GP Lens Case Grand Rounds Troubleshooting Guide – 25

Back Toric GP Design: Terry Scheid, OD

Background

Patient MO was a 10-year soft toric lens wearer with intermittent OD esotropia and mild OD amblyopia. She developed persistent contact lens papillary conjunctivitis (CLPC) and unacceptable visual acuity fluctuation even with various soft toric designs and modalities.

Test Procedures, Fitting/Refitting, Design and Ordering

It was decided to fit gas permeable (GP) lenses due to the CLPC and the visual acuity difficulty. Refractive findings were:

Manifest Refraction:

OD: +3.75 -4.25 x 150 20/25
OS: +2.25 -3.25 x 05 20/20

Keratometry:

OD: 45.00 @ 150/48.00 @ 060
OS: 45.75 @ 005/48.00 @ 95
Mires clear and regular OU

Corneal Topography: Regular astigmatism with central measurements very consistent with keratometry readings.

Diagnostic spherical base curve GP fitting revealed with-the-rule astigmatic patterns with bearing along the flat axis and excessive standoff along the steeper meridian. In addition, significant residual astigmatism (often defined as ≥ 0.75D) was present with over-refraction.

Gas Permeable lens calculation of residual astigmatism:
Refractive astigmatism — corneal astigmatism (minus cylinder)
The total power in each meridian was less than 4D and did not require effective power vertex distance correction.

OD: -4.25 x 150 – (-)3.00 x 150 = -1.25 x 150
OS: -3.25 x 005 – (-)2.25 x 005 = -1.00 x 005

A back toric base curve GP lens was selected to enhance the fitting characteristics on this toric cornea. By using a back toric with a spherical front surface, the residual astigmatism optical problem can be addressed.

When a toric base curve GP lens is placed on the cornea, a cylinder over-correction results from the difference in index of refraction of the contact lens versus the cornea and tear film index of refractions. The induced over-correction is a minus cylinder at the same axis as the flatter principal meridian. The higher the contact lens index of refraction, the greater the induced cylinder.

For simplification, a PMMA lens of 1.49 index of refraction induces approximately ½ the back surface toricity. GP lenses range in index of refraction from 1.41 to 1.48. Selecting a material with a lower index of refraction can reduce the amount of induced astigmatism closer to 40% of the back surface toricity.

As the corneal cylinder in this case was 3D OD and 2.25D OS, a “saddle fit” can be utilized whereby both meridians are fit “on K.” With higher amounts of corneal astigmatism usually a “low toric simulation” fit is utilized with the flatter meridian fit “on K” to 0.25D flatter than “K,” and the steeper meridian fit .75 – 1.00D flatter than “K.” The saddle fit tends to represent a tighter fit and induces more cylinder due to the maximum corneal cylinder correction.

Lens Design

OD Base Curve Radii(BCR): OD 45.00/48.00(7.5/7.03MM)
Powers: +3.75D/ -.50D (total power in each meridian)
Overall/Optical Zone Diameters (OAD/OZD): 9.2/7.8mm
Toric Secondary Curve Radii (SCR/W): 9.0/8.5/ .4mm
Peripheral Curve Radius (PCR/W): 11.0/.3mm
Center Thickness calculation based on most plus meridian
(Equivalent to a spherical lens with a power equal to +3.75D)
Material: Boston EO(Dk = 58 and index of refraction = 1.429

OS Base Curve Radii(BCR): 45.75/48.00(7.38/7.03mm)
Power: +2.25D/ -.1.00D (total power in each meridian)
Overall/Optical Zone Diameters (OAD/OZD):9.2/7.8
Toric Secondary Curve Radii (SCR/W): 8.5/8.1/.4mm
Peripheral Curve Radius (PCR/W):11.0/.3mm
Center Thickness calculation based on most plus
meridian (Equivalent to a spherical lens with a power equal to +2.25D)
Material: Boston EO(Dk = 58 and index of refraction = 1.429)

Follow-Up Care/Final Outcome

The resultant lenses fit well on these toric corneas. The patient’s best visual acuities of OD 20/25 and OS 20/20 at distance were obtained with the back toric design (front surface spherical) as the OD 3D back surface toricity with a material of refractive index of 1.429 induced close to 40% of the 3D giving the full cylinder correction of about -4.25D.

The left lens back surface toricity of 2.25 D with the same material, induced about 1D of minus cylinder giving the total in air correction of -3.25D. She adapted well to these back surface toric GP lenses and the CLPC was well controlled.

Discussion/Alternative Management Options

Back toric designs theoretically are indicated when residual astigmatism is present where the spectacle cylinder is approximately 1.5 times the corneal astigmatism as in the presented case. The back toricity can then induce the full corrective cylinder.

However, due to the easier manufacturing considerations and lesser lens cost, GP lens laboratories may prefer to use a back toric design even when refractive and corneal cylinder are equal (no residual astigmatism). This requires using 2/3 of the spectacle cylinder as the difference between the flat and steep base curve. Thus if the keratometry readings were: 43.00 @ 180/46.00 @ 90 and spectacle Rx is Plano -3.00 x 180 , two-thirds of 3D = 2D and the contact lens base curve radius can be designed as 42/44 with the induced cylinder providing the needed full in-air correction.

However, with higher amounts of corneal and spectacle astigmatism, this design may result in too flat a fit in the steeper meridian and unlike a spherical power effect design (SPE), any lens rotation can result in optical cross cylinder problems. Therefore, unless it is a case similar to this one in which the astigmatism induced via having a back surface toric lens on the eye essentially corrects the residual astigmatism, a back toric lens design is not indicated (1-3).

Bibliography/Supplemental Readings

  1. Byrnes S, Cannella A. Fitting the Astigmatic Eye: Various Rigid Gas Permeable Contact Lens Designs. In Scheid T . Clinical Manual of Specialized Contact Lens Prescribing. Butterworth-Heinemann. Boston, MA, 2002: 103- 125.
  2. Silbert J. Rigid Contact Lenses and Astigmatism. In Bennett E, Weissman B. Clinical Contact Lens Practice (2nd ed.) Lippincott Williams & Wilkins, Philadelphia, PA, 2005; 498- 513.
  3. Johnson M. Utilizing Back Toric Designs in Your RGP Practice and Bitoric, Back toric or aspheric? Choosing the Right RGP for High Toric Corneas. On the Art Optical Contact Lens Inc. (Grand Rapids, Michigan) website at artoptical.com.

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