GP Lens Case Grand Rounds Troubleshooting Guide – 27

Bitoric Spherical Power Effect: Tom Quinn, OD, MS

Background

PE was a 22-year-old waitress interested in contact lenses.

Test Procedures, Fitting/Refitting, Design and Ordering

Manifest Refraction:

OD: -1.00-3.00 X 180 20/20
OS: -2.00-3.00 X 175 20/20

Keratometry Readings:

OD: 41.00 @ 180; 44.00 @ 090
OS: 43.00 @ 175; 45.50 @ 085

Spherical Diagnostic Lens Applied OD:

BCROAD/OZDPowerCT

8.03mm (42.00D) 9.2/7.8mm -2.00D .16mm

BCR = Base Curve Radius; CT = Center Thickness
OAD/OZD = Overall Diameter/Optical Zone Diameter


Spherical diagnostic lens on right eye demonstrates poor centration; therefore elected to pursue toric GP fit.

Design of Toric GP:

1. Overall Diameter selection: Since upper lid covers upper limbus, attempt lid attachment, so bias diameter large. Patient has small aperture, so select intermediate diameter: 9.2 mm

2. Base curve radii selection: Fit horizontal meridian near alignment, vertical meridian slightly flat to promote vertical lens movement with blink (see table).

Recommended Toric GP:

Base Curve Radii Selection with Varying Lens Diameter

Base Curve/Cornea Relationship:

Lens Diameter (mm)HorizontalVertical

Small (8.0 to 8.6) 0.25 D STK* 0.50 D FTK**
Intermediate (8.7 to 9.3) On K 0.75 D FTK
Large (9.4 to 10.2) 0.25 D FTK 1.00 D FTK

Base Curve Radii Chosen:

OD: 41.00 (8.23mm)/ 43.25 (7.80mm)
OS: 43.00 (7.85mm)/ 44.75 (7.54mm)

3. Power Selection: Adjust spectacle power:

a) for vertex effects in any meridian with power greater than -4.00
b) to account for tear lens power by amount of tear lens power (see summary diagram for OD)

OD: -1.00/ -3.25
OS: -2.00/ -4.00

Lens Design Analysis:

If the difference in base curve power (BCR) between major meridians is equal to the difference in lens power (BVP) between major meridians, then the lens is a Spherical Power Effect (SPE) Design and you do not need to be concerned about lens rotation.

OD: Difference between BCR (41.00 / 43.25) = 2.25 D
Difference between BVP (-1.00/-3.25) = 2.25 D
OS: Difference between BCR (43.00 / 44.75) = 1.75D
Difference between BVP (-2.00/ -3.75) = 1.75D
Adjust power to -2.00/ -3.75 so difference = 1.75 D

Patient Education/Consultation

When the refraction was completed, visual acuity through the phoropter with and without cylinder correction was demonstrated to show need to correct astigmatism. This helps the patient understand the need for a special lens design.

Follow-Up Care/Final Outcome

Patient experienced excellent vision with a stable, well positioned lens (see photo below).


SPE Toric GP on right eye demonstration good centration and movement.

Discussion/Alternative Management Options

A spherical GP lens positioned poorly, so a toric lens was designed to provide an optimal fit. When power was calculated, it was determined that the difference in power between the two meridians of the right lens was equal to the difference in base curves in the lens. When this relationship exists it is a SPE design, which can rotate without adversely affecting vision. A toric soft lens was not chosen due to the high degree of astigmatism relative to the spherical refractive error. Any rotation of such a toric lens would result in variable vision.

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