Potential Influence Of Extraocular Muscles On Surgically Induced Astigmatism Aftercataract Surgery In Eyes With Superonasal Or Superotemporal Clear Cornealincisions

In order to achieve excellent postoperative visual quality, modern refractive cataract surgery puts forward higher requirements for the accuracy of lens power calculation and astigmatism control. Accurate treatment of astigmatism requires precise evaluation of the preoperative corneal astigmatism and surgically induced astigmatism (SIA). Lots of factors could affect the stablity of SIA,and even with small incisions and fixed meridians, SIA is highly variable. In this study, factors that may influence the SIA and cause the variability of SIA between different corneal incisions  were discussed.

This retrospective cohort study was performed at the First Affiliated Hospital of Northwest University.

The first operatied-eye of each patient who underwent a 2.4mm standard phacoemulsification with toric IOL implantation  from September 2021 to February 2022 were included. The SIAs of anterior and posterior corneal surface in 3mm, 5mm, and 7mm  zone were calculated using double angle plot tool, and compared between superotemporal (Group A: n=32) and superonasal incision (Group B: n=37) with independent t test. Correlation analysis and multiple linear regression were preformed between SIA and age, axial length,white-to-white (WTW), anterior chamber depth, and corneal thickness, respectively.

Significant differences were found in Y component of SIA of simulated keratometry and anterior surface at 3mm, 5mm, and 7mm(t=2.467, P=0.016; t=3.254, P=0.002; t=3.105, P=0.003; t=3.162, P=0.002, respectively) between groups, while no difference was found in that of posterior surface and in X component of SIA between groups. The mean absolute values of SIA of group B were overall larger than those of group A, however, significant difference was found only in the posterior surface at 5mm (t=-2.025, P=0.047). The centroids SIA plot indicated the mean axis of anterior surface SIA torque more clockwise from the meridian of the incision in group B. Correlation analysis showed that WTW was the main independent factor for altering SIA in group B.

The orientation of SIA was significant different, especially at anterior corneal surface. SIA torqued more clockwise from the meridian of the incision in the left eye with superonasal incision.  The relative distances between the extraocular muscle terminations and the two types of incisions were different. In the left eye the incision is adjacent to the medial rectus muscle and superior rectus muscle, while in the right eye the incision is adjacent to the lateral and superior rectus muscle. According to the anatomy of the eyeball, the medial rectus and superior rectus muscle terminations were much closer to the superonasal incision. Traction force of extraocular muscle on the incision may play an important role in the orientation of SIA.