Toric IOL’s transitional conic design enhances tolerance to misalignment

Take-home: A novel toric IOL features an aberration-free, transitional conic anterior optic surface shows pupil independence and improved tolerance to misalignment in bench testing and excellent clinical outcomes.

Reviewed by José L Güell, MD

A new aberration-free, transitional conic toric IOL (Precizon, Ophtec) with good rotational stability and tolerance to misalignment is providing excellent visual and refractive outcomes for patients with pre-existing regular corneal astigmatism undergoing cataract surgery in Europe.

“As little as 0.5 D of astigmatism can have a significant effect on the quality of vision after cataract surgery,” said José L Güell, MD. “Toric IOLs are an option for accurate correction of pre-existing astigmatism.”

The results depend in part on precise IOL alignment because there is a 3.3% loss of cylindrical power correction for every degree that the IOL is off-axis.

“Although newer techniques have improved the accuracy of intraoperative alignment to the intended meridian and newer toric IOLs demonstrate good rotational stability, it would also be advantageous if an IOL has a high tolerance of misalignment in case there is postoperative rotation,” explained Dr. Güell, associate professor, ophthalmology, Autonoma University; director of Cornea and Refractive Surgery, Instituto Microcirugía Ocular (IMO), Barcelona, Spain.

“Because of its unique transitional conic toric surface, the Precizon IOL has a broader toric meridian that might be more tolerant to misalignment,” he added. “Clinical experience indicates that the refractive outcome is not significantly affected by minimal misalignment (<5°).”

IOL’s design

The transitional conic toric IOL is a single-piece hydrophilic acrylic lens with C-loop haptics that measures 12.5 mm in overall length. It is available in a wide dioptric range and in cylinder powers of +1.00 D to +10.00 D in 0.5 D increments.

It has a 6-mm biconvex optic with a 360° square edge. The toricity correction is on the anterior surface, which has neutral asphericity, and the posterior surface is spherical.

The hallmark of the IOL is its transitional conic surface that is designed to confer pupil independence and enhanced tolerance to misalignment. Evidence of success in achieving these goals is available from a bench study by Kim et al. that evaluated four toric IOLs [J Cataract Refract Surg. 2015;41(10):2274-2282].

“These investigators reported that the transitional conic implant and another aberration-free toric IOL provided better image quality across different pupil diameters and despite decentration compared with two negative aspheric models,” said Dr. Güell. “Of the four IOLs, however, the transitional conic toric IOL also had the greatest tolerance to rotation-induced loss of contrast.

“The excellent pupil-independent behavior of this IOL makes it an ideal choice for younger patients who tend to have larger pupils,” he added.

Tolerance to misalignment was also demonstrated in a clinical study reported by Erik Mertens, MD, at the XXXII Congress of the European Society of Refractive Surgery. Dr. Mertens, director and ophthalmic surgeon, Antwerp Eye Centre, Belgium, used intraoperative aberrometry to measure refraction in patients receiving the transitional toric conic IOL when the implant was properly aligned and when it was rotated off-axis by 5° and 10°.

Personal experience

Dr. Güell evaluated the clinical performance of the transitional conic toric IOL for correcting regular corneal astigmatism >-0.75 D in a prospective, nonrandomized case series of 165 eyes of 144 patients who underwent uncomplicated cataract surgery and were followed for a minimum of 6 months. Biometrical data for IOL power calculation were obtained using optical coherence biometry (IOLMaster 500, Carl Zeiss Meditec) and IOL power calculations were performed using Ophtec’s software.

Preoperatively, eyes were marked with a manual marker (Robomarker, Surgilum). In some cases, a computer-guided, visualization system was used to assist with intraoperative alignment of the toric IOL (TrueGuide, TrueVision 3D Surgical). In most patients, a blended micromonovision approach was used to improve both near and distance uncorrected visual acuity (UCVA).

Preoperatively, axial length averaged about 24 mm, but about one-fourth of eyes had an axial length >25 mm and 4% were <21 mm. Mean SE was -0.59 D ± 3.84 D and mean refractive cylinder was -1.61 D ± 1.38 D. Thirteen eyes had a history of LASIK, 7 eyes had a phakic IOL, and 3 eyes had undergone penetrating keratoplasty.

There were no intraoperative complications, and Dr. Güell highlighted the user-friendliness of the manufacturer’s disposable injector and cartridge system (DualTec Kit, Ophtec) and implantation.

“Loading the IOL into the cartridge is easy,” Dr. Güell said. “The delivery can be done by pushing on the plunger with one hand or–in what is my favorite technique­­–with a twisting motion using two hands. Once released into the capsular bag, the IOL can be gently manipulated and rotated until reading its correct position.”

At 6 months after surgery, mean binocular distance UCVA had improved from 0.32 D preoperatively to 0.77 D. Distance UCVA was 20/40 or better in 89.4% of eyes, and mean binocular near UCVA improved from 0.41 D preoperatively to 0.66 D.

Mean binocular distance BCVA also improved significantly from 0.73 D preoperatively to 0.86 D. No eyes lost BCVA, almost 60% gained one or more lines, and 95% achieved 20/40 or better distance BCVA.

Both the overall refractive and cylinder results showed good accuracy. Attempted mean spherical equivalent (SE) was -0.46 D and was -0.57 D at 6 months. The SE results were stable over time with minimal change between 3 and 6 months.

Refractive cylinder was reduced from an average of -1.05 D at 3 months to -0.87 D at 6 months. The relatively high residual astigmatism in this series was accounted for by suture placement and removal in eyes that underwent phakic IOL explantation.

“The explantation procedure requires a 5-mm incision that needs to be sutured,” Dr. Güell explained. “Until the suture is removed, there is a significant amount of induced astigmatism.”

Postoperatively, no IOL rotated more than 3°, but three (1.7%) IOLs required realignment due to intraoperative positioning error. In all three eyes, intraoperative alignment was guided by the preoperative manual markings.

“Perhaps routine use of the computer-guided surgical system will reduce alignment errors,” said Dr. Güell. “As with other toric IOLs, there may be greater potential for rotation of the Precizon in longer eyes that have larger capsular bags.  Implantation with an intracapsular tension ring may provide improved stability.”

Twelve (7%) eyes with a residual spherical defect underwent LASIK enhancement and were corrected to achieve the targeted micromonovision.

José L Güell, MD

E: guell@imo.es

This article is based on a presentation given by Dr. Güell at the 2016 American Society of Cataract and Refractive Surgery meeting. He is a consultant to Ophtec.