Article Date: 4/1/2001

Instrumental Strategies
OPD - Let's Get Clinical
Gather information in a snap with this multi-use instrument.
By Philip M. Buscemi, O.D.

I recently attended the Second annual International Congress of Wavefront Sensing and Aberration-Free Refractive Correction. With that name, you'd expect to hear topics such as, "Deconvolution of 6th Order Zernike Polynomials -- Is it Enough?" and all kinds of stuff about whether it's a Hartmann-Shack or Shack-Hartmann Sensor and whether or not they're inferior to the Tracey or Tscherning aberrometers.

A shining star

Nidek's OPD (optical path difference) Scan really stood out at this meeting. Instead of lasers, it uses spatial dynamic skiascopy to determine the eye's optical aberrations. You might not realize this, but we've used skiascopy on autorefractors for years. This is just skiascopy on hormones. Rather than only giving you one prescription, the machine plots more than 1,400 power points on the cornea out to about 6 mm.

Each point has an easy-to-understand dioptric power value associated with it. So basically, you get a true power map of the cornea in addition to the prescription. It also measures corneal topography in less than 1 second during the same examination.

Humphrey is also coming out with a unit that does aberrometry and topography as well. Hopefully, they'll display it at the American Society of Cataract Refractive Surgeons (ASCRS) meeting, so if you're there, check it out for yourself.

The OPD-Scan from Nidek.

Wavefront data and aberrations

The OPD-Scan uses its power data to calculate wavefront data. Wavefront data are used to calculate ocular aberrations. Those good old Zernike Deconvolutions, that I mentioned earlier, are mathematical ways to describe the aberrations.

Theoretically, if you correct for the aberrations using an excimer laser, you can give the patient better vision than he or she has with glasses.

A future offering

The OPD-Scan will connect to Nidek's EC-5000 excimer laser using the company's Final Fit software (which isn't for sale yet and will need regulatory approval). Final Fit is nice because it breaks down the ablation into three clinically understandable components: spherical, cylindrical and irregular.

Technically speaking

Now, some of the OPD's competitors say that you don't need to plug corneal topography data into your wavefront data to get an accurate ablation. They cite the Munnerlyn equation used to aid in this calculation, but they use a simplified form of this equation. Actually, if you examine the full Munnerlyn equation, you'll find you do need corneal curvature to properly calculate ablation, especially in higher powers of an irregular periphery where there's more induced aberration. If you don't use the full equation on an 8.00D myope who has an irregular or very steep cornea, the peripheral part of the ablation can be off by more than 1.00D.

Bottom line: You need topographic data, but you also need to register it accurately with your wavefront aberration data. Because the OPD does these two exams on the same machine, Nidek believes that it accomplishes this.

Wave of the future

There's really a lot more to learn about wavefront science in vision care. It's going to go way beyond refractive surgery.

I've already been asked to consult on customized designs for contact lenses that correct aberrations. Yes, we're talking about customized disposables. So don't yawn too much when you hear about Zernike's. And don't break out your old calculus book either. We'll translate all of this mathematical hooey into easy-to-use clinical terms and procedures. In the meantime, check out the OPD-Scan because, pardon the pun -- it's definitely the wave of the future.

Dr. Buscemi has a financial interest in Fortune Technologies, now a wholly owned subsidiary of Nidek, Co, Ltd. Dr. Buscemi submitted this column shortly before accepting the position of Vice President and General Manager of Nidek Technologies America. He'll soon write a farewell column.

Optometric Management, Issue: April 2001