الفهرس 
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Abstract
Refractive surgical correction of ametropia in patients with keratoconus remains challenging. Cross-linking alone stabilizes and stiffens the cornea by inducing more corneal collagen cross-links, and the remaining refractive errors will still need to be corrected.
Laser refractive surgery in patients with such irregular corneas has long been contraindicated because of the risk of postoperative progression of the disease process. This is especially true with laser in situ keratomileusis (LASIK). The flap created in LASIK weakens the corneal tissue and renders the cornea more prone to keratectasia. However, numerous studies claim the safety of surface ablation in suspected keratoconus or in “forme fruste keratoconus” as in photorefractive keratectomy (PRK). In patients with mild to moderate keratoconus, combined PRK and collagen cross-linking has been proven to be a safe and effective alternative to correct minor refractive error, stabilizing the remaining stromal bed and avoiding progression of ectatic disease.
Modern aberrometers that was introduced to the ophthalmic armamentarium could read the aberrations of the irregular cornea and generate a dependable ablation profile out of them. Yet, there was some difficulties in reading a dependable, repeatable and reliable wavefront maps in many cases of highly aberrated corneas.
Recently, a new version of high definition aberrometers were introduced to the field of refractive surgery with recent report of their abilities to read and treat precisely the highly aberrated corneas.
The study was carried out on 34 eyes of 25 patients of either sex after documented corneal stability at least 1 year after corneal collagen cross-linking. Age not less than 21, corneal thickness not less than 400 µ and manifest refraction spherical equivalent of no more than 6 D.
Every patient was subjected to complete ophthalmologic evaluation; including UDVA and CDVA, manifest refraction, Pentacam, and wavefront measurement using iDesign system®.
Wavefront-guided PRK was performed under topical anesthesia (Benoxinate eye drops) with VISX STAR S4 IR using the ablation profile generated by iDesign aberrometer.
Postoperatively, the patients were followed up at the 3rd day and one week for the state of epithelial healing. Then at 1 month, 3 months, 6 months and 12 months. In each follow up visit the patient was examined regarding UDVA, CDVA, manifest refraction, full pentacam study and wavefront measurement.
At the last follow up visit, the preoperative logMAR UDVA of 0.93 ±0.33 (mean ± SD) has been improved to 0.14±0.11 postoperatively; p-value <0.001, and logMAR CDVA has been improved from 0.28 ±0.24 preoperatively to 0.05 ±0.06 (mean ±SD) postoperatively; p-value <0.001. Results also showed reduction in MRSE from -3.22 ±1.32 D (mean ±SD) preoperatively to -0.68 ±0.64 D at the last follow up visit. About the predictability of the procedure, at the 6th month postop, 76.5% of eyes had a manifest refraction within ±1.00 D of emmetropia, 47.1% were within ±0.50 D. At the last follow up visit 62% of the cases reached within ±0.50 D. 97.1% of our patients reached ≥ 20/40 UDVA at the last follow up compared to 73.5% only reaching CDVA of ≥ 20/40 preoperatively. 100% of our patients reached ≥ 20/50 UDVA at the final follow up. 44.1% of our patients gained 2 or more lines in the CDVA at month 12.
There is a statistically significant reduction in the higher-order aberrations in general demonstrated by the significant reduction of total RMS p-value <0.001 and the RMS HOA p-value 0.003, and reduction of coma , trefoil with p-value 0.001 , <0.001 respectively. It is to be mentioned that the reduction of HOA in these eyes is considered number one reason behind the postopertive improvement in both UDVA and CDVA.
In this study, vector analysis of ocular astigmatic changes was done (Alpins method) for better understanding and presentation of astigmatic correction. Targeted induced astigmatic correction (TIA) was 2.79 ±1.82 (mean ±SD), while the real change achieved at the last follow up visit (SIA) was 2.36 ±1.72 (mean ±SD) and this showed a magnitude of error (ME) of 0.43 ±0.86 (mean ±SD) which is the difference between SIA and TIA (positive number means undercorrection). The correction index (CI) was evaluated and found to be 0.88 ±0.29 (mean ±SD) which means slight undercorrection.
Postoperatively, there were no complications of surgery and all our patients completed the whole follow up. As regards visual acuity, there was a statistically significant improvement in UDVA and CDVA. A statistically significant reduction in the manifest refraction spherical equivalent and higher-order aberrations was demonstrated. This technique has proven a statistically significant safety and efficacy.
Wavefront-guided PRK should be considered for visual rehabilitation of stable keratoconic eyes after corneal CXL especially in cases with low refractive errors in which we can obtain a reliable dependent wavefront map.