The pre-operative assessment procedure
In order to safely alter the focus of the eye very detailed analysis of many parameters is necessary. On your visit to the clinic your eyes will be measured on many machines. None of these measurements are uncomfortable or dangerous and there is little or no risk involved in them. An important measurement is the actual refractive state of your eye, i.e. what is your exact spectacle prescription. This must be measured with more than usual accuracy than would be the case just for glasses or contact lenses. Another measurement is the mapping of the both the front and back surfaces of the cornea. Also the cornea must be sufficiently thick to allow the laser to safely operate. The eyes are also examined to exclude any other possible eye diseases which could affect the outcome. Diseases such as keratoconus, where the shape of the cornea is abnormal, preclude laser surgery. The lens of the eye which sits just behind the pupil must also be healthy and the retina too must be free of significant disease. Most of these tests involve sitting in front of a machine and staring at a target. They are not invasive and do not put the eye at risk. They yield highly important information to assess whether the eye is suitable for treatment and more importantly how much treatment is required.
A brief summary of the machines follows below:
- The Sirius Corneal Topographer
This device maps the anatomy of the front of the eye. It measures the shape of the cornea and looks for abnormal shapes or contours. It can analyse both the front surface and also the back surface of the cornea. It can also analyse the shape of the interior of the front of the eye for those patients who might need an implant inside their eye to correct their vision.
- Wavefront Analyser
This device measures how the eye handles light. It tracks 256 separate laser beams through the eye and sees where they pass. It then analyses the optical quality of the eye and gives very valuable information for surgical planning and assessing the success of the outcome.
Optical Coherence Tomography (OCT) is a device that allows us to examine the retina and the cornea in very considerable detail. It yields very valuable information regarding the morphology of these tissues. With it we can see the rods and cones of the retina and assess their health. We can also analyse the anatomy of the optic nerve and make sure it is healthy.
This is a device that maps the contours of the cornea. It gives valuable information in cases where there is a complicated spectacle prescription. In Presbyond surgery data from this machine is fed electronically into the laser as part of the surgical planning.
- Optical Biometry
The technology is similar to OCT and it measures the length of the eye from the front of the cornea to the very back of the retina. It also measures other distances within the eye and gives information about the refracting power of the cornea.
This is the examination that carefully measures exactly what spectacle prescription will put your eye into the best focus. It is essentially the same as a detailed spectacle examination with a good optician. In some patients we also have to put drops into the eye to paralyse the focusing muscles for a short while. This allows more precise measurement in those cases. This is the measurement that is used to calculate the exact treatment and as you can imagine is very important. It is more precise than for ordinary glasses or contact lenses.