In this experimental model, using porcine eyes, we prospectively evaluated the changes in IOP from the application of the suction ring through the end of the passage of the mechanical microkeratome (M2; Moria, Antony, France) or creation of the nonmechanical flap with the femtosecond laser (IntraLase Corp., Irvine, CA).
Fourteen freshly enucleated porcine eyes were separated into two groups of seven eyes each: the Moria group and the IntraLase group. All eyes were free of corneal damage when inspected by slit lamp microscopy.
The eyes were inflated with a 5% glycosylated solution through the optic nerve (in the same manner described by Kasetsuwan et al.
6 ) to obtain an IOP of 8 to 20 mm Hg checked with a Perkins applanation tonometer; the eyes were placed on a stand with sufficient support to withstand the surgical procedure. The IOP was measured in the anterior chamber using a 27-gauge winged infusion (Set REF 387412 Valu-Set BD Biosciences, Hull, UK) that was inserted through the limbus in such a way that the suction ring could be applied over the sclera without touching the needle. Pressure measurements were obtained with a reusable blood pressure transducer (MLT0380 Reusable BP Transducer, Power Laboratory; AD Instruments, Racine, WI). The transducer is an external sensor for coupling to vascular pressure (in our experiment the IOP in the anterior chamber) via a liquid-filled catheter. A saline-filled silicone tube attached to the catheter was connected to the transducer. The transducer was prepared according to the instructions of the manufacturer, to assure a tight seal and that all air was flushed from the system. The recorder was set to 0 to initialize the transducer. Before starting the procedure, the transducer was checked to verify that the pressure would be registered correctly. For calibration, we connected the transducer to a mercury-calibrated column and then checked that the pressure in the mercury column and the display connected to the transducer were the same.
The suction ring was applied, and a flap was created in the eyes in both groups. The same experienced surgeon (JMR) performed all procedures on 1 day under direct microscopy visualization. During the procedure, the IOP was recorded continuously with the amplifier (ML110 Bridge Amplifier; AD Instruments, Castle Hill, Australia) connected to the barometric transducer, from the time of the application of the suction ring through the end of the microkeratome pass. IOP also was measured before and after the suction ring was placed, by using a Perkins handheld tonometer (Clement Clarke, Essex, UK). The IOP level after the procedure had to be at least 6 mm Hg, to rule out any substantial fluid leakage from the eye during the experiment,
Statistical analysis was performed using Student’s t-test and the nonparametric Wilcoxon signed-rank test. P < 0.05 was considered significant.