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Biometry: The Measurements Needed Before Cataract And Refractive Lens Exchange Surgery
“Biometry” is the term used by eye surgeons to describe the eye measurements that need to be made before a cataract or Refractive Lens Exchange operation. In both of these procedures the natural lens of the eye is removed and replaced with a plastic artificial lens, called “the lens implant”. In a cataract operation the natural cloudy lens of the eye is removed to restore vision. In Refractive Lens Exchange the natural lens remains clear but is removed and replaced so as to correct a focus problem with the eye. Technically the operations are virtually identical.
The pre-operative measurements are needed in order to calculate the power of the lens implant required to give the desired focus to the eye following surgery. These measurements are simple, quick and painless to perform. There are two main measurements. These are: (1) the curvature of the cornea (the window of the eye), and (2) 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 the required lens implant power. In other words the lens implant power is calculated and chosen for each individual eye.
The curvature of the cornea is determined 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 (axial length) 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 anesthetic 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 alternative instruments that use laser light to determine the axial length do not need to touch the eye. The patient places their chin and forehead against rests just in front of the instrument.
The formulae that calculate the required lens implant power are complex but the principle is simple. To see clearly an image must be in focus on the retina. The total focus needed is related to the length of the eye. The two parts of the eye that “do the focusing” are the cornea and the lens. If the focus provided by the cornea is known then the amount of focus that must be provided by the lens implant can be worked out. 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.
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. There is a consensus amongst eye specialists that about 90% of eyes should be within one dioptre of the desired focus following surgery.
The information within this article is for general interest only. These are the personal views of Consultant Ophthalmologist Mr C J Heaven. If you have a problem with your vision then always seek the advice of your own eye specialist. Mr Heaven is based at The Royal Albert Edward Infirmary in Wigan, in the north west of England. His special interests include small incision cataract surgery and various forms of refractive surgery. For more details see: http://www.lens-exchange.co.uk
Source: www.isnare.com