contact lens primer
Tear Lens = Base Curve Corneal Curve
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
The tear lens (TL) is an often ignored and under-appreciated component of rigid contact lens success. The tear lens is formed by the back surface, or base curve (BC), of the contact lens and the front surface of the cornea (K). Since the cornea flattens in radius of curvature from its apex towards the limbus, you generally need to prescribe a base curve flatter than the average measured central keratometry readings in order to obtain an alignment fitting relationship. This will create a minus tear lens. If the average keratometry reading is 43.00 diopters, a reasonable base curve to achieve an alignment fit is approximately 42.50 diopters. The tear lens in this case is 0.50D.
Factor the Tear Lens into the Prescription
The tear lens contributes power to the optical correction afforded by the rigid contact lens. A minus tear lens (base curve flatter than corneal curve) contributes minus power. You must therefore offset this effect by adding the same amount of plus power to the rigid contact lens prescription. In the above example, a 0.50D tear lens would dictate that you order the contact lens power with +0.50D more than the vertexed equivalent sphere of the manifest refraction. You can verify this relationship by over-refracting with a trial rigid lens.
Figure 1. The tear lens affects the power in RGP fitting.
Conversely, if you prescribe a plus tear lens (steep fitting relationship), plus power is contributed to the optical system. There-fore, additional minus power is indicated for the lens power.
Many of us memorized the abbreviations SAM (steep add minus) and FAP (flat add plus) (Table 1).
"Dye"ing to See the Tear Lens
We observe the tear lens when we evaluate fluorescein patterns. If we prescribe a spherical base curve, the toricity we see in the fluorescein pattern is the same amount and orientation as the patient's corneal toricity. If we prescribe a toric back surface rigid lens, the amount of toricity in the fluorescein pattern will equal the difference in toricity between the base curve and cornea. If the base curve toricity equals the corneal toricity, we anticipate a spherical fluorescein pattern.
Remember the exchange of tears beneath a rigid contact lens allows for removal of debris and flow of fresh oxygenated tears.
Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry. E-mail him at firstname.lastname@example.org.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of Optometry