In addition, the visual acuity levels were sustained over time in these patients compared with those who underwent a mechanical procedure for channel creation and experienced a substantial decrease in visual acuity over time, said Dr. Daya. He is director and consultant, Corneoplastic Unit and Eye Bank, Queen Victoria Hospital, East Grinstead, and medical director, Centre for Sight, London and East Grinstead, England.
"[The corneal implants] have traditionally been implanted using mechanical means," Dr. Daya said. "An issue associated with this procedure is the accuracy of the ring depth, which can result in extrusions and infections. With experience this improves.
"However, using the [femtosecond laser] the depth is a maximum of 400 μm and the inner and outer ring diameters can be changed," he added.
Dr. Daya has changed the nomogram from the manufacturer's recommendation based on his mechanical outcomes, he explained. "I decreased the size of the inner ring to 6.3 mm [from 6.6 mm] and outer channel to 7.1 mm [from 7.5 mm], making the channel diameter also smaller to 0.4 mm. The rationale for this change is that the smaller outer diameter forces more flattening as the rings, which are larger in diameter than the channels, exert vector forces on the cornea outwards," he said.
Dr. Daya and his colleagues have used both the mechanical method and the femtosecond method. The incision axis, he explained, is based on the steep axis. The asymmetrical rings were used only in pellucid-like keratoconus, which requires a horizontal incision.
He described a patient who had a mean keratometry value of 52.3 D. Two months after corneal insert implantation that value decreased to a mean of about 42.5 D.
When the investigators compared their outcomes from the two groups, there were more patients in the laser group. There were 32 eyes of 30 patients in the laser group and 17 eyes of 16 patients in the mechanical group.
"We abandoned doing this surgery with the mechanical method because the outcomes were very poor," he said. Following installation of femtosecond laser, Dr. Daya recommenced corneal insert implantation.
"The mean keratometry value was slightly higher with the group that underwent the mechanical procedure," he said. "We previously had done the surgery on all patients with a clear cornea and very high keratometry values. Since then we have decided that this is a bad idea. We no longer do the procedure on any patient with greater than 57 D of keratometry."
The only difference between the two methods, he pointed out, is that with the mechanical procedure the channels can be deeper than 400 μm, which is the limit with the laser method. Regarding the changes in keratometry, the patients who underwent the conventional procedure and the laser procedure both had a gradual decline in keratometry values, which was sustained over time. The spherical equivalent also decreased in both groups, but the degree of decrease was greater in the laser group.
The laser group had a better best-corrected visual outcome compared with the conventional group.
"The patients in the mechanical group hovered around the 20/50 visual acuity level, while the patients in the [laser] group had an average visual acuity of 20/40 to 20/30," Dr. Daya reported.
In the mechanical group, 38% of patients had 20/40 or better visual acuity 3 months after the procedure; however, only 8% at 6 months sustained that level of visual acuity. In the laser group, 29% of patients had 20/40 acuity or better 3 months after the procedure, and this increased to 67% at 6 months, he said.
One perforation and one infection occurred in the conventional group. Three infections occurred in the laser group.