, trefoil RMS 
, and the Zernike term corresponding to the primary spherical aberration (Z 40).
Postoperatively , patients should be examined the third day after surgery and at 1 week postoperatively to assess the status of the corneal epithelial healing and if a complete epithelialization is present, the therapeutic contact lens fitted has to be removed. Afterward, patients should be examined at 1, 3, 6, and 12 months after surgery. UDVA and CDVA testing, manifest refraction, corneal topography, ocular aberrometry, and biomicroscopic examination are performed in these four visits. In all postoperative visits, the level of corneal haze has to be evaluated on a scale from 0 to 4 (0 = clear cornea, 1 = mild haze, 2 = moderate haze, 3 = severe haze, and 4 = reticular haze obstructing iris details) [5].
26.7 Surgical Procedure
All surgical procedures are performed under topical anesthesia (benoxinate hydrochloride 0.4 % Sterile Ophthalmic Solution). In all cases, the corneal epithelium is removed using the 9.0-mm Amoils brush (Innovative Excimer Solutions, Inc., Toronto, Canada) and the laser ablation iw then applied. A WFG-PRK laser treatment is applied using the VISX Star S4IR excimer laser (Abbott Medical Optics, Santa Ana, California) and an ablation profile generated according to the measurements obtained with the iDesign system. An adjustment to the profile is applied as needed to reduce the maximum ablation depth to a maximum of 15 % of the corneal thickness at the thinnest location, reducing only the sphere component without changing the cylinder. The sphere adjustment is limited to 2.50 D, which is the maximum adjustment allowed by the manufacturer.
After laser ablation, a 6-mm cellulose disc soaked in Mitomycin C (MMC) 0.02 % solution is applied over the ablated tissue for 20 s followed by irrigation with 30 mL of chilled balanced salt solution. A bandage contact lens is then fitted, remaining in place until full reepithelialization. The postoperative treatment regimen consists of Topical Antibiotics/Corticosteroids, e.g., moxifloxacin hydrochloride 0.5 % (Vigamox, Alcon Laboratories) applied four times daily up to 2 weeks after complete reepithelialization, a combination of tobramycin 3 mg/mL with dexamethasone 1 mg/mL (Tobradex, Alcon Laboratories, UK) applied four times daily until complete reepithelialization and then gradual tapering over 2 weeks, preservative-free tear substitutes applied every 2 h for 2 weeks (e.g., Refresh plus, Allergan, Inc, USA), and oral analgesic nonsteroidal anti-inflammatory drug applied three times daily for 3 days (Catafast, Novartis, Switzerland).
26.8 Clinical Results
A study by Shafik Shaheen et al. was done on 34 eyes with stage I and II keratoconus meeting the above-mentioned criteria who had previous CXL at least 1 year before treatment to demonstrate the effect of this wavefront-guided laser surface correction on visual, refractive, corneal morphology, and aberrometric data (Recently published: Shafik Shaheen M, Bardan AS, Pinero D, Ezzeldin H, El-Kateb M, Helaly H, Khalifa M. Wave Front–Guided Photorefractive Keratectomy Using a High-Resolution Aberrometer After Corneal Collagen Cross-Linking in Keratoconus. Cornea 2016;35(7):946–953).).
It demonstrated a statistically significant improvement in the mean logMAR UDVA from 0.93 ± 0.33 (mean ± SD) preoperatively to 0.14 ± 0.11 (mean ± SD) at the last follow-up visit and the mean logMAR CDVA has been improved from 0.28 ± 0.24 (mean ± SD) preoperatively to 0.05 ± 0.06 (mean ± SD) at the last follow-up visit; (p-value <0.001) for both. The mean MRSE has been reduced from −3.22 preoperatively to −0.68 at the 12th month postoperatively. The magnitude of reduction is shown in Fig. 26.1. The study demonstrates a high safety and efficacy indices at the last follow-up visit. Efficacy index was 1.58 ± 1.11 (mean ± SD), p < 0.001, safety index was 1.96 ± 1.52 (p < 0.001) proving safety and efficacy regarding VA improvement.
Fig. 26.1
Summary of the predictability outcomes after surgery in the analyzed sample
About the predictability of the procedure, at the sixth month postoperatively, 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. Summary is shown in Fig. 26.1.
As shown in Fig. 26.2, 97.1 % of the study subjects reached ≥ 20/40 UDVA at the last follow-up visit compared to 73.5 % only reaching CDVA of ≥ 20/40 preoperatively. Hundred percent reached ≥ 20/50 UDVA at the last follow-up visit at 1 year postoperatively.
Fig. 26.2
Comparison of the distribution of the postoperative UDVA (after 1 year) and the preoperative CDVA outcomes
The study demonstrated many changes in the corneal morphology . First of all the reduction of the maximum K reading from a mean of 48.32 D preoperatively to a mean of about 45.8 D at month 3 which remained in the same range over the first year postoperatively as shown in Fig. 26.3. Mean Q-value has decreased from −0.44 to −0.26. Central corneal thickness has decreased from 491.12 ± 39.19 μm (mean ± SD) preoperatively to 429.56 ± 61.81 μm at the last follow-up visit. Minimal corneal thickness decreased from 478.32 μm (mean) preoperatively to about 414 μm (mean) which remained stable over the period of follow-up as shown in Fig. 26.4. All the results are statistically significant (p < 0.001).
Fig. 26.3
Stability of the maximum keratometry (K-MAX) over the postoperative follow-up
Fig. 26.4
Stability of the minimal corneal thickness (MCT) over the postoperative follow-up
About the corneal indices, the results showed decrease in the values of the eight Pentacam indices studied. The reduction in IVA, IHA, IHD, and CKI was not statistically significant. While the reduction in ISV, R min, and KI was statistically significant. The study showed also stability of the indices through the period of follow-up.
There was 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 = 0.003), and reduction of coma (p = 0.001) and trefoil (p < 0.001). The primary coma RMS has decreased from 0.60 ± 0.59 μm (mean ± SD) preoperatively to 0.37 ± 0.37 μm at the last follow-up visit (p = 0.001). Mean Trefoil RMS has decreased from 0.35 μm preoperatively to 0.21 μm at the last follow-up visit.
Astigmatic analysis was done by vector analysis (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. This is described in detail in Table 26.1.
Table 26.1
Summary of the parameters derived from the vector analysis of ocular astigmatic changes at the end of the follow-up in the analyzed sample (Alpins method)
Vector parameters | Mean (SD) |
|---|---|
Median (range) | |
TIA (D) | 2.79 (1.82) |
3.00 (0.00 to 7.00) | |
SIA (D) | 2.36 (1.72) |
2.39 (0.00 to 7.95) | |
DV (D) | 1.06 (0.92) |
0.75 (0.00 to 3.25) | |
ME | 0.43 (0.86) |
0.28 (2.58 to −0.95) | |
CI | 0.88 (0.29) |
0.83 (0.26 to 1.61) | |
AE (°) | 2.57 (15.43) |
2.34 (48.76 to −44.86) |
The following vectors were determined and evaluated: targeted induced astigmatism (TIA) as the vector of intended change in cylinder for each treatment, surgically induced astigmatism (SIA) as the vector of the real change achieved, and difference vector (DV) as the additional astigmatic change that would enable the initial surgery to achieve its intended target. Additionally, the following parameters derived from the relationship between these vectors were calculated and analyzed in each postoperative visit:
Magnitude of error (ME): the arithmetic difference between the magnitudes of the SIA and TIA. Positive: undercorrection; negative: overcorrection.
Angle of error (AE): the angle described by the vectors of the achieved correction (SIA) and the intended correction (TIA): Negative: achieved correction is clockwise to its intended axis. Positive: achieved correction is counterclockwise to its intended axis
Correction index (CI): The ratio of the SIA to the TIA; what the surgery actually induced versus what the surgery was meant to induce. The CI is preferably 1; it is greater than 1 if an overcorrection occurs and less than 1 if there is an undercorrection.
Case Example 1
A 25-year-old female patient. Wavefront-guided PRK was performed 2 years after corneal collagen cross-linking. Table 26.2 represents the preoperative clinical data , Pentacam and iDesign wavefront measurements compared to the findings at the final postoperative follow-up visit (12 months postop).
Table 26.2
Summary of preoperative and postoperative data of case 1
Preoperative data | Final postoperative data | |
|---|---|---|
UDVA (logMAR) | +1.30 | +0.10 |
CDVA (logMAR) | +0.50 | 0.00 |
Refraction | +0.75 − 4.00 × 110 | −0.25 − 1.00 × 155 |
Pentacam corneal data | ||
Thinnest location | 422 μm | 374 μm |
ISV | 67 | 51 |
IVA | 0.83 | 0.49 |
KI | 1.19 | 1.17 |
CKI | 1.03 | 1.05 |
R min | 6.78 | 6.98 |
IHA | 9.8 | 15.3 |
IHD | 0.061 | 0.057 |
iDesign data at 5 mm WFD | ||
RMS total | 3.65 μm | 1.75 μm |
RMS HOA | 2.72 μm | 1.32 μm |
Coma | 1.16574 | 0.47882 |
Trefoil | 0.67390 | 0.40287 |
Spherical aberrations | −0.00306 | −0.21724 |
In this case there is a noticeable reduction in the total RMS value by (52 %), RMS HOA by (51.4 %), Coma reduction by (58.9 %), trefoil reduction by (40.2 %). This improvement was reflected on the patient’s visual outcome (logMAR CDVA was improved from +0.50 to 0.00). Figures 26.5, 26.6, 26.7, 26.8, and 26.9 show the pre- and postoperative investigations and the ablation profile of this patient.
