At 20°C, the most viscous solutions were COM (3.02 cP) and neutralized UC (1.26 cP), which were significantly different from each other (p < 0.001) and from the remaining products (p < 0.001). Neutralized AO (0.96 cP) and CC (0.96 cP) were the least viscous solutions. They were less viscous than all other products (p < 0.05), except for SWS (0.97 cP), to which they were not dissimilar (p = NS). The remaining solutions showed minor differences in viscosities.
At 34°C all the viscosities were lower (p < 0.001), but the order of solutions from most viscous to least viscous remained the same. COM at 1.92 cP remained the most viscous solution, followed by neutralized UC (0.86 cP). These were different from each other (p < 0.001) and were also higher than all other products (p < 0.05). The least viscous solutions were AO and CC, which were both lower in viscosity than all other products (p < 0.001), with the exception of SWS and OFR (p = NS).
Un-Neutralized Peroxide Solutions
The un-neutralized peroxide solutions had very different physical properties than the rest of the solutions examined, as shown in Table 2. The pH values of un-neutralized UC (3.33), AO (6.40), and CC (6.53) were significantly more acidic than any of the other solutions tested (p < 0.001). AO and CC were not different (p = NS), but were significantly higher in pH than UC (p < 0.001).
These solutions also had significantly higher (p < 0.001) osmolality values than any other products (Table 2). As with pH, there was no difference between the osmolality of un-neutralized AO and CC (p = NS), but un-neutralized UC was significantly lower than both these solutions (p < 0.001).
After neutralization, the osmolality of all three peroxide systems dramatically reduced. The osmolality for the two disc-based systems (AO and CC) reduced from approximately 1320 to 292 mOsm/kg, when compared with the tablet-based system (UC), which reduced from 945 to 329 mOsm/kg.
The ST of un-neutralized AO (67.7 mN/m) and un-neutralized UC (66.0 mN/m) were similar (p = NS) to the ST of SWS (67.9 mN/m). However, un-neutralized CC (42.9 mN/m) had a much lower ST than the other un-neutralized peroxides (p < 0.001), and exhibited a similar ST to that of the one-bottle multipurpose solutions (Fig. 3).
At 20°C un-neutralized CC (0.99 cP) had a statistically higher (p < 0.05) viscosity than either AO or UC, which were not different from each other (p = NS). When the un-neutralized peroxide solutions were re-tested at 34°C, all three solutions reduced in viscosity (p < 0.001). The three solutions were statistically different from each other (p < 0.001), with CC having the highest and UC the lowest viscosity.
This study was designed to measure the physical properties of many soft contact lens solutions, which are not readily available from manufacturers.
pH, which is a measure of a solution's acidity or alkalinity, is also an indication of it's buffering capacity. It is an important property of soft contact lens solutions and has been linked directly to patient comfort.5–7 Reportedly, the human eye is capable of tolerating pH values in the range of 6.2 to 9.0 at 0.2 M strength.19 Although some patients may be more sensitive to changes in pH than others,3 solutions with pH values above this range have a tendency to cause discomfort.5,6 Buffering agents used in soft contact lens solutions directly affect their pH, and it is quite possible that the type of buffer used in a particular solution could also affect subsequent patient comfort.
Within the group of solutions tested, small but statistically significant differences existed, but the clinical significance of these differences on patient comfort is impossible to predict. However, one clear difference was the acidic pH seen in the un-neutralized peroxide solutions (particularly UC), which did not fall within the ocular pH tolerability range. These solutions must be neutralized before lens insertion or, understandably, discomfort will occur. Patients who may be especially sensitive to solution pH could potentially report some mild discomfort when using those products with slightly higher (OFX or OFR) or slightly lower (AO or CC) than average pH, but this requires further clinical investigation.
Neutralization had a significant effect on the pH of all peroxide systems (Fig. 1, Table 2). AO and CC, both of which use a platinum disc method of neutralization, showed only a minor alteration in their pH after neutralization. In comparison, UC, which uses a tablet method of neutralization, demonstrated a large change in pH after neutralization.
Osmolality is a quantitative measure of the amount of dissolved solutes in a solution. These dissolved solutes, such as biocides, surfactants, chelating agents, and buffers, exert an osmotic pressure that is recorded in milliosmoles per kilogram (mOsm/Kg).
On average, the osmolality of tears is 305 mOsm/kg.8,11–13 Our results indicate that the majority of soft contact lens solutions are hypo-osmotic compared with human tears,1 with our osmolality values falling within the 275 to 310 mOsm/kg range. OFX and neutralized UC were the only solutions that had significantly different osmolalities from the group of solutions indicated for direct ocular contact. The remaining lens care solutions fell into two groups, higher and lower osmolalities. Examination of the results indicated that the two products with significantly different osmolalities (OFX at 225 mOsm/kg and neutralized UC at 329 mOsm/kg) could potentially cause patient discomfort, as they are quite different than human tears. However, both products are clearly commercially successful and thus the impact of osmolality on ocular comfort requires further investigation. All other solutions had relatively small differences in osmolality between them and differed only slightly from that of tears. Previous studies have shown that a relationship exists between a higher tear film osmolality and increased ocular discomfort in patients with dry eyes,8,11–13 so it seems logical that a dry eye patient has the potential to be more sensitive to the osmolality of their contact lens solution than a patient with a healthy, robust tear film.
The large difference in pH and osmolality of the peroxide systems before neutralization, along with the chemical properties of peroxide itself, likely contribute to the burning, stinging, and epithelial cell damage experienced by patients who are unfortunate enough to insert these solutions directly onto the ocular surface, as shown in a previous study.20
ST describes the residual inward attraction of molecules at the surface of a solution, and is measured in milli-Newtons per meter (mN/m). Pure water has a ST of approximately 72 mN/m, whereas human tears have a ST in the range of 40 to 46 mN/m.14,15 The dissolved proteins, mucins, and other molecules present in tears reduce their ST relative to pure water. The presence of surfactants in a solution, such as those often found in contact lens care systems, also reduces ST.
Examination of Fig. 3 and Table 2 shows that AO (un-neutralized and neutralized), UC (un-neutralized), and SWS all had STs of approximately 67 mN/m, which were significantly higher than any other solution. The ST of these solutions resembles that of water, because none of them contain a surfactant (Table 1). All the solutions that contain one or more surfactants had significantly lower STs, with values that were closer to that of human tears.14,15 OFX and OFR had significantly lower STs from that of human tears, as well as from the rest of the solutions tested.
After neutralization, the ST of AO and CC were only marginally altered (although the change for AO was statistically significant at p = 0.004). However, the ST of UC decreased significantly. This is likely related to the components of the neutralizing tablet used in this system. AO and CC use a platinum-disc method of neutralization, resulting in only water and oxygen remaining after the chemical reaction and thus there are markedly lower pre- and post-neutralization ST measurements.
Viscosity is a quantitative measure of the resistance of a fluid to flow when subjected to a shear stress of a particular shear rate, and is measured in centipoise (cP). The viscosity of the products tested was evaluated at two temperatures. As viscosity is influenced by temperature, we were curious to see what effect the temperature change at the ocular surface would have on the solutions tested. Figs. 4 and 5 show that all soft contact lens solutions had lower viscosities at the ocular surface temperature (34°C)18 than those at room temperature (20°C). The viscosity of water is 1.0 cP, whereas human tears demonstrate a non-Newtonian viscosity, such that the viscosity of the tear film demonstrates marked shear-thinning, with values of viscosity decreasing as shear-rate increases, from approximately 5.0 to 1.5 cP at 25°C for normal subjects.15,16
COM was by far the most viscous of any of the solutions tested, largely in part due to presence of propylene glycol and hydroxypropyl methylcellulose (HPMC) (Table 1), which are used as wetting agents in this product. At 25°C propylene glycol has a viscosity of 56 cP and HPMC has a range of viscosities from 4 to 78 000 cP. The impact of solution components on viscosity can also be shown by comparing neutralized UC to un-neutralized UC (1.26 vs. 0.96 cP at 20°C; p = 0.002). The neutralizing tablet for UC is coated with HPMC (Table 1) and, after its dissolution in the UC case, the HPMC is released and the solution viscosity clearly increases.
This study demonstrates that soft contact lens solutions differ in certain physical properties. Clinically, these differences may have the potential to influence patient comfort initially and/or at the end of the day. However, whether or not the differences noted are clinically significant is unclear at this time and warrants further investigation.
The authors thank the Natural Science and Engineering Research Council of Canada (NSERC) for financial support through a Discovery Grant to LJ and Advanced Instruments for providing the Osmometer.
Centre for Contact Lens Research
School of Optometry, University of Waterloo
200, University Avenue West
Waterloo, ON, N2L 3G1
1. Morgan PB, Efron N. A decade of contact lens prescribing trends in the United Kingdom (1996–2005). Cont Lens Anterior Eye 2006;29:59–68.
2. Morgan PB, Woods C, Jones D, Efron N, Tan KO, Gonzalez M, Pesinova A, Grein H-J, Runberg S, Tranoudis I, Chandrinos A, Fine P, Montani G, Marani E, Itoi M, Bendoriene J, van der Worp E, Helland M, Phillips G, Belousov V, Barr JT. International contact lens prescribing in 2006. Contact Lens Spectrum 2007;22:34–8.
3. Carney LG, Hill RM. Human tear pH. Diurnal variations. Arch Ophthalmol 1976;94:821–4.
4. Janoff L. The effective disinfection of soft contact lenses using hydrogen peroxide. Contacto 1979;23:37–40.
5. Tang I, Wong DM, Yee DJ, Harris MG. The pH of multi-purpose soft contact lens solutions. Optom Vis Sci 1996;73:746–9.
6. Harris MG, Torres J, Tracewell L. pH and H2
concentration of hydrogen peroxide disinfection systems. Am J Optom Physiol Opt 1988;65:527–35.
7. Harris MG, Higa CK, Lacey LL, Barnhart LA. The pH of aerosol saline solution. Optom Vis Sci 1990;67:84–8.
8. Tomlinson A, Khanal S. Assessment of tear film dynamics: quantification approach. Ocul Surf 2005;3:81–95.
9. Farris RL, Gilbard JP, Stuchell RN, Mandel ID. Diagnostic tests in keratoconjunctivitis sicca. CLAO J 1983;9:23–8.
10. Gilbard JP, Farris RL, Santamaria J, II. Osmolarity of tear microvolumes in keratoconjunctivitis sicca. Arch Ophthalmol 1978;96:677–81.
11. Gilbard JP, Rossi SR, Gray KL. A new rabbit model for keratoconjunctivitis sicca. Invest Ophthalmol Vis Sci 1987;28:225–8.
12. Gilbard JP, Rossi SR, Heyda KG. Tear film and ocular surface changes after closure of the meibomian gland orifices in the rabbit. Ophthalmology 1989;96:1180–6.
13. Farris RL. Tear osmolarity—a new gold standard? Adv Exp Med Biol 1994;350:495–503.
14. Tiffany J. Soluble mucin and the physical properties of tears. In: Sullivan DA, Dartt DA, Meneray MR, eds. Lacrimal Gland, Tear Film and Dry Eye Syndromes 2: Basic Science and Clinical Relevance. New York: Plenum Press; 1998:229–34.
15. Pandit JC, Nagyova B, Bron AJ, Tiffany JM. Physical properties of stimulated and unstimulated tears. Exp Eye Res 1999;68:247–53.
16. Tiffany JM. The viscosity of human tears. Int Ophthalmol 1991;15:371–6.
17. Jones L, Senchyna M. Soft contact lens solutions review. Part 1. Components of modern care regimens. Optom Practice 2007;8:45–56.
18. Efron N, Young G, Brennan NA. Ocular surface temperature. Curr Eye Res 1989;8:901–6.
19. Carney LG, Fullard RJ. Ocular irritation and environmental pH. Aust J Optom 1979;62:335–6.
20. Paugh JR, Brennan NA, Efron N. Ocular response to hydrogen peroxide. Am J Optom Physiol Opt 1988;65:91–8.
21. Tiffany JM, Pandit JC, Bron AJ. Soluble mucin and the physical properties of tears. Adv Exp Med Biol 1998;438:229–34.
Keywords:© 2008 American Academy of Optometry
contact lens solution; pH; osmolality; surface tension; viscosity