(1)
Newcastle Eye Centre Royal Victoria Infirmary, Newcastle upon Tyne, UK
Over time, experienced surgeons will develop a preferred technique for creating the corneal wound. This will automatically take account of several factors: available access to the corneal surface, corneal astigmatism, anterior chamber depth, ocular movement and ocular co-pathologies.
During rear-ended modular training, novice surgeons have the opportunity to observe many corneal incisions. Despite this, novice surgeons commonly hesitate when performing the incision themselves. This frequently occurs if: the patient’s eye is moving slightly, the chosen site is obscured by the lids, or if an unfamiliar incision site is chosen.
This chapter deals with the fundamentals of making the main incision. It assumes that Trainees have completed all of the previous modules of cataract surgery training.
14.1 Fundamentals
14.1.1 Corneal Orientation
The choice of where to place the main corneal incision varies between surgeons. Some surgeons prefer to always operate from the temporal aspect, others from the superior aspect. Some surgeons vary the site according to the corneal topography – by adjusting the incision in relation to the greatest degree of astigmatism (on-axis surgery) the amount of postoperative corneal astigmatism may be decreased.
For novice surgeons, the decision where to place the main wound site is usually dictated by their supervising surgeon. Novices may be instructed to place the incision in the temporal or superior positions. Alternatively, instructions may include the number of degrees at which the incision should be placed. Convention dictates the horizontal meridian (when looking down the microscope from the superior position) is labelled as the 0 to 180 degree line (Fig. 14.1). For left eye temporal incisions the cut is made at about 0°, and for a right eye the temporal incision is made at about 180°.
Fig. 14.1
Corneal orientation (right or left eye) given as degrees of a circle as if viewing from a superior operating position looking down the microscope
14.1.2 The Keratome
The choice of blade used to make the corneal incision will vary between ophthalmology units. Whilst some will prefer reusable diamond knives, the more commonly used disposable keratome is described in this chapter.
The keratome is angled at about 45-degrees. The tip and sharp side edges are used to cut the cornea. The distance between the widest points defines the size of the keratome. These two points will be referred to as the shoulders of the keratome blade. It is important to appreciate that, once past the shoulders of the cutting edge, the remaining edge is blunt and cannot be used to enlarge or cut the corneal section (Fig. 14.2).
Fig. 14.2
Disposable steel keratome
14.2 Keratome Movements
14.2.1 Tip Up/Heel Down Verses Tip Down/Heel Up
A variety of movements are required to create a stepped, self-sealing corneal incision. Additional movements may be required to help overcome the resistance of the corneal stroma whilst entering the eye with the blade. It is important to appreciate the various movements and various positions the keratome may need to make.
The cutting direction of the keratome can be altered by pivoting the keratome on the vertical axis into three positions (Fig. 14.3):
- 1.
The keratome blade can be kept flat.
- 2.
The keratome can be tilted heel down causing the tip to point upwards.
- 3.
The keratome can be tilted heel up causing the tip to point downwards.
Fig. 14.3
Keratome positions as seen in side view. (a) Flat, position. (b) Tip up, heel down. (c) Tip down, heel up
14.2.2 Keratome ‘Snake-Like’ Movement
It is common for novice surgeons to find the keratome occasionally sticking whilst trying to cut through the corneal stroma. Additional force may be needed to overcome the resistance, and if force is applied in an uncontrolled fashion it can lead to rapid keratome entry into the anterior chamber. To avoid this, counter-traction may be applied by holding the side port (if already made) or using a fixation device. It is also possible to perform a snake-like cutting action as a means of maintaining control of the keratome.
The snake-like movement is performed directing the tip from one side of the wound to the other. Each small change of direction will make a cut, gradually enlarging the internal ostium. The sideward movement encourages the cutting edges of the keratome to enlarge one lateral edge of the wound and then the other. As the resistance is overcome, and the internal ostium is widened, a controlled, gradual forward motion of the keratome can be achieved.
This technique is a useful is a useful one for novices to learn if a fixation device is not routinely used. Appreciation of when to use a snake-like keratome movement is useful as it can avoid the necessity of gripping the side port or any part of the ocular surface (Fig. 14.4).
Fig. 14.4
Keratome ‘snake-like’ movement. Keratome encounters corneal resistance and struggles to pass through the corneal tissue. (a–c) Pressure is applied to the cutting edge on one side of the blade (open arrow) by pivoting the blade tip to one side (arrow) of the incision and then the other, following a snake-like movement (curvy-linear arrow) as the keratome blade is inserted. The successive small cutting actions enlarge the internal ostium
Box 14.1 Surgical Tip
Novice surgeons may encounter several problems during the initial learning phase:
- 1.
Difficulty making continuous incision movements.
- 2.
Getting stuck in the corneal stroma (with the keratome tip failing to breach the endothelium).
- 3.
Shallowing of the anterior chamber whilst making the incision.
- 4.
Pranging of the capsule with the keratome blade.
- 1.
Learning a controlled, stepped incision takes practice and confidence. Using rear-ended training, Trainees should be able to continue with the surgery even with a wound that is not perfect.
14.3 Obtaining an Adequate View
Making an incision on the temporal aspect of the cornea allows unhindered access to the intended temporal incision site , as the lids to not obstruct the limbal view (Fig. 14.5).
Fig. 14.5
Temporal based view of surgical field of left eye. The temporal limbus (arrow) is not obstructed by the lid margins
If the intended corneal incision site is more superior , it may be obscured by the upper lid, even after speculum insertion. This can be due to patient factors, for example: a tight palpebral aperture, a strong Bell’s phenomenon, or the patient trying to squeeze both eyes shut. Although a view can be obtained by asking the patient to look down, or by manually pulling the eye down, this requires the eye to move out of the primary position. Also, conjunctival trauma may result from gripping the eye. Alternatively, the following manoeuvres can be tried in any order to gain a better view of the incision site:
- 1.
Inspect the drape and check to see if it has been adequately cut.
This should not be an issue in rear-ended training as normally the Trainer would have performed this part.
- 2.
Ask the patient to open both eyes. This simple request may help gain an adequate view.
Giving a direct instruction is recommended, as opposed to, for example, asking the patient to relax.
- 3.
Using closed forceps (held in the non-dominant hand) place the opposed tips over the draped lid margin and retract the upper lid (Fig. 14.6). It is helpful to have the have the blade ready to start the incision when an adequate view of the intended incision site is obtained. The major advantage of this technique is that the globe is maintained in the primary position. The surgeon is not required to either manually rotate the globe downwards, or ask the patient to look down.
Fig. 14.6
Obtaining an improved view of the superior corneal incision site. (a) Superior limbus is obscured by the draped lid (arrow). (b) Forceps are positioned on the draped lid with the tip of the keratome blade poised ready above the intended incision site. Direction of intended retraction (arrow). (c) Forceps (arrowhead) are used to retract upper lid. (d) Limbus view obtained and keratome tip (open arrow) begins incision. Forceps just visible (arrow head)
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