Toric IOL alignment
New toric IOL alignment system provides easier workflow but same results as manual technique
A new toric intraocular lens (IOL) axis alignment system, IntelliAxis™ (Topcon), that combines corneal shape analysis with automated iris registration appears to provide only slight advantages in the reduction of astigmatism compared to manual toric IOL axis alignment in femtosecond laser-assisted cataract surgery (FLACS) procedures, according to a study presented by Philipp Hagen PhD at the 21st ESCRS Winter Meeting in Maastricht, The Netherlands.
“There was less scatter in our results with IntelliAxis than with the manual technique, but the differences between the groups were not significant. However, data transfer, workflow and prevention of read-off errors is better with IntelliAxis,” said Dr Hagen, who is a physicist working as a study coordinator and research analyst in a private eye surgery in Düsseldorf, Germany.
In their retrospective study, Dr Hagen and his associates compared the results they achieved in two groups of consecutive patients who underwent FLACS and implantation of a toric IOL. In one group of 84 patients they performed axis alignment with the IntelliAxis system, and in another group of 41 patients they aligned the axis manually.
All eyes underwent implantation of Lentis® Comfort MF15 multifocal toric IOL (Oculentis). The lens has purely refractive optics, with no diffractive ring segments and provides high light transmission. It has a rotationally asymmetric anterior surface with a sector-shaped segment with +1.5D add for improved intermediate visual acuity, Dr Hagen said.
In both groups there was an effective reduction of astigmatism. Mean cylinder was reduced to 0.40D from a preoperative value of 3.00D in the IntelliAxis group, and from 2.77D to 0.45D in the manual group, he noted.
The mean correction index and index of success were closer to ideal values in the IntelliAxis group, with smaller standard deviations than in the manual group. All these advantages were of the order of 10% but differences did not reach statistical significance, Dr Hagen pointed out.
FEWER STEPS FOR FEWER ERRORS
In the manual axis alignment group, Dr Hagen and his team first determined the steepest axis by aberrometry or with a Scheimpflug camera and created two epithelial markings on the cornea’s horizontal axis with a YAG laser.
They then performed lens fragmentation and capsulorhexis with the femtosecond laser and performed phacoemulsification and lens implantation. They then adjusted the steepest axis on Stacy protractor and then aligned the IOL to the steepest axis on that basis.
In the IntelliAxis group, they determined the steepest axis in relation to iris structures with the Cassini (i-Optics) topographer. They then transferred the data from the topographer – using a wireless streamline data link – to the LENSAR® femtosecond laser with IntelliAxis axis alignment software. Just prior to performing FLACS and phacoemulsification, they marked the steepest axis with two opposing laser shots.
“By using corneal shape analysis combined with iris detection, the preoperatively measured steep corneal axis can be marked during cataract surgery by a femtosecond laser in a way that compensates for cyclorotation. Using these landmarks in aligning the toric IOL, this new technique aims for a better correction of astigmatism during FLACS procedures,” Dr Hagen said.
In both groups, they measured manifest refraction and evaluated the reduction of astigmatism using the vector-based Alpins method. He noted that the Alpins formula transforms the astigmatic values into a two-dimensional vector space with components J0 and J45. In this vector space one can consider preoperative, postoperative and target astigmatism and define all their difference vectors and relative angles yielding characteristic quantities such as angle of error, correction index and index of success, Dr Hagen explained.
He noted that the manual method for alignment requires four manual angular matching steps whereas the IntelliAxis system only requires one. Assuming Gaussian error propagation, one can predict a narrower standard deviation in the index of success and the angle of error with IntelliAxis.
“Our findings suggest the IntelliAxis system may provide greater precision. The question that remains is whether this trend will reach significance in larger studies,” Dr Hagen added.
Philipp Hagen: firstname.lastname@example.org