Influence of lens material and intra-ocular pressure on the outcome of non-contact tonometry over soft contact lenses.
OBJECTIVE
To quantify the influence of soft lens rigidity and power on the difference between intraocular pressure values (DeltaIOP) obtained by non-contact tonometry through soft lenses in situ and over the cornea (IOP).


METHODS
Both eyes of 25 patients, attending for either routine contact lens check up, were fitted with a (I) low water content silicone hydrogel lenses of relatively high modulus of rigidity (modulus of rigidity 1.2MPa, Focus Night and Day, Cibavision) and (II) high water content daily disposable of relatively low modulus of rigidity (0.91MPa, Focus Dailies, Cibavision). IOP measurements were taken over the cornea and repeated over the lens after insertion. Lens powers ranged from -7.50D to +6.00D.


RESULTS
ANOVA revealed DeltaIOP was associated with both lens power and material (p<0.05). Multiple linear regression revealed relationships between DeltaIOP (y) lens power (x(1)) and IOP (x(2)) as follows (I), right eyes, y = 0.899x(1) - 0.172x(2) + 5.659 (F = 15.615, r = 0.766, p < 0.001, n = 25) (I), left eyes, y = 0.993x(1) - 0.101x(2) + 4.694 (F = 23.368, r = 0.825, p < 0.001, n = 25) (II) right eyes, y = 0.399x(1) - 0.370x(2) + 6.595 (F = 11.804, r = 0.719, p < 0.001, n = 25) (II) left eyes, y = 0.561(1) - 0.225x(2) + 4.153 (F = 28.736, r = 0.723, p < 0.001, n = 25)


CONCLUSIONS
DeltaIOP appears to be related to lens power, material and to a lesser extent IOP. Practitioners should derive their own empirical relationship between DeltaIOP, lens power and IOP for the specific types of soft lenses they commonly use. This would improve efficiency in the screening of IOP in more susceptible soft lens wearers.
Patel
Sudi
S
Practitioner Services, National Services Scotland, NHSScotland, Edinburgh, UK. sudhir.patel@psd.csa.scot.nhs.uk
Stevenson
Graeme
G
D000328:Adult; D000368:Aged; D001672:Biocompatible Materials; D003263:Contact Lenses, Hydrophilic; D004548:Elasticity; D004867:Equipment Design; D006801:Humans; D007429:Intraocular Pressure; D016014:Linear Models; D008875:Middle Aged; D055620:Optical Phenomena; D014065:Tonometry, Ocular / Q000379:methods*; D055815:Young Adult
D001672:Biocompatible Materials