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Astigmatism correction: cutting-edge technology and surgical experience
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We put some questions to Noel Alpins, an Australian ophthalmologist specialising in cataract and refractive surgery and one of the leading experts in the technological aspects of planning surgical procedures for astigmatism correction and analysing post-operative results.
We put some questions to Noel Alpins, an Australian ophthalmologist specialising in cataract and refractive surgery and one of the leading experts in the technological aspects of planning surgical procedures for astigmatism correction and analysing post-operative results.
When and why did you start working on technological issues related to improving eye surgery results? N.A.: My specific interest in outcome analysis started in the early 1990s. At that time, I used the radial keratotomy technique for the correction of astigmatism and placed the incisions on the most curved corneal meridian. In contrast, in excimer laser surgery, which had already been introduced, the maximum ablation for astigmatism correction was 90° from the most curved meridian on which the incision was made. This was my first step in understanding the issues involved in planning surgical techniques for astigmatism correction and analysing the results. This led to the Alpins method for astigmatism analysis that can be applied to both refractive and corneal outcomes. This method involves identifying astigmatic targets and how to achieve them by also considering non-zero targets where corneal and refractive values are different and not all pre-operative astigmatism can be corrected surgically. For example, when selecting a toric IOL from a stock, in which it is available in both 0.5 D and 0.75 D steps - the neutralised corneal astigmatism may be between these two steps, since the measurements are now accurate to 1/100 diopter, so that it is theoretically not possible to reach the plano cylinder in the post-operative refraction in most cases.
Why is the position of the incision so important when calculating the power of toric IOLs? N.A.: The position of the phaco incision for toric IOLs is of paramount importance. Whatever effect it has on pre-operative corneal astigmatism must be taken into account in the IOL selection process, as well as in its placement. The surgeon must calculate the effect of the incision on pre-operative corneal astigmatism - as far as magnitude changes and/or meridian changes are concerned. The selection of IOL toricity will, therefore, be more accurate. It is important to note that placing the phaco incision on the most curved meridian of pre-operative astigmatism will have the greatest flattening effect. Placing the incision in a location other than the most curved meridian will cause a lesser effect along with the rotation of the pre-operative corneal astigmatism. As one gets closer to 90° from the most curved meridian, the astigmatism increases and less rotation is caused.
Why is it better to refer to the Flattening Effect of the primary incision instead of the overall SIA (Surgically induced astigmatism) vector for the calculation of IOL power? N.A.: The SIA is composed of theFlattening Effect and Torsion (Torque).
The Flattening Effect is the measure of how much the magnitude of the preoperative corneal astigmatism has changed while the Twist is the component involved in the rotation of astigmatism from the preoperative meridian to the postoperative meridian, but not in its reduction.
Using the entire SIA overestimates the effect of the incision in reducing astigmatism and leads to an inaccurate calculation of the corneal astigmatism or its location; consequently to an inappropriate calculation of the IOL toricity required to neutralise the corneal astigmatism.
Do you think that the latest technological innovations can really improve the management of 'refractive surprise' in post-operative refractive cataract surgery for the correction of astigmatism? N.A.: There are basically three options for managing refractive surprise after toric IOL surgery:
Rotation of the implanted toric IOL - using the Assort calculator for toric IOLs (www.assort.com) the minimum value of the refractive cylinder that can be achieved by rotating the toric IOL is calculated - if this reduces the refractive cylinder surprise significantly below 1.0 D, then it is only a matter of rotating the implanted toric IOL towards the calculated axis. Ideally, this should be done 4-6 weeks after cataract surgery.
Replacement of implanted toric IOL - Again using the Assort calculator for toric IOLs, the analysis with the Alpins Method shows the Error Magnitude (ME) which is the difference in magnitude between the Target Induced Astigmatism (TIA) and the Surgically Induced Astigmatism (SIA) vector and allows us to know if the implanted toric IOL is too strong or too weak for astigmatism correction. If this parameter is significant (0.75 D or higher) then replacement of the toric IOL with a toric corrected lens or the use of an additional lens in the ciliary sulcus is recommended.
Excimer laser surgery - if rotation of the implanted toric IOL does not significantly reduce the surprise of the refractive cylinder and the ME is less than +/- 0.75 D, then laser refractive surgery is advisable to correct the residual astigmatism that is probably the result of a multiple set of causes.
Finally, are there major differences in the treatment approach to astigmatism in Europe and Australia? N.A.: The approach to astigmatism is very similar in Europe and Australia - in both contexts we have the advantage of having the most advanced technology available, including the latest in IOL design, surgical and diagnostic equipment.
My personal approach to surgical correction of astigmatism depends on the extent of the astigmatism to be corrected:
– 0,50 - 0,75 D a single 2.2 mm incision in a clear cornea
– 1,00 - 2,00 D a single clear corneal axis incision (3.0 mm) + a single LRI (180° from the clear corneal incision).
– >2.00 D Toric IOL with 2.2 mm phaco incision
If there are no signs of lenticular opacities and the patient is suitable for laser refractive surgery I use vector planning for optimal treatment of astigmatism.
In many cases where there is residual ocular astigmatism (ORA - Ocular Residual Astigmatism) due to corneo-refractive differences, vector planning reduces the amount of residual corneal astigmatism post-operatively compared to treatment based on refractive parameters alone without increasing the residual refractive cylinder post-operatively. Vector planning incorporates both refractive cylinder and corneal astigmatism into the treatment plan, using an optimised approach that takes into account both corneal parameters and refractive cylinder.
Noel Alpins Dr Alpins has dedicated himself to cataract and refractive surgery since founding NewVision Clinics in the 1990s. He is regarded nationally and internationally as one of the leading cataract and refractive surgeons and a worldwide reference for the treatment of myopia and astigmatism with excimer lasers.
Dr Alpins began performing refractive treatments in 1985, introducing the technique of radial keratotomy at that time, and was one of the first surgeons to use Laser Vision correction techniques in Australia. Since 1991, he has treated more than 20,000 patients with excimer lasers.
Dr Alpins developed a new method for treating and analysing astigmatism and developed the ASSORT software programme for analysing planned surgical results, which is currently used by ophthalmologists in Australia and overseas.
Dr Alpins often speaks on these topics as a speaker at national and international congresses and on many occasions has been the keynote speaker and chair of the relevant sessions.
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