Perkins Mk2 Manuel D'instructions page 17

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4. Taking a Pressure Measurement
- continued
The major advantage of applanation tonometry is the small bulbous deformation
that amounts to only 0.56 mm 3 . The values found by this method of tonometry are
only slightly influenced by scleral rigidity and radius of corneal curvature.
The principle of applanation tonometry is simple. The careful construction of the
apparatus is a guarantee of its continued correct performance.
Nevertheless, in order to obtain exact results, the method of use indicated must be
strictly followed.
The readings obtained are based on the assumption that a cornea with a 'normal'
thickness is being measured - alteration of the corneal thickness leads to changes
in the measured IOP.
A 'normal' corneal thickness is considered to be within the area of 530-560
microns. There is no universally agreed formula for the adjustment of the measured
tonometry reading against altered corneal thickness.
The users should bear these two facts in mind and keep themselves informed,
through the current literature, with regard to the various formulae for adjusting
measured tonometry readings in the event of thinned, or thickened, corneas
being measured.
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