Biometry
Biometry is the term used for the
pre-operative calculation of the strength of lens implant that will
give an eye normal focus after cataract surgery. It is individual
to each eye.
Why is biometry needed?
Before cataracts develop, human eyes are
capable of 60 dioptres of focussing power, as if there were a huge
magnifying glass inside the eye bending light on to the retina.
Most of the focussing (approx 40 dioptres) is done by the cornea,
the curved transparent collagen window at the front of the eye. The
other 20 dioptres is achieved by the natural lens, effectively a
bag of protein inside the eye. When the protein of the lens goes
hazy with age we call it ‘cataract’. At cataract surgery, the lens
is removed and replaced with an artificial lens implant of the
right strength to achieve ‘normal’ vision after the operation.
Biometry uses measurements to predict this strength so that we can
customise the focus of the eye post-operatively. The choice of
required focus is usually discussed between patient and
ophthalmologist prior to surgery.
What measurements are done for ’biometry’?
Two measurements of the eye are needed. The
first (keratometry) is a measure of the curve of the cornea and its
power to bend light rays (refraction). The second is a measure of
the length of the eye, to judge how much refraction is required.
Most eyes are approximately 23 millimetres long. Short sighted eyes
(myopic) are longer, and longsighted eyes (hypermetropic) are
shorter. (Don’t be confused - much of ophthalmology sounds upside
down!). For most eyes, the tests are quick and painless using
non-contact laser technology, and the results are available in
minutes. Occasionally other tests are needed. A very dense cataract
will require A-scan ultrasound to measure of the length of the eye
accurately, and an irregular or steep cornea may need manual
keratometry.
How are the measurement used to calculate the lens
strength?
The results of the tests are fed into a
computer that uses mathematical formulae to calculate the lens
implant power customised to the chosen lens implant style and
surgeon. Different mathematical formulae apply to different length
eyes, and all formulae are less accurate at the extremes of
measurement. To reduce avoidable error, it is usual to test both
eyes and cross-check the results.