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Astigmatism correction: high-tech and surgical expertise

We have asked a few questions to Noel Alpins, Australian cataract and refractive surgeon, who is one of the most renowned expert in the technological aspects related to the planning of surgical procedures for astigmatism correction and analyzing the postoperative outcomes.

When and why have you started to study in depth the technological problems related to a best surgical outcome in eye surgery?
N.A.: My interest in analyzing outcomes started in the early 90s. At the time I was performing AK (Astigmatic Keratotomy) procedures for astigmatism and placing the incisions on the steepest corneal meridian. In contrast, for excimer laser surgery which had just been introduced, the maximum ablation activity for correcting astigmatism was 90 degrees away from the steepest meridian where the incisional action occurred. This was my first step into understanding planning of surgical procedures for astigmatism and analyzing outcomes. This led to the Alpins Method of astigmatism analysis which can be applied to both refractive and corneal outcomes. It involves understanding astigmatic targets and how to achieve them including non-zero targets where corneal and refractive values differ and all the preoperative astigmatism cannot be surgically corrected. For example, when selecting stock toric IOL which come in either 0.50 D or 0.75 D steps – the corneal astigmatism that is neutralized may be in between these steps as measurements are now accurate to 1/100’ths of a diopter, so from the outset you cannot theoretically achieve plano cylinder in the postoperative refraction in the majority of cases

Why is the position of the incision so important for toric IOLs power calculation?
N.A.: The position of the phaco incision for toric IOLs is of paramount importance. Any effect it has on the preoperative corneal astigmatism must be factored into the toric IOL selection process, as well as its placement. The surgeon must calculate the effect of the incision on the preoperative corneal astigmatism – both for change in magnitude and/or change in meridian. The selection of the toricity for the IOL will then be more accurate. It is important to note that placing the phaco incision on the steep meridian of the preoperative astigmatism will have the maximum flattening effect. Placing the incision away from the steep corneal meridian will cause less of an effect together with rotation of the preoperative corneal astigmatism. As it approaches 90 degrees away from the steep meridian, so the astigmatism increases and less rotation occurs.

Why is it better to refer to the Flattening Effect of the primary incision instead of the whole SIA (Surgically induced astigmatism vector) for IOL power calculation?
N.A.: The SIA is composed of both the Flattening Effect (FE) and the Torque:
The FE is the amount that has acted on changing the magnitude of the preoperative corneal astigmatism AND the Torque is the component involved in rotating the astigmatism from its preoperative meridian to its postoperative meridian, but not reducing it.
Using the whole SIA over estimates the effect of the incision in reducing astigmatism and leads to an inaccurate calculation of the corneal astigmatism or positioning of it; consequently and inaccurate calculation of the toricity of the IOL required to neutralize the corneal astigmatism.

In your opinion the latest technological innovation can really improve the management of the “refractive surprise” in the post-op of astigmatism cataract refractive surgery?
N.A.: There are basically 3 options available to manage refractive surprise after toric IOL surgery:

  1. Rotation of the implanted toric IOL – using the Assort toric IOL calculator (www.assort.com) the minimum amount of refractive cylinder that can be achieved by rotating the toric IOL is calculated – if this reduces the refractive cylinder surprise significantly to below 1.0D, then it is just a matter of rotating the implanted toric IOL to the calculated axis. Ideally this should be done 4-6 weeks post cataract surgery.
  2. Replacement of the implanted toric IOL – again using the Assort toric IOL calculator, analysis using the Alpins Method displays the Magnitude of Error (ME) which is the difference in magnitude between the Target induced astigmatism vector (TIA) and the Surgically induced astigmatism vector (SIA) and informs us about whether the toric IOL implanted is too strong or too weak for the astigmatism correction. If this is significant (0.75 D or greater) then exchange of the toric IOL with an adjusted toricity or the use of a supplementary lens in the ciliary sulcus is recommended.
  3. Excimer laser surgery – if rotation of the toric IOL implanted does not significantly reduce the refractive cylinder surprise and the ME is less than +/- 0.75 D, then refractive laser surgery is recommended to correct the remaining astigmatism from what is likely multiple causes.

Finally, are there great differences in the approach to astigmatism treatment in Europe and in Australia?
N.A.:
The approach to astigmatism in Europe and Australia is very similar- we both have the advantage of the latest technology available which includes the latest design IOLs, surgical and diagnostic devices.
My personal approach to the surgical correction of astigmatism depends on the magnitude of astigmatism that requires correcting:
– 0.50 – 0.75 D single clear corneal incision of 2.2 mm
– 1.00 – 2.00 D single on axis clear corneal incision (3.0 mm) + single LRI (180 degrees away from clear corneal incision)
– >2.00 D toric IOL with 2.2 mm phaco incision

When there are no signs of lenticular opacities and patient is suitable for refractive laser surgery I use vector planning to optimally treat astigmatism.
In many cases where ocular residual astigmatism (ORA) exists due to the corneo-refractive differences, vector planning reduces the amount of corneal astigmatism remaining postoperatively compared to treatment by refractive parameters alone without increasing the refractive cylinder remaining postoperatively. Vector planning incorporates both refractive cylinder and corneal astigmatism into the treatment plan by using an optimized approach with both corneal and refractive cylinder parameters taken care of.

Noel Alpins, MD
Dr Alpins has been practising cataract surgery and refractive surgery since founding NewVision Clinics in the 1990’s. He nationally and internationally recognised as a leading cataract and refractive surgeon and a leading world authority on myopia and astigmatism treatment with the excimer laser. Dr Alpins began performing refractive surgery in 1985 introducing the technique of radial keratotomy at that time, and was one of the first surgeons in Australia to use the Laser Vision correction techniques. Since 1991, he has treated over 20,000 patients with the Excimer laser.
He has invented new methods for treating and analysing astigmatism, and has developed the ASSORT surgical planning outcomes analysis software program that is used by ophthalmologists in Australia and overseas. He speaks widely on these topics at national and international meetings and has been a keynote speaker and chair on many occasions.

Read this  in Italian Correzione dell’astigmatismo: tecnologie d’avanguardia e esperienza chirurgica

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Pubblicato il
venerdì, 29 aprile 2016
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Area Chirurgica