Back Toric GP Design: Terry R. Scheid, OD, FAAO
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 lens designs and modalities.
Test Procedures, Fitting/Refitting, Design and Ordering
It was decided to fit corneal gas permeable (GP) lenses due to the CLPC and the visual acuity difficulty. Refractive findings were:
OD: +3.75 -4.25 x 150 20/25
OS: +2.25 -3.25 x 005 20/20
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 corneal 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-surface toric base curve corneal 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.
|Base Curve Radii(BCR):
|+3.75D/ -.50D (total power in each meridian)
|+2.25D/ -1.00D (total power in each meridian)
|Overall/Optical Zone Diameters (OAD/OZD):
|Toric Secondary Curve Radii (SCR/W):
|Peripheral Curve Radius (PCR/W):
|Center Thickness calculation based on most plus meridian (Equivalent to a spherical lens with a power equal to +3.75D)
|Center Thickness calculation based on most plus meridian (Equivalent to a spherical lens with a power equal to +2.25D)
|Dk = 58 and index of refraction = 1.429
|Dk = 58 and index of refraction = 1.429
Follow-Up Care/Final Outcome
The dispensed lenses fit well on both 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 corneal GP lenses and the CLPC was well controlled.
Back-surface toric designs (toric back surface with spherical front surface) 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 surface toricity can then induce the full corrective cylinder.
There are two lens modification advantages to prescribing a toric back-surface, spherical front-surface corneal GP contact lens when compared to a bitoric GP. One, you can add small amounts of minus or plus power; and two, you can polish the front-surface to enhance wetting or to remove scratches. If you attempt to add power or polish the front surface of a bitoric, you will distort the visual optics of the lens.
One of the advantages of prescribing a spherical power effect (SPE) bitoric lens is that there is no unwanted over-refraction cylinder induced by lens rotation and misalignment. With a back-surface toric, spherical front surface corneal GP lens, misaligned or rotated lenses will induce unwanted astigmatism because of cross-cylinder effects.
Because most eye care practitioners do not have extensive bitoric trial lens sets, generally you should consider ordering your initial trial lenses empirically.
Patient MO was diagnosed with contact lens induced papillary conjunctivitis (CLPC). Historically, CLPC was often caused by extended wear of yearly replacement soft contact lenses. Currently, the overwhelming majority of soft contact lens wearers replace their lenses monthly, biweekly, or daily. This along with fewer patients wearing their lenses overnight has resulted in decreased incidence and severity of CLPC because of decreased protein buildup on the lens.
CLPC can also occur with GP contact lenses, but generally not as frequently as with soft contact lenses. Bound protein may be a factor for GP CLPC, but more likely it is due to a bad edge or too much edge lift. Prescribing back toric and bitoric contact lenses generally reduce the occurrence of CLPC when compared to a spherical lens prescribed on a significantly toric cornea. This is due to the decrease of lid interaction with the toric back surface.
- 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.
- 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.
- 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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