Three-step manual marking method skews toric IOL alignment, researchers find

Study shows how errors in reference axis markings, alignment axis markings and IOL alignment may result in significant residual astigmatism.

Nienke Visser

A popular three-step ink marking procedure for toric pseudophakic and phakic IOL implantation produced significant alignment error, a study found.

The manual marking method yielded a mean alignment error of about 5°, the study authors said.

“Every degree of misalignment contributes to residual astigmatism,” Nienke Visser, MD, the lead author, said in an email interview. “Orienting the toric IOL with great accuracy is necessary in all patients to achieve the most optimal cylinder correction.”

Study results were published in the Journal of Cataract and Refractive Surgery.

IOL alignment, astigmatism correction

The prospective cohort study included 40 eyes of 31 patients; 26 eyes were pseudophakic and 14 were phakic. Mean patient age was 58.8 years in the pseudophakic group and 40.1 years in the phakic group. The between-group age difference was statistically significant (P = .001).

Patients receiving toric pseudophakic IOLs (26 eyes) underwent cataract extraction and implantation of an AcrySof toric SN60T3-T9 pseudophakic IOL (Alcon). Eight eyes were implanted with an Artiflex toric phakic IOL (Ophtec); six eyes received an Artisan toric phakic IOL (Ophtec).

The three steps of toric IOL implantation — reference axis marking, alignment axis marking and IOL alignment — were analyzed using digital images. In addition, manifest refraction was performed in all patients 3 months postoperatively.

Study results showed that mean error in reference axis marking was 2.4°. The mean error in alignment axis marking was 3.3·. Mean error in toric IOL alignment was 2.6°.

Cyclotorsion of the eye with and without reference marks was no more than 5° in any case. The reference marks deviated more than 5° from the calibrated horizontal axis in one eye. Combined, these two errors resulted in the mean error in reference axis marking of 2.4°.

The mean difference between the marked alignment axis and the intended alignment axis was more than 5° in six eyes (15%). No eye had a difference of more than 10°.

Mean IOL alignment error was more than 5° in four eyes (10%) and more than 10° in one eye (2%).

Combined, the three marking steps resulted in a mean total error in toric IOL alignment of 4.9°. Differences in mean error between pseudophakic IOL and phakic IOL alignment were statistically insignificant.

Other factors contributing to alignment error include faded ink markings, horizontal or vertical translocation of ink marks or erasure of ink marks at the time of surgery, she said.

Vector analysis of errors

Investigators used the Alpins method of vector analysis to obtain mathematical confirmation of the physical accuracy of toric IOL placement, Dr. Visser said.

Vector analysis showed a mean angle or error of –2 ± 8° for pseudophakic IOLs and 6 ± 14° for phakic IOLs, the authors reported.

“The angle of error obtained in the vector analysis is not directly comparable to the physical error in toric IOL alignment because of the subjective component of the refractive outcome, the influence of the incision and possibly the effect of other refractive surfaces of the eye (posterior corneal surface and vitreous),” she said. “However, the relatively large [standard deviations] of the calculated angles of error indicate that this angle of error was much larger in some individual patients.”

Overall results showed that 99% of astigmatism was corrected in the toric pseudophakic IOL group and 98% in the toric phakic IOL group. In addition, the toric pseudophakic IOL corrected astigmatism more effectively than did the toric phakic IOL, Dr. Visser said.

“We believe this may be the result of not incorporating the flattening effect of the incision in the [phakic] IOL power calculation,” she said.

Dr. Visser elucidated two analytic approaches based on literature concerning the effect of toric IOL misalignment on residual astigmatism.

“The first approach is based on the flattening effect,” she said. “Vector analysis is used to determine the amount of astigmatism reduction achieved at the intended meridian of treatment (flattening effect). Using this method, an error of 4.9° would lead to 1.5% of preoperative astigmatism remaining at the intended meridian of treatment.”

The second approach determined overall magnitude of residual astigmatism remaining. It is calculated by determining the vector difference between the targeted and achieved astigmatic outcomes. Using this method, an error of 4.9° would result in residual astigmatism that equaled 17% of the preoperative value, Dr. Visser said. – by Matt Hasson

Reference:

• Visser N, Berendschot TT, Bauer NJ, et al. Accuracy of toric intraocular lens implantation in cataract and refractive surgery. J Cataract Refract Surg. 2011;37(8):1394-1402.

• Nienke Visser, MD, can be reached at Department of Ophthalmology, Academic Hospital Maastricht, P. Debyelaan 25, 6202 AZ Maastricht, Netherlands; email: nienke.visser@mumc.nl.
• Disclosure: Dr. Visser has no relevant financial disclosures.