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New laser system for intraoperative
measurement of LASIK flap thickness
By
Roibeard O'hÉineacháin
NICE
- A new laser imaging system may provide more accurate corneal pachymetry
than ultrasound instruments and allow for the intraoperative measurement
of LASIK flap thickness.
The new system uses triangulation between a laser and a high-resolution
digital camera to create a three-dimensional map of the entire corneal
surface, instead of just one point as in ultrasound pachymetry,
an Italian specialist told told the XX ESCRS Congress.
Moreover, the system could be mounted onto an excimer laser system
and provide data to guide ablation patterns, Francesco Carones MD
said.
"Microkeratomes may produce unpredictable flap thicknesses
which can in turn lead to inadequate residual stromal thickness
and an increased risk of ectasia. That is why there is a need for
intraoperative pachymetry.
"However, ultrasound pachymetry only measures single points
as regards the thickness and there can be difficulty in finding
the thinnest corneal point and the center of ablation. The intraoperative
corneal topographer was designed to address these issues,"
he explained.
The system works by using an ultraviolet 193 nm laser source to
project a grid or pattern of light that reaches the cornea where
it causes the surface to fluoresce. A high-resolution digital camera
detects the pattern.
A 30° angle between laser and camera provides the triangulation
necessary to interpret the distortions in the laser pattern as a
3D representation of the cornea's contours.
The laser pattern consists of approximately 100 lines with line
width of 50 microns and 10 line-pairs/mm on the central cornea.
"When the fringes are projected on a plane there is no distortion
and the lines are straight up and down. However, on a surface like
the cornea the lines assume a curvature with a difference in dimension
that is proportional to the contour of the surface.
"To obtain a flap thickness measurement, the surgeon has merely
to measure the difference in the fringe pattern of the cornea before
and after flap creation," he said.
In a small trial to evaluate the accuracy of the instrument, Dr
Carones and his associates used the ICT instrument to measure the
corneal thickness in six LASIK patients before and after flap creation
with an SKBM microkeratome.
The microkeratome was set to cut flaps 150 microns in thickness.
However, measurements with the ICT showed that the thickness of
LASIK was much greater (above 200 microns) in the region where the
microkeratome blade enters the cornea.
In fact, the nominal thickness was only achieved in the 6.0 mm diameter
central area of the flap.
Dr Carones emphasised that the ICT Corneal Topographer is at present
only an investigational instrument and its accuracy remains to be
proved.
Moreover, its registration and calibration will need improvement
before it is ready for clinical use. But when fully implemented
the technology could enhance the precision of refractive surgery.
"The accuracy of the instrument seems to be quite consistent
and gives enough information to measure surface regularity, particularly
with regard to the correction of higher order aberrations.
"This technology could therefore be integrated into a laser
system in order to guide the laser for custom ablation," he
noted.
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