Lens Exchange - The Operation

This section provides more details about the Lens Exchange operation.

Lens Exchange involves removing the natural lens of the eye using a surgical procedure called Phacoemulsification.

Phacoemulsification

With phacoemulsification the solid core of the natural lens of the eye is broken up and disintegrated within the eye. The microscopic lens debris is then sucked out of the eye. Because the natural lens is fragmented inside the eye the incision can be kept small. It can be as little as 2 to 3 mms in length, just enough to allow the phacoemulsification instrument to be inserted into the eye.

Once the natural lens has been removed the Lens Implant is put into the eye. In order to avoid having to make the incision any larger many surgeons use foldable lens implants. As the name suggests these are made from a pliable plastic and are folded to allow them to pass through the small incision. Once inside the eye they unfold and are manoeuvred into place.

The best position for the artificial lens implant is the same location as the natural lens of the eye, i.e. behind the pupil and iris. In order to provide support for the lens implant in this position the thin membranous outer shell of the natural lens is preserved. Eye surgeons call this the capsular bag.

The natural lens of the eye is similar in a size and shape to a smartie (a sweet made by Nestle, formerly Rowntrees). The capsular bag is like the thin outer sugar coating of the smartie that surrounds the chocolate within. In a phaco' cataract operation a small circular opening, usually between 4 and 7 mm in size, is made in the front layer of the capsular bag. This is like removing part of the sugar coating on one surface of a smartie. Saline fluid is then gently flushed around the lens material within the capsular bag to loosen it. The lens material is then disintegrated and removed to leave an empty capsular bag. Every effort in made to preserve the integrity of the posterior (back) layer of the capsular bag. Using the smartie analogy it is as if the chocolate had been removed leaving the sugar coating behind. An empty sugar shell with a small hole in one surface. The Lens Implant is then put into this empty capsular bag. If the capsular bag breaks then the lens implant may need to be placed in a different location to ensure that it is secure and stable.

The Phacoemulsification Instrument and Machine

The phacoemulsification hand piece is about the size and shape of a fountain pen. This is held by the surgeon. At its tip is a needle like probe. This is the bit that is placed into the eye. This needle has a metal core surrounded by a plastic sheath, which together have a diameter of 2-3 mms; manufactures are though continually trying to further miniaturise the designs.

This phaco' tip does three things at once. The needle moves backwards and forwards very slightly and very rapidly. The movement is too small to be seen and occurs about 40,000 times per second. It is this oscillation that directly or indirectly disintegrates the natural lens immediately adjacent to the needle tip. The disintegrated lens debris is then sucked out of the eye up through the core of the needle. At the same time saline like fluid is infused into the eye from beneath the sheath that surrounds the needle. The volume of fluid entering the eye matches that sucked out and so the eye remains inflated. The fluid flow also cools the needle as the rapid oscillations do generate some heat. The sheath also protects the incision from this heat where the instrument passes into the eye.

The phaco' hand piece is connected to a computer driven control box by a cable and tubing. The settings on the control box determine the performance of the hand piece and what is happening within the eye.

Once the harder core of the natural lens, called the nucleus, has been removed by the phacoemulsification machine a layer of softer material often remains. This is called soft lens matter or cortex. The phaco' hand piece can then be exchanged for another probe that will remove this material. This is a simpler device which just infuses fluid and vacuums up this soft lens matter. This part of the procedure is called irrigation/aspiration. Once this has been completed the artificial lens implant is inserted.