Aspheric IOLs put to the test

March 1, 2007

Research into the remarkable workings of the human eye have led to the acknowledgement of the presence of inherent higher order aberrations (HOAs) of the cornea, such as coma and spherical aberration. These aberrations are known to lead to optical degradation with the most significant culprit being spherical aberration.

In response to these discoveries, several companies have been developing a new generation of intraocular lenses (IOLs) - the aspheric IOLs. These lenses either balance or counteract the positive spherical aberrations found on the cornea in order to provide better contrast sensitivity and functional visual acuity, particularly under mesopic and scotopic conditions.

Aspheric IOLs can either be based on negative spherical aberration designs (to correct existing corneal asphericity), or they can be aberration-neutral, to reduce the occurrence of additional and unwanted optical aberrations.

In a Swedish multicentre study, Anders Behndig, MD, PhD, led a team to evaluate these two aspheric lenses, to compare visual acuity outcomes, contrast sensitivity and induction of HOAs following implantation of each lens.

One patient, two lenses: the test begins

Eighty patients with bilateral cataract were enrolled and, using random allocation, one eye received an Akreos AO and the other eye a Tecnis lens. Follow-up was performed one day and ten weeks after surgery, however, the team were able to collect data for 77 Akreos AO and 79 Tecnis implantations 12 weeks post surgery.

Outcomes assessed included high (90%) and low (12.5%) contrast uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA) (logMar, ETDRS chart), photopic and mesopic contrast sensitivity (FACT charts), HOAs (Orbscan – Zywave evaluation; Bausch & Lomb) and responses from a patient satisfaction questionnaire.

Mean high contrast BCVA was also similar for both lenses (Tecnis: 0.06±0.09; Akreos AO: -0.06±0.09), as was low contrast BCVA (Tecnis: 0.12±0.14; Akreos AO: 0.10±0.12). There was no statistically significant difference between the lenses at any frequency regarding contrast sensitivity in either photopic or mesopic conditions.

"We found slightly more aberrations in eyes implanted with the Akreos lens. This was expected and was a consequence of the presence of residual corneal spherical aberration," said Dr Behndig.

Depth of field was significantly greater for the Akreos lens (0.85±0.38 versus 0.71±0.42 for Tecnis), however, perceived quality of vision was similar for both.