Accuracy of Topography Guided Automatic Marking of the Intraocular Lens (IOL) -Axis

In cataract surgery, the opaque lens is replaced by an artificial intraocular lens. Based on a preoperative measurement of the eye (biometry) the power of the artificial lens is calculated so that refractive errors may be corrected. If high corneal astigmatism is present a correction of this astigmatism is also possible and offered to the patients. This may lead to a certain degree of spectacle independence. The astigmatism is corrected by the implantation of a special toric lens (spherical and cylindrical correction). This toric lens has to be positioned in the eye so that the axis of the IOL-cylinder aligns 90° to the axis of the corneal cylinder. The axis of the corneal cylinder has to be measured and marked before the intervention. Usually, the intended axis of placement for the toric IOL is marked with ink at the slitlamp or with the aid of a manual axis instrument. Anatomical landmarks without preoperative marking are also used. A clinical trends survey of the European society of cataract and refractive surgery (ESCRS) for 2021 has revealed that in 15% of all cataract cases (totally 550'000 cases assessed) a special toric IOL was implanted to correct a clinically significant astigmatism. About 75% use manual marking with ink. Only 25% of cases were operated using a digital image registration. The manual marking using a marking device is currently the gold standard. However, the manual marking might be not very precise. The thick ink-marks on the cornea and the limbus are not very well visible. Sometimes the marks are not perfectly centred on the optical axis of the eye. Head tilt and rotation of the eyeball if patients are not fixating may lead to a deviation up to several degrees. One possible solution of this problem are digital imaging technologies. The Cassini topography instrument for example provides additional to the measurement of the axis and the cylindrical power of the cornea a digital image of the iris. If the cataract surgery is performed assisted by a femtosecond laser (FLACS), this digital image is transferred to the femtosecond laser. Based on this image the software of the laser is capable to determine the correct position of the axis of the cylinder of the cornea and the femtosecond laser can make two intrastromal marks in the cornea. These marks allow the surgeon to align the toric IOL without any preoperative manipulations with ink and marking tools. The aim of this study is to assess the accuracy of topography guided automatic marking of the intended IOL-axis in toric IOL-implantation in femtosecond laser assisted cataract surgery (FLACS)