Cornea relaxing incisions are an established technique to correct astigmatism. Transverse
incisions flatten the cornea in the meridian of application and steepen 90 degrees
away. The problem in the past was the need to take the patient to an operating room
to perform the surgery.
Incisions can be made at the slit lamp with a few modifications of technique which
we will describe here.
The first modification is that the patient is now seated at the slit lamp. Anaesthesia
is accomplished with topical drops and with the lids dried, the assistant holds the
lids open instead of the lid speculum used in the OR. The surgeon’s arm is stabilized
by resting the elbow on the slit lamp table and the wrist against the chinbar of
the slit lamp as seen in figure B and C.
In the OR making incisions in different locations merely requires repositioning of
the hand and arm. At the slit lamp the hand and arm are now fixed and a different
approach is dictated. The incision can be made at the slit lamp by grasping the knife
with the fingertips and using a “bellows” action to move the knife up and down as
seen in figure D. If an incision is to be placed in a location not access able to
the “bellows” movement of the fingers, simply have the patient look up or down.
Of course not all patients are candidates for even minor surgery at the slit lamp.
The patient must be calm and be able to hold fixation. Nonetheless most patients
tolerate the shot procedure well and are amazed by the improvement of vision within
a few minutes.
The operation requires one special instrument, a diamond knife which will not bump
into the optics of the slit lamp. In the OR the operating microscope is much farther
away from the patient than at the slit lamp The knife needs to have an angled head
and there are several models available. Duckworth and Kent have a diamond knife with
an angled head and adjustable depth which can be found here (figure E). Mastel Precision
has a set depth knife specifically designed for the slit lamp which can be found
here. There are many other short diamond knives available on the market which will
also work. In addition, a 7mm zone marker is used, coated with gentian violet and
centered on the pupil as seen in figure F.
The procedure begins with topical anaesthesia and placing the head in the slit lamp,
eye held open by the assistant. The diamond knife is set to 0.5mm or use a preset
diamond knife without pachymetry since the cornea is almost uniformly thicker than
0.5mm in the midperiphery. After marking the cornea with a 7mm marker, the incisions
are made along the steep axis (figure A). The result is shown in figure G for a patient
with steep axis at 180 degrees (with the rule). The nomogram is 1mm for a 1 diopter
and 2mm for a 2 diopter correction. For a larger correction I use the Canrobert procedure
(or “C” procedure) which is described here. With the “C” procedure one can obtain
up to 6 diopters of correction although I don’t try for more than 5 diopters due
to unpredictability of the result.
The procedure is easiest along the horizontal meridian. If the steep axis is at 90
degrees, make the bottom incision and have the patient look down for the to incision.
For oblique astigmatism a useful technique is to place the plus cylinder correction
in the phoropter and use it as a guide to the proper axis.
The Blue Line Incision and Refractive Phacoemulsification, Slack Inc., published
2000, Kurt A. Buzard, M.D., Miles H. Friedlander, M.D., Jean-Luc Febrarro, M.D.