Dr Hirnschall described a recent prospective, multicentre study performed in Vienna (Austria), Castrop-Rauxel (Germany) and London (UK), in which the insufficient astigmatism reduction after toric lens implantation was assessed.
Pre-op measurements cause errors
"Preoperative measurements seem to be the main source of error for our toric intraocular lens (t-IOL) calculations," confessed Dr Nino D. Hirnschall [VIROS (Vienna Institute for Research in Ocular Surgery), Hanusch Hospital, Vienna, Austria] during the 'Assessment of Astigmatism' session, which took place on the first day of this year's ESCRS meeting. Dr Hirnschall described a recent prospective, multicentre study performed in Vienna (Austria), Castrop-Rauxel (Germany) and London (UK), in which the insufficient astigmatism reduction after toric lens implantation was assessed.
Residual astigmatism is associated with significantly reduced postoperative uncorrected visual acuity outcomes for patients; therefore identifying and, if possible, addressing the source of this residual error could help surgeons to offer patients improved postoperative outcomes, increasing patient satisfaction with the technique.
So, to assess at least some of these inaccuracies, Dr Hirnschall and his peers examined three different lenses, the T-flex (Rayner, Hove, UK), AcrySof (Alcon, Fort Worth, Texas, USA) and another toric lens. "Measurements we performed were Scheimpflug images immediately after surgery and then 3 months later," added Dr Hirnschall. "Autorefraction and subjective refraction 3 months post-surgery and then keratometry obviously before surgery and 3 months later." Initial measurements included corneal topography and Scheimpflug imaging and were only performed in 100 eyes of 100 patients.
This multicentre study included 235 eyes of 200 patients who were scheduled for cataract surgery with implantation of a t-IOL. "It was a typical cataract population," said Dr Hirnschall. "Mean corneal astigmatism before surgery was 2.24 D and went up to 5.75 D."
Misalignment of the lens
The first point of the investigation was misalignment of the lens, he noted. This was defined as the deviation between the meridian degree of what was desired from the steep axis and the final positioning of the lens. "So, here we did not consider the measurement error of keratometry as we just simply did not know how large it was," he said.
"So, our mean absolute misalignment was 4º with 50% of our patients within 3º, and we had some severe outliers at 17º," Dr Hirnschall added. "However, this is, of course, just one factor and we wanted to know if there were others that could explain why some of our patients had residual astigmatism after surgery."
Pre-op error of keratometry
"We also tried to, at least, assess the pre-op error of keratometry," he continued. "Of course, you don't actually know what that error is, but we used a difference vector between the keratometry and the topography and then we used a regression model to predict the remianing astigmatism after surgery."
This analysis demonstrated that there was a significant correlation with patients who had a large difference between kertometry and topography readings and those who had quite severe post-op residual astigmatism.
"We can state that t-IOLs, in general, can correct astigmatism very well. Lens misalignment only happens in very severe cases, at least it is not common," said Dr Hirnschall. "The influence of surgically induced corneal astigmatism was very small in our study and we found no other influence relating to age or type of lens and the post-op ACD was only relevant for high toric power lenses."
Therefore, he concluded that although misalignment of the t-IOL is related to residual astigmatism there are other factors and preoperative measurements may be a major factor in causing erroneous IOL calculations.
Dr Hirnschall has indicated no financial interests in the subject matter.