CASE OVERVIEW:
A 49-year-old woman, who had radial keratotomy (RK) 15 years ago, was referred for refractive surgery. Her refractive error was:
- Right eye: +5.25 -3.25 × 60 (corrected to 20/25)
- Left eye: +3.00 -1.75 × 70 (corrected to 20/30)
She had a 16-cut RK in the right eye with a small optical zone and an eight-cut RK in the left eye. Her high hyperopia resulted from excessive corneal flattening— a recognized long-term RK complication.
After discussing the risks and benefits, including unpredictable IOL measurements and the potential need for a secondary procedure, she proceeded with refractive lens exchange (RLE).
SURGICAL PROCEDURE:
An aspheric toric intraocular lens (IOL) was implanted in both eyes. However, despite advanced IOL measurements, the postoperative refractive error was significantly off:
- Right eye: +7.25 -4.00 × 90 (corrected to 20/25)
- Left eye: +3.75 -1.75 × 85 (corrected to 20/25)
Given this refractive surprise, options were discussed, including an IOL exchange or a piggyback lens implant. The decision was made to proceed with a piggyback IOL, custom-ordered from Rayner in Europe.
SURGICAL APPROACH:
- Right eye: A scleral tunnel was created due to the presence of 16 RK incisions and inadequate corneal incision space.
- Left eye: A corneal incision was used instead.
- No intraoperative complications occurred.
Post-Operative Outcome
At one month post-op, the patient achieved 20/25 uncorrected vision in both eyes and was delighted with the outcome.
Understanding The Refractive Surprise
RK corneas differ significantly from normal prolate corneas:
- Flatter central curvature
- Steeper peripheral curvature
Standard keratometry (K) readings at 3.0 mm are used in IOL formulas, but in RK patients, these can be misleading.
Case-Specific Measurements:
- Lenstar keratometry (K): 32.80 × 33.68 D
- Pentacam flattest K: 28.2 D
Calculation of IOL Power Error:
- Average Lenstar K: (32.80 + 33.68) / 2 = 33.24 D
- Difference from flattest Pentacam K: 33.24 – 28.2 = 5.04 D
- Adjustment factor: 5.04 × 1.25 = 6.3 D additional IOL power needed
Corrected IOL power should have been 29.5 D instead of 23.0 D.
KEY LEARNING POINTS:
1. RK PATIENTS POSE UNIQUE IOL CALCULATION CHALLENGES
- RK-induced central corneal flattening, in a zone less than 3.0 mm, makes IOL power estimation difficult.
- Patients must be counseled about the risk of a refractive surprise and possible secondary procedures.
2. CORNEAL TOPOGRAPHY IS ESSENTIAL
- Standard biometers may miss the flattest K in RK eyes.
- Preoperative corneal topography should be analyzed to identify the flattest K.
- A modified IOL power formula (difference between mean K and flattest K × 1.25) improves accuracy.
3. REFRACTIVE SURPRISE MANAGEMENT OPTIONS
- IOL Exchange:
- Requires cutting and removing the IOL through a small incision.
- More challenging with 16 RK incisions and the need for a scleral tunnel.
- Riskier if the capsule is compromised by a prior YAG capsulotomy.
- Piggyback Lens (Preferred in This Case):
- Custom-ordered from Rayner or Staar Surgical.
- Inserted into the ciliary sulcus in front of the primary IOL.
- Can correct hyperopia, myopia, and astigmatism.
- Better refractive predictability, safer surgical technique, and less trauma than an IOL exchange.
- PRK (Preferred Laser Option):
- LASIK is contraindicated in RK eyes.
- PRK has slower healing compared to IOL-based solutions.
4. IOL SELECTION CONSIDERATIONS IN RK PATIENTS
- Multifocal IOLs are generally not recommended due to RK-induced higher-order aberrations.
- Aspheric or extended depth-of-focus (EDOF) IOLs are better choices.
- Light Adjustable Lenses (LALs) are not ideal due to delayed visual recovery and post-op fluctuations.
5. AIM FOR SLIGHT MYOPIA
- Continued corneal flattening postoperatively from the RK incisions is likely.
- Targeting a mildly myopic outcome is preferred to avoid hyperopia.
FINAL THOUGHTS:
RK patients can achieve successful outcomes with refractive lens exchange if careful corneal topography analysis is performed. Identifying the flattest K can improve IOL calculations and reduce the risk of refractive surprises. When unexpected refractive outcomes occur, piggyback IOLs, IOL exchanges, or PRK are viable solutions for optimizing vision.
