Study finds problems with intraoperative subtraction pachymetry

Intraoperative subtraction pachymetry measurement of flap thickness is less accurate and moresubject to measurement error than flap thickness measurements made via postoperative anterioroptical coherence tomography (OCT), according to results of a prospective, randomized clinicaltrial presented by Edward E. Manche, MD, of Stanford University.

The research also demonstrated that a femtosecond laser (IntraLase FS, Advanced Medical Optics)could make flaps of more precise thickness, and with a tighter standard deviation, than could amechanical microkeratome (Hansatome, Bausch & Lomb). Additionally, the study showed that flapsin eyes treated with the femtosecond laser had a planar architecture compared with the meniscusarchitecture found in the flaps made by the mechanical microkeratome.

The study included 100 eyes of 50 consecutive patients who had been treated with customwavefront-guided LASIK for myopia using a proprietary excimer laser (VISX STAR S4, Advanced MedicalOptics). One eye was treated with the mechanical microkeratome, and the fellow eye was treatedwith the femtosecond laser. A single masked observer performed the pachymetry and OCTmeasurements on that 12 patients/24 eyes available for analysis at 1 year.

Pachymetry measured a flap thickness of 119 ± 19 um in the femtosecond laser groupand 137 ± 22 µm in the mechanical microkeratome group, but OCT imaging revealed anachieved flap thickness of 107 ± 8 µm in the femtosecond group and 141 ±15µm in the mechanical group.

"There's really poor correlation between OCT and pachymetry in the [femtosecond laser] group,"Dr. Manche said. Sometimes, flap thickness was overestimated, and sometimes it wasunderestimated. The correlation between the two measurements also was poor in the mechanicalmicrokeratome group, for which pachymetry tended to underestimate the actual thickness of theflaps, he added.