INCISION PRECISION

Incisional techniques can provide visual benefits to patients with mild to moderate amounts of astigmatism, and new technologies and improved nomograms for planning and creating the incisions are likely to improve their still suboptimal predictability, reports Thomas Kohnen MD, PhD, FEBO, Goethe University, Frankfurt, Germany.

“We have to work on standardising our nomograms and we also have to look at the long-term clinical outcomes. The published data is still a little bit scarce,” he told the XXXII ESCRS Congress in London.

The indications for astigmatic cuts in the cornea include natural astigmatism, astigmatism after penetrating keratoplasty, and induced or residual astigmatism after cataract or refractive surgery . The basic principle of incisional techniques is to create cuts that are perpendicular to the steep meridian in order to flatten the cornea on that meridian.

In eyes with astigmatism after penetrating keratoplasty, the incisions are performed on the donor tissue no more than 7.0mm from the centre. The aim of these incisions is to reduce astigmatism, refraction can then be fine-tuned with an excimer procedure. Limbal relaxing incisions, on the other hand, are performed on the peripheral cornea, close to the limbus, and are generally used for the purpose of fine-tuning the refraction after cataract and refractive procedures.

He noted that peer-reviewed studies suggest that limbal relaxing incisions are effective for the treatment of 1.5D to 2.0D of astigmatism and that they should only be performed when the spherical equivalent is within plus or minus 0.5D to 0.75D of plano, because otherwise the patient will have residual myopic error and will not be satisfied, if emmetropia is the goal.

“We see a pretty quick recovery and usually excellent optical outcomes with limbal relaxing incisions. It is a safe procedure in cataract procedures with a low rate of complications and rarely any loss of best-corrected visual acuity. I think they are also useful for fine-tuning patients after procedures like PRK or LASIK. For example, if you don’t have enough tissue and you cannot do any further treatment on the cornea,” Prof Kohnen said.

Improving predictability

One drawback of incisional techniques is that, although on average they provide reasonably good results, there is actually a lot of variability in outcomes. The residual or induced cylinder following the procedures may result from a number of factors, including incorrect measurement of the astigmatism and/or the incorrect placement of the incisions.

Prof Kohnen noted that if the meridian of the anti-astigmatic incision is off by just 10 degrees it will lose 30 per cent of its effect, and if it's off by 30 degrees it will lose all of its effect. If the transversely incised meridian is off by more than 30 degrees, the astigmatism starts to increase. He noted there are also as yet several incompletely understood factors of corneal healing and biomechanics that can effect outcomes.

There are now numerous nomograms available for performing incisional astigmatic corrections, Prof Kohnen noted. However, the long-term predictability they afford is unknown because the studies published to date generally have follow-up periods of less than a year.

The advent of femtosecond cataract surgery systems with online OCT guidance for placing the incisions may help remove some of the uncertainties from the procedures. He noted that he and his associates have been using the laser
and their results so far appear promising. Femtosecond
laser-assisted corneal surgery can also be used to create intrastromal peripheral relaxing incisions.

He added that Douglas Koch MD and his team have had impressive results in a series of 19 patients who received intrastromal incisions using the Optimedica® femtosecond laser system.

“I think that in the future we will see a growing use of femtosecond lasers for performing penetrating and intrastromal keratotomies with better predictability than the manually driven devices,” Prof Kohnen said.

Thomas Kohnen:
Kohnen@em.uni-frankfurt.de