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Boston lens
The B.C.L.A. held a special Forum on Gas Permeable Hard Lenses at their meeting in March 1981. Papers were given by D. P. Auerbach, K. H. Edwards and Nigel Burnett-Hodd, and are presented on the following pages.
Boston Lens D. P. Auerbach, BSc(Hons), FBCO David Auerbach is Head of Clinical Research at Kelvin. Chart I
As with the other two materials under discussion the Boston Polymer is basically a cocktail of PMMA and siloxane. From a clinical aspect, the greatest asset of the Boston lens must lie with its high oxygen permeability. This is given in Table I at 25°C. At a
THE BOSTON LENSTM CONTACT LENS EOP CONVERSION* ( DK=12.3 xlO-"ml 0 z cmZ/sec ml mmHg )**
Table I O X Y G E N P E R M E A B I L I T Y (DK)* A T 25°C The Boston Polymer Alberta (XL30) Menicon O2 CAB Polycon * ml O2 cm 2/sec.ml mmHg
12.3 10.5 7.0 4.5 3.9
× x x x x
10" 10" 10" 10" 10"
greater temperature as in vivo, then, of course, the permeability is somewhat higher, in fact, we expect a gain of some 20 to 25%, but its measurement becomes less reliable. The DK value can vary somewhat between batches, and with the Alberta material, the permeability may vary along the rod, so as that a button cut from the bottom of the rod has a slightly lower D K value than a lens button cut from the top of the rod. These variances do appear to be significant and are given in Table II. For the Boston Polymer, Table II Alberta (XL30) D K for given batch rod 5.9 x 10" to8.2 x 10 ~' D K for other batch rod 11.5 × 10" to 15 x 10" the Polymer Technology Corporation has made careful measurements of permeability as a function of position in the rod, and they have found no variation larger than the standard deviation of measurement (less than 3%). Other important characteristics as hardness and refractive index also have shown no measurable variation along the rod. If we now progress to relate for Boston the oxygen equivalent % beneath the lens to lens centre thickness, then we can see, from Chart I that the oxygen transmission through stationary lenses of Boston
13~
Air
21
~14 ~.. 12
8
_~
. . . . - ~
4
2 0
1
0.1
I
0.2
I
03
I
0.4
Sleep _ Glycogen Depletion Edema I
0.5
LENS CENTER THICKNESS (ram)
* Loshoek,S.ond HdI,RM., ICLC Nov Dec. 26-28,77 Irving Fair, Ph.D material, at practically any clinically used thickness, ought to be sufficient to avoid measurable corneal swelling. Josh Josephson has commented that he has not found clinically observable oedema to be a particularly significant observation with any regular occurrence. We have observed oedema when this has related to poor or experimental lens designs. We do not believe DK or permeability to be the last word, and believe rather that design is essentially important. Whilst studying varying reduced edge lift designs then in certain instances, at certain prescriptions, it is quite conceivable that a Boston lens may affect a significant oedema which may be alleviated not through simple fenestration, but with a greater and improved peripheral corneal clearance. The Boston Polymer, like PMMA, features a very low water absorption, given as less than 1% in the Data Table, and this of course enhances its stability with hydration. We investigatcd the lens stability for
lot. naj oj the BI aish ( omact Lens Association
just a small series of cut lenses of relatively steep base curves and relatively flat base curves and have found the stability of the len~ to be quite acceptable. (Table iii). In fact, the greatest flattening in base curve was one of .09 which occurred for - 1 5 , with a centre thickness of .08. Whereas for the o t h e r - - 15 lens the change was .06.
Table III B.V.P, --3.00 +3.00 --15.00 + 14,00 --3.00 +3.00 --15.00 +14.00
BCOR BCOR BCOR BCOR BCOR BCOR Centre BCOR BCOR thickness dry w e t 20°C w e t 20°C w e t 20°C w e t 20°C w e t 20°C w e t 20°C w e t 20°C 15 mins. 30 rains, 1 hour 2hours 3hours 12 hours 24 hours .10 7.30 7.29 7.29 7.28 7.28 7.29 7.30 7.30 .27 7.27 7.27 7.28 7.29 7.28 7.28 7.28 7.28 ,08 7.27 7.32 7.32 7.33 7.35 7.36 7.33 7.33 .46 7.29 7.30 7.30 7.30 .7.29 7.29 7.29 7.29 .10 8.18 8.20 8.19 8.17 8.19 8.18 8.18 8.18 .27 8.18 8.19 8.18 8.19 8.19 8.19 8.19 8.19 .08 8,17 8.24 8.26 8.25 8.27 8.26 8,26 8.26 .45 8,18 8.20 8.21 8.20 8.19 8.18 8.'19 8.19
Wetting, or rather non-wetting of gas permeable lenses is naturally an important issue amongst practitioners. According to the Polyme~ Technology Corporation, the polymer's chemical structure incorporates an ionic charge that enhances the material's wettability, so negating the need for any surface treatment. The figures given in Table IV relate to work undertaken by Brian Tighe at Aston University on the Boston material, and quantify
Table IV Boston Lens (Stored Dry) Water 68° Boston Lens (Stored in unbuffered saline) Water 60° - 65° Boston Lens (Stored in buffered borax)* Water 56° Boston Lens (Boiled in sodium bicarbonate)* Water 56° Boston Lens (Stored in Soaclens) Water 60° - 65° Wetting Solutions about 20° below these figures. wetting angle not with wetting agent, but rather with the pre-soak. The wetting angle is with water. We may learn that a pre-soak in solutions buffered to a slightly alkaline pH would appear to improve wettability, whilst wetting solutions in general give a gain of some 20°. In practice, we have found the wetting of Boston lenses to be quite acceptable, and to pose no real difficulties. A daily surfactant cleaning being adequate to promote or encourage a complete and even tear film over the lens surface. As regards surface resistance then despite the equivalent Rockwell Hardness figure, we have found that the lenses do scratch more easily than PMMA, and patients ought to be advised accordingly. Lens
140
fragility has been reported as a possible cause for concern from both manufacturer and practitioner. The breakage rate at the manufacturing stage, was, in fact, negligible, so much so that it had not warranted being recorded. On the question of practitioner breakage, or reported breakage rate from practitioners supplied by Kelvin, then over a three month period, this ran at
1.3% (1335 lenses). Over a nine month period; we have experienced a breakage rate of less than 5%. The greater proportion of these lenses having a lenticular construction. Brian Tigbe has quantified the "toughness" of Polycon, Boston Alberta and PMMA lenses from consideration of the area beneath the appropriate stress-strain curves shown in Chart Ii. For these materials we may also express their relative elasticity or brittleness from consideration of the percentage elongation at break. This is indicated for each individual material by the arrowheads beneath the respective curve. As regards greater toughness and least brittleness, then the order of merit on both counts ought to read Polycon, then PMMA, Alberta, and finally Boston. However, what is more important is to recognise that for all "normal" stresses and providing we don't approach our lens with a sledge hammer then all.these materials ought to mechanically perform adequately and to physically answer our needs. My own experiences indicate that breakages only occur under certain stress handling situations, where the patient is applying undue pressure to the lens as when squeezing a lens that has landed inverted upon a surface.
Further Technical Data Include: Refractive Index Specific Gravity Water Absorption Wetting Angle (Poster) with Soaclens Scratch Resistance
Heat Resistance
1.471 1.14 Less than 1% Boston 33.3% PMMA 25,4% About same as PMMA, Rockwell hardness M Scale 80 Cannot be boiled
Journal of the British Contact Lens Association
Chart II
Z 0 (..)
<
O
,d <
< Z~
Z © ©
÷ ELONGATION
For patients who have experienced difficulties with the lens whilst cleaning, then we recommend that they use the Hydra-Mat. We encourage all patients to keep their lenses flat rather than to flex or fold them. On the question of the compatibility of benzalkonium chloride with Boston lens material, we a r e able to report upon some preliminary work undertaken by Brian Meakin of Bath University. He has been concerned with the question of the interaction of polymer powder and lenses with benzalkonium chloride in neutral isotonic buffer solution over a concentration range of 0.002% to .015%. He has found the level of interaction over this concentration range to be similar or less than that for PMMA. On the effect of storage in benzalkonium chloride on Boston lens surface and optical parameters, then we have recorded no signs of surface degradation or any significant shift in lens parameters. In almost 12 months we have issued over 200 Boston lenses to the Kelvin Boston design. That is an 8.20 optic a n d . 1 E/L at an overall size of 9.50. The Kelvin Boston lens has successfully resolved an enormous volume of grief cases. In general terms the lens has proved valuable in resolving hema complications. These do include chronic oedemas with new vessel growth, certain lid conditions, and visual
Journal of the British Contact Lens Association
difficulties. The material has proved valuable in resolving PMMA oedema cases providing greater patient comfort with no restriction on day wearing time, and an absence of spectacle blur. However, some patients do experience a slight tackiness to the lens surface. The lens provides for a more rapid adaptation, and a greater physical and visual stability, especially for many astigmatic patients. From the patients, the lens has been well received. Possibly, what we ought to appreciate most with any of these materials is that whilst gas permeability or gas transmission is an excellent asset to a good material, it is only one part of the story. The general. characteristics of the lens, how it has been designed and, how is it manufactured, must remain paramount in the success or failure of the lens. For a simple mechanical deficiency, as in the edge of a lens, can destroy much of what we stand to gain. Finally, I would like to acknowledge my thanks to Brian Tighe and Brian Meakin in generating certain information in time for this meeting. Address for further correspondence: Kelvin Lenses Ltd., Kelvin House, Manchester Road, Denton, Manchester.
141
Boston Lens D. P. Auerbach, BSc(Hons), FBCO David Auerbach is Head of Clinical Research at Kelvin. Chart I
As with the other two materials under discussion the Boston Polymer is basically a cocktail of PMMA and siloxane. From a clinical aspect, the greatest asset of the Boston lens must lie with its high oxygen permeability. This is given in Table I at 25°C. At a
THE BOSTON LENSTM CONTACT LENS EOP CONVERSION* ( DK=12.3 xlO-"ml 0 z cmZ/sec ml mmHg )**
Table I O X Y G E N P E R M E A B I L I T Y (DK)* A T 25°C The Boston Polymer Alberta (XL30) Menicon O2 CAB Polycon * ml O2 cm 2/sec.ml mmHg
12.3 10.5 7.0 4.5 3.9
× x x x x
10" 10" 10" 10" 10"
greater temperature as in vivo, then, of course, the permeability is somewhat higher, in fact, we expect a gain of some 20 to 25%, but its measurement becomes less reliable. The DK value can vary somewhat between batches, and with the Alberta material, the permeability may vary along the rod, so as that a button cut from the bottom of the rod has a slightly lower D K value than a lens button cut from the top of the rod. These variances do appear to be significant and are given in Table II. For the Boston Polymer, Table II Alberta (XL30) D K for given batch rod 5.9 x 10" to8.2 x 10 ~' D K for other batch rod 11.5 × 10" to 15 x 10" the Polymer Technology Corporation has made careful measurements of permeability as a function of position in the rod, and they have found no variation larger than the standard deviation of measurement (less than 3%). Other important characteristics as hardness and refractive index also have shown no measurable variation along the rod. If we now progress to relate for Boston the oxygen equivalent % beneath the lens to lens centre thickness, then we can see, from Chart I that the oxygen transmission through stationary lenses of Boston
13~
Air
21
~14 ~.. 12
8
_~
. . . . - ~
4
2 0
1
0.1
I
0.2
I
03
I
0.4
Sleep _ Glycogen Depletion Edema I
0.5
LENS CENTER THICKNESS (ram)
* Loshoek,S.ond HdI,RM., ICLC Nov Dec. 26-28,77 Irving Fair, Ph.D material, at practically any clinically used thickness, ought to be sufficient to avoid measurable corneal swelling. Josh Josephson has commented that he has not found clinically observable oedema to be a particularly significant observation with any regular occurrence. We have observed oedema when this has related to poor or experimental lens designs. We do not believe DK or permeability to be the last word, and believe rather that design is essentially important. Whilst studying varying reduced edge lift designs then in certain instances, at certain prescriptions, it is quite conceivable that a Boston lens may affect a significant oedema which may be alleviated not through simple fenestration, but with a greater and improved peripheral corneal clearance. The Boston Polymer, like PMMA, features a very low water absorption, given as less than 1% in the Data Table, and this of course enhances its stability with hydration. We investigatcd the lens stability for
lot. naj oj the BI aish ( omact Lens Association
just a small series of cut lenses of relatively steep base curves and relatively flat base curves and have found the stability of the len~ to be quite acceptable. (Table iii). In fact, the greatest flattening in base curve was one of .09 which occurred for - 1 5 , with a centre thickness of .08. Whereas for the o t h e r - - 15 lens the change was .06.
Table III B.V.P, --3.00 +3.00 --15.00 + 14,00 --3.00 +3.00 --15.00 +14.00
BCOR BCOR BCOR BCOR BCOR BCOR Centre BCOR BCOR thickness dry w e t 20°C w e t 20°C w e t 20°C w e t 20°C w e t 20°C w e t 20°C w e t 20°C 15 mins. 30 rains, 1 hour 2hours 3hours 12 hours 24 hours .10 7.30 7.29 7.29 7.28 7.28 7.29 7.30 7.30 .27 7.27 7.27 7.28 7.29 7.28 7.28 7.28 7.28 ,08 7.27 7.32 7.32 7.33 7.35 7.36 7.33 7.33 .46 7.29 7.30 7.30 7.30 .7.29 7.29 7.29 7.29 .10 8.18 8.20 8.19 8.17 8.19 8.18 8.18 8.18 .27 8.18 8.19 8.18 8.19 8.19 8.19 8.19 8.19 .08 8,17 8.24 8.26 8.25 8.27 8.26 8,26 8.26 .45 8,18 8.20 8.21 8.20 8.19 8.18 8.'19 8.19
Wetting, or rather non-wetting of gas permeable lenses is naturally an important issue amongst practitioners. According to the Polyme~ Technology Corporation, the polymer's chemical structure incorporates an ionic charge that enhances the material's wettability, so negating the need for any surface treatment. The figures given in Table IV relate to work undertaken by Brian Tighe at Aston University on the Boston material, and quantify
Table IV Boston Lens (Stored Dry) Water 68° Boston Lens (Stored in unbuffered saline) Water 60° - 65° Boston Lens (Stored in buffered borax)* Water 56° Boston Lens (Boiled in sodium bicarbonate)* Water 56° Boston Lens (Stored in Soaclens) Water 60° - 65° Wetting Solutions about 20° below these figures. wetting angle not with wetting agent, but rather with the pre-soak. The wetting angle is with water. We may learn that a pre-soak in solutions buffered to a slightly alkaline pH would appear to improve wettability, whilst wetting solutions in general give a gain of some 20°. In practice, we have found the wetting of Boston lenses to be quite acceptable, and to pose no real difficulties. A daily surfactant cleaning being adequate to promote or encourage a complete and even tear film over the lens surface. As regards surface resistance then despite the equivalent Rockwell Hardness figure, we have found that the lenses do scratch more easily than PMMA, and patients ought to be advised accordingly. Lens
140
fragility has been reported as a possible cause for concern from both manufacturer and practitioner. The breakage rate at the manufacturing stage, was, in fact, negligible, so much so that it had not warranted being recorded. On the question of practitioner breakage, or reported breakage rate from practitioners supplied by Kelvin, then over a three month period, this ran at
1.3% (1335 lenses). Over a nine month period; we have experienced a breakage rate of less than 5%. The greater proportion of these lenses having a lenticular construction. Brian Tigbe has quantified the "toughness" of Polycon, Boston Alberta and PMMA lenses from consideration of the area beneath the appropriate stress-strain curves shown in Chart Ii. For these materials we may also express their relative elasticity or brittleness from consideration of the percentage elongation at break. This is indicated for each individual material by the arrowheads beneath the respective curve. As regards greater toughness and least brittleness, then the order of merit on both counts ought to read Polycon, then PMMA, Alberta, and finally Boston. However, what is more important is to recognise that for all "normal" stresses and providing we don't approach our lens with a sledge hammer then all.these materials ought to mechanically perform adequately and to physically answer our needs. My own experiences indicate that breakages only occur under certain stress handling situations, where the patient is applying undue pressure to the lens as when squeezing a lens that has landed inverted upon a surface.
Further Technical Data Include: Refractive Index Specific Gravity Water Absorption Wetting Angle (Poster) with Soaclens Scratch Resistance
Heat Resistance
1.471 1.14 Less than 1% Boston 33.3% PMMA 25,4% About same as PMMA, Rockwell hardness M Scale 80 Cannot be boiled
Journal of the British Contact Lens Association
Chart II
Z 0 (..)
<
O
,d <
< Z~
Z © ©
÷ ELONGATION
For patients who have experienced difficulties with the lens whilst cleaning, then we recommend that they use the Hydra-Mat. We encourage all patients to keep their lenses flat rather than to flex or fold them. On the question of the compatibility of benzalkonium chloride with Boston lens material, we a r e able to report upon some preliminary work undertaken by Brian Meakin of Bath University. He has been concerned with the question of the interaction of polymer powder and lenses with benzalkonium chloride in neutral isotonic buffer solution over a concentration range of 0.002% to .015%. He has found the level of interaction over this concentration range to be similar or less than that for PMMA. On the effect of storage in benzalkonium chloride on Boston lens surface and optical parameters, then we have recorded no signs of surface degradation or any significant shift in lens parameters. In almost 12 months we have issued over 200 Boston lenses to the Kelvin Boston design. That is an 8.20 optic a n d . 1 E/L at an overall size of 9.50. The Kelvin Boston lens has successfully resolved an enormous volume of grief cases. In general terms the lens has proved valuable in resolving hema complications. These do include chronic oedemas with new vessel growth, certain lid conditions, and visual
Journal of the British Contact Lens Association
difficulties. The material has proved valuable in resolving PMMA oedema cases providing greater patient comfort with no restriction on day wearing time, and an absence of spectacle blur. However, some patients do experience a slight tackiness to the lens surface. The lens provides for a more rapid adaptation, and a greater physical and visual stability, especially for many astigmatic patients. From the patients, the lens has been well received. Possibly, what we ought to appreciate most with any of these materials is that whilst gas permeability or gas transmission is an excellent asset to a good material, it is only one part of the story. The general. characteristics of the lens, how it has been designed and, how is it manufactured, must remain paramount in the success or failure of the lens. For a simple mechanical deficiency, as in the edge of a lens, can destroy much of what we stand to gain. Finally, I would like to acknowledge my thanks to Brian Tighe and Brian Meakin in generating certain information in time for this meeting. Address for further correspondence: Kelvin Lenses Ltd., Kelvin House, Manchester Road, Denton, Manchester.
141