Week 3 - RGP fit and problem solving

Question 1

What are the different methods of correcting a flat fit?

Answer 1

• Steepen the BOZR (will have the greatest effect)
• Increase the TD to stabilize the lens
• Increase the BOZD
• Use a thinner lens to reduce movement

N.B. If you increase the BOZD by 0.5mm then the BOZR requires to be flattened by 0.05mm to maintain the same pattern with a spherical lens (change in BOZD of 0.7mm for aspherics)

Question 2

What are the different methods of correcting a steep fit?

Answer 2

• Flatten the BOZR (greatest effect)
• Decrease the TD to increase mobility
• Decrease the BOZD (be careful with the 2nd and 3rd options with the pupil size)
• Use a thicker lens

Question 3

Why is over-refraction performed?

Answer 3

• Tells us a lot about the fit
• If the fit is good then the over refraction will be zero (or only a small difference) as the BVP is based on the ocular refraction accounting for BVD

• More plus/less minus - flat fit due to negative liquid lens
- E.g. -3.00 lens with a +0.50 over refraction is flat

• More minus/less plus - steep fit due to positive liquid lens
- E.g. -3.00 lens with a -0.50 over refraction is steep

Question 4

So, if over refraction out by 0.50Ds, how much do we change lens by?

Answer 4

• 0.25Ds = 0.05mm
Therefore 0.5Ds = 0.1mm