Influence of lens material and intra-ocular pressure on the outcome of non-contact tonometry over soft contact lenses

Abstract 

Purpose

To quantify the influence of soft lens rigidity and power on the difference between intraocular pressure values (ΔIOP) 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 ΔIOP was associated with both lens power and material (p<0.05). Multiple linear regression revealed relationships between ΔIOP (y) lens power (x₁) and IOP (x₂) as follows

(I), right eyes,y=0.899x₁−0.172x₂+5.659 (F=15.615, r=0.766, p<0.001, n=25)

(I), left eyes,y=0.993x₁−0.101x₂+4.694 (F=23.368, r=0.825, p<0.001, n=25)

(II) right eyes,y=0.399x₁−0.370x₂+6.595 (F=11.804, r=0.719, p<0.001, n=25)

(II) left eyes,y=0.561₁−0.225x₂+4.153 (F=28.736, r=0.723, p<0.001, n=25)

Conclusion

ΔIOP appears to be related to lens power, material and to a lesser extent IOP. Practitioners should derive their own empirical relationship between ΔIOP, 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.

Keywords: IOP, Soft contact lens power, Rigidity