In the current prospective study on 80 eyes of 40 patients, we compared Pentacam and AS-OCT measurements to determine the real depth of ablation and correlated it to the intended ablation depth as calculated by the WaveLight EX500 excimer laser (Alcon) computer software. After 3 months of follow-up, the mean spherical equivalent was − 6.15 ± 3.14 D. In the present study, the WaveLight EX500 excimer laser (Alcon) computer software system was predictable and accurate. For example, 100% (80 eyes) had a post-operation refraction within 0.50 D of the tried adjustment. Comparatively, in one study by Wallerstein et al., they operated on 114 eyes from 78 cases who experienced LASIK at the Canadian Institute of Refractive Surgery. After 24 months post-surgery, the scatterplot of the tried against actual refractive adjustment showed an expected technique with 51.7%, 71.9%, 84.2%, and 93.8% existing in ± 0.25 D, ± 0.50 D, ± 0.75 D, and ± 1.00 D, of SEQ aim . Kanellopoulos and Asimellis operated on 160 eyes with high myopia ⩾ − 6.00 D, with postoperative average spherical equivalents at − 0.37, − 0.43, and − 0.25 D for the 3, 6, and 12 months, which was also predictable .
With respect to corneal thickness measurement, in the present study, the mean value of preoperative CCT was 581.44 ± 31.07 μm when measured by Pentacam and 590.85 ± 29.18 μm when measured by the AS-OCT. A strong positive correlation between the two measurement devices was noted. However, Pentacam measurements were statistically significantly thinner compared to AS-OCT (P = 0.032). Pentacam measurements underestimated the cornea thickness by 9.13 µm compared with the AS-OCT.
In the third month postoperative results, the mean value of CCT was 502.68 ± 26.15 μm when measured by Pentacam, while the CCT was 512.8 ± 29.15 μm when measured by the AS-OCT. CCT measurements also strongly correlated for the two measurement devices. Pentacam measurements significantly underestimated the cornea thickness by 9.5 µm compared with the AS-OCT (P = 0.098).
Interestingly, O’Donnell et al. found poor correlation between CCT measurements using Pentacam and Visante AS-OCT with a coefficient of agreement 37.36 μm and limits of agreement 25.61 μm to − 49.11 μm . Conversely, Fu et al.  found the opposite of our study. In their study, the mean CCT as measured by the AS-OCT was 519.23 ± 34.37 μm, which was 18.53 μm lower than the Pentacam measurement of 537.76 ± 31.84 μm.
Comparative measurements of CCTs were discovered by Muriel Doors et al. from AS-OCT (Carl Zeiss Meditec), Orbscan II, and Pentacam (Oculus, Wentzler) . CCT measurements using the Cirrus OCT, employing the new anterior segment lens fixtures and the Pentacam HR, were reported by Baghdasaryan et al. , with a high P value of < 0.0001. Furthermore, Xuepei Li et al. concluded that the CCT readings were well agreed between Pentacam and CASIA2-OCT. The two instruments can be regarded as interchangeable for monitoring corneal problems or for planning eye surgery for these CCT assessments in healthy patients. No significant changes were found between CAISA2 and Pentacam estimates for CCT or ACD (P > 0.05) .
In the current study, the mean intended ablation depth was 78.60 ± 34.05 µm. The mean clinically observed ablation depth (calculated by measuring the difference between CCT at the third postoperative month from the preoperative CCT) was 78.75 ± 34.21 µm when measured by Pentacam and 78.38 ± 36 µm when measured by the AS-OCT. No statistically significant disparity was observed between the intended ablation depth and clinically observed ablation depth estimated by the two devices (P > 0.05).
The two devices showed an extremely important association between the changes in corneal pachymetry and the anticipated depth of ablation. It was best to correlate with Pentacam (Pearson RC 0.974, P < 0.001) and AS-OCT (Pearson RC 0.863 and P < 0.001), respectively. Carl-Arnold Lackerbauer et al. evaluated for 3 months post-microscopic laser in the site of keratomileusis (labsic) dissimilarity from measured to intended after operative central corneal depth (CCT). They found that the average variance between measured and intended after operative corneal thickness is between 11.1 μm 6 weeks postoperatively and 13.8 μm 6 weeks postoperatively . Giacomo Savini et al. investigated the conformity of the expected depth of ablation 3 months postoperatively as determined by the EX500 excimer laser with a rotating Scheimpflug camera. No average expected depth of ablation (66.33 ± 24.15 μm) or calculations of the depth of ablation in the smallest core were found to be statistically significant . Anastasios Kanellopoulos et al. evaluated stroma depth decrease in a retrospective of 205 successive eyes of 205 cases experiencing myopic and myopic astigmatism LASIK . Front segment OCT was done preoperationally and 3 months post-operationally. The achieved ablation depth was 86.01 ± 28.28 µm, comparable to the usual programmed greatest ablation deepness of 88.48 ± 26.05 µm. Actual stroma depth decrease following myopic LASIK relates well with the tried versus actual refractive alteration. Febbraro et al. compared the preoperative estimate of greatest ablation depth offered by the laser software with actual extent using Scheimpflug pachmetry (Pentacam HR; Oculus) in myopic laser-aided in-site keratomileusis. They discovered a solid linear association between laser software assessment of greatest ablation depth and Scheimpflug pachmetry (P < 0.001). The average ablation depth calculated using Scheimpflug pachmetry was more than that estimated by the laser software, with an average global variance of 2.15 μm (P < 0.05) . Arbelaez et al. compared measurements of ultrasound (Pachette pachymeter DGH Technology), Scheimpflug (Pentacam, Oculus), and OCP (Heidelberg Engineering GmbH, Heidelberg, Germany) methods to define ablation depth post-myopic astigmatism cornea laser refractive operation. A high correlation of preoperative pachymetry has been reported for both Scheimpflug and OCP (r = 0.84, P = 0.0001 for Scheimpflug; r = 0.83 and P = 0.0001 for OCP) using ultrasound pachymetry as a reference measurement .
The central removal depth of ultrasound, Scheimpflug, and OCP measurements (r2 = 0.60, P = 0.0001 for ultrasound; r = 0.75, P = 0.0001 for Scheimpflug; r = 0.76, P = 0.0001 for OCP) was well correlated with the central ablation depth presented on the laser screen as the reference measurement. The predicted ablation depth did not differ much from the values of Scheimpflug and ultrasonography (P = 0.47) .
In our study, Pentacam is more sensitive but as specific as AS-OCT on measuring ablation depth. The sensitivity of Pentacam is 88.9%, and the specificity measures around 60%. The sensitivity and specificity of AS-OCT are 80% and about 60%, respectively.
Several papers have studied the sensitivity and specificity of Pentacam and AS-OCT in measuring the anterior chamber angle. Studies have also been conducted to evaluate the sensitivity and specificity in detecting keratoconus and early diagnosis. However, to the best of our knowledge, this is the first report to assess the sensitivity and specificity of these devices in measuring the ablation depth.
The current study was limited by the sample size. Future studies with larger numbers of cases are necessary. Additional Pentacam, OCT, and LASIK devices should also be included with different manufacturers. Furthermore, another aspect of refractive errors, hypermetropia, should be thoroughly investigated and compared with the myopia results. Finally, future studies with longer duration (beyond 3 months) of follow-up would be ideal.