HIGH ASTIGMATISM: Designs, Indications, Parameters, Evaluation and Decision-Making Process, Part Two: A Quick Word About GP Toric Lenses
Clarke D. Newman, OD, FAAO, FBCLA, FSLS
For contact lens wear to be successful, so the truism goes, one must have a lens material that is biocompatible with the ocular milieu, a posterior lens surface alignment that is physiologically tolerable, and an anterior surface curvature that neutralizes the residual refractive power of the eye in question.
When prescribing GP corneal lenses, the optic and haptic zones are a geometry problem. One can vault the optic zone by increasing the lens sagittal relationship as described by Caroline and Kojima; however, in corneas with significant anterior toricity, this project is still difficult without toric lens surfaces.
There are three types of toric GP lenses. First, there is a front toric lens. This design is only indicated when lens alignment is achieved with an iso-curve (non-toroidal) posterior surface, but the patient has significant residual astigmatism. This lens design is rarely used in corneal, but widely used in scleral lens designs. The second type is a base toric lens. This lens type is indicated when a toric posterior surface is required for physiological alignment, but the residual refractive error is equal to the induced cylinder of the posterior surface. This design is rarely used in corneal GP lenses.
The most common GP toric lens design is the bitoric. Again, there are three types here. The first is the spherical power effect lens, or SPE. The SPE lens has a toric front surface that corrects only the induced cylinder from the base toric surface. This lens is indicated in patients who have a significant anterior corneal toricity (greater than about 2.50 Diopters), but no residual cylinder. This type becomes more useful with increasing anterior corneal cylinder. All toric GP diagnostic sets should be SPE’s to make the optics much easier to determine.
The second type is the cylindrical power effect lens, or CPE. This design is indicated when there is both significant anterior corneal toricity and significant residual astigmatism. The third type is a crossed cylinder effect, or CCE. This bitoric lens is indicated when the flat corneal meridian is more than twenty degrees misaligned to the residual cylindrical axis.
When aligning the posterior lens surface with the anterior cornea, aligning the horizontal meridian while being 1.00 D flatter than the vertical meridian will create a low-toric simulation that will center horizontally and travel easily vertically. Aligning both meridians is best when the corneal toricity is oblique.
What would you prescribe for the following patient?
| Manifest Refraction | Keratometric Values |
| OD: -2.00 -2.75 X 180 | 42.50 @ 180 / 46.50 @ 090 |
| OS: -1.50 -2.50 X 180 | 42.50 @ 180 / 45.00 @ 090 |
As the keratometric and refractive axes align for both eyes, a CCE is not indicated. As the refractive cylinder of the right is not equal to the corneal toricity, a CPE is indicated. However, the refractive cylinder of the left eye has the same magnitude as the corneal toricity, so, an SPE is likely indicated. One can make sure by performing a diagnostic evaluation with an SPE diagnostic lens and an over-refraction.
Source
- Bennett ES, Henry VA, Kinoshita BT, Lampa M. Correction of Astigmatism. In Bennett ES, Henry VA. Clinical Manual of Contact Lenses (5th ed.) Wolters Kluwer, Philadelphia, 2020:388-439.