In Contact - August 2005
Big Zones
I was recently referred an RGP patient by a colleague. She?d been fitted in Germany with a Conflex RGP design with a relatively large optic zone [OZ] - or as the PC-nannies will tell us BOZD - which I measured at 8mm. My Auckland based colleague who?d examined this patient had sent the lenses away to an NZ contact lens manufacturer to ask if they could replicate this large OZ design. Apparently the lab told the practitioner that they did not think lenses of this design could be manufactured (by them?). The patient then consulted me explaining her concerns about changing her fifteen year old RGPs, which had worked relatively well. She was worried any new lenses would aggravate her comfort and particularly her night vision, if small optic zone lenses were made.
I explained that there should be no problems with this and demonstrated this with simulated fluorescein pattern RGP ?fitting? which is part of the Medmont E300 topographer and Studio software package.
Another simulated RGP fitting and manufacturing programme known as Focal Point is also gaining wider use for custom manufacture of RGPs. It's apparently proving to be a great aid for optometrists, who in recent years, have often been awarded their degrees with little or no RGP training or experience?
So much for broad ranging skills and competence.
Inventor and developer of Focal Point, Renato Liffredo was awarded an EFCLIN technology award for his sterling work in the field, in 2002.
An excellent article on simulated RGP fitting and integrated manufacture was featured in Polymer Technologies? Boston Newsletter, Volume 16 May 2005. This edition also contains excellent info on Dk and wetting angles.
They see a day where an RGP can be designed and fitted to high levels of accuracy without ever placing a trial lens on the eye.
Of course topographers don't have ?eyelids? and are unaffected by gravity so unfortunately although simulated RGP designs may work well in some cases there will be situations where lens diameter, centration and edge lift - as mentioned in previous In Contact articles - could be way off the mark.
At this point in time I still advocate trial lenses to determine such things. I do however use the simulations to ?tweak? designs and to see what effect changes to curves will have on tear thickness, edge lift and so on, especially in ?abnormal? eyes.
In the Zone
In the aforementioned case I proceeded to design and manufacture custom tetra-curve RGPs and it was interesting to note that after fifteen years, the lenses provided by me were only 0.03mm flatter in base curve radius and the power was within 0.12D of her habitual lenses. The old lenses were in relatively poor condition - as one would expect.
I increased the optic zone diameter to 8.6mm on the right and 8.7mm on the left with an overall diameter of 9.9mm on the right and 10.0mm on the left. The design is as follows:
R 8.17wet(8.6)8.9(9.0)10.25(9.4)12.00/9.9 /-1.50- green Optimum Extra UV
L 8.18wet(8.7)8.9(9.0)10.25(9.4)12.00/10.0/-1.50- blue Boston XO UV
DAC blend 0.2
Simulated K readings were
R 41.00/42.00 (8.24/8.04mm)
L 41.10/42.20 (8.22/8.00mm).
She had rather large pupils in mesopic conditions, which were even larger in scotopic conditions and thus the increase in optic zone should provide better vision in reduced light conditions resulting in less flare, reflections around lights, and so on.
On fitting the new lenses she immediately commented that they felt extremely comfortable and she obtained excellent 6/6 vision right and left, which was slightly improved compared to the previous lenses. Base curve relationship, diameter, and periphery were excellent. She was somewhat relieved as she had been rather apprehensive in obtaining new lenses, one of the reasons she had delayed so long.
At aftercare she reported that vision was excellent and night vision much better. A recent visit to the theatre confirmed this in that there was an almost total elimination of ?flare?, relatively speaking. Her only complaint was of slight physical awareness/irritation, after a few hours of wear, with the left lens. She was 100% happy with the right.
Slit lamp exam confirmed a mild, left arcuate stain and I redesigned the lens with a 0.2mm smaller overall diameter and similar reduction in OZ. In addition I increased the blend to 0.4 [or 40%] in order to soften the ?junctions? between zones. Generally I use around a 0.2 blend for 7 to 8mm optic zones and as much as 0.9 on complex five curve, steep designs as one would fit on a severe nipple cone.
As one may imagine when fitting a large OZ there may well be an increase in the effect of lid pressure and other factors that will ?bump? a large optic zone/second curve junction into the mid-peripheral cornea where the cornea?s relative flattening kicks in - outside the ?optic cap? of the cornea. The typical cornea does not change much within a central area of around 4 to 6mm in diameter. It then starts flattening fairly fast toward the periphery. Thus a large OZ will be more likely to affect sag and place the zone junction into physical ?touch? with the flatter mid periphery. Hence a heavier degree of blending will help make the junction ?softer? and relatively aspheric.
This tweak seems to have done the trick.
One must bear in mind that blending will effectively reduce the true optical zone by ?blurring the junctions?, so one has a trade off between desired optic zones and practical, blended optic zones. A blend of 0.2 will typically reduce the OZ by around 0.2mm with a 0.9 blend reducing an OZ by as much as 1.0mm. Either way, in this case, the resultant OZ was still bigger than her old lenses.
As with many aspects of contact lens design, some of the desired properties are often mutually exclusive. That is where the Art and Science of Contact Lens Speciality kicks in: Finding the best balance between vision, flare, comfort, fit, optics, physiology, function, wetting, material, Dk, tear lens thickness, edge lift, centration, lid interaction and many other factors, is what counts.
This is where hands on training and experience pays off. A hundred hours of Kanski and expensive courses on BIO and Gonio don't teach you these things.
Godzone?
When I initially came to New Zealand, some twelve years ago, I was somewhat flabbergasted to see that most of the standard lens designs and trial lens sets on the New Zealand market had relatively small optic zones of around 6 to 7mm. Even larger diameter lenses were limited to optics zones around 7mm. I discussed this at time with various manufacturers and commented that we had been fitting larger 7.5 to 8mm zones for many years, at altitude. I also made the point that small optic zones had only been necessary in the days of PMMA in order to minimise corneal oedema and enhance tear exchange. Since the late 1970s low Dk lenses were coming to market [e.g. Polycon Dk ≈ 8 units]. The early ?80s saw Paragon O2 [aka XL40] provide the first lens with a Dk in the teens, at 16 units. By the mid 1980s Dks of 20 to 30 units were easily obtained.
In the late ?80s materials in the 60-90 Dk range were readily available and hence restrictive optic zones were no longer necessary. It has however taken a long time for Dks in this range to catch on, both here and elsewhere. Today there is rarely any reason, except in cases of allergic reaction or wetting issues, to fit anything less than a material in the 50-100 Dk range.
In the past few years I have taken the plunge and moved onto optic zones that are bigger than 8mm. There is no reason why with larger diameter lenses, such as 10 to 10.5 millimetres, that optic zones of up to 9 mm cannot be accommodated. In fact in the past month I delivered a pair of lenses with a 9.0mm optic zone. They?ve worked a treat. As mentioned, increased blending will almost certainly be necessary. One also needs to ?flatten off? the base curve and periphery to prevent overly steep lenses and tight peripheries.
I must say the results we obtain are very promising. Such lenses almost always centre better, are more stable and wearers report that they are much less aware of reductions in visual quality with night driving, poor light conditions and so on.
Check out this excellent article by Heather Power on Contact Lens and RGP history, especially as far as material development and Dk is concerned.
The Great Zeiss Secret? is another excellent article on the history of CLs, with some good images, references and links.
Sport Zone
Full credit to the ABs for an excellent series-win in the Lions tour. I saw some sublime tries and the boys in black certainly seem to have developed the modern game to a fine level of skill. The spirit and atmosphere at the final test at Eden Park was terrific and one has to compliment the Lions supporters for their good sportsmanship. Even in defeat their singing and spirit was amazing and local rugby supporters could learn a thing or two from them. Unfortunately the following Mandela Plate game between the Wallabies and Boks was not that marvellous and the Boks were rather disappointing?
Bugger.
It seems to me that only the Aussies have any hope of beating the All Blacks?
Formula One has also had its ups and downs but It's looking good for young Alonso potentially becoming the youngest World Champion, ever.
I hope he does.
It will be great to see a non-Ferrari diver and an alternative to the great Schumacher achieve this.
When NZ stopped playing rugby against the Boks during the Apartheid years, it was, I believe, one of the final nails in the coffin of the Racist Regime. After all life was hardly worth living anymore, sans rugby!
Rugby was in those days pretty much an Apartheid flagship-sport.
Although there were not many in SA that enjoyed sanctions and embargoes, they worked.
I thus find it strange that NZ doesn?t have the resolve to can the Zimbabwe cricket tour - what with Mugabe off the rails and some pretty nasty stuff taking place. It seems It's all about the mighty dollar. Funny that.
Terminate the tour. Simple.
Going over and lending credibility to Zimbabwe?s cricket patron Mugabe, is patently ridiculous.
It's just not cricket!
For more information or any comments email Alan at incontact@optom.co.nz.