Accurate corneal assessment, including measurements of corneal refractive power, corneal shape, and corneal thickness, is crucial before cataract and refractive surgery. Generally, a keratometry or corneal topography system is used to measure the anterior corneal curvature.1 The refractive power of the entire cornea can be determined by measuring the anterior corneal curvature, together with the hypothetical standard keratometric refractive index of 1.3375.2 In this regard, the standard keratometric refractive index is based on the assumption that the normal cornea maintains a relatively constant thickness and that the relationship between the curvatures of anterior and posterior surfaces remains constant.3 Therefore, this simple calculation is applicable for patients who have not undergone previous corneal surgery. For patients who previously underwent LASIK treatment or other corneal operations, the curvature of the anterior corneal surface cannot be precisely measured; in addition, the standard corneal refractive index is not applicable. Both of these measurement complications are obstacles when conducting post-LASIK calculation of intraocular lens power for cataract surgery.4 Therefore, many researchers have endeavored to develop more reliable approaches that precisely measure corneal conditions before and after LASIK. Several methods utilize a modern optical apparatus to calculate the true refractive power and curvature of the cornea. These approaches calculate the total corneal refractive power by measuring the corneal curvature of both anterior and posterior surfaces. Thus far, several imaging techniques, including slit-scanning tomography, Scheimpflug photography, and optical coherence tomography, are available for such measurement.8,14,15
Currently, several studies have shown profound clinical applications for the Pentacam (a single rotating Scheimpflug camera; Oculus Optikgeräte GmbH, Wetzlar, Germany) in measuring corneal thickness; these investigations have provided valid definitions of the derived corneal refractive powers, including total corneal refractive power (TCRP) (i.e. corneal power determined by ray tracing) and true net power (TNP) (i.e. corneal power calculated by using the Gaussian optics formula).4,5 However, there have a few reports regarding potential applications of the newer biometer GALILEI G6 (a dual rotating Scheimpflug imaging system; Ziemer Ophthalmic System AG, Zurich, Switzerland) in measuring total corneal power (TCP) and TCP-intraocular lens (TCP-IOL) values. The aim of this study was to identify the corneal power parameters of Pentacam that were in agreement with those of GALILEI G6. Because previous studies have indicated that the total corneal refractive powers (TCRP) of Pentacam, measured at points whose radii from the center are 3 and 4 mm zones, respectively, and 2 mm ring apex, are more representative of the true refractive power following LASIK treatment,4–6 we aimed to further identify which corneal power parameters of GALILEI G6 can provide TCRPs that are in greatest agreement with those of Pentacam readings at the 3 and 4 mm zones and 2 mm ring. Additionally, values of central corneal thickness (CCT) and thinnest corneal thickness (TCT) were compared between the two machines.
Volunteers who had previously undergone LASIK surgery and who matched the following criteria were recruited as subjects of this study: (1) more than 6 months had elapsed from the completion of LASIK surgery; (2) they were otherwise healthy adults (without any prior intraocular operations) older than 20 years of age; (3) they did not wear contact lenses within 2 weeks prior to the examination; (4) they had no difficulty cooperating during the evaluation. These subjects were publicly and fully informed of the details pertaining to this study and provided written consent. The institutional review board at our center approved this study.
2.2. Study design
Unassisted vision and optimally corrected vision in both eyes of the subjects were evaluated by an experienced optometrist. Subjects underwent slit-lamp examination to assess their corneal conditions, as well as their levels of eyelid tightness. This examination was followed by a dilated-pupil fundus exam to inspect whether the subjects had retinal or vitreous diseases. Subjects that met the recruitment conditions then underwent Pentacam and GALILEI G6-based corneal assessments as follows.
The inspection was performed in a dark environment; the camera sequence was randomly determined. Subjects were asked to blink their eyes and administer artificial tears to each eye prior to measurement. Each machine assessed the right eye of each subject; the maximum inspection period was approximately 30 min. Quality values for each measurement were shown on both machines. For GALILEI G6 measurements, the components included: motion compensation, Placido, Scheimpflug, and motion distance, which were used to evaluate its quality values. Regarding the Pentacam-based measurement, the apparatus was set to a “25-picture scan” mode, as described previously.7 To reduce errors attributed to anthropic factors, the auto mode of photography was selected.
2.3. Evaluation and statistics
In order to identify which parameters of GALILEI G6 resulted in total corneal refractive powers that strongly accorded with those of Pentacam readings at 3 and 4 mm zones and at the 2 mm ring, the following parameters for the right eye were separately measured by the GALILEI G6: average TCP over the central 4-mm area, mean TCP, and TCP-IOL. The TCP is divided into two types, TCP1 and TCP2, on the basis of which refractive index (ncornea = 1.376 for TCP1; naqueous humor = 1.336 for TCP2) is used to convert ray-traced focal length to power. When determining focal length, the defined reference plane for both is the anterior corneal surface. The value of TCP-IOL is calculated by using the aqueous index of refraction (naqueous humor = 1.336). The posterior corneal surface is regarded as the reference plane when determining the focal length of TCP-IOL. TCP data derived from the GALILEI G6 measurement were compared with those of Pentacam readings at 3 and 4 mm zones and at the 2 mm ring. Additionally, CCT and TCT were compared between the two machines. Paired t-tests were used to analyze statistical differences between Pentacam and GALILEI G6 measurements; Bland–Altman analyses were used to validate agreement between the two measurements.8
A total of 50 otherwise healthy volunteers who had undergone LASIK and met the inclusion criteria were enrolled in this study. Patient characteristics are shown in Table 1. First, the average TCP over the central 4-mm area, mean TCP, and TCP-IOL for the right eyes were measured by the GALILEI G6 system; TCRP values at the 2 mm ring apex, 3 mm zone apex, and 4 mm zone apex were evaluated by the Pentacam system. The mean values of each measurement item ± its standard deviation are shown in Table 2. When comparing data derived from GALILEI G6 measurements with those from Pentacam measurements, we found that mean and central TCP1 values alone were consistent with TCRP values. In contrast, the mean/central TCP2 and TCP-IOL values were significantly lower than the TCRP values at the 2 mm ring apex, 3 mm zone apex, and 4 mm zone apex (P < 0.001). However, Bland–Altman plots of the mean and central TCP1 values (Fig. 1A,B, respectively) demonstrated that the limits of agreement were wide when compared with TCRP values at the 2 mm ring apex, 3 mm zone apex, and 4 mm zone apex (3.2D, 3.2D, and 2.9D; 2.8D, 2.8D, and 2.6D, respectively); thus, the two measurements exhibited poor agreement with each other.
Second, we attempted to compare the agreement of corneal thickness detected by the GALILEI G6 and Pentacam systems. As indicated in Table 3, the average CCT (obtained by GALILEI G6 measurement) of the 50 subjects was 463.64 ± 55.67 μm. Notably, the CCT and TCT values by Pentacam analysis were slightly greater than those from the GALILEI analysis. Nevertheless, paired t-test analyses showed no significant differences in average CCT values between the two datasets. However, Bland–Altman plots of the CCT and TCT values (Fig. 2A,B, respectively) demonstrated that the limits of agreement were wide when the two measurements were compared (140.2 μm and 44.9 μm, respectively). Altogether, our study demonstrated that the corneal refractive power parameters of GALILEI G6 (central TCP1 and mean TCP1) are consistent with, but not identical to, the TCRP readings of Pentacam at 3 and 4 mm zones and at the 2 mm ring. Furthermore, CCT is the most consistent (but not identical) measurement item between the two apparatuses GALILEI G6 and Pentacam for examination of the corneal thickness of subjects who had undergone LASIK.
Corneal assessment is a prerequisite process prior to refractive and cataract surgery. However, corneal power calculation after LASIK remains a challenging task. With the advancement of corneal tomography technology, new measurement approaches have gradually replaced traditional automatic optometry instruments or topography systems. In this study, we measured the agreement of post-LASIK corneal power and corneal thickness measurements by two modern corneal tomography systems—GALILEI G6 and Pentacam—and showed that the values of TCP1 and TCPR and those of both CCT measurements were not significantly different, but were not identical. Therefore, we suggest that the post-LASIK refractive power and corneal thickness measured by GALILEI and Pentacam are sufficiently disparate that the two devices cannot be regarded as identical.
Currently, there are several correction approaches regarding preoperative evaluation of crystalline lens cornea for cataract surgery in patients who have undergone LASIK treatment. For example, pre-treatment refractive power, as well as thickness and curvature changes following corneal incisions, are used to perform post-operative calculation of the total corneal refractive power (TCRP) after LASIK treatment.5 The disadvantage of this method is the lack of pre-operative information (i.e., prior to LASIK treatment). An alternative approach is to conduct regulation analysis by using the Koch and Wang method, or the Shammas method, to calculate the corrected corneal refractive power.9,10 A comparatively more popular approach recognized by clinical physicians, due to its good precision and reliability, is to use regression analysis to design a new intraocular lens power formula (e.g., Shammas post-LAISK formula and the most well-known Hagis-L formula).11–13 However, the true refractive power cannot be acquired by these methods. There are several other methods that utilize a more updated optical apparatus to calculate the true refractive power and curvature of the cornea. This approach can calculate the total corneal refractive power by measuring the corneal curvature of both anterior and posterior surfaces. Currently, several imaging techniques, including slit-scanning tomography, Scheimpflug photography, and optical coherence tomography, are available for such measurements.8,14,15 A growing body of literature has attempted to address the repeatability and reproducibility of these newer corneal tomography machines, as well as their comparability (regarding the corneal refractive power and curvature) with traditional optometry instruments or topography systems.2,8,16–18 Subjects in these studies primarily comprise healthy individuals who have not undergone corneal operations. A minority of these subjects have received corneal refractive surgery and exhibit corneas that are thin with a cone-like bulge. Many studies aimed to determine how to measure the true refractive power of the entire cornea; the recent advancement of corneal topography systems (e.g., Pentacam and GALILEI) has provided a new perspective.
Pentacam is an earlier developed technique that uses rotating Scheimpflug photography to perform corneal assessment.19 A number of previous studies have indicated high repeatability and reproducibility of the Pentacam measurement, regardless of the study population. It has been noted that the total corneal refractive power through the Pentacam ray-tracing method, measured at the corneal radius of 2, 3, and 4 mm, is more representative of the post-LASIK refractive power of the cornea.4–6 GALILEI G6, which utilizes the Placido disc-combined rotating dual-Scheimpflug photography, is a measurement that can acquire more precise parameters between anterior and posterior corneal surfaces. A previous study has shown that GALILEI G6-based measurement can acquire highly repeatable and reproducible data, regardless of whether the subjects have undergone LASIK treatment.20,21 Furthermore, a study showed that the accuracy of the total corneal power and simulated keratometric value derived from GALILEI G6 is statistically equivalent to that of related corneal values demonstrated by the traditional corneal topography apparatus.22 Nevertheless, the subjects of this study were ophthalmology patients who had not undergone corneal operations, and the authors did not indicate which total corneal power parameters (TCP1 and TCP2, or central TCP and mean TCP) were measured in their study. With respect to the comparison between the two apparatuses (Pentacam and GALILEI G6), a study demonstrated that both measurements can acquire highly repeatable and reproducible data for individuals who have not undergone corneal surgery. However, Pentacam can allow for more precise measurement of corneal curvature and refractive power, compared with GALILEI G6.23 Crawford et al. performed a comparison of corneal power measurements obtained by Orbscan II, Pentacam, and GALILEI corneal tomography systems; they concluded that the corneal parameters (e.g., keratometry and corneal thickness) derived from these measurements cannot be regarded as identical.8 Further, GALILEI measurement acquired the most highly repeatable data as compared to the other ones; the subjects of this research also did not undergo corneal surgery. Likewise, Jay et al. compared the measurements carried out in keratoconus patients by Orbscan, Pentacam and GALILEI machines, and suggested that both Pentacam and GALILEI measurements exhibit high reproducibility, though are incapable of producing identical numerical values of the corneal parameters.24 In this study, our data showed that the TCP derived from GALILEI measurement was not agreeable with that from Pentacam measurements, indicating that GALILEI and Pentacam are not interchangeable for post-LASIK evaluation.
Corneal thickness is an important post-LASIK corneal parameter. It also plays a crucial role in risk assessment before undergoing correction of intraocular pressure and second-time corneal refractive surgery.25 The numerical items of corneal thickness demonstrating clinical significance include CCT and TCT. The gold-standard approach used to measure the thickness of cornea is contact ultrasound pachymetry. However, the external ultrasound probe must contact the patient’s cornea; therefore, this procedure potentially results in bacterial infection. It does not measure the true value of CCT, but offers an indirect surrogate approach. In addition, it results in a greater range of variation in different subjects.26 Currently, researchers have dedicated to identify alternative methods that can replace ultrasound pachymetry to measure corneal thickness. Dalraj et al. have suggested that Pentacam tomography is comparatively more advantageous than ultrasound pachymetry in measuring the corneal thickness of the individuals who have undergone corneal refractive surgery or display corneal opacity; it also results in smaller erroneous deviation between examinees.26 Several previous studies have demonstrated that both Pentacam and GALILEI measurements result in highly repeatable and reproducible data with respect to the corneal thickness in subjects before and after undergoing corneal refractive surgery.4,16,27 Park et al. have explored the difference in results of corneal thickness measured by Orbscan, GALILEI, and Pentacam, and compared such data with that derived from ultrasonic measurement. They concluded that both GALILEI and Pentacam measurements, in different subject types, provide comparable results, which are statistically consistent with those measured by ultrasonic pachymetry.7 In our study, however, our data suggested that neither CCT nor TCT is an agreeable parameter when performing measurements in patients who have undergone LASIK treatment.
The limitation of our study is that we only measured once in each machine, which may lead to an erroneous result. However, several previous studies have indicated high repeatability of the Pentacam and Galilei measurement, regardless of the study population.4–6,20,21 Thus, the error may be small. Further, we did not compare to the gold standards for corneal tomography and pachymetry measurements, which may greatly influence the interpretation of the differences between these two machines; currently, it is not appropriate to state which device has the most accurate measurements. Further investigations would help to clarify this problem.
In conclusion, in this study, we showed that the data obtained from GALILEI measurements were not identical to those from Pentacam measurements, when assessing post-LASIK patients. This indicates that GALILEI and Pentacam are not interchangeable for post-LASIK evaluation. Further studies regarding which measurement can more precisely reflect the changes in corneal power before and after LASIK treatment must be performed.
Yun-Hsiu Hsieh was supported by Tri-Service General Hospital, Taiwan -TSGH-C107-178.
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