Modern cataract surgery can be an effective refractive procedure. There is a growing demand for this surgery considering the influence of astigmatism on uncorrected visual acuity and minimizing it during cataract surgery. This study observed the changes in ocular astigmatism in patients with low to moderate corneal astigmatism after cataract surgery. It analysed the differences in astigmatism changes caused by the temporal and superiortemporal corneal incisions.
Lenticular astigmatism has been considered the most common cause of internal astigmatism.13 Toam Katz et al.14 found that the magnitude of the ocular residual astigmatism (ORA) decreased after cataract surgery. Consistent with Toam Katz’s study, this study also found that the magnitude of non-corneal ocular residual astigmatism significantly decreased post-operatively, that is, from a mean magnitude of 1.17D orientated along an axis of 51.44 to a mean magnitude of 0.73D orientated along an axis of 46.58. As known, astigmatism arising from the anterior corneal and anterior lens surfaces is almost completely compensated for by the posterior corneal and posterior lens surface in young healthy subjects.15 In the process of cataract formation, as the lens opacity increases, internal astigmatism caused by the lens opacity may gradually increase. The posterior lens surface cannot compensate for it. Therefore, after cataract extraction, the internal astigmatism of cataract patients decreases significantly.
It is believed that ORA in pseudophakic eyes mainly results from the posterior corneal surface.14 A significant but weak correlation was found between the magnitudes of internal astigmatism and posterior corneal astigmatism in the pseudophakic eyes.16 However, calculation of ORA in previous studies was determined from the vector difference between the refractive cylinder and the anterior topographic astigmatism, which did not exclude the influence of the posterior surface of the cornea. In this study, the N-CORA, which was determined from the vector difference between the refractive cylinder and the total corneal surface topographic astigmatism, was calculated. The magnitude of corneal astigmatism, including anterior, posterior, and total cornea surface, remained stable after surgery. Moreover, to remove the influence of corneal astigmatism, there was still residual astigmatism of 0.73D. Hence, ORA in pseudophakic eyes perhaps also resulted from the implanted IOL.
Previous studies have shown that the tilt of the IOL can induce ORA, and IOL-induced astigmatism increased with increasing IOL tilt.17 Five degrees of IOL tilt induced astigmatism from 0.08D to 0.14D and ten degrees of IOL tilt induced astigmatism from 0.33D to 0.56D. In this study, the average tilt of IOL was 1.78 degrees, which was small. At the same time, variations in the N-CORA according to IOL tilt and IOL decentration were not significant. Hence, an internal astigmatism caused by this degree of IOL tilt may be relatively small. The ORA in pseudophakic eyes might also be the main result of the shrinkage and proliferation of the posterior capsule during the 6 months after cataract surgery.
Incision sizes have a significant influence on postoperative astigmatism, and a 3-mm temporal incision is considered astigmatically neutral.18 Many previous studied demonstrated that surgically induced astigmatism (SIA) reduced significantly when the incision size was reduced from 3.0 mm to 2.2 mm.19 However, this difference decreases over a more extended period of time.20 At the same time, this significant decrease was not found when the reduction of incision size from 2.6 mm to 2.2 mm with respect that SIA was similar between the 2.2-mm and 2.6-mm groups.21 In this study, cataract surgery using a 2.6-mm incision size and follow-up for 6 months after the operation was performed. No significant changes in postoperative astigmatism compared with preoperative were observed, indicating that corneal astigmatism did not worsen postoperatively using the 2.6-mm incision size, no matter whether it is using the superotemporal or temporal clear corneal incision.
Cataract surgery can be performed with clear corneal incisions in several sites, and different incision sites can induce different degrees of SIA. A nasal clear incision was usually not used because it often associated with a higher SIA. Though an on-axis incision method is the most effective method for astigmatism correction, temporal and superotemporal sites were more popular than on-axis owing to better exposure of the surgical field and more comfort while operating. Hence, in this study, the differences in astigmatism change caused by the superotemporal and temporal incisions were compared.
Previous studies generally considered that temporal incisions are to be associated with less SIA. However, some studies have demonstrated22 that superior and temporal incision sites make no difference in regular and irregular SIA after surgery. In this study, a significant difference in J45 of surgically induced corneal astigmatism at the anterior cornea and total cornea surface between the R and L groups was found. A significant difference in the meridian of postoperative anterior corneal and postoperative total corneal surface astigmatism was also found. This significant difference represented the difference in oblique astigmatic change components with axes at 45 degrees and 135 degrees. However, this difference only causes a change in the astigmatism axis after surgery but has no significant effect on the total magnitude of astigmatism. This finding is because no significant difference between the two groups in the magnitude of corneal SICA persists after considering J0. There is no difference in the magnitude of corneal astigmatism postoperatively. Hence, superotemporal and temporal incisions could induce meridian differences in oblique astigmatic change components. And this difference is ultimately presented in the significant difference in the meridian of postoperative corneal astigmatism in the anterior corneal and total corneal surface. However, it does not influence the magnitude of each astigmatism component.