on the lens or the eye
there is an instrument now a machine that reads your eye measurements. these re then programmed into another machine in the lab and that matches what you have form teh first machine.
here is an article that I found that might explain it better....
Issue: August 2005
SOFT LENS FITTING
Techniques for Improved Soft Lens Fitting
Applying predictable, scientific fitting strategies can offer your patients the best possible lens fit.
By Robert L. Davis, OD, FAAO, and P. Douglas Becherer, OD, FAAO
Fitting strategies for soft lenses lack a sound scientific basis compared to GP lenses, which we typically fit based on corneal curvature. Fitting soft lenses based on predictable corneal shape factors may increase your fitting success by allowing for comparisons between different lens polymers, by improving office efficiency and by limiting the trial-and-error aspect of fitting soft lenses.
In fact, developing a unique soft contact lens design for each patient makes more sense than blindly choosing a manufacturer's lens that happens to perform for that patient. The statement, "One lens will not fit every patient," is true because we have no control over the specifications required to optimize comfort and vision for our patients.
With the clinical deployment of aberrometers, which can measure down to one-tenth of a diopter and diagnose issues of poor visual quality, we'll soon be able to offer our patients prescriptions with exact precision.
A Need for Predictable Fitting
How do you fit a soft lens in a predictable manner? What do we know? First, lens diameter and base curve are the most important parameters controlling the lens design. Second, the most important corneal shape measurement is the corneal sagittal height because it defines the most appropriate contact lens-to-cornea fitting relationship. Central corneal radius, eccentricity, corneal diameter, peripheral corneal slope and radius of the para-limbal sclera all define the sagittal height.
In the past we measured corneal curvature with a keratometer and then selected a base curve without regard to the corneal diameter. The emphasis on keratometry was taken from fitting strategies for GP lenses from the 1950s. Applying this strategy to soft lens fitting is time consuming for practitioners, often frustrating for patients and labor intensive for manufacturers.
The keratometer is limited in that it derives corneal curvature from measurements of four points from within the central 3mm of the cornea. Making contact lens choices based on measurements of such a small, central corneal area may be error prone. Given that the average contact lens measures 14.0mm in diameter, keratometer limitations are obvious.
In spite of the apparent over simplification of using keratometric measurements to define corneal shape, the majority of patients achieve a successful contact lens fit. However, many patients just don't fit within the present fitting paradigm. Industry sources indicate that fitters experience only a 75 percent success rate, and even if the lens "fits," the fitter and patient often compromise for the "best fit available." How do we arrive at a scientific fitting design for these patients?
Factors that Improve Soft Lens Fitting
Base Curve Many lens fabricators suggest starting with a base curve radius (BCR) that is 4.00D to 5.00D flatter than flat K. Another method is to trial-and-error fit by selecting the middle BCR available. Most soft lens designs feature three BCRs: Steep, median and flat. In this method, you would first try the median BCR. If the lens is too flat, then you'd change to the steeper BCR and if the lens is too steep, then you'd try the flatter BCR. Still another method is to use the following as a guide for selecting base curves:
If Low K is >45.00D, then fit the steeper BCR.
If Low K ranges from 41.00D to 45.00D, then fit the median BCR.
If Low K is <41.00D, then fit the flatter BCR.
Yet another method of selecting the BCR is to order the contact lens approximately 0.8mm to 1.0mm flatter than the flat K to ensure proper movement. The contact lens should cover 1.5mm of the conjunctiva in all directions, and it should move 0.5mm to 1.0mm with blinking in primary gaze. Visual acuity should equal or exceed that with the spectacle Rx.
Material Lens polymers have a profound effect on fitting characteristics. The prescribing technique will change greatly in relation to the polymer used. A 38 percent HEMA lens responds differently compared to a 59 percent GMA lens. Lens hydration controls how fast a lens stabilizes and can affect chair time. Lens dehydration can change fitting strategies because a lens applied to the eye in the morning will act differently eight hours later. Lens thickness, water content and material all affect base curve determination. Lens dehydration, lens glide, lens stiffness (durometer), tensile strength and fitting parameters take on different values depending on lens material.
TABLE 1 - Corneal Diameter Compensation
CORNEAL ADD TO
DIAMETER FLAT K
Horizontal Visible Iris Diameter (HVID) Because the typical HVID is approximately 11mm to 12mm, most manufacturers offer standard 13.8mm to 14.5mm soft lens diameters and rely on overlapping of the soft lens at the limbus or the wrapping phenomenon. However, eyes that have a small HVID may need a smaller diameter or flatter base curve, and eyes that have a large HVID may require a larger diameter or steeper base curve. Soft lens wrapping is forgiving and may mask the disparity between HVID and standard base curves, which illustrates the inexactness of standard soft lens fitting methods.
One study (Caroline, André, 2002) measured 200 right-eye corneas and found an average HVID (a surrogate marker for corneal diameter) of 11.8mm with a range of 10.2mm to 13.0mm. Fifty percent of the HVIDs fell between 11.6mm and 12.0mm. Twenty-five percent were smaller than 11.6mm and 25 percent were greater than 12.0mm. They found that patients who had HVIDs between 11.6mm and 12.0mm had a high percentage of success using a traditional fitting approach. Patients who had corneal diameters outside of this normal range needed a corrective factor to maintain a high degree of contact lens fitting success.
Analysis of their data demonstrates the need to adjust the keratometric value based on the sagittal height influence on corneal diameter. The average HVID is 11.8mm. For every 0.2mm larger than 12.0mm, you should add 1.00D to the flat K value, resulting in a steeper lens. For every 0.2mm smaller than 11.6mm, you should subtract 1.00D from the flat K value, resulting in a flatter lens (Table 1). The HVID or corneal diameter is an important measure in matching the eye's sagittal height with the appropriate lens design. A suitable location for measuring the HVID for with-the-rule (WTR) and against-the-rule (ATR) astigmats is at axis 45 or axis 135 for the average iris diameter if the corneal diameter isn't an exact circle.
For example, say you have a cornea with an HVID of 12.2mm and K value of 43.00D X 44.00D. According to Table 1, you'd need to add 1.00D to the 43.00D flat base curve. The corrective base curve equals 44.00D or 7.65mm.
To summarize, hydrophilic lens base curves depend on the cornea's sagittal height and central keratometry values. The HVID may help determine the adjusted base curve.
For this example, once you adjust for HVID, you must make an additional correction to determine the final hydrophilic base curve: Because the measurement is based on an ellipse and not a straight line, add 0.3mm to the adjusted base curve radius of curvature to determine the BCR of the contact lens. We ordered a soft lens base curve for this patient of 7.95mm. We based these calculations on a GMA (hioxifilcon A 59 percent) material that experiences little lens shrinkage throughout the wearing time. Depending on the curvature in diopters, the lens will fit 1.00D to 2.50D flatter than the adjusted base curve. The flatter the central K reading, the smaller the adjustment in diopters. The steeper the central K reading, the larger the adjustment in diopters.
Several methods are available to determine the HVID or corneal diameter measurement. Some topographers have incorporated corneal diameter measurement into their software. Biomicroscopes
integrate a reticule in the eyepiece for a direct measurement. You can place surgical calipers in front of the corneal diameter or horizontal iris diameter to obtain the measurement. Pupilometers also have a software modification to include corneal diameter measurements.
Lens Diameter Soft lens overall diameter must exhibit proper corneal coverage and adequate movement so as not to compromise the limbus. A 1.5mm scleral overlap on each side is sufficient to provide adequate coverage.
Selection of an appropriate lens diameter ensures corneal coverage, good centration, lack of limbal compression, lack of edge standoff and sufficient lens movement (and tear circulation), which helps to ensure that the contact lens provides stable vision. In the example above, we ordered a lens diameter equal to the 12.2mm HVID plus 3.0mm (1.5mm on each side of the cornea) for a final diameter of 15.2mm.
A method to recall these soft lens fitting guidelines is the "Rules of 3":
Adjusted base curve in mm + 0.3mm = soft lens base curve
Iris diameter (HVID) in mm + 3.0mm = lens diameter
A simple nomogram is available for corneal diameters between 11.6mm and 12.0mm (Figure 1). Measuring the flat K will determine the lens diameter and optimal base curve with 59 percent hioxifilcon A and 49 percent hioxifilcon B materials. This fitting nomogram can predict the lens parameters without going through calculations to determine the lens specifications. It helps customize soft contact lens fitting.
Figure 1. This prescribing nomogram can help you choose lens parameters for hioxifilcon A and hioxifilcon B materials.
Determining Lens Power and Thickness
You can determine lens power by taking a patient's spherical equivalent from his spectacle Rx and correcting for vertex distance. You can look this up on a table such as the one that appears in the contact lenses and Solutions Summary, or you can use a computer-based program for "vertexing" the sphere and cylinder to the corneal plane. Traditionally, practitioners would convert the sphere and cylinder to cross-cylinder form and then convert this to the corneal plane using a 'Vertex Chart' before reconverting to a minus-cylinder prescription.
The optimal lens thickness results in the least mass. Ideal lens design provides full corneal coverage while taking into consideration individual patient factors such as comfort, overall corneal oxygen requirements and patient dexterity. If lens manipulation or durability is a problem, use a thicker lens.
Fitting the Lens
Now that you've designed the best lens for the patient, it's time to actually fit it. Apply the lens and allow 10 minutes to 15 minutes (for lens dehydration) to assess proper fitting relationships. Some lenses dehydrate more than others. Lenses that retain water content don't bind and dry at the end of the day. You can evaluate these lenses soon after you apply them, saving time for both you and the patient. Most lenses can lose up to 11 percent of their water content over the course of the day, which causes excessive binding after eight hours of wear.
The optimal lens has the flattest base curve that centers consistently and doesn't move excessively as a result of blinking or exaggerated eye movements. Remember, lenses may tighten over time, so if in doubt go with the flatter lens. A simple push-up test will verify lens movement and tightness.
The quality of the keratometer mires or retinoscopic reflex can help you ascertain the proper lens fitting relationships. Determine visual acuity and over-refraction with the lens on the eye after total lens adaptation. Over-keratometry or retinoscopic reflex will help establish the optimal lens fit. A steep fit results in clear but wobbly mires immediately after the blink that then become distorted and blurry. A flat fit results in mire distortion that becomes more distorted on the blink. Visual acuity will sharpen momentarily after the blink, but then blur. Additionally, lenses that dehydrate more than expected will have a dull reflex and are associated with decreased visual quality, especially towards the late afternoon and evening.
Evaluating Lens Comfort
Patient comfort depends on lens edge awareness upon the lower lid. A simple clinical test to check this is to pull the lower lid down away from the eye and question the patient about the difference in lens edge awareness. Always check the peripheral lens design for material gapping or gathering as a reason for discomfort. Move the lens away from the lower lid edge by increasing overall diameter, which tucks the lower edge of the lens into the lid sulcus, or fit the lens closer to the corneal surface so the lid can roll over the lens to improve the comfort.
Knowledge about lens polymer interactions can solve many patient-related problems that result from the wearing environment. An example is a computer user who can't wait to get home after work to take out his lenses, although he seems to wear them comfortably on the weekends. Patients blink less frequently during computer use and the air quality in offices may be lower than that in the home, and so lenses tend to dry out more in such environments.
Increased chair time and lens remakes often frustrate both patients and practitioners. However, utilizing a scientific approach to soft lens fitting that has a high percentage of success will increase your fitting success with these designs. Identifying problem areas during the fitting process optimizes a systematic fitting approach and may eliminate soft lens fitting problems.
Although corneal topography may provide some useful information, topography doesn't replace a practitioner's assessment of proper lens fit. While the instruments provide considerable information about corneal surface characteristics, they aren't consistent predictors of the optimal fit.
Most soft lens wearers aren't difficult to fit, although with the development of new silicone hydrogel materials, fitting problems have arisen that may take up more chair time. Practitioners are fitting increasing numbers of difficult-to-fit patients without knowledge of lens parameter options to obtain a proper fit. Lens movement, centration and stability of hydrophilic lenses depend on limbal and scleral shape as well as peripheral corneal shape. We anticipate that the next few years will see a dramatic increase in practitioner interest in ocular (corneal, limbal and scleral) topography rather than just corneal topography. We believe this new interest will result from documented problems of how the new materials change corneal physiology.
Nonetheless, if practitioners will keep in mind the importance of better lens fitting relationships, then improved contact lens visual acuity will result. The knowledge of peripheral corneal and scleral topography will dictate the proper lens design. This understanding will reduce the likelihood of abrasion and minimize the risk for acute red eye syndrome while increasing visual clarity and lens comfort, resulting in fewer patient dropouts.
Last edited by LENS101; 06-03-2008 at 12:58 AM..