- Email: [email protected]
The design of the back surface ofgas permeable lenses
The Design of the back surface of Gas Permeable Lenses T. C. O. Atkinson, FBCO, DCLP
Terry Atkinson is a member o f the senior staff" at the London Refraction Hospital and Consultant Clinician at City University. He has a mixed spectacle and contact lens practice in Essex and is a former President o f both the B C L A and the Contact Lens Society.
The introduction of gas permeable contact lenses into general practice has opened up a new era for practitioners, and the advantages over hard PMMA and soft lenses are well documented. For many new patients it will be quite possible to fit the standard hard lens designs which are favoured by the individual practitioner. However, for those wearers who subsequently suffer from flare, lens instability or lid irritation it is necessary to consider lenses with larger back central optic diameters and/or overall size and less edge lift. Similarly the soft lens wearer who is having to be refitted with a hard type lens will prefer immediate flare free stable vision which is not always immediately available with the standard hard lens constructions. In order to fulfil these requirements there have been several new designs offered for gas permeable lenses. To date these have received a mixed reception from practitioners and it is the purpose of this paper to compare these designs with the familiar PMMA constructions and offer new constructions which practitioners may find useful as an adjunct to those already available. Historically the initial interest with gas permeables goes back to CAB, but the lack of stability of lathe cutting reduced the effectivity of the material and forced us to consider moulded lenses such as the Hartflex from Wohlk. The edge lift of these lenses is, however, as little as 0.045mm, to 0.06ram,' and when fitted in the standard British fashion would lead to a very tight lens although giving often an amazing amount of tear interchange. Satisfaction is only really obtained utilizing a flatter than 'K' approach or by adding a flat edge curve about 0.5mm wide and blended into the lens periphery. The second lens introduced, the Menicon 02 works in a similar
16
fashion. The lens like the Hartflex has a very reduced edge lift, varying from 0.06mm on steeper radii. These figures the manufacturers will not vary. Fitting this lens with apical clearance gives absolutely no edge lift, but fitted 0.15mm flatter than 'K' as suggested by the manufacturers means again we depart from conventional methods of fitting. At the same time as these lenses were introduced to Great Britain a similar investigation was being undertaken by Williams in the USA 2. He designed for the Syntex Corporation two series of lenses made from Polycon. The first set of lenses (Table 1) had an overall size of 9.50mm with a BCOD of 8.40ram, and an axial edge lift of about 0.12mm. No mention was A.E.L.A.E.L.
@ 9.10 @ 9.50
(mm) (nun) 7 . 2 0 : 8 . 4 0 / 8.45:9.10/10.00:9.50 0.056 0.120 7.40 : / 9.10 /10.50 0.065 0.128 7.60 : / 9.30 /11.00 0.060 0.121 7.80 : / 9.70 /11.50 0.060 0.120 8.00 : /10.00 /12.00 0.058 0.117 8.20 : /10.20 /12.50 0.055 0.112 8.40 : /10.60 /13.00 0.055 0.111 Table 1. Tricurve Design (after Williams USA) O.S. 9.50ram. BCOD 8.40 made of how the peripheral curves were fashioned. The second series (Table 2) were of overall size 8.50mm, with a BCOD of 7.00mm, and an edge lift of 0.05 - 0.08mm. The lenses were made very thin with a centre thickness of 0.08ram, and an edge thickness of 0.07 - 0.09mm. This meant that lenses above -4.00 were of lenticular construction. This value for edge thickness was determined in trials which showed that less than 10% of patients were aware of lid sensation
Journal of the British Contact Lens Association
A.E.L.A.E.L.
@ 8.30 @ 8.5o (mm) (nun) 7.20:7.00/ 7.70:8.30/17.00:8.50 0.036 0.082 7.40 : / 7.90 /17.00 0.033 0.076 7.60 : / 8.10 /17.00 0.031 0.071 7.80 : / 8.30 /17.00 0.028 0.067 8.00 : / 8.50 /17.00 0.026 0.063 8.20 : / 8.70 /17.00 0.025 0.059 8.40 : / 8.90 /17.00 0.023 0.055 Table 2. Tricurve Design (after Williams U S A ) O.S. 8.50 B C O D 7.00mm. at this thickness. The r e c o m m e n d e d fitting was related to the a m o u n t of corneal astigmatism. For a 9.50ram lens on a cornea with up to 0.75D of astigmatism, the lens was fitted 0.20 - 0.30mm. flatter than the flattest 'K'. A s the corneal astigmatism increased the fitting steepened with respect to flattest ' K ' (Table 3). T h e prime aim was to fit large, flat and u n d e r the lid. T h e smaller 8.50ram lenses were fitted on alignment. B C O D / ' K ' Relationship for Polycon Lenses of 9.50mm. Overall Size
Corneal Cylinder (Dioptres)
BCOR/'K' Relationship
0 - 0.75 1.00 - 1.75 2.00 - 2.75 3.00 - 3.75 4 D and o v e r
0.30 - 0.20ram. flatter than 'K' 0.25 - 0.15mm. ,, 0.20 - 0.10ram. ,, 0.10 - 0.05ram. ,, On 'K' Table 3.
A.E.L.A.E.L.
@ 8.10 @ 8.5o (mm) (ram) 7.20:7.40/ 8.40:8.10/12.00:8.50 0.042 0.109 7.40 : / 8.70 /13.00 0.041 0.109 7.60 : / 8.90 /12.50 0.038 0.098 7.80 : / 9.10 /13.50 0.036 0.097 8.00 : / 9.30 /14.00 0.033 0.093 8.20 : / 9.40 /14.50 0.029 0.087 8.40 : / 9.70 /15.00 0.029 0.086 Table 5. Tricurve Design after Syntex (GB). O.S. 8.50 B C O D 7.40ram. A.E.L.A.E.L.
In o r d e r to find out the peripheral radii and their diameters so that I could try some of their designs I contacted Syntex. They informed me that the lenses p r o d u c e d in the U S A related to a special edge blend and finish and in order to achieve the same effective fit from a lens m a d e by one of the laboratories in G r e a t Britain the lenses should be both designed and fitted differently. They also included a 9.00ram lens. F o r a 9.50mm lens (Table 4) they suggested a lens be fitted 0.05ram flatter than 'K' against the 0.20ram to 0.30turn flatter in the U S A For a 8.50ram lens (Table A.E.L.A.E.L.
@ 9.10 @ 9.10 (mm) (mm) 7.20:7.90/ 8.25:9.10/ 9.00:9.50 0.0'79 0.127 7.40 : / 8.55 / 9.50 0.078 0.127 7.60 : / 8.75 /10.00 0.072 0.121 7.80 : / 9.05 /10.50 0.071 0.121 8.00 : / 9.25 /11.00 (I.066 0.116 8.20 : / 9.50 /11.00 0.064 0.108 8.40 : / 9.85 /11.00 0.065 0.105 Table 4. Tricurve Design after Syntex (GB). O.S. 9.50mm. B C O D 7.90ram.
18
5) they should be fitted 0.05ram steeper than 'K' against the alignment fitting in the U S A , and with a m u c h larger edge lift of 0.10mm. For a 9.00ram lens (Table 6) they should be on 'K'. A s the 9.50mm lens m a d e in Great Britain has a B C O D of only 0.5ram smaller I am surprised there is such a large difference in the way the lens is fitted, hearing in m i n d the edge lift is the same. It is slightly easier to understand why the 8.50mm lens from Great Britain with the large B C O D has to be fitted steeper r a t h e r than flatter because of the increased edge lift f r o m 0.07 to 0.10mm. I do, however, find it difficult to understand the true link up.
@ 8.60 @ 9.00 (ram) (mm) 7.20:7.60/ 8.30:8.60/10.00:9.00 0.061 0.117 7.40 : / 8.60 /11.00 0.060 0.121 7.60 : / 8.75 /11.00 0.054 0.109 7.80 : / 9.10 /11.50 0.055 0.109 8.00 : / 9.40 /12.00 0.055 0.108 8.20 : / 9.50 /12.50 0.048 0.100 8.40 : / 9.75 /13.00 0.047 0.098 Table 6. Tricurve Design after Syntex (GB). O.S. 9.00mm. B C O D 7.60ram. So far, as you can see, the trend has been towards flatter fitting techniques and I began to wonder w h e t h e r the days of alignment and slight apical clearance giving the classic fluorescein picture were n u m b e r e d , but the recent introduction of the Kelvin B o s t o n lens findings by colleagues Christopher W e s t o n and R o b e r t Grave 3 plus m y own findings have convinced me otherwise. To reconstruct lenses to fulfil these requirements means we have to look at the following factors:
1. Determination of BCOD. My first contention was that because of the superior wettability and oxygen permeability of the gas p e r m e a b l e materials it should be possible to replace the B C O R and the first peripheral curve of a tetracurve, or part of the secondary curve in the case of a tricurve, by one curve, the new B C O R and B C O D , and make the remainder of the lens flatter than the cornea to give the necessary tear flow under
Journal of tile British Contact Lens Associatioll
the lens. This would reduce the number of points along the edge of the lens where an error in a choice of d i a m e t e r or radius would lead to reduced tear flow. 1. D e t e r m i n a t i o n of B C O D .
My first contention was that because of the superior wettability and oxygen permeability of the gas p e r m e a b l e materials it should be possible to replace the B C O R and the first peripheral curve of a tetracurve, or part of the secondary curve in the case of a tricurve, by one curve, the new B C O R and B C O D , and make the remainder of the lens flatter than the c o r n e a to give the necessary tear flow under the lens. This would reduce the number of points along the edge of the lens where an error in a choice of d i a m e t e r or radius w o u l d lead to reduced tear flow. F r o m nr 2 (Fig. I) it is possible to work out the area of bearing which is the apical clearance and/or alignment areas as a percentage of the total lens. In o r d e r to try and ascertain what area should contribute to bearing and therefore determine the B C O D and what area should contribute to edge lift I compared the seven trial sets I use in practice or at the L R H to find out w h e t h e r this was a c o m m o n denominator in the success or failure of the designs. O n e must r e m e m b e r that a lightly blended lens will not have m u c h effect on the parameters of the lens, but a well b l e n d e d lens could effectively reduce the widths of each curve by 0.20mm overall, hence flattening the fit. i would also add that it is my custom to lightly blend the transition b e t w e e n the B C O R and the first peripheral curve w h e t h e r I ask for a light, medium or heavy b l e n d e d lens as heavy blending at this point tends to tighten the fit. Taking each lens design in turn:
rs
~i
e~
(5
"
aS
/ Bearing A r e a = All Apical Clearance and/or A l i g n m e n t Areas Bearing A r e a =It(_BP~_Q. D ) 2
T o t a l A r e a = ,i (.O~__~.)2 Bearing A r e a as % of Total A r e a : (BP, OD)~, (-O.S.) ~ x 100 Fig. 1
L e n s (1) 7 . 8 0 : 7 . 0 0 / 9 . 0 0 : 7 . 8 0 / 1 0 . 9 0 : 8.60
This lens, a tricurve, giving 0.12 e. 1. is fitted steep of ' K ' by 0.1ram. With a lightly blended lens the bearing area is 7.8mm wide and is 82.2% of the whole lens. O n a well blended lens where the second d i a m e t e r is reduced to 7.6 from 7.80 the area is 78.1%. O n the whole this lens is usually on the tight side.
can be a little small on occasions. The bearing d i a m e t e r is 7.80mm, giving a percentage cover of 75.1%. W h e n fully blended it is a good fitting lens but if the B C O R is slightly flat the final fitting would be on the loose side. T h e bearing area would then be 7.60mm. (71.3%).
L e n s (2) 7 . 8 0 : 7 . 0 0 / 8 . 4 5 : 7 . 8 0 / 9 . 3 0 : 8 . 4 0 / 1 1 . 0 0 : 9.00
L e n s (3) 7 . 8 0 : 7 . 0 0 / 8 . 7 5 : 7 . 8 0 / 1 0 . 1 0 : 8 . 4 0 / 1 1 . 3 0 : 9.00
A tetracurve giving an axial edge lift of 0.125ram. W h e n lightly blended it is a good lens but the edge lift
A tetracurve giving an axial edge lift of 0.15ram which w h e n fully blended is often too loose, but with Med. A.E.L.
1. 2. 3. 4. 5. 6. 7.
7.80 7.80 7.80 7.80 7.80 7.80 7.80
: 7.00/9.00 : 7.00/8.45 : 7.00/8.75 : 7.00/8.30 : 7.50/8.60 : 7.50/8.60 : 8.00/8.60
Lt. Med. Blend %
Fit
: 7.80/10.90 : 8.60 0.120 82.2 Tight : 7.80/9.30 : 8.40/11.00 : 9.00 0.125 75.1 Good : 7.80/10.10 : 8.40/11.30 : 9.00 0.150 75.1 Good : 8.00/10.50 : 8.50/12.50 : 9.00 0.150 79.0 Tight : 8.00/11.20 : 9.00 0.135 79.0 Tight : 8.20/9.60 : 8.90/10.60 : 9.50 0.150 74.5 Good : 8.50/10.60 : 9.50 0.138 80.0 Tight G o o d Bearing A r e a = 75 - 76% (70.9% - 82.2%) Table 7. Bearing A r e a as percentage of Overall Size of Lens.
Heavy Blend %
Fit
78.1 71.3 71.3 75.1 74.2 70.9 76.3
Tight Loose Loose Good Good Loose Good
(continued on page 25)
Journal of the British Contact Lens Association
19
(continued from page 19) a light b l e n d is fine. Diameter of bearing area is 7.80mm, w h e n lightly blended giving an area of 7 5 . l % , but when fully blended a bearing area of 71.3%. L e n s (4) 7 . 8 0 : 7 . 0 0 / 8 . 3 0 : 8 . 0 0 / 1 0 . 5 0 : 8 . 5 0 / 1 2 . 5 0 : 9.00
A tetracurve giving an axial lift of 0.15mm, requiring a meticulous edge blend. The biggest problem with these lenses has b e e n between the second and third curves. Should a fifth curve be introduced? W h e n poorly blended the lens gives a beating area of 79% but w h e n well blended a comfortable lens with a bearing area of 75.1%. L e n s (5) 7 . 8 0 : 7 . 5 0 / 8 . 6 0 : 8 . 0 0 / 1 1 . 2 0 : 9.00
A good lens when fully blended, giving an axial edge lift of 0.135mm. W h e n lightly blended it tends to be tight, the area covered being 79%. W h e n fully b l e n d e d it tends to become a good fitting bicurve. The area then covered is 74.2%. L e n s (6) 7 . 8 0 : 7 . 5 0 / 8 . 6 0 : 8 . 2 0 / 9 . 6 0 : 8 . 9 0 / 1 0 . 6 0 : 9.50
This lens gives an axial edge lift of 0.15ram. In m a n y cases when lightly blended it is quite good giving a bearing area of 74.5%, but with a heavier b l e n d gives a loose fitting with a bearing area of 70.9%. L e n s (7) 7 . 8 0 : 8 . 0 0 / 8 . 6 0 : 8 . 5 0 / 1 0 . 6 0 : 9.50
This lens with an axial edge lift of 0.138 has to be well b l e n d e d if it is to be successful and covers an area
of 76.3%. O n occasions when poorly made it can be on the tight side, covering an area of 80%. This lens design has b e e n used more and more with gas p e r m e a b l e materials. It is interesting to note from the above, which are summarized in Table 7, that the average area covered by lenses is between 70.9% on the loose side and 82.2% o n the tight side. The majority of satisfactory fittings fall with a beating area of 75 - 76%. For ease of calculation I have taken a figure of 75%. Therefore, assuming the superior wettability and permeability of gas permeables, we should be able to replace the whole of this bearing area by one curve and fit lenses as follows with a B C O D of: 7.40 for 8.50mm lenses (instead of the customary 7.00) 7.80 for 9.00mm lenses (instead of the customary 7 7.50) 8.25 for 9.50mm lenses (instead of the customary 7.50 - 8.00) and still remain within the requirements of apical clearance or at worst alignment. 2. C h o i c e o f E d g e Lift
This factor is truly very difficult to determine exactly for any one patient. It can be set for a trial set but can only be deemed correct after careful e x a m i n a t i o n of the patient. It has been customary to suggest an axial edge lift of 0.15ram for most standard P M M A wearers, but on the evidence of Hartflex,
T I T M U S E U R O C O N HAVE DEVELOPED A NEW COMPUTER CONTROLLED LATHE TECHNOLOGY WHICH PERMITS THE FULLY AUTOMATED PRODUCTION OF CONSISTENTLY REPRODUCIBLE ASPHERIC SURFACES OF THEIR WEICON LENSES.
F
WEICON
m S(
Further details from: TE PROFESSIONAL SERVICES DEP,~ Ashville House, Havant Road, Drayton, Portsmouth PO6 1 PA. or T E L : 0705 388716
Journal of the British Contact Lens Association
25
M e n i c o n and general experience, i feel 0.10 - 0.12 is as good a point to start as any. One should also r e m e m b e r that, assuming the clearance from the c o r n e a is the same for all lenses, a 7.80 : 7.00 will need m o r e edge lift to give the same corneal clearance at the edge of the lens than a 7.85 : 7.50 or 7.90 : 8.00. Theoretically, by increasing the B C O D on a 9.00mm lens from 7.00 to 7.80 means we can reduce the edge lift by 0.02mm without really affecting the fit.
3. Choice of Peripheral Curves and Diameters My i m m e d i a t e reaction was that it should be possible to m a k e all lenses with a bicurve design and this was confirmed by the introduction of the Kelvin B o s t o n lens. This lens is available at present only in a 9.50mm overall size, with a B C O D of 8.20mm, giving a 0.10mm axial edge lift from the following bicurve construction: 7.80 : 8.20/9.80 : 9.50 I understand a touch of 12.30mm curve 0.1ram wide is added to the periphery but this is really only part of the edge process and is fully blended in. The lens is fitted with slight apical clearance, and initial results as presented by A u e r b a c k 4 have been very encouraging. M y own experience would to some degree endorse their choice of a bicurve. A s stated previously, my choice of lens, based on my old trial sets, has been: 7.80 : 8.00/8.60 : 8.50/10.60 : 9.50
W h e n this is very fully blended the second curve tends to disappear. This can be replaced partly by the larger B C O D ; 8.25 has been suggested but has been m o d i f i e d to 8.30 for reasons that will become a p p a r e n t later, and by one single peripheral curve to give any edge lift required. I have found between 0.10 to 0.12mm the most satisfactory. This would give us the following constructions: 7.80 : 8.30/9.55 : 9.50 giving an edge lift of 0.10mm 9.80 : 9.50 giving an edge lift of 0.1 l m m 10.00 : 9.50 giving an edge lift of 0.12mm It is quite easy utilizing a programmable calculator such as the now outdated Texas SR52 or the latest T.I.59 to design for all radii lenses with any edge lift required. Similarly it would be possible to design two-curve lenses for an overall size of 8.50 with a B C O D of 7.40mm, and a 9.00mm lens with a B C O D of 7.80mm. (Table 8) With regard to the tricurve lenses, the advantage h e r e is that one can stay much closer to the changing periphery of the cornea and compensate the reduced tear v o l u m e under the first peripheral curve of the lens by having a flat enough tertiary curve to give corneal clearance as the lens rides up. W e s t o n and G r a v e based their ideas on a similar t e c h n i q u e to myself. T h e y have used the Menicon fitting set which offers a 9.20mm lens with an 8.00mm optic to obtain the correct central clearance and then
TITMUS EUROCON HAVE DEVELOPED A NEW COMPUTER CONTROLLED LATHE TECHNOLOGY WHICH PERMITS THE FULLY AUTOMATED PRODUCTION OF CONSISTENTLY REPRODUCIBLE ASPHERIC SURFACES OF THEIR PERSECON LENSES.
I
PERSECON
Further details from: TE PROFESSIONAL SERVICES DEPA Ashville House, Havant Road, Drayton, Portsmouth PO6 1 PA. or TEL: 0705 388716
26
.lournal of the British Contact Lens Association
BCOR 7.20 7.40 7.60 7.80 8.00 8.20 8.40
PERIPHERAL CURVES FOR BICURVE LENSES Overall Size 8.50ram. Overall Size 9.00mm. Overall Size 9.50mm. BCOD 7.40mm. (BCOD 7.80mm.) (BCOD 8.30mm) Axial Edge Lift (mm.) Axial Edge Lift (nun.) Axial Edge Lift (mm.) 0.10mm. 0.11mm. 0.12mm. 0.10ram. 0.11ram. 0.12mm. 0.10mm. 0.11ram. 0.12mm. 9.20 9.45 9.80 8.70 8.90 9.10 8.45 8.60 8.80 9.60 9.90 10.30 8.80 9.00 9.20 9.05 9.30 9.55 10.00 10.40 10.80 9.15 9.40 9.60 9.40 9.70 10.00 10.45 10.85 11.35 9.55 9.80 10.00 9.80 10.15 10.45 10.90 11.35 11.90 9.90 10.20 10.45 10.20 10.55 10.90 11.40 11.90 12.45 10.30 10.60 10.90 10.60 11.00 11.40 11.90 12.45 13.05 10.70 11.00 11.40 11.05 11.45 11.90 Table 8
redesigned the edge to give a 0.12mm edge lift. They have achieved excellent results. The design of the M e n i c o n is: a . e . l , at 8.60 a.e.1, at 9.20
7.80 : 8.00/8.50 : 8.60/8.80 : 9.20 0.021 0.053 and has b e e n modified by them to 7.80 : 8.00/8.80 : 8.60/12.30 : 9.20 0.028 0.120 This lens conforms to all my prerequisites, a 75% bearing area, 0.12 edge lift tricurve and with slight apical clearance. They have also made, by adjusting the B C O D and overall size, two other lenses, both with 0.12 edge lift and with 0.30ram wide peripheral Curves: a.e.1, at 8.40 a.e.1, at 9.00
7.80 : 7.80/8.85 : 8.40/12.65 : 9.00
0.028
0.120
a . e . l . a t 8.90 a.e.1, at 9.50
and 7.80 : 8.30/8.70 : 8.90/11.80 : 9.50 0.028 0.120 T h e only fault I can find in their theory is that the axial lift at the second diameter (8.60mm) is the same for all three lens diameters. My feeling is that the smaller lens should have a lower axial lift at this d i a m e t e r than the larger lens which is bearing on a flatter part of the cornea. If we now c o m p a r e the design by Williams in the U S A and G r e a t Britain plus the designs of Weston and G r a v e , we can compromise these and make theoretical computations for trial sets. For a 9.50ram lens: Williams U S A 7.80 : 8.40/9.70 : 9.10/11.50 : 9.50 Syntex G B 7.80 : 7.90/9.05 : 9.10/10.50 : 9.50
7.20 7.40 7.60 7.80 8.00 8.20 8.40
: 8.30/8.00 : /8.30 : /8.55 : /8.85 : /9.15 : /9.50 : /9.80
W e s t o n & G r a v e 7.80 : 8.30/8.70 : 8.90/12.50 : 9.50 L o o k i n g at the lens designed by Williams in the U S A it is easy to understand why it has to be fitted flatter, although why as much as 0.20 to 0.30mm I do not understand. The secondary curve is slightly wider, and just possibly too wide to be a full bearing surface and may I suppose dig into the peripheral cornea. The Syntex lens has an immediate disadvantage in having a B C O D of 7.9 which defeats the o b j e c t of what we set out to achieve. The design by W e s t o n and G r a v e I feel is very good, but gives a large difference b e t w e e n second and third radii. Personally I have always used the following guide for flattening the second curve: F o r a B C O D of 7.00mm 0.50 - 0.60mm ,, 7.50mm 0.70 - 0.80mm ,, 8.00mm 0.90- 1.00mm ,, 8.50mm 1.10- 1.20mm In this case we are nearer a B C O D of 8.50 than 8.00 and t h e r e f o r e we should consider the following: 7.80 : 8.30/8.90 : 8.90/11.50 : 9.50 T a b l e 9 shows the full range of parameters for all B C O R ' s for an axial lift of 0.10, 0.11 and 0.12mm but maintaining the axial lift at 8.90 as 0.032mm. F o r lenses of 9.00mm overall size we have: Williams U S A n/a Syntex G B 7.80 : 7.60/9.10 : 8.60/11.50 : 9.00 W e s t o n & G r a v e 7.80 : 7.80/8.85 : 8.40/12.65 : 9.00 T h e Syntex G B lens has again the disadvantage of having a slightly too small B C O D and cannot be
A.E.L. of 0.12mm. @ 9.50mm. : 8.90/9.70 : 9.50 : /10.30 /10.85 /11.50 /12.15 /12.80 /13.55
A.E.L. of 0.11ram. @ 9.50mm. 9.25 : 9.50 9.75 10.30 10.85 11.40 11.90 12.50
A.E.L. of 0.10mm. @ 9.50mm. 8.90 : 9.50 9.30 9.80 10.25 10.75 11.15 11.70
Table 9. Tricurve Designs (after Atkinson) Overall Size 9.50mm., B C O D 8.30mm., Constant Axial Edge Lift 0.10 - 0.12mm. maintaining an Axial Lift of 0.032mm. at 8.90mm.
28
Journal of the British Contact Lens Association
considered. The Weston and Grave tens again appears satisfactory. If however we apply the original criteria the secondary curve should be 0.90mm flatter, and my suggestion would be, against what I said f admit for the 9.50mm lens, to steepen the second curve very slightly and flatten the edge: 7.80 : 7.80/8.70 : 8.40/13.00 : 9.00 This has the disadvantage of introducing a very flat third curve for very flat radii. If, however, we were to increase the axial lift at 8.40mm to 0.028 as suggested by W e s t o n and Grave, this would have the effect of reducing the third curve on a lens of B C O R of 8.40mm f r o m 15.85mm to 15.20mm which is a minimal change at this level. Table 10 shows the full range of B C O R ' s for a fitting set for edge lifts of 0.10 0.12mm, maintaining the axial lift at 8.40mm as 0.024mm. F o r the 9.20mm lens we have only the Weston and G r a v e to go on, which is: 7.80 : 8.00/8.80 : 8.60/12.35 : 9.20 This gives an axial lift of 0.028mm, and based on the earlier criteria this fits in very well. Table 11 shows the full trial set for axial e d g e lifts of 0.10 - 0.12mm. I w o u l d add that I am only attempting here to satisfy the academic side and am in no way critical of their designs. In practice slight errors, even within British Standards, will m e a n the final decision about any lens must be down to the practitioner and modified if incorrect. Finally, for an 8.50mm lens we have the following: Williams U S A 7.80 : 7.00/8.30 : 8.30/17.00 : 8.50
Syntex G B
a.e. 1.0.07mm. 7.80 : 7.40/9.10 : 8.10/13.50:8.50 a.e.1.0.10mm
Weston & Grave (assumed) 7.80 : 7.30/9.00 : 7.90/13.80 : 8.50 a.e.1.0.12mm The lens designed by Williams is fitted on flattest ' K ' , hence the reason for keeping the secondary curve tight, although I feel in this instance too wide, and very meticulous blending will be required. The Syntex G B lens is fitted fractionally steeper than 'K' and varies little from the assumed construction of W e s t o n and Grave. My own feelings are that we should reduce the B C O D to 7.30mm instead of 7.40mm, so that the peripheral curves can be made of equal width and we should in this instance reduce the edge lift to 0.11mm, otherwise the fit is too flat. My suggestion would be: 7.80 : 7.30/9.00 : 7.90/12.60 : 8.50 a.e.1.0.11mm. T h e full set with edge lifts of 0.09 - 0. l l m m is given in Table 12. The above discussion, therefore, offers in my opinion variations to the standard P M M A fitting sets. E x p e r i m e n t a l work with these lenses to date shows s o m e very impressive findings. In many cases it has b e e n possible to fit these designs with a quite stable fitting without going flatter than alignment and often with a trace of fluorescein centrally. (Fig.2). The overall size is chosen just as before with the majority of patients being fitted with 9.00mm lenses and the 9.50mm lens only used on large palpebral apertures.
A.E.L. of A . E . L . of A.E.L. of 0.12mm. 0.11ram. 0.10ram. @ 9.00mm. @ 9.00mm. @ 9.00mm. 7.20 : 7.80/7.85 : 8.40/10.85 : 9.00 10.20 : 9.00 9.65 : 9.00 7.40 : /8.15 /11.55 10.80 10.15 7.60 : /8.40 /12.30 11.45 10.70 7.80: /8.70 /13.10 12.10 11.25 8.00 : /8.95 /13.95 12.80 11.85 8.20 : /9.25 /14.85 13.55 12.45 8.40 : /9.50 /15.85 14.30 13.10 Table 10. Tricurve Designs (after Atkinson) Overall Size 9.00mm. B C O D 7.80mm., Constant Axial Edge Lift 0.10 - 0.12mm. maintaining an Axial Lift of 0.024mm. at 8.40mm.
7.20 7.40 7.60 7.80 8.00 8.20 8.40
: 8.00/7.95 : /8.25 : /8.50 : /8.80 : /9.10 : /9.40 : /9.65
A.E.L. of 0.12mm. @ 9.20mm. : 8.60/10.30 : 9.20 /10.90 /11.60 /12.30 /13.05 /13.85 /14.70
A.E.L. of 0.11ram. @ 9.20mm. 9.75 : 9.20 10.30 10.85 11.45 12.10 12.75 13.40
A.E.L. of 0.10mm. @ 9.20ram. 9.30 : 9.20 9.75 10.25 10.75 11.25 11.80 12.40
T a b l e 11. Tricurve Designs (after Atkinson) Overall Size 9.20mm. B C O D 8.00mm., Constant Axial Edge Lift 0.10 - 0.12mm. maintaining an Axial Lift of 0.028mm. @ 8.60mm.
J o u r n a l of the British Contact Lens Association
29
7.20 7.40 7.60 7.80 8.00 8.20 8.40
A.E.L. of 0.11mm. @ 8.50mm. : 7.30/8.15:7.90/10.60:8.50 : /8.45 /11.20 : /8.70 /11.90 : /9.00 /12.60 : /9.30 /13.30 : /9.65 /14,10 : /9.95 /14,90
A.E.L. of 0.10ram. @ 8.50mm. 9.95 lOA5 11.00 ll.60 12.20 12.80 13.50
A.E.L. of 0.09mm. @ 8.50mm. 9.40 9.85 10.30 10.80 11.30 11.80 12.35
Tabte 12. Tricurve Designs (after Atkinson) Overall Size 8.50mm., B C O D 7.30mm. Constant Axial Edge Lift 0.09 - 0.1 l m m . maintaining an Axial Lift of 0.028ram. at 3.90ram.
Fig. 2. The main criteria to be applied has been is the lens stable and is the lens moving, the latter being just as essential for gas permeable materials as P M M A in o r d e r to p u m p the tears under the lens. I feel that a tricurve fitting is the more stable but the bicurve with an edge lift of 0.10mm to 0.12mm has worked well. At present, without calculations however to back me up, I feel a bicurve with a 0.10mm edge lift is probably equivalent in tear volume beneath the lens periphery to a tricurve with an edge lift of 0.12mm. The disadvantage of a tricurve is that there is much more d e m a n d on the accuracy of the diameters and more blending is often required, even with a trusty laboratory, which is slightly more time-consuming, and sometimes leads to breakage (with Boston especially). With a bicurve there is only one blending area. O f the materials available I have been happy using these constructions with XL30, Boston and Polycon, but most disappointed with XL20 which seems to require a standard P M M A design in order to p r e v e n t o e d e m a . With the latter material many cases have exhibited marked o e d e m a and staining which has been resolved with the others, even Polycon and C A B , which have a similar D K value. I have however found the designs unimpressive when a lens rides up due to tight lids as they tend to stick, and of reduced value on toric corneas where the fitting tends to bite on the flatter cornea or lie too fiat on the steeper cornea. In some cases I have had to revert to a toric
30
lens or a small spherical lens to obtain the least corneal disturbance. Finally, 1 do not think it is necessary to make the lens as thin as those designed by Williams for Syntex as we are not attempting to Obtain the same fluorescein picture. O n the whole it would seem preferable to m a k e the centre thickness a minimum of 0.12mm to avoid breakage or distortion. In plus lens cases this presents no problems and the practitioner can decide w h e n they require a reduced optic to give a r e d u c t i o n in lens weight or a means of keeping the lens under the lid. In minus lens cases this can present a p r o b l e m above - 5 . 5 0 O D . A t this point a mid peripheral " l u m p " can occur where the Front Central O p t i c D i a m e t e r ( F C O D ) which should be larger than the B C O D m e e t s the lenticular curve, the thickness at this point being greater than the centre or edge. In o r d e r to reduce this " l u m p " it is either necessary to r e d u c e the F C O D to a value smaller than the B C O D , and risk inducing flare or by introducing a third curve on the front surface. T h e designs offered in this paper are a compromise b e t w e e n the standard lathe cut lenses used in the past and Some of the newer concepts. No doubt, however, we shall soon be seeing alternative designs offered using A s p h e r i c or Elliptical curves, but in the m e a n t i m e the parameters discussed will probably b e c o m e the rule rather than the exception as new better materials gradually replace P M M A .
References 1. 2. 3. 4.
Atkinson, T. C. O., Journal British Contact Lens Association 3:3 July 1980 pp. I05 - 112. Williams, C. E. Journal American Optometric Association 50:3 March 1979 pp. 331 - 336 Weston, C. & Grave, R. Personal Communication A u e r b a c k , D. P. Optician 180 D e c e m b e r 6th 1980 pp. 20 - 24.
A d d r e s s for further correspondence: 24 Forest R o a d , L o u g h t o n , Essex.
Journal of the British Contact Lens Association
Terry Atkinson is a member o f the senior staff" at the London Refraction Hospital and Consultant Clinician at City University. He has a mixed spectacle and contact lens practice in Essex and is a former President o f both the B C L A and the Contact Lens Society.
The introduction of gas permeable contact lenses into general practice has opened up a new era for practitioners, and the advantages over hard PMMA and soft lenses are well documented. For many new patients it will be quite possible to fit the standard hard lens designs which are favoured by the individual practitioner. However, for those wearers who subsequently suffer from flare, lens instability or lid irritation it is necessary to consider lenses with larger back central optic diameters and/or overall size and less edge lift. Similarly the soft lens wearer who is having to be refitted with a hard type lens will prefer immediate flare free stable vision which is not always immediately available with the standard hard lens constructions. In order to fulfil these requirements there have been several new designs offered for gas permeable lenses. To date these have received a mixed reception from practitioners and it is the purpose of this paper to compare these designs with the familiar PMMA constructions and offer new constructions which practitioners may find useful as an adjunct to those already available. Historically the initial interest with gas permeables goes back to CAB, but the lack of stability of lathe cutting reduced the effectivity of the material and forced us to consider moulded lenses such as the Hartflex from Wohlk. The edge lift of these lenses is, however, as little as 0.045mm, to 0.06ram,' and when fitted in the standard British fashion would lead to a very tight lens although giving often an amazing amount of tear interchange. Satisfaction is only really obtained utilizing a flatter than 'K' approach or by adding a flat edge curve about 0.5mm wide and blended into the lens periphery. The second lens introduced, the Menicon 02 works in a similar
16
fashion. The lens like the Hartflex has a very reduced edge lift, varying from 0.06mm on steeper radii. These figures the manufacturers will not vary. Fitting this lens with apical clearance gives absolutely no edge lift, but fitted 0.15mm flatter than 'K' as suggested by the manufacturers means again we depart from conventional methods of fitting. At the same time as these lenses were introduced to Great Britain a similar investigation was being undertaken by Williams in the USA 2. He designed for the Syntex Corporation two series of lenses made from Polycon. The first set of lenses (Table 1) had an overall size of 9.50mm with a BCOD of 8.40ram, and an axial edge lift of about 0.12mm. No mention was A.E.L.A.E.L.
@ 9.10 @ 9.50
(mm) (nun) 7 . 2 0 : 8 . 4 0 / 8.45:9.10/10.00:9.50 0.056 0.120 7.40 : / 9.10 /10.50 0.065 0.128 7.60 : / 9.30 /11.00 0.060 0.121 7.80 : / 9.70 /11.50 0.060 0.120 8.00 : /10.00 /12.00 0.058 0.117 8.20 : /10.20 /12.50 0.055 0.112 8.40 : /10.60 /13.00 0.055 0.111 Table 1. Tricurve Design (after Williams USA) O.S. 9.50ram. BCOD 8.40 made of how the peripheral curves were fashioned. The second series (Table 2) were of overall size 8.50mm, with a BCOD of 7.00mm, and an edge lift of 0.05 - 0.08mm. The lenses were made very thin with a centre thickness of 0.08ram, and an edge thickness of 0.07 - 0.09mm. This meant that lenses above -4.00 were of lenticular construction. This value for edge thickness was determined in trials which showed that less than 10% of patients were aware of lid sensation
Journal of the British Contact Lens Association
A.E.L.A.E.L.
@ 8.30 @ 8.5o (mm) (nun) 7.20:7.00/ 7.70:8.30/17.00:8.50 0.036 0.082 7.40 : / 7.90 /17.00 0.033 0.076 7.60 : / 8.10 /17.00 0.031 0.071 7.80 : / 8.30 /17.00 0.028 0.067 8.00 : / 8.50 /17.00 0.026 0.063 8.20 : / 8.70 /17.00 0.025 0.059 8.40 : / 8.90 /17.00 0.023 0.055 Table 2. Tricurve Design (after Williams U S A ) O.S. 8.50 B C O D 7.00mm. at this thickness. The r e c o m m e n d e d fitting was related to the a m o u n t of corneal astigmatism. For a 9.50ram lens on a cornea with up to 0.75D of astigmatism, the lens was fitted 0.20 - 0.30mm. flatter than the flattest 'K'. A s the corneal astigmatism increased the fitting steepened with respect to flattest ' K ' (Table 3). T h e prime aim was to fit large, flat and u n d e r the lid. T h e smaller 8.50ram lenses were fitted on alignment. B C O D / ' K ' Relationship for Polycon Lenses of 9.50mm. Overall Size
Corneal Cylinder (Dioptres)
BCOR/'K' Relationship
0 - 0.75 1.00 - 1.75 2.00 - 2.75 3.00 - 3.75 4 D and o v e r
0.30 - 0.20ram. flatter than 'K' 0.25 - 0.15mm. ,, 0.20 - 0.10ram. ,, 0.10 - 0.05ram. ,, On 'K' Table 3.
A.E.L.A.E.L.
@ 8.10 @ 8.5o (mm) (ram) 7.20:7.40/ 8.40:8.10/12.00:8.50 0.042 0.109 7.40 : / 8.70 /13.00 0.041 0.109 7.60 : / 8.90 /12.50 0.038 0.098 7.80 : / 9.10 /13.50 0.036 0.097 8.00 : / 9.30 /14.00 0.033 0.093 8.20 : / 9.40 /14.50 0.029 0.087 8.40 : / 9.70 /15.00 0.029 0.086 Table 5. Tricurve Design after Syntex (GB). O.S. 8.50 B C O D 7.40ram. A.E.L.A.E.L.
In o r d e r to find out the peripheral radii and their diameters so that I could try some of their designs I contacted Syntex. They informed me that the lenses p r o d u c e d in the U S A related to a special edge blend and finish and in order to achieve the same effective fit from a lens m a d e by one of the laboratories in G r e a t Britain the lenses should be both designed and fitted differently. They also included a 9.00ram lens. F o r a 9.50mm lens (Table 4) they suggested a lens be fitted 0.05ram flatter than 'K' against the 0.20ram to 0.30turn flatter in the U S A For a 8.50ram lens (Table A.E.L.A.E.L.
@ 9.10 @ 9.10 (mm) (mm) 7.20:7.90/ 8.25:9.10/ 9.00:9.50 0.0'79 0.127 7.40 : / 8.55 / 9.50 0.078 0.127 7.60 : / 8.75 /10.00 0.072 0.121 7.80 : / 9.05 /10.50 0.071 0.121 8.00 : / 9.25 /11.00 (I.066 0.116 8.20 : / 9.50 /11.00 0.064 0.108 8.40 : / 9.85 /11.00 0.065 0.105 Table 4. Tricurve Design after Syntex (GB). O.S. 9.50mm. B C O D 7.90ram.
18
5) they should be fitted 0.05ram steeper than 'K' against the alignment fitting in the U S A , and with a m u c h larger edge lift of 0.10mm. For a 9.00ram lens (Table 6) they should be on 'K'. A s the 9.50mm lens m a d e in Great Britain has a B C O D of only 0.5ram smaller I am surprised there is such a large difference in the way the lens is fitted, hearing in m i n d the edge lift is the same. It is slightly easier to understand why the 8.50mm lens from Great Britain with the large B C O D has to be fitted steeper r a t h e r than flatter because of the increased edge lift f r o m 0.07 to 0.10mm. I do, however, find it difficult to understand the true link up.
@ 8.60 @ 9.00 (ram) (mm) 7.20:7.60/ 8.30:8.60/10.00:9.00 0.061 0.117 7.40 : / 8.60 /11.00 0.060 0.121 7.60 : / 8.75 /11.00 0.054 0.109 7.80 : / 9.10 /11.50 0.055 0.109 8.00 : / 9.40 /12.00 0.055 0.108 8.20 : / 9.50 /12.50 0.048 0.100 8.40 : / 9.75 /13.00 0.047 0.098 Table 6. Tricurve Design after Syntex (GB). O.S. 9.00mm. B C O D 7.60ram. So far, as you can see, the trend has been towards flatter fitting techniques and I began to wonder w h e t h e r the days of alignment and slight apical clearance giving the classic fluorescein picture were n u m b e r e d , but the recent introduction of the Kelvin B o s t o n lens findings by colleagues Christopher W e s t o n and R o b e r t Grave 3 plus m y own findings have convinced me otherwise. To reconstruct lenses to fulfil these requirements means we have to look at the following factors:
1. Determination of BCOD. My first contention was that because of the superior wettability and oxygen permeability of the gas p e r m e a b l e materials it should be possible to replace the B C O R and the first peripheral curve of a tetracurve, or part of the secondary curve in the case of a tricurve, by one curve, the new B C O R and B C O D , and make the remainder of the lens flatter than the cornea to give the necessary tear flow under
Journal of tile British Contact Lens Associatioll
the lens. This would reduce the number of points along the edge of the lens where an error in a choice of d i a m e t e r or radius would lead to reduced tear flow. 1. D e t e r m i n a t i o n of B C O D .
My first contention was that because of the superior wettability and oxygen permeability of the gas p e r m e a b l e materials it should be possible to replace the B C O R and the first peripheral curve of a tetracurve, or part of the secondary curve in the case of a tricurve, by one curve, the new B C O R and B C O D , and make the remainder of the lens flatter than the c o r n e a to give the necessary tear flow under the lens. This would reduce the number of points along the edge of the lens where an error in a choice of d i a m e t e r or radius w o u l d lead to reduced tear flow. F r o m nr 2 (Fig. I) it is possible to work out the area of bearing which is the apical clearance and/or alignment areas as a percentage of the total lens. In o r d e r to try and ascertain what area should contribute to bearing and therefore determine the B C O D and what area should contribute to edge lift I compared the seven trial sets I use in practice or at the L R H to find out w h e t h e r this was a c o m m o n denominator in the success or failure of the designs. O n e must r e m e m b e r that a lightly blended lens will not have m u c h effect on the parameters of the lens, but a well b l e n d e d lens could effectively reduce the widths of each curve by 0.20mm overall, hence flattening the fit. i would also add that it is my custom to lightly blend the transition b e t w e e n the B C O R and the first peripheral curve w h e t h e r I ask for a light, medium or heavy b l e n d e d lens as heavy blending at this point tends to tighten the fit. Taking each lens design in turn:
rs
~i
e~
(5
"
aS
/ Bearing A r e a = All Apical Clearance and/or A l i g n m e n t Areas Bearing A r e a =It(_BP~_Q. D ) 2
T o t a l A r e a = ,i (.O~__~.)2 Bearing A r e a as % of Total A r e a : (BP, OD)~, (-O.S.) ~ x 100 Fig. 1
L e n s (1) 7 . 8 0 : 7 . 0 0 / 9 . 0 0 : 7 . 8 0 / 1 0 . 9 0 : 8.60
This lens, a tricurve, giving 0.12 e. 1. is fitted steep of ' K ' by 0.1ram. With a lightly blended lens the bearing area is 7.8mm wide and is 82.2% of the whole lens. O n a well blended lens where the second d i a m e t e r is reduced to 7.6 from 7.80 the area is 78.1%. O n the whole this lens is usually on the tight side.
can be a little small on occasions. The bearing d i a m e t e r is 7.80mm, giving a percentage cover of 75.1%. W h e n fully blended it is a good fitting lens but if the B C O R is slightly flat the final fitting would be on the loose side. T h e bearing area would then be 7.60mm. (71.3%).
L e n s (2) 7 . 8 0 : 7 . 0 0 / 8 . 4 5 : 7 . 8 0 / 9 . 3 0 : 8 . 4 0 / 1 1 . 0 0 : 9.00
L e n s (3) 7 . 8 0 : 7 . 0 0 / 8 . 7 5 : 7 . 8 0 / 1 0 . 1 0 : 8 . 4 0 / 1 1 . 3 0 : 9.00
A tetracurve giving an axial edge lift of 0.125ram. W h e n lightly blended it is a good lens but the edge lift
A tetracurve giving an axial edge lift of 0.15ram which w h e n fully blended is often too loose, but with Med. A.E.L.
1. 2. 3. 4. 5. 6. 7.
7.80 7.80 7.80 7.80 7.80 7.80 7.80
: 7.00/9.00 : 7.00/8.45 : 7.00/8.75 : 7.00/8.30 : 7.50/8.60 : 7.50/8.60 : 8.00/8.60
Lt. Med. Blend %
Fit
: 7.80/10.90 : 8.60 0.120 82.2 Tight : 7.80/9.30 : 8.40/11.00 : 9.00 0.125 75.1 Good : 7.80/10.10 : 8.40/11.30 : 9.00 0.150 75.1 Good : 8.00/10.50 : 8.50/12.50 : 9.00 0.150 79.0 Tight : 8.00/11.20 : 9.00 0.135 79.0 Tight : 8.20/9.60 : 8.90/10.60 : 9.50 0.150 74.5 Good : 8.50/10.60 : 9.50 0.138 80.0 Tight G o o d Bearing A r e a = 75 - 76% (70.9% - 82.2%) Table 7. Bearing A r e a as percentage of Overall Size of Lens.
Heavy Blend %
Fit
78.1 71.3 71.3 75.1 74.2 70.9 76.3
Tight Loose Loose Good Good Loose Good
(continued on page 25)
Journal of the British Contact Lens Association
19
(continued from page 19) a light b l e n d is fine. Diameter of bearing area is 7.80mm, w h e n lightly blended giving an area of 7 5 . l % , but when fully blended a bearing area of 71.3%. L e n s (4) 7 . 8 0 : 7 . 0 0 / 8 . 3 0 : 8 . 0 0 / 1 0 . 5 0 : 8 . 5 0 / 1 2 . 5 0 : 9.00
A tetracurve giving an axial lift of 0.15mm, requiring a meticulous edge blend. The biggest problem with these lenses has b e e n between the second and third curves. Should a fifth curve be introduced? W h e n poorly blended the lens gives a beating area of 79% but w h e n well blended a comfortable lens with a bearing area of 75.1%. L e n s (5) 7 . 8 0 : 7 . 5 0 / 8 . 6 0 : 8 . 0 0 / 1 1 . 2 0 : 9.00
A good lens when fully blended, giving an axial edge lift of 0.135mm. W h e n lightly blended it tends to be tight, the area covered being 79%. W h e n fully b l e n d e d it tends to become a good fitting bicurve. The area then covered is 74.2%. L e n s (6) 7 . 8 0 : 7 . 5 0 / 8 . 6 0 : 8 . 2 0 / 9 . 6 0 : 8 . 9 0 / 1 0 . 6 0 : 9.50
This lens gives an axial edge lift of 0.15ram. In m a n y cases when lightly blended it is quite good giving a bearing area of 74.5%, but with a heavier b l e n d gives a loose fitting with a bearing area of 70.9%. L e n s (7) 7 . 8 0 : 8 . 0 0 / 8 . 6 0 : 8 . 5 0 / 1 0 . 6 0 : 9.50
This lens with an axial edge lift of 0.138 has to be well b l e n d e d if it is to be successful and covers an area
of 76.3%. O n occasions when poorly made it can be on the tight side, covering an area of 80%. This lens design has b e e n used more and more with gas p e r m e a b l e materials. It is interesting to note from the above, which are summarized in Table 7, that the average area covered by lenses is between 70.9% on the loose side and 82.2% o n the tight side. The majority of satisfactory fittings fall with a beating area of 75 - 76%. For ease of calculation I have taken a figure of 75%. Therefore, assuming the superior wettability and permeability of gas permeables, we should be able to replace the whole of this bearing area by one curve and fit lenses as follows with a B C O D of: 7.40 for 8.50mm lenses (instead of the customary 7.00) 7.80 for 9.00mm lenses (instead of the customary 7 7.50) 8.25 for 9.50mm lenses (instead of the customary 7.50 - 8.00) and still remain within the requirements of apical clearance or at worst alignment. 2. C h o i c e o f E d g e Lift
This factor is truly very difficult to determine exactly for any one patient. It can be set for a trial set but can only be deemed correct after careful e x a m i n a t i o n of the patient. It has been customary to suggest an axial edge lift of 0.15ram for most standard P M M A wearers, but on the evidence of Hartflex,
T I T M U S E U R O C O N HAVE DEVELOPED A NEW COMPUTER CONTROLLED LATHE TECHNOLOGY WHICH PERMITS THE FULLY AUTOMATED PRODUCTION OF CONSISTENTLY REPRODUCIBLE ASPHERIC SURFACES OF THEIR WEICON LENSES.
F
WEICON
m S(
Further details from: TE PROFESSIONAL SERVICES DEP,~ Ashville House, Havant Road, Drayton, Portsmouth PO6 1 PA. or T E L : 0705 388716
Journal of the British Contact Lens Association
25
M e n i c o n and general experience, i feel 0.10 - 0.12 is as good a point to start as any. One should also r e m e m b e r that, assuming the clearance from the c o r n e a is the same for all lenses, a 7.80 : 7.00 will need m o r e edge lift to give the same corneal clearance at the edge of the lens than a 7.85 : 7.50 or 7.90 : 8.00. Theoretically, by increasing the B C O D on a 9.00mm lens from 7.00 to 7.80 means we can reduce the edge lift by 0.02mm without really affecting the fit.
3. Choice of Peripheral Curves and Diameters My i m m e d i a t e reaction was that it should be possible to m a k e all lenses with a bicurve design and this was confirmed by the introduction of the Kelvin B o s t o n lens. This lens is available at present only in a 9.50mm overall size, with a B C O D of 8.20mm, giving a 0.10mm axial edge lift from the following bicurve construction: 7.80 : 8.20/9.80 : 9.50 I understand a touch of 12.30mm curve 0.1ram wide is added to the periphery but this is really only part of the edge process and is fully blended in. The lens is fitted with slight apical clearance, and initial results as presented by A u e r b a c k 4 have been very encouraging. M y own experience would to some degree endorse their choice of a bicurve. A s stated previously, my choice of lens, based on my old trial sets, has been: 7.80 : 8.00/8.60 : 8.50/10.60 : 9.50
W h e n this is very fully blended the second curve tends to disappear. This can be replaced partly by the larger B C O D ; 8.25 has been suggested but has been m o d i f i e d to 8.30 for reasons that will become a p p a r e n t later, and by one single peripheral curve to give any edge lift required. I have found between 0.10 to 0.12mm the most satisfactory. This would give us the following constructions: 7.80 : 8.30/9.55 : 9.50 giving an edge lift of 0.10mm 9.80 : 9.50 giving an edge lift of 0.1 l m m 10.00 : 9.50 giving an edge lift of 0.12mm It is quite easy utilizing a programmable calculator such as the now outdated Texas SR52 or the latest T.I.59 to design for all radii lenses with any edge lift required. Similarly it would be possible to design two-curve lenses for an overall size of 8.50 with a B C O D of 7.40mm, and a 9.00mm lens with a B C O D of 7.80mm. (Table 8) With regard to the tricurve lenses, the advantage h e r e is that one can stay much closer to the changing periphery of the cornea and compensate the reduced tear v o l u m e under the first peripheral curve of the lens by having a flat enough tertiary curve to give corneal clearance as the lens rides up. W e s t o n and G r a v e based their ideas on a similar t e c h n i q u e to myself. T h e y have used the Menicon fitting set which offers a 9.20mm lens with an 8.00mm optic to obtain the correct central clearance and then
TITMUS EUROCON HAVE DEVELOPED A NEW COMPUTER CONTROLLED LATHE TECHNOLOGY WHICH PERMITS THE FULLY AUTOMATED PRODUCTION OF CONSISTENTLY REPRODUCIBLE ASPHERIC SURFACES OF THEIR PERSECON LENSES.
I
PERSECON
Further details from: TE PROFESSIONAL SERVICES DEPA Ashville House, Havant Road, Drayton, Portsmouth PO6 1 PA. or TEL: 0705 388716
26
.lournal of the British Contact Lens Association
BCOR 7.20 7.40 7.60 7.80 8.00 8.20 8.40
PERIPHERAL CURVES FOR BICURVE LENSES Overall Size 8.50ram. Overall Size 9.00mm. Overall Size 9.50mm. BCOD 7.40mm. (BCOD 7.80mm.) (BCOD 8.30mm) Axial Edge Lift (mm.) Axial Edge Lift (nun.) Axial Edge Lift (mm.) 0.10mm. 0.11mm. 0.12mm. 0.10ram. 0.11ram. 0.12mm. 0.10mm. 0.11ram. 0.12mm. 9.20 9.45 9.80 8.70 8.90 9.10 8.45 8.60 8.80 9.60 9.90 10.30 8.80 9.00 9.20 9.05 9.30 9.55 10.00 10.40 10.80 9.15 9.40 9.60 9.40 9.70 10.00 10.45 10.85 11.35 9.55 9.80 10.00 9.80 10.15 10.45 10.90 11.35 11.90 9.90 10.20 10.45 10.20 10.55 10.90 11.40 11.90 12.45 10.30 10.60 10.90 10.60 11.00 11.40 11.90 12.45 13.05 10.70 11.00 11.40 11.05 11.45 11.90 Table 8
redesigned the edge to give a 0.12mm edge lift. They have achieved excellent results. The design of the M e n i c o n is: a . e . l , at 8.60 a.e.1, at 9.20
7.80 : 8.00/8.50 : 8.60/8.80 : 9.20 0.021 0.053 and has b e e n modified by them to 7.80 : 8.00/8.80 : 8.60/12.30 : 9.20 0.028 0.120 This lens conforms to all my prerequisites, a 75% bearing area, 0.12 edge lift tricurve and with slight apical clearance. They have also made, by adjusting the B C O D and overall size, two other lenses, both with 0.12 edge lift and with 0.30ram wide peripheral Curves: a.e.1, at 8.40 a.e.1, at 9.00
7.80 : 7.80/8.85 : 8.40/12.65 : 9.00
0.028
0.120
a . e . l . a t 8.90 a.e.1, at 9.50
and 7.80 : 8.30/8.70 : 8.90/11.80 : 9.50 0.028 0.120 T h e only fault I can find in their theory is that the axial lift at the second diameter (8.60mm) is the same for all three lens diameters. My feeling is that the smaller lens should have a lower axial lift at this d i a m e t e r than the larger lens which is bearing on a flatter part of the cornea. If we now c o m p a r e the design by Williams in the U S A and G r e a t Britain plus the designs of Weston and G r a v e , we can compromise these and make theoretical computations for trial sets. For a 9.50ram lens: Williams U S A 7.80 : 8.40/9.70 : 9.10/11.50 : 9.50 Syntex G B 7.80 : 7.90/9.05 : 9.10/10.50 : 9.50
7.20 7.40 7.60 7.80 8.00 8.20 8.40
: 8.30/8.00 : /8.30 : /8.55 : /8.85 : /9.15 : /9.50 : /9.80
W e s t o n & G r a v e 7.80 : 8.30/8.70 : 8.90/12.50 : 9.50 L o o k i n g at the lens designed by Williams in the U S A it is easy to understand why it has to be fitted flatter, although why as much as 0.20 to 0.30mm I do not understand. The secondary curve is slightly wider, and just possibly too wide to be a full bearing surface and may I suppose dig into the peripheral cornea. The Syntex lens has an immediate disadvantage in having a B C O D of 7.9 which defeats the o b j e c t of what we set out to achieve. The design by W e s t o n and G r a v e I feel is very good, but gives a large difference b e t w e e n second and third radii. Personally I have always used the following guide for flattening the second curve: F o r a B C O D of 7.00mm 0.50 - 0.60mm ,, 7.50mm 0.70 - 0.80mm ,, 8.00mm 0.90- 1.00mm ,, 8.50mm 1.10- 1.20mm In this case we are nearer a B C O D of 8.50 than 8.00 and t h e r e f o r e we should consider the following: 7.80 : 8.30/8.90 : 8.90/11.50 : 9.50 T a b l e 9 shows the full range of parameters for all B C O R ' s for an axial lift of 0.10, 0.11 and 0.12mm but maintaining the axial lift at 8.90 as 0.032mm. F o r lenses of 9.00mm overall size we have: Williams U S A n/a Syntex G B 7.80 : 7.60/9.10 : 8.60/11.50 : 9.00 W e s t o n & G r a v e 7.80 : 7.80/8.85 : 8.40/12.65 : 9.00 T h e Syntex G B lens has again the disadvantage of having a slightly too small B C O D and cannot be
A.E.L. of 0.12mm. @ 9.50mm. : 8.90/9.70 : 9.50 : /10.30 /10.85 /11.50 /12.15 /12.80 /13.55
A.E.L. of 0.11ram. @ 9.50mm. 9.25 : 9.50 9.75 10.30 10.85 11.40 11.90 12.50
A.E.L. of 0.10mm. @ 9.50mm. 8.90 : 9.50 9.30 9.80 10.25 10.75 11.15 11.70
Table 9. Tricurve Designs (after Atkinson) Overall Size 9.50mm., B C O D 8.30mm., Constant Axial Edge Lift 0.10 - 0.12mm. maintaining an Axial Lift of 0.032mm. at 8.90mm.
28
Journal of the British Contact Lens Association
considered. The Weston and Grave tens again appears satisfactory. If however we apply the original criteria the secondary curve should be 0.90mm flatter, and my suggestion would be, against what I said f admit for the 9.50mm lens, to steepen the second curve very slightly and flatten the edge: 7.80 : 7.80/8.70 : 8.40/13.00 : 9.00 This has the disadvantage of introducing a very flat third curve for very flat radii. If, however, we were to increase the axial lift at 8.40mm to 0.028 as suggested by W e s t o n and Grave, this would have the effect of reducing the third curve on a lens of B C O R of 8.40mm f r o m 15.85mm to 15.20mm which is a minimal change at this level. Table 10 shows the full range of B C O R ' s for a fitting set for edge lifts of 0.10 0.12mm, maintaining the axial lift at 8.40mm as 0.024mm. F o r the 9.20mm lens we have only the Weston and G r a v e to go on, which is: 7.80 : 8.00/8.80 : 8.60/12.35 : 9.20 This gives an axial lift of 0.028mm, and based on the earlier criteria this fits in very well. Table 11 shows the full trial set for axial e d g e lifts of 0.10 - 0.12mm. I w o u l d add that I am only attempting here to satisfy the academic side and am in no way critical of their designs. In practice slight errors, even within British Standards, will m e a n the final decision about any lens must be down to the practitioner and modified if incorrect. Finally, for an 8.50mm lens we have the following: Williams U S A 7.80 : 7.00/8.30 : 8.30/17.00 : 8.50
Syntex G B
a.e. 1.0.07mm. 7.80 : 7.40/9.10 : 8.10/13.50:8.50 a.e.1.0.10mm
Weston & Grave (assumed) 7.80 : 7.30/9.00 : 7.90/13.80 : 8.50 a.e.1.0.12mm The lens designed by Williams is fitted on flattest ' K ' , hence the reason for keeping the secondary curve tight, although I feel in this instance too wide, and very meticulous blending will be required. The Syntex G B lens is fitted fractionally steeper than 'K' and varies little from the assumed construction of W e s t o n and Grave. My own feelings are that we should reduce the B C O D to 7.30mm instead of 7.40mm, so that the peripheral curves can be made of equal width and we should in this instance reduce the edge lift to 0.11mm, otherwise the fit is too flat. My suggestion would be: 7.80 : 7.30/9.00 : 7.90/12.60 : 8.50 a.e.1.0.11mm. T h e full set with edge lifts of 0.09 - 0. l l m m is given in Table 12. The above discussion, therefore, offers in my opinion variations to the standard P M M A fitting sets. E x p e r i m e n t a l work with these lenses to date shows s o m e very impressive findings. In many cases it has b e e n possible to fit these designs with a quite stable fitting without going flatter than alignment and often with a trace of fluorescein centrally. (Fig.2). The overall size is chosen just as before with the majority of patients being fitted with 9.00mm lenses and the 9.50mm lens only used on large palpebral apertures.
A.E.L. of A . E . L . of A.E.L. of 0.12mm. 0.11ram. 0.10ram. @ 9.00mm. @ 9.00mm. @ 9.00mm. 7.20 : 7.80/7.85 : 8.40/10.85 : 9.00 10.20 : 9.00 9.65 : 9.00 7.40 : /8.15 /11.55 10.80 10.15 7.60 : /8.40 /12.30 11.45 10.70 7.80: /8.70 /13.10 12.10 11.25 8.00 : /8.95 /13.95 12.80 11.85 8.20 : /9.25 /14.85 13.55 12.45 8.40 : /9.50 /15.85 14.30 13.10 Table 10. Tricurve Designs (after Atkinson) Overall Size 9.00mm. B C O D 7.80mm., Constant Axial Edge Lift 0.10 - 0.12mm. maintaining an Axial Lift of 0.024mm. at 8.40mm.
7.20 7.40 7.60 7.80 8.00 8.20 8.40
: 8.00/7.95 : /8.25 : /8.50 : /8.80 : /9.10 : /9.40 : /9.65
A.E.L. of 0.12mm. @ 9.20mm. : 8.60/10.30 : 9.20 /10.90 /11.60 /12.30 /13.05 /13.85 /14.70
A.E.L. of 0.11ram. @ 9.20mm. 9.75 : 9.20 10.30 10.85 11.45 12.10 12.75 13.40
A.E.L. of 0.10mm. @ 9.20ram. 9.30 : 9.20 9.75 10.25 10.75 11.25 11.80 12.40
T a b l e 11. Tricurve Designs (after Atkinson) Overall Size 9.20mm. B C O D 8.00mm., Constant Axial Edge Lift 0.10 - 0.12mm. maintaining an Axial Lift of 0.028mm. @ 8.60mm.
J o u r n a l of the British Contact Lens Association
29
7.20 7.40 7.60 7.80 8.00 8.20 8.40
A.E.L. of 0.11mm. @ 8.50mm. : 7.30/8.15:7.90/10.60:8.50 : /8.45 /11.20 : /8.70 /11.90 : /9.00 /12.60 : /9.30 /13.30 : /9.65 /14,10 : /9.95 /14,90
A.E.L. of 0.10ram. @ 8.50mm. 9.95 lOA5 11.00 ll.60 12.20 12.80 13.50
A.E.L. of 0.09mm. @ 8.50mm. 9.40 9.85 10.30 10.80 11.30 11.80 12.35
Tabte 12. Tricurve Designs (after Atkinson) Overall Size 8.50mm., B C O D 7.30mm. Constant Axial Edge Lift 0.09 - 0.1 l m m . maintaining an Axial Lift of 0.028ram. at 3.90ram.
Fig. 2. The main criteria to be applied has been is the lens stable and is the lens moving, the latter being just as essential for gas permeable materials as P M M A in o r d e r to p u m p the tears under the lens. I feel that a tricurve fitting is the more stable but the bicurve with an edge lift of 0.10mm to 0.12mm has worked well. At present, without calculations however to back me up, I feel a bicurve with a 0.10mm edge lift is probably equivalent in tear volume beneath the lens periphery to a tricurve with an edge lift of 0.12mm. The disadvantage of a tricurve is that there is much more d e m a n d on the accuracy of the diameters and more blending is often required, even with a trusty laboratory, which is slightly more time-consuming, and sometimes leads to breakage (with Boston especially). With a bicurve there is only one blending area. O f the materials available I have been happy using these constructions with XL30, Boston and Polycon, but most disappointed with XL20 which seems to require a standard P M M A design in order to p r e v e n t o e d e m a . With the latter material many cases have exhibited marked o e d e m a and staining which has been resolved with the others, even Polycon and C A B , which have a similar D K value. I have however found the designs unimpressive when a lens rides up due to tight lids as they tend to stick, and of reduced value on toric corneas where the fitting tends to bite on the flatter cornea or lie too fiat on the steeper cornea. In some cases I have had to revert to a toric
30
lens or a small spherical lens to obtain the least corneal disturbance. Finally, 1 do not think it is necessary to make the lens as thin as those designed by Williams for Syntex as we are not attempting to Obtain the same fluorescein picture. O n the whole it would seem preferable to m a k e the centre thickness a minimum of 0.12mm to avoid breakage or distortion. In plus lens cases this presents no problems and the practitioner can decide w h e n they require a reduced optic to give a r e d u c t i o n in lens weight or a means of keeping the lens under the lid. In minus lens cases this can present a p r o b l e m above - 5 . 5 0 O D . A t this point a mid peripheral " l u m p " can occur where the Front Central O p t i c D i a m e t e r ( F C O D ) which should be larger than the B C O D m e e t s the lenticular curve, the thickness at this point being greater than the centre or edge. In o r d e r to reduce this " l u m p " it is either necessary to r e d u c e the F C O D to a value smaller than the B C O D , and risk inducing flare or by introducing a third curve on the front surface. T h e designs offered in this paper are a compromise b e t w e e n the standard lathe cut lenses used in the past and Some of the newer concepts. No doubt, however, we shall soon be seeing alternative designs offered using A s p h e r i c or Elliptical curves, but in the m e a n t i m e the parameters discussed will probably b e c o m e the rule rather than the exception as new better materials gradually replace P M M A .
References 1. 2. 3. 4.
Atkinson, T. C. O., Journal British Contact Lens Association 3:3 July 1980 pp. I05 - 112. Williams, C. E. Journal American Optometric Association 50:3 March 1979 pp. 331 - 336 Weston, C. & Grave, R. Personal Communication A u e r b a c k , D. P. Optician 180 D e c e m b e r 6th 1980 pp. 20 - 24.
A d d r e s s for further correspondence: 24 Forest R o a d , L o u g h t o n , Essex.
Journal of the British Contact Lens Association