|Year : 2020 | Volume
| Issue : 2 | Page : 101-107
Visual and corneal biomechanical changes after femtosecond laser-assisted intracorneal ring segment implantation in patients with keratoconus
Paula S Abdel Malek1, Alaa A El-Dorghamy2, Ahmed M Ghoneim2, Hazem A Elbedewy2
1 Cornea and Refractive Surgery Unit, Tanta Ophthalmology Hospital, Tanta, Egypt
2 Department of Ophthalmology, Faculty of Medicine, Tanta University, Tanta, Egypt
|Date of Submission||11-Jan-2020|
|Date of Decision||07-Mar-2020|
|Date of Acceptance||16-Mar-2020|
|Date of Web Publication||26-Jun-2020|
MD, FRCS Hazem A Elbedewy
Department of Ophthalmology, Tanta University, Tanta 31511l
Source of Support: None, Conflict of Interest: None
Purpose The aim of this study was to evaluate the effect of femtosecond laser-assisted intrastromal corneal ring segment (ICRS) implantation in patients with keratoconus on the visual outcome and corneal biomechanics using CORVIS ST.
Patients and methods This is a prospective interventional study that was carried out on 30 keratoconic eyes which underwent femtosecond laser-assisted ICRS implantation. All the included eyes were assessed before ICRS implantation for uncorrected/best-corrected visual acuity, Pentacam and Corvis ST imaging. They were assessed again at 1 and 3 months after the implantation for the same parameters.
Results The mean age of the studied group was 24.00±3.80 years. The uncorrected corrected visual acuity improved significantly at the first (0.36±0.04, P<0.001) and third month (0.53±0.09, P<0.001) after ICRS implantation compared with the preoperative level (0.22±0.05), whereas the best-corrected visual acuity at the third postoperative month (0.73±0.12) was significantly better than the preoperative value (0.60±0.29, P=0.082) and the first postoperative month value (0.60±0.08, P<0.001). The corneal biomechanical index measured by CORVIS ST remained stable from the preoperative level (1.0) till the first month after ICRS implantation but decreased significantly at the third postoperative month to 0.86. There were no significant changes regarding Applanation 1 (A1) or Applanation 2 (A2) times but A1 and A2 lengths and biomechanically corrected intraocular pressure decreased significantly at the first and third month after ICRS implantation than the preoperative level.
Conclusion Implantation of ICRS in eyes with keratoconus improved visual acuity and changed significantly the corneal biomechanical parameters measured by CORVIS ST. Corneal biomechanical index, A1, and A2 were significantly decreased with no changes in A1 or A2 times. ICRS implantation can be considered safe, as the complications were almost nil.
Keywords: corneal biomechanics, emtolaser, intrastromal corneal ring segment, keratoconus
|How to cite this article:|
Abdel Malek PS, El-Dorghamy AA, Ghoneim AM, Elbedewy HA. Visual and corneal biomechanical changes after femtosecond laser-assisted intracorneal ring segment implantation in patients with keratoconus. Delta J Ophthalmol 2020;21:101-7
|How to cite this URL:|
Abdel Malek PS, El-Dorghamy AA, Ghoneim AM, Elbedewy HA. Visual and corneal biomechanical changes after femtosecond laser-assisted intracorneal ring segment implantation in patients with keratoconus. Delta J Ophthalmol [serial online] 2020 [cited 2022 May 26];21:101-7. Available from: http://www.djo.eg.net/text.asp?2020/21/2/101/287460
| Introduction|| |
Keratoconus is one of the sight-threatening diseases that affect vision in adolescence. It has a wide range of clinical manifestations, from disturbed visual acuity and astigmatism to total opacification of the cornea with deteriorated vision .
In very early cases of keratoconus, spectacles can correct the regular astigmatism and the very low amount of irregular astigmatism . Contact lenses for keratoconus have been of hard or rigid gas-permeable variety, although specialized soft or hydrophilic lenses have the tendency to conform to the conical shape of the cornea . Corneal collagen cross-linking with riboflavin plays an important role in decreasing the progression of keratoconus by strengthening the corneal collagen bundles and prevention of corneal thinning ,.
Between 10 and 25% of the cases of keratoconus progress to the point where conservative visual rehabilitation is no longer possible, in which deep anterior lamellar keratoplasty or penetrating keratoplasty is a must, especially in those who present at young age and with keratometry measurement more than 60 D and corneal thickness less than 400 μm at the thinnest location .
Recently, intrastromal corneal ring segments (ICRS; Keraring) have been designed to achieve refractive adjustment by flattening the cornea. They are a pair of semicircular polymethyl methacrylate ring segments with inner and outer diameter and certain thickness used to flatten the cornea in an attempt of reducing visual impairment in keratoconus . ICRS act by an arc-shortening effect to flatten the center of the cornea and provide biomechanical support for the thin ectatic cornea .
Corneal hysteresis is a corneal biomechanical response through which the corneal tissue is able to absorb and dissipate energy owing to the viscoelasticity of its collagen fibers . It can be measured by ocular response analyzer or CORVIS ST .
CORVIS ST is a new device that can measure corneal biomechanics, intraocular pressure, and central corneal thickness (CCT) in a more complicated and analyzing technique ,,. A precisely metered air pulse causes the cornea to flatten (the first corneal applanation). The cornea continues to move inward until reaching a point of the highest concavity . Because the cornea is viscoelastic, it rebounds from this concavity to another point of applanation (the second applanation) and then to its normal convex curvature. CORVIS ST records throughout the deformation process and therefore takes information concerning the cornea’s viscoelastic properties and stiffness, as well as recording standard tonometry and pachymetry data .
CORVIS ST parameters are recorded when the air pulse pressure increases and the cornea deforms inward through the first applanation stage. At this point, the device measures the length of the first applanation segment (A1 length in mm), the time of applanation (A1 time in ms), and the velocity of the corneal apex (A1 velocity in m/s) as well as the measurement of the corneal deflection amplitude (A1 DeflAmp). The next two parameters are the deformation amplitude (DA) ratio at 2 mm and deflection amplitude ratio at 2 mm. Then the cornea changes its shape from convex to concave, approaching the oscillatory phase. During this time, the device measures stiffness coefficients, correspondingly, stiffness parameters A1 and HC. At the moment of highest corneal concavity (HC), the following parameters are determined: time of occurrence of the highest corneal concavity moment (HC time), radius of corneal curvature at its highest concavity (HC radius), maximum DA, distance between two corneal peaks during its maximum bending (peak distance), and the maximum value of the inverse of the radius of corneal curvature at the moment of highest concavity (InvRadMax). One of the last stages is the moment of the second applanation, in which the length of the second applanation (A2 length), the time of the second applanation (A2 time), and the velocity of the cornea (A2 velocity) are measured, correspondingly ,,. An attempt to standardize the available biomechanical parameters is the development of the CORVIS ST biomechanical index (CBI) ,,.
This study aimed to evaluate the effect of femtosecond laser-assisted ICRS implantation on the visual outcome and corneal biomechanics using CORVIS ST.
| Patients and methods|| |
This is a prospective interventional study that was carried out from August 2018 to August 2019. It included 30 keratoconic eyes that underwent femtosecond laser-assisted ICRS implantation at Elnokhba Eye and Laser Center, Tanta, Egypt. The study was performed in accordance with the Declaration of Helsinki and was approved by the Ethical Committee of the Faculty of Medicine, Tanta University. An informed written consent was signed by every patient or his/her parents or guardians (if <21 years) to participate in the study and for publication of data before enrollment in the study.
The study included keratoconus eyes that were naïve to treatment and were eligible for ICRS implantation in patients with the following criteria: adults more than 18 years old with clear central cornea, keratometric (K) readings less than 58 D, and pachymetry of at least 350 μm at the thinnest location and at least 450 μm at the incision and segment sites.
It included history of intraocular surgeries or penetrating eye trauma, concurrent ocular or systemic disease (e.g. diabetes mellitus and collagen tissue diseases), patients diagnosed with posterior segment pathology, patients who had previous refractive surgery or corneal cross-linking, and patients who developed intraoperative or postoperative complications.
All the patients were asked about ocular, medical, and surgical history. They underwent complete ophthalmic examination, including uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA), and slit-lamp biomicroscopy including fundus examination using 90 D lens. This was followed by imaging by Pentacam (Allegro-Oculyzer WaveLight, GmbH, Erlangen, Germany) and oculus CORVIS ST (Oculus Scheimpflug Technology, Wetzlar, Germany).
The proper ring size, thickness, and number to be inserted were chosen according to the manufacturer’s nomogram and the patient’s manifest refraction (both sphere and cylinder components). Using VisuMax device (Zeiss, Jena, Thuringia, Germany), the tunnel of the desired depth was formed all the way through. The femtosecond laser’s patient interface kit consisted of a curved cone without applanation of the cornea and limbal suction ring without pressure on the conjunctiva with power of 35 mmHg to decrease pain and vacuum tubing.
Ultrafast laser (10−15 of a s), femtosecond laser, was used to create the tunnel and the incision with a wavelength of about 1030±5 nm. The laser spot size was 5 μm, spot separation was 6.5 μm, and the line separation was 6.5 μm. The laser pulse repetition rate was 200 KHz. Tunnels were set to be 80% of corneal thickness at their sites. The energy used was less than 1.7 mJ, with a procedure time of ∼5 s for femtolaser to create all tunnels and incisions. The incision was made on the steepest corneal topographic axis.
The keraring (Vistaeyes, Elsternwick, Victoria, Australia), according to the chosen nomogram (there are three types of nomograms, according to the shape of the cone in every case), was inserted after tunnel creation. The segment was centered in the middle of its tunnel at equal distance from the line bisecting the cornea at the incision site.
The postoperative follow-up evaluation included UCVA, BCVA, and corneal biomechanical changes using CORVIS ST at 1 and 3 months postoperatively.
Results were analyzed by SPSS, version 23 (Released 2015, IBM SPSS statistics for Windows; SPSS Inc., Armnok, New York, USA). Data were expressed in number, percentage, mean, and SD. Repeated measures analysis of variance (with or without Bonferroni correction) with Mauchly’s test of sphericity was used for comparison among three or more consecutive data measures in the same group (before, 1 month, and 3 months postoperatively) for quantitative variables. Assumed sphericity was used for normally distributed data, whereas Greenhouse-Geisser was used for not normally distributed ones. Two-sided P value less than 0.05 was considered statistically significant.
| Results|| |
The mean age of the studied patients was 24.00±3.80 years. Seventeen (56.7%) cases were males and 13 (43.3%) were females. Twelve (40.0%) eyes were right and 18 (60.0%) were left.
The UCVA (in decimal) increased significantly from the preoperative level of 0.22±0.05 to 0.36±0.04 at the first postoperative month and to 0.53±0.09 at the third postoperative month (P<0.001). The BCVA (in decimal) increased significantly at the third postoperative month (0.73±0.12) than both the preoperative level (0.60±0.29) and the first postoperative month (0.60±0.08). There was no significant difference regarding the spherical power (−3.62±2.18, −2.98±2.75, and −3.01±2.32 D, respectively), but the cylindrical power (D), mean refractive spherical equivalent, and mean K (D) showed significant improvement at the first postoperative month (−1.98±1.25, −3.97±2.04, and 46.29±2.33, respectively) and third postoperative month (−1.97±1.14, −3.99±1.99, and 45.97±2.97, respectively) than the preoperative level (−3.18±2.98, −5.21±2.23, and 50.30±3.02, respectively). This is detailed in [Table 1].
The CORVIS ST parameters are detailed in [Table 2] and [Figure 1]. CBI decreased significantly at the third month (0.86±0.19) than both preoperative and first postoperative month (1.00±0.0). DA ratio increased significantly at the third postoperative month (4.93±0.39) than both preoperatively and first postoperative month (4.63±0.46 and 4.66±0.50, respectively) ([Figure 2]). CCT decreased significantly at the first postoperative month (515.33±11.81 μm) than the preoperative level (520.0±9.24 μm) and then increased significantly again at the third postoperative month (524.00±7.61 μm).
|Table 2 Central corneal thickness and other biomechanical parameters at all time measures|
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|Figure 2 (a) Preoperative Pentacam of a male patient aged 29 y. (b) Corvis ST parameters of the same patient at the preoperative assessment. (c & d) are his Corvis ST parameters at the 1st and 3rd postoperative months.|
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Stiffness parameter A1 (SP.A1) increased significantly at the first postoperative month (82.20±19.53) compared with the preoperative level (72.73±5.65) and then decreased significantly again at the third postoperative month (73.43±10.00) to be insignificant from the preoperative level. A1 and A2 times and HC radius of curvature did not show any significant difference preoperatively or postoperatively.
A1 length and A2 length decreased significantly at the first postoperative month (2.00±0.13 and 1.70±0.51, respectively) and third postoperative month (1.77±0.34 and 1.40±0.36, respectively) compared with the preoperative level (2.20±0.28 and 1.85±0.32, respectively). In addition, A1 length at the third postoperative month decreased significantly than the first postoperative month. A1 velocity increased significantly at the third postoperative month (0.17±0.02) than the preoperative value (0.16±0.01) and the first postoperative month (0.15±0.03). A2 velocity decreased significantly at the third postoperative month (−0.34±0.02) than the preoperative level (−0.29±0.03) and the first postoperative month (−0.26±0.12). Highest concavity peak distance and highest concavity deformation amplitude decreased significantly at the first postoperative month (4.92±0.79 and 1.06±0.23, respectively) than the preoperative level (5.25±0.14 and 1.19±0.05, respectively) and then increased significantly again at the third postoperative month (5.37±0.22 and 1.21±0.07, respectively).
| Discussion|| |
The cornea exhibits both elastic and viscoelastic properties. Using an air impulse, CORVIS ST can measure corneal thickness, intraocular pressure, and other specific biomechanical parameters through bidirectional applanation process.
In the current study, the mean UCVA and BCVA increased significantly at the first and third postoperative month than the preoperative state. This improvement in vision could be owing to corneal flattening from the cone area toward the center after ring segment implantation. It was observed that long-term efficacy of ICRS implantation depends on the progression pattern of keratoconus at the time of surgery. Thus, in those cases with the stable form of the disease, ICRS implantation does not cause significant changes after a long period of follow-up . Hence, we observed most changes of UCVA and BCVA during the first postoperative month and stabilized afterward in 3 months.
Similarly, Fahd et al.  conducted a retrospective case series on 30 eyes with moderate to severe keratoconus and found significant improvement of the preoperative UCVA and BCVA (0.633±0.303 LogMAR and 0.322±0.156 LogMAR, respectively) to that measured after 6 months (0.202±0.135 LogMAR and 0.134±0.131 LogMAR, respectively). The first month and third month follow-ups were not recorded in their article; therefore, it was not noted if these improvements were found earlier on. They stated that the reason behind the improvement in BCVA could be the upward shifting of the cone toward the center of the cornea. Shabayek and Alió  conducted a prospective consecutive interventional study on 21 eyes and found that keraring implantation significantly increased UCVA from 0.06 to 0.3 (P=0.0001) and BCVA from 0.54 to 0.71 (P=0.0003), which is still in agreement with our results. Hashemi et al.  reported an improvement of UCVA from 0.84±0.48 LogMAR preoperatively to 0.51±0.48 LogMAR 12 months after surgery (P=0.001).The mean BCVA showed improvement in the final follow-up (0.29±0.17 LogMAR), as compared with the mean BCVA before the operation (0.21±0.2 LogMAR) (P=0.008). These results are also similar to ours.Regarding corneal biomechanical parameters, most of them significantly changed from the preoperative state to the first and third postoperative months. CBI decreased significantly at the third postoperative month than both the preoperative and the first postoperative value. CCT decreased significantly at the first postoperative month than the preoperative level and then increased significantly again at the third postoperative month. First and second applanation times (A1 and A2 times) did not show any significant differences preoperatively or postoperatively. First and second applanation lengths (A1 and A2 length) decreased significantly at the first and third postoperative month than the preoperative level. They also decreased significantly at the third postoperative month than the first postoperative month.
Hassan et al.  performed a study to analyze the early results of a new device measuring ocular biomechanics after corneal refractive surgery. They examined 78 eyes of 39 refractive surgery patients (mean age: 32.6±9.9 years). The day after the procedure, the radius values showed significant differences compared with the preoperative data. One month after surgery, radius values, velocity of the second applanation, and pachymetric values showed statistically significant differences compared with the preoperative data. They observed significant differences in intraocular pressure, maximum amplitude at the apex, A1 time, A2 velocity, and HC time. One month after surgery, there were no differences in specific parameters compared with the preoperative data except in pachymetry.
| Conclusion|| |
Implantation of ICRS in eyes with keratoconus improved visual acuity and changed significantly the corneal biomechanical parameters measured by CORVIS ST. CBI, A1, and A2 were significantly decreased with no changes in A1 or A2 times. ICRS implantation was considered safe as the complications were almost nil.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]