If central keratometry does not predict the base curve nor the sagittal height of the potential lens, then what does? And what information does a clinician now use to best select a lens? With the advent of corneal topographers and optical coherence tomography OCT imaging, we can gain a complete characterization of the shape of the anterior ocular structures. With OCT, a number of additional parameters that were not readily available to us can now be measured.
This includes the profile and angle of the corneoscleral junctions, sagittal height at various depths, corneal diameter, corneal eccentricity, and scleral radius. The question that remains is: How does knowing all of this help us to predict the fit of a lens or select the best-fitting lens? Hall et al 17 used OCT imaging to characterize various aspects of the eye and showed that some of the above anatomical parameters play a significant role in predicting clinical lens performance on the eye.
One of the findings was that measuring the horizontal corneal diameter using OCT was much more reliable compared to measuring horizontal visible iris diameter HVID at a slit lamp. The HVID has previously been known to be unreliable because it depends largely on the rate of transparency loss of the tissue, and while it correlates with true corneal diameter, it is almost always an underestimation. Another interesting finding is that the eye is not rotationally symmetrical in the sense that the profiles of the corneoscleral junctions differ from each other in each of the four quadrants.
The differences in the corneoscleral junction angles were vastly greater in the temporal-nasal meridian than in the superior-inferior meridian. The nasal corneoscleral junction angle was sharpest, followed by that in the inferior, temporal, and superior aspect. In addition, the difference in horizontal corneoscleral junction angles is associated with lens tightness on push up in a silicone hydrogel lens, making this a potentially important parameter to predict future lens fits. Figure 2.
Left The nasal corneoscleral junction angle was measured to be Right The temporal corneoscleral junction angle was measured to be This small difference may have a profound effect on contact lens behavior on the eye. This information can be combined with data from slit lamp measurements.
An aspect of lens fitting that is often overlooked is patient demographics. Age and sex have a large effect on various ocular parameters, including HVID, palpebral aperture size, corneoscleral junction profiles, and scleral curvatures. For example, Caucasians and males tend to have a higher sagittal height compared to Asians and females.
There is still much to learn about what factors affect lens fit. Understanding these factors could provide some clues or steps for improving and honing the fitting process and may even have implications for future lens design. Once the lenses have been successfully applied onto the eye, they need to be assessed. Some of the lens fit parameters are directly related to each other, and using a reticle to measure these parameters is valuable in determining the quality of the fit.
It is important to consider the time of lens assessment. The lenses should not be assessed immediately after application because they take time to settle. Lenses tend to show much mobility immediately after application, but this movement slowly decreases over the first half-hour and then increases again throughout the day. Therefore, the five-minute mark is a good time for soft hydrogel lens assessment.
Post-blink movement and tightness at 10 to 20 minutes post-application was similar to that at the eight-hour mark. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk. Current Issue October There are two principal factors that influence the selection of base curves and their resulting lens forms : Mechanical factors Optical factors.
Current designs involve sensors embedded in a soft contact lens that connect with an external wireless device. They developed a contact lens that can show real-time changes in eye surface moisture and intraocular pressure by changing colors. Researchers used to think that eye color was determined by a single gene and followed a simple inheritance pattern in which brown eyes were dominant to blue eyes.
However, later studies showed that this model was too simplistic. The inheritance of eye color is more complex than originally suspected because multiple genes are involved. Most of the genes associated with eye color are involved in the production, transport, or storage of the pigment melanin.
Eye color is directly related to the amount and quality of melanin in the front layers of the iris. People with brown eyes have a large amount of melanin in the iris, while people with blue eyes have much less. Power is written as a unit of measurement called a diopter. Diopters can range from 0. Plus is for farsighted and minus is for nearsighted.
The average number for both far and nearsighted is 3. The additional numbers almost always include the curve and size of a contact lens, but may contain other information as well. The base curve, or BC, is the radius of the back of the contact lens, the part that sits on your eye 3. According to Laser Eye Surgery Review , the average measurement of a base curve is between 8.
An example of a common base curve would be BC 8. All contact lenses have a measurement called the diameter.
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