This patient was very myopic and desired surgical correction many years before the advent of the excimer laser. As such, she had 20 cuts of radial keratotomy (RK) in the cornea in order to induce enough flattening to result in an emmetropic outcome. This worked reasonably for many years, but over time she slowly became hyperopic and then developed a cataract. Doing cataract surgery in this eye will not be routine: there are challenges in both the lens power estimation as well as in the intra-operative surgical technique.
Radial Keratotomy (RK) is an antiquated technique for the most part and it is no longer routinely performed. I first started doing ocular surgery more than 20 years ago and I have never performed an RK procedure, so it has been decades since it was routinely done in the United States. Since most RK patients had the surgery done to their eyes about 30 years ago, these same patients are now entering the age where cataracts naturally develop. In their careers, all ophthalmologists are expected to encounter multiple RK patients who now need cataract surgery.
IOL Power Calculations
IOL calculations must be adjusted for the highly unusual cornea. The axial length of the eye is straightforward to measure but the keratometry is not. Look at the above photo to see that the central optical zone created by the 20 RK cuts is relatively small, about 2 mm in diameter. Most of our keratometers and biometric devices will measure the corneal at a ring of about 3.5 mm and therefore, they will over-estimate the K values.
Many ophthalmologists have come up with methods to adjust the lens power calculations in post-RK eyes in order to improve accuracy and some of these are included at the ASCRS post-RK calculator. In doing your calculations, it helps to look at multiple different corneal power measurements such as auto-keratometry, readings from biometers such as the Haag-Streit Lenstar or Zeiss IOL Master, corneal topography, dual-scheimpflug corneal tomography, and more. Choose the lowest central K value from these devices and then for good measure either aim for mild post-operative myopia or add at least some dioptric power to the IOL (see method below):
Approximate IOL power adjustment to account for RK effect on keratometry
Step 1: calculate IOL power at www.IOLCalc.com based on lowest central K values measured and aim for -0.25:
Step 2: make the following adjustment to IOL power:
- 4-cut RK Add +0.5 D to IOL power
- 8-cut RK (>3mm OZ) Add +1.0 D to IOL power
- 8-cut RK (<3mm OZ) Add +1.5 D to IOL power
- 12, 16, 20, 24, or 32-cut RK Add at least +2.0 to IOL power
Here is a table of the number of incisions and optical zones that were historically performed in Radial Keratotomy.
Use this as a reference to estimate the patient’s pre-RK level of myopia.
The primary issues intra-operatively are avoiding the RK with your cataract surgery incisions and ensuring that these RK cuts do not leak during the procedure. For patients with 4, 6, or 8-cut RK, the phaco incisions can be placed at the limbus-cornea edge with care taken not to intersect the RK cuts. For those with 12, 16, or 20-cut RK, the best option is to avoid the cornea and make a scleral tunnel incision instead. This scleral tunnel can be made superiorly or temporally and it should be sutured for maximum stability.
To be gentle on the RK-weakened cornea, I prefer lower flow and a lower bottle height with a smaller phaco needle to ensure that the fluid inflow still stays greater than the fluid outflow. If the RK incisions open during surgery, be aware that there could be sudden instability and shallowing of the anterior segment, and the chance for capsule rupture is increased. At the end of these surgeries, I like to paint the entire cornea with fluorescein dye to check for any leaks, which can easily be sutured while the patient is still in the operating room.
Phaco incision carefully placed between two RK cuts without intersecting them.
The RK incisions swell during even the gentlest cataract surgery, and this swelling can induce central corneal flattening, which results in excessive hyperopia immediately postop. These RK patients will experience fluctuations in their refractive state for many weeks after their cataract surgery, so a mild amount of initial hyperopia should not be a cause of concern. After waiting at least 6 weeks, if the patient is still significantly hyperopic, a second procedure can be performed.
Perhaps the most important issues in RK patients with cataracts are explaining to them that their IOL calculations are, at best, estimations and that their surgery and post-op recovery will likely be more challenging for both the surgeon and the patient.
Here is a video showing the creation of a scleral tunnel to perform cataract surgery in a patient with prior 20-cut RK:
Want even more challenges?
here is a patient of mine with prior 32-cut RK:
and here is a patient of mine with prior RK and prior LASIK:
©2018 Uday Devgan MD. This copyright includes all text, figures, photos, videos, and other content.