The LASIK Workup at LA Sight

Automated Refraction, Topography and Wavefront Analysis: Using infrared sensors and auto-focus technology, autorefractors are able to measure the optical state of the eye, (nearsightedness, farsightedness, astigmatism) called the refraction. While very accurate, the readings are not always perfect, so the process of 'subjective refraction' (having the examiner ask “Which is better, one or two?”) is still performed to fine-tune the measurements. In the near future, wavefront analysis may replace even this time-honored method of measuring the prescriptive needs for optimum vision.

At LA Sight we use the Marco 3D-Wave system for initial testing. This instrument combines capability of autorefraction, corneal topography measurement and wavefront analysis.

Orbscan Corneal Topography and Thickness Mapping: The cornea accounts for about 65% of the eyes’ focusing power. In the normal case, corneal curvature is smooth, symmetric, and regular in contour. However, certain conditions are associated with irregularity, distortion, or significant asymmetry of corneal curvature. Examples include keratoconus, a disorder characterized by abnormal thinning of the cornea, with distortion and irregular, abnormally steep curvature over the area of thinning. Current laser systems are not able to correct this particular situation, so topography helps identify those who would not be good candidates for laser sculpting surgery. Orbscan analysis may also help guide recommendations about which procedure would be the most appropriate for you (sometimes we advise PRK instead of LASIK). In certain cases of high-magnitude astigmatism, especially involving topographically asymmetric astigmatism, laser vision correction is contraindicated and should not be performed.

The Orbscan is a very elaborate and sophisticated instrument, designed to measure and map corneal topography for both front and back corneal surfaces, and to map corneal thickness across the entire corneal surface. As a diagnostic tool this shows the overall architecture of the cornea including surface power, front and back shape, and thickness variation across the entire surface. Only about 3% of all laser vision correction centers in the US have access to this technology due to its cost.

Using an illuminated ring pattern and a beam of light that scans across the cornea, the image sensors in the Orbscan acquire over 9000 points of data in less than 2 seconds. That data is transformed into a wealth of information about the architecture of the cornea. The Orbscan is unique among corneal topographers in its ability to image the posterior surface of the cornea. Early signs of certain corneal diseases or abnormalities are seen first on the posterior corneal surface.

Wavefront Analysis: We perform wavefront measurement with the Wavelight Allegro analyzer as well as the Marco 3D-Wave. LA Sight is one of the only centers in the US to perform wavefront testing with two separate diagnostic instruments.

While the cornea is one of the important optical elements of the eye, it is not the only one. Wavefront analysis measures the unique optical characteristics including subtle optical aberrations of the whole eye. This information can then be used to guide customized laser treatment to optimize visual performance for each eye. Think of a tailor or dress-maker custom-tailoring a garment to the particular measurements of each client, compared to buying something "off the rack" and you will appreciate the difference between wavefront-guided laser treatment and the methods that predate this capability. You may see or hear ads promoting wavefront treatment featuring different buzz-words -- "Custom-Cornea" with the LADAR laser system, and "Custom-Vue" with the Visx laser system. These are both marketing terms for wavefront-guided therapy.

You might ask, "If clothes off the rack fit well, why pay extra for custom tailoring?" The same analogy applies very aptly to laser treatment. Not everyone needs customized, wavefront-guided treatment. At LA Sight, we recognize this and employ wavefront mapping to guide our recommendations, but we do not ask all patients to pay a premium for custom treatment if it is not likely to yield significantly better results in their particular case. For a more detailed description of wavefront imaging, technology and capability, see Wavefront Imaging Explained.

Prescription Analysis: Modern instruments called lens analyzers can measure the existing prescription in your current eyeglasses. It is helpful to know this information, even if your glasses are several years old. By comparing the prescription in the existing glasses to the current one measured at the time of consultation, we know if there has been any change in the prescription over time; and we also know by how much the prescription has changed.

Pupillometry: Measurement of pupil size in darkness (by a modified night-vision scope) is important for several reasons. First, it is preferable if the intended diameter of the laser sculpting treatment (called the “optical zone”) is at least as large as the pupil diameter. If not, then some light entering the eye will come from outside the optical zone, and contribute to glare, halo, blur, and image degradation. Second, when correcting nearsightedness, the depth of the treatment is proportional to the diameter of the optical zone. In order to calculate how deep the treatment will extend, we must also know how wide a diameter needs to be treated. We use both the (digital, automated) Neuroptics pupillometer and a more conventional analog instrument called the Colvard pupillometer.

Subjective Refraction: The process of measuring and fine-tuning the prescription measurement. Typically a person sits behind an instrument containing many types of lenses (a phoropter) and the examiner carefully adjusts the combination of lenses to optimize vision clarity. This is a subjective, interactive process where feedback is essential (but often boring), which is why the examiner will ask, “Which is better, one or two?”

Slit-Lamp Microscopy : The eyelids and eyes are examined in high magnification using a special microscope with a slit-shaped beam. This affords a close-up view of the eyelids, conjunctiva (inside lining of the lids and membrane covering the sclera), cornea, sclera, iris, anterior chamber, pupil, lens, and anterior vitreous. Eye pressure can also be measured with a tonometer attached to the slit-lamp microscope.

Tonometry: Measurement of eye pressure is performed as a screening test to aid in the detection of glaucoma. Elevation of eye pressure is one of the major risk factors in this condition. Dr. Wallace is a recognized authority on the measurement of eye pressure, being one of the inventors and developers of the Tono-Pen tonometer, one of the two most popular instruments for measuring eye pressure in the world today.

Pachymetry: Corneal thickness is important to measure, as laser treatment involves delicately sculpting – and therefore slightly thinning - the cornea. It is generally accepted that the cornea can retain its’ structural integrity as long as such sculpting does not reduce thickness below a safe level. Dr. Wallace is recognized professionally as one of the inventors and developers of the Pach-Pen pachymeter, one of the smallest and most accurate instruments of its kind.