The cornea is the first surface of light gateway to the retina, representing two-thirds of the dioptric power of the human eye, making it the most important refractive element. The parameter most used to describe how the curvature of a parabola differs from the curve of a circle is the asphericity,Q value is one of the most common and important parameters for describing the asphericity of the cornea. The Q value characterizes the change on cornea curvature from the center to the periphery. When Q = 0, it represents a circle, but if Q < 0 or Q > 0, it represents a prolate or oblate ellipse, respectively. 11 The anterior and posterior surface corneal Q value with a zone of 6.0 mm were observed to be -0.11 ± 0.17 and − 0.41 ± 0.30, respectively in this study, whereas, in some earlier studies the corneal mean Q values were reported as -0.22 (Cheung (Chinese), 12 -0.08 (Horner (Indian)), 13 -0.20 (Fuller (American Caucasian)) 4 and − 0.19 (Read (Australian)). 14 The subtle differences between our study and the earlier studies could have been possibly due to the impact of various factors like age, race, sample size, and the differences in testing equipment.
The anterior and posterior corneal surfaces were separately calibrated using the Sirius system in the study, which we had used for a related study before.15 While some other studies had utilized varying methods, like: Pentacam HR system, TMSI mapping system, and the EyeSys corneal topography. Sirius system was routinely employed in research and clinical use, and preceding studies that measured anterior segment parameters have demonstrated the system’s high degree of repeatability and reproducibility. The system’s repeatability is akin to that reported for the Pentacam tomographers.16, 17
Regarding the study of Q values across different ages, Dubbleman et al.18 investigated corneal asphericity in 114 cases (aged between 18–65 years old), finding that the Q values increased with age. Age-related changes in corneal thickness and asphericity are believed to be due to the increase in the number of patients with corneal arcus senilis with age. Guirao et al.,19 examined changes in corneal curvature across three groups, namely, young people (between 20–30 years old), middle-aged people (between 40–50 years old), and elderly people (between 60–70 years old), finding that corneal asphericity progressed and became increasingly circular (from oblate to round) with age. Out of the 1,991 subjects in the study conducted by Davis 20 for the children in the age group of 6 to 15 years old, the mean Q value they determined was found to be -0.346. The subjects in Zhang’s 5 studies were Chinese youngsters at an average age of 25.4 years old with − 0.30 as the Q value. In our study, the average age of the subjects was 67.44 years, the corneal Q value of the anterior surface with 6.0 mm zone was found to be much higher in the group with an age above 70 years than in the group ranging from 38–49 years, which is consistent with the previous study conducted by Dubbleman. However, few studies have measured the Q values of the posterior corneal surface. Our results have shown that the Q value decreased significantly with age across the zone of 4.0 mm, 5.0 mm, and 6.0 mm. A correlation between sex and the Q value was aptly established in our study. Scholz21 and Chan22 arrived at similar conclusions. For the anterior surface in our study, the female group was found to have a smaller Q value than the male group. For the posterior surface, the opposite was found and the difference was statistically significant. However, sex was found to be an irrelevant factor for the corneal Q value in the study conducted by Fuller 4, it may be attributed to the difference in the subject’s age and race.
Wavefront aberration refers to the optical path difference between wavefront and ideal wavefront at each point on the imaging plane of the eye. Some aberrations are the primary causes of glare, halos, and decreased night vision in patients following cataract surgery, including total HOAs, spherical aberration, coma, and trefoil aberration.23 The corneal Q value is a morphological parameter representing the geometric shape of the cornea, whereas corneal aberration (e.g., spherical aberration) describes the optical quality of the cornea and is representative of the degree of corneal optical error. However, few studies have investigated the correlation between Q values and HOAs. Calossi11 analyzed the relationship between asphericity and the degree of spherical aberration of the anterior corneal surface. It was found that, as long as the corneal refractive index and the pupil diameter remained constant, the flatter corneal surfaces from the center to the periphery (negative Q value), the lesser degree of spherical aberration, and steeper corneal surfaces from the center to the periphery (positive Q value) equated to greater degrees of spherical aberration. Our results indicated that both the anterior and posterior surfaces of the cornea correlate significantly with the degree of spherical aberration of the corresponding ranges. That is to say that, alongside higher Q value, the degree of spherical aberration increases accordingly, which is consistent with previous studies. Coma and trefoil were both third-order aberrations, which reflected the asymmetry of refractive characteristics of the eye and were the representation of irregularity, inclination, eccentricity, and other symmetry of the eye. Related studies have indicated that spherical and coma aberration is associated with decreased visual acuity and contrast sensitivity in healthy people. In this study, a positive correlation was found to exist between Q value and the degree of trefoil aberration of the front corneal surface, while a negative correlation exists between Q value and total coma aberration, coma aberration of the corneal back surface, total trefoil aberration, and trefoil aberration of the front corneal surface. These findings suggest that evaluation of corneal Q-value before cataract surgery may have certain significant to the postoperative recovery of visual function and enhancing patients’ visual quality.
There was no significant change in Q value before and after surgery, which indicated that the surgery did not cause corneal morphology change. The possible reason may be that all patients underwent limbal incision with a length of 2.2 mm. Firstly, the incision was small enough and relatively far away from the cornea. Secondly, the change of corneal vertical diameter was limited by the outer edge of the large curved tunnel, so the change of corneal morphology was limited. It is important that the phacoemulsification did not change the corneal Q value, because it is meaningful to observe whether the preoperative corneal Q value can be used as a parameter for personalized selection of IOL, only when there is no significant change in the corneal Q value before and after surgery.
The present study differs from the previous studies in that the corneal Q values for 3.0mm, 4.0mm, 5.0mm, and 6.0 mm zone were tallied separately. It was found that the Q value relates to aperture size and that the average values vary statistically across different diameters. Meanwhile, the Q values of the posterior corneal surface were also examined. Although the posterior corneal surface is seldom relevant in the design of refractive products, it does affect the optical properties of the eye. Prior research around the Q value of the posterior corneal surface is limited. In conclusion, there were great individual differences between the corneal Q values of the cataract patients. Age, sex, and HOAs seemed to be correlated with the Q value. The preoperative Q value can be used as one of the parameters for personalized selection of intraocular lens.