b) An aspherical design with BOZR of 7.65 and a TD of 10.4mm
c) A spherical design with a BOZR of 7.65 and a TD of 8.5mm
The measurement of central corneal curvature by keratometry purely for the purpose of choosing the BOZR of the initial soft lens is, therefore, not particularly helpful.
A common theory is that as a lens dehydrates there is a steepening of the BOZR, the consequences of which are a reduction in lens movement.
Typically, the BOZR
is reduced (creating a steeper curve) to create a tighter fit to eye as a result of an increase in lens sagittal depth (sag), and increased to loosen the fit due to a decrease in lens sag.
* If a lens is fitted steeper than K, a positive tear lens is created, requiring more negative power in the over-refraction to neutralise the effect (that is the tear lens power increases by +0.25 for each 0.05mm that the BOZR
is made steeper, therefore an extra -0.25 over-refraction) (see Figure 1i
* For BOZD between 7.00 and 7.40, each principal BOZR
is fitted 0.05mm flatter than K readings
Generally speaking, practitioners using 'in house or system lenses' only have to deliberate about the total diameter (TD), back optic zone radius (BOZR
) and back vertex power (BVP).
* For toric corneas with 1.00-2.50 D of astigmatism (0.2mm and 0.5mm difference between K readings), the lens should be fitted with a BOZR
that is 2/3 of the way towards the flattest K
b) Alter the BOZR
to 7.90mm and reduce the maximum wearing time
a) Back optic zone radius (BOZR
) and lens thickness b) Back optic zone radius (BOZR
) and back optic zone diameter (BOZD) c) Back optic zone radius (BOZR
) and back vertex power (BVP) d) Back optic zone diameter (BOZD) and lens thickness
If excessive rotation is observed, then the BOZR
should be steepened and the TD increased.