contact lens primer
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
The benefits (and drawbacks) of rigid gas permeable contact lenses are well documented. RGP contact lenses provide excellent vision and corneal physiology. Initial comfort is the bane to their popularity. Rigid lenses are invaluable for managing patients with large amounts of regular or irregular astigmatism. Even though the recent crop of soft toric lenses may be prescribed for large amounts of refractive astigmatism, misalignment or rotational instability may result in unacceptable vision. These rotational effects are more pronounced in patients with higher amounts of astigmatism. Toric RGPs may be the best contact lens option for these patients.
Spherical lenses work well for patients with 0.75D or less of residual astigmatism and less than 2.50D of corneal toricity. A toric back surface design is indicated dependent on the eyecare practitioner's fitting philosophy and other fitting variables such as corneal topography, lid structure and lid position. Rigid contact lenses fit lid attached may be suitable for patients with larger amounts of corneal toricity than intrapalpebral fits.
Front, Back or Front and Back?
Lens design and parameters may be determined by empirically ordering from keratometry and manifest refraction findings or by obtaining additional data from a diagnostic contact lens fitting.
We prefer a diagnostic lens in most instances. Apply a non-flexing spherical rigid contact lens, then perform a sphero-cylinder over-refraction and fluorescein pattern evaluation. Based on fitting considerations (corneal toricity and spherical lens position on the eye) and the amount of over-refraction cylinder, a lens design is determined. A bitoric diagnostic set can be helpful in assessing the fluorescein pattern and lens positioning, as well as fine-tuning the over-refraction. If the patient has more than 2.50D of corneal toricity, we recommend starting with a bitoric diagnostic lens.
Choose a base curve toric, spherical front surface design when there is sufficient corneal toricity (>=1.50D) to minimize lens rotation and the patient's over-refraction cylinder through a spherical rigid contact lens is approximately one-half the corneal toricity and at the same axis as the corneal toricity. Choose a bitoric RGP lens if a spherical design does not provide an acceptable fitting relationship due to the amount of corneal toricity, and a base curve toric does not sufficiently correct the refractive astigmatism. Choose a front surface toric, with a spherical base curve, when the corneal toricity is minimal (¾1.00D), and the residual cylinder in the over-refraction (typically lenticular astigmatism) with a spherical lens is unacceptable.
Table 1 lists designs from simplest to more complex. The simpler the lens design, the better the optical properties of the lens, the more economical the lens will be for you and your patient, the better the ability of your laboratory to fabricate the lens, and the easier you will be able to analyze, modify and duplicate the lens.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at
Dr. Barr is editor of
Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of
Contact Lens Spectrum, Issue: November 2001