Pre-operative Measurement
In Lens Exchange surgery the natural lens of the eye is removed and replaced by a plastic lens (called "the intraocular lens implant"). Before the operation the eye needs to be measured in order to calculate the optical power of the intra ocular lens implant that is needed to achieve the desired change in focus of the eye. Taking these measurements is called Biometry. The measurements are simple, quick and painless.
Biometry
There are two main measurements:
- The curvature of the cornea (the window of the eye).
- The overall length of the eye; called the axial length.
Sometimes the depth of the front chamber of the eye is also measured.
Once these measurements are known formulae are used to calculate (by computer) the Lens Implant power needed in order to obtain the desired focus of the eye after the operation. In other words the Lens Implant power is calculated and chosen for each individual eye.
Accuracy
The formulae used are well tried and the best available but they are not perfect. In the large majority of cases they will predict with reasonable accuracy the power of Lens Implant that should be used. However in a minority of cases the focus of the eye after surgery may not be quite as expected. In the UK at present there is a consensus amongst eye specialists that about 85 - 90% of eyes should be within one dioptre of the desired focus following surgery. The reason that the formulae, and therefore the focus results of surgery, are not perfect is "biological variability". For example two eyes may appear to be identical. They may give identical biometry measurements. However when implanted with the same power of lens they may not achieve identical focus after the operation. One reason may be that the Lens Implants sit in a very slightly different position within the eye. There may also be minor variability in the taking of the measurements and in the surgery itself which can also have an effect on the resulting focus of the eye.
Further information on Biometry can be found Here.
