Analyzing small-incision cataract surgery by Orbscan II fourth-dimensional pachymetry mapping.

Purpose: To assess the changes in pachymetry after routine corneal and scleral tunnel phacoemulsification.

Setting: Discipline of Ophthalmology, University of Auckland, Auckland, New Zealand.

Method: This prospective study comprised 174 eyes of 174 consecutive patients having uneventful, small-incision, sutureless, phacoemulsification cataract surgery; 124 patients had a clear corneal incision and 50, a superior scleral tunnel incision. Difference pachymetry maps were derived from Orbscan II elevation topography data obtained before and 4 weeks after surgery. Corneal thickness changes at 12 midperipheral areas located in 12 meridians were derived from difference (subtraction) maps. The mean corneal thickness within a single area corresponding to the center of the surgical incision was compared to the mean pachymetry readings of all 12 midperipheral measurements.

Results: The overall mean midperipheral corneal thickness (12 samples per cornea) increased by a mean of 5.89 microm +/- 16.09 (SD) in the clear corneal incision group and 6.89 +/- 14.50 microm in the scleral tunnel group. The mean central corneal thickness increased by 7.28 +/- 20.98 microm and 7.74 +/- 21.34 microm, respectively. The corneal thickness in the meridian closest to the incision was significantly higher than the mean value of the 12 meridians measured in both groups, with means of 14.95 +/- 26.86 microm in the clear corneal group (P =.001) and 16.22 +/- 21.23 microm in the scleral tunnel group (P =.002). There were no statistically significant postoperative differences in the central, midperipheral, or incision-site corneal thickness measurements between the 2 surgical techniques (P =.77).

Conclusions: Time-dependent (fourth-dimensional) pachymetry subtraction maps are useful for following corneal dynamics over time. This study found that small-incision phacoemulsification techniques--scleral and corneal tunnel--had a minor but similar effect on corneal thickness 4 weeks after surgery.