Often the front optic zone diameter and BOZR are adjusted to reduce or increase the centre thickness (CT) to ensure suitable lens handling, optical performance and oxygen transmission.
The sagittal depth of the lens is emerging as a more suitable parameter for controlling the fit due to being a better comparator between different lens designs, rather than BOZR and TD.
Optical microspherometers (also known as radiuscopes) use optical principals to measure the BOZR and are relatively simple to use with rigid lenses measured in air.
By using a chord of a known diameter, the equation can be rearranged to calculate the BOZR from S, which is commonly measured using a mechanical probe but ultrasound techniques can also be used.
All of these methods require knowledge of the contact lens refractive index (RI), CT and BOZR to obtain in air power readings, in addition to taking into account the influence of the wet cell itself.
25mm that the BOZR is made flatter, therefore an extra +0.
To correct a flat fit: use a steeper BOZR to improve centration; increase the TD to stabilise the lens, use tighter peripheral curves, and use a thinner lens to reduce mobility.
If the lids are not the cause, then either fit a lens with a larger diameter, or steepen the BOZR.
Table 1 Suggested BOZR of initial trial lens, based on K readings.