RMS values ?

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RMS values ?

Postby paz » Tue Oct 31, 2006 12:46 am

I've been doing some research (web based) on laser platforms, namely the Allegretto Wavelight system (wavefront optimized) and Visx Star S4 (wavefront guided).

Based on some studies, it appears that wavefront optimized LASIK offers similar results as wavefront guided "provided that RMS levels are <0.3 microns. " Patients with RMS levels > 0.3 microns obtain better resutls with wavefront guided LASIK.

My questions:
1) Are RMS results a measurement of higher order aberations?
2) What instrument is used to measure RMS?
3) Would the same instrument/measurement be used for a wavefront optimized patient consultation vs. a wavefron guided patient consultation?

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Re: RMS values ?

Postby LasikExpert » Tue Oct 31, 2006 2:21 am

RMS means Root Mean Squared. It is a mathematical evaluation of lower and/or higher order aberrations (HOA) measured by a wavefront aberrometer and represented by Zernike polynomials. (We just lost half our readers.)

Lower order aberrations are sphere and cylinder - essentially your eyeglass prescription. The RMS to which you refer is actually the RMS of HOA. RMS is an important measurement, but RMS is not the final word on what laser system is the best for a particular patient.

The different Zernike representations have names such as spherical aberration, coma, secondary astigmatism and form a pyramid with defocus (essentially myopia or hyperopia) at the top. As mathematical representations of optics, Zernike polynomials are infinite. Current laser systems are able to effectively limit induction of, or even reduce, only the first three or four HOA levels of this infinitely descending pyramid. If one of the top aberrations as represented in Zernike is elevated, but all lower levels are well below the norm, it is possible that the RMS may be low yet the potential benefit of wavefront-guided ablation may be very high. The hard and fast rule of HOA RMS >=.30 microns is not really so hard and fast.

As it happens I was discussing the Allegretto with one of its developers today. Their studies have indicated that RMS below the .30 level does not convert into improved vision as perceived by the patient. As an example, if a patient were to have RMS of .20 and was raised to .30, the patient would likely not have symptomatic vision problems. If a patient were .30 and lowered to .20, the patient would not have perceptible vision improvement. The grain of salt with this analysis is my point made above. If all of the reason for that increase from .20 to .30 was due to spherical aberration (one of the higher levels of the Zernike pyramid), then the patient would very likely be symptomatic.

The manufacturer’s studies also indicate that the wavefront-optimized does not discernibly raise the RMS of HOA. Again, consider that if spherical aberration – which simply means that the refractive error at the periphery of the measured optic is different than the refractive error at the center and would commonly present as halos around light sources at night – were to be astronomically raised the patient would be symptomatic, but due to the muting of a single Zernike polynomial by the inclusion of all HOA in RMS, the patient’s RMS may not significantly raise. In other words a patient can have poor vision represented by a wavefront analysis, but not have a significant rise in HOA RMS.

The (very) general rule is that if preoperative HOA RMS is above .30, then the patient will likely benefit from wavefront-guided ablation such as the Visx S4, Alcon LADARVision, or Bausch & Lomb Zyoptix. If the HOA RMS is below .30 then the benefit of wavefront-guided compared to wavefront-optimized is negligible however this general rule can be and often is inaccurate if the patient has a specific elevation in a more important HOA as represented in Zernike.

One rule that almost always is true is that wavefront-optimized and wavefront-guided ablations are less likely to induce HOA than conventional ablation. For a less technical description, visit Wavefront Lasik.

The bottom line is that every patient is unique and no hard and fast rule is going to be accurate in every case. An analysis of all the facts by a competent surgeon is necessary to determine what is actually best, not just what generally appears to be best.

paz wrote:1) Are RMS results a measurement of higher order aberations?

RMS can be of lower order aberrations (LOA), higher order aberrations (HOA), specific LOA or HOA as defined by Zernike, or a combination. The number to which you refer is HOA RMS.

paz wrote:2) What instrument is used to measure RMS?

Wavefront aberrometer with the data converted into mathamatical representations.

paz wrote:3) Would the same instrument/measurement be used for a wavefront optimized patient consultation vs. a wavefron guided patient consultation?

The same and it should be the same from the same manufacturer. The readings from an Alcon aberrometer will likely be different than the readings from a Visx or B&L. Also, individual aberrometers have "personalities" and can have readings slightly different than others of the same manufacturer even when properly calibrated. This is one of the reasons why a doctor's practical experience with the same equipment is important.
Glenn Hagele
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Postby paz » Wed Nov 01, 2006 3:00 am

Glen - thank you very much for your informative response - much appreciated.
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