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The Eye and the Contact Lens In Search of the Ultimate Soft Lens Solution W ith the marketplace now well supplied (some would say literally saturated) with soft lens solutions, this would seem an ideal time for retros- pection, i.e., to ask “ just what the ideal solution should be?” Put another way: After two historic decades of serious formulation, trial and error ventures, and other sundry strategies applied to the soft solu- tions art, can one now detect a “con- vergence” toward the ultimate sofl lens solution? Properties You Can’t Refuse? One way to test for such trends might be to look for industry agreement on the most fundamental chemical- physical properties of these prod- ucts. This was done recently with a survey of 17 solutions whose labels listed varied objectives and expec- tations, but all of which were directed toward soft lens care.’ Four inescapable properties that can offer clues to intended actions (and sometimes their reception by the eye) are osmolality (primarily “salt balance”), pH (relative acidity/alka- linity), buffering capacity (pH stabil- ity), and viscosity (“stickiness”). Figures l-4 show the distributions found for each property among the 17 products studied (the numbers along the horizontal scales in each figure indicate position within that se- quence only, i.e., they do not stand % NaCl 1.1 1 2 3 4 5 6 7 8 9 1011121314151617 Soft Lens Solution Figure 1. The osmolality (equivalent salt balance) of 17 soft lens solutions intended for one or more of the following purposes: disinfection (hot), disinfection (cold), storage, rinsing, or soaking. The numbering (x-axis) indicates ordering within the group for this particular property, i.e., not for specific product designations. pH Value 7.5 7.4 ACIDITY / ALKALINITY- 7.3 7.2 7.1 7 6.9 6.8 . . s 1. 8.. 8 I.. . . 8. I 1 2 3 4 5 6 7 8 9 1011121314151617 Soft Lens Solution Figure 2. The pH (measured acidity/alkalinity) of the 17 soft lens solutions shown in Figure 1. The numbering indicates order only. Volume 16, Numbers 7 & 8 July/August 1989 237

In search of the ultimate soft lens solution

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Page 1: In search of the ultimate soft lens solution

The Eye and the Contact Lens

In Search of the Ultimate Soft Lens Solution

W ith the marketplace now well supplied (some would say literally saturated) with soft lens solutions, this would seem an ideal time for retros- pection, i.e., to ask “just what the ideal solution should be?”

Put another way: After two historic decades of serious formulation, trial and error ventures, and other sundry strategies applied to the soft solu- tions art, can one now detect a “con- vergence” toward the ultimate sofl lens solution?

Properties You Can’t Refuse? One way to test for such trends might be to look for industry agreement on the most fundamental chemical- physical properties of these prod- ucts. This was done recently with a survey of 17 solutions whose labels listed varied objectives and expec- tations, but all of which were directed toward soft lens care.’

Four inescapable properties that can offer clues to intended actions (and sometimes their reception by the eye) are osmolality (primarily “salt balance”), pH (relative acidity/alka- linity), buffering capacity (pH stabil- ity) , and viscosity (“stickiness”). Figures l-4 show the distributions found for each property among the 17 products studied (the numbers along the horizontal scales in each figure indicate position within that se- quence only, i.e., they do not stand

% NaCl 1.1

1 2 3 4 5 6 7 8 9 1011121314151617 Soft Lens Solution

Figure 1. The osmolality (equivalent salt balance) of 17 soft lens solutions intended for one or more of the following purposes: disinfection (hot), disinfection (cold), storage, rinsing, or soaking. The numbering (x-axis) indicates ordering within the group for this particular property, i.e., not for specific product designations.

pH Value 7.5

7.4 ACIDITY / ALKALINITY-

7.3

7.2

7.1

7

6.9

6.8 . . s 1. 8.. 8 I.. . . 8. I 1 2 3 4 5 6 7 8 9 1011121314151617

Soft Lens Solution

Figure 2. The pH (measured acidity/alkalinity) of the 17 soft lens solutions shown in Figure 1. The numbering indicates order only.

Volume 16, Numbers 7 & 8 July/August 1989 237

Page 2: In search of the ultimate soft lens solution

3.5

3

2.5

2

1.5

1

0.5

0

(Weak)

(Strong)1 2 3 4 5 6 7 8 9 1011121314151617

Soft Lens Solution

Figure 3. The buffering capacity (resistance to pH challenge) responses of the 17 soft lens solutions in Figure 1. The numbering indicates order only.

(High) 4.5

4

3.5

3

2.5

2

1.5

1

0.5

0 (LOW) 1 2 3 4 5 6 7 8 9 1011121314151617

Soft Lens Solution

Figure 4. The relative viscosities (compared to-water = 1) of the 17 soft lens solutions in Figure 1. The numbering indicates order only.

for the same product from figure to figure).

Prevailing Wisdoms From Fig- ure 1, the individuality of the market- place can be seen still to persist. While the osmolalities (tonicities) of most of these solutions are clustered be- tween 0.90 and 0.95% (measured in equivalent % NaCI), three did fall somewhat outside of those limits.* One might conclude from an average value for this group, however, that the trend for such solutions is to move gradually away from the traditional 0.90% level and more toward the am- bient value of the open eye tear en- vironment (0.97%).2

*Although the published sodium chloride con- tent for the most “salty” representative is 0.85%, it would appear to contain some additional osmotic constituents, giving it the mildly hy- pertonic value measured here.

Solutions of higher or lower tonicity (i.e., than was a lens at the time of fitting) might have the effects, re- spectively, of tightening or loosening it on the eye, at least initially. High- water ionic lens types would seem the most susceptible.

Similarly, as may be seen in Figure 2, a spectrum of pH values exists in the marketplace ranging from the open eye tear average of 7.4 down to 6.9.3 Again, the high-water ionic representatives among hydrophilic lenses would appear the most vul- nerable to pH shifts from the lens value at the time of fitting, an acidic shift having the effect on some of shortening their back surface radii.

In addition, as shown in Figure 3, most are buffered (those particularly with the shorter bars) in order to maintain their container pH while in use. Only one (#l) showed virtually

fi

!!

tl tc k C V

0

jt

%

51 S b a n r( n

IO buffering capacity, i.e., shifting pH bn test challenge by about the same lumber of pH units as does unbuf- sred distilled saline.4

One further property measured lere was viscosity. Although differ- !nces were found among the 17 so- Jtions studied, the range was, in eality, a modest one, spanning from ‘cry near to 1 .O to just over four times be viscosity of water. The common ‘alue for tears, however, often lies in ie 2-4 interval as well.5

Will They Ever Get It Right? In act, collectively, the laboratories robably have gotten it as close to right” as may possibly be. Only if iere were just one eye environment I deal with, and just one hydrophilic ?ns type to service, would the con- ept of a single supreme solution be alid. This can be useful to keep in mind

In those occasions when solutions 1st do not easily match a particular latient’s needs. With the array of lroperty combinations among solu- ons currently in the marketplace, as emonstrated by even this limited urvey, the odds are that there will le a best combination for your case .s well. Careful history taking, good scords, and a lot of solution survey sading, however, seem the only measures currently available to speed ou on this matching task.

References

1. Hill, R.M., Carney, L.G., Barr, J.T.. Jochum, K.L. Soft Lens Solutions: A Look at What is Not on the Label. Contact Lens Spectrum 3:43,1988.

2. Terry, J.E. Human Tear Osmotic Pressure. Arch. Ophthalmol. 96:120, 1978.

3. Carney, L.G., Hill, R.M. Human Tear pH. Arch. Ophthalmol. 94;821, 1976.

4. Carney, L.G., Hill, R.M. Human Tear Buffering Capacity, Arch. Ophthalmol. 97:951, 1979.

5. Schuller, W., Young, W., Hill, R.M. Clinical Measurements of the Tears: Viscosity. J. Am. Optom. Assoc. 43: 1358, 1972.

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