christie JP, Chandra NS, Chou B, Korn T, Nepomuceno R. Intacs for keratoconus. Ophthalmology. 2003;110:1031–1040. Erratum in: Ophthalmology. 2003;110:1475. doi:10.1016/S0161-6420(03)00094-0 [CrossRef]
  25. Pesando PM, Ghiringhello MP, Di Meglio G, Romeo S. Treatment of keratoconus with Ferrara ICRS and consideration of the efficacy of the ferrara nomogram in a 5-year follow-up. Eur J Ophthalmol. 2010;20:865–873. doi:10.1177/112067211002000509 [CrossRef]
  26. Yeung SN, Lichtinger A, Ku JY, Kim P, Low SA, Rootman DS. Intracorneal ring segment explantation after intracorneal ring segment implantation combined with same-day corneal collagen crosslinking in keratoconus. Cornea. 2013;32:1617–1620. doi:10.1097/ICO.0b013e3182a738ba [CrossRef]
  27. Prakash G, Srivastava D, Choudhuri S, Thirumalai SM, Bacero R. Differences in central and non-central keratoconus, and their effect on the objective screening thresholds for keratoconus. Acta Ophthalmol. 2016;94:118–129. doi:10.1111/aos.12899 [CrossRef]
  28. Burris TE, Holmes-Higgin DK, Silvestrini TA, Scholl JA, Proudfoot RA, Baker PC. Corneal asphericity in eye bank eyes implanted with the intrastromal corneal ring. J Refract Surg. 1997;13:556–567.
  29. Holmes-Higgin DK, Baker PC, Burris TE, Silvestrini TA. Characterization of the aspheric corneal surface with intrastromal corneal ring segments. J Refract Surg. 1999;15:520–528.
  30. Shetty R, Nuijts RM, Nicholson M, et al. Cone location-dependent outcomes after combined topography-guided photorefractive keratectomy and collagen cross-linking. Am J Ophthalmol. 2015;159:419–425. doi:10.1016/j.ajo.2014.11.020 [CrossRef]
  31. Greenstein SA, Fry KL, Hersh PS. Effect of topographic cone location on outcomes of corneal collagen cross-linking for keratoconus and corneal ectasia. J Refract Surg. 2012;28:397–405. doi:10.3928/1081597X-20120518-02 [CrossRef]

Demographic Data of the Study Group

CharacteristicValue
No. of patients19
Mean age ± standard deviation (y)38.3 ± 6.4
Male:female ratio12:7
Right eye:left eye ratio9:10
Keratoconus stage 2:stage 3 ratio7:12
Contact lens intolerance11
Previous corneal cross-linking0

Summary of the Visual, Refractive, and Corneal Topographic and Tomographic Outcomes After ICRS Implantation in Patients With Keratoconus

ParameterPreoperativePostoperativeP
UDVA (logMAR)1.19 ± 0.410.71 ± 0.33< .0001a
CDVA (logMAR)0.31 ± 0.220.10 ± 0.16< .0001b
Spherical equivalent (D)−3.39 ± 3.37−1.66 ± 3.03.0284a
Cylinder (D)−7.58 ± 2.59−3.95 ± 1.99< .0001b
Kmax (D)58.77 ± 3.9754.69 ± 3.35< .0001a
K1 (D)47.53 ± 3.3846.06 ± 2.92.001a
K2 (D)51.98 ± 3.2949.53 ± 3.16< .0001a
Coma (μm)−4.17 ± 1.39−2.73 ± 0.95< .0001a
CCT (μm)462.2 ± 34.3466.6 ± 31.8.2849a
Corneal thinnest point (μm)428.6 ± 36.9441 ± 36.8.0020a
DKmax (mm)1.59 ± 1.270.87 ± 1.67.0141b
DTh (mm)1.82 ± 0.591.02 ± 0.34.0188a
Authors

From Ophthalmica Institute, Thessaloniki, Greece (ZG, GDP, NK, MB); Queen Victoria Hospital, Corneo-Plastic Unit, East Grinstead, United Kingdom (ZG, ME, AK, SH, DL); and The Research Institute of Ophthalmology, Cairo, Egypt (ME).

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (ZG, SH); data collection (ZG, GDP, ME, MB); analysis and interpretation of data (ZG, GDP, AK, DL, NK); writing the manuscript (ZG, GDP, NK, MB); critical revision of the manuscript (GDP, ME, AK, SH, DL, MB); statistical expertise (ZG, GDP); administrative, technical, or material support (GDP, NK); supervision (SH)

Correspondence: Zisis Gatzioufas, MD, PhD, Queen Victoria Hospital, Holtye Road, RH19 3DZ East Grinstead, United Kingdom. E-mail: zisisg@hotmail.com

Received: April 04, 2017
Accepted: December 20, 2017

10.3928/1081597X-20180115-02

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