Each step of toric IOL procedures poses a potential source of error
Determining the cause of astigmatic refractive surprise in patients with toric IOLs involves a step-by-step review of the case, from preoperative biometry and on to the early postoperative weeks, said Alja Crnej MD, Slovenia, at the 22nd ESCRS Winter Meeting in Belgrade Serbia
“You need to be thinking of the time prior to the operation, during the operation and of course after the operation. All three different times, things can go wrong,” said Dr Crnej, who presented the paper on behalf of Nino Hirnschall MD, PhD, Hanusch Hospital in Vienna, Austria.
Dr Crnej noted that the process of elimination should start with the most basic questions about the lens and the patient. A quick check should be made to ensure there were no mix-ups in the paperwork leading up to the operation that might have resulted in the patient receiving the wrong lens.
If the patient received the assigned lens in the correct eye, the next step of the investigation is to redo the keratometry. She noted that, in a study Dr Hirnschall and his associates conducted involving 1,500 patients who had undergone IOL implantation, the main source of error was measurement of the cornea. Other factors, such as the axial eye length measurement and the measurement of the cornea, are a much smaller source of error.
Irregular astigmatism can also reduce the efficacy of toric IOLs, Dr Crnej said. Patients with a high level of irregular astigmatism probably have a lot of higher-order aberrations and are unlikely to gain much visual improvement through such lenses.
Another thing to consider is whether the correct formula was used. The same data entered into different formulas for toric IOL calculation will often yield different results.
“Probably the best thing to do before we decide which toric IOL to implant is to do calculations with more formulas. We can compare them and then decide,” Dr Crnej said
Surgically induced astigmatism can also be a factor in disappointing toric IOL results. Although modern cataract surgery induces on average only around 0.6D of astigmatism, there can be outliers with significant amounts of induced astigmatism, she pointed out.
And finally, there is the question of alignment errors, which can result from improper orientation of the patients’ head during examination, mistakes during surgery and spontaneous IOL rotation postoperatively, she added.