Purpose To investigate the effect of corneal collagen cross-linking (CCL) on tonopen measurements of intraocular pressure (IOP).

Methods CCL with 0.1% riboflavin solution and 30 min of ultraviolet A radiation was performed on the right eye of 15 New Zealand albino adult rabbits (1.8–2.4 kg) (15 eyes). The left eye served as control. IOP was measured by a pressure transducer system (true IOP) and by the tonopen hand-held device (corneal applanation tonometer) before treatment, at 1 week, 1 month and 3 months following CCL. Reference pressure in the globe was increased by increments of 10 mm Hg from 10 to 40 mm Hg, using an anterior chamber infusion on a stand with variable height, and tonopen IOP measurements were recorded for each reference pressure in both eyes.

Results Before CCL, tonopen readings were similar between the two eyes (p>0.05). Tonopen underestimated the true IOP in all cases. Following CCL treatment, IOP measurements were significantly higher in the treated eye, at all time intervals (0.005<p<0.03). The most significant difference between true and measured IOP was noted at a reference pressure of 20 mm Hg.

Conclusions IOP measurements following CCL are overestimated by the tonopen, probably due to increased stiffness of the treated cornea.