PowerPoint Presentation

TORIC IOLS andCATARACT SURGERY

Dr Brad TownendBSc(Med), MBBS, MPH, MMed, FRANZCOGosford and Wyong Eye Surgery

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Toric IOL

Correction of corneal astigmatism

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Conoid of Sturm

Q1: Astigmatism is:AWhat you have when you dont have a stigmaBUncorrectable with glasses or intraocular lensesCAble to be ignored in intraocular lens selectionDA difference in the curvature and therefore focusing power of the cornea at 90 degree axes

WHY ARE TORICS GOOD?Usually result in low residual astigmatism

Higher visual outcome demands from patients

Toric IOLs are superior and more predictable in reducing corneal astigmatism than limbal relaxing incisions (Mingo-Boton et al, JCRS 2010)

Toric lenses have been around for almost 10 years now, and they seem to be gaining popularity with time.The first key papers on their use were published in 2000 (Ophthalmology, Sun et al)

Mingo-Boton et al in JCRS 2010 compared toric intraocular lenses with LRI and found that toric lenses were superior and more predictable in reducing corneal astigmatism10

TORIC LIMITATIONSNeutralise corneal astigmatism in the plane of the Effective Lens Position (ELP) Higher order aberrations (Hayashi et al, JCRS 2012)Time consuming

Reliability of outcomes

So what are some of the limitations of toric lenses?Neutralise corneal astigmatism in the plane of the effective lens position (ELP)This will inevitably result in some higher order aberrations (such as spherical and chromatic aberration or coma) as the primary astigmatism is in the plane of the cornea, and we are correcting it in a different plane.We often see phakic lenticular astigmatism naturally neutralising corneal astigmatism, so the body often successfully utilises this method of correction in the optical system.I often see a patient for surgery who doesnt wear glasses pre-operatively yet has enough corneal cyl to warrant the use of toric lenses. In other words, while they have a lot of corneal astigmatism, they also have a lot of lenticular astigmatism that neutralises it. If I dont use a toric lens to replace this lenticular astigmatism then even though their cataract will be gone they will be underwhelmed by the visual outcome because of the residual astigmatism that occurs post-operatively. Using toric lenses can really slow down your theatre listFrom a distance, toric lenses look like the perfect solution to corneal astigmatism, but their ability to completely eradicate all residual astigmatism is by no means perfect and there are a number of reasons for this.My aim with toric IOLs is to minimise corneal astigmatism I accept that I usually cant completely eradicate it.11

THRESHOLD FOR USE1.5 Dioptres of corneal astigmatism

I use 1.5D as my threshold, and this results in about 10% of toric lens use in my lists. 12

WHY LESS THAN PERFECT?Not an exact science

Firstly, the eye is a dynamic structure with natural variation. It doesnt always follow all the rules and formulas.13

WHY LESS THAN PERFECT?

Limited range of lens powers

Its unreasonable to expect consistent plano results when the range of cyl powers comes in 0.5 cyl increments at best.14

WHY LESS THAN PERFECT?

Approximation of final anterior chamber depth

ACD is critical with toric lenses, especially with high powered cyl lenses because the cyl correction is not occurring at the corneal plane. The power of cyl required to neutralise a given corneal astigmatism can vary greatly depending on the ACD or in effect the distance from the cornea where the correcting power is being applied.15

WHY LESS THAN PERFECT?

Keratometry axis variable

This is potentially a major source of error.I wanted to look into this a little more myself, so conducted my own n=3 study into this.I asked my orthoptist on three consecutive IOL master measurements, to have the patient retire in their seat after the calculations had been done, removing their chin from the chinrest and then re-engaging with the machine to have their keratometry re-measured by the IOL master.The variation in keratometry axis readings was considerable:16

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So you can see what were up against from the start! Getting the lens on the correct axis is obviously critical to the refractive outcome especially if the power of the cylinder in the lens is particularly high.19

WHY LESS THAN PERFECT?MARKINGPatients head not straightBig and blotchy pen tipsMarkings wash away!Parallax error

This is probably the hardest part of the operation with a toric IOL

Patients head not straightBig and blotchy pen tips (some marks may span 5-10 degrees!)

These errors, especially the parallax errors, can result in considerable errors in the final refraction particular with lenses that have a high powered cyl that finishes up lying off axis.In the worst case scenario, if all the planets align in a negative sense and all these errors compound on each other, the result can be considerably adverse.20

WHY LESS THAN PERFECT?Surgically Induced Astigmatism variableSize of keratome bladeSuperior vs Temporal incision locationAnterior vs Posterior wound

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WHY LESS THAN PERFECT?Lens rotation in bag post-operativelyWorse with long axial length (Shah et al)Most occurs in first week? Intra-operative re-positioning