Week 7: Prescribing for High Ametropia & Prismatic in PALs

Question 1

How do you find prism thinning in PAL for major reference point?

Answer 1

• Add power of lens is a major factor in determining amount of prism thinning required
• Add power increases, amount of prism thinning required in order to equalise the thickness difference & slim the lens black also increases

Question 2

Problem: People with prescribed prism have problems with their progressive lenses. How do you resolve this?

Answer 2

• Person measuring for progressive lenses needs to anticipate the amount the eyes will be displaced by the prism
• Amount is about 0.3 mm for every Δ of prescribed prism

Question 3

How do you Prescribe Vertical Prism in PALs?

Answer 3

• Measure fitting cross heights
• Change fitting cross height by 0.3 x prescribed vertical prism as calculated clinically using the phoropter
• Base down prism = add fitting cross height
• Base up prism = minus fitting cross height

Question 4

How do you Prescribe Horizontals Prism in PALs?

Answer 4

• Measure monocular PDs using a pupillometer
• Modify the monocular PDs by 0.3x prescribed horizontal prism as calculated clinically using phoropter
• Base in prism = add PD
• Base out prism = minus PD

Question 5

How do you prescribe prism for PALs?

Answer 5

• This has implications for prescribing prism in PALs as the near corridor is relatively narrow
• Must be more careful to prescribe the prism by adjusting the location of the fitting cross, unlike bifocals where near seg size is increased

Question 6

What is Prescribed Vertical Prism Verification?

Answer 6

• For prism thinning, we expect vertical prism at the R & L MRPs to be equal
• However, if vertical prism is prescribed, we expect that there should be a difference in the prism measured at the R & L MRPs

Question 7

What is Prescribed Horizontal Prism Verification?

Answer 7

Measure the R & L prism at MRP and expect the net prism to equal the total prism ordered = desired prism + decentration prism

Question 8

How to Decrease Lens Thickness?

Answer 8

• Refractive index
• Centre/edge thickness
• Stock v. Grind lenses
• Frame Selection

Question 9

What Prescription Require Thinning?

Answer 9

> +6.00D → definitely
+2.00D - +4.00D → good to consider
-2.00D - +2.00D → typically not a concern
-4.00D - -2.00D → good to consider
<-6.00D – definitely

Question 10

What is the relationship when you increase refractive index of the lens?

Answer 10

If we increase refractive index of the lens (n’), then to achieve same dioptric power increasing curvature of the lens (r) makes the lens flatter

Question 11

Describe what is required for lens thickness?

Answer 11

• Influence on back vertex power for thick lenses
• Standards specify that lenses should be thick enough to avoid shattering for safety reasons
• Practical considerations – must be thin enough to be cosmetically acceptable, but thick enough to be robust to avoid shattering, breakage or chipping in daily wear

Question 12

What are the minimum thickness required for centre and edge thickness?

Answer 12

• Positive lenses (minimum thickness = 1-1.5 mm for CR39)
• Negative lenses (minimum thickness = 2.00 mm for CR39)
• Hence we want to be able to minimise the centre or edge thickness whilst meeting requirements for safety

Question 13

Describe stock vs grind lenses

Answer 13

Stock lens:
- Lens kept in supply by lens company, cheaper as made in bulk
- Comes in set diameters (eg 65, 70, 75 mm)

Grind lens:
- Lens made specifically for a patient, such that diameter is ground to suit the frame the script will be fitted
- More expensive