Secondary Logo

Journal Logo

Article

Comparative study of 3 intracorneal implant types to manage central keratoconus

Yousif, Mohamed Omar MD, FRCS; Said, Azza Mohamed Ahmed MD*

Author Information
Journal of Cataract & Refractive Surgery: March 2018 - Volume 44 - Issue 3 - p 295-305
doi: 10.1016/j.jcrs.2017.12.020
  • Free

Abstract

Keratoconus is an ectatic corneal disorder characterized by cone-like steepening of the cornea. The progressive thinning and subsequent anterior bulging of the cornea lead to severe astigmatism and central scarring, producing visual distortion, increased sensitivity to light, and an associated reduction in corrected visual acuity.1

There are several therapeutic choices for management of this condition, such as contact lens use, thermokeratoplasty procedures, corneal crosslinking (CXL), intrastromal corneal ring segment (ICRS) implantation, and lamellar and penetrating keratoplasty.2

Intrastromal corneal ring segments are small devices made of rigid poly(methyl methacrylate) (PMMA) that are implanted within the corneal stroma to induce a change in the geometry and the refractive power of the tissue. The concept of inserting segments as corneal inserts was first introduced by Fleming and Schanzlin in 19873; the aim at that time was myopia correction.

Intacs (Addition Technology, Inc.), one of the first ICRS, received Conformité Européenne certification in 1996 and U.S. Food and Drug Administration approval in 1999 for the correction of low to moderate myopia.4

Colin at al.2 found that ICRS could flatten the central cornea and regularize the asymmetry of tissue, leading to a reduction in keratometry (K) readings and an improvement in the refraction and vision of patients with keratoconus. Since then, several authors have reported the benefit of implanting ICRS in keratoconic eyes and in delaying or avoiding more complex interventions, such as keratoplasty procedures.4

Four types of ICRS are available, and various reports in the ophthalmic literature describe their effectiveness in treating keratoconus2 In addition is the newly developed Keratacx Plus rings (Imperial Medical Technologies Europe GmbH), and there is only 1 report of their effectiveness in management of keratoconus.5

Keratacx Plus rings have a smaller visual zone than and dimensions similar to those of Kerarings ICRS (Mediphacos Ltda.) and Ferrara ICRS (Ferrara Ophthalmics Ltd.). The design is meant to prevent the visual aberrations, halos, glare, sparkles, and field defects sometimes encountered with the other 2 designs. The decrease in visual problems is the result of the rings’ domed edges. Also, smooth borders were incorporated to help protect the corneal stroma from erosion over time (Figure 1). Keratacx Plus rings are more affordable than other types of ICRS6 and are available in several radii (45 degrees, 90 degrees, 120 degrees, 160 degrees, 210 degrees, 320 degrees, 355 degrees) to allow for precise diopter (D) corrections in eyes with all types of corneal topography. Table 1 shows the basic characteristics of different ring segments.5,6

Figure 1.
Figure 1.:
A: Cesign of the 160-degree 2-ring segment. B: Dome edges and smooth borders of the segments.
Table 1
Table 1:
Basic characteristics of different ring segments. 5,6

A central cone is defined when 50% or more of the cone is within the 3.0 mm zone on the posterior elevation map of the Pentacam rotating Scheimpflug device (Oculus Optikgeräte GmbH).7 Eccentric cones tend to have larger higher-order aberrations and astigmatism, and central cones produce higher refractive errors.8

The decision to implant symmetric ICRS versus asymmetric ICRS is based on the cone location. Symmetric segments are typically used to manage central ectatic conditions, whereas asymmetric segments are used for eccentric ectatic conditions.7 The results of the first experiments with intracorneal continuous rings in the early 1980s were unacceptable.9

The development of new technologies, new designs of intracorneal implants, and new surgical approaches, such as femtosecond laser corneal tunnel creation, made the procedure faster, easier, and more comfortable for patients and surgeons. The main advantages of this method over mechanical tunnel creation are that the depth of implantation is more precise and there are fewer complications.10

The Myoring intracorneal implant, a continuous intracorneal ring (ICR) (Dioptex GmbH), is another surgical option in which a flexible 360-degree, full-ring PMMA implant is inserted into a corneal pocket for treatment of keratoconus, keratectasia after laser in situ keratomileusis (LASIK), and moderate to high myopia. This ICR is available in diameters ranging from 5.0 to 6.0 mm and thickness ranging from 200 to 400 μm in 20 μm increments. The anterior surface is convex, and the posterior surface is concave.11

Because the Myoring ICR is a continuous, full-ring implant with no disruption of continuity along its circumference, it acts in the cornea as a second (artificial) limbus and supports the cornea biomechanically in the same way as a ceiling beam supports the ceiling of a room under load by separating the ceiling (cornea) into 2 compartments and reducing the load on each compartment. Specifically, the ICR takes up a significant amount of the load acting on the cornea.12

Another difference between ring segments and the Myoring ICR is that the latter provides all 3 possible degrees of freedom (implant thickness, implant diameter, and implant position) with the goal of achieving the optimum result in any given case, whereas ring segments provide 1 degree of freedom (implant thickness).12 Daxer et al.13 concluded that the ICR has the potential to produce excellent long-term vision results in cases of mild, moderate, and advanced keratoconus, regardless of the cone position and disease progression.

The aim of this study was to compare the efficacy (visual acuity, refraction, corneal topography, corneal asphericity) of intrastromal corneal implantation of Keratacx 160-degree 2 symmetrical ring segments, the Keratacx 320-degree near-total ring, and the Myoring ICR in central keratoconus. To our knowledge, this is the first study to do such a comparison.

PATIENTS AND METHODS

This prospective nonrandomized comparative interventional case study included eyes with a diagnosis of keratoconus of the central cone or symmetric bowtie type. All patients were not contact lens wearers. Patients were recruited from a private practice and Ain Shams University cornea clinics from March 2015 to November 2016, and all provided written informed consent to enroll in the study. All surgeries were performed and followed at a private ophthalmic subspecialty center by same surgeon (M.O.Y.). Approval of the Ethical Committee of Ain Shams University was obtained before the patients’ enrollment.

Inclusion and Exclusion Criteria

Inclusion criteria were a diagnosis of moderate to severe keratoconus according to keratoconus study classification using the steepest K reading14 with a history of progression over the previous 12 months in the form of a change in the steepest K reading of 1.0 D or more,15 age between 18 years and 40 years, maximum K reading less than 65.0 D, minimum corneal thickness at the proposed tunnel or pocket site of greater than 400 μm, presence if central cone (≥50% of cone within 3.0 mm zone on the rotating Scheimpflug posterior elevation map), and ability to complete a follow-up of 6 months postoperatively. Exclusion criteria were previous corneal surgery; evidence of infectious corneal disease in the study eye; unclear visual axis resulting from corneal opacity, scars, and/or cataract; pregnancy or nursing; and systemic collagen vascular disease.

Preoperative Assessment

Preoperative evaluation comprised uncorrected (UDVA) and corrected (CDVA) distance visual acuity measurement, manifest and cycloplegic refractions, slitlamp examination, fundus examination, and corneal topography with a rotating Scheimpflug device.

Surgical Technique

All Groups

The surgical procedures were performed using topical anesthesia and a femtosecond laser (Wavelight-FS 200, Alcon Surgical, Inc.) for tunnel creation in eyes that had Keratacx 2, 160-degree symmetric ring segment implantation (Group 1) or Keratacx 320-degree near-total ring segment implantation (Group 2) and for pocket creation in eyes that had Myoring ICR implantation (Group 3). Postoperatively all patients were prescribed to use gatifloxacin and dexamethasone eyedrops 4 times daily for 1 week.

Group 1

In step 1, the femtosecond laser was to create a tunnel with the following parameters: inner diameter 4.9 mm, outer diameter 5.7 mm, and depth 75% of the thinnest point along the proposed tunnel site. The incision leading to the tunnel was made on the steep meridian.

Step 2 comprised selection of the thickness of the ring segments and insertion of the symmetric 160-degree ICRS. The ICRS were implanted using a purpose-designed forceps based on the surgeon-specific nomogram. The nomogram was created by combining the well-known third-generation Ferrara ring nomogram, in which ring selection depends on corneal thickness, the amount of topographic corneal astigmatism (simulated K), and the ectatic area distribution on the corneal surface, with the fourth-generation nomogram, which considers the corneal asphericity as the first parameter for ring selection.

The 2 symmetric ring segments were placed on both sides of the steep corneal meridian. The choice of ring thickness was based on the K readings, corneal thickness, and asphericity. The ring segment thickness cannot exceed 50% of the thickness of the cornea on the track of the ring and cannot exceed the thickness required to keep the cornea slightly prolate, with a corneal asphericity (Q) value close to −0.23.6

Group 2

In step 1, the femtosecond laser was to create a tunnel with the following parameters: inner diameter 4.8 mm, outer diameter 5.7 mm; depth was set at 75% of the thinnest point along the proposed tunnel site. The incision leading to the tunnel was made on the steep meridian.

Step 2 comprised selection of the thickness of the ring segments and insertion of the 320-degree near-total ring ICRS. The ICRS were implanted using a purpose-designed forceps based on the surgeon-specific nomogram and on his experience; the ring thickness choice (200 μm, 250 μm, 300 μm) depended on the spherical equivalent (SE) while also respecting the Q value (Table 2). The ring segment thickness cannot exceed 50% of the thickness of the cornea on the track of the ring and cannot exceed the thickness required to keep the cornea slightly prolate, with a Q value close to −0.23.6Figure 2 shows the surgical steps of the 320-degree segment implantation.

Table 2
Table 2:
Choice of the ring thickness in Group 2.
Figure 2.
Figure 2.:
Surgical steps of 320-degree segment implantation. A: Ring design. B: Femtosecond laser corneal tunnel creation. C: Ring is held to be implanted into the tunnel. D: Final intrastromal corneal position.

Group 3

In step 1, the femtosecond laser was to create a corneal pocket with the following parameters: 9.0 mm diameter, 300 μm depth, and a 5.0 mm small-incision tunnel located on the temporal periphery of the cornea.

Step 2 comprised selection of the diameter and thickness of the ring and its insertion. The diameters of the rings used in this study were 5.0 or 6.0 according to the scotopic pupil and degree of keratoconus, and the thickness was 240 μm, 280 μm, or 320 μm according to the mean central K reading. The ring choice followed the nomogram recommended by the manufacturer (Middle East modified nomogram) but with modifications based on the surgeon's experience (Table 3).16

Table 3
Table 3:
Surgeon-modified nomogram for ring diameter and thickness choice in Group 3. 16

The ring was inserted using an implantation forceps to ovalize the deformable 360-degree ICR in 1 dimension to enter into the corneal pocket via a 5.0 mm temporal incision tunnel. Once placed in the pocket, the ICR inflates to its original preoperative rounded shape. The access to the pocket was self-sealing and did not require suturing. Figure 3 shows the surgical steps of ICR implantation.

Figure 3.
Figure 3.:
Surgical steps of ICR implantation. A: Full 360-degree ring design. B: Femtosecond laser corneal pocket creation. C: Implantation forceps are used to ovalize the deformable ICR in 1 dimension to enter into the corneal pocket via the 5.0 mm temporal incision tunnel. D: The ICR final position inside the corneal pocket with proper centration.

Postoperative Assessment

The patients' follow-up was at 1 day, 1 week, and 1, 3, and 6 months. Postoperative evaluation and data collection were performed at 6 months and comprised UDVA and CDVA measurement, manifest and cycloplegic refractions, slitlamp examination, and corneal topography with the rotating Scheimpflug device. Lines of improvement in acuity were calculated in logarithmic scales according to logarithm of the minimum angle of resolution (logMAR) notation.

Statistical Analysis

Sample size was calculated using Power Analysis Sample Size software (version 15, NCSS, LLC) setting the type −1 error (α) at 0.05 (95% confidence interval [CI]) and the power (1 − β) at 0.8. A successful outcome was defined as an improvement in refractive measures to the least dioptric measurable power of 0.25 D. A sample size was calculated that could significantly detect a difference of at least 0.25 D in sphere and cylinder between before surgery and after surgery. The result of the sample-size calculation was 39 eyes.

Data were analyzed using SPSS software (version 13, SPSS, Inc.). The Kolmogorov-Smirnov test was used to assess normality of data. Nonparametric quantitative data were expressed as the median, interquartile range, and 95% CI of the mean. Comparison between variables of 2 independent dependent samples was performed using the Wilcoxon test and Mann-Whitney test. The Kruskal-Wallis test was used for comparisons between more than 2 groups.

Parametric continuous variables were expressed as the mean ± SD. The independent sample t test was used to compare quantitative variables between 2 groups. One-way analysis of variance was used to compare means between more than 2 groups; the Bonferroni post hoc test was used for comparison between groups if statistically significant results were obtained. Qualitative variables are given as numbers and percentages. The chi-square test was used to compare between qualitative data. The Spearman correlation coefficient was used to assess the correlation between different variables. The level of significance was set at a P value less than 0.05.

RESULTS

The study included 73 eyes of 63 patients. The mean age of the 36 (57%) men and 27 women (43%) was 28.74 years (range 22 to 38 years). Surgical procedures were performed in 38 right eyes (52%) and 35 left eyes (48%). Group 1 comprised 36 eyes of 28 patients; 4 had bilateral implantation of the segments. Group 2 comprised 23 eyes of 23 patients. Group 3 comprised 14 eyes of 12 patients; in 2, the rings were implanted sequentially in both eyes. Table 4 shows demographic and preoperative data in each study group; there were no statistically significant between-group differences in the preoperative parameters (P > .05). In all study groups, there was a statistically significant improvement in all data between preoperatively and postoperatively (P < .01) (Table 5).

Table 4
Table 4:
Comparison of demographic data and preoperative parameters between study groups.
Table 5
Table 5:
Comparison (Wilcoxon signed-ranks test) between preoperative and postoperative parameters by group.

Group 1

Six months after surgery, the median UDVA was significantly improved (P < .01), with a median change of 0.85 ± 0.70 logMAR. The postoperative UDVA was better than 0.10 logMAR (20/200) in all patients at the end of follow-up. The median CDVA was significantly better (P < .01), with a median change of 0.15 ± 0.20 logMAR. The postoperative CDVA was 0.30 logMAR (20/40) or better in 34 patients (94.4%) at the end of follow-up. After 6 months of follow-up, 20 (55.5%) of 36 eyes had a change in CDVA of 0.10 to 0.20 logMAR. No change occurred in 2 eyes (5.6%), and worsening of 0.15 logMAR occurred in 2 eyes (5.6%) (Figure 4).

Figure 4.
Figure 4.:
Changes in logMAR CDVA (percentage of eyes) by group (CDVA = corrected distance visual acuity; logMAR = logarithm of the minimum angle of resolution).

The median SE decreased significantly, with a median change of 2.62 ± 1.50 D at the end of follow-up (P < .01). Fourteen of 36 eyes (40%) had a change in SE of 1.00 to 2.00 D. Fourteen eyes (40%) had a change in SE from 2.25 to 3.00 D (Figure 5).

Figure 5.
Figure 5.:
Changes in SE (percentage of eyes) by group.

There was also a significant decrease in the mean corneal astigmatism (median change 1.43 ± 2.30 D), median refractive cylinder (mean change 1.30 ± 1.96 D), median K mean (mean change 5.96 ± 2.09 D), mean K maximum (mean change 6.92 ± 2.78 D), and median Q value (median change −0.43 ± 0.98) (all P < .01).

Group 2

Six months after surgery, the median UDVA was significantly improved (P < .01), with a mean change of 1.30 ± 0.34 D. The postoperative UDVA was better than 0.10 logMAR (20/200) in 22 patients (95.7%). The mean CDVA improved significantly (P < .01), with a mean change of 0.30 ± 0.20 D. The postoperative CDVA was 0.30 logMAR (20/40) or better in 22 patients (95.7%) at the end of follow-up. After 6 months of follow-up, 10 (43.5%) of 23 eyes had a change in CDVA of 0.10 to 0.20 logMAR. One eye (4.3%) had no change in CDVA (Figure 4).

The mean SE decreased significantly, with a mean change of 4.34 ± 2.28 D at the end of follow-up (P < .01). Seven (30.4%) of 23 eyes had a change in SE of 3.25 to 4.00 D. One eye had no change in SE (Figure 5).

At 6 months, there was also a significant decrease in the mean corneal astigmatism (median change 1.00 ± 0.70 D), median refractive cylinder (median change 2.50 ± 3.00 D), median K mean (mean change 5.33 ± 4.06 D), mean K maximum (mean change 6.35 ± 3.45 D), and median Q value (median change −0.70 ± 0.70) (all P < .01).

Group 3

Six months after surgery, the median UDVA was significantly improved (P < .01), with a median change of 1.30 ± 0.38. The postoperative UDVA was better than 0.1 logMAR (20/200) in all patients. The mean CDVA was significantly better as well (P < .01), with a mean change of 0.30 ± 0.10 logMAR. The postoperative CDVA was better than 0.1 logMAR (20/200) in 12 patients (85.7%) and 0.30 logMAR (20/40) or better in all patients. After 6 months of follow-up, 8 of 14 eyes (57.2.5%) had a change in CDVA of 0.25 to 0.35 logMAR (Figure 4).

The median SE decreased significantly postoperatively, with a mean change of 5.03 ± 2.41 D at the end of follow-up (P < .01). Six (42.8%) of 14 eyes had a change in SE of more than 7.00 D (Figure 5).

There was also a significant decrease in the mean corneal astigmatism (median change 1.85 ± 4.46 D), median refractive cylinder (mean change 2.20 ± 1.70 D), median K mean (mean change 5.98 ± 1.69 D), mean K maximum (mean change 7.13 ± 2.38 D), and median Q value (median change −0.86 ± 0.72 D) (all P < .01).

Group 3 had no significant surgical complications except that the procedure had to be stopped and postponed for 1 week in 1 patient as a result of suction loss during pocket creation.

All Groups

Figure 6 shows the preoperative and postoperative corneal topographies in all groups.

Figure 6.
Figure 6.:
Preoperative and postoperative corneal topographies. Before (A) and after (B) 160-degree 2-ring segment implantation. Before (C and E) and after (D and F) 320-degree near-complete ring segment implantation. Before (G) after (H) ICR implantation.

Comparison Between Groups

There was a statistically significant difference in the postoperative UDVA, CDVA, and corneal astigmatism between the 3 study groups (P ≤ .01) (Table 6). There was a statistically significant increase in the mean UDVA in Group 2 compared with Group 1 (P < .01). There was also a statistically significant difference in the mean CDVA in Group 1 compared with Group 2 (P < .01) and in Group 2 compared with Group 3 (P = .013). Group 2 had the most significant improvement.

Table 6
Table 6:
Comparison of mean postoperative parameters between study groups.

There was a statistically significant difference in the median SE between Group 1 and each of Group 2 and Group 3 (P = .008 and P = .003, respectively). Group 2 and Group 3 had a more effective reduction in SE.

The postoperative median corneal cylinder was statistically significantly higher in Group 1 than in Group 3 (P = .008). There was no statistically significant difference in the postoperative refractive cylinder, K readings (mean and maximum), or corneal asphericity between the 3 groups (Table 6).

The only statistically significant difference in the mean or median changes in visual acuity, refraction, keratometry, and asphericity between groups was the change in UDVA and SE (P = .003 and P = .001, respectively) (Table 7). The median change in UDVA was significantly higher in Group 2 and Group 3 than in to Group 1 (P = .002 and P = .016, respectively). There was no statistically significant difference between Group 2 and Group 3 (P = .72). The median change in SE was significantly higher in Groups 2 and 3 than in Group 1 (P = .001 and P = .02, respectively). There was no statistically significant difference between Group 2 and Group 3 (P = .47). Figure 7 shows the mean and median changes in all studied parameters.

Table 7
Table 7:
Changes in visual acuity, refraction, topography, and asphericity in each group.
Figure 7.
Figure 7.:
Changes in all measured parameters by group (CDVA = corrected spectacle distance visual acuity; cyl = cylinder; K = keratometry; SE = spherical equivalent; logMAR = logarithm of the minimum angle of resolution; Q = corneal asphericity; UDVA = uncorrected distance visual acuity).

Fourteen eyes (8 in Group 1, 5 in Group 2, and 1 in Group 3) had crosslinking after the 6-month follow-up. These eyes had progressive steepening (≥1 D increase in mean K reading) while comparing the 3-month and 6-month corneal topographies.

In all groups, there was a statistically significant positive correlation between the postoperative UDVA and CDVA (logMAR) and postoperative maximum K (r = 0.27, P = .02 and r = 0.3, P = .009, respectively) and between the postoperative UDVA and CDVA (logMAR) and corneal astigmatism (r = 0.35, P = .003 and r = 0.32, P = .005, respectively).

There was a statistically significant positive correlation between the postoperative logMAR CDVA and postoperative refractive cylinder (r = 0.36, P = .002). There was also a statistically significant positive correlation between the degree of change in logMAR CDVA and degree of change in the Q value (r = 0.48, P < .001).

DISCUSSION

In the present study, implantation of Keratacx 160-degree 2 symmetrical ring segments, the Keratacx 320-degree near-total ring segment, and the Myoring ICR using a femtosecond laser improved the UDVA, CDVA, corneal and refractive astigmatism, K readings, and Q values in patients with central keratoconus or symmetric bowtie disease. The ICR and 320-degree segment were more effective than 2 160-degree symmetric ring segments in improving the UDVA and SE.

Only 1 study5 evaluated the optical value of implanting the Keratacx 2 symmetrical 160-degree segments and near-total ring Myoring ICR 320 degree in patients with keratoconus and quantified subsequent changes in corneal topography and asphericity. The small arc segments were implanted in 25 eyes (for keratoconus and post-LASIK ectasia) using a mechanical method or femtosecond laser implantation with significant improvement in all preoperative parameters. Only 4 eyes had the near-total 320-degree ring segments implanted, with an improvement in the mean UDVA, CDVA, sphere, cylinder, and K values; however, this improvement did not reach statistical significance. This may be a limited number of cases.

The high efficiency of Intacs ICRS and Ferrara rings in correcting keratoconic eyes has been reported in several studies.2,6,17 Although Intacs ICRS provided refractive, topographic, and optical quality outcomes comparable to those of Ferrara rings, eyes with Ferrara rings had a greater decrease in scotopic contrast sensitivity under glare, which was significantly correlated with pupil diameter.17

In the present study, 2 eyes (5.6%) no change in CDVA and 2 eyes (5.6%) had worsening of 0.15 logMAR in after 160-degree 2 rings implantation for moderate to severe keratoconus. In the near-total ring 320-degree segment group, 1 eye (4.3%) had no change. All eyes in the ICR group had improved CDVA.

Ertan and Kamburoğlu18 and Alfonso et al.19 evaluated the efficacy of Intacs ICRS implantation and analyzed outcomes according to different keratoconus stages. They found that the CDVA, UDVA, and refraction improved significantly for in eyes with mild to moderate keratoconus after ICRS implantation. In eyes with severe keratoconus, the improvement in refractive outcomes and UDVA was lower. In the study by Alfonso et al., 3.2% of the eyes lost 2 or more lines of CDVA. Ertan and Kamburoğlu pointed out that the induced irregular astigmatism might be the reason for the loss of vision after ICRS implantation.

In the present study, the majority of eyes (80.0%) in the 2-segments group had a change in SE from 1.00 to 3.00 D compared with 52.1% of eyes in the 320-degree segment group, which had a change in SE from 2.25 to 4.00 D, and 57.1% eyes in the ICR group, which had a change in SE from 5.25 to 7.75 D. A statistically significant decrease in myopia and cylinder occurred in the ICR groups. At 6 months, the mean decrease in sphere was 5.03 ± 2.41 D and the mean change in refractive cylinder was 2.20 ± 1.70 D. These degrees of refractive changes were comparable to those in an earlier study of Myoring ICR insertion.19–21

Hosny et al.22 compared complete ring (Myoring) versus incomplete ring (Keraring) segments implantation for keratoconus correction using femtosecond laser implantation. They found that both were effective in improving corneal and visual parameters in eyes with keratoconus and that the complete ring might have a greater flattening effect on the anterior corneal surface, with a mean change in sphere of 4.45 D and a mean change in refractive cylinder of 2.32 D. These levels of refractive change were consistent with our results and those previously reported after Myoring ICR implantation with mechanical dissection.11,12,17,23

It looks like the ICR inserts have a higher possibility than ICRS of improving myopia and astigmatism in eyes with keratoconus. The explanation behind the changes in the refraction and visual acuity could be the changes in the corneal surface as a result of the arc shortening effect of ICR implantation, in particular during the early weeks after surgery. The flexible nature of the Myoring ICR, its ability to touch the center of the cornea, and its central effect of flattening in the inferior and superior parts of the cornea might reasons for these improvements.22,24

A goal of the treatment of keratoconus is to improve the quality of vision beyond the simple corneal flattening and stabilization of the disease.7 Significant asphericity changes can occur after any corneal surgery,25 and these changes might explain the increase in spherical aberration and deterioration in the quality of monocular and binocular vision.26

Intrastromal corneal ring segments can effectively reduce the excess of prolateness in eyes with keratoconus by modifying the cornea so it has a more physiologic aspheric shape. The thicker the implanted segment or segments, the greater the reduction in corneal asphericity. This is especially important for ring selection based on the preoperative Q value, as Torquetti and Ferrara27 concluded. In their study, intrastromal Ferrara ring segments were placed in 135 eyes of 123 patients with keratoconus; the mean follow-up was 16.46 months. They found that the ICRS implantation significantly reduced the mean corneal asphericity, from −0.85 to −0.32 (P = .000). There was significant Q value reduction after implantation of all ring thicknesses except by the 150 μm single-ring segment. The significant reduction in Q values occurred with all grades of keratoconus.27

In another study, Torquetti et al.28 evaluated the reversibility of the visual, refractive, and topographic changes occurring in keratoconic eyes after ICRS exchange, reposition, addition, or removal. They found that asphericity and SE did not improve in these patients having subsequent surgery, perhaps due to the scarring of corneal tissue and/or stroma secondary to the first procedure.

We found a statistically significant positive correlation between the postoperative UDVA and CDVA and the postoperative maximum K, corneal astigmatism, and refractive cylinder. Also, a statistically significant positive correlation was found between the degree of change in the CDVA and degree of change in the Q value. This does not agree with results in a previous study by Amanzadeh et al.,29 who correlated the changes in visual acuity and topographic indices after implantation of single-segment Intacs. They found that no statistically significant correlation between the changes in topographic indices and UDVA and CDVA changes. They concluded that Intacs implantation in keratoconic eyes increased visual acuity and made the corneal shape less irregular. However, the improvements in visual acuity and corneal shape were not strongly correlated. They stated “[i]t seems that more regular corneal surface does not mean better subjectively tested visual acuity. In other words, visual acuity values do not correlate with keratoconus severity.” They used 7 Pentacam topographical indices for correlation with changes of visual acuity not used in the present study.

We used a femtosecond laser for tunnel and pocket creation, and no ICR required explantation in the current study. In a study by Alió et al.,30 there was only 1 case of explantation. Jabbarvand et al.,24 who used the conventional method for pocket creation, reported 4 cases of explantation. Use of the femtosecond laser offers a uniform depth and provides better results with fewer complications than the manual method.31

We believe that this is the first study to compare the effectiveness of 2 symmetric Keratacx 160-degree segments, the Keratacx 320-degree near-total ring, and the continuous Myoring ICR in improving visual acuity, corneal topography, and asphericity in a considerable number of cases of central keratoconus using femtosecond laser–assisted tunnel and pocket creation.

In conclusion, Keratacx ICRS and Myoring implantation using femtosecond laser tunnel and pocket creation yielded marked improvement in all visual and topographic parameters and reduces the excess prolateness in the central type of keratoconus. The 320-degree near-total ring the continuous ICR were more effective in increasing the UDVA and reducing the SE.

WHAT WAS KNOWN

  • Intracorneal ring segment implantation has become one of the most approved lines of treatment for keratoconus and ectatic corneas; it yields satisfactory results and is a minimally invasive and reversible technique.
  • Two symmetrical ring segments, a 320-degree near-total ring, and a continuous ICR intrastromal corneal implantation, are used with success in managing central and symmetric bowtie keratoconus. They improve topographic and refractive parameters.

WHAT THIS PAPER ADDS

  • The 320-degree near-total ring and the continuous ICR were more effective than the 160-degree 2 symmetric segments in improving UDVA and reducing the SE, especially in moderate to severe cases of central keratoconus.
  • There was a strong correlation between the degree of postoperative topographic and corneal asphericity improvement and the degree of postoperative CDVA improvement in cases with central keratoconus that were managed using all 3 intracorneal implants.

REFERENCES

1.Rabinowitz YS. (1998). Keratoconus. Surv Ophthalmol, 42, 297-319, Available at: http://www.keratoconus.com/resources/Major+Review-Keratoconus.pdf.
2.Colin J, Cochener B, Savary G, Malet F. Correcting keratoconus with intracorneal rings. J Cataract Refract Surg. 2000;26:1117-1122.
3.Fleming JF, Reynolds AE, Kilmer L, Burris TE, Abbott RL, Schanzlin DJ. The intrastromal corneal ring: two cases in rabbits. J Refract Surg. 1987;3:227-232.
4.Vega-Estrada A, Alio JL. The use of intracorneal ring segments in keratoconus. Eye Vis. 3. 2016. 8. Available at: https://eandv.biomedcentral.com/track/pdf/10.1186/s40662-016-0040-z?site=eandv.biomedcentral.com.
5.Israel M, Yousif MO, Osman NA, Nashed M, Abdelfattah NS. (2016). Keratoconus correction using a new model of intrastromal corneal ring segments. J Cataract Refract Surg, 42, 444-454, Available at: http://www.jcrsjournal.org/article/S0886-3350(16)00138-3/pdf.
6.Zuberbuhler B, Tuft S, Garty D, Spokes D. Corneal Surgery; Essential Techniques. 2013. Springer Verlag. Berlin. 94-111.
7.Torquetti L, Berbel RF, Ferrara P. Long-term follow-up of intrastromal corneal ring segments in keratoconus. J Cataract Refract Surg. 2009;35:1768-1773.
8.Tan B, Baker K, Chen Y-L, Lewis JWL, Shi L, Swartz T, Wang M. (2008). How keratoconus influences optical performance of the eye. J Vis, 8, 13.1-13.10, Available at: http://jov.arvojournals.org/article.aspx?articleid=2158188.
9.Binder PS. Hydrogel implants for the correction of myopia. Curr Eye Res. 1982-1983;2:435-441.
10.Coskunseven E, Kymionis GD, Tsiklis NS, Atun S, Arslan E, Jankov MR, Pallikaris IG. One-year results of intrastromal corneal ring segment implantation (KeraRing) using femtosecond laser in patients with keratoconus. Am J Ophthalmol. 2008;145:775-779.
11.Mahmood H, Venkateswaran RS, Daxer A. Implantation of a complete corneal ring in an intrastromal pocket for keratoconus. J Refract Surg. 2011;27:63-68.
12.Daxer A. (2015). Biomechanics of corneal ring implants. Cornea, 34, 1493-1498, Available at: http://journals.lww.com/corneajrnl/Fulltext/2015/11000/Biomechanics_of_Corneal_Ring_Implants.28.aspx.
13.Daxer A, Ettl A, Hörantner R. (2017). Long-term results of MyoRing treatment of keratoconus. J Optom, 10, 123-129, Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5383460/pdf/main.pdf.
14.Zadnik K, Barr JT, Edrington TB, Everett DF, Jameson M, McMahon TT, Shin JA, Sterling JL, Wagner H, Gordon MO., the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study Group. (1998). Baseline findings in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Invest Ophthalmol Vis Sci, 39, 2537-2546, Available at: http://iovs.arvojournals.org/article.aspx?articleid=2161642,.
15.Chatzis N, Hafezi F. Progression of keratoconus and efficacy of pediatric corneal collagen cross-linking in children and adolescents. J Refract Surg. 2012;28:753-758. errata 2013; 29:72.
16.Daxer B, Mahmood H, Daxer A. (2012). MyoRing treatment for keratoconus: DIOPTEX PocketMaker vs Ziemer LDV for corneal pocket creation. Int J Kerat Ect Cor Dis, 1, 151-152, Available at: http://www.jaypeejournals.com/eJournals/ShowText.aspx?ID=4128&Type=FREE&TYP=TOP&IN=_eJournals/images/JPLOGO.gif&IID=323&isPDF=YES.
17.Siganos CS, Kymionis GD, Kartakis N, Theodorakis MA, Astyrakakis N, Pallikaris IG. Management of keratoconus with Intacs. Am J Ophthalmol. 2003;135:64-70.
18.Ertan A, Kamburoğlu 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.
19.Alfonso JF, Lisa C, Fernández-Vega L, Madrid-Costa D, Montés-Micó R. Intrastromal corneal ring segment implantation in 219 keratoconic eyes at different stages. Graefes Arch Clin Exp Ophthalmol. 2011;249:1705-1712.
20.Nasrollahi K, Rezaei L, Ghoreishi M, Kashfi A, Mahboubi M. Clinical outcomes of MyoRing implantation in keratoconic eyes by using the femtosecond laser technology. J Med Life. 8. special issue 3. 2015. 66-71. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5348932/pdf/SIJMedLife-08-03-66.pdf.
21.Mohebbi M, Hashemi H, Asgari S, Bigdeli S, Zamani KA. Visual outcomes after femtosecond-assisted intracorneal MyoRing implantation: 18 months of follow-up. Graefes Arch Clin Exp Ophthalmol. 2016;254:917-922.
22.Hosny M, El–Mayah E, Sidky MK, Anis M. (2015). Femtosecond laser-assisted implantation of complete versus incomplete rings for keratoconus treatment. Clin Ophthalmol, 9, 121-127, Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4315561/pdf/opth-9-121.pdf.
23.Janani L, Jadidi K, Mosavi SA, Nejat F, Naderi M, Nourijelyani K. (2016). MyoRing implantation in keratoconic patients: 3 years follow-up data. J Ophthalmic Vis Res, 11, 26-31, Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4860982/pdf/JOVR-11-26.pdf.
24.Jabbarvand M, SalamatRad A, Hashemian H, Mazloumi M, Khodaparast M. Continuous intracorneal ring implantation for keratoconus using a femtosecond laser. J Cataract Refract Surg. 2013;39:1081-1087.
25.Anera RG, Jiménez JR, Jiménez del Barco L, Bermúdez J, Hita E. Changes in corneal asphericity after laser in situ keratomileusis. J Cataract Refract Surg. 2003;29:762-768.
26.Jiménez JR, Anera RG, Jiménez del Barco L. Equation for corneal asphericity after corneal refractive surgery. J Refract Surg. 2003;19:65-69.
27.Torquetti L, Ferrara P. (2010). Corneal asphericity changes after implantation of intrastromal corneal ring segments in keratoconus. J Emmetropia, 1, 178-181, Available at: http://www.journalofemmetropia.org/2171-4703/jemmetropia.2010.1.178.181.php.
28.Torquetti L, Ferrara G, Almeida F, Cunha L, Ferrara P, Merayo-Lloves J. (2013). Clinical outcomes after intrastromal corneal ring segments reoperation in keratoconus patients. Int J Ophthalmol, 6, 796-800, Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3874518/pdf/ijo-06-06-796.pdf.
29.Amanzadeh K, Elham R, Jafarzadepur E. Effects of single-segment Intacs implantation on visual acuity and corneal topographic indices of keratoconus. J Curr Ophthalmol. 2017;29:189-193.
30.Alio JL, Piñero DP, Daxer A. Clinical outcomes after complete ring implantation in corneal ectasia using the femtosecond technology; a pilot study. Ophthalmology. 2011;118:1282-1290.
31.Kubaloglu A, Sari ES, Cinar Y, Cingu K, Koytak A, Coşkun E, Özertürk Y. Comparison of mechanical and femtosecond laser tunnel creation for intrastromal corneal ring segment implantation in keratoconus; prospective randomized clinical trial. J Cataract Refract Surg. 2010;36:1556-1561.

Disclosures:Neither author has a financial or proprietary interest in any material or method mentioned.

© 2018 by Lippincott Williams & Wilkins, Inc.