How most refractive surgeons do it
Those who do not use Alpins astigmatism-analysis methodology do not record -- and have no way of knowing -- any axis shifts that may have occurred. If a patient has a preoperative reading of 4 D of astigmatism at axis 45 degrees and the postoperative reading is 1 D of astigmatism at axis 45 degrees, they have no way to differentiate the success of that surgery from a procedure that reduced astigmatism to 1 D but left it at an axis of 135 degrees. They have no way of knowing if their laser or their technique was consistently shifting the axis clockwise or counterclockwise. They have no way of knowing if their laser was consistently over- or undercorrecting, especially if a shift of axis was occurring. They have no way of knowing exactly why all their patients didn't get 20/20-or-better vision. And they have no way of refining their future surgical procedures to correct for all but the grossest surgical errors.
And they have no way to determine what they are doing to the corneal topography when they treat solely for refractive astigmatism. And they have no well-defined, rational methodology for "splitting the difference" between topographic and refractive astigmatism on those occasions when their intuition tells them to do so.
Most refractive surgeons, in fact, do not use a state-of-the-art outcomes-analysis database to analyze their procedures and refine their techniques. Most refractive surgeons do not publish their results, so feel no need to follow patients so closely. Most refractive surgeons lose a lot of patients to follow-up. Most refractive surgeons are quick to point out that they "get good results" and are content to leave it at that -- and hope their patients are content to leave it at that.
Patients would be wise to ask themselves: Am I content to leave it at that?
Why the Alpins method?
The Alpins astigmatism-analysis method builds on earlier vector-analysis approaches, but introduces important new concepts. Alpins describes parameters such as the TIA (target induced astigmatism), which is the treatment that the surgeon is aiming to accomplish; the SIA (surgically induced astigmatism), which is what the surgery actually accomplished; and the DV (difference vector), which is how far the surgeon missed.
Alpins employs a golf analogy in describing SIA, TIA and DV. For a simple graphical explanation, click here.
Alpins' "Correction Index" is determined by the ratio of the SIA to the TIA (what the surgery actually produced versus what the surgery was meant to produce), and is optimally 1 (it is greater than 1 if an overcorrection has occurred and less than 1 if there has been an undercorrection). It is calculated by dividing SIA (actual effect) by TIA (target effect). The Coefficient of Adjustment is the inverse of the Correction Index and quantifies the modification needed to the initial surgery plan to have achieved a Correction Index of 1, the ideal correction. If the surgeon achieved an overcorrection, for example, the Coefficient of Adjustment might be .90, indicating that the surgeon should have selected a correction 90% of what he actually selected. The Coefficient of Adjustment can be used to refine future procedures.
The Index of Success is also an important new concept. It is calculated by dividing the DV (how far you missed the target) by the TIA (the original target). While the DV is an absolute measure, the Index of Success is a relative measure that can be useful in comparing different procedures and even different surgeons. Ask your surgeon what his or her Index of Success is, based on the Alpins method. If they don't know and would like to find out, have them come to this Web site or contact me by e-mail (assort@assort.com).
Refraction versus topography
The Alpins methodology provides a link between topography and refraction. In effect, it calculates a vector (representing laser settings) between treating 100% refraction or 100% topography or any surgical emphasis in between (for example, 80% refraction and 20% topography), leaving the absolute minimal astigmatism on the eye. Because you can't get rid of both refractive and topographic astigmatism, Alpins calls the unavoidable remaining astigmatism "ocular residual astigmatism," or ORA. More than 90% of people with astigmatism demonstrate differences in corneal and refractive axes as well as magnitude.
For a simple graphical representation of the ORA, click here.
In a classic Duke-Elder text, ORA is described as the refractive astigmatism measured when a spherical contact lens is placed on the eye (in other words, the cornea is spherical, but astigmatism can be measured by refraction). Some ophthalmologists describe the ORA as "lenticular astigmatism" -- that is, related to the natural crystalline lens of the eye. Alpins believes that astigmatism can be introduced at any of the optical interfaces of the eye -- the front of the cornea, the back of the cornea, the front of the lens, the lens, the back of the lens, the vitreous, and the plane of the retina -- and in terms of the patient's subjective appreciation of astigmatism, may even have a perceptual component related to the visual cortex of the brain. Astigmatism is also measurable after implantation of a spherical intraocular lens at cataract surgery. Consequently, the term "ocular residual astigmatism" seems more appropriate than "lenticular astigmatism."
The Alpins method quantifies the intuitive approach many experienced refractive surgeons already take when refractive astigmatism does not coincide, either in magnitude or axis, with topographic astigmatism. What Alpins calls the "optimal treatment" incorporates a rational, well-defined built-in bias toward leaving corneal astigmatism in a favored orientation -- toward 90 degrees, or with-the-rule -- which probably goes beyond the intuition of even the most experienced refractive surgeon. Studies suggest that Alpins' "optimal treatment" produces better refractive results, despite the fact that in some instances the "optimal treatment" does not aim to treat 100% of the refractive astigmatism. This may be due to spherical aberration introduced by corneal astigmatism (described recently by Seiler), as well as natural physiologic advantages of with-the-rule as opposed to against-the-rule astigmatism.
Alpins has shown that in some patients, treating 100% on refractive astigmatism actually increases corneal topographic astigmatism. That can't be a good thing.
For a simple graphical representation of Alpins' optimal treatment, click here.
The Alpins method allows surgical planning calculations that are a natural for inclusion in refractive lasers or corneal topography machines. The way it could work: The surgeon (or an assistant) enters preoperative measurements of sphere, cylinder (astigmatism) and axis based on refraction and topography. The target and any "adjustment algorithm" are entered. The calculations show the vector (laser settings) necessary to reach the calculated astigmatic target, and the effect any selected target will have on topography values. Should he desire, the surgeon can choose Alpins' "optimal treatment" or his own approach for any particular patient and be assured of maximal astigmatic treatment.
The visibility of the Alpins astigmatism-analysis method will continue to grow in the months and years ahead. An important paper by Alpins, addressing his approach to irregular astigmatism, was recently published in a formal journal (to review an abstract of the paper, click here). He is presenting a number of papers at major ophthalmic meetings this year. In addition, I think the field is ripe for the Alpins astigmatism-analysis system, and that the need for such a system is reaching critical mass.
What program does your surgeon use?
Remember, 20/40-or-better is better than nothing, but the Index of Success and the other Alpins indices described above truly contribute to a surgeon's understanding of refractive surgical procedures and the refinement of results. And to your well-being.
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