THE PERFECT INCISION

Mike Adams MA, MB BChir, FRCOphth

The creation of the perfect incision in cataract surgery is an exercise in adaptation to the patient, the surgeon and the available instruments, with the goal of avoiding induced astigmatism and creating a watertight wound with as little trauma to the eye as possible, said Mike Adams MA, MB BChir, FRCOphth, Oxford University Hospital, Oxford, UK.

“To achieve a perfect incision you’re going to need four things: the perfect patient, the perfect eye, the perfect instruments and the perfect surgeon. We all know the first two are impossible, but we have to hope that the last two are possible,” Mr Adams told a Young Ophthalmologists Symposium at the 19th ESCRS Winter Meeting in Istanbul, Turkey.

The first requirement for a good cataract incision is that it provides good access to the eye for surgery. Temporal incisions are better than superior incisions in that regard and are necessary when a patient has a large brow. In addition, in studies conducted using 3.0mm blades, temporal incisions induced less astigmatism than superior incisions, due to the ellipsoidal shape of the cornea.

But superior incisions also have their advantages. For example, they are protected by the eyelid from postoperative trauma. In addition, the brow supports the surgeon’s hands, which can be very helpful for trainee surgeons. Furthermore, in research conducted at the Bascom Palmer Institute, inferotemporal incisions appeared to be associated with a high risk of infection. That may be a result of the wound’s location in the tear lake, where it is more exposed to bacteria.

Meanwhile, there have been reports that smaller, 2.2mm blades may reduce the disadvantage of superior incisions regarding astigmatism. Smaller temporal wounds may also be less vulnerable to infection than the 3.0mm temporal incisions used in the past.

Incisions for cataract surgery must also provide good access to the anterior chamber. In most countries corneal incisions are by far the most commonly used. They are quicker to perform and easier to learn. An important rule to remember is that the further from the optical zone the incision is, the less astigmatism it will induce.

Scleral wounds by contrast involve a peritomy, diathermy dissection of the tunnel with a crescent knife and then finally entering the eye with the keratome. However, scleral wounds are very useful when it is necessary to implant a rigid PMMA lens, since the large incisions needed produce a negligible astigmatic effect compared to corneal incisions. Scleral wounds also have far less risk of leaking than corneal wounds and also tend to have a more consistent shape postoperatively.

Mr Adams noted that there remains some controversy over which corneal wound architecture provides the best postoperative coaptation with the epithelium. In a study carried out by Howard Fine MD in Oregon, USA, in 2007, optical coherence tomography (OCT) examination showed that, unexpectedly, the three-plane incisions actually had poorer coaptation than the stab incisions. Moreover, the stab incision did not result in a vertical wound straight into the cornea, but instead the internal stromal faces of the wound realigned into a curve, forming a very good seal and a smooth surface.

However, in another study carried out that same year by Dan Calladine FRCOphth and Richard Packard FRCS in the UK, stab incisions resulted in loss of coaptation compared to multi-plane incisions, with gaps in the stromal layers. That may be because they injected viscoelastic into the eye through a paracentesis prior to making the incisions.

“When you press the knife in, the cornea flattens a little ahead of your blade and when you take the blade out and the cornea springs back to its natural shape, you find you’ve created a curve with the stab incision. However, if the eye is hard because you've filled it up with Healon there is less flattening of the cornea with a stab wound when you put a blade in,” said Mr Adams.

A cataract incision also needs to be a good fit for the phaco instrument, Mr Adams said. It should be tight enough to ensure a stable anterior chamber, but loose enough to avoid pinching the phaco probe’s irrigation sleeve, or stretching the wound and impairing its closure at the end of the procedure. Moreover, with a too-tight wound, movement of the phaco probe will cause the eye to move as well.

Mr Adams noted that different blades perform differently when making a surgical incision in the cornea. For example, a steel trapezoidal blade with a pronounced shoulder may surge into the anterior chamber once the shoulder has passed through the cornea and inadvertently catch the capsule as a result. Similarly, diamond knives are so sharp that in the hands of an inexperienced surgeon they may pass much further through the cornea than intended.

Mike Adams:
mwjadams@gmail.com