contact lens case reports

A Case of Irregular Astigmatism

BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO

One of the most difficult, yet rewarding challenges in clinical practice is the successful management of irregular astigmatism with contact lenses. In recent years our fitting techniques have been enhanced through the recent merging of a number of technologies that include:

1. Advances in large-diameter aspheric lens designs
2. Advances in computer controlled lathing technology, allowing accurate fabrication of more complex lens designs
3. The development of stable, wettable, high-Dk GP materials
4. Advances in computerized corneal mapping techniques, providing greater understanding of corneal shape
5. Advances in computer simulated fluorescein programs to aid in the selection of optimum lens design parameters

These five technologies recently came together in the case of a 52-year-old female who underwent penetrating keratoplasty in both eyes in the early 1980s. Her left eye has been successfully managed with a traditional 10.0mm tri-curve lens design with a visual acuity of 20/20 and all-day lens wearing comfort. However, over a period of two months she had experienced increased discomfort and a decrease in wearing time with the right contact lens.

Slit lamp examination showed that the habitual right lens was decentered and riding temporally. The decentered lens had resulted in some lens-induced corneal flattening and distortion. We asked the patient to discontinue the right contact lens for five days and then to return to the clinic for a more accurate corneal topography and diagnostic fitting.

Technology-Driven Lens Design

Upon the patient's return we performed corneal mapping and found that the cornea had returned to a more "normal" shape. The topographic map was evaluated with the Medmont (Precision Technology Services) lens design software, and a simulated K-Max (MedLens Innovations) lens was designed. The K-Max is a 10.0mm to 12.0mm diameter lens design that incorporates a 7.2mm spherical central posterior optical zone and a wide aspheric zone with a radius 1.00mm flatter than the base curve radius.

The software of the Medmont system allowed us to simulate the K-Max lens design and simply adjust the base curve radius (steeper and steeper) until the lens landed along the flat corneal meridian (Figure 1). In this case, the optimum base curve radius of 6.80mm (49.50D) resulted in 37 microns of apical clearance and midperipheral "landing" at approximately 2 o'clock and 8 o'clock. Figure 2 shows the actual fluorescein pattern of this lens next to the simulated fluorescein pattern. We performed a simple over-refraction and ordered the lens in Paragon HDS 100 (Paragon Vision Sciences) material.

Figure 1. The simulated fluorescein pattern from the Medmont corneal topographer.

Figure 2. Corneal topography, simulated fluorescein pattern, and actual fluorescein pattern.

This case nicely demonstrates how modern corneal mapping techniques and a contemporary lens design and material have merged to aid in fitting a complex corneal condition such as postpenetrating keratoplasty. CLS

Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.