Purpose
To evaluate the effect of keratoconus cone location on the change in refractive outcomes, corneal aberrations, and biomechanics after combined topography-guided photorefractive keratectomy (PRK) and collagen cross-linking (CXL).
Design
Prospective, comparative case series.
Methods
Topography-guided PRK was performed followed by accelerated CXL using riboflavin A and enhanced-intensity (30 mW/cm 2 ) ultraviolet light. Outcome parameters including uncorrected distance visual acuity (UDVA) and best-corrected distance visual acuity (BDVA), corneal tomography and biomechanics (corneal hysteresis [CH] and corneal resistance factor [CRF]), and corneal wavefront aberrations were assessed before and a year after the procedure. Eyes were subdivided into 2 groups preoperatively for statistical analysis: Group 1, cone located within the central 2-mm zone; and Group 2, cone located outside the central 2-mm zone.
Results
UDVA, BDVA, sphere, cylinder, and simulated keratometry improved after treatment in both groups ( P < .05). However, BDVA improved more in Group 1 than in Group 2 ( P = .04) and the other variables were not affected by cone location. A few corneal wavefront Zernike aberrations changed after treatment ( P < .05) but none were affected by cone location ( P > .05). CH and CRF increased after treatment in both groups ( P > .05). Interestingly, the increases in CH and CRF were greater in Group 2 than in Group 1 ( P > .05).
Conclusions
Cone location appeared to impact only visual acuity and biomechanics after the combined procedure. The greater increase in CH and CRF in Group 2 may indicate differences in the ablation profile and variability in CXL outcomes and requires further study.
Wavefront-guided surgery was the first ablative procedure that used the total ocular aberrations to determine the ablation pattern on the anterior corneal surface of highly aberrated corneas (eg, keratoconus). However, the results of wavefront-guided treatments were unsatisfactory in highly irregular corneas because repeatability of whole-eye or ocular aberrations was unsatisfactory. Topography-guided photorefractive keratectomy (PRK) was introduced primarily for the treatment of highly irregular corneas. Unlike conventional PRK, which used ocular wavefront, topography-guided PRK used only the corneal wavefront to reduce lower- (sphere, cylinder) and higher-order corneal aberrations (coma, spherical aberration). Its use gained popularity especially in keratoconus patients who were unable to tolerate spectacles or contact lenses. It was targeted to reduce the steepness of the anterior corneal surface and improve the central corneal symmetry by reducing the asymmetric higher-order aberrations. Topography-guided PRK was restricted to a small degree of ablation with the maximum depth of tissue loss typically being less than 50 μm.
Keratoconus is a degenerative collagen disorder and suffers from loss of stromal stiffness owing to lack of ordered collagen distribution seen in normal corneas. A few studies on corneal microstructure attributed this loss of stiffness to the reduction of interlamellar adhesion caused by cross-links, observed primarily in the anterior stroma. Significant risks were associated with the ablation of keratoconic corneas because they were biomechanically weaker. To overcome this, collagen cross-linking (CXL) was combined with topography-guided PRK to prevent further reduction in biomechanical stiffness of the cornea. Further, topography-guided PRK reduced the steepness of the anterior corneal surface but it did not eliminate the in situ biomechanical weakness in the corneal stroma. The outcome of CXL itself may be affected by the location of the cone or steepest point relative to the geometric center of the topography map because the magnitude of flattening was observed to be inversely proportional to the distance from the geometric center. This effect has not been investigated in keratoconus patients that have undergone combined topography-guided PRK and CXL. CXL alone differed in mechanism from the combined approach because it reduced the steepness of the cone as a secondary effect of treatment without any ablation. Combined topography-guided PRK and CXL reduced the steepness of the cone and corneal aberrations by combining ablation and tissue stiffening. Therefore, the purpose of this study was to evaluate the influence of the cone location–affected refractive outcomes (visual acuity, spherical and cylindrical error), corneal aberrations, and corneal biomechanics after the combined treatment.
Methods
The prospective, comparative case series study was approved by the institutional research and ethics committee of Narayana Nethralaya, India and conducted in accordance with the Declaration of Helsinki after taking an informed written consent from each patient.
Study Population
Patients between 18 and 60 years of age were included in the study. Inclusion criteria were mild to moderate keratoconus (grades 1 and 2 on the Amsler-Kruemeich keratoconus severity scale), intolerance to contact lens wear, thinnest pachymetry greater than 450 μm, or a predicted post-topography-guided PRK thinnest pachymetry of at least 400 μm. Patients with active allergic eye disease, active ocular inflammation, or central scarring of the cornea were excluded from the study. Progression of keratoconus was defined as an increase of 0.5 diopter (D) or more in 2 or more keratometry values in the steep meridian between 2 axial curvature maps or a decrease in corneal thickness of 10% or more at the thinnest point between 2 pachymetry maps on Scheimpflug imaging (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) in the preceding 6 months.
Study Design
All measurements including refractive (sphere and cylinder) and visual (uncorrected distance visual acuity [UDVA] in logMAR; best-corrected distance visual acuity [BDVA] in logMAR) outcomes were assessed preoperatively and at 12 months postoperatively. Intraoperative tomography was not approved by the institutional research and ethics committee as this would have required the patient to be physically displaced. Corneal topography (flat [K1] and steep [K2] axis keratometry) and thickness measurements were performed using Scheimpflug imaging (Pentacam; Oculus Optikgeräte GmbH) preoperatively and at 12 months postoperatively. Corneal wavefront aberrations (anterior plus posterior surface) were computed by Pentacam using ray tracing. The maximum tangential curvature on corneal topography was used for classifying the cones. The x and y coordinate of the location of the maximum tangential curvature was noted in the tangential curvature map. The distance (L) from the geometric center (x coordinate = 0, y coordinate = 0) of the corneal tangential curvature map was evaluated as the square root of the sum of squares of x and y coordinate. Eyes were subdivided into 2 groups preoperatively based on L: Group 1, cones located within the central 2-mm zone; and Group 2, cones located outside the central 2-mm zone. Corneal biomechanics were assessed with the Ocular Response Analyzer (Reichert Inc, Depew, New York, USA) and reported as corneal hysteresis (CH) and corneal resistance factor (CRF). Patients also underwent corneal endothelial evaluation using a noncontact specular microscope (EM-3000; TOMEY Inc, Aichi, Japan). Endothelium cell density (ECD) was measured as cells/mm 2 .
Study Treatments and Procedures
After instillation of a topical anesthetic solution (proparacaine 0.5%; Alcon Inc, Fort Worth, Texas, USA), an 8-mm-diameter zone of corneal epithelium was mechanically removed. By linking the Topolyzer (Alcon Inc) with the Wavelight Allegretto Wave Excimer Laser System (Alcon Inc), topography-guided PRK was performed. In order to ablate the least amount of tissue, the optical zone diameter was restricted between 5.5 and 6.5 mm. The maximum ablation depth at the thinnest region of the cornea was limited to 50 μm. The aim of the procedure was to regularize the anterior corneal shape and to not necessarily reduce the refractive error of the eye. After ablation, the residual stromal bed was irrigated with balanced salt solution. Then, riboflavin (0.1% solution of 10 mg riboflavin in 10 mL 20% dextran-T-500) was applied every 2 minutes for the first 20 minutes. Ultraviolet-A (wavelength of 365 nm) and incident intensity of 30 mW/cm 2 was applied for the next 4 minutes (Avedro Inc, Waltham, Massachusetts, USA), followed by a thorough irrigation with balanced salt solution. This cross-linking procedure delivered a total energy of 7.2 J/cm 2 to the cornea. A bandage contact lens (BCL) was placed for 3 days or until complete healing of the epithelium. The patient was put on a tapering dose of prednisolone acetate 1% (Allergan Inc, Irvine, California, USA) 3 times a day for 3 days, 2 times a day for the next 3 days, and 1 time a day for the next 2 days. Topical antibiotic (Vigamox; Alcon Inc) was administered 3 times a day for 1 week. Lubricating eye drop (Systane; Alcon Inc) was administered 6–8 times a day for 3 months.
Statistical Analysis
All continuous variables were assessed for normality of distribution. The preoperative values of the variables of Group 1 and 2 were assessed with 1-way analysis of variance (ANOVA). The postoperative outcomes were compared with preoperative values within each group using repeated measures ANOVA, which performed a paired comparison of the outcome variables between pre- and postoperative time points with cone location treated as between-group discriminator. The variables assessed were UDVA, BDVA, sphere, cylinder, K1 (flat K), K2 (steep K), astigmatism, axis, corneal thickness, and total (anterior and posterior surface combined) corneal wavefront aberrations, CH, and CRF. The aberrations were fitted with Zernike polynomials up to the 8th order and with an analysis zone of 8 mm diameter. The individual Zernike coefficients up to the 4th order (defocus, coma, trefoil, spherical aberration) and the root mean square (RMS) of Zernike coefficients were assessed. Sample size was estimated using mean keratometry (K1 and K2) and a type I/II error of .05 ( P value)/0.2 (80% power). Based on mean preoperative K1, K2, and 80% power, the required sample size to detect differences between the 2 groups was only 4. All statistical analyses and samples size calculations were performed with MedCalc v12.5.0.0 (Medcalc Inc, Ostend, Belgium). A P value <.05 was considered statistically significant.
Results
The mean age of the patients was 25 years (range: 23–47 years). The numbers of eyes in Groups 1 and 2 were 17 and 12, respectively. Thus, the study sample size was adequate. Table 1 lists the preoperative mean and standard error of the mean (SEM) of UDVA, BDVA, sphere, cylinder, spherical equivalent, corneal wavefront aberrations up to the 4th order, RMS of total corneal wavefront aberrations (total RMS), RMS of higher-order aberrations (HOA RMS), CH, and CRF. Mean BDVA ( P = .21), UDVA ( P = .3), sphere ( P = .09), cylinder ( P = .89), and spherical equivalent ( P = .08) were similar between Groups 1 and 2, though UDVA and BDVA were slightly better in Group 2 than in Group 1. Mean K1 ( P = .03), K2 ( P = .004), and astigmatism ( P = .04) were significantly different between Group 1 and Group 2 preoperatively. Here astigmatism was the difference between K2 and K1. Preoperatively, mean total ( P = .86) and higher-order ( P = .73) RMS of corneal wavefront aberrations were similar between Group 1 and Group 2. Only mean defocus and spherical aberration were significantly different between Group 1 ( P = .02) and Group 2 ( P = .04) preoperatively. Mean CH ( P = .32) and CRF ( P = .43) were similar between Group 1 and Group 2. Mean central corneal thickness was similar between Groups 1 and 2 ( P = .96). Mean endothelium cell density was similar between the 2 groups preoperatively ( P = .25 in Table 1 ).
| Cone Location | P Value c | ||||
|---|---|---|---|---|---|
| Group 1 a | Group 2 b | ||||
| Mean | Standard Error of the Mean | Mean | Standard Error of the Mean | ||
| UDVA (logMAR) | 0.88 | 0.10 | 0.71 | 0.13 | .30 |
| BDVA (logMAR) | 0.28 | 0.05 | 0.19 | 0.06 | .21 |
| Sphere (D) | −2.94 | 0.83 | −0.73 | 0.90 | .09 |
| Cylinder (D) | −3.75 | 0.40 | −3.88 | 0.84 | .89 |
| Spherical equivalent (D) | −4.82 | 0.84 | −2.67 | 0.78 | .08 |
| K1 (D) | 48.57 | 0.94 | 45.66 | 0.78 | .03 c |
| K2 (D) | 54.49 | 1.03 | 49.87 | 0.96 | .004 c |
| Astigmatism (D) | 5.92 | 0.47 | 4.21 | 0.71 | .04 c |
| CCT (μm) | 456.71 | 6.66 | 457.33 | 12.75 | .96 |
| Total RMS (μm) | 17.81 | 1.43 | 17.34 | 2.35 | .86 |
| HOA RMS (μm) | 4.30 | 0.41 | 4.55 | 0.59 | .73 |
| Defocus (μm) | −6.57 | 2.26 | 0.71 | 1.38 | .02 c |
| Astigmatism horizontal (μm) | −2.52 | 1.13 | −1.41 | 1.25 | .52 |
| Astigmatism oblique(μm) | 0.05 | 1.17 | 0.56 | 1.32 | .78 |
| Coma horizontal (μm) | −0.93 | 0.48 | −1.55 | 0.75 | .47 |
| Coma vertical (μm) | −2.09 | 0.48 | −2.12 | 0.77 | .97 |
| Trefoil horizontal (μm) | 0.21 | 0.16 | 0.34 | 0.14 | .98 |
| Trefoil oblique (μm) | 0.15 | 0.10 | 0.37 | 0.22 | .33 |
| Spherical aberration (μm) | −1.38 | 0.46 | −0.13 | 0.20 | .04 c |
| CH (mm Hg) | 8.79 | 0.76 | 7.70 | 0.34 | .32 |
| CRF (mm Hg) | 8.66 | 0.57 | 8.00 | 0.30 | .43 |
| Endothelium cell density (cells/mm 2 ) | 2650 | 20 | 2615 | 30 | .25 |
a In Group 1, the cone is located within the central 2-mm (diameter) zone of the tangential curvature topographic map of the anterior cornea.
b In Group 2, the cone is located outside the central 2-mm zone.
c P value <.05 indicates statistically significant difference between Groups 1 and 2 preoperatively.
Table 2 lists the postoperative mean and SEM of UDVA, BDVA, sphere, cylinder, spherical equivalent, corneal wavefront aberrations up to the 4th order, RMS of total corneal wavefront aberrations (total RMS), RMS of higher-order aberrations (HOA RMS), CH, and CRF. No eye lost any line of vision up to 1 year after treatment. Altogether, 27.6% (8 out of 29) of the total eyes remained unchanged after 1 year of follow-up; 44.8% (13 out of 29) and 27.6% (8 out of 29) gained 1 and 2 lines of vision after 1 year of follow-up ( Table 2 ). Table 2 also lists 2 P values: the first P value describes whether the treatment caused a statistically significant change in the variable; the second P value describes whether the change in the variable owing to the combined treatment may be attributable to preoperative location of the cone relative to the geometric center. The number of lines gained after the treatment was significantly better in Group 1 (1.5 ± 0.21) than in Group 2 (0.67 ± 0.26) ( P = .02). Mean UDVA ( P = .002) and BDVA ( P = .001) improved in both groups after treatment. Group 1 had significantly better mean BDVA than Group 2 ( P = .04), though mean UDVA was similar in both groups. Mean sphere ( P = .001), cylinder ( P = .001), spherical equivalent ( P = .001), K1 ( P = .001), and K2 ( P = .001) reduced after the treatment in both groups and the cone location did not influence the magnitude of reduction ( P > .05). Mean total RMS ( P = .02), horizontal trefoil ( P = .048), and spherical aberration ( P = .048) changed after treatment and were marginally significant. The cone location did not influence the change in aberrations (specific terms and RMS) between Groups 1 and 2 ( P > .05). Mean CH ( P = .35) and CRF ( P = .38) increased after treatment, but the cone location did not influence the magnitude of increase between Group 1 ( P = .44) and Group 2 ( P = .47). Mean CCT was higher in Group 2 than in Group 1 ( P = .001) but the decrease was independent of the cone location ( P = .37). Neither the treatment ( P = .28) nor the cone location ( P = .34) had any significant effect on the endothelium cell density ( Table 2 ).
| Cone Location | P Value: Effect of Treatment c | P Value: Effect of Cone Location on Treatment Outcome d | ||||
|---|---|---|---|---|---|---|
| Group 1 a | Group 2 b | |||||
| Mean | Standard Error of the Mean | Mean | Standard Error of the Mean | |||
| Lines gained | 1.50 | 0.21 | 0.67 | 0.26 | – | .02 d |
| UDVA (logMAR) | 0.70 | 0.10 | 0.40 | 0.07 | .002 c | .36 |
| BDVA (logMAR) | 0.07 | 0.02 | 0.08 | 0.03 | .001 c | .04 d |
| Sphere (D) | −1.44 | 0.62 | 0.19 | 0.68 | .009 c | .51 |
| Cylinder (D) | −2.71 | 0.26 | −2.08 | 0.46 | .001 c | .34 |
| Spherical equivalent (D) | −2.79 | 0.64 | −0.85 | 0.76 | .001 c | .81 |
| K1 (D) | 46.99 | 0.80 | 44.05 | 0.94 | .001 c | .96 |
| K2 (D) | 52.14 | 0.88 | 48.25 | 1.09 | .001 c | .28 |
| Astigmatism (D) | 5.14 | 0.60 | 4.20 | 0.62 | .16 | .17 |
| CCT (μm) | 357.65 | 29.43 | 391.17 | 12.94 | .001 c | .37 |
| Total RMS (μm) | 15.31 | 2.04 | 12.53 | 1.51 | .02 c | .40 |
| HOA RMS (μm) | 4.66 | 0.73 | 4.02 | 0.48 | .40 | .20 |
| Defocus (μm) | −1.48 | 2.69 | −0.90 | 1.50 | .16 | .17 |
| Astigmatism horizontal (μm) | −0.92 | 1.36 | −0.99 | 1.35 | .13 | .42 |
| Astigmatism oblique (μm) | 0.22 | 1.42 | 1.32 | 1.22 | .66 | .75 |
| Coma horizontal (μm) | −1.66 | 0.69 | −1.47 | 0.71 | .28 | .27 |
| Coma vertical (μm) | −2.25 | 0.71 | −1.66 | 0.73 | .87 | .42 |
| Trefoil horizontal (μm) | −0.11 | 0.15 | −0.12 | 0.18 | .047 c | .73 |
| Trefoil oblique (μm) | 0.04 | 0.20 | 0.37 | 0.19 | .63 | .55 |
| Spherical aberration (μm) | −0.39 | 0.61 | −0.01 | 0.30 | .048 c | .16 |
| CH (mm Hg) | 9.04 | 0.53 | 8.83 | 0.48 | .35 | .44 |
| CRF (mm Hg) | 8.84 | 0.29 | 8.82 | 0.32 | .38 | .47 |
| Endothelial cell density (cells/mm 2 ) | 2575 | 19 | 2568 | 30 | .28 | .34 |
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