Purpose
We sought to compare the accuracy of different corneal curvature parameters in estimating corneal power change in eyes that underwent myopic small-incision lenticule extraction (SMILE).
Design
Prospective cross-sectional trial.
Methods
89 eyes of 48 patients who had undergone SMILE at Shanghai Eye Disease and Prevention & Treatment Center were enrolled. Total corneal refractive power (TCRP) and total keratometry (TK) values, along with corrected parameters calculated using the Haigis, Shammas, Maloney, and Actual K a+p methods, were compared with data obtained using the clinical history method (CHM). We also compared the surgically induced changes in TCRP (ΔTCRP) and TK (ΔTK) to changes in the spherical equivalent on the corneal plane (ΔSEco).
Results
After SMILE, differences were smallest between values of TK (0.41 ± 0.48 D) and CHM. The width of the 95% limits of agreement of TK (1.88 D) was narrowest, followed by that of K a+p (1.94 D). Pearson analysis showed that each parameter had a good correlation with CHM data. The differences between ΔTK and ΔSEco was 0.42 D, when between ΔTCRP and ΔSEco was 0.62 D. The width of the 95% limits of agreement of ΔTK (1.88 D) and the correlation coefficient of ΔTK (0.964) were greater than those of ΔTCRP. A separate case of cataract surgery following SMILE confirmed the validity of the above points in our study (the absolute error was 0.005 D).
Conclusions
The TK parameter of the IOLMaster 700 can provide accurate and objective corneal power evaluation after SMILE.
INTRODUCTION
S mall-incision lenticule extraction (SMILE) represents a cutting-edge technology in refractive surgery, being extensively employed for correcting refractive errors. There are few data pertaining to the precise intraocular lens (IOL) power calculations required in eyes with cataract that have undergone SMILE. 2 Anticipating the burgeoning popularity of the SMILE technique and demographic shifts, it is expected that the number of patients who have undergone SMILE and subsequently require cataract surgery will escalate exponentially in the coming years. Consequently, accurate refractive predictions have become paramount in cataract surgeries to determine the IOL power for these patients.
The traditional clinical history method (CHM) was applied as a standard method, which requires information from before the SMILE. However, the data from before laser in situ keratomileusis (LASIK) or SMILE are usually not available or reliable, and the literature has shown that nonhistorical methods are superior and are currently widely used in clinical practice. The alteration in the ratio of the anterior and posterior corneal surface curvatures renders the traditional corneal refractive index (1.3375), which was historically used to calculate corneal curvature, no longer applicable. Therefore, it is necessary to modify the simulated keratometry (SimK) value or directly measure the total corneal curvature when calculating the IOL power after corneal refractive surgery. Thus, securing precise measurements of the anterior as well as the posterior corneal curvature radii to calculate total corneal power might help to minimize the margin of error in IOL power calculation in eyes with previous keratorefractive surgery. Currently, there is a diverse array of devices that possess the capability to concurrently assess both the anterior and posterior surfaces of the cornea, thereby enabling the acquisition of the total corneal curvature, eg, real keratometry based on the optical coherence tomography (OCT) principle and total corneal refractive power (TCRP) based on the Scheimpflug camera technique. Study demonstrates that true net power (TNP) corneal power difference maps best reflect surgically induced corneal power changes after LASIK and PRK, while TCRP corneal power difference maps are optimal post-SMILE.
Recently, the total keratometry (TK) method, which incorporates both anterior and posterior corneal radii measurements through a combination of telecentric keratometry and swept-source OCT, has been introduced for measuring the total corneal power. The use of TK-based IOL power calculation has been demonstrated to be advantageous, particularly in eyes that have undergone previous myopic excimer-based keratorefractive surgery, though this approach is also beneficial in eyes with untreated corneas. In addition, the physical ray-tracing approach, which is independent of any prior clinical history, has yielded exceptional results in IOL power calculation for both untreated eyes and eyes that have undergone keratorefractive surgery. Previous studies have confirmed that TK can be used to calculate the IOL power of eyes after LASIK.
To date, only a few reports of IOL calculation and implantation after SMILE have been published in the peer-reviewed literature, , and the amount of data available is not large. Furthermore, the legitimacy of formulas specifically tailored for excimer-based photoablative techniques, notably LASIK and photorefractive keratotomy, including the Masket formula, appears to be in question. This arises from the significant differences in corneal shape alterations that SMILE procedures elicit, in comparison to those observed in femtosecond LASIK.
Therefore, the present study evaluated the accuracy of corneal curvature values obtained by different methods in reflecting the corneal refractive state and tracking the changes in corneal curvature caused by SMILE.
MATERIALS AND METHODS
This prospective cross-sectional study included 89 eyes of 48 patients who underwent SMILE for the treatment of myopia and myopic astigmatism. All eyes underwent uneventful SMILE procedures performed by a single surgeon (L.Q.Z.) at the Shanghai Eye Disease Prevention & Treatment Center from July 2022 to September 2022. The study inclusion criteria were (1) a preoperative refractive error between −1.00 and −10.00 diopters (D) of myopia and refractive astigmatism of ≤−5.00 D; (2) no other eye diseases other than ametropia, including corneal diseases, dry eye, cataract, glaucoma, and fundus diseases; (3) no previous ocular surgeries or history of ocular trauma; (4) contact lenses were not worn within 1 month; and (5) preoperative corrected distance visual acuity values of 20/25 or better, together with stable refraction (ie, ±1.00 D of manifest refraction spherical equivalent [SE]) over 2 years prior to surgery. Separately, the exclusion criteria were (1) postoperative uncorrected visual acuity worse than 20/20; (2) the occurrence of complications, such as dry eye and corneal opacity; and (3) the intraocular pressure was not within the reference range. The study was approved by the local ethics committee of the Shanghai Eye Diseases Prevention & Treatment Center (2021SQ015) and carried out in accordance with the principles outlined in the Declaration of Helsinki. All patients provided written informed consent prior to inclusion in the study.
preoperative measurements
The corneal curvature of each patient was measured by the same operator under the same indoor light source before and 3 months after SMILE. The examined eyes did not undergo cycloplegia, and the total examination time did not exceed 0.5 hours. Qualified data were recorded. Manifest refraction SE was converted into corneal-plane SE using the formula SE/(1-0.012 × SE). The changes of SE (ΔSEco) in the corneal plane caused by corneal refractive surgery were recorded.
corneal curvature measurements
The following approaches were used for corneal curvature measurement :
- (1)
CHM: The postoperative corneal curvature is calculated according to the corneal curvature before corneal refractive surgery and the change in refractive power before and after surgery. In this study, the CHM corneal curvature was calculated as the SimK value provided by the IOLMaster 700 biometer (Carl Zeiss Meditec AG) before SMILE minus ΔSEco.
- (2)
TK: This parameter was obtained from the IOLMaster 700 biometer. The measurement range was a circular area with a diameter of 2.5 mm at the center of the corneal apex.
- (3)
TCRP: This parameter was obtained from Pentacam AXL panoramic biometry (Oculus). The measurement range was defined as a circular area with a diameter of 4 mm centered on the corneal apex.
- (4)
SimK value-correction method : Four common SimK correction methods were used in this study—namely, the Haigis method, Shamma’s method, the Maloney method, and the Actual K a+p method. Their respective formulas are: K Haigis = 1.119 × postKm − 5.78, K Shammas = 1.14 × postKm − 6.8, K Maloney = 1.114 × postKm − 6.2, and K a+ p = 1.114 × postKm + Kp ( the Actual K a +p method ), respectively. The postKm is the SimK value measured by the IOLMaster 700 biometer after SMILE, and Kp is the posterior corneal curvature surface measured by the Pentacam AXL.
statistical analysis
The data distribution for normality was checked using the Kolmogorov–Smirnov test. Continuous variables (eg, keratometry) were expressed as means and standard deviations. A linear mixed effect model was used by adjusting the clustering effects from two-sided eyes to compare postoperative corneal curvature and CHM corneal curvature and to compare the changes in TK, TCRP (ΔTK, ΔTCRP), and ΔSEco before and after surgery. The interparameter agreement was assessed using Bland–Altman analysis, and 95% limits of agreement were calculated. Pearson correlation analysis was used to evaluate the correlation between parameters. P < .05 was considered statistically significant.
Statistical analysis was performed using SPSS software (version 27.0; IBM Corporation), and a probability of <5% ( P < .05) was considered statistically significant.
RESULTS
A total of 48 patients (89 eyes) with myopia, including 23 men and 25 women aged 25.01 ± 5.52 years (17-36 years), were included. Their axial length was 25.65 ± 0.99 mm, and the mean SE was −4.56 ± 1.83 D before and 0.43 ± 0.37 D after SMILE ( Table 1 ). The mean posterior-to-anterior corneal curvature radii ratio (P/A ratio) was 82.08% ± 1.41% before and 73.77% ± 2.61% after SMILE.
| Parameter | Mean ± SD | Range |
|---|---|---|
| Age (y) | 25.01 ± 5.52 | 17-36 |
| Preoperative manifest refraction (D) | ||
| Sphere | −4.12 ± 1.79 | −1.00 to −7.75 |
| Cylinder | −0.75 ± 0.55 | 0 to −2.50 |
| Spherical equivalent | −4.56 ± 1.83 | −1.25 to −8.00 |
| Postoperative manifest refraction (D) | ||
| Sphere | 0.60 ± 0.38 | −0.50 to 1.50 |
| Cylinder | −0.34 ± 0.21 | 0.00 to −1.00 |
| Spherical equivalent | 0.43 ± 0.37 | −0.62 to 1.38 |
| Preoperative axial length (mm) | 25.65 ± 0.99 | |
| Preoperative keratometry (D) | ||
| K mean | 42.96 ± 1.36 | 40.29-45.86 |
| TCRP m | 42.96 ± 1.37 | 40.14-45.95 |
| TK m | 42.20 ± 1.38 | 39.45-45.55 |
| Preoperative P/A ratio | 82.08% ± 1.41% | 79.10%-85.40% |
| Postoperative keratometry (D) | ||
| K mean | 38.42 ± 1.78 | 34.63-42.13 |
| TCRP m | 36.85 ± 1.92 | 32.65-40.60 |
| TK m | 37.82 ± 1.88 | 33.79-41.83 |
| Postoperative P/A ratio | 73.77% ± 2.61% | 69.00%-79.50% |
comparison of corneal curvature obtained by different methods after smile
The CHM corneal curvature, TK, TCRP, TCRP cornea, K Haigis , K Shammas , K Maloney , and K a+p after SMILE and their comparisons are shown in Table 2 . TK was closest to the CHM corneal curvature, with a difference of 0.41 ± 0.48 D. The difference was statistically significant ( t = 9.77, P < .001). The differences in corneal curvature obtained by other methods and CHM were also statistically significant ( t = 26.04,22.53,26.4,36.31,37.01; P < .001). The corneal curvatures obtained by other methods were all smaller than the CHM corneal curvatures.
| Corneal Power Measurement | Mean ± SD (D) | Mean difference a vs CHM (D) | t Value | P Value | 95% LOA vs CHM (D) | Correlation Coefficient ( r ) |
|---|---|---|---|---|---|---|
| CHM | 38.23 ± 1.90 | |||||
| TK | 37.82 ± 1.89 | 0.41 ± 0.48 | 9.77 | <.001 | −0.53 to 1.35 | 0.978 |
| TCRP | 36.85 ± 1.92 | 1.37 ± 0.60 | 26.04 | <.001 | 0.20-2.55 | 0.966 |
| K Haigis | 37.21 ± 1.99 | 1.02 ± 0.51 | 22.53 | <.001 | 0.01-2.02 | 0.977 |
| K Shammas | 37.00 ± 2.03 | 1.23 ± 0.53 | 26.4 | <.001 | 0.19-2.27 | 0.977 |
| KMaloney | 36.60 ± 1.98 | 1.63 ± 0.51 | 36.31 | <.001 | 0.63-2.63 | 0.977 |
| K a+p | 36.62 ± 1.84 | 1.61 ± 0.49 | 37.01 | <.001 | 0.64-2.58 | 0.976 |
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