Biometry

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 measurement are simple, quick and painless.

There are two main measurements:

(a) The curvature of the cornea (the window of the eye).

(b) 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.

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% 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.

The curvature of the cornea is found by analysing reflections from its surface. Instruments that do this shine an array of lights or bright rings onto the eye and then measure the size of their reflections. Some instruments are table mounted. With these the patient places their chin and forehead against rests just in front of the instrument. Others are hand held by the examiner who positions the instrument just in front of the eye. Although the instrument may need to come close to the eye there is no need to touch the eye. These instruments are called keratometers. The corneal measurements they take are called the "k" readings or "k" values.

The overall length of the eye is called the "axial length". It is the distance from the front central point on the cornea to the centre of the retina at the back of the eye. This distance can be measured either using ultrasound or a weak laser light.

The instruments that use the ultrasound method need to touch the eye. An anaesthetic eye drop is used to numb the surface of the eye. The measurement is then taken either using a pencil like probe which lightly touches the centre of the cornea, or with a saline bath placed over the eye. An ultra sound signal is passed into the eye. This signal bounces back from each surface within the eye, rather like sonar. The axial length can be calculated from the time taken for the ultra sound signal to bounce back from the retina. The depth of the front chamber of the eye is calculated from the time taken for the signal to bounce back from the front surface of the lens of the eye.

The calculation makes certain assumptions about the density of the contents of the eye. As the density of the structures within the eye may vary a little from patient to patient there is the possibility of a small error in determining the axial length. Instruments that use laser light to determine the axial length do not suffer from this potential error. They may therefore be more accurate but are considerably more expensive. These instruments do not need to touch the eye.

Once the corneal curvature and the length of the eye are known formulae are used to calculate the Intra Ocular Lens Implant power required for the desired post-operative focus of the eye. Although these formulae are complex the principle is simple. To see clearly an image must be in focus on the retina at the back of the eye. Most of the focusing of the eye (about ¾) is done by the cornea. If this amount is known (from the corneal curvature) then the additional focus needed can be calculated. This remaining part of the focus of the eye must be provided by the Lens Implant.

The precise power of the Lens Implant will depend on its exact position within the eye. This can vary with different lens designs and types of surgery.

All of these factors are taken into account when calculating the most appropriate lens power to use in any individual patient. Because it is a complex calculation there is always the possibility that the focus of the eye after surgery may not be exactly what was aimed for. However in the great majority of patients the result is close to that intended. The errors and uncertainty tends to be greater for eyes that were either very long or very short sighted before the operation.