It is essential to know about the repeatability of instruments under normal conditions to establish its validity in clinical practices and studies. The i.ProfilerPlus, as a wavefront autorefractor, can provide the quantification of both objective refraction and wavefront aberration[10–12]. More information about this device is needed as growing interest in image quality assessment for refractive surgery design, intraocular lens selection, contact lenses evaluation, corneal diseases diagnose, and so forth[13–16]. The present study might provide evidence to help learn more about this wavefront-based device.
The present study showed a high repeatability for measuring corneal and ocular HOAs by i.Profiler, with ICC (both greater than 0.9) and Sw (both less than 0.01µm). Meanwhile, this study verified that the values of corneal HOA [SA (Z40), coma (Z3 − 1, Z31), and trefoil (Z3 − 3, Z33)] and tHOA significantly depended on the pupil diameters; at 2mm ~ 7mm pupil diameter, these values all rose with the increase of pupil diameter. At 3mm pupil diameter, the amount of ocular HOAs was found to be comparable to the previous studies[17]. When the pupil diameter was ≥ 5mm, the increase amplitude of SA became the largest. It implied that SA plays an important role in the factors influencing visual quality. Randazzo[18] pointed out that when the pupil diameter was ≥ 5mm, SA doubled for every 1mm increase, which was consistent with the results of this study. Raymond et al.[19] also compared the corneal HOAs at 3mm ~ 7mm pupil size, and found that HOAs increased with the pupil dilation, among which the corneal SA had the largest variation.
SA, as the variation of focus with aperture, has a great impact on the image quality. With the increase of pupil size, the diffuse spot centered on the optical axis becomes larger and the visual quality becomes worse. In this study, the cornea and ocular SA showed larger ICC and smaller Sw values under different pupil diameters. The TRT values of SA at all pupil diameters were not more than 0.020 µm, except for the TRT of corneal SA (Z40) with 0.022µm at 7mm pupil, indicating that the repeatability of measuring corneal and ocular SA with i.Profiler was excellent. The results also showed that the corneal SA was 0.049 ± 0.016 µm and 0.270 ± 0.040 µm at 4 mm and 6 mm pupil diameters respectively, which were close to our previous measurements of 0.05 ± 0.03µm and 0.28 ± 0.07µm by a wavefront aberration instrument KR-1W based on Hartmann-Shack principle[20]. Similarly, Beiko et al.[21] also obtained a corneal SA value of 0.270 ± 0.089µm at pupil diameter of 6mm by measuring 696 healthy subjects earlier.
Unlike SA, coma and trefoil is not on-axis aberration with axial symmetry. The decentration of pupil increases coma and trefoil, so pupil size is associated with both. In this study, ICC and Sw of the coma and trefoil are similar to these of SA at different pupil. ICC values of negative vertical coma (Z3 − 1) even reached 1.0 at 6mm pupil diameter. Likewise, all the TRT values of coma and trefoil were not more than 0.020 µm, except for ocular trefoil (Z33) 0.022 µm at 5mm pupil diameter. These data all indicated that the repeatability was excellent in measuring the coma and trefoil of cornea and oculi with i.Profiler. It was also found that the corneal and ocular Z3 − 1 was larger than Z31, and this trend increased with the increase of pupil diameter. Studies have shown that a vertical coma increased the depth of field on the vertical meridian and helps improve reading[22, 23]. This feature was used in the optical surface of rotationally asymmetric multifocal IOL (intraocular lens) and was correlated with postoperative visual acuity corresponding to near defocus[24]. In addition, Omaret al.[25] compared the HOAs in keratoconus before and after CXL (corneal collagen cross-linking), and suggested that the corneal vertical coma and trefoil could add value beside keratometric readings.
Ocular aberrations consist of corneal and intraocular (mainly lenticular) aberrations. In this study, the corneal SA at 3mm (mesopic) and 5mm (scotopic) pupil diameter were 0.016 ± 0.007 µm and 0.116 ± 0.031 µm respectively, while the ocular SA were 0.010 ± 0.008 µm and 0.073 ± 0.052 µm respectively. The ocular SA was smaller than corneal SA at the same pupil diameters, thus intraocular SA in young adults is negative. Just like the coupling of two optical systems, the corneal positive aberration and lenticular negative aberrations tend to neutralize each other, thereby achieving optimal image quality and visual function. In our previous studies, it was also confirmed that aspheric IOL can compensate corneal SA and reduce ocular SA, and improve the postoperative visual quality in the darkness[20]. Moreover, the i.ProfilerPlus is capable of obtaining measurements for pupil sizes of 2 to 7 mm. As shown by the measurements, the aberrations are pupil size dependent. This needs to be taken into special consideration when measuring different participants or under different conditions, especially when the measurements are used for clinical applications such as corneal refractive surgery and refractive cataract surgery.
In the present study, a few deficiencies were found in the i.ProfilerPlus. For example, only corneal and ocular aberrations but not intraocular aberrations could be directly measured; further, ocular aberrations were provided only with the pupil diameter of 3mm and 5mm. The values of Kappa angle and Alpha angle were also not available. The improvement in these aspects will facilitate the device to be widely used in clinical practice. This study also had its limitations. Firstly, the included subjects were all normal young volunteers, so the accuracy of the device in measuring diseases affecting the refractive transparency (e.g., cataract) remains to be determined. Secondly, the inclusion criteria did not include moderate to severe refractive errors, so the measurement accuracy of the device in this case is not quite clear. In subsequent studies, the sample should be expanded to these participants of different ages, refractive status, and eye diseases.
In summary, the i.Profiler comprehensively evaluates visual quality by measuring both refraction and wavefront aberrations. The present study demonstrates that measurements of wavefront aberrations by i.Profiler are highly repeatable. The i.Profiler can be used as a reliable aberrometer to measure wavefront aberrations in the clinical practice.