contact lens primer
Contact Lens Management Of the Irregular Cornea
BY TIMOTHY B. EDRINGTON, OD, MS,
FAAO, & JOSEPH T. BARR, OD, MS, FAAO
An irregular corneal surface may result from corneal disease, corneal surgery, contact lens wear or trauma. If corneal distortion reduces manifest refraction visual acuity to an unsatisfactory level, rigid contact lenses may optimize vision. The tear lens that forms between the cornea and the lens base curve optically minimizes corneal irregularity, and the front surface of the lens provides a smooth anterior refracting surface.
Corneal Disease and Surgery
Rigid contact lenses can enhance the vision of most patients with corneal thinning conditions like keratoconus and pellucid margin-al degeneration. Be sure to minimize harsh apical bearing, avoid areas of excessive clearance beneath the optic zone and maximize tear exchange through adequate peripheral clearance.
Figure 1. Reverse geometry RGP fitted on a post-PK cornea.
Surgeries such as penetrating keratoplasty (PK), radial keratotomy (RK), photorefractive keratectomy (PRK) and LASIK may cause corneal distortion. Post-surgical corneal contour dictates the most appropriate lens design. For post-PK patients with raised or steep grafts, prescribe spherical or aspheric rigid lenses with large overall and optic zone diameters. For sunken or flat grafts, a reverse geometry lens design helps avoid excessive central clearance and optimize lens centration. Reverse geometry designs also benefit most post-RK patients. If corneal distortion results subsequent to PRK or LASIK, try spherical or aspheric rigid lens designs. Corneal topography is extremely helpful in fitting post-surgical corneas.
Contact Lens Wear
Long-term RGP wear may cause corneal distortion or warpage. Corneal warpage may occur due to inadequate oxygen or localized harsh lens bearing. Lid-attached RGPs as well as soft toric lenses with thick profiles may cause inferior corneal steepening with keratoconus-like topography. Temporary disposable sphere and toric soft contact lens use may allow for corneal rehabilitation. Refitting the patient into an alignment RGP fit (no areas of harsh lens bearing) is often the best strategy to allow the patient's cornea to "heal," while providing excellent vision.
Scarring from corneal trauma can lead to areas of localized corneal steepening. Prescribe rigid lenses for post-corneal trauma to minimize areas of localized, harsh lens bearing and/or excessive clearance and to enhance tear exchange.
Generally manage irregular astigmatism with spherical or aspheric base curve RGPs, even when K readings indicate a large amount of corneal toricity. Consider reverse geometry, bitoric, soft toric, hybrid and piggyback designs for individual cases. Although keratometry or videokeratography can help select the initial diagnostic lens, evaluate the fluorescein pattern and lens position and movement to obtain the best lens parameters.
Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry. E-mail him at tedrington @scco.edu. Dr. Barr is editor of Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of Optometry.