contact lens primer
Toric Soft Contact Lenses
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
Patient selection and education are crucial to soft toric lens success. Patients with high sphere or cylinder prescription are not ideal candidates for toric soft lenses. Sphere powers greater than +5.00D
+5.00D or 7.00D generate thick lens profiles that may cause edema. Patients with more than 3.00D of cylinder might suffer from reduced vision if the correcting cylinder misaligns or is rotationally unstable. To soften vision expectations, educate patients that their vision will be "comparable" to their spectacles.
Stability Is the Key
Stabilize lens orientation and rotation to ensure that the correcting cylinder is aligned properly and consistently with the patient's axis of astigmatism. Most of today's lenses achieve positional and rotational stability with prism ballast and/or eccentric lenticulation and thin zones. Cylinder power is achieved with a toric front or back optical surface. Many practitioners feel that a back surface toric design provides additional rotational stability for patients whose astigmatism is primarily corneal.
When assessing the fit of a toric soft lens, allow it to equilibrate for at least 15 minutes. Evaluate for movement (on a blink and on horizontal and vertical excursions), coverage and centration. Change the base curve or overall diameter if the fit is unsatisfactory. Also, evaluate for orientation of the prism base and rotational stability. Some lenses are marked at 3 o'clock and 9 o'clock instead of at the prism base. The lens base location is not crucial, although many practitioners prefer it within 30 degrees of the 6 o'clock position. The critical aspect is the locational stability of the lens. The lens should consistently locate to the same orientation each time it is applied, and should not rotate excessively. (We recommend less than 10 degrees rotation on a blink.)
Positional and rotational stabi-lity are more critical for larger cylinder power prescriptions. A lens cylinder that positions 15 degrees away from the refractive cylinder axis induces a cylinder of one-half of the correcting cylinder in the over-refraction.
A Little Help From LARS
Assess lens marks to determine the cylinder axis prescription. For example, if a prism base marking is to your left of the 6 o'clock position, add the amount of rotation to the cylinder axis of the patient's refraction. If the prism base marking is right of the 6 o'clock position, subtract. This is the LARS principle (Left Add, Right Subtract). You want the lens marking of the prescribed lens to position as it did with the diagnostic lens in place.
Empirical ordering based on keratometry readings and manifest refraction can reduce chair time and provide a favorable first experience for the patient. Trial fitting provides fitting information, primarily base curve performance and prism base orientation and stability. However, patients are sometimes disappointed with their initial visual experience.
Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry. E-mail him at email@example.com.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of Optometry.