Test Your GP Fitting Skills – page 2

Case 1 – Discussion Item 2

After reading the Patient History, read the Discussion Items below. To test your understanding of this case, click on the word “select” to read the reasoning behind each choice.

Patient History: 16 y/o female student has never worn contact lenses. She plays no extra curricular sports, and her mother wears GP lenses. No contraindications for lens wear.

Spectacle Rx

Corneal Measurements

Ocular Measurements (OU)
Corneal diameter
Pupil
Fissure width

OD -2.00 -0.75 x 180
OS -2.25 -1.00 x 180
OD 43.50/44.25 @ 90
OS 43.25/44.25 @ 9011.5mm
4.0mm
10.5mm

Discussion Item 2: When designing a lens empirically or selecting the first diagnostic lens, which parameter would you choose first?

a) Diameter select

b) Base curve select

c) Power select

d) Peripheral curves select

e) Material select

Go to Discussion Item 3 >>


Spectacle Rx

Corneal Measurements

Ocular Measurements (OU)
Corneal diameter
Pupil
Fissure width

OD -2.00 -0.75 x 180
OS -2.25 -1.00 x 180
OD 43.50/44.25 @ 90
OS 43.25/44.25 @ 9011.5mm
4.0mm
10.5mm

a) More than one fitting philosophy will usually work for most patients. A lid attachment philosophy is great for patients whose upper lids cover the superior limbus, as is the situation in this case. Larger diameters in the range of 9.4mm – 9.8mm will usually achieve lid attachment. Therefore in this case diameter would be the 1st parameter selection to make. TIP – If making changes in fit by altering diameter, change the diameter at least 0.3mm to make any significant change. back


Spectacle Rx

Corneal Measurements

Ocular Measurements (OU)
Corneal diameter
Pupil
Fissure width

OD -2.00 -0.75 x 180
OS -2.25 -1.00 x 180
OD 43.50/44.25 @ 90
OS 43.25/44.25 @ 9011.5mm
4.0mm
10.5mm

b) This is usually the second parameter to be selected. Larger diameter, lid attachment philosophy lenses (9.4mm – 9.8mm) require a base curve fit flatter than K to achieve an alignment fit while smaller intrapalpebral philosophy lenses (8.8mm – 9.2mm) require an on K or steeper than K base curve selection to achieve the desirable lens to cornea relationship. TIP – If making changes by altering base curve, alter at least 0.50D (0.10mm) for a significant change. back


Spectacle Rx

Corneal Measurements

Ocular Measurements (OU)
Corneal diameter
Pupil
Fissure width

OD -2.00 -0.75 x 180
OS -2.25 -1.00 x 180
OD 43.50/44.25 @ 90
OS 43.25/44.25 @ 9011.5mm
4.0mm
10.5mm

c) The lens power cannot be selected until the diameter and base curve is determined. The power can then be calculated using the rule of thumb – Steeper Add Minus and Flatter Add Plus (SAM/FAP). back


Spectacle Rx

Corneal Measurements

Ocular Measurements (OU)
Corneal diameter
Pupil
Fissure width

OD -2.00 -0.75 x 180
OS -2.25 -1.00 x 180
OD 43.50/44.25 @ 90
OS 43.25/44.25 @ 9011.5mm
4.0mm
10.5mm

d) Today it is not necessary to design the peripheral curves. Your CLMA member laboratory can provide standardized parameters to complete the data necessary for the posterior optical zone, peripheral curves, edge shape, center thickness and the anterior control of power and lens mass. The CLMA member laboratory will use standardized parameters based on philosophies established by your colleagues and peers. They are tried and proven averages, which will be successful for the greatest percentage of patients fit. back


Spectacle Rx

Corneal Measurements

Ocular Measurements (OU)
Corneal diameter
Pupil
Fissure width

OD -2.00 -0.75 x 180
OS -2.25 -1.00 x 180
OD 43.50/44.25 @ 90
OS 43.25/44.25 @ 9011.5mm
4.0mm
10.5mm

e) Today, the choice of material doesn’t affect the parameters of a lens chosen. Instead the material choice is based upon oxygen demands, wetting characteristics, etc. back