Systematic examination of the influence of the corneal thickness on IOP measurements after excimer laser photoablation with a Mackay Marg type tonometer

Purpose: IOP measurements with applanation tonometers are dependent on the corneal thickness. Thick corneae show false low IOP values and thin corneae show false elevated readings. We examined the influence of the corneal thickness on Mackay Marg type tonometers experimentally. Methods: We varied the corneal thickness using an excimer laser on 5 enucleated porcine eyes and on 2 enucleated human eyes. Constant IOP in the cannulated bulbi was given via an infusion system. The cornea was ablated stepwise in a 7mm zone and IOP readings were taken with the tonometer. To enable us to take IOP readings on the denuded stroma we applied a therapeutical contact lens. The influence of the contact lens on the IOP reading was determined in advance. Measurements were taken during the stepwise ablation of the cornea until a perforation occured. The required number of laser pulses for the perforation was taken into account to determine the relative thickness of the cornea. Results: A significant IOP elevation was recognized after the ablation of 50% of the stroma of the porcine eyes p<0.001. We could not detect any significant IOP elevation in the human enucleated bulbi even if 80% of the stroma was ablated. Conclusion: These results are not in accordance with the findings of the influence of the stromal thickness on IOP measurements on applanation tonometers published in the 50s and 60s. Mackay Marg tonometers could therefor proof to be an adequate instrument to measure patient treated with excimer lasers where applanation tonometry is theoretically not applicable.