The College of Optometrists

When asked to write out a full specification for a rigid cornal contact lens I am never certain how to calculate the peripheral curves of the lens. I know the first peripheral curve should be at least 0.70mm flatter than the BOZR but I get stuck after that...

Hi -this is not a big deal. To calculate them would be tedious - and most people actually use standard curves. That's because you can't polish on acurately curves much different from 0.7 flatter. The final curve is typically 12.25 or there abouts - 'cos that is the radius of the globe and you don't want it to dig in. The narrowest you can get a curve is 0.4 (by polishing). So creating a lens 7.85, 7.00/ 8.60,7.85/9.40, 8.40/12.25,9.2 is typical. Rigorously, you would need to calculate all the little sags and subtract them from the sag of the central radius - almost no-one does this because the whole thing is made less acurate by polishing the lens.

I hope this helps.

steve

-also changing the diameter alters the sag/edge lift twice as much as changing the radius.

so you create a typical C3 lens

7.85, 7.00/ 8.50, 7.85/ 9.50, 8.4/ 12.25, 9.0

but you think its ride is non central and it seems to have excessive edge lift but when centred the central zone aligns.

So what do you do?

try 7.85, 7.00/ 8.40, 7.70/ 9.30, 8.40/ 12, 9.0

-this would fit tighter somewhat.

You could always make the lens smaller by polishing away at the edge

you could also make the lens smaller and thinner

7.85,7.00/ 8.50, 7.70/ 9.40, 8.2/ 12.00, 8.8

But all this is truely only inspired guessing. To do it acurately you would need to get out the calculator.

I would decide the overall size - which is get the eye lid aperture and take 3mm away. Add 0.5 for a big cornea and take 0.5mm for a small cornea.

Then how many peripheral curves you want?

Most common is the tricurve. The BCOD (forgive me if my terminology is sl. out of date) should be at least 1 mm more than the average pupil size (6mm)

So then you have BCOD 7mm

Overal diameter 9mm (ave lid aperture is 12mm)

You have 2mm to play with to make your edge lift of 0.12mm (ave)

Note more than 0.2mm causes bubbled to go underneeth.

Less than 0.12mm causes lens binding and digging in at the limbus.

The final curve is nearly always 12.25 and 0.3mm wide.

The two inner curves carry the gradually flattening geometry.

You can't have curves less than 0.4mmwide or 0.6mm flatter - cos it can't be made accurately.

So to put it all together: 7.70, 7.00/... , .../... , 8.4/12.25,9.00 average curve width is (8.40 - 7.00 )/2 = 0.7

7.70,7.00/... , 7.70/ ... , 8.40/ 12.25, 9.00 -you want a smooth curve between BCOR 0f 7.70 to final radius of 12.25 -so averaging lets go for steps of 12.25 - 7.70 / 3 = 1.51mm but as you know the inner curves affect the edge lift more so we can take from the inner curve and give to the outer curve:

ie - lets take (average) of 0.6mm away from 1.51mm = 0.91mm and give that to the first curve ie First curve = 7.70 plus 0.91 = 8,6mm

So second curve = 1.51 plus the 0.6 mm = 2.11mm -we add that to the BCOR = 9.81mm

So final Rx 7.70,7.00 / 8.60, 7.70/ 9.80, 8.40/ 12.25, 9.00

Any inacuracy is polished out in the manufacture. As you can see its not very accurate - but it works. To do it accurately you average the curves dioptrically and add that. Far more fiddly and not needed in practice.

Good luck -steve ps: sorry for the delay - I was wondering if anyone else would answer you !