Front Surface Cylinder
A Front Toric design is normally utilized on those patients with little or no corneal astigmatism yet significant refractive cylinder, resulting in residual astigmatism.
Keratometry Spectacle Refraction
Front Toric Candidate: 43.00 @ 180 / 43.00 @ 90 -1.00 -1.00 X 90
A Front Toric design combines a spherical base curve with a front surface cylinder ballasted with prism to orient the cylinder at the proper axis. In the above example no corneal astigmatism is present yet the refractive cylinder is -1.00 X 90. This -1.00 X 90 is residual physiological astigmatism resulting from internal components of the eye. The tear lens will not correct internal residual astigmatism.
Back Toric
Back Toric designs are normally utilized on those patients with both significant corneal toricity (>2.50 Diopters) and refractive cylinder approximately 1.5 times the corneal astigmatism.
Keratometry Spectacle Refraction
Back Toric Candidate: 43.00 @ 180 / 46.00 @ 90 -1.00 -4.50 X 180
A Back Toric design combines toric base curves (selected to fit both major meridians of the cornea) and a spherical front surface. Even though the front surface is spherical, the base curve toricity will induce an in air cylindrical power approximately 1.5 times the actual base curve toricity. A 3-diopter base curve toricity (radiuscope) will result in an approximate 4.5-diopter cylinder power (lensometer). The percentage of induced cylinder will vary based on the specific refractive index of the material. The 1.5 figure will provide a quick calculation for design purposes. For the reasons just presented, a change in base curve toricity will also alter the resultant cylinder power. Do not compromise the lens to cornea fitting relationship in order to achieve a specific cylinder power. A Bi-Toric may be required to provide both the best fit and appropriate meridian powers.
Bi-Toric
Bi-Toric designs are normally utilized on patients with significant corneal astigmatism (>2.50 Diopters). Under the Bi-Toric design category two specific types are available, the Spherical Power Effect (SPE) and the Cylindrical Power Effect (CPE).
The SPE Bi-Toric greatly simplifies meridian power calculations and should be considered when fitting corneal astigmatism of 2.50 diopters or more and the corneal astigmatism is equal to the refractive cylinder. Your base curves should be selected to promote a natural tear pump; flatter than K in the vertical and aligned in the horizontal.
Keratometry Spectacle Refraction
SPE Candidate: 43.00 @ 180 / 47.00 @ 90 -0.50 -4.00 X 180
Against the rule corneal astigmatism (flat meridian vertical) should also be fit to achieve vertical clearance and horizontal alignment. This will require attention to the axis of the flat and steep meridians of the cornea when selecting the corresponding base curves. Regardless of your fitting approach, (small & steep, large & flat or aligned saddle) the SPE Bi-Toric power computation is simple. The base curve toricity in diopters should be equal to the difference between the meridian powers of the contact lens. The SPE Bi-Toric design will not correct internal or residual astigmatism. Residual astigmatism would require a CPE Bi-Toric or Back Toric design.
CPE Bi-Toric
The CPE Bi-Toric should be used when corneal and refractive astigmatism are not equal.
Keratometry Spectacle Refraction
CPE Candidate: 43.00 @ 180 / 47.00 @ 90 -1.00 -2.50 X 180
CPE should also be considered when a SPE or Back Toric doesn't satisfy both the physical fit and in air cylinder requirements. If the refractive cylinder is less than the corneal astigmatism, a CPE Bi-Toric is indicated. If the refractive cylinder is more than the corneal astigmatism, CPE Bi-Toric or possibly a Back Toric is indicated.