Hunter Newsletter
Welcome to 2014 and our new blog! Our optometry night seems like yesterday but time has moved quietly along. Enjoy the remainder of the year!
If ever you have queries or concerns we would be delighted to assist!
Here’s a few things you might find interesting between seeing patients:
Toric Lenses
I had several questions about toric intraocular lenses after my presentation at the meeting.
They related to the thought process in choosing a toric implant and the question of subjective refraction versus astigmatism after the cataract is out.
In essence, I mostly disregard the subjective refraction when assessing for a toric lens implant. The critical element is the anterior corneal topography which will contribute the majority of the residual astigmatism. Posterior corneal astigmatism is an unknown but can be assumed to be against the rule in 87% of cases and I factor it in arbitrarily.
The topography may show regular or slightly irregular astigmatism, which makes an axis estimation tricky. Likewise, an elderly tear film is the commonest culprit for variable and irregular topography. . I use both an Orbscan (BandL) and an Atlas (Zeiss) topographer because the combined information can often predict the correct axis better than one topographic assessment. I also use 2 different autokeratometers to get additional data. So every patient has their keratometry assessed with 4 different machines in 4 slightly different ways. If it doesn’t work out as planned it is not for want of trying!
I don’t use corneal arcuate incisions to correct corneal toricity because they induce foreign body sensations, may heal irregularly with resultant small irregularities in topography and they are quite variable between individuals in terms of their clinical effect. I would rather use a toric lens implant because of its better reliability and the avoidance of extra surgical wounds from corneal cuts (even the neat cuts made by using a femtosecond laser). There is a saying I like : “arcuate corneal cuts are an art, but toric lenses are a science”!
Raynor Sulcoflex lens
This is a great new lens implant which is used as a piggy back onto an existing acrylic IOL. It is super thin and safely sits in front of the IOL and behind the iris without causing pupil block. Both lenses together are still thinner than the crystalline lens.
Case1) A case was referred to me recently with the patient having seen another surgeon a few years ago and ending up +2D. The patient wondered what sort of refractive surgery may help him and originally asked me for LASIK. The sulcoflex lens (a +3D for effectivity) was inserted and the patient ended up 6/5 and ecstatic!
Case2) A patient who had bilateral IOLs with emmetropia and who changed his mind and wanted minimonovision 2 years later. Sulcoflex solved the problem nicely with a final refraction of -125 in the non dominant eye.
Lining up the astigmatism for LASIK:
When a patient lies down for LASIK, did you ever wonder what happens to the astigmatism? Of course supine posture causes cyclorotation and the cylindrical component of the ablation could be up to 10 degrees misplaced in some patients. So when we perform the wavefront capture we also take an iris image (upright) which is programmed into the VISX laser. Before the laser fires, the iris appearance is ‘registered’ and iris landmarks are compared so that any supine cyclotorsional rotation is compensated for. The software ‘twists’ the laser ablation pattern to the same number of degrees to ensure that the cylindrical component end up axis perfect!
What about the pupil in LASIK?
As well as determining the cyclorotation, the VISX laser notes the pupil centre with the expected miosis under the illumination of the operating microscope. The wavescan is captured in mesopic light to get the widest possible wavefront which helps in preventing night driving problems. But if the patient mioses during surgery, then the ablation will be centred on the miotic pupil centre instead of the mesopic pupil centre! This will cause aberrations and problems in the dark.
The solution is to shift the ablation laterally back towards the original mesopic pupil centre as measured and recorded by the iris registration and pupil detection process preoperatively.
Blending
Reduced night vision symptoms are also achieved by making the ablation profile aspheric and by ‘blending’ the ablation. The central 6 mm ablation ‘optical zone’ gives the majority of the refractive effect. The next 2 mm (from 6 to 8mm) is the ‘blend zone’ to ensure that there is not a sudden demarcation which will caused aberration in a dilated patient. The individual wavefront errors measured in the preoperative wavescan are overlayed over this spherocylindrical optical and blend zone base.
