ELIMINATING REFRACTIVE SURPRISES

New technologies that enable intraoperative aberrometry in eyes undergoing cataract surgery may provide better visual outcomes than surgery performed with conventional preoperative aberrometry alone, but new preoperative optical measurement techniques may produce equally accurate results, according to participants in a debate held at the JCRS session of the XXX Congress of the ESCRS. stephen Lane MD opened the discussion by stating that intraoperative aberrometry can bring the refractive results of cataract surgery closer to those commonly achieved with LASIK.

“Greater than 95 per cent of LASIK surgery outcomes are within half a dioptre of their nomogram-predicted refractive target and 90 per cent of eyes achieve an uncorrected distance visual acuity of 20/20 or better and the enhancement rate of less than three per cent. if we could do that in our present day cataract surgery we would certainly be doing our patients a tremendous service,†said Dr Lane, University of Minnesota, st Paul, Us.

He noted that data from the swedish national cataract registry showed that among 17,000 eyes undergoing cataract surgery procedures aiming for emmetropia, only 55 per cent had a spherical equivalent within half a dioptre of emmetropia and had less than one dioptre of astigmatism (Behndig et al, J Cat Refract Surg 2012 ; 38: 1181-1186).

In contrast, of 1,314 patients who underwent cataract surgery with intraoperative aberrometry, 80 per cent achieved a postoperative refraction within half a dioptre of target values. The intraoperative aberrometry technology used in the patients was the Optiwave™ refractive analysis system or ORA system™ by WaveTec. “This was not quite the 94 per cent that we were aiming for with LASIK, but it’s certainly a lot better than what we see with standard preoperative biometry,†Dr Lane said.

The ORA system attaches to the base of most surgical microscopes and enables a real-time evaluation of true refractive power of the eye in each case. in addition, every ORA system is connected live to the Wavetec servers in order to continually update the IOL calculation algorithms based on the results achieved, Dr Lane said.

He pointed out that, in 72 per cent of cases, surgeons changed the power in conventional IOLs and the power and/ or cylinder axis rotation in toric iOLs based on the findings of intraoperative aberrometry. Preoperative biometry alone would have resulted in at most around 70 per cent of eyes being within half a dioptre of target refraction.

Furthermore, the intraoperative refractive information and potential for surgical course correction the ORA system provides makes it particularly useful with toric iOLs, he said. The intraoperative prediction error was less than one half of a dioptre in 82 per cent of patients. The system is also very effective in patients that have undergone previous myopic LAsiK, among whom 72 per cent were within half a dioptre of intended refraction. “The refractive surprises that we sometimes see in these very difficult to manage patients disappear with the use of this technology.

Preoperative aberrometry improving in accuracy

Intraoperative aberrometry shares some inherent limitations with preoperative aberrometry and it cannot be used in all patients, countered Paul-Rolf Preussner MD, PhD, University Clinic Mainz, Germany. The optical characteristics of the eye can change after surgery. The shrinkage of the capsular bag can change the defocus and centration of the iOL, and can cause toric iOLs to rotate, he said.

In addition, intraoperative hydration of corneal incisions can result in corneal thickness variations, which in turn cause an asymmetry of the cornea. in addition, since the patient needs to fixate in order to align the measurement with the visual axis, intraoperative aberrometry is only possible when using topical anaesthesia. it cannot be used in eyes requiring peribulbar or general anaesthesia. Furthermore, true state-of-the-art technology in preoperative aberrometry can produce refractive results with a refractive predictability very close to that of spectacle prescription.

To illustrate his point, he presented the results he and his associates achieved in 115 eyes that underwent cataract surgery with iOL calculations performed using OKULiX ray tracing software (Tedics) and the LenstarR optical biometer (haag-streit). The patients in the study were part of a larger trial and were selected for the analysis because they had achieved best corrected visual acuity of 20/20 or better and were implanted with aspheric iOLs (Hoffmann et al, J Refract Surg 2012; 28:650-655).

In all eyes, Dr Preussner and his associates measured axial length, corneal radii and the position and thickness of the crystalline lens with the Lenstar system and entered the data into the OKULiX ray tracing program.

At a follow-up of at least one month the standard deviation of the prediction error was 0.29 D, the mean absolute error was 0.21 D and the median absolute error was 0.17 D. One hundred per cent were within 1.0 D and 91 per cent were within 0.5 D. The said prediction errors are approximately a factor of two smaller than the corresponding errors obtained after re-digitising the data of Chen M, Clin Ophthalmol 2011; 5:197-198 who compared measurements of intraoperative aberrometry with post-surgery refraction. The same ray tracing approach also produced highly accurate results in 80 eyes with toric iOLs, where the mean and median cylinder vector prediction errors were 0.51 D and 0.41D, Dr Preussner said (data presented by P Hoffmann on the meeting in Milan).

Furthermore, the ray tracing and optical biometry approach to preoperative aberrometry largely solves the problem of biometry in patients who have previously undergone myopic LASIK, he said. As an illustration, he presented his findings in 12 eyes with a history of myopic LASIK all of which had a prediction error of less than 0.5 D. “The problem of biometry in postmyopic LASIK eyes is a solved problem when we use a physical rather than a statistical approach. That means we have to measure the whole cornea, both the anterior and posterior corneal surface. When we submit that data to the ray tracing software we end up with the same accuracy as in virgin eyes,†he added.