Gives Way to Precision Corneal Molding
BY DONALD S. TEIG, O.D., F.A.A.O.
Precision corneal molding devotees say that using computerized videokeratography makes this a predictable method for reducing myopia.
Many people predicted that photorefractive keratectomy would put an end to orthokeratology. In a sense, that's what has happened in my practice. It seems that the more publicity PRK receives, the more patients become interested in a nonsurgical approach to vision correction. But I don't recommend orthokeratology. I practice precision corneal molding (PCM).
ADVANCES IN LENS DESIGNS AND CORNEAL MAPPING IMPROVE PREDICTABILITY
The two major drawbacks of ortho-k are its unpredictable nature and the length of time it takes to see results. PCM, however, is very predictable and I have seen results overnight. What makes the difference is the type of lenses I use to perform PCM and the technology used to design those lenses.
Reverse geometry lenses, recently popularized by Nick Stoyan of Contex, have a flat central base curve and a steep intermediate zone. This unusual shape forces the cornea to flatten evenly under the central optic zone. When combined with a high Dk material, the result is a lens suitable for overnight wear which greatly accelerates the PCM process.
Advances in how we measure corneal topography have improved precision. Now that we can map the contour of most of the central cornea, we know more precisely what we're starting with, where we'd like to end up and if that goal is feasible.
PREDICTIONS AND PATIENT EXPECTATIONS
One of the greatest advantages of PCM is that we can calculate the probable results before placing a lens on the eye by examining the eccentricity of the corneal surface. Eccentricity is the rate of change in the corneal contour as it moves out from the apex. A negative e-value indicates a flat central zone with a steep mid-periphery. This patient is a poor candidate for PCM because the cornea's shape is already similar to the contour we would be attempting to make. Similarly, a patient with an e-value of zero, indicating a perfectly spherical cornea, is also a poor candidate. Luckily, the most common finding is a positive e-value, which indicates that the cornea is steep centrally and flattens peripherally. Eyes with this configuration are good candidates for PCM.
In my experience, we can reduce one diopter of myopia for each 0.21 of positive e-value measured prior to fitting. To date, a reduction of four diopters is the maximum I've achieved. I've also found that PCM works well to reduce with-the-rule astigmatism up to 2.50D.
Be sure to counsel patients carefully about goals and expectations. Although I can predict results fairly accurately with PCM, it's not a panacea. Set realistic goals, particularly for patients with higher myopia for whom total reduction of correction is probably not feasible. Many of these patients will, however, opt to try PCM if only to reduce their current prescription.
FITTING FOR PCM
I rely on topography and a computer software program provided by SciOptic, Hawthorne, N.Y., to determine the base curve of the lens. I then select a lens from a diagnostic fitting set (all lenses in this set have plano power). I then overrefract to determine which lens power to order. I use the SciOptic EZM (Eccentricity Zero Molding) lens for my PCM patients. Other companies make similar reverse geometry lens designs.
The ideal fluorescein pattern resembles a doughnut -- a flat, dark central portion surrounded by a bright, clear ring expressing the steep mid-periphery of the lens (Fig. 1). The lens should center well, allowing an average of one to two millimeters of movement on the blink. Because patients wear these lenses overnight, I use a large diameter. I have found a 10.6mm lens with an 8.8mm optic zone is adequate for most patients. I use 11.2mm lenses only for patients with extremely wide palpebral apertures. Due to the large diameter, SciOptic lenses include three fenestrations in a triangular pattern to reduce lens mass and improve movement.
FIG. 1: A WELL-FITTED REVERSE GEOMETRY LENS EXHIBITS A DOUGHNUT- LIKE PATTERN WITH A FLAT CENTRAL ZONE AND SIGNIFICANT POOLING IN THE MID- PERIPHERY.
I order the base curve and diameter in either Equalens2 (Polymer Technology) or Quantum (Gelflex of Australia) material. I prefer a high Dk material to minimize risk of adverse reaction. For most patients, the lens power will be close to plano because the central portion is substantially flat.
I instruct patients in lens handling and wearing schedule (at least eight hours while sleeping), and give them a home eye chart and instructions for recording acuities. Follow-up is usually 10 days after dispensing.
FAST VS. MODERATE RESPONDERS
By careful screening using topographic e-values, I can categorize PCM patients as either fast or moderate responders. Fast responders often achieve 20/20 vision after only one night of lens wear. Moderate responders experience a more gradual reduction in myopia.
For patients who haven't yet achieved plano, but who no longer require their habitual correction, I dispense daily disposable lenses in approximately one-half the pretreatment prescription with the first set of PCM lenses. Although this may over- or undercorrect slightly, it's a convenience patients appreciate. They wear the daily disposables at school or the office and then replace them with the RGPs at home.
I advise patients that there will be some regression for the first few
days of the procedure, for example from
-1.00D in the morning to -2.00D in the evening, but this effect rarely lasts more than three days. During this initial period, the patient is likely to need the daily disposable lenses. I caution all PCM patients to continue overnight wear even after they've achieved 20/20 vision to 'lock' the results into place.
THE 10-DAY FOLLOW-UP
It's rare not to see an ecstatic patient at the 10-day visit. At this visit, we ask the patient about his success, his daily a.m. acuity readings, any occurrences of regression, if he needed the daily disposables and if so, for how many days. I then check acuity and take a new topography complete with color map and e-value calculation. I check fluorescein pattern and perform a new refraction.
If the refraction is near plano, the patient has no daily regression and is no longer using the daily disposables, the procedure is essentially complete. I instruct patients to continue overnight lens wear and to return for follow-up in three months. This scenario is often the case, particularly with low-to-moderate prescriptions (<-2.00D).
If the patient still has refractive error but topography shows a negative or zero e-value, we have gone as far as possible. Again, nightly lens wear will retain the correction, but the patient will need a daytime correction. Many patients are happy with this compromise even if they haven't achieved "emmetropia."
A positive e-value coupled with some residual refractive error indicate the need for a flatter reverse geometry lens. I calculate the lens parameters from the new topography and evaluate a new trial lens. I repeat the procedure and evaluate the result 10 days after the second dispensing visit. Two pairs of lenses are usually required to complete the PCM process for moderate myopia.
I recently performed PCM on a 15-year-old girl who was a 3.50D myope. With the first lens, her e-value dropped from 0.5 to 0.24, and her unaided acuity improved from 20/400 to 20/60. Because the e-value was still positive, I could predict about another diopter reduction in myopia with a flatter reverse geometry lens.
In rare cases, you may need to prescribe a third pair of lenses. Any additional lens changes will likely produce little or no improvement. It's critical to remember that the topography, rather than the number of lenses or the reduction in acuity, dictates when the procedure is complete. Once the e-value approaches zero, very little change will occur no matter how flat the central zone of the lens.
After wearing PCM retainer lenses for three months, patients may reduce the number of nights of wear. I dispense another eye chart and instruct them to reduce retainer wear to three nights per week, checking acuity regularly to detect regression. Some patients continue nightly wear simply because it's easier to remember.
A HEDGE AGAINST MANAGED CARE & PRK
I consider PCM a hedge against both managed care and PRK. I spend a great deal of time with these patients initially, counseling them and explaining the procedure. But, with fees averaging $2,000, my time is well-spent.
By my calculations, one PCM patient per day is the financial equivalent of 12 managed care patients with less work and fewer headaches for me and my staff. PCM patients are primarily adults who are interested in PRK but hesitant about surgery or afraid of lasers, and children too young to be PRK candidates. Adolescent athletes are perfect PCM candidates. The fact that the procedure is reversible appeals to their parents, who are often myopic themselves.
Precision corneal molding is a fast, precise and predictable answer to many forms of myopia with an added advantage -- it's fun. The personal gratification I get from each PCM patient is the contact lens prescriber's equivalent of a home run. CLS
Dr. Teig is the founder of Sports Performance Centers of America and director of the Institute for Sports Vision in Ridgefield, Conn.He is a technical advisor for SciOptic, Inc., manufacturer of the Sharper Image contact lens product line.