Purpose
To evaluate the effect of mechanical implantation of a continuous intrastromal ring in keratoconus.
Design
Prospective, interventional, nonrandomized, case series.
Methods
The MyoRing (Dioptex GmbH) was implanted after creation of an intrastromal pocket for 95 eyes of 95 patients with moderate and advanced keratoconus. All patients had at least 12 months of follow-up. Preoperative and postoperative visual acuity, keratometry, aberrometry, and refraction were the main outcome measures of the study.
Results
A significant improvement in uncorrected and corrected distance visual acuity was observed 1 month after surgery, which was consistent with the significant reduction in sphere (5.74 diopters [D]) and cylinder (3.02 D). No significant changes were detected in these parameters afterward. Furthermore, a significant corneal flattening of a mean value of 9.78 D was found. Both spherical myopia and astigmatism underwent reduction, but the reduction in myopia was more remarkable than astigmatism. Higher-order aberrations and coma-like aberrations decreased significantly, but spherical aberrations increased after surgery. No significant change in central corneal thickness was observed at any point after operation. There were no significant differences between 2 keratometry groups (higher or lower than 53 D) in visual gain after the procedure. There were no major complications during or after surgery. MyoRing explantation was performed in 4 eyes (4%). The refraction, visual acuity, and corneal topography returned to the preoperative status 1 month later for all 4 eyes.
Conclusions
MyoRing implantation has an acceptable efficacy profile in moderate and advanced keratoconus.
Keratoconus is a rare (prevalence of 1 in 2000) chronic corneal disease affecting a young population. The cornea assumes a conical shape as a result of progressive, noninflammatory thinning. Nonsurgical therapeutic options for keratoconus are spectacles and contact lenses. In more advanced cases and in cases of deformed or opaque cornea, corneal grafts, either lamellar or penetrating, are the main treatment options. Although keratoplasty has acceptable results, ongoing research seeks less invasive methods, including corneal collagen cross-linking and intrastromal corneal rings, to treat keratoconus. Intrastromal corneal ring segment implantation has been proved to be safe approach to reinforce corneal structure in mild to moderate keratoconus and other ecstatic disorders. However, they are not as effective in more advanced cases.
The recently proposed MyoRing (Dioptex GmbH, Linz, Austria) is a complete intrastromal ring designed to be placed into a corneal pocket. A potential advantage of the MyoRing over ring segments is its effectiveness on advanced keratoconus and also its ability to reduce keratometric power of the cornea much more. The present study aimed to evaluate the effect of MyoRing implantation in eyes with moderate to severe keratoconus.
Methods
The present study was an interventional case series. All procedures were performed by 2 surgeons (M.J. and A.S.) in 2010, with identical surgical techniques. All patients gave informed consent to participate in research and to undergo the proposed treatment, after explanation of other options of treatment. The tenets of the Helsinki Declaration were followed, and the Institutional Review Board of Tehran University approved both the procedures and use of the MyoRing for keratoconic patients prospectively.
We included keratoconic patients with clear central corneas, contact lens intolerance, mesopic pupil diameter of 5.5 mm or less (according to Orbscan (Bausch & Lomb, Rochester, New York, USA) report in mesopic conditions), minimum central corneal thickness of 360 μm, and a corneal thickness of at least 400 μm at the location of the proposed incision site and along the location of the proposed MyoRing placement. We graded the severity of keratoconus according to Amsler-Krumeich classification :
- 1.
Stage I: eccentric steeping; myopia or induced astigmatism of less than 5.00 diopters (D), or both; and mean central K readings less than 48.00 D.
- 2.
Stage II: myopia or induced astigmatism from 5.00 to 8.00 D, or both; mean central keratometry readings of less than 53.00 D; absence of scarring; and minimum corneal thickness more than 400 μm.
- 3.
Stage III: myopia or induced astigmatism from 8.00 to 10.00 D, or both; mean central keratometry readings of more than 53.00 D; absence of scarring; and minimum corneal thickness of 300 to 400 μm.
- 4.
Stage IV: nonmeasurable refraction, mean central keratometry readings of more than 55.00 D, central corneal scarring, and minimum corneal thickness of 200 μm.
We excluded patients with stage I keratoconus according to the Amsler-Krumeich classification, incomplete follow-up of less than 1 year (6 eyes), or ring extrusion (4 eyes) in the first postoperative year. Other exclusion criteria were additional severe ocular pathologic features (eg, glaucoma, cataract, and diabetic retinopathy), spherical equivalent of plano or hyperopic, previous ocular surgery, history of herpes keratitis, diagnosed autoimmune disease, systemic connective tissue disease, pregnancy, and age younger than 19 years. We had no upper limit for age.
Ninety-five eyes of 95 patients with a mean age of 27.1 ± 4.79 years were included in the study and completed the 12-month follow-up. Most of the patients were male (70.5%). According to Amsler-Krumeich classification, there were 56 eyes with stage II keratoconus, 18 eyes with stage III keratoconus, and 21 eyes with stage IV keratoconus.
A complete ocular examination, including ultrasound pachymetry (Nidek UP1000; Nidek Technologies, Gamagori, Japan), total aberrometry using iTrace (Tracey Technologies, Houston, Texas, USA), and Orbscan IIz (Bausch & Lomb, Rochester, New York, USA) imaging were performed before surgery and also 1 week, 1 month, 6 months, and 1 year after surgery. We calculated aberration coefficients and root mean square (RMS) values for a 5.5-mm pupil. Also, the following aberrometric values were calculated: higher-order RMS, coma-like RMS (third-order component Z3, fifth-order component Z5, and seventh-order component Z7), and spherical-like RMS (fourth-order component Z4 and sixth-order component Z6) in each visit. We used ultrasound pachymetry to measure peripheral and central corneal thickness at the time of surgery.
The stromal pocket was created at the depth of 300 μm and diameter of 9 mm using the PocketMaker microkeratome (Dioptex GmbH) as described in detail by Daxer ( Supplemental Video available at AJO.com ). The microkeratome consists of a suction ring and a motor-driven blade. First suction ring fixates the microkeratome to the cornea, and then the blade creates the stromal pocket. It also creates a temporally located 4.0-mm wide incision tunnel. In the next step, the suction ring is removed and the MyoRing is inserted into the pocket. We used our adapted nomogram to calculate the size of the MyoRings ( Table 1 ).
| Ring Dimension | Mesopic Pupil (mm) | K Value | |
|---|---|---|---|
| Thickness (μm) | Diameter (mm) | ||
| 240 | 6 | <5.5 | K ≤ 44 |
| 240 | 5 | <4.5 | 44< K ≤ 48 |
| 280 | 6 | ≥4.5 | |
| 280 | 5 | <4.5 | 48< K ≤ 52 |
| 320 | 6 | ≥4.5 | |
| 320 | 5 | <4.5 | K > 52 |
No complications occurred to any case during the procedure. Safety of the procedure was defined as the percentage of eyes losing more than 2 lines of Snellen corrected distance visual acuity (CDVA). The safety index was calculated by dividing mean postoperative CDVA by mean preoperative CDVA. Uncorrected distance visual acuity (UDVA), CDVA, manifest refraction, keratometry, and pachymetry also were the main outcome measures.
Secondary measures included procedure complications, efficacy, and stability index. Efficacy of a refractive procedure was defined as the percentage of operated eyes, achieving a UDVA 20/40 or more. The efficacy index was calculated by dividing the mean postoperative UDVA by the mean preoperative CDVA, and the stability index was the percentage of eyes with a less than 1-D change in spherical equivalent (SE) between the first and twelfth postoperative months.
We analyzed the preoperative versus 1-month, 6-month, and 1-year postoperative data using a paired t test. If not indicated otherwise, statistical measures are presented as mean ± standard deviation and significant P values are < .05. Statistical analysis was performed using SPSS software version 11 (SPSS Inc, Chicago, Illinois, USA).
Results
The mean preoperative sphere was −4.6 ± 4.36 D (range, −16.00 to 2.00 D), mean cylinder was −5.3 ± 2.17 D (range, −2.00 to −11.00 D), and mean spherical equivalent was −7.28 ± 4.69 D. Also, the mean keratometry reading was 51.77 ± 3.65 D (range, 48 to 59 D).
Preoperative and postoperative measurements are shown in Table 2 . Before surgery, 58% eyes had a UDVA of 20/200 or worse, whereas after 12 months, 42% eyes had a UDVA of 20/40 or better. No significant change in central corneal thickness was observed at any point before or after operation.
| Before Surgery | After Surgery | P Value | |||||
|---|---|---|---|---|---|---|---|
| 1 Month | 6 Months | 1 Year | Before Surgery vs 1 Month | 1 Month vs 1 Year | Before Surgery vs. 1 Year | ||
| UDVA (logMAR) | 1.02 ± 0.24 | 0.39 ± 0.18 | 0.4 ± 0.19 | 0.39 ± 0.15 | <.001 | .948 | <.001 |
| CDVA (logMAR) | 0.43 ± 0.2 | 0.16 ± 0.15 | 0.17 ± 0.15 | 0.17 ± 0.1 | <.001 | .909 | <.001 |
| Sphere (D) | −4.63 ± 4.36 | 1.15 ± 1.95 | 1.12 ± 1.95 | 1.11 ± 1.94 | <.001 | .486 | <.001 |
| Cylinder (D) | −5.3 ± 2.17 | −2.30 ± 1.17 | −2.26 ± 1.09 | −2.28 ± 1.01 | <.001 | .709 | <.001 |
| SE (D) | −7.28 ± 4.69 | 0.03 ± 2.24 | −0.05 ± 2.14 | −0.03 ± 2.13 | <.001 | .186 | <.001 |
| CCT (μm) | 431 ± 47.7 | 430 ± 48.4 | 429 ± 47.9 | 430 ± 47.8 | .105 | .681 | .101 |
| Km (D) | 51.77 ± 3.6 | 42 ± 3.66 | 42 ± 3.64 | 41.99 ± 3.56 | <.001 | .818 | <.001 |
| HOA (μm) | 5.01 ± 2.46 | 4.67 ± 2.6 | 4.66 ± 2.58 | 4.56 ± 2.62 | .022 | .239 | .014 |
| Coma-like (μm) | 4.7 ± 1.62 | 4.51 ± 1.73 | 4.49 ± 1.8 | 4.35 ± 1.79 | .010 | .044 | .001 |
| Spherical-like (μm) | 1.42 ± 0.65 | 2.03 ± 1.07 | 2.13 ± 1.12 | 2.04 ± 1.19 | <.001 | .93 | <.001 |
Table 2 sums up aberrometric outcomes after MyoRing implantation during the 1-year follow-up. A statistically significant increase in spherical aberration was found at all follow-up points compared with preoperative values. Between postoperative follow-up time points, spherical aberration remained constant with no significant changes. Both higher-order and coma-like aberrations decreased significantly after operation, but remained constant with no significant changes between follow-up visits ( P values are presented in Table 2 ).
According to the proposed definition for safety, this procedure was 100% safe because no patient lost more than 2 lines of Snellen CDVA. The safety index (mean postoperative CDVA/mean preoperative CDVA) was 1.8 at 1 year, indicating an 80% increase of visual acuity (visual acuities changed to Snellen to calculate ratios). Efficacy (percentage of eyes reaching UDVA of 20/40 or better) of our study was 40%. The efficacy index (mean postoperative UDVA/mean preoperative CDVA) was 0.9 at 1 year, suggesting that after 1 year, the MyoRing alone could achieve 90% of the baseline best spectacle-corrected visual acuity. Stability (percentage of eyes with SE change of less than 1 D from 1 month to 1 year) of the study was 97.9% (93 patients).
We divided patients to 2 groups based on their preoperative keratometry (group 1, keratometry < 53; group 2, keratometry ≥ 53) and compared the changing parameters between the 2 groups. No significant difference was noted between the identical values of the 2 groups ( Table 3 ).
